Documents published in Scipedia

• Evaluating the Corrosion Level of Bare Steel Bars with Pitting Corrosionby DOFS

  S. Zhang, Y. Li

  DBMC 2023.

  
  

  Abstract

  Steel corrosion is one of the main causes of the deterioration of reinforced concrete structures. Localized pitting corrosion of rebar is particularly harmful, as it can severely damage mechanical properties of steel rebar including both the load and deformation capacities. Moreover, unlike uniform corrosion which can give a warning by causing extensive longitudinal cracking, pitting corrosion is often more hidden with the absence of obvious corrosion cracks. Traditional non-destructive methods based on electrochemistry may encounter large errors when estimating the pitting corrosion level in concrete; as a result, more effective methods/tools are in necessity for a timely and accurate detection of localized pitting corrosion. This study investigates the ability of distributed optical fiber sensors (DOFS) to measure pitting corrosion of steel bars, which is based on the principle that pitting corrosion causes strain localization of steel bar under tension and DOFS enable to capture the strain distribution with high spatial resolution. DOFS were attached on bare steel bars, which have mechanical notches to simulate corrosion pits, to measure the strain distribution along the notched bars under direct tension. Through experiments, the present study explores the possibility of attaching DOFS on the surface of a steel bar to monitor its pitting corrosion, and the positive results are of interest to the development of non-destructive detection method of steel pitting corrosion in concrete structures. Further quantitative analysis is required to find the correlations between the strain distribution along the notched bars and notch geometries, so that the pitting corrosion level could be assessed from the monitored DOFS strains of rebar.

  Abstract
  Steel corrosion is one of the main causes of the deterioration of reinforced concrete structures. Localized pitting corrosion of rebar is particularly harmful, as it can severely [...]

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• Research on the Corrosion Behavior and Cathodic Protection of Defected-Epoxy-coated Reinforcement Exposed to Chloride Environment

  D. Zhang, G. Liu, Z. Fan

  DBMC 2023.

  
  

  Abstract

  The application of epoxy coated rebar (ECR) is an effective method to improve the durability of reinforcement in chloride environment. However, the coating defects caused in construction will significantly decrease its corrosion resistance to chloride. Herein, the corrosion behaviors of different defects rate in ECR under chloride attack in South China are investigated. The results indicate that the corrosion of ECR is due to the coating defects, the corrosion potentials of defected-ECR are shifted to negative, and the corrosion current density is significantly increased with increasing the defect rate. The corrosion resistance of defected-ECR is improved by applying the anode-mortar cathodic protection system, both the exposure test and engineering application all show that the durability of defected-ECR is increased with the high polarization potential (≥ 200mV) and negative protection potential (＜-800 mV).

  Abstract
  The application of epoxy coated rebar (ECR) is an effective method to improve the durability of reinforcement in chloride environment. However, the coating defects caused [...]

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• Hygrothermal Behaviour of Non-traditional Mortars and ConcretesEva Barreira1, Ricardo M.S.F. Almeida1,2 and Margarida Pais

  E. Barreira, R. Almeida, M. Pais

  DBMC 2023.

  
  

  Abstract

  In 2015, the European Commission adopted a Circular Economy Action Plan to stimulate the transition of European countries towards the circular economy. In the 2030 Agenda, which includes the 17 Sustainable Development Goals, the United Nations has also defined targets to be implemented by the construction sector based on circular economy concepts. In this context, the importance of directing the development of the construction sector towards sustainable solutions to address the challenges of energy consumption, climate change, resource reduction and waste production is undeniable. Waste incorporation in thermal mortars is already under study by several authors. Among the possibilities of producing sustainable mortars, the reduction of natural aggregates in their preparation, such as sand, or of binders, such as cement, stands out. The incorporation of residual materials in mortars is, therefore, a possible alternative to guarantee more sustainable solutions. The objective of this work was to evaluate the hygrothermal behaviour of non-traditional mortars and concretes, such as fibre-reinforced mortar (P1); sprayable thermal insulation mortar (P2); mortar with granulated cork (P3); concrete with expanded clay (P4); concrete with metallic fibres (P5); cement mortar with construction and demolition waste (P6). The thermal conductivity of these materials, with different moisture contents, from totally dry to saturated after 24 hours of total immersion was determined. The results showed that the thermal conductivity values of the dry materials were similar to those found in the literature. After the 24 hours of humidification, there was, as expected, an increase in mass, with a minimum of 2% for specimen P3 and a maximum of 51% for specimen P2. Regarding the thermal conductivity, there was also an increase in its value for higher moisture contents, being that increase more relevant in specimen P2, with a variation of 294%, and less relevant in specimen P5, with a variation of 18%.

  Abstract
  In 2015, the European Commission adopted a Circular Economy Action Plan to stimulate the transition of European countries towards the circular economy. In the 2030 Agenda, [...]

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• Experimental Study on Macro-performance of Long-age HydraulicConcrete Based on High Temperature Accelerated Curing

  Z. Li, Y. Huang, Z. Liao

  DBMC 2023.

  
  

  Abstract

  In view of time-consuming and expensive for the long-age mechanical property test of hydraulic concrete, the change rule for the mechanical properties of hydraulic concrete with long-age is still unclear. Based on the equivalent age theory, the high-temperature curing method was employed to accelerate test. First, the macro-mechanical properties tests of hydraulic concrete at different curing ages (90d, 180d, 1a, 2a, 3a) and different fly ash contents (0%, 15%, 35%) were designed and carried out. Then the change rule of mechanical properties of hydraulic concrete at long-age were analyzed. In addition, the macro test results of concrete core specimen of a gravity dam which has been operated more than 40 years were adopted to feedback the above test results. The research results showed that the fly ash content had a significant influence on the activation energy Ea of hydraulic concrete. To reach the same hydration degree of design long-age, the curing time increased with the increasing of fly ash content. Within the curing age of 3a, the compressive and splitting tensile strength of concrete increased with the increasing of curing age. The strength values of cement concrete and concrete with 15% fly ash content were close to each other, while the strength values of concrete with 35% fly ash content were smaller than the cement concrete and concrete with 15% fly ash content. The consistency and reliability of the rule that the concrete strength continues to increase with age was further verified by combining the macroscopic test results and the strength growth rate calculation results of a gravity dam concrete core specimen that had been in service for more than 40 years.

  Abstract
  In view of time-consuming and expensive for the long-age mechanical property test of hydraulic concrete, the change rule for the mechanical properties of hydraulic concrete [...]

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• Evaluating the Energy and Mechanical Properties of CSH/nano-CC Interface: An Atomic Investigation

  R. Wen, Q. Zeng, B. Gao

  DBMC 2023.

  
  

  Abstract

  Carbonation of concrete generally leads to decalcification of calcium silicate hydrates (CSH) and generation of calcium carbonate (CC), however, the structure and mechanical properties of CSHCC nano composites are far from being fully addressed. The CC formed by CSH carbonization mainly has three polymorphs, including calcite, vaterite and aragonite. Although different polymorphs have the same chemical composition, they belong to different crystal systems and have completely different unit cell structures. In this paper, the CSH and three polymorphs nano-CC are constructed at the atomic level, and the interface properties between them are explored by reactive molecular dynamics (MD) simulations. The results show the greater interfacial bonding energy, the better the mechanical properties of the CSH-CC composite. Moreover, interface transition region (ITR) emerges between CSH and CC polymorphs, and the ITR thickness is different. The atomic structure in ITR is different from that in the middle region, the former is more disorderly and the coordination number in ITR is significantly reduced, thus showing a metastable state. The findings would deepen the mechanistic understanding of interface properties between CSH and nano-CC.

  Abstract
  Carbonation of concrete generally leads to decalcification of calcium silicate hydrates (CSH) and generation of calcium carbonate (CC), however, the structure and mechanical [...]

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• Damage Process Assessment of Mortar Samples under Freeze-thaw Cycleswith Micro-CT and Expansion Measurements

  J. Xi, Y. Takahashi, Y. Kawabata

  DBMC 2023.

  
  

  Abstract

  The aim of this study is to comprehensively investigate the relationship between 3- dimensional crack development and mechanical degradation during Freeze-thaw cycles (FTC). An FTC test was designed in this study to relate the micro-scale crack patterns to the macro-scale expansion and mechanical property deterioration at different FTC damage levels. Mortar specimens with waterto-cement (w/c) ratios of 50% and 75% were cast in two sizes (i.e., Ø5×10 cm, Ø2×2.5 cm) and were subjected to FTCs. For Ø5×10 cm specimens, strain in the center part of the specimens were monitored by embedded mold gauges and compression tests were conducted at different expansion levels. For Ø2×2.5 cm specimens, X-ray micro computed tomography (micro-CT) and compression tests were conducted after different FTC durations. By comparing the test results of these two groups of specimens, the expansion, mechanical degradation, and development of micro-cracks in the mortar specimens during the FTC damage process were correlated. It is indicated that with similar mechanical reduction, the damage pattern differs in two w/c cases. This research provides a test method for investigating internal swelling damage and proposes the direction for further improvement of FTC simulation model.

  Abstract
  The aim of this study is to comprehensively investigate the relationship between 3- dimensional crack development and mechanical degradation during Freeze-thaw cycles (FTC). [...]

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• Sensitivity of Capillary Absorption and Sorptivity to Water Retention, Permeability and Other Relevant Characteristics for Cement Mortars

  Y. Zhang, F. Ren, C. Zhou

  DBMC 2023.

  
  

  Abstract

  Concrete materials and structures are neither totally saturated nor dry, making the efficient capillary absorption of water deserve much attention. Basically, absorption of water is a process of unsaturated permeation driven by capillary pressure. This dependence of capillary pressure on water content plays an essential role in predicting capillary absorption and thus calculating sorptivity, which is also affected by inherent permeability, tortuosity coefficient significantly. Considering the evolution of pore structure of cement-based materials (CBMs) upon wetting, the long-term absorption of water into mortars and sorptivity can be predicted. However, a few investigations have been done to understand the precision of measured sorptivity, which is tried numerically in this paper focusing on the influences of varying parameters including water retention characteristics, tortuosity parameter, inherent permeability and swelling time. Based on reported experimental data of two cement mortars, 100 curves of capillary absorption in 10 days are predicted with artificially random parameters. Both the first and secondary sorptivity are further calculated and evaluated with emphases on the coefficient of variation of sorptivity and its sensitivity to varying parameters. Water retention characteristics make certain contribution to the dispersion of both initial and secondary sorptivity. Sorptivity is also sensitive to the variations of porosity and tortuosity representing the heterogeneity of their pore structure. The swelling time brings observable effects on the precision of secondary sorptivity only.

  Abstract
  Concrete materials and structures are neither totally saturated nor dry, making the efficient capillary absorption of water deserve much attention. Basically, absorption of [...]

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• Application of Ultrasonic Computerized Tomography Technique to Detect the Internal Defects in Cement-based Mortar

  Z. Li, K. Li, J. Wang, L. Li

  DBMC 2023.

  
  

  Abstract

  It is widely acknowledged that ultrasonic technology provides a considerable detection approach while describing the internal fractures in concrete, which is a serious issue when evaluating the durability performance of cement-based materials. This paper aims to introduce the work which is concerned with the use of ultrasonic computerized tomography (UCT) technique to evaluate the micro-cracks inside mortar. The result shows that UCT is a relatively promising method which is capable of imaging the positions of tiny cracks and local damage, moreover, the application of X-ray computerized tomography (XCT) also verifies the accuracy and reliability of UCT. Effective evaluating the internal cracks depends on the detection of attenuation of ultrasonic signals that perpendicular to internal cracks, furthermore, ultrasonic technology is more likely to detect the crack with larger surface of damage, but not sensitive to local fracture.

  Abstract
  It is widely acknowledged that ultrasonic technology provides a considerable detection approach while describing the internal fractures in concrete, which is a serious issue [...]

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• Experimental Evaluation of Flexural Strength and Ductility of One-Way Concrete Slab Panels Reinforced with Welded Wires and Deformed Bars

  M. Mahdi, F. Ahmed, I. Ahmed, N. Islam

  DBMC 2023.

  
  

  Abstract

  This study evaluates the flexural strength and ductility of one-way concrete slab panels reinforced with deformed bar and welded wire under the application of pure bending. An experimental database of flexural strength and ductility for a set of three slab panels reinforced with 10 mm deformed bar, three panels with 6 mm welded wire reinforcement (WWR), and three panels with 8 mm WWR has been developed, with each slab panel having a dimension of 762 mm by 2286 mm. The deflection and ductility factors of the specimens are investigated. The results show that the slab panels reinforced with 6 mm WWR exhibit less vertical deflection at fracture in comparison to those with 8 mm WWR. Welded wire reinforced slab panels demonstrated uniformly distributed crack propagation in comparison to deformed bars. In addition, the slab panels with 8 mm WWR exhibit higher flexural strength than the 6 mm WWR reinforced panels. The 10 mm deformed bar-reinforced slab panels exhibited greater deflection at fracture compared to WWR specimens. The reason for the lower ductile behavior of slab panels with 6 mm WWR is due to the fact that 6 mm WWR, produced locally in Bangladesh, is manufactured by the cold-drawn method and has a lower ductility in compliance with BDS ISO 6935 Class A, which does not conform to ASTM A1064. Both 8 mm WWR and 10 mm deformed bars conform to BDS ISO 6935 Class D and ASTM A1064. Hence, the 6 mm WWR with Class A ductility is not recommended for reinforced concrete (RC) slab panels based on the experimental results conducted in this study, whereas the 8 mm WWR and the 10 mm deformed bar with Class D ductility are suitable for structural use as recommended in ACI 318.

  Abstract
  This study evaluates the flexural strength and ductility of one-way concrete slab panels reinforced with deformed bar and welded wire under the application of pure bending. [...]

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• Pitfalls in Using Electrical Conductivity to Monitor the Chloride Ingress of Concrete

  L. Huang, L. Tang, Z. Yang

  DBMC 2023.

  
  

  Abstract

  Chloride ingress in the field structure is influenced by many factors. A non-destructive monitoring is a useful tool for assessing the health of reinforced structures. This study used array sensors to measure the temperature and electrical conductivity of concrete at depths from 10 mm to 140 mm. The electrical conductivity in concrete showed a continuous decrease during the exposure to 3% NaCl solution. A numerical modelling of multi-ion species migration in pore solution can explain the evolution of conductivity profile over exposing time. By comparing with several previous investigations, this study identified the pitfalls in using electrical conductivity or resistivity to monitor the chloride ingress in the exposed concrete. To obtain a correct information from the electrical monitoring system, the experimental and analysing process should consider the saturation degree of concrete, the hydration induced structure change, the leaching of ions, the ingress of chloride and composition of binders.

  Abstract
  Chloride ingress in the field structure is influenced by many factors. A non-destructive monitoring is a useful tool for assessing the health of reinforced structures. This [...]

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