Scipedia: Documents published in 2020

Revisión sobre el comportamiento de las armaduras galvanizadas en el hormigón

María Jesús Samper — Fri, 22 May 2020 11:19:38 +0200

Esta comunicación pretende ofrecer una panorámica de las ventajas e inconvenientes derivados del empleo de las armaduras de acero galvanizados en el hormigón.

Se resumen las causas que pueden provocar la corrosión de las armaduras embebidas en el hormigón y se analiza la protección suplementaria que el galvanizado puede conferir a dichas armaduras en condiciones especialmente agresivas.

Se mencionan los datos publicados sobre los límites tolerables de cloruros para las armaduras normales, punto en el que se echa en falta la existencia de datos cuantitativos similares para las armaduras galvanizadas y se examina, asimismo, las repercusiones sobre ellas de la carbonatación del hormigón y de la presencia de pilas de aireación diferencial, así como la influencia sobre la adherencia hormigón-armaduras, y el peligro de fragilización de estas, por el hidrógeno desprendido en el curso del fraguado. Finalmente se nombran algunas de las obras donde se han empleado armaduras galvanizadas.

Electrochemical Behavior of Galvanized Steel in a Calcium Hydroxide-Saturated Solution. A Quantitative Assessment of Corrosion Rate.

María Jesús Samper — Fri, 22 May 2020 10:20:34 +0200

An electrochemical study has been carried out involving immersion of galvanized steel in a Ca(OH)//2-saturated solution with and without chloride additions. Results show that galvanized steel corrodes steadily during the first few days; after approximately a week, however, it becomes passive, the passivity being perfectly stable both in the presence and absence of chlorides for the entire test period which lasted for more than 60 days' exposure.

Quantitative measurements of corrosion rate of reinforcing steels embedded in concrete using polarization resistance measurement

María Jesús Samper — Fri, 22 May 2020 10:07:18 +0200

Up to this time all the methods to measure the corrosion rate of reinforcing steels embedded in hardened concrete have been of the destructive type, which requires much time and materials and makes the full scale study of the phenomenon difficult. The authors have applied the polarization resistance method to bars embedded in hardened mortar and immersed in Ca(OH)2‐saturated solution, and have found an acceptable agreement between the gravimetric and the electrochemical weight loss data.

Valores del potencial de corrosión y de a resistencia de polarización de armaduras galvanizadas embebidas en hormigón

María Jesús Samper — Fri, 22 May 2020 09:43:36 +0200

Muchas de las estructuras de hormigón armado se muestran exentas de corrosión aun después de periodos muy prolongados de servicio. Ello es debido a que en el hormigón el electrolito está constituido por la fase líquida que rellena sus poros y que es, fundamentalmente, una disolución saturada de CA(OH)2, con un pH alrededor de 13. A este elevado pH y a los potenciales de corrosión Ecorr, que presentan las armaduras, el acero se encuentra en la zona de pasividad descrita por Pourbaix, permaneciendo protegido indefinidamente mientras no cambien las condiciones del medio.

Este cambio en las condiciones de servicio del hormigón capaz de introducir en la zona de corrosión al acero de las armaduras, se puede producir con relativa frecuencia. Así, la carbonatación del cemento disminuye el pH del hormigón y provoca una corrosión generalizada, la presencia de iones despasivantes o de fisura conduce a corrosión localizada y, finalmente, la acción simultánea de tensiones mecánicas y factores agresivos puede ser la causa de corrosión bajo tensiones.

Así pues, es aconsejable recurrir a una protección suplementaria de los aceros cuando se prevean situaciones que puedas provocar la corrosión de las armaduras. Entre los varios métodos de protección que se utilizan se viene ensayando desde hace varios lustros, con resultados muy contradictorios, la galvanización en caliente de los redondos.

Corrosion of Reinforcing Bars in Carbonated Concrete

María Jesús Samper — Fri, 22 May 2020 08:50:58 +0200

In all published studies of cement carbonation there has been general agreement that it must be considered as a cause of generalised corrosion in reinforcements, but quantitative data have been provided in very few cases. In the present work the intensity of attack has been modified by means of the accelerated carbonation of mortar specimens without additives, with 2% CaCl2, and with 3% NaNO2 It has been shown that a critical level of atmospheric moisture must also be present for considerable attack to occur. The measurement of polarisation resistance (Rp) has been used to evaluate the intensity of corrosion of the reinforcements. This method has already been usefully applied by the authors to investigations of corrosion of steel bars in hardened concrete specimens.

Influencia de la dosificación en cemento sobre la corrosión de armaduras, negras y galvanizadas provocado por lo penetración de Cl- en el mortero

María Jesús Samper — Thu, 21 May 2020 16:01:58 +0200

En el presente trabajo se aborda el estudio de la corrosión de armaduras, tanto desnudas como galvanizadas, producida por la penetración de agua de mar en nueve calidades (dosificaciones y por lo tanto porosidades) diferentes de mortero de cemento portland en el que se embebieron redondos a tres profundidades (0,5-1,5-2,5 cm). Las probeta de 7 X 7 X 7 cm se conservaron sumergidas durante 1 año. Para la evaluación de la velocidad de corrosión se ha utilizado la Resistencia de Polarización, técnica que permite el seguimiento continuo de la despasivación progresiva que provocan los Cl^- al llegar a la armadura. Se aportan resultados sobre la penetración de Cl^-, discutiéndose las relaciones encontradas entre: corrosión, % de Cl^-, porosidad y dosificación de cemento. Finalmente se compara el comportamiento frente a la corrosión de las armaduras de acero desnudo respecto de las galvanizadas.

Effect of Carbonation, Chlorides and Relative Ambient Humidity on the Corrosion of Galvanized Rebars Embedded in Concrete

María Jesús Samper — Thu, 21 May 2020 15:25:01 +0200

The variation with time of the corrosion rates of galvanized steel and bare steel rebars has been studied as a function of the relative ambient humidity, mortar carbonation and the presence or absence of chloride ions. Corrosion rates are estimated from Rp measurements with electronic compensation for the ohmic drop between the working imd reference electrodes.

It is shown that a normal galvanized coating with a continuous layer of practically pure zinc in contact with mortar gives effective protection to the steel reinforcing bars against the aggressive action of carbonation and chlorides. The protective effect of this galvanized coating is much greater under partial immersion conditions of the mortar than in very humid atmospheres, although in the latter case the corrosion rate decreases with time

Techniques electrochimiques qualitatives et quantitatives pour mesurer les effets des additions sur la corrosion des armatures

María Jesús Samper — Thu, 21 May 2020 15:12:25 +0200

Les auteurs resument une serie de considerations theoriques sur l'incidence que les additions actives peuvent avoir dans la corrosion des armatures. Ensuite, ils attirent l'attention sur le manque de normes ou recommandations de travail, qui s'appuient sur des essais suffisants, et qui pourraient permettre de delimiter les causes et conditions qui influent sur la duree des armatures ; finalement ils exposent une systematique d'essais qu'ils ont realisee pour etudier cette incidence au moyen de techniques electrochimiques, et les differences de comportement trouvees entre les differents ciments.

Les effets des cycles d’humidité sur la corrosion des armatures galvanisées dans les mortiers carbonatés et non carbonatés

María Jesús Samper — Thu, 21 May 2020 14:17:41 +0200

Galvanisation of steel reinforcement has been usual practice for the last decades as a protection against certain types of aggressive agents. However, recent results, mostly European, have thrown doubts on the usefulness of galvanisation.

The results presented here contribute to explain this controversy. We have identified three factors that may influence the corrosion kinetics of galvanized steel bars embedded in concrete, although they usually go unheeded: type of galvanized coating, type of cement and humidity during curing and hardening.

From these factors, this report deals mostly with ambient humidity. Corrosion rate was basically estimated from the Polarisation Resistance in reinforced mortar specimens that were cyclically subjected during curing and hardening to different ambient humidities. The influence on the corrosion kinetics of the structure of the galvanized coating which, in turn, depends on the chemical composition of the steel, is also discussed. Finally, the behaviour of bare and galvanized steel are compared, and criteria are given on the usefulness of galvanized coatings.

Corrosion Rate of Galvanized Steel Immersed in Saturated Solutions of Ca(OH)2 in the pH Range 12–13 8

María Jesús Samper — Thu, 21 May 2020 13:45:38 +0200

The influence of the alkaline content of cement on the corrosion kinetics of galvanized reinforcements has been studied, by means of the polarisation resistance method, in saturated solutions of Ca(OH)2 with additions of NaOH, KOH and SO2 4, thus simulating the liquid phase of concrete.It has been established that below a threshold pH the coating protects the surface; above this pH the coating is attacked, disappearing in a few days. Cements with low alkali contents and proper concrete dosage must therefore be used to ensure that the pH remains below the threshold value.

Corrosion rate of galvanized steel immersed in saturated solutions of Ca(OH)2 in the pH Range 12-13.8

María Jesús Samper — Thu, 21 May 2020 13:23:41 +0200

The influence of the alkaline content of cement on the corrosion kinetics of galvanized reinforcements has been studied, by means of the polarisation resistance method, in saturated solutions of Ca(OH)2 with additions of NaOH, KOH and SO²4, thus simulating the liquid phase of concrete. It has been established that below a threshold pH the coating protects the surface; above this pH the coating is attacked, disappearing in a few days. Cements with low alkali contents and proper concrete dosage must therefore be used to ensure that the pH remains below the threshold value.

SEM Study of the Corrosion Products of Galvanized Reinforcements Immersed in Solutions in the pH Range 12·6 to 13·6

María Jesús Samper — Thu, 21 May 2020 12:51:15 +0200

In previous work on galvanized reinforcements in contact with Ca-containing highly alkaline media the authors have reported the existence of a threshold pH below which the Zn in contact with such a medium is passivated, but at pH values above this it corrodes continuously until it totally disappears. In this paper, after a brief literature review concerning the identification of the corrosion products in these media, they report that the passivation detectedfrom the data of the polarisation technique agreed with the results of an SEM study. This showed that below the threshold pH value the surface is covered with a continuous layer of calcium hydr.oxyzinccite, Ca (Zn(OH)3)2.2H2O, and above this pH this layer is increasingly composed ofnon-passivating ZnO, to the detriment of the calcium hydroxyzincate content. They have also established that it is the (Ca2+) ions that control the amount of calcium hydroxyzincate formed, and therefore the likelihood of the formation of a continuous passivating layer. Finally, they report that if the calcium hydroxyzincate isperfectly developed, a later increase of thepH does not affect the stability.

Corrosion rate of three different types of galvanized coatings of steel reinforcements in contact with mortar

María Jesús Samper — Thu, 21 May 2020 11:57:45 +0200

Corrosion of galvanized reinforcements in concrete is controlled by three factors: the type of the cement, the type of metallographic structure and the humidity of conservation of the concrete. The behaviour of three different types of galvanized coating embedded in mortars made with three different Portland cements: low, medium and high alkali content, was studied. The corrosion rate was measured by means of the linear polarization (Polarization Resistance) technique. The results show that the thicker the pure zinc layer is, the greater the resistance against the corrosion of the chlorides.

Effect of four coating structures on corrosion kinetic of galvanized reinforcement in concrete

María Jesús Samper — Thu, 21 May 2020 11:24:11 +0200

In the present work is described the effect of the characteristics of the galvanized coating on corrosion rate of reinforcing bars embedded in a concrete without additives and in another with 2% CaCl2 in terms of cement weight. To this end, the following materials were subjected to the action of a moisture saturated atmosphere for a period of400 days: an annealed-galvanized coating; another one obtained by dipping in a molten Zn bath with0.025% Al and some others produced at450 and430°C in an ordinary bath.

The attack evolution vs time was controlled by measuring the polarization resistance. The electrochemical determinations were gravimetrically verified and supplemented by a metallographic examination of tests coatings.

In the absence of chlorides, the attack is fully tolerated. Chlorides radically change the corrosion behaviour of galvanized steel and increase the attack by a factor of2.8 in the most favourable case, but in these conditions, coatings with thick, homogeneous η layer outperform the usual ones, and specially the annealed ones.

Corrosion of galvanized steel reinforcements in alkaline solutions: Part 2: SEM study and identification of corrosion products

María Jesús Samper — Thu, 21 May 2020 11:09:58 +0200

A report is presented of the morphology and composition of the corrosion products formed on galvanized corrugated steel reinforcing bars immersed in 0·05–1·5M KOH solutions designed to simulate concrete pore environments. The morphology was studied by SEM of the in situ corrosion products and optical examination of transverse sections of the bar; phase identification was performed by X-ray diffraction and IR absorption analyses. Passivation events on the corrosion potential and corrosion intensity curves can be related to precipitation of prismatic Zn (OH)2 crystals which form a compact layer on the surface. Active corrosion is associated with formation of porous ZnO layers or precipitation of Zn(OH)2 in the solution rather than on the galvanized surface. A solution-precipitation mechanism involving ZnO and Zn(OH)2− 4 intermediate species is proposed for formation of the passivating Zn(OH)2 coating, although this scheme must be complemented to take account of the localized corrosion observed at pH < 12 in the first part of this study

Corrosion of galvanized steel in dilute Ca(OH) 2 solutions (pH 11·1–12·6)

María Jesús Samper — Thu, 21 May 2020 10:03:47 +0200

The corrosion behaviour of. galvanized bars was studied in 0·1–3 g Ca(OH)2 (pH 11·1–12·6) solutions designed to simulate the aqueous phase found in concrete pores. Corrosion potential and current density were monitored over the course of the 33 day tests and the corrosion products were examined by light and electron microscopy, X-ray diffraction, and IR absorption spectroscopy. Most specimens showed high initial corrosion rates which fell to a lower, nearly constant level after ∼6 days. The corrosion kinetics were found to depend only on solution pH. Ca2+ concentration had no effect other than on the nature of the corrosion products: Ca(Zn(OH)3)2.2H2O is the passivation product in the presence of calcium ions, Zn(OH)2 in their absence. Localized corrosion was observed at pH <11·5.

Corrosion of galvanized steel reinforcements in alkaline solutions: Part 1: Electrochemical results

María Jesús Samper — Thu, 21 May 2020 09:20:12 +0200

The behaviour of galvanized corrugated reinforcing bars has been studied in 0·001–1·5M solutions of KOH and NaOH designed to simulate aqueous environments found in concrete pores. Specimens were immersed for 33 days in polyethylene cells, corrosion potential, corrosion intensity, and pH being monitored throughout. Polarization curves were also obtained after 50 min and ∼ 33 days' immersion. It was possible to establish pH ranges in which localized corrosion (< 12), stability (12–13·4), and complete dissolution of the galvanic coating (> 13·4) occurred. A white, dusty corrosion product was observed on most specimens at the conclusion of the tests. Hydrogen dissolution on the galvanized bars occurred at pH values higher than the threshold level of 12·8±0·1.

Equilibria of the chemical composition of the concrete pore solution. Part I: Comparative study of synthetic and extracted solutions

María Jesús Samper — Thu, 21 May 2020 08:49:26 +0200

A critical review is made on Frattini and Hansen's results on the equilibria of the different ions present in the aqueous solution of cement paste after mixing. These values are compared with those obtained from results of squeezed cement paste specimens drawing the conclusions that the cement paste solution is oversaturated in Ca(OH)2 just after mixing but the ions present in it are not in thermodynamic equilibrium because the solution is continuously evolving during early ages.

Also the increase in the solubility of the Ca+2 ion because of the presence of SO4= is discussed and attributed to the increase in the ionic strength of the solution.

Galvanized steel behaviour in Ca(OH)2 saturated solutions containing SO4 ions

María Jesús Samper — Thu, 21 May 2020 08:04:52 +0200

In previous work on galvanized reinforcements in contact with Ca-containing highly alkaline media the authors have reported the existence of a threshold pH of 13.3 ± 0.1 below which the Zn in contact with such a medium is passivated, but at pH values above this it corrodes continuously until it totally disappears. In this paper, the influence of the presence of SO4 ions on the electrochemical behaviour of the galvanized has been studied.

It has been established that the precipitation of CaSO4 with takes place in the media studied makes that the pH never surpasses the value of 13.3 ± 0.1. This coincidence with the pH threshold previously reported for passivation of the galvanized bars allows conclude that the sulfates not only are innocuous with respect to the galvanized behaviour in media which simulate the concrete but they are beneficial.

Some unpublished results on the research in corrosion of steel in concrete

María Jesús Samper — Wed, 20 May 2020 16:35:29 +0200

Accuracy of different electrochemical laboratory techniques for evaluating corrosion rates of galvanized reinforcements

María Jesús Samper — Wed, 20 May 2020 16:21:42 +0200

In the present paper the accuracy of three electrochemical laboratory techniques in order to evaluate the corrosion rate of galvanized rods inmersed in solutions which simulate the pore solution, is commented. The techniques used were: polarization resistance, electrochemical impedance and harmonic analysis. The corrosion intensity values obtained through these techniques were compared with the gravimetric ones. Values of constant B obtained through polarization curves and the harmonic analysis were compared.

Evaluation of steel durability in a construction system of reinforced mortar-polystyrene-mortar panels

María Jesús Samper — Wed, 20 May 2020 16:13:49 +0200

This paper sumarises the research into steel durability in reinforced sandwich panels made of mortar-polystyrene mortar, which is a new construction system for buildings. In this type of system the main risk of steel corrosion is that of the steel connectors, which cross the polystyrene, and the corrosion of the reinforced mesh, owing to the carbonatation of the mortar coating. The evaluation of different suplementary protection methods which increase expected service life in aggressive environments is also presented.

The different tests carried out are described. The specimens were of small, medium and full size. The corrosion was monitored with electrochemical techniques (polarization resistance). The specimens were submitted to wet and dry cycles and some of them were carbonated. Galvanized, stainless and bare steel were tested. Also addition in the mix of calcium nitrite as an inhibitor was used. Results show that the steel connectors are not the weakest point of the system as previously thought.

Finally, practical recomendations are given which allow a reasonable increase in the service life of the reinforced mortar-polystyrene-mortar system. The specifications are presented considerinng three levels of environmental aggressivity.

Equilibria of the chemical composition of the pore concrete solution Part II: Calculation of the equilibria constants of the synthetic solutions

María Jesús Samper — Wed, 20 May 2020 15:31:06 +0200

The chemical composition of synthetic solutions used to simulate the pore concrete solution was analyzed in Part I. It was not possible to match the experimental results obtained from concentration measurements of the ions present in the solutions with the results obtained from calculations based on the values of pK of Ca(OH)2 and CaSO4. The inconsistency between the experimental values and the theoretical calculations was attributed to the elevated concentrations that exist, and, therefore, the elevated ionic strength.The errors that are committed when applying the Debye-Huckel theory make it imprecise for proper calculations at such high concentrations. As an alternative the use of “apparent constants” is proposed. These are calculated from the values of the concentrations directly measured in the solutions.

Polypropylene Fibre Reinforced Concrete Air Permeability

María Jesús Samper — Wed, 20 May 2020 14:14:50 +0200

The permeability of concrete to gases is of direct importance to the durability of concrete structures, because of carbon dioxide flowing through the concrete favour lime carbonation and reinforcing steel corrosion.

Mortar with and without polypropylene fibres having water/cementitious ratios of 0.30, 0.35 and 0.40 and a cement/sand ratio of 1/1 were studied. Polypropylene dosage varied from 0.1 to 0.3% by volume of cement.

The characterization of mortar permeability was made using cylindrical shaped samples (3 cm height and 15 cm diameter). These specimens were 28 days cured and then dried before the test.

The addition of fibres results in a decrease of air permeability. Variation of the water/cement ratio is of lesser importance than fiber addition.

Quantitative measurement of the corrosion rate using a small counter electrode in the boundary of passive and corroded zones of a long concrete beam

María Jesús Samper — Wed, 20 May 2020 13:54:36 +0200

Recently, in CORROSION-87, one paper was presented which, assuming the electrical model of the 'transmission line' to represent the nonuniform distribution of the current in a concrete beam, demonstrated the calculation of the instantaneous corrosion rate by unit of length of the rebar through mathematical equations. In the present contribution those equations are used in order to determine whether the equations enable distinction between the corroded zones of a rebar and those which remain passive. A concrete beam was fabricated, half of it by mixing chlorides with the cement (and therefore inducing a corrosion in the rebars) and the other half without admixtures (passivated zones of the rebars). The apparent polarization resistance was measured along the beam using a small counter electrode. Through the equations previously proposed, the corrosion rate was calculated. The results obtained confirm that the boundary between corroded and passivated zones can be clearly identified using these formulae. Finally, some comments are given on the relative activity of galvanic macrocells compared with the corrosion microcells.

Initial effort to use the corrosion rate measurements for estimating rebar durability

María Jesús Samper — Wed, 20 May 2020 13:20:06 +0200

The approach presented here is an attempt to implement the values of corrosion intensity which have been measured in laboratory tests, in the framework of the service life prediction analysis for corroding structures. First of all, the model on service life suggested by Tuuti was considered, and only the propagation period model has been analyzed in this paper. In order to expand the proposal, different steps were covered: (1) the definition of an unacceptable level of deterioration, taking into consideration the levels suggested by the Comite Eurointernational du Beton (CEB) in its Bulletin No. 162, in order to define the urgency of intervention in a damaged structure; (2) the reduction in bar diameter or bar section was taken as the determining parameter in failure risk, assuming that this decrease in section occurs either in a generalized form or in the zones of the structure in which the load-carrying capacity may be significantly affected; and (3) the ranges of possible corrosion intensity values were introduced in Tuuti's model for calculating the reduction in bar section in function of the life of the structure. Some examples for bars of 10 and 20 mm Φ were presented. Finally, the limitations and the improvements of the proposal are discussed.

The behaviour of galvanized steel in chloride-containing alkaline solutions-I. The influence of the cation

María Jesús Samper — Wed, 20 May 2020 12:08:23 +0200

The behaviour of galvanized reinforced bars (rebars) in chloride-contaminated concrete is a very controversial matter not yet resolved. An extensive study of the behaviour of galvanized steel in alkaline solutions which simulate the aqueous phase present in concrete pores has been undertaken. This paper describes the modifications that the presence of chloride has on the corrosion behaviour of galvanized steel in alkaline solutions, as well as the influence of the accompanying cation. It was confirmed that the corrosion behaviour of galvanized steel in the presence of chlorides is controlled by the medium pH, which depends on the calcium or sodium salt which provides the chloride ions.

Identificación de presencia de cemento aluminoso en hormigones mediante el uso del pachómetro

María Jesús Samper — Wed, 20 May 2020 11:37:49 +0200

The observation of the premature degradations detected in High Alumina concretes due to the transformation and carbonation of this kind of cement has promoted the interest to develop in-situ testing methods for the fast Identification of HAC presence. In present work, preliminary results are given on the use of magnetic methods (pachometer), usually employed in the concrete cover verification, to identify the existence of HAC in a concrete. The laboratory results have been verified in practice and show that HAC can be clearly distinguished of an OPC concrete with the pachometer used in present experimentation. The research has also indicated that fly ash cements as aluminous one, have a lower magnetic reluctance. La constatación de los daños que se producen en estructuras de hormigón de cemento aluminoso, debido a su rápida transformación y carbonatación, ha llevado a intentar poner a punto diversos métodos de detección acelerada in situ de la presencia del citado cemento. En el presente trabajo se aportan unos primeros resultados sobre el empleo de métodos magnéticos (pachómetro) habituales en la medida del espesor del recubrimiento de armaduras para identificar si un hormigón está fabricado o no con cemento aluminoso. Los resultados de laboratorio se han contrastado en obra pudiéndose comprobar que el pachómetro utilizado es capaz de detectar perfectamente la presencia de aluminoso previa calibración con un hormigón de portland puro. Esta investigación preliminar también ha permitido comprobar que los hormigones de cenizas volantes presentan, como el aluminoso, una menor reluctancia magnética

A reply to the discussion by A. Xu and S. Chandra of the Paper “calculation of chloride diffusion coefficients in concrete from ionic migration measurements”

María Jesús Samper — Wed, 20 May 2020 10:33:07 +0200

Calculation of chloride diffusion coefficients in concrete from ionic migration measurements

María Jesús Samper — Wed, 20 May 2020 10:00:36 +0200

A critical review is offered on the Rapid Chloride Permeability Test standarized by AASHTO, pointing out its limitations and errors but recognizing its contribution to the developing of a simple and quick test for chloride migration. Then another review is made on the electrochemical fundaments of the processes developped in concrete when an electrical field is applied and on the basic equations of mass transport (Nernst-Plank and Nernst-Einstein) which can be applied to calculate ionic movements. The limitations and assumptions needed for a simplified resolution of these equations, are presented, as well as numerical examples of calculation of the Effective Diffusion Coefficient of chlorides, Deff, in steady state condition. Finally, considerations on the possibility of calculation of Deff from simple resistivity measurements are also offered.

Suitability of Assessment Methods for Identifying Active and Passive Zones in Reinforced Concrete

María Jesús Samper — Wed, 20 May 2020 09:19:47 +0200

The ability of various electrochemical techniques to differentiate between areas of reinforcing steel (rebar) with high and low corrosion activity and to define the boundary between such areas was studied. These techniques are based on determining the corrosion potential (E[sub corr]), the galvanic current between an external stainless steel (SS) sensor and the bar (I[sub g]), the apparent polarization resistance (R[sub p][sup ap]), or the polarization resistance (R[sub p]) under electric confinement conditions. Values of E[sub corr] and I[sub g] (the current flowing between a SS counter electrode [CE] and the rebars) were shown to distinguish active areas from passive ones very well when the areas were isolated electrically, but with difficulty when the areas were in electric contact, which is usually the case. Values of R[sub p][sup ap] or i[sub corr] (both calculated using the Stern-Geary equation) were shown to define areas of high corrosion rates in a structure where passivity prevailed much better. However, excessive i[sub corr] values were estimated because the polarized area of the rebar (especially with passive rebars) was much greater than the CE area. Finally, the confinement of signal distribution permitted definition of the exact boundary between active and passive areas, regardless of whether they were isolated or interconnected. In addition, i[sub corr] values obtained with this technique were shown to provide a reliable quantitative index of the damage rate in reinforced concrete structures.

La realcalinización y la extracción electroquímica de los cloruros en las construcciones de hormigón armado

María Jesús Samper — Wed, 20 May 2020 08:47:35 +0200

Realkalisation and electrochemical chloride removal techniques, developed for rehabiliting carbonated and chloride-containing structures, are presented. Electrolysis and electromigration mechanisms and consequences as well as electrochemical conditions at the reinforcement surface are discussed and compared with cathodic protection ones. Furthermore, possible side effects are commented.

Se presentan técnicas electroquímicas de realcanización y extracción de cloruros, desarrolladas con el fin de rehabilitar estructuras carbonatadas y que contengan cloruros. Se discuten y comparan los mecanismos y consecuencias de la electrólisis y la electromigración, así como las condiciones electroquímicas en la superficie de las armaduras con los de protección catódica. Se comentan, también, los posibles efectos secundarios.

Mathematical modelling of a concrete surface skin effect on diffusion in chloride contaminated media

María Jesús Samper — Tue, 19 May 2020 16:11:23 +0200

Concrete skin is considered the closest zone to the surface of concrete cover of reinforcements. It usually has a different composition than the internal concrete due to phenomena such as contact with molds or segregation of aggregates. In addition, environmental actions induce a gradient of moisture along the cover depth. These circumstances sometimes produce an irregular chloride profile in the cover, which either exhibits a maximum of chloride content some millimeters inside the outer surface or sometimes shows an anomalously high chloride concentration right at the concrete surface. In the present paper, analytical modeling of chloride diffusion is used to study the relative influence of the skin thickness. This theoretical analysis aims to show that there are cases where, if the diffusivity of the skin and the bulk concrete are very different, an error is introduced when the “skin effect” is not accounted for. The clarification of this error may contribute to understanding the differences found between laboratory experiments and the long-term record of chloride profiles in the same concrete.

Evolution of pH during in-situ leaching in small concrete cavities

María Jesús Samper — Tue, 19 May 2020 14:18:34 +0200

Small amounts (0.4 cc) of neutral water placed in small cylindrical cavities (5 mm diameter) in concrete exposed to 100% relative humidity first developed a pH comparable to that of a saturated Ca(OH)2 solution. The pH then increased over a period of days-weeks toward a higher terminal value. A micro pH electrode arrangement was used. This behavior was observed in samples of 12 different concrete mix designs, including some with pozzolanic additions. The average terminal cavity pH closely approached that of expressed pore water from the same concretes. A simplified mathematical model reproduced the experimentally observed behavior. The model assumed inward diffusional transport of the pH-determining species in the surrounding concrete pore solution. The experimental results were consistent with the model predictions when using diffusion parameters on the order of those previously reported for alkali cations in concrete. The cavity size, cavity water content, and exposure to atmospheric CO2 should be minimized when attempting to obtain cavity pH values approaching those of the surrounding pore water.

Progress Report 1994-97

María Jesús Samper — Tue, 19 May 2020 13:41:35 +0200

L Aniversario

María Jesús Samper — Tue, 19 May 2020 13:26:44 +0200

Editorial

María Jesús Samper — Tue, 19 May 2020 13:20:25 +0200

An accelerated leaching test to evaluate the long term behaviour of concrete in waste disposal

María Jesús Samper — Tue, 19 May 2020 12:21:28 +0200

Towards excellence

María Jesús Samper — Tue, 19 May 2020 11:52:45 +0200

Progress has been made in American public education in the last 20 years, and the pending American Defense Education Act, inspired by the National Defense Education Act, would enable school districts to address many of the current problems of public schools. (MSE)

Statements and Conclusions

María Jesús Samper — Tue, 19 May 2020 11:41:48 +0200

Engineers have been dealing with deterioration in concrete structures since shortly after their initial use. Before 1980 deterioration was taken into consideration predominantly using rules of thumb developed through experience. After 1980 simple engineering models, such as Fick's law, the Hetek's model and time dependent models, were developed and became more widely used. With time these models, both deterministic and stochastic, are becoming increasingly more sophisticated and are increasingly based on "physical" parameters. This trend will continue and future models will be based even more so on the physical, chemical and mechanical laws and principles that govern material and structural behaviour.

Calculation of initiation and propagation periods of service life of reinforcements by using the electrical resistivity

María Jesús Samper — Tue, 19 May 2020 11:09:39 +0200

Reinforcement corrosion is attracting research interest in many areas due to the economic consequences of the damage generated by the process. Several proposals can be found on prediction of the time to reinforcement corrosion and service life duration. In the present communication a proposal is made on using the electrical resistivity to calculate both the initiation and propagation periods. For the time period to corrosion onset, the electrical resistivity serves to model the porosity and its connectivity and therefore can be used to calculate transport processes. Due to the reaction of chlorides and carbon dioxide with cement phases, the resistivity has to be factored by a “reaction factor”, r, to account for this. Concerning the propagation period, the electrical resistivity is an indication of the moisture content of concrete and therefore, it has a relationship with the corrosion current. The service life can be expressed with the following equation: t_l = t_i + t_p = x^2 rho_es r_CICO2 / k_CI,CO2 + P_x . rho_ef / k_corr Based on this, minimum resistivity values can be established according to cover thickness and as a function of exposure classes.

Evaluacion del comportamiento a largo plazo de hormigones en almacenamientos de residuos radiactivos de alta actividad

María Jesús Samper — Tue, 19 May 2020 10:18:48 +0200

The present work describes an accelerated leaching method that with a rapid process allows to develop and evaluate cements for use in a nuclear disposal, and the understanding of the long term effects. The method has been developed to study the stability of cementitious materials in contact with bentonite, to be used in high level radioactivity waste repositories. Nitric acid has been selected to simulate in an accelerated way the pH decreasing produced when concrete is in contact with groundwaters. Evaluacion del comportamiento a largo plazo de hormigones en almacenamientos de residuos radiactivos de alta actividad El presente trabajo describe un ensayo acelerado de lixiviación, que mediante un proceso rápido, permite desarrollar y evaluar cementos para su uso en instalaciones nucleares, y la comprensión de su comportamiento a largo plazo. El método se ha desarrollado para estudiar la estabilidad de materiales de base cemento, en contacto con bentonita, que serán utilizados en almacenamientos de resíduos radiactivos de alta actividad. Como agente lixiviante se seleccionó el ácido nítrico, con objeto de simular de forma acelerada, la disminución del pH que se produce cuando el hormigón entra en contacto con aguas subterráneas.

Are service life models appropriate for use in design of reinforced concrete structures for exposure to chloride salts?

María Jesús Samper — Tue, 19 May 2020 09:32:26 +0200

Chloride ingress leading to corrosion of embedded steel is the most detrimental process affecting the durability of reinforced concrete structures. Various authors have suggested methods of calculating the time to depassivation of reinforcing steel by modelling chloride penetration on the basis of a solution of Fick's second law of diffusion. This involves several assumptions, including boundary and initial conditions that may be fulfilled in idealised laboratory tests but not in real structures. The situation calls for an explicit recognition of the limited knowledge that exists regarding both detailed mechanisms of chloride ingress and threshold chloride concentrations that cause depassivation of steel in different types of concrete subjected to different conditions of exposure. These uncertainties can result in major inaccuracies when models are used to predict the long-term performance of structures. Despite the admitted lack of detailed understanding of the process of chloride penetration, tentative proposals are outlined for alternative methods of determining performance indices useful in some circumstances for specifying the durability of structures exposed to chloride salts. They depend on measurement of one parameter (steady-state diffusion coefficient or electrical resistivity) to account for the transport process and a separate parameter to account for chloride binding. The values obtained provide a means of ranking different types of concrete in terms of their expected performance when exposed to chloride environments in which diffusion is likely to be the dominant transport mechanism.

Model for prediction of reinforced concrete service life based on electrical resistivity

María Jesús Samper — Tue, 19 May 2020 09:07:00 +0200

Reinforcement corrosion is attracting research interest in many areas due to the economical consequences of the damage generated by the process. Several proposals can be found on prediction of the time to reinforcement corrosion and service life duration. In present communication a proposals is made on using the electrical resistivity to calculate both the initiation and propagation periods. For the time to corrosion onset, the electrical resistivity represents the porosity and its connectivity and therefore can be used to model transport processes. Due to the reaction of chlorides and carbon dioxide with cement phases, the resistivity has to be factorised by a “reaction factor” accounting for it. Concerning the propagation period, the electrical resistivity is an indication of the moisture content of concrete and therefore, it has a certain relationship with the corrosion cement. The service life can be expressed with the following equation: corr x ef Cl CO es ClCO l i p k P k x r t t t ρ ·ρ 2 2 , 2 = + = + Based in it, minimum resistivity values can be established according to cover thickness and in function of exposure classes.

Reinforced-concrete cover cracking due to the pressure of corrosion products

María Jesús Samper — Tue, 19 May 2020 08:35:10 +0200

Reinforcement corrosion is one of the most important phenomena that reduce the service life of the reinforced concrete structures. Steel corrosion reduces the strength and bond of the reinforcement and the oxides formed cause internal stresses that crack the concrete cover. The prediction of the evolution of these effects is a problem that involves chemical and mechanical aspects. The phenomenon is not well known and quantitative descriptions of development and magnitude of stresses produced by a corroding rebar to concrete are scarce and although there are several models in the literature they do not universally reproduce the experimental results. This work presents a summary of the available experimental evidence on the amount of internal expansion needed at the reinforcement level for concrete cover to crack and formulas to estimate the expansion. Also, results are presented on the pressure needed for concrete cover to crack together with the experimental technique used for the verification of the assumptions and crack propagation analysis.

Elasto-plastic model for beams damaged by corrosion

María Jesús Samper — Tue, 19 May 2020 08:15:34 +0200

This article describes the elasto-plastic model that has been applied to analyse the beams damaged by corrosion. CONTECVET Manual, which contains the principles of this kind of structural evaluation, has been adopted. This Manual recommends considering the reduced section of the concrete and of the reinforcements as well as the possible reduction of the concrete steel bond. The first part of the article describes the basis of the structural model and, the mechanical behaviour of the materials. In the second part, the results of the experimental tests are presented and compared with the results of the model. The results indicate an enough good accordance, if the typical scatter of this type of experimental tests is taken into account.

Stress corrosion cracking and fracture toughness variation of High Strength Steels

María Jesús Samper — Tue, 19 May 2020 08:07:40 +0200

The stress corrosion cracking (SCC) process is at present a not fully elucidated mechanism of deterioration. It is a surface process that implies a corrosion and stress synergy, but the most practical consequence is that stress corrosion cracking can modify the mechanical characteristics of the metal causing brittle failure. Previously, we present some results about stress corrosion cracking, crack propagation rate or, even, crack arrest conditions in High Strength Steels. This kind of steels is usually used in prestressed and postensioned structures. These wires are of eutectoid composition and cold drawn. It is well established that failures occur when the wires are in contact with electrolytes of specific compositions while under stress. In the case of concrete, the electrolyte is its pore solution and the stress levels result from the different loads applied due to structural requirements. In this work we suggest some improvements of the Mechanism of SCC based in the Surface Mobility of vacancies on the crack surface proposed by Galvele. Improvements consist in incorporating the electrochemical corrosion as one of the sources for the creation of vacancies and some mechanical effects, both produce synergic effect in the crack propagation rate and they are important for a more comprehensive explanation of the process. On the other hand, the Fracture Toughness change when the steel corrodes, questioning the idea that is an intrinsic characteristic of the material. The reduction in the fracture toughness of steel wires when they are in contact to aggressive media involve that the material becomes less damage tolerant, which implies that it is necessary to detect defects of smaller size, as for example, small notch, pits or superficial cracks.

Structural Assessment of Corroded Beams

María Jesús Samper — Mon, 18 May 2020 17:55:00 +0200

This paper shows how the corrosion and mechanical actions can be taken into account in a simple model for the assessment of structural elements. This work is framed in the Project Benchmark des Poutres de La Rance in which several partners have participated. The Project tried to characterize the degradation of reinforced and prestressed concrete structures after an aging in marine atmosphere with non-destructive methods and to evaluate different existing models for the structural assessment. For this aim a 4-point bending test was performed on six prestressed concrete beams. The model gives a reasonable approach for the assessment of structural elements. Similar rules can be adopted for other failure modes. It is important to notice the need of reliable input data for the assessment and also the need of integration of deterioration processes in design.

Molecular Dynamics simulations of Hydrogen Embrittlement: preliminary results

María Jesús Samper — Mon, 18 May 2020 16:58:28 +0200

Hydrogen embrittlement is believed to be one of the main reasons for cracking of structures under stress. High strength steels in these structures often include a ferritic core made of alpha-iron (body centered cubic lattice). Previous work [1] was concerned with the interaction of atomic hydrogen with iron using first principles calculations. We studied the effect of interstitial hydrogen in the iron lattice and the stress induced by the interstitial hydrogen in the host lattice. In this paper we study the dynamical behaviour of hydrogen inside the iron lattice. Using ab-initio Molecular Dynamics we obtain hydrogen diffusion paths and by taking statistical averages we extract diffusion coefficients from Einstein’s equation. Depending on temperature, the diffusion path involve going through tetrahedral or octahedral sites. Simulations where a number of hydrogens occasionally coincide in one unit cell have been performed to elucidate the effect of interactions between hydrogens

Estudio del efecto sumidero de CO2 de los materiales de base cemento

María Jesús Samper — Mon, 18 May 2020 16:42:30 +0200

Uno de los principales problemas relacionados con el cambio climático es el efecto invernadero, producido por la absorción parcial de la radiación infrarroja emitida por la Tierra por parte de los gases denominados "de efecto invernadero", entre los que se incluye el dióxido de carbono.

El trabajo aquí planteado se enmarca dentro de las iniciativas acometidas por la industria del cemento para contribuir a la reducción del efecto invernadero y los estudios realizados para estimar la contribución de los materiales de base cemento en la lucha contra el cambio climático. Para ello se estudia el alcance del efecto sumidero de CO2 que supone la carbonatación del hormigón.

La Convención Marco de las Naciones Unidas sobre el Cambio Climático de 1992 recoge en su artículo 1 la definición de sumidero: "Por sumidero se entiende cualquier proceso, actividad o mecanismo que absorbe un gas de efecto invernadero, un aerosol o un precursor de un gas de efecto invernadero de la atmósfera".

CO 2 sink effect of concrete carbonation

María Jesús Samper — Mon, 18 May 2020 16:28:01 +0200

Although carbonation of cement phases is well known, the amount of carbon dioxide combined during the process has been much less investigated. Related to the greenhouse effect much more attention is being paid to the sinks for C02 in order to correctly compute the gases emission during production of materials. In the case of cement a strict calculation should discount the C02 emitted from that fixed by the concrete. This is the aim of present work which is a study of the cement based materials ability to combine C02. Using Thermogravimetry and Differential Thermal Analysis as well as phenolphthalein indicator, quantities of C02 combined and carbonation depth in several cement pastes and concretes exposed to different environments have been measured. A calculation to estimate the contribution of cement based materials carbonation to the partial re-absorption of the C02 emitted in cement production is proposed.

The use of electrical resistivity as a NDT for the specification of concrete durability

María Jesús Samper — Mon, 18 May 2020 15:48:26 +0200

The calculation of the time to reinforcement corrosion and service life dura-tion is a subject of increasing interest. In present communication is explained how to use the electrical resistivity to calculate both the initiation and propagation periods in the case of carbonation and chloride penetration, although only the case of chlorides is illustrated here. The main advantage is that resistivity is an inexpensive non destructive measurement that can be used for routine quality control. The resistivity is an indication of the porosity and pore connectivity. It increases with time as hydration proceeds which is an indication of aging or pore refinement. In addition to the measurement of the resistivity it is necessary to calculate a "reaction factor" for accounting of the binding with cement phases. Concerning the propagation period, the electrical resistivity is an indication of the moisture content of concrete and therefore, it has a relationship with the corrosion cement. A model is proposed in which the resistivity is introduced in the square root of time law.

Correlación entre ancho de grieta del recubrimiento del concreto y corrosión del refuerzo en elementos expuestos a un ambiente natural contaminado por cloruros

María Jesús Samper — Mon, 18 May 2020 15:26:51 +0200

La corrosión del refuerzo genera daños variados a estructuras de concreto que influyen en la capacidad de carga; entre estos daños se puede mencionar el agrietamiento del recubrimiento del concreto. Este trabajo proporciona resultados de campo en el tema de propagación de grietas del recubrimiento del concreto por corrosión del refuerzo después de una exposición natural por varios años. Los elementos estructurales evaluados fueron una viga T y una columna cuadrada expuestos al ambiente atmosférico de Madrid, España. Dos formulas empíricas fueron usadas para correlacionar los valores obtenidos del agrietamiento del recubrimiento por corrosión: w=k Px/(C/φ), y w=k Px/Ro, en donde w es el ancho de la grieta por corrosión, k es una constante de proporcionalidad, Px es la penetración por la corrosión en el tiempo, C/φ es la relación recubrimiento de concreto/diámetro del refuerzo, y Ro es el radio original del refuerzo.

Determination of chloride diffusivity through partially saturated Portland cement concrete by a simplified procedure

María Jesús Samper — Mon, 18 May 2020 14:52:55 +0200

A recently proposed methodology for measuring the diffusion coefficient of chloride ions through concrete partially saturated with water, has been applied to five high-early-strength Portland cement mortars. The method is based on putting solid NaCl in contact with the concrete surface during the diffusion test. The results obtained indicate a strong dependence of the diffusion coefficient on the water saturation degree. The ionic diffusivities through these mortars are higher than those obtained with the same approach for five pozzolanic cement mortars of similar compositions. It has been also shown that the proposed test methodology and a different one based on the interaction of HCl(g) with the tested concrete surface, can yield comparable results. The D values obtained are of similar magnitude, and the same type of dependence on the water saturation degree is found through both methods, when applied to similar Portland cement concrete mixes with cement content of about 350 kg/m3 and water/cement ratios between 0.5 and 0.6.

Observations on the Morphology of Oxide Formation due to Reinforcement Corrosion

María Jesús Samper — Mon, 18 May 2020 14:21:44 +0200

The oxide formed during corrosion has an expansive character which induces the cracking of concrete cover. The properties of this oxide layer are focussing the interest due the need to introduce its mechanical characteristics in the models to calculate crack width. In a previous paper it was suggested that the oxide behave as “water” and in consequence it has to be modelled so, which fitted very well in the experiments and model associated an that time. The attribution to the water was based in the observation that the rust has not mechanical consistency and was like a suspension. However, much recent papers insist in attributing other much higher young modulus to the rust. Present paper tries to insist in the original concept by illustrating the formation of the rust in solution and in mortar. The oxides are of mixed colours evolving in function of the time to contact to open air and they have not mechanical consistency, being apparently hydrophilic by retaining large amount of water.

High Strength Steels Fracture Toughness Variation by the Media

María Jesús Samper — Mon, 18 May 2020 14:11:35 +0200

The stress corrosion cracking process is at this moment an unknown mechanism of deterioration. It is a process that implies the joint action of the media, the presence of corrosion or a surface defect and of stress in the metal. Prestressing tendons can suffer SCC jointly with hydrogen embrittlement which dramatically changes not only the type of fracture (from ductile to brittle) but also the kinetics of the process leading to unexpected collapses. The metal should be resistant to this type of process which can be characterized by its toughness and therefore by its damage tolerance. This research shows that the Fracture Toughness change when the steel corrodes, questioning the idea that is an intrinsic characteristic of the material. The reduction in the fracture toughness of steel wires when they are in contact to aggressive media involve that the material fractures with a lower crack depth for the same stress level. That means that the material becomes less damage tolerant, which implies that it is necessary to detect defects of smaller size, as for example, small notch, pits or superficial cracks. In the paper some results of the percentage of decrease of the toughness of prestressing wires suffering corrosion are presented

Bond Response in Structural Concrete with Corroded Steel Bars. Experimental Results

María Jesús Samper — Mon, 18 May 2020 13:36:38 +0200

The growing interest in upgrading existing reinforced concrete structures or extending their service life, and in ensuring greater durability in new designs, has led to a need for resistance models that take deterioration processes into account to verify structural safety. Bond activation between reinforcing steel and concrete is of cardinal importance in this context. A number of experimental studies have been conducted in recent years on bond failure, which normally leads to brittle behaviour. The findings have diverged rather widely, however, due primarily to differing test conditions. The present paper presents an experimental programme for eccentric pull-out tests in which specimens were subjected to both accelerated and natural corrosion in an attempt to surmount these inconsistencies. It also introduces an embedded fibre-optic sensing system with corrosion-resistant fibre Bragg grating sensors and discusses some of the findings.

Corrosion Resistance Performance of Steel-Reinforced Engineered Cementitious Composite Beams

María Jesús Samper — Mon, 18 May 2020 13:16:14 +0200

This paper presents the results of an experimental investigation on steel-reinforced engineered cementitious composite (ECC) beams subjected to accelerated corrosion by an electrochemical method. ECC is a micromechanically-based designed, high-performance, fiber-reinforced cementitious composite with high ductility and improved durability due to tight crack width. An accelerated corrosion test method, which was carried out by imposing a constant potential, was used to induce different degrees of corrosion into the reinforcement embedded in ECC prismatic specimens. Mortar specimens that have an equal compressive strength to the ECC specimens were also used as reference specimens. After inducing different degrees of accelerated corrosion, the cracks and the residual flexural load capacity of the test specimens and the mass loss of reinforcing bars embedded in specimens were determined. From the results of this study, it is concluded that due to its high tensile strain capacity and microcracking behaviors, ECC significantly prolonged the corrosion propagation period while enhancing the ability to maintain the load capacity of the beam. These performances of reinforced ECC (R/ECC) are expected to contribute substantially to improving infrastructure sustainability by reducing the amount of repair and maintenance during the service life of the infrastructure.

Hydrogen induced changes in structural properties of iron: Ab initio calculations

María Jesús Samper — Mon, 18 May 2020 12:50:05 +0200

We have used Ab initio calculations to study the structural changes produced by the inclusion of light impurities into pure bcc iron. In order to obtain a clear picture of the mechanics of the phase changes Bain’s pathway was studied in detail for pure iron. The position that hydrogen atoms tend to occupy at high densities favours octahedral sites inside the bcc matrix, producing an internal stress field that suggests a deformation that matches the prediction of martensitic transformation predicted by Bain’s pathway. We have used Density Functional Theory in order to optimize the structures studied, obtaining the enthalpy of the configuration as a function of c/a, allowing a better understanding of the dynamics of the process of phase changes.

Steel Corrosion in a Chloride Contaminated Concrete Pore Solution with Low Oxygen Availability

María Jesús Samper — Mon, 18 May 2020 12:44:07 +0200

It is commonly mentioned that in concrete chloride induced corrosion is controlled by the oxygen content in such a manner that in water saturated conditions no oxygen will be present and thus no corrosion can develop. In the present paper, experimentation has been made in low oxygen availability “pore” solutions with several amounts of chlorides. These situations may represent the case of a water saturated concrete. The results indicate that at very low oxygen contents, i.e. almost negligible because complete removal is very difficult, corrosion may develop in presence of chlorides. The presence or absence of corrosion is influenced by the amount of chloride, its corrosion potential and the steel surface condition.

From corrosion rate to accumulated corrosion depth or loss in cross section of reinforcements

María Jesús Samper — Mon, 18 May 2020 12:38:33 +0200

In marine environments, reinforcement corrosion develops due to the penetration of chlorides through the concrete pores. Initially the corrosion appears as localized attack, but when the chlorides arrive in high quantities, it extends to the bar surfaces which corrode entirely. In numerous previous studies, the detection of corrosion and its evolution have been monitored by means of measuring and presenting the variation of corrosion rate over time. This representation can be used to deduce when corrosion initiates on a steel bar and how it evolves with time. However, as the corrosion rate changes with temperature and the corrosion process itself evolves, this representation may not be clear enough to be used for comparative purposes. The present work proposes a parallel use of accumulated corrosion depth, which is obtained by the integration of each age of the corrosion rate-time curve. This kind of representation enables the determination of corrosion depth at each age and appears more appropriate for comparative purposes. The procedure is applied to concrete specimens that have been in contact with natural sea water from the Mediterranean Sea for 20 years. The specimens are prepared with different binders in different proportions for purposes of comparison.

Corrosion Data Interpretation in Concrete Structures

María Jesús Samper — Mon, 18 May 2020 11:34:53 +0200

Concrete protects the steel embedded until carbonation front or the chloride threshold reaches the rebar. Then a corrosion process develops. In present paper the corrosion process is analyzed from its onset a definition of which is given. The techniques for its measurement are described and also are given a model for the corrosion propagation period as well as some alternatives to assume a value of the corrosion rate, even based in the values of the concrete resistivity. Finally, the functions of relation between the corrosion and the structural consequences are commented.

From the instantaneous corrosion rate to a representative value

María Jesús Samper — Mon, 18 May 2020 11:24:05 +0200

Reinforcement corrosion develops due to the penetration of chlorides or of the carbonation front through the concrete pores and corrosion develops if enough moisture is present. However as the corrosion rate changes with temperature and the evolution of the corrosion itself, the representation may not be clear enough to be used for comparative purposes or for prediction of future evolution. The present work proposes a parallel use of accumulated corrosion depth, which is obtained by the integration of each age of the corrosion rate-time curve. This kind of representation enables the determination of corrosion depth at each age and appears the adequate to establish a Representative annual average value for comparative purposes. The procedure is applied to concrete specimens and real size elements exposed to Madrid outdoor conditions.

Corrosión y protección de armaduras

María Jesús Samper — Mon, 18 May 2020 11:11:36 +0200

La investigación en el campo de la corrosión de armaduras ha ido siempre muchos años con retraso respecto de los avances conseguidos en otros campos de la corrosión del acero, principalmente por la dificultad que supone el manejo de un electrólito (fase acuosa) que está encerrado en los poros de una estructura pétrea que dificulta el seguimiento visual de los procesos. Electrólito que evoluciona constantemente en el tiempo debido a la progresiva hidratación del cemento y, por lo tanto, no presenta una composición definida y fácilmente reproducible en todos sus componentes.

Determinación electroquímica cuantitativa de la velocidad de corrosión de un acero de construcción. Evaluación de la susceptibilidad al ataque por picaduras. Parte II: Ensayos en mortero

María Jesús Samper — Mon, 18 May 2020 11:02:50 +0200

El estudio del efecto de un determinado aditivo sobre la corrosión de las armaduras, o el de un hormigón con fisuras, o sometido a ambientes marinos o, en fin, de cualquier factor o circunstancia calificada como agresiva, suele exigir un laborioso y extenso programa de trabajo que contemple todas las posibilidades por las que puedan pasar las armaduras y el hormigón que las contiene. En éste, como en tantos otros campos de la tecnología, es importante la búsqueda de métodos de ensayo acelerados, que sean a la vez de lo más representativos posible una ayuda para economizar esfuerzo y materiales.

Corrosión de armaduras embebidas en hormigón carbonatado

María Jesús Samper — Mon, 18 May 2020 10:32:47 +0200

Los componentes ácidos de la atmósfera, en particular el CO₂, pueden neutralizar (carbonatar) el electrólito que llena los poros del hormigón y las fases del clínker, transformándolos en variedades del CO₃Ca. La carbonatación no sólo modifica la estructura de la pasta de cemento y por lo tanto sus propiedades, sino que también tiene una influencia decisiva sobre la velocidad de corrosión de las armaduras (1) (2). En un reciente informe del Comité 12-CRC de la RILEM (3) se afirmaba que: "si se exceptúa la corrosión causada por Cl^-, actualmente se está de acuerdo en que la carbonatación del hormigón es la condición esencial para la corrosión de las armaduras". (Aquí se debería añadir que esto se aplica a los hormigones simplemente armados, pues en los pretensados también es muy peligroso el fenómeno conocido como corrosión bajo tensión). La carbonatación puede desencadenar el proceso de corrosión, debido a que el descenso de pH que provoca hace posible el paso de las armaduras del estado pasivo al activo. La velocidad de ataque depende, sin embargo, de otros factores externos: un contenido mínimo de humedad en el medio ambiente que garantice la existencia del electrólito en los poros del hormigón, y un libre acceso de O₂ hasta la superficie metálica para que pueda producirse la semirreacción catódica. Numerosos autores están de acuerdo con estos puntos, pero sin aportar pruebas cuantitativas de los mismos. Es objeto principal de este trabajo contribuir a llenar este vacío.

La agresividad ambiental y la durabilidad de las estructuras de hormigón

María Jesús Samper — Mon, 18 May 2020 10:02:58 +0200

The deterioration of concrete structures is the result of the interaction between them and the environment. In this study, comments are made on the most decisive environmental factors and their interactions. Special reference is made to the corrosion of the reinforcements and also to the different proposals of classification of the aggressiveness. El deterioro de las estructuras de hormigón es el resultado de la interacción entre ellas y el medio ambiente. En el presente trabajo se comentan los factores ambientales más determinantes y sus interacciones, con especial referencia a la corrosión de las armaduras, así como diferentes propuestas de clasificación de la agresividad.

Corrosión de armaduras galvanizadas y sin proteger embebidas en cemento aluminoso estabilizado

María Jesús Samper — Mon, 18 May 2020 09:51:57 +0200

El cemento aluminoso, cuya obtención se basa en la fusión de bauxita y caliza, ha demostrado desde que fue patentado en Francia, en 1908 por Cementos Lafarge, unas propiedades excepcionales que lo hacen idóneo para ciertas aplicaciones. Tales propiedades son fundamentalmente el desarrollo de elevadas resistencias mecánicas en las primeras edades de hidratación, posibilidad de hormigonar en tiempo frío, una gran resistencia química al agua de mar y a los sulfatos, propiedades refractarias, etc. Todo ello hizo que el cemento aluminoso del tipo fundido, con un 40 % de Al₂O₃, se utilizara ampliamente en la construcción. Sin embargo, a partir de un determinado momento empezaron a observarse alteraciones en algunas obras construidas con hormigón de este tipo, manifestándose importantes pérdidas de sus resistencias mecánicas (1). Las causas de tales deterioros han sido ampliamente estudiadas (2)-(13), atribuyéndose la principal a la "conversión" de los aluminatos cálcicos hidratados hexagonales en hidratos cúbicos, según reacción [1]

Gas distribution network topology problem

Dejan Brkić — Sat, 16 May 2020 16:09:03 +0200

Problem of gas fow in looped network is nonlinear problem and these kind of problems have to be solved using some kind of iterative procedure. For the pipeline network, two topology matrices can be written; i.e. loop matrix and node matrix. The node matrix is related to the first Kirchho 's law while the loop matrix is related to second Kirchho 's law. Here will be shown effcient method in which both matrices, i.e. the node and the loop matrix are united in one coherent procedure for solution of looped gas pipeline problem.

Lambert W function in hydraulic problems

Dejan Brkić — Sat, 16 May 2020 15:58:02 +0200

Darcy’s flow friction factor is expressed in implicit form in some of the relations such as Colebrook’s and have to be solved by iteration procedure because the unknown friction factor appears on both sides of the equation. Lambert W function is implicitly elementary but is not, itself, an elementary function. Implicit form of the Lambert W function allows us to transform other implicit functions in explicit form without any kind of approximations or simplifications involved. But unfortunately, the Lambert W function itself cannot be solved easily without approximation. Two original transformations in explicit form of Colebrook’s relation using Lambert W function will also be shown. Here will be shown efficient procedure for approximate solutions of the transformed relations.

Intelligent flow friction estimation

Dejan Brkić — Sat, 16 May 2020 15:53:02 +0200

Nowadays, the Colebrook equation is used as a mostly accepted relation for the calculation of fluid flow friction factor. However, the Colebrook equation is implicit with respect to the friction factor (). In the present study, a noniterative approach using Artificial Neural Network (ANN) was developed to calculate the friction factor. To configure the ANN model, the input parameters of the Reynolds Number (Re) and the relative roughness of pipe () were transformed to logarithmic scales. The 90,000 sets of data were fed to the ANN model involving three layers: input, hidden, and output layers with, 2, 50, and 1 neurons, respectively. This configuration was capable of predicting the values of friction factor in the Colebrook equation for any given values of the Reynolds number (Re) and the relative roughness () ranging between 5000 and 108 and between 10−7 and 0.1, respectively. The proposed ANN demonstrates the relative error up to 0.07% which had the high accuracy compared with the vast majority of the precise explicit approximations of the Colebrook equation.

A note on explicit approximations to Colebrook’s friction factor in rough pipes under highly turbulent cases

Dejan Brkić — Sat, 16 May 2020 15:47:02 +0200

New explicit approximations to the implicitly given Colebrook equation for flow friction factor are given. They are with improved accuracy compared with the one recently published [Shaikh et al.: Int. J. Heat Mass Tran. 88 (2015) 538–543]. The new approximations are highly accurate only in rough pipes under fully developed turbulent cases of flow.

Spreadsheet-based pipe networks analysis for teaching and learning purpose

Dejan Brkić — Sat, 16 May 2020 15:41:02 +0200

An example of hydraulic design project for teaching purpose is presented. Students’ task is to develop a looped distribution network for water (i.e. to determinate node consumptions, disposal of pipes, and finally to calculate flow rates in the network’s pipes and their optimal diameters). This can be accomplished by using the original Hardy Cross method, the improved Hardy Cross method, the node-loop method, etc. For the improved Hardy Cross method and the node-loop method, use of matrix calculation is mandatory. Because the analysis of water distribution networks is an essential component of civil engineering water resources curricula, the adequate technique better than the hand-oriented one is desired in order to increase students’ understanding of this kind of engineering systems and of relevant design issues in more concise and effective way. The described use of spreadsheet solvers is more than suitable for the purpose, especially knowing that spreadsheet solvers are much more matrix friendly compared with the hand-orientated calculation. Although matrix calculation is not mandatory for the original Hardy Cross method, even in that case it is preferred for better understanding of the problem. The application of commonly available spreadsheet software (Microsoft Excel) including two real classroom tasks is presented.

Study on the impacts of possible amendments to the ATEX, the Machinery, and the Pressure Equipment Directives with respect to equipment intended for use in the offshore oil and …

Dejan Brkić — Sat, 16 May 2020 15:22:02 +0200

The purpose of this study is to provide the Commission with the information necessary for the assessment of the economic, social, and environmental impacts of a possible extension of the scope of the EU product safety legislation (ATEX Directive 2014/34/EU, Pressure Equipment Directive 2014/68/EU and Machinery Directive 2006/42/EC) with respect to equipment intended for the use in the offshore oil and gas industry. Whereas equipment on fixed units is in the scope of these three Directives, mobile offshore units and equipment installed on them are currently in general excluded. In addition the Pressure Equipment Directive excludes also well-control equipment. The study investigates whether there are safety issues which could be addressed by extending the scope of the Directives and what would be the impacts of such an extension. Title Study on the impacts of possible amendments to the ATEX, the Machinery, and the Pressure Equipment Directive with respect to equipment intended for use in the offshore oil and gas industry.

Evolutionary optimization of Colebrook’s turbulent flow friction approximations

Dejan Brkić — Sat, 16 May 2020 15:13:03 +0200

This paper presents evolutionary optimization of explicit approximations of the empirical Colebrook’s equation that is used for the calculation of the turbulent friction factor (λ), i.e., for the calculation of turbulent hydraulic resistance in hydraulically smooth and rough pipes including the transient zone between them. The empirical Colebrook’s equation relates the unknown flow friction factor (λ) with the known Reynolds number (R) and the known relative roughness of the inner pipe surface (ε/D). It is implicit in the unknown friction factor (λ). The implicit Colebrook’s equation cannot be rearranged to derive the friction factor (λ) directly, and therefore, it can be solved only iteratively [λ = f(λ, R, ε/D)] or using its explicit approximations [λ≈f(R, ε/D)], which introduce certain error compared with the iterative solution. The optimization of explicit approximations of Colebrook’s equation is performed with the aim to improve their accuracy, and the proposed optimization strategy is demonstrated on a large number of explicit approximations published up to date where numerical values of the parameters in various existing approximations are changed (optimized) using genetic algorithms to reduce maximal relative error. After that improvement, the computational burden stays unchanged while the accuracy of approximations increases in some of the cases very significantly.

Solution of the implicit Colebrook equation for flow friction using Excel

Dejan Brkić — Sat, 16 May 2020 15:09:02 +0200

Empirical Colebrook equation implicit in unknown flow friction factor (λ) is an accepted standard forcalculation of hydraulic resistance in hydraulically smooth and rough pipes. The Colebrook equation givesfriction factor (λ) implicitly as a function of the Reynolds number (Re) and relative roughness (ε/D) of innerpipe surface; i.e. λ0=f(λ0, Re, ε/D). The paper presents a problem that requires iterative methods for thesolution. In particular, the implicit method used for calculating the friction factor λ0is an application of fixed-point iterations. The type of problem discussed in this "in the classroom paper" is commonly encountered influid dynamics, and this paper provides readers with the tools necessary to solve similar problems. Students’task is to solve the equation using Excel where the procedure for that is explained in this “in the classroom”paper. Also, up to date numerous explicit approximations of the Colebrook equation are available where as an additional task for students can be evaluation of the error introduced by these explicit approximations λ≈f(Re,ε/D) compared with the iterative solution of implicit equation which can be treated as accurate.

Unified friction formulation from laminar to fully rough turbulent flow

Dejan Brkić — Sat, 16 May 2020 14:54:02 +0200

This paper provides a new unified formula for Newtonian fluids valid for all pipe flow regimes from laminar to fully rough turbulent flow.This includes laminar flow; the unstable sharp jump from laminar to turbulent flow; and all types of turbulent regimes, including the smooth turbulent regime, the partial non-fully developed turbulent regime, and the fully developed rough turbulent regime. The new unified formula follows the inflectional form of curves suggested in Nikuradse’s experiment rather than the monotonic shape proposed by Colebrook and White. The composition of the proposed unified formula uses switching functions and interchangeable formulas for the laminar, smooth turbulent, and fully rough turbulent flow regimes. Thus, the formulation presented below represents a coherent hydraulic model suitable for engineering use. This new flow friction model is more flexible than existing literature models and provides smooth and computationally cheap transitions between hydraulic regimes.

Advanced iterative procedures for solving the implicit Colebrook equation for fluid flow friction

Dejan Brkić — Sat, 16 May 2020 14:45:03 +0200

The empirical Colebrook equation from 1939 is still accepted as an informal standard way to calculate the friction factor of turbulent flows (4000 < Re < 108) through pipes with roughness between negligible relative roughness (ε/D ⟶ 0) to very rough (up to ε/D = 0.05). The Colebrook equation includes the flow friction factor λ in an implicit logarithmic form, λ being a function of the Reynolds number Re and the relative roughness of inner pipe surface ε/D: λ = f(λ, Re, ε/D). To evaluate the error introduced by the many available explicit approximations to the Colebrook equation, λ ≈ f(Re, ε/D), it is necessary to determinate the value of the friction factor λ from the Colebrook equation as accurately as possible. The most accurate way to achieve that is by using some kind of the iterative method. The most used iterative approach is the simple fixed-point method, which requires up to 10 iterations to achieve a good level of accuracy. The simple fixed-point method does not require derivatives of the Colebrook function, while the most of the other presented methods in this paper do require. The methods based on the accelerated Householder’s approach (3rd order, 2nd order: Halley’s and Schröder’s method, and 1st order: Newton–Raphson) require few iterations less, while the three-point iterative methods require only 1 to 3 iterations to achieve the same level of accuracy. The paper also discusses strategies for finding the derivatives of the Colebrook function in symbolic form, for avoiding the use of the derivatives (secant method), and for choosing an optimal starting point for the iterative procedure. The Householder approach to the Colebrook’ equations expressed through the Lambert W-function is also analyzed. Finally, it is presented one approximation to the Colebrook equation with an error of no more than 0.0617%.

One-log call iterative solution of the Colebrook equation for flow friction based on Padé polynomials

Dejan Brkić — Sat, 16 May 2020 14:36:03 +0200

The 80 year-old empirical Colebrook function ξ, widely used as an informal standard for hydraulic resistance, relates implicitly the unknown flow friction factor λ, with the known Reynolds number R e and the known relative roughness of a pipe inner surface ε*; λ= ξ (R e, ε*, λ). It is based on logarithmic law in the form that captures the unknown flow friction factor λ in a way that it cannot be extracted analytically. As an alternative to the explicit approximations or to the iterative procedures that require at least a few evaluations of computationally expensive logarithmic function or non-integer powers, this paper offers an accurate and computationally cheap iterative algorithm based on Padé polynomials with only one l o g-call in total for the whole procedure (expensive l o g-calls are substituted with Padé polynomials in each iteration with the exception of the first). The proposed modification is computationally less demanding compared with the standard approaches of engineering practice, but does not influence the accuracy or the number of iterations required to reach the final balanced solution

Choosing the optimal multi-point iterative method for the Colebrook flow friction equation

Dejan Brkić — Sat, 16 May 2020 14:29:03 +0200

The Colebrook equation is implicitly given in respect to the unknown flow friction factor λ; λ=ζ(Re, ε∗, λ)which cannot be expressed explicitly in exact way without simplifications and use of approximate calculus. A common approach to solve it is through the Newton–Raphson iterative procedure or through the fixed-point iterative procedure. Both require in some cases, up to seven iterations. On the other hand, numerous more powerful iterative methods such as threeor two-point methods, etc. are available. The purpose is to choose optimal iterative method in order to solve the implicit Colebrook equation for flow friction accurately using the least possible number of iterations. The methods are thoroughly tested and those which require the least possible number of iterations to reach the accurate solution are identified. The most powerful three-point methods require, in the worst case, only two iterations to reach the final solution. The recommended representatives are Sharma–Guha–Gupta, Sharma–Sharma, Sharma–Arora, Džuni´c–Petkovi´c–Petkovi´c; Bi–Ren–Wu, Chun–Neta based on Kung–Traub, Neta, and the Jain method based on the Steffensen scheme. The recommended iterative methods can reach the final accurate solution with the least possible number of iterations. The approach is hybrid between the iterative procedure and one-step explicit approximations and can be used in engineering design for initial rough, but also for final fine calculations.

Symbolic regression-based genetic approximations of the Colebrook equation for flow friction

Dejan Brkić — Sat, 16 May 2020 14:25:02 +0200

Widely used in hydraulics, the Colebrook equation for flow friction relates implicitly to the input parameters; the Reynolds number, Re and the relative roughness of an inner pipe surface, ε/D with an unknown output parameter; the flow friction factor, λ; λ = f (λ, Re, ε/D). In this paper, a few explicit approximations to the Colebrook equation; λ≈f (Re, ε/D), are generated using the ability of artificial intelligence to make inner patterns to connect input and output parameters in an explicit way not knowing their nature or the physical law that connects them, but only knowing raw numbers, {Re, ε/D}→{λ}. The fact that the used genetic programming tool does not know the structure of the Colebrook equation, which is based on computationally expensive logarithmic law, is used to obtain a better structure of the approximations, which is less demanding for calculation but also enough accurate. All generated approximations have low computational cost because they contain a limited number of logarithmic forms used for normalization of input parameters or for acceleration, but they are also sufficiently accurate. The relative error regarding the friction factor λ, in in the best case is up to 0.13% with only two logarithmic forms used. As the second logarithm can be accurately approximated by the Padé approximation, practically the same error is obtained also using only one logarithm.

Accurate and efficient explicit approximations of the Colebrook flow friction equation based on the Wright ω-function: Reply to Discussion

Dejan Brkić — Sat, 16 May 2020 14:18:02 +0200

This reply gives two corrections of typographical errors in respect to the commented article, and then provides few comments in respect to the discussion and one improved version of the approximation of the Colebrook equation for flow friction, based on the Wright ω-function. Finally, this reply gives an exact explicit version of the Colebrook equation expressed through the Wright ω-function, which does not introduce any additional errors in respect to the original equation. All mentioned approximations are computationally efficient and also very accurate. Results are verified using more than 2 million of Quasi Monte-Carlo samples.

Accurate and efficient explicit approximations of the Colebrook flow friction equation based on the Wright ω-function

Dejan Brkić — Sat, 16 May 2020 14:12:02 +0200

The Colebrook equation is a popular model for estimating friction loss coefficients in water and gas pipes. The model is implicit in the unknown flow friction factor, f . To date, the captured flow friction factor, f , can be extracted from the logarithmic form analytically only in the term of the Lambert W-function. The purpose of this study is to find an accurate and computationally efficient solution based on the shifted Lambert W-function also known as the Wright ω-function. The Wright ω-function is more suitable because it overcomes the problem with the overflow error by switching the fast growing term, y=W(ex), of the Lambert W-function to series expansions that further can be easily evaluated in computers without causing overflow run-time errors. Although the Colebrook equation transformed through the Lambert W-function is identical to the original expression in terms of accuracy, a further evaluation of the Lambert W-function can be only approximate. Very accurate explicit approximations of the Colebrook equation that contain only one or two logarithms are shown. The final result is an accurate explicit approximation of the Colebrook equation with a relative error of no more than 0.0096%. The presented approximations are in a form suitable for everyday engineering use, and are both accurate and computationally efficient.

Short overview of early developments of the Hardy Cross type methods for computation of flow distribution in pipe networks

Dejan Brkić — Sat, 16 May 2020 14:07:03 +0200

Hardy Cross originally proposed a method for analysis of flow in networks of conduits or conductors in 1936. His method was the first really useful engineering method in the field of pipe network calculation. Only electrical analogs of hydraulic networks were used before the Hardy Cross method. A problem with flow resistance versus electrical resistance makes these electrical analog methods obsolete. The method by Hardy Cross is taught extensively at faculties, and it remains an important tool for the analysis of looped pipe systems. Engineers today mostly use a modified Hardy Cross method that considers the whole looped network of pipes simultaneously (use of these methods without computers is practically impossible). A method from a Russian practice published during the 1930s, which is similar to the Hardy Cross method, is described, too. Some notes from the work of Hardy Cross are also presented. Finally, an improved version of the Hardy Cross method, which significantly reduces the number of iterations, is presented and discussed. We also tested multi-point iterative methods, which can be used as a substitution for the Newton–Raphson approach used by Hardy Cross, but in this case this approach did not reduce the number of iterations. Although many new models have been developed since the time of Hardy Cross, the main purpose of this paper is to illustrate the very beginning of modeling of gas and water pipe networks and ventilation systems. As a novelty, a new multi-point iterative solver is introduced and compared with the standard Newton–Raphson iterative method.

An efficient iterative method for looped pipe network hydraulics free of flow-corrections

Dejan Brkić — Sat, 16 May 2020 14:02:02 +0200

The original and improved versions of the Hardy Cross iterative method with related modifications are today widely used for the calculation of fluid flow through conduits in loop-like distribution networks of pipes with known node fluid consumptions. Fluid in these networks is usually natural gas for distribution in municipalities, water in waterworks or hot water in district heating systems, air in ventilation systems in buildings and mines, etc. Since the resistances in these networks depend on flow, the problem is not linear like in electrical circuits, and an iterative procedure must be used. In both versions of the Hardy Cross method, in the original and in the improved one, the initial result of calculations in the iteration procedure is not flow, but rather a correction of flow. Unfortunately, these corrections should be added to or subtracted from flow calculated in the previous iteration according to complicated algebraic rules. Unlike the Hardy Cross method, which requires complicated formulas for flow corrections, the new Node-loop method does not need these corrections, as flow is computed directly. This is the main advantage of the new Node-loop method, as the number of iterations is the same as in the modified Hardy Cross method. Consequently, a complex algebraic scheme for the sign of the flow correction is avoided, while the final results remain accurate.

Colebrook’s flow friction explicit approximations based on fixed-point iterative cycles and symbolic regression

Dejan Brkić — Sat, 16 May 2020 13:57:01 +0200

The logarithmic Colebrook flow friction equation is implicitly given in respect to an unknown flow friction factor. Traditionally, an explicit approximation of the Colebrook equation requires evaluation of computationally demanding transcendental functions, such as logarithmic, exponential, non-integer power, Lambert W and WrightΩ functions. Conversely, we herein present several computationally cheap explicit approximations of the Colebrook equation that require only one logarithmic function in the initial stage, whilst for the remaining iterations the cheap Padé approximant of the first order is used instead. Moreover, symbolic regression was used for the development of a novel starting point, which significantly reduces the error of internal iterations compared with the fixed value staring point. Despite the starting point using a simple rational function, it reduces the relative error of the approximation with one internal cycle from 1.81% to 0.156% (i.e., by a factor of 11.6), whereas the relative error of the approximation with two internal cycles is reduced from 0.317% to 0.0259% (i.e., by a factor of 12.24). This error analysis uses a sample with 2 million quasi-Monte Carlo points and the Sobol sequence.

What can students learn while solving Colebrook’s flow friction equation?

Dejan Brkić — Sat, 16 May 2020 13:51:02 +0200

Even a relatively simple equation such as Colebrook’s offers a lot of possibilities to students to increase their computational skills. The Colebrook’s equation is implicit in the flow friction factor and, therefore, it needs to be solved iteratively or using explicit approximations, which need to be developed using different approaches. Various procedures can be used for iterative methods, such as single the fixed-point iterative method, Newton–Raphson, and other types of multi-point iterative methods, iterative methods in a combination with Padé polynomials, special functions such as Lambert W, artificial intelligence such as neural networks, etc. In addition, to develop explicit approximations or to improve their accuracy, regression analysis, genetic algorithms, and curve fitting techniques can be used too. In this learning numerical exercise, a few numerical examples will be shown along with the explanation of the estimated pedagogical impact for university students. Students can see what the difference is between the classical vs. floating-point algebra used in computers.

Accurate and Efficient Explicit Approximations of the Colebrook Flow Friction Equation Based on the Wright ω-Function: Reply to the Discussion by Majid Niazkar

Dejan Brkić — Sat, 16 May 2020 13:38:02 +0200

In this reply, we present updated approximations to the Colebrook equation for flow friction. The equations are equally computational simple, but with increased accuracy thanks to the optimization procedure, which was proposed by the discusser, Dr. Majid Niazkar. Our large-scale quasi-Monte Carlo verifications confirm that the here presented novel optimized numerical parameters further significantly increase accuracy of the estimated flow friction factor.

FIRE SIMULATIONS OF A FISHING RESEARCH VESSEL WITH FRP STRUCTURES

Tuula Hakkarainen — Fri, 15 May 2020 15:33:02 +0200

The fire safety effect of using fibre-reinforced polymer (FRP) as the primary construction material in a fishing research vessel was studied by fire simulations. The effect of FRP structures on fire development was assessed by comparing the simulated gas temperatures and potential heat releases with FRP and steel structures. The structural integrity of FRP structures was assessed using simulated temperatures of the structures as indicators of integrity. The effect of protective mineral wool and intumescent coating layers was also quantified. The results showed that despite the protection, the structural integrity of FRP bulkheads could be compromised in fire conditions. Mineral wool was found to be better protection than the intumescent coating: it can either prevent or postpone the pyrolysis of the FRP bulkhead, depending on the fire exposure.

Kuitulujitteisten muovien paloturvallisuus laivoissa / Fire safety of fibre-reinforced polymers in ships

Tuula Hakkarainen — Fri, 15 May 2020 15:23:02 +0200

Eurooppalaisen FIBRESHIP-tutkimusprojektin tavoitteena on kehittää suunnittelu- ja tuotantomenetelmäkokonaisuus, joka mahdollistaa yli 50 metrin pituisen laivan rungon ja kansirakenteen valmistamisen kuitulujitteisista muovikomposiiteista. Projektissa suoritettiin laaja kaksivaiheinen koeohjelma, jossa tutkittiin kuitulujitemuovimateriaalien ja -rakenteiden mekaanisia, väsymis- ja palo-ominaisuuksia. Tässä artikkelissa esitellään kuitulujitteisten muovikomposiittien pienen mittakaavan palokokeiden ja pyrolyysimallinnuksen tuloksia.

Kuitulujitteisten muovien palo-ominaisuuksia voidaan parantaa merkittävästi paisuvilla pinnoitteilla. Pinnoitteet voivat muuttaa olennaisesti lämmön- ja savuntuottokäyrien muotoa, alentaa niiden maksimiarvoja ja pidentää syttymisaikaa.

Vaikka palo-ominaisuudet ovat merkittävä tekijä kuitulujitteisten muovien käytössä laivateollisuudessa, myös muita näkökohtia kuten mekaaniset ja valmistustekniset ominaisuudet on otettava huomioon. Osa projektissa alustavasti tutkituista tuotteista ei päässyt jatkotarkasteluihin tästä syystä.

Kokeissa tehdyt havainnot korostavat toistettavan ja hallitun valmistusprosessin tärkeyttä. Prosessi on ohjeistettava, valvottava ja raportoitava perusteellisesti. Laminaattien ja pinnoitteiden tulee olla tasalaatuisia, jotta kokeiden perusteella määritetyt palo-ominaisuudet voidaan varmistaa. Tarkat määrittelyt ja laadunvalvonta ovat keskeisessä roolissa, kun halutaan varmistaa materiaalien ja tuotteiden paloturvallisuus.

Kovettujen hartsien massahäviö alkaa tyypillisesti vähän yli 300 °C:n lämpötilassa. Tällöin lujitemuovista tehty rakenne alkaa tuottaa palavia kaasuja ja myötävaikuttaa paloon. Lujitemuovien lasisiirtymälämpötilat ovat tyypillisesti 100 °C:n suuruusluokkaa. Tässä lämpötilassa materiaali pehmenee ja alkaa menettää kuormankantokykyään. Lujitemuoveista valmistetuilla rakenteilla on taipumus lämmetä paikallisesti, koska niiden lämmönjohtavuus on suhteellisen alhainen. Paikallisen palon tapauksessa keskeiset paloturvallisuusriskit ovat palavien kaasujen muodostuminen ja lämmöntuotto. Jos palo uhkaa suurta rakennetta, kuten osastopalon tapauksessa, pääongelma on materiaalin pehmeneminen ja kantokyvyn menetys.

Fire performance assessment of FRP materials

Tuula Hakkarainen — Fri, 15 May 2020 15:11:02 +0200

The European FIBRESHIP research project aims to develop a comprehensive set of methods that would enable the building of the complete hull and superstructure of over 50-metre-long ships in fibre reinforced polymer (FRP) materials. In the work package dedicated to materials selection, an extensive experimental campaign is performed to characterize the fatigue and fire performance of a range of FRP materials and solutions. In this paper, the cone calorimetry results of FRP laminates are introduced in terms of ignitability, heat release, smoke production and effective heat of combustion.

The fire performance of FRP materials can be considerably improved by suitable intumescent coatings. Coatings can significantly change the shape of the heat release and smoke production rate curves, and reduce their maximum values.

Even though fire performance is of high importance in the use of FRP materials, also other properties, such as mechanical properties and manufacturing, need to be taken into account. In some cases, products with excellent fire performance have to be discarded from further considerations due to other issues.

Estudio por SEM de los productos de corrosión de armaduras galvanizadas sumergidas en disoluciones de pH comprendido entre 12,6 y 13,6

María Jesús Samper — Fri, 15 May 2020 15:04:08 +0200

In previous work on galvanized reinforcements in contact with Ca-containing highly alkaline media, the authors have reported the existence of a threshold pH below whith such a medium is passivated, but at pH values above this it corrodes continuously until it totally disappears. In this paper, after a brief literature review concerning the identification of the corrosion products in these media, they report that the passivation detected from the data of the polarisation technique agreed with the results of an SEM study. They show that below the threshold pH value the surface is covered with a continuous layer of calcium hydroxyzincate, Ca[Zn(0II)j2-^ HjO, and above this pH, the layer increase its proportion of non-passivating ZnO, to the detriment of the calcium hydroxyzincate contents. They have also established that it is the (Ca^'^) ions that control the amount of calcium hydroxizincate formed, and therefore the likelihood of the formation of a continuous passivating layer. Finally, they report that if the calcium hydroxyzincate is perfectly developed, a later increase of the pH does not affect the stability.

FIBRESHIP - Paloturvallisia materiaaleja kevyisiin laivoihin / FIBRESHIP - Fire safe materials for lightweight ships

Tuula Hakkarainen — Fri, 15 May 2020 15:00:03 +0200

Kuituvahvisteisia polymeereja eli lujitemuoveja käytetään nykyisin laajalti kevyiden alusten runkorakenteissa, kun aluksen pituus on enintään 50 metriä. Näitä komposiittimateriaaleja käyttämällä voidaan saavuttaa suurempi lastikapasiteetti ja merkittäviä polttoainesäästöjä. Lujitemuovit eivät ruostu, mikä vähentää huoltotarvetta ja –kustannuksia. Niiden avulla voidaan myös parantaa laivan stabiilisuutta ja vähentää vedenalaista melua. Yli 50 metrin pituisissa aluksissa lujitemuovien käyttö on nykyisin kuitenkin rajoitettu sekundaarirakenteisiin ja –komponentteihin. Tämä johtuu pääosin näiden materiaalien kestävyyteen ja palo-ominaisuuksiin liittyvistä kysymyksistä, suunnittelu- ja tuotantomenetelmien kehitystarpeista sekä ohjeistuksen puutteesta.

Euroopan unionin rahoittaman ”Engineering, production and life‐cycle management for the complete construction of large‐length fibre‐based ships” (FIBRESHIP) –tutkimushankkeen tavoitteena on mahdollistaa yli 50 metrin pituisten laivojen rungon ja kansirakenteiden rakentaminen muovikomposiiteista. FIBRESHIP-projektissa kehitetään innovatiivisia komposiittimateriaaleja laivateollisuuden tarpeisiin, laaditaan uusia suunnittelu- ja tuotantomenetelmiä ja –ohjeita sekä kehitetään ja validoidaan simulointityökaluja.

FIBRESHIP-projekti alkaa toukokuussa 2017 ja kestää kolme vuotta. Projektiin osallistuu laivanrakennusteollisuutta, laivanvarustamoja, meriteknologia- ja komposiittimateriaaliyrityksiä, luokituslaitoksia ja tutkimuslaitoksia Espanjasta, Irlannista, Iso-Britanniasta, Italiasta, Kreikasta, Kyprokselta, Ranskasta, Romaniasta, Suomesta, Tanskasta ja Unkarista. Hankkeen koordinaattori on espanjalainen TSI (Técnicas y Servicios de Ingeniería).

VTT on päävastuussa FIBRESHIP-projektissa kehitettävien komposiittimateriaalien ja -tuotteiden palo-ominaisuuksien arvioinnista palokokeiden ja simuloinnin avulla. VTT kehittää ja varmentaa numeerisia pyrolyysimalleja näille materiaaleille. VTT vastaa myös palomallinnuksen ja rakenteellisen analyysin yhdistävän simulointityökalun kehityksestä.

Palotutkimuksen päivillä esitellään projektin tavoitteita, työsuunnitelmaa ja ensimmäisiä tuloksia elokuun 2017 tilanteen mukaisesti. Esityksessä keskitytään kehitettävien materiaalien ja tuotteiden palo-ominaisuuksiin, pyrolyysimallinnukseen ja palosimulointiin.

Tämä projekti on saanut rahoitusta Euroopan unionin Horisontti 2020 –tutkimus- ja innovaatio-ohjelmasta perustuen rahoitussopimukseen nro 723360.

Durabilidad de las armaduras en el sistema Dragados-Plastbau

María Jesús Samper — Fri, 15 May 2020 14:37:27 +0200

En un sistema de las características del presente, los elementos metálicos que pueden sufrir corrosión son principalmente la malla y los conectores que atraviesan el poliestireno. La porosidad del microhormigón y el no siempre suficiente espesor de recubrimiento pueden permitir una carbonatación del microhormigón que alcance la malla durante el período de vida útil previsto para la estructura. Este riesgo de corrosión por carbonatación viene acrecentado en este sistema particular por una posible aparición de fisuras (retracción, movimientos de dilatación y asentamientos diferenciales, etc.).

Rational approximation for solving an implicitly given Colebrook flow friction equation

Dejan Brkić — Fri, 15 May 2020 14:28:02 +0200

The empirical logarithmic Colebrook equation for hydraulic resistance in pipes implicitly considers the unknown flow friction factor. Its explicit approximations, used to avoid iterative computations, should be accurate but also computationally efficient. We present a rational approximate procedure that completely avoids the use of transcendental functions, such as logarithm or non-integer power, which require execution of the additional number of floating-point operations in computer processor units. Instead of these, we use only rational expressions that are executed directly in the processor unit. The rational approximation was found using a combination of a Padé approximant and artificial intelligence (symbolic regression). Numerical experiments in Matlab using 2 million quasi-Monte Carlo samples indicate that the relative error of this new rational approximation does not exceed 0.866%. Moreover, these numerical experiments show that the novel rational approximation is approximately two times faster than the exact solution given by the Wright omega function.

La importancia del curado en el comportamiento posterior frente a la corrosión de las estructuras de hormigón armado

María Jesús Samper — Fri, 15 May 2020 14:14:48 +0200

Steel in contact with fresh concrete is covered by a layer of passivating oxydes due to the alkaline nature of the pore solution. The presence of oxygen plays an important role in this passivation process as well as in the corrosión one when is developped due to the ingress of aggressives, such as, chioride ions. In the present paper, results are commented on the behaviour found when oxygen is limited due to the concrete curing conditions, in a completely saturated chamber Surprisely, the results indícate that the oxygen availability during the curing has a marked influence on the future corrosión beabieviour This allows to deduce that the mechanisms involved are not so simple as was currently accepted and that new specific tests are needed to clarify the controversial points.

Study of corrosion rate variability in indoor and outdoor specimens

María Jesús Samper — Fri, 15 May 2020 13:25:04 +0200

Behavior, time evolution and variability of the corrosion process in reinforced concrete structures is an important factor to have in mind if structural service life is to be determined. Corrosion development and its consequences over time are often empirically deduced. It would be significant to improve as much as possible the prediction accuracy of corrosion process behavior. In the present paper, a series of hygrothermal and corrosion data obtained for indoor and outdoor reinforced concrete specimens are summarized by means of descriptive statistics. For a T-beam exposed to continental weather conditions, temperature, relative humidity, corrosion potential and corrosion current density measurements were taken since the year of 1993. The same measurements were made for an outdoor pile exposed to the same conditions. The indoor studied specimens are two small slabs exposed to lab conditions for which corrosion measurements were taken in the first 96 days period. Chloride was added to the concrete mixture of one specimen in order to accelerate the corrosion process. Electrochemical corrosion rate measurements using modulated confinement of the current were taken by means of Gecor 8 system device. Temperature and relative humidity were measured both inside and outside of the outdoor specimen using a probe. Hygrothermal and corrosion results were plotted on time scale in order to visualize possible correlations. A first approach to a statistical analysis is made by describing the raw data obtained by means of average value and standard deviation for different variables. Results show that the accumulated corrosion progress linearly in time. In the outdoor exposed specimen two corrosion rates Icorr were noticed, being the highest for the first period where no cracks were yet produced. After cracking occurs corrosion rate decreases. No direct relation between RH and Icorr was observed.

Ruptures of prestressing cables

María Jesús Samper — Fri, 15 May 2020 13:14:46 +0200

The structures involved are prestressed concrete structures. The aim is to detect either failures of prestressing cables (or strands) or as a preventive measure to detect corrosion or damages before failure.

Corrosión del acero desnudo y galvanizado en yeso

María Jesús Samper — Fri, 15 May 2020 13:03:49 +0200

Gypsum is a relatively lovi/-cost building material much abounding in our country. When it is put in contact with steel, it may produce high corrosion rates due to its pH value (close to 7). This work reports the results obtained in studying the corrosion rates of bare and galvanized steel in contact with gypsum and plaster, as well as the influence curing thermal treatment applied to gypsum, enviromental relative humidity and addition of compounds with different natures and purposes may have in such process. In-situ observations, as well as the measurement of the Polarization Resistance and the weight loss have been used as measurement technics. From the results obtained it has been possible to deduce that galvanized steel has better behaviour in dry enviroments than bare steel in the same conditions and moist atmosphere induces proportionally more corrosion in galvanized steel than in bare one. Additions to gypsum do not modified these conclusions, though it may be pointed out that addition of nitrites or lime improves the behaviour of bare steel, while galvanized behaviour is not modified. The addition of lime is not recommended because phenomena of dilated along time expansion may take place.

Influencia del contenido en álcalis de los cementos sobre la corrosión de armaduras galvanizadas

María Jesús Samper — Fri, 15 May 2020 12:01:13 +0200

Las armaduras galvanizadas han mostrado un comportamiento errático en los ensayos llevados a cabo hasta ahora, encontrándose en la literatura datos muy contradictorios. Los autores han encontrado que las siguientes tres variables tienen una decisiva importancia: a) pH del mortero u hormigón, que depende básicamente de la cantidad de álcalis del cemento, b) tipo de estructura metalográfica del recubrimiento galvanizado, que depende principalmente del tipo de acero y su contenido en Si y C, y c) la humedad contenida en los poros del hormigón. En este trabajo se presentan resultados relacionados sólo con el punto a). Se ha podido comprobar que existe un valor de pH umbral o crítico por encima del cual el recubrimiento galvanizado se corroe severamente, debido a la formación de capas no-protectoras de productos de corrosión. La evolución de la velocidad de corrosión se ha seguido mediante la Resistencia de Polarización, y las pérdidas de peso se han determinado también, gravimétricamente