Ultra- ductile waterborne epoxy-concrete composite repair material

With the extension of service time, the building structure is in urgent need of repair and reinforcement due to its deterioration in safety and durability with the years. Normal cement concrete exposes shortcomings such as high brittleness and low flexural-tensile strength for which fails to meet the requirement of construction repairing. In this paper, a novel waterborne epoxy-concrete composite repair material (WECM) was prepared by using self-synthesized water-based epoxy resin based on the molecular structure-activity relationship. The key flexural and tensile performance parameters of WECM were accurately obtained

Molecular simulation of "hydrolytic weakening": A case study on silica

Water molecules have a great influence on the mechanical performance of silicate materials. In this study, the hydrolytic weakening mechanisms of crystal alpha-quartz and amorphous silica glass were investigated in light of molecular simulation. A reactive force field was used to simulate the structure, dynamics and mechanical properties of the silica with water molecules during a uniaxial tensile process with loading rates from 0.02 to 0.16 ps(-1). Dry silica samples, obtained by the temperature quenching method, and wet silica samples, formed by Grand Canonical Monte Carlo water adsorption, were prepared for mechanical tests. Structurally, water penetrating the silica cavity can form H-bonds with the neighboring bridging oxygen in the silicate network, leading to the elongation of the Si-O bonds and the enlargement of the Si-O-Si angles. Dynamically, the confined water molecules, with a high diffusion rate, collide with the silicate network, reducing the stability of the siloxane bonds. The reactive force field, coupled with the chemical and mechanical responses, provides new insights into the molecular structural evolution. During the tensile process, the siloxane bonds and Si-O-Si angles are initially stretched in the elastic region. Subsequently, the silica network depolymerizes into branch structures and, finally, the structure undergoes a tensioned fracture in a catastrophic manner. When the stress exceeds 30% of the tensile strength, water molecules in the silica system begin to dissociate into two reaction pathways, i.e. one governed by the chemical adsorption of water at a low stress level and the other dominated by the breakage of siloxane bonds at a high stress level. The stress-dependent water dissociation accelerates the depolymerization of the silica network, and brings about further irreversible deformation, which weakens the cohesive force of the silica. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Chloride Diffusion and Induced Reinforcement Corrosion in Concrete with Fly Ash and Ground-Granulated Blast-Furnace Slag Exposed to Marine Submerged Zone

This paper investigates the influence of mineral admixtures fly ash (FA) and ground-granulated blast-furnace slag (GGBS), the byproducts of industry, on chloride ions migration and corrosion resistance performance. A novel preparation method of wire beam electrode (WEB) was also introduced to explore the excellent corrosion-resistant capacity of concrete with mineral admixtures. By comparing concrete specimens with and without FA and GGBS, the test result of wire beam electrode, rapid chloride migration (RCM), and electrochemical tests highlight the positive impact of fly ash and GGBS against chloride ions migration, respectively. Concrete with fly ash and GGBS supplies an advanced protection effect of ordinary Portland cement; meanwhile, CO2 emission amount can be significantly reduced. Moreover, homemade wire beam electrode was proved to be a novel and reliable test method against corrosion, which has agreement with the test result of an electrochemical device

Effect of SiO2 Sol/Silane Emulsion in Reducing Water and Chloride Ion Penetration in Concrete

Here, a new concrete hydrophobic treatment method is developed using SiO2 sol and silane emulsion. The effectiveness of the modification for concrete protection is evaluated through testing water absorption and chloride diffusion. Two types of concrete with different strength grades (C40, C50) are used as the research object. The results show that the water capillary absorption coefficient and chloride ion diffusion coefficient of concrete decrease greatly under the protection of SiO2 sol and silane emulsion. Additionally, the protection effect is better with the increase of SiO2 consumption. Contact angle test results reveal that when the coating amount of SiO2 sol and silane emulsion is 300 g/m2, respectively, the contact angle reaches 150.2°, indicating the concrete (C40) surface reaches the superhydrophobic state. Through scanning electron microscope (SEM) observation, it is found that the hydrophobic effect of the SiO2 sol/silane emulsion is mainly due to the change in the surface morphology of concrete (C40)

Experimental and Numerical Study on Chloride Transport in Unsaturated Concrete: Highlighting Temperature, Humidity, and Mineral Admixtures

Chloride transport within concrete is critical for the durability of reinforced concrete structures; however, its diffusion under the coupling action of temperature and humidity has not been fully comprehended. Therefore, in this work, the coupling effects of temperature, relative humidity, and mineral admixtures on chloride transport in concrete were investigated through experimental and numerical simulation work. The results show that the chloride diffusion coefficient decreases with the decreased temperature and growth of relative humidity; however, the chloride concentration on the concrete surface is increased with the growth of temperature and relative humidity. Moreover, compounding about 15% fly ash (FA) and 30% granulated ground blast furnace slag (GGBS) to replace the cement is the most beneficial for improving the antichloride capacity of concrete, considering also the strength. In addition, the numerical simulation considering the coupled effect of temperature and relative humidity of chloride transport in concrete has good agreement with that of experimental results

The Effect of Temperatures on the Passivation Behavior of Q235 Steel in the Simulated Concrete Pore Solution

Concrete, especially mass concrete, releases a large amount of heat during the hydration process, resulting in the passivation of reinforcement at high temperatures. However, the passivation study of reinforced concrete is mostly conducted at room temperature, and the influence of temperature on passive film behavior is not clear at present. The passivation film of reinforcing steel directly determines the corrosion resistance of reinforcing steel and affects the service life of reinforced concrete. Herein, the passivation of Q235 steel soaking in simulated concrete pore (SCP) solution at 20 °C, 40 °C, and 60 °C is explored. It is found that the passivation process is divided into two stages, with 24 h as the boundary; within 24 h the passivation was carried out rapidly, and the passive film is in a relatively stable state after 24 h. In addition, the higher the temperature, the faster the passivation. Moreover, under the condition of higher temperatures, more Fe3+ compounds are produced, and the semiconductor properties of passivated films are more stable. Based on experiments, the passivation mechanism affected by temperature was analyzed in detail

New Self-Repairing System for Brittle Matrix Composites Using Corrosion-Induced Intelligent Fiber

Brittle matrix composites such as concrete are susceptible to damage in the form of cracks. Most of the current self-repair and self-healing techniques have repair limits on crack widths or high costs of an external stimulator, or have an unfavorable effect on the composite’s strength. This paper proposes a new concept of corrosion-induced intelligent fiber (CIF) and a new self-repairing system that uses the CIFs to close cracks in brittle matrix composites within a corrosive environment without external help, and without compromising the strength. The CIF comprises an inner core fiber and an outer corrodible coating that are in equilibrium, with the core fiber in tension and the corrodible coating in compression. The preparation steps and shape recovery mechanism of the CIF and the self-repair mechanism of the CIF composites are explained. Based on these concepts, this paper also describes several mechanical models built to predict the magnitude of pre-stress stored in the core fiber, and the maximum pre-stress released to the matrix composites, and the minimum length of the reliable anchor ends of CIF. The sample calculation results show that the recovery strain was 0.5% for the CIF with the steel core fiber and 12.7% for the CIF with the nylon core fiber; the maximum crack closing force provided by the CIF to concrete can be increased by increasing the amount of the CIFs in concrete and the initial tensile stress of the core fiber. This paper provides some suggestions for enhancing the self-repair capability of brittle composites in complex working environments

Influence of Moisture Content on Electromagnetic Response of Concrete Studied Using a Homemade Apparatus

In this study, we examined the influence of moisture content on the electromagnetic response of concrete. A novel homemade electromagnetic monitoring apparatus was developed and used to evaluate the Hall effect voltage at both ends of concrete based on our previous study of the Hall effect. We used four different concrete mix water/binder ratios: 0.30, 0.28, 0.26, and 0.24, and three conditions (relative humidity, carbonation, and water absorption) were examined in this experiment. The results show that the moisture content inside concrete influences the relative permeability of concrete. The variation in the Hall effect voltage is more influenced by carbonation than changes in relative humidity; water absorption increases the Hall effect voltage the least amongst the other examined factors. According to the experiment, a calibration system was established, and the relevant correction factors are provided

Molecular Simulation of Calcium Silicate Composites: Structure, Dynamics, and Mechanical Properties

Calcium silicate composite (CaO)x(SiO2)1-x has significant applications in the bioactive materials in medical treatment and cementitious materials in construction engineering. In this study, to unravel the role of calcium atoms on the silicate composite, the molecular dynamics (MD) technique was used to simulate the structures, dynamics, and mechanical properties of (CaO)x(SiO2)1-x systems, with x varying from 0 to 0.6. The Feuston-Garofalini model was employed to describe the interatomic interactions in the systems. Q species, the connectivity factor, shows that the increase in calcium content in the silicate composite can lead to the depolymerization of the silicate network. Due to the high diffusion rate, the presence of Ca atoms also weakens the stability of the chemical bonds in the system. With the increasing calcium content, the molecular structure of the silicate skeleton is transformed from an integrity network to separated short chains, which significantly decreases the stiffness and cohesive force of the calcium silicate composites. On the other hand, the uniaxial tension response of the calcium silicate composites suggests that at the postfailure stage, Ca atoms associate with the nonbridging oxygen atoms and the reconstructed Ca-O connection slows down the irreversible damage of the composite, hereby enhancing the plasticity

Effect of hoop restraint on the degradation behavior of cement paste exposed to sodium sulfate solution

Concrete filled steel tube (CFST) is an effective method to alleviate sulfate attack in saline soil environments. In this study, the effect of hoop restraint on the degradation behavior due to sulfate attack in ordinary Portland cement (OPC) paste and sulfate-resistance Portland cement (SRPC) pastes is studied. The cracking behavior and mass variation are determined to investigate deterioration process of the pastes. EPMA-WDS, nanoindentation and SEM-EDS are performed to analyse the deterioration mechanism of the cement paste under hoop restraint condition. As the results shown: The deterioration behaviors of cement pastes is strongly affected by hoop restraint. Specifically, the hoop restraint inhibited crack formation of the specimens and formation of sulfate products. The average effective sulfate diffusion coefficient is reduced by 30.26% and 5.81% for OPC and SRPC paste during 365 days of exposure, respectively, compared with the reference group. Moreover, the mechanism of deterioration of cement paste under hoop restraint condition is proposed based on change in chemical component of C–S–H phase. The hoop restraint from steel tube can effectively inhibit the sulfate attack on cement composite with important engineering significance