Documents published in Scipedia

• 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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• Corrosion Mechanisms of Reinforced Alkali-Activated Concrete

  F. Zhang, S. Yang

  DBMC 2023.

  
  

  Abstract

  Alkali-activated concrete (AAC), which exhibits good mechanical strength and chemical resistance properties, has attracted emerging interest from the research perspective considering the sustainable development of construction materials. However, the corrosion mechanism at the steel-AAC interface is not yet well understood including the physical and chemical aspects, which leads to different accumulation and evaluation of corrosion products, compared to ordinary Portland cement (OPC) concrete. In this paper, concrete pull-out test and electrochemical techniques were used to investigate the bond-slip behaviour and the evolution of deterioration of AAC respectively. In addition, the current guidance of corrosion evaluation used for OPC concrete based on ASTM C876 is not suitable for AAC. Five mixed ratios of blended fly ash and slag AAC were investigated under two chloride environments and one non-chloride environment, i.e., (1) 3.5% NaCl salt fog spray in the environmental chamber; (2) 3.5% NaCl saltwater immersion; (3) tap water immersion. Electrochemical techniques include half-cell potential, linear polarization resistance and Tafel extrapolation method were used to determine the corrosion rate. The electrochemical results are validated through the comparison of the gravimetric loss of steel after corrosion and electrochemical loss from calculation.

  Abstract
  Alkali-activated concrete (AAC), which exhibits good mechanical strength and chemical resistance properties, has attracted emerging interest from the research perspective [...]

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• Effect of Moisture History on ASR Expansion and Microstructural Properties

  O. Olajide, M. Nokken, L. Sanchez

  DBMC 2023.

  
  

  Abstract

  Alkali Silica reaction has been a great menace to the durability of concrete infrastructure since its discovery. The mechanism is caused by the reaction between poorly crystallized silica and alkalis in the presence of sufficient amount of water. Just as water plays a critical role in several durability challenges in concrete, the limitation of moisture has been prominently used as a technique for the maintenance of ASR affected structures. The variation in moisture condition to which structures are exposed could lead to alternate wet and dry regimes. Drying aids the mitigation of the reaction, however, the cyclic phenomenon can modify the kinetics of the reaction and exercebate inner damage. This paper focuses on the development of the reaction over an alternate wetting and drying cycle involving aggregates of different levels of reactivity. The influence of the moisture history on the microscopic features were appraised using the damage rating index. The kinetics of the reaction and ASR induced deterioration in specimens undergoing cycles of wet and dry conditions differ when compared to those stored at constant moisture. Furthermore, ASR induced expansion and petrographic features are influenced by the difference in the reactivity level of aggregates.

  Abstract
  Alkali Silica reaction has been a great menace to the durability of concrete infrastructure since its discovery. The mechanism is caused by the reaction between poorly crystallized [...]

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• Comparison Between Parameters of Corrosion, Obtained by Destructive and Non-destructive Electrochemical Techniques

  C. Alves, A. Oliveira, T. Cardoso, O. Cascudo

  DBMC 2023.

  
  

  Abstract

  This research aims to compare corrosion parameters obtained by linear polarization technique and Tafel extrapolation method in steel bars embedded in concrete. It was analyzing the intervention of mineral additions, such as metakaolin (10%) in binary concrete mixtures and silica fume (9% ) and nano silica (1%) in ternary concrete mixtures of water/binder ratio of 0.4 and 0.6 after induction of corrosion by wetting and drying cycles in a solution containing chlorides. Thus, the corrosion potential (Ecorr), polarization resistance (Rp) and corrosion rate (icorr) were measured. As a main result, a good correlation (R² of 0.82 and 0.96) was obtained with the values of polarization resistance (Rp) and corrosion rate (icorr), obtained by Rp and by Tafel slope, respectively. Binary and ternary concretes mixtures showed higher performance than reference concretes. Reference concrete with w/ratio of 0.6 showed a high corrosion rate.

  Abstract
  This research aims to compare corrosion parameters obtained by linear polarization technique and Tafel extrapolation method in steel bars embedded in concrete. It was analyzing [...]

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• Durability of Concrete and Reinforced Concrete Structures Subjected to Temperature Cycles: Insight from Multiscale Thermomechanical Analysis

  B. Pichler

  DBMC 2023.

  
  

  Abstract

  This document refers to thermal expansion of cementitious materials and to related thermal stresses activated in samples of concrete as well as in unreinforced and reinforced concrete structures. It provides an overview of multiscale research which was carried at the Institute for Mechanics of Materials and Structures, TU Wien, Vienna, Austria, in cooperation with Tongji University and Shanghai Jiao Tong University, China. The overview starts with the nanoscopic origin of the thermal expansion behavior of cement paste, which is triggered by the heating-induced release and the cooling-induced uptake of water by calcium-silicate-hydrates (C-S-H). Temperature changes lead to thermal strains. Thermal stresses are activated provided that the thermal strains are constrained. Such constraints prevail at microstructural scales of concrete as well as at cross-sectional and macrostructural scales of unreinforced and reinforced concrete structures such as beams, plates, and frames. Diurnal temperature changes result in daily cycles of self-equilibrated thermal stresses. They represent a considerable loading for interfacial transition zones (ITZs) separating the aggregates from the cement paste matrix. Extreme events such as sudden hail showers or moderate fires are very likely to result in thermal cracking, at least in the microscopic region of the ITZ, but frequently also at the larger scale of concrete.

  Abstract
  This document refers to thermal expansion of cementitious materials and to related thermal stresses activated in samples of concrete as well as in unreinforced and reinforced [...]

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