In this study, finite element (FE) analyses were conducted on a reinforced-concrete (RC) shear wall that is affected by an alkali–aggregate reaction (AAR), which were then applied for a benchmark studies in OECD/NEA/CNSI/ASCET (Organization for Economic Co-operation and Development/Nuclear Energy Agency/Committee on Safety of Nuclear Installations/Assessment of Structures subjected to Concrete Pathologies) Phases II and III assessments. A commercial software has been modified to account for this AAR expansion, which is affected by the stress field and change in physical properties of the concrete. The impacts of boundary conditions, modeling in two and three dimensions, and material properties on the load–displacement curve and crack patterns were carefully evaluated. Finally, although similar load–displacement curves and crack patterns were obtained, the peak load due to brittle failure of an RC shear wall affected by AAR could not be reproduced.
Consequently, it was found that the rotation of the loading stub and anchoring procedure of the base stub were critical conditions for load-displacement relationship of RC shear wall, and meshing capturing the arrangement of reinforcement bars is crucial for FE analysis with two-dimensional (2D) condition, and finally, the occurrence of initial cracks and the loading capacity could not be clearly reproduced. This suggests that consideration of the placement of rebars and covering concrete in the mash setting in three-dimensional (3D) model affected the failure mode of the concrete. It is necessary to consider the possible failure mechanism and to reflect such features in numerical modeling.
Municipal waste incineration ash contains heavy metals, and the safety of its disposal and reuse is an important issue. In this study, we discussed a safe recycling technology for bottom ash (BA) by utilizing the excellent heavy metal immobilization feature of geopolymer (GP). First, the differences in chemical compositions, physical properties, and heavy metal contents of BAs discharged in different months were investigated. Next, the reaction products and strength of the mixture of BA and alkali-activator (AA) solution were examined to clarify the reactivity of BA in the AA solution. We also investigated the effects of the discharge time of BA, ingredients of AA solution, curing method and mixing ratio of BFS on the setting time, strength and heavy metal immobilization capacity of GP mortar using ground granulated blast furnace slag (BFS) and coal fly ash (CFA) as precursors, and BA as fine aggregate, and discussed reaction products and micro-structure of the GP mortar. The main results are as follows: 1) BA contained a small amount of amorphous phase. Hardened GP monolith using BA and AA solution was not dense and had a very low strength. 2) The BFS/CFA-based GP mortar with BA as fine aggregate had a higher strength and a longer setting time when sodium silicate solution (WG) was used as AA solution than when sodium hydroxide was added or used entirely. The GP mortars using the BAs discharged in the warm season had longer setting time and higher strength. The reaction products of the GP mortar with WG solution and BA were mainly C-A-S-H gels. The leaching of heavy metal elements (HME) from the GP mortars increased with decreasing the alkalinity of leachate, but the effect of BA’s discharge season was not found in this study. The HME leaching concentrations from the GP mortars in non-acidic water environment were less than the HME leaching limits specified for recycled construction materials directly contacting with water, thus the GP materials with BA can be used in dry or non-acidic water environment. However, when used in acidic water environment, the BA content in the GP materials should be reduced.
The purpose of this study is to improve the interfacial performance between the concrete and polymer cement mortar (PCM) by using nano-SiO2. This study examined the bond properties of the inclusion of nano-SiO2 in the PCM based on splitting-tensile tests. In addition, the bonding mechanism was investigated with SEM. The results demonstrate that the inclusion of 2% nano-SiO2 in the PCM is beneficial to compressive strength and microstructure so as to obtain good interfacial bond strength as a repair layer mortar. The results also show that the increase in the surface roughness, the improvement of substrate concrete strength, vertical casting, and wet saturated interface state facilitate bond strength and change the failure mode of nano-SiO2 PCM/concrete composite specimens. At that time, the interfacial strength is predominantly influenced by the interface roughness and the old concrete strength through one-way analysis of variance. The results of SEM present that the interface of nano-SiO2 PCM/concrete composite specimen is more compact than that of the PCM/concrete composite specimen due to the transformation of harmful Ca(OH)2 into more C-S-H gels and the formation of a better polymer film structure at the interface. As a result, mixing nano-SiO2 into PCM accompanied by adopting effective treatment method of surface and higher compressive strength of substrate concrete is significantly beneficial to bond strength.
Few reports have described practice-based models assessing ultimate strengths of existing reinforced concrete (RC) members subjected to frost damage. This paper presents a kinematic model for shear assessment of damaged RC short columns based on the upper bound theorem. Without regressive functions, the developed model predicts the shear strength contribution of damaged concrete when the displacement field is divided into undamaged and damaged zones based on damage depths obtained from core sampling. The model accuracy is verified by comparison of its predictions with those of earlier test results of 14 RC columns presenting shear failure after freeze–thaw exposure. The analytical predictions show good agreement with experimentally obtained results within error of 20%. Shear strength predictions for different damage depths are presented for an existing RC bridge pier with severe frost damage. Rational shear assessment was achieved because the kinematic analysis directly correlates the damage depth with shear strength reduction.
Concrete structures serving in cold and wet regions usually suffer frost damage and thus have server deterioration. Many researches have been conducted to reveal the damaging mechanism and damaged mechanical properties of concrete under the effect of frost action. It has been widely known that the strength and stiffness of frost damaged concrete without using air-entraining agent decrease under room temperature. However, there will be a different story if the frost-damaged concrete is saturated and loaded under freezing temperature. Water existing in pores and cracks will freeze into ice, which provides additional strengthening effects. This paper presents a multi-scale modeling and simulation work on the static and fatigue behaviors of frost damaged concrete with consideration of such ice-strengthening effects. The micro-mesoscale damaging and strengthening effects induced by ice formation are modeled and integrated into the mesoscale analytical approach - Rigid Body Spring Model, and the macroscale static and fatigue behaviors are simulated. It is found that the freezing temperature has a positive (strengthening) effect on the static strength, while it has a negative effect on the fatigue life for both intact and frost-damaged concrete. Test is also conducted with available experimental evidence to validate the developed approach. Satisfactory correlation is found through the comparison between simulation and experiment.
The effect of bacterial and fiber combination to enhance the properties of GGBS based geopolymer concrete to be used as paver block is investigated in this study. In this study, Bacterial combinations such as Bacillus Subtillis and Bacillus Sphaericus and high modulus glass fibers and low modulus polypropylene fibers were incorporated to produce hybrid fiber reinforced bacterial geopolymer concrete with increased energy absorption characteristics and better post cracking behavior under heavy loads. Combined and the discrete performance of bacteria and fiber over the mechanical properties of the geopolymer concrete were investigated. The influence of bacterial strain combination over self-healing of concrete is also studied by inducing artificial cracks of 1mm over concrete. The self-healed products were subjected to microstructural investigation such as SEM analysis and XRD analysis to understand the microstructure of precipitated products and the bio- remedial action exhibited by the bacteria. Finally, the paver blocks were produced for the optimum specimens of geopolymer concrete along with fiber and bacteria and its performance over compressive, split tensile, flexural and water absorption characteristics were assessed to determine its adaptability to sustain loading under heavy and very heavy traffic conditions as per Indian Standard 15658 (2006). This research work lays a path for the sustainable development of production of eco-friendly self-healing high strength paver blocks.
Self-healing cement composites are considered to be an effective solution towards the enhancement of sustainability and service-life of cement and concrete structures, as well as the reduction of repair and maintenance cost. Among the several self-healing technologies, encapsulated healing agents present benefits that include healing of larger cracks and timeless healing potential upon damage. This paper presents a critical overview of the progress made in the development of encapsulated healing agents, along with the main achievements related to their integration in the cement mixtures. Encapsulated healing agents were classified according to their size in two main categories: (i) spherical microcapsules up to 1 mm, and (ii) macrocapsules that include larger spherical capsules (>1 mm) and/or tubular capsules up to 100 millimeters long. The review emphasizes on the performance characteristics of the shell material and the capsule system that are necessary in order to protect the different types of healing agents in the long-term, to provide even distribution and survivability and finally, to ensure efficient triggering and release of the healing agent during crack propagation. The relevant literature is analyzed and discussed according to the above thematic priorities, aiming to locate research gaps and best practices and thus, to enable and facilitate the development of effective encapsulated healing agents that could be scaled-up. To this end, particular emphasis is given on the effect of capsules integration on the properties of both fresh mixtures and hardened cement specimens.