Abstract

Chemical reactions that lead to decreased durability in concrete structures may result from chemical interactions between aggressive agents present in the external environment and cement paste compounds or may result from internal reactions. The chemical reactions are manifested by the physical properties of the concrete, such as porosity and permeability and the decrease of resistance and cracking. Among the several aggressive agents that attack the reinforced concrete structures it can be highlighted chloride ions, sulphate ions and carbon dioxide. In this context, the study presents the analysis of different traces of mortar executed with cement type CPII-F-32 (OPC Cement, in Brazil one of the most used cements) with w/c ratios of 0.4 and 0.7, when subjected to the attack of sulphates, chlorides and carbonation. The analyzes quantitatively evaluated the amount of ions present after exposure to the agents and demonstrate the importance of evaluating the dosage of water in the mortars to minimize future problems with durability.

Abstract

The aim of the current study is to investigate the carbonation behavior of cement hydrates at different environments. Hardened cement pastes are prepared using OPC and BFS and crushed into ~90μm powder. The powder samples are carbonated at 5/20% CO2 and at 60/85%RH. TGA, XRD and phenolphthalein tests are conducted before and after carbonation. The results show that CH remain uncarbonated even after CO2 uptake apparently ended and that the amount of the residual CH affects the results of phenolphthalein test. External humidity strongly affects the carbonation behavior of both CH and other hydrates. The degree of CH carbonation is greater at higher humidity, while the degree of other hydrates carbonation is greater at lower humidity. This conflicting effect can occur at surface of bulk concretes and changes in its properties (coarsening of pore structure and cracking) can be caused without apparent carbonation detected by phenol phthalein test. The effect of relative humidity on CSH carbonation should be taken into consideration when estimating long-term performance of carbonated concretes.

Abstract

The negative effects of climate change are underway despite the global efforts to mitigate them and the future scenario are unsettling. Climate change poses critical challenges to urban environments and highlights the need for research its impacts on the built environment. One of the most significant effects of climate change on reinforced concrete structures is associated with the carbonation of these structures. The increase of parameters such as temperature and carbon dioxide jeopardise the degradation of such structures by carbonation-induced corrosion. This paper presents the results of the monitoring and analysis of a set of buildings that determine carbonation as the main degradation mechanism of structures in Paraguay. Through the application of a previously developed carbonation model, the worsening of the carbonation-induced degradation has been determined after considering the climate change effects in the coming 50 years. The outcomes of the study determined that the poor quality of the structures in Paraguay cause premature degradation in them. Furthermore, considering climate change effects, it has been determined that this phenomenon could accelerate corrosion failure times in reinforced concrete structures.

Abstract

Many environments to which concrete is exposed are highly aggressive due to various chemical components. In such environments, concrete is subjected to processes of chemical degradation, among which carbonation is one of the most frequently seen degradation processes. Though, the influence of saturation degree (or relative humidity - RH) of the specimen and CO2 concentration on the carbonation of cementitious materials is still not comprehensively described with respect to carbonation rate/degree as well as alteration in microstructure and mineralogy. This work aims at thoroughly investigating how these two key parameters affect the carbonation under accelerated conditions. Furthermore, the effect of initial moisture state of the specimen on the carbonation rate is also demonstrated. For such purpose, a numerical model at continuum scale is developed to investigate the effects of RH and CO2 concentration on the carbonation depth, phase changes in phases and porosity of hardened cement pastes due to carbonation under accelerated conditions. Verification with experimental results from accelerated carbonation tests shows a good agreement. The modelling results with supporting experimental data help to better understand the modification of material properties under different carbonation conditions and to optimize the carbonation conditions.

Abstract

This study aimed to evaluate the influence of different types of metakaolin and curing conditions on mortars depth of carbonation. To do so, prismatic mortar specimens were produced by using three different 1:2 mix proportions (binder: sand): a reference mix proportion (without mineral addition) and two mixes with partial replacement of the cement by metakaolin, in the content of 10% by mass of cement. Two types of metakaolin with different characteristics were used in order to verify their capacity of changing the mortar microstructure and the pore solution, modifying this way the carbonation behavior of these materials. Three curing conditions for the test specimens were defined: no wet curing and wet curing (by immersion in water) for 3, 9 and 28 days. After the accelerated carbonation procedure in a CO2 chamber (in an atmosphere of 10% of CO2), with a relative humidity of 70% and a temperature of 20°C, for a period of 7 days, the carbonation depth was measured by using pH-indicators based on phenolphthalein and thymolphthalein. The results indicated influence of the type of curing in the carbonated depth and the significant interaction between curing and the type of metakaolin. When wet curing was not performed, the carbonation was more intense in the reference mortar. The effect of curing in the mortars with metakaolin was much less significant compared to that in the reference mortar. A global discussion about carbonation behavior of metakaolin modified mortars will be performed, based on the characteristics of the metakaolins.

Abstract

The work summarizes the results of the interlaboratory testing made in the committee CEN/TC51/WG12 on the standardized procedures for carbonation (EN 12390 – 10 and part 12) and chloride diffusion (EN 12390-12). The exercise was made by 13 different laboratories on 8 different concrete compositions with w/c ratios of 0.4, 0.5 and 0.6. The results indicated repeatability of 10-22% for natural carbonation (outdoors protected from rain and controlled chamber respectively), of around 5% for accelerated carbonation (at 4% of CO2 concentration), and 23% in the Apparent diffusion coefficient for natural chloride diffusion and 13% for the chloride surface concentration.

Abstract

The fly ash concrete (FAC) has often been used in chloride contaminate environments. The aluminates present in fly ash (FA) react with free chlorides binding them and reducing the amount of chlorides able to penetrate into concrete. However, Ca(OH)2 present in FAC is easily consumed by pozzolanic reactions, making easier the advance of carbonation. In this context, it is important to know the FAC behaviour in environments subjected simultaneously to chlorides and carbonation. Chloride diffusion coefficient (D) increases in environments containing chlorides and CO2 when compared to one containing only chlorides. This increase is related to the effects of carbonation on concrete. Therefore, this paper investigates the efficiency of FAC in context of the combined action of chlorides and carbonation. Two different concretes were cast, a FAC (40% FA) and a reference concrete (RC), that is a concrete with similar properties to FAC but without FA. After 90 days humid curing, half of specimens were subjected to accelerated carbonation (20ºC, 55% RH and 4% CO2) during 1, 2 and 7 months. The other half was protected and remained in laboratory during the same period. Afterwards, non-steady-state migration tests were performed. The results show that, under the studied conditions, the efficiency of the FAC against chloride penetration is not affected when compared to the results obtained for RC. The carbonation depth in FAC is higher than in RC, but the D in FAC remain lower than in RC. This behaviour can be attributed to the higher capacity of FAC binding chlorides.

Abstract

In order to reduce carbon dioxide emissions, the construction sector is increasingly using clinker-reduced concretes. Concrete with high amounts of ground granulated blast furnace slag (GGBFS) show increased durability performance in certain environmental conditions compared to concrete with Ordinary Portland cement (OPC). However, in laboratory studies, concrete with GGBFS show an increased initial scaling rate in comparison with OPC concrete. Changing weather conditions and high relative humidities > 70 % in field lead to less carbonation and thus, to a better freeze-thaw deicing salt resistance (FTDSR). Therefore, the current lab performance test penalizes GGBFS concrete. This contribution studied the impact of carbonation and intermittent dry periods on FTDSR of GGBFS concrete compared to that of OPC concrete. It could be shown that for concrete with OPC, intermittent dry periods lead to an accumulation of salt in the near surface, which causes a reduction of the scaling. GGBFS concrete, on the other hand, shows an opposite behavior: carbonation causes an increase in scaling after the dry period. GGBFS concrete samples with CO2-free pre-storage show the lowest scaling of all samples investigated. A better simulation of the behavior of GGBFS concrete could lead to economical as well as optimized solutions for the local environment conditions and enable the large-scale use of more environmentally-friendly concretes. Therefore, by taking the effect of carbonation and intermittent dry periods into account in a simple engineering model, the prediction of concrete scaling under consideration of real climatic conditions is possible in an eco-efficient manner.

Abstract

A model building made out of concrete block specimens whose surfaces were coated with various finishing materials was constructed for the outdoor exposure test. After 30 years of outdoor exposure, the deterioration of finishes and their carbonation and corrosion inhibition effects were investigated. As a result, the following conclusions were obtained: Though the deterioration levels differed depending on their types and thickness, all finishing materials showed some kind of deterioration after 30 years‘ long-term outdoor exposure. The progress of carbonation and corrosion are prevented when the surface is coated with finishing materials of sufficient thickness or waterproof effect. Regarding the relationship between corrosion depth and carbonation depth, corrosion is initiated after the carbonation front reaches the area. It usually takes a while to initiate corrosion in the carbonated area, not immediately after the carbonation reaction. When the concrete surface is properly coated with finishing material, corrosion may not be initiated in non-carbonated area in concrete where the pH level is not lowered.