Documents published in Scipedia

• Effects of Surface Layer on Moisture Transport in Cementitious Materials:Experimental Evidence and Simulation Results

  Z. Zhang, U. Angst

  DBMC 2023.

  
  

  Abstract

  Moisture in concrete is the essential factor of many degradation mechanisms to reinforced concrete structures, such as carbonation, chloride ingress, and frost attack. Therefore, it is important to arcuately determine and predict moisture state and moisture distribution inside the material. Most moisture transport models generally view concrete as a homogeneous material, which might be appropriate for the large scale. However, it is commonly found that the properties (chemical and microstructure) of the concrete surface layer are different from the bulk concrete. When considering the moisture exchange with the surrounding environment, this surface layer may unavoidably affect the process of moisture transport. In this study, water absorption tests were performed to investigate moisture transport in uncarbonated cement pastes and mortars. The results show that during the process of sample preparation, the microstructure of the surface layer was altered, leading to anomalous moisture transport, in which the conventional models can not predict the measured mass change. Based on the experimental results, the non-homogeneous simulation domain was created to represent the more realistic microstructure of concrete. The numerical simulation results show a similar trend of the mass change due to moisture transport to the experimental results. Therefore, the nonhomogeneous microstructure of concrete, in particular the different microstructure of the surface layer from the bulk concrete, can be one of the reasons that cause the anomalous moisture transport in cementitious materials. This also emphasizes the importance of sample preparation in the lab, which must avoid creating a surface layer with the different microstructure from the bulk concrete

  Abstract
  Moisture in concrete is the essential factor of many degradation mechanisms to reinforced concrete structures, such as carbonation, chloride ingress, and frost attack. Therefore, [...]

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• Degradation Behavior of GO-ternary Blended Nano-concrete Compositesunder Accelerated Sulfuric Acid Attack

  A. Anwar, X. Liu, L. Zhang

  DBMC 2023.

  
  

  Abstract

  Biogenic corrosion of concrete in wastewater sewer systems is caused by the in-situ production of sulfuric acid (H2SO4) formed by the bacterial action is considered the principal cause of structural degradation. The intense corrosive environment disintegrates the service lifespan of concrete structures significantly sooner in its advanced state, resulting in structural collapse within 10-20 years and jeopardising its durability. Thus, the degradation of concrete structures in the aggressive acidic environment of sewerage systems remains a global concern for industries and stakeholders, resulting in economic losses of several billion dollars annually. This research experimentally investigates the degradation behavior of developed ternary blended nano-concrete composites reinforced with graphene oxide (GO-TBNCCs) under accelerated H2SO4 exposure (pH 1.0) for 28 days and 90 days. The experimental results revealed that TBNCCs reinforced with GO are more resistant to aggressive acidic attack in contrast to its control composites, hence increasing its acidic resistivity performance with lower degradation depth that may cater to the durability problem of concrete structures in the wastewater sewer systems.

  Abstract
  Biogenic corrosion of concrete in wastewater sewer systems is caused by the in-situ production of sulfuric acid (H2SO4) formed by the bacterial action is considered the principal [...]

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• An Experimental and Numerical Study of the Effect of Heat and ThermalShock in Precast Concrete Wall System

  C. Cremonesi, G. Akyo, O. Oliveira, L. Alves de

  DBMC 2023.

  
  

  Abstract

  The aim of the paper is to analyse the performance of a precast concrete wall system under heat and thermal shock (physical phenomena). The understanding of thermal shocks in building systems is a challenge because of the requirements and complexities of the phenomena behaviour through cycles of heating and cooling provided by the environment (sun followed by suddenly rain). The period of duration of the phenomenal during the cycles makes changes in the systems responses in terms of mechanical and physical aspects. The responses of the building system to heat and thermal shock were analysed during the monitoring of the changes in temperature and deformation using sensors in a scalable prototype of a precast concrete wall system. In addition, a numerical simulation based on the finite element method (FEM) in commercial software was conducted to verify and analyse the variables in a digital model of the wall as a complementary study of the effects of heat and thermal shock. The results demonstrated the feasibility to use numerical simulation to analyses the phenomenal of heat and thermal shock on the behaviour of a precast concrete wall system.

  Abstract
  The aim of the paper is to analyse the performance of a precast concrete wall system under heat and thermal shock (physical phenomena). The understanding of thermal shocks [...]

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• Effect of Repeated Loading on the Flexural Load Carrying Capacity ofMarine Concrete Beams Exposed to Chloride Environment

  C. Lu, C. Feng, Y. Zheng

  DBMC 2023.

  
  

  Abstract

  To investigate the effect of repeated loading on the flexural load carrying capacity of marine concrete beams suffering from chloride attack, a total of 10 marine concrete beams and 27 concrete cubes were designed for flexural performance test and compressive strength test, respectively. Three damage mechanisms, including repeated loading with a stress level of 0.4, chloride salt dry-wet cycles and coupling action of repeated loading history and chloride salt dry-wet cycles, were applied for concrete beams and cubes. Test results show that the effect of repeated loading on the degradation of compressive strength of concrete is significantly higher than that on the flexural properties of concrete beams, and the corresponding degradation ratio between them is maintained at about 1.5. The effect of chloride salt dry-wet cycles on concrete compressive strength is 1.8 times higher than that on the flexural load carrying capacity of test beams. Combined with the test data proposed in this paper and some existing studies, the influence of compressive strength loss on the flexural load carrying capacity of marine concrete beams was discussed. The analysis shows that the compressive strength loss rate can effectively reflect the remaining flexural load carrying capacity of marine concrete beams and there is an exponential relationship between them.

  Abstract
  To investigate the effect of repeated loading on the flexural load carrying capacity of marine concrete beams suffering from chloride attack, a total of 10 marine concrete [...]

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• Monitoring Steel Rebar Corrosion-induced Cracking in ECC Cover byDOFS

  E. Chen, H. Ye, S. Zhang

  DBMC 2023.

  
  

  Abstract

  Steel corrosion is the main threat to the durability of concrete structures, and the cracking or spalling of concrete cover caused by the expansion of corroded steel can largely accelerate the deterioration process. Engineered cementitious composites (ECCs), as one type of high-performance fiber-reinforced cementitious composites with high tensile ductility and excellent ability of crack width control, have a great potential to be able to improve the durability of structures under corrosive environment. Nevertheless, corrosion of steel rebar may still happen in ECC if aggressive agents depassivate the rebar; therefore, the relationship between the corrosion level and cracking degree in ECC cover is essential for accurately estimating the corrosion condition as well as service life of steel reinforced ECC structures. This study investigates the cracking process of ECC under accelerated steel corrosion implemented by impressed current, during which the distributed optical fiber sensors (DOFS) were employed both on the steel rebar surface and surface of ECC specimens to monitor the internal strain and surface strain generated by the corrosion expansion. The results show that DOFS provide an effective non-destructive tool for detecting the corrosion of steel rebar at early stage when the surface cracks in ECC are not visible.

  Abstract
  Steel corrosion is the main threat to the durability of concrete structures, and the cracking or spalling of concrete cover caused by the expansion of corroded steel can largely [...]

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• Evaluation of Alteration in Cement Paste Due to Calcium Leaching withIntegrated CT-XRD Method and Its Effect on Ionic Diffusions

  Y. Tan, T. Sugiyama, K. Hashimoto

  DBMC 2023.

  
  

  Abstract

  In 2011, Tohoku earthquake and subsequent tsunami hit Fukushima Daiichi Nuclear Power Plant, and they lead to Fukushima nuclear disaster. For more than a decade after the disaster, concrete materials in submerged structures of nuclear power plants have been chronically in contact with water and resulted in calcium leaching. To assess the alteration of the concrete property, it’s necessary to evaluate the permeability change due to calcium leaching and its effect on radioactive ion diffusion. In this study, small scale cylindrical cement paste specimens with 3mm in diameter and 6mm in height were prepared with water to cement ratios of 0.5 and 0.6 respectively, and they were tested in the static leaching. Specimens were subjected to carbonation before immersed into deionized water for different periods. CT image of each specimen at a resolution of 2.46μm/voxel was acquired in SPring-8, Hyogo, Japan. Combining CT images and X-ray diffraction data, dissolution front of portlandite under different leaching periods was determined, and the time dependent development law of dissolution front was evaluated. Introducing Buil’s model as a local equilibrium, time and position dependent porosity of cement paste due to leaching was calculated and correspondingly diffusion coefficient of Sr ion in the numerical simulation was modified. Simulative results on Sr ion diffusivity for 10-year showed the impact of carbonation followed by leaching on radioactive ion diffusion.

  Abstract
  In 2011, Tohoku earthquake and subsequent tsunami hit Fukushima Daiichi Nuclear Power Plant, and they lead to Fukushima nuclear disaster. For more than a decade after the [...]

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• 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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