Influence of Fly Ash on Surface Chloride Concentration Under Shallow Immersion Condition

In this paper, the influence of fly ash content on concrete surface chloride concentration was investigated through periodical tests of surface chloride concentration of concrete by immersing three kinds of concrete specimens in 5.0 wt.% sodium chloride solution. One kind of specimen is common concrete without fly ash, whereas the other two kinds of specimens are mixed with fly ash. The results show that the surface chloride ion concentration ranges from 0.295 to 0.777 wt.% for the immersed concrete samples in this study; in the initial stage of immersion, the concrete surface chloride ion concentration is affected by added fly ash, and after 30 days of immersion, the surface chloride concentration of concrete with fly ash is 1.3 times that of concrete without fly ash at the same water–binder ratio; for the concretes without fly ash, the surface chloride concentration is linear with immersion time, whereas for the concrete with fly ash, the surface chloride concentration is nearly linear with square root of immersion time; for the concrete with fly ash, fly ash contents have little impact on the surface chloride concentration, and the surface chloride content of concrete without fly ash is gradually close to that of concrete with 15 and 30 wt.% of fly ash

Investigation of a Novel 3D Non-Destructive Evaluation for Corrosion Process in Reinforced Concrete

X-ray computed microtomography (X-ray μCT) method is proposed to trace the corrosion features of steel bar in cementitious materials. Three-dimensional corrosion morphology (including morphology of steel corrosion, corrosion products and cracks formation) are characterized and reconstructed. Experimental results demonstrate that X-ray μCT can track time-dependent development of corrosion products and the subsequent initiation and propagation of corrosion-induced cracks. In addition, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) are used to morphologically and chemically analyze the corrosion result. The microscopic analysis result is found to be in a very good agreement with the image analysis measured by X-ray μCT. Furthermore, the total corrosion ratio of steel, obtained by the X-ray μCT, is calculated to be in a good correlation with the data measured by Faraday\u27s law as well as gravimetric method

Behavior of CFRP Plate in Simulated ICCP System of Concrete Structures

An innovative effort was made to utilize carbon fiber reinforced polymer (CFRP) plate as an anode in impressed current cathodic protection (ICCP) of reinforced concrete structure. The feasibility was explored by bonding CFRP strips to concrete and then applying protection current through CFRP strips to steel rebar in concrete. Service life and performance of CFRP plate were investigated in simulated ICCP systems with various configurations. Steel potential results confirmed that the steel rebar in concrete maintained at a satisfactory level with the applied protection current. It is shown that CFRP strips can be successfully used in various solutions as an anode material

The Feasibility of Waterproof Microcapsule System for Bacteria-Based Self-Healing Cementitious Material

In this study, a waterproof material was used to fabricate microcapsule by interfacial curing reaction to encapsulate an alkaliphilic spore-forming bacterium. The technical feasibility of encapsulated spores and the influence of three kinds of curing agent on the calcium precipitation activity (CPA) of the bacterium were studied. Furthermore, micromorphology of microcapsules was observed by Scanning Electron Microscopy (SEM). Afterwards, the thermal stability and thermolysis temperature were determined by TGA thermal analyzer. Moreover, the CPA of broken/ unbroken microcapsules was evaluated. In addition, water resistance was evaluated by adding microcapsules in the water for 1, 3, 7, 14, 28, and 56 days. Finally, light microscope was applied to trace the self-healing behavior of encapsulated mineralization bacterium in cement paste specimens. The results showed that compared with unbroken microcapsules, higher CPA was achieved by breaking the microcapsule to release the bacterium, suggesting good protection for the encapsulated spores. Three curing agents showed nearly similar influence on the spores, while KH792 performed relatively better, and thus was used to fabricate microcapsule with the core/shell weight ratio being 1:1. Our results also indicated that ER microcapsules could keep unbroken in the water for 2 months. Compared with the specimens without embedded bacterium, the healed crack area of specimens embedded with bacterial microcapsules was monitored, suggesting effective self-healing of concrete crack can be achieved by introducing encapsulated mineralization microorganisms into concrete structures. Therefore, we put forward that this waterproof epoxy resin microcapsules could be potential for the application of self-healing concrete

Strength Deterioration of Concrete in Sulfate Environment: An Experimental Study and Theoretical Modeling

Sulfate corrosion is one of the most important factors responsible for the performance degradation of concrete materials. In this paper, an accelerated corrosion by a sulfate solution in a dry-wet cycle was introduced to simulate the external sulfate corrosion environment. The deterioration trend of concrete strength and development law of sulfate-induced concrete corrosion depth under sulfate attacks were experimentally studied. The damaged concrete section is simply but reasonably divided into uncorroded and corroded layers and the two layers can be demarcated by the sulfate corrosion depth of concrete. The accelerated corrosion test results indicated that the strength degradation of concrete by sulfate attack had a significant relation with the corrosion depth. Consequently, this paper aims to reveal such relation and thus model the strength degradation law. A large amount of experimental data has finally verified the validity and applicability of the models, and a theoretical basis is thus provided for the strength degradation prediction and the residual life assessment of in-service concrete structures under sulfate attacks

INVESTIGATION OF SELF-HEALING BY USING ETHYL CELLULOSE ENCAPSULATED BACTERIUM IN CEMENTITIOUS MATERIALS

Abstract: A new approach to microcapsule based microbial self-healing system is presented that aims to heal the crack in cementitious materials. In this work, ethyl cellulose (EC) was designed to fabricate microcapsule as a protection strategy to encapsulate an alkaliphilic spore-forming bacterium. The technical feasibility of encapsulated spores and the influence factors were studied by calcium precipitation activity (CPA) of the bacterium. The CPA of broken/unbroken microcapsules was evaluated. The micro-morphology of the precipitation produced by the bacterium was investigated through Environmental Scanning Electron Microscopy (ESEM), X-ray Diffraction (XRD) and X-ray energy dispersive spectroscopy (EDS). X-ray Computed Tomography (XCT) was applied to trace the crack development and self-healing behavior of encapsulated mineralization bacterium in cement paste specimens in three dimensions. The experimental results showed that compared with unbroken microcapsules, higher CPA was achieved by breaking the microcapsule to release the bacterium, suggesting good protection for the encapsulated spores. Subsequent production of calcium carbonate confirmed by ESEM and EDS indicated activation of encapsulated mineralization bacterium. The XCT results showed that formation of crack successfully triggered the breakage of embedded microcapsules. Compared with the specimens without embedded bacterium, the healed crack area of specimens embedded with bacterial microcapsules was monitored, suggesting effective self healing of concrete crack can be achieved by introducing encapsulated mineralization microorganisms into concrete structures

Research on Microscopic Pore Structure and Permeability of Air-Entrained Fly–Ash Concrete Subjected to Freezing and Thawing Action

In this paper, rapid freezing and thawing test of air-entrained fly–ash concrete was carried out. Gas adsorption of concrete pores, mass loss, dynamic elastic modulus, and chloride diffusion coefficient of concrete after different freezing and thawing cycles was tested; scanning electron image was used to study the microscopic pore structure of the specimens. Results show that after certain freezing and thawing cycles, microscopic pores of the sample had a brittle failure, which results in sudden changes of cumulative desorption pore volume and cumulative desorption surface area obtained by Barrett, Joyner, and Halenda method. It was found that the variation of maximum cumulative desorption surface area is in good agreement with that of mass loss rate and chloride diffusion coefficient. At different stages of freezing and thawing, rate of mass loss shows a linear relationship with maximum cumulative desorption pore volume and maximum cumulative desorption surface area. In the range of freezing and thawing cycle between 0 and 150, chloride diffusion coefficient demonstrates a linear relationship with maximum cumulative desorption pore volume and maximum cumulative desorption surface area; in the range of freezing and thawing cycle between 200 and 300, this relationship becomes nonlinear

Axial stress-strain behaviour of pre-damaged square concrete column repaired with FRP jackets

This paper presents the results of an experimental study on the behaviour of pre-damaged concrete square columns that have been strengthened with carbon fibre-reinforced polymer (CFRP) wrap. Tests were conducted on 46 concrete columns involving variations of corner radius ratio, damage degree, and confinement pressure. The effect of corner radius on the main parameters of stress-strain curve for FRP strengthened pre-damaged specimens, such as compressive strength, strain capacity, initial elastic modulus, was investigated. The test results demonstrated that the efficiency of repairing damaged columns significantly depends on the corner radius ratio, the strength gain after FRP-strengthening decreases as the corner radius ratio reduces. Using those test data, new ultimate strength and ultimate strain model are proposed. The proposed models involved the effect of both corner radius and damage level, showing good agreement with the experimental results

Axial stress-strain behaviour of pre-damaged square concrete column repaired with FRP jackets

This paper presents the results of an experimental study on the behaviour of pre-damaged concrete square columns that have been strengthened with carbon fibre-reinforced polymer (CFRP) wrap. Tests were conducted on 46 concrete columns involving variations of corner radius ratio, damage degree, and confinement pressure. The effect of corner radius on the main parameters of stress-strain curve for FRP strengthened pre-damaged specimens, such as compressive strength, strain capacity, initial elastic modulus, was investigated. The test results demonstrated that the efficiency of repairing damaged columns significantly depends on the corner radius ratio, the strength gain after FRP-strengthening decreases as the corner radius ratio reduces. Using those test data, new ultimate strength and ultimate strain model are proposed. The proposed models involved the effect of both corner radius and damage level, showing good agreement with the experimental results

Cementing mechanism of potassium phosphate based magnesium phosphate cement

Magnesium phosphate cements (MPCs) are materials that belong to chemically bonded ceramic materials. They have a wide range of potential applications, due to their superior performance. In this paper, the reaction products and cementing mechanism of magnesium phosphate bonded cement based on the dead burned magnesia and the mono-potassium phosphate (M PP) are investigated. Fine powder and grains of dead burned magnesia were used to prepare pure cement paste and bonding cluster samples, respectively. The cement reaction products and their micro-morphology in the both different samples are examined. The microstructure of specimens is analyzed by SEM, TEM, XDR, and optical microscopy. Struvite of potassium (MgKPO4 . 6H(2)O) is observed in the reaction products. According to the analysis, it is found that struvite exists in both crystalline and amorphous form. There is also residual magnesia in the hardened cement paste. By means of microscopy observation, it can be seen that reaction products form around the unreacted magnesia and can develop into a continuum structure, which further produces the hardened paste. Struvite can grow up to form the more perfect crystal in a long term curing age, if large enough space is available during the hydration process. (C) 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved