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Microbial-induced calcite precipitation (MICP) techniques are promising solutions for repairing cracks in reinforced concrete (RC) structures when exposed to a harsh chloride environment. By means of the lattice network method, this study numerically investigates the entire MICP-induced healing process from crack closure to inhibition of chloride transport of the microbial self-healing concrete by coupling the following two models: (1) the crack self-healing model; and (2) chloride diffusion model. The crack self-healing model provides visualization of the reduction of local crack widths induced by MICP. The chloride concentration distribution is quantitatively reflected through the chloride diffusion model. In addition, an empirical formula is employed to represent the relationship between diffusion coefficients of cracks in the chloride diffusion model and the time-varying crack width obtained from the crack self-healing model. The reduction of crack widths and the distribution of chloride within microbial self-healing concrete are determined through numerical calculations. The simulation reliability is validated based on comparisons with the available literature. Simulation results demonstrate that the MICP-induced crack healing effectively impedes chloride transport. [ABSTRACT FROM AUTHOR]

Highlights • Dynamic biaxial compressive experiments on SFRC were conducted. • Combined effects of strain rate and steel fiber on dynamic strength were analyzed. • Dynamic strength criterion of SFRC considering stress ratio was proposed. • Model results well agreed with experiments at strain rates from 10−5/s to 10−2/s. Abstract Understanding the biaxial mechanical behavior of steel fiber reinforced concrete (SFRC) is of critical significance when the concrete structures are subjected to dynamic loading. This paper presents the results of an experimental investigation on the dynamic mechanical behavior of SFRC subjected to biaxial compressive loadings. Aiming at the combined effects of steel fiber and strain rate on strength criterion, a series of dynamic biaxial compressive experiments on 100 mm-length SFRC cubic specimens with various steel fiber contents (0%, 1%, 2%, and 3% by volume) were conducted by using a large dynamic triaxial servo-hydraulic testing apparatus. The specimens were loaded by biaxial compression with five stress ratios (i.e. 0:1, 0.25:1, 0.5:1, 0.75:1 and 1:1, respectively) at various strain rates ranging from 10−5/s to 10−2/s. A comprehensive dynamic biaxial strength criterion taking into account the combined effects of uniaxial strength, strain rate and stress ratio was proposed. The effect of steel fiber content and strain rate on the biaxial mechanical behavior of SFRC specimens was discussed, including the failure mode, stress-strain curve, biaxial compressive strength, dynamic strength criterion. It suggested that at given strain rate, the biaxial stress ratio is the major factor dominating the strength increment, and the incorporation of steel fiber has a remarkable enhancement on the strength for all stress ratios. Additionally, the dynamic compressive strengths of SFRC were found to increase with the increase of strain rate, while the failure pattern and dynamic ultimate strength were closely dependent on the magnitude of lateral stress exerted on the specimens. The comparison indicates that the proposed model results of dynamic strength criterion were in good agreement with the experimental data for SFRC specimens. [ABSTRACT FROM AUTHOR]

The effect of saturation coupled with loading rate on the behavior of dam concrete was investigated under the biaxial compression. Dynamic biaxial compressive experiments on dam concrete cubes with an edge length of 250 mm (dry and saturated) were carried out using a large static and dynamic triaxial electro-hydraulic servo multiaxial testing system. The specimens were loaded in biaxial compressive stress states (with the stress ratios of 0:1, 0.25:1, 0.5:1, 0.75:1 and 1:1 respectively) under static and a series of dynamic loading velocities (with strain rates ranging from 10 −5 /s to 10 −2 /s). The ultimate strengths of dry and saturated concretes were found to increase with the increase of strain rate, while the damage pattern and ultimate strength are closely related to the magnitude of lateral pressure exerted on the specimen. In addition, the dynamic failure criterion is proposed to characterize both the effects of strain rate and water content on the ultimate strength of dam concrete under biaxial compressive stress states. By testing dry and saturated specimens, the effect of water content on concrete strength was also examined. The experimental results indicate that the static compressive strengths of saturated concrete are lower than those of concrete in dry state, but on the contrary the dynamic strengths of saturated concrete is higher than those of concrete in dry state. The strain rate effect on strength of saturated concrete is more significant than that of dry concrete in lateral confining pressure, indicating that the saturated concrete is more rate-sensitive than dry concrete. As explained through a basic mechanical analysis, this dissimilarity is mainly attributed to the inertia effect and the viscosity of pore-water inside the saturated concrete during fast (quasi-static or dynamic) loading. [ABSTRACT FROM AUTHOR]

• Hydration degrees of the concrete with gap-graded size fly ash were calculated. • A primary mathematical model was established and verified. • Accuracy of the model was improved by calculation error correction. Fly ash (FA) was commonly used for admixture in concrete to change the durability and workability of concrete due to its dilution effect, enhanced pozzolanic reaction effect, nucleation effect and physical filling effect. The efficient utilization of FA based on particle size has now become a hot research topic at present. In this work, FA particles with gap-graded sizes were prepared by sieving method and then prepared for concrete mixing. The hydration heat, compressive strength and pore structure of concrete were studied using isothermal calorimetry, mercury intrusion porosimetry (MIP) analysis. Hydration degrees of the concrete with gap-graded FA particles were calculated, and a primary mathematical model was tentatively established and verified according to the particle sizes of FA and early compressive strength (7 days) of concrete based on Neville's theory, where the hydration degree was used as an intermediate variable. However, the mathematical model can just roughly reflect the relationship between FA particle sizes and the early compressive strength of concrete due to the lack of consideration of porosity in Neville's theory. Therefore, to improve the accuracy of the model, calculation error of the model with pore characteristic parameters were used as correction coefficients. The calculation error was significantly reduced from 30 ~70% to 3~17%. The modified model was proved to be effective to predict the early compressive strength of concrete prepared with gap-graded sizes of FA. It is of great significance to the prediction of concrete strength and the efficient utilization of FA. [ABSTRACT FROM AUTHOR]

Microcracks, which are always present in concrete, play a critical role in the durability of concrete because they provide additional pathways for aggressive agents such as chlorides to penetrate into the material. A thorough understanding of the transport properties of cracked/damaged concrete is thus essential for predicting its long-term durability. In addition, for concrete in an unsaturated state, water acts as the main medium for the ingress of chloride ions into concrete. This paper presents an experimental investigation into the capillary absorption of concrete after being subjected to compressive or tensile loading. Sorptivity is chosen as a parameter to describe the rate of water penetrating into concrete since it can characterise the tendency of cementitious materials to absorb and transmit water by the capillary mechanism. A series of water absorption experiments were conducted on concrete samples after being subjected to uniaxial compressive or tensile loading. Three load levels were considered - 70%, 80% and 90% of the corresponding compression and tension loading capacity. An improved gravimetrical test device was used to measure the cumulative water absorbed by concrete at the given time of exposure. The results show that mechanical loading, compression or tension, has an important influence on the water absorption of concrete. The rate of capillary absorption of load-damaged concrete can be up to double that of sound concrete samples for the ranges of load level studied. [ABSTRACT FROM AUTHOR]

Abstract: Water penetration into concrete by capillary absorption plays a very important role in the ingress of contaminative substances when the structures are built in aggressive environments. In the present paper the lattice network model is proposed based on the unsaturated flow theory to predict the water penetration into concrete. On the mesocale level, concrete is treated as a three-phase composite. By means of the Voronoi diagram meshing strategy, the lattice network model of concrete with different types of lattice elements is developed. The corresponding transport properties are assigned to the lattice elements in the network falling in different phases. As a result, the lattice elements are idealized as conductive “pipes” in which uni-directional flow can be realized between the two nodes of the elements. Parameters in the lattice network model, such as the sorptivity and porosity of the mortar and the ITZs are quantitatively determined. With help of the approach, the water content distribution within a concrete sample after any elapsed time, especially the penetration depth of water frontier, can be easily predicted. The cumulative water absorption calculated by the lattice network model is shown to be well agreed with the experimental results. [Copyright &y& Elsevier]

• The cumulative water content rises when loading cycles and stress level increase. • An improved test apparatus is used to record in real time the cumulative water absorption of specimens after fatigue loading. • The initial sorptivity and secondary sorptivity increases generally with an increase of loading cycles and stress level. • A theoretical model for predicting water absorption of concrete damaged by compressive fatigue loading is developed in terms of the residual strain. For reinforced concrete (RC) structures in coastal regions, fatigue loading is a common loading type, which has the potential to progressively promote micro-cracks generation and development, and in turn, greatly to facilitate the water and aggressive agents (such as chloride and sulfate ions, which are usually sucked by water) penetration in concrete. Therefore, it is imperative to investigate the behavior of water movement in concrete after fatigue loading. This paper presented an experimental investigation on the influence of fatigue stress level and loading cycles, which may induce different degree of damage for concrete, on the transport rate and volume of water absorbed by concrete after compressive fatigue loading. The residual strain was measured by strain gauges to represent the damage evolution of concrete and estimate the water absorption in concrete under cyclic loading. Moreover, the cumulative water content of specimens under fatigue loading was recorded in real time through an improved test apparatus. The results show that the residual strain and cumulative water content of concrete increase with the increase of stress level and loading cycles. The initial and secondary sorptivities, i.e., the slope of the two linear parts of the cumulative water content-time1/2 curves, both increase dramatically when stress level and loading cycles increase because of the development of the cumulative pore volume in concrete corresponding to various pore radius. In addition, a theoretical model for predicting water absorption of concrete damaged by compressive fatigue loading was developed in terms of the residual strain, which shows good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Thick-walled cylinder models are the most commonly used analytical models for corrosion-induced cracking of concrete cover which have been developed for decades and have become a mature theoretical series. This paper reviews their development history and classifies their characteristics. Furthermore, the performances of these models on predicting cracking time of concrete cover are tested by comparing with collected test data, and the effects of controlling factors are analyzed. It is found that the main characteristics of these models include the differences in the assumptions and constitutive laws for cracked concrete, number of cylinders and hypotheses of crack morphologies. Among the evaluated models, El Maaddawy and Soudki (2007) and Chernin et al. (2010) have the best predicting performances. And the cracking time extends with the increase of concrete cover thickness, tensile strength, porous zone thickness and creep coefficient, while it decreases with the increase of concrete elastic modulus, rust volumetric expansion factor and corrosion current density. The main limitations include the ill considerations of heterogeneity of concrete, non-uniform corrosion along rebar, applied load, corner rebar or multi rebars conditions, biaxial stress state of concrete, compatibility conditions between the cracked and uncracked concrete, rust production rate and the values of involving parameters. • About 20 thick-walled cylinder models for corrosion-induced cracking are reviewed and analyzed. • 57 test data of corrosion-induced cracking time are collected to test the performances of these models. • Characteristics of different models include assumptions for cracked concrete, number of cylinders and crack morphologies. • Limitations of these models include corrosion morphology, compatibility conditions and values of some parameters. [ABSTRACT FROM AUTHOR]

The article discusses the article "Capillary absorption of concrete after mechanical loading," by L. Wang and S. Li. It says that it presents an experimental examination into capillary absorption of concrete after being subjected to uniaxial compressive or tensile loads. It adds that a series of water absorption experiments were done to investigate the extent to which load level and loading patterns influence the permeation of concrete.