Search

In saline-alkali or salt-lake areas, the influence of pH value of underground aqueous solution and carbonation on the deterioration of cement-based materials remains an open question. This study utilizes various techniques to investigate the physical properties and degradation mechanism of partially immersed mortar incorporated with different dosages of fly ash exposed to combined action of salt solutions (10 wt% Na 2 SO 4 , 3.5 wt% NaCl + 10 wt% Na 2 SO 4 , and tap water) at different pH values (pH 5, 7, and 9) and carbonation (accelerated carbonation: 20 vol% CO 2 and natural carbonation: 0.035 vol% CO 2). The results indicate that the degree of deterioration of mortar increases with dosage of fly ash and pH value of salt solution, with the maximum mass loss exceeding 25 %. Physical sulfate attack (PSA) is responsible for the partially immersed mortar exposed to combined action of salt attack at various pH values and natural carbonation. However, when carbonation is accelerated, a more severe deterioration occurs, with the durability time of mortar reduced by nearly 60 %. PSA and decomposition of C-S-H are mainly responsible for the deterioration of mortar, while the formation of thaumasite and natrite is partially responsible for the deterioration of mortar. The presence of NaCl in mixed salt solution inhibits the precipitation of Na 2 SO 4 crystals; the precipitation of NaCl clogs the pore network of mortar, which impedes the progressive penetration of CO 2. The presence of NaCl in mixed salt solution is beneficial to salt resistance and carbonation resistance of mortar. • The impact of pH of salt solution and carbonation on physical sulfate attack (PSA) of mortar is carried out in this study. • Degree of deterioration of partially immersed mortar increases with dosage of fly ash and pH value of salt solution. • The deterioration of partially immersed mortar exposed to salt solution at various pH and natural carbonation is PSA. • Under accelerated carbonation, PSA, decomposition of C-S-H, formation of thaumasite are all responsible for deterioration. [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]

This paper presents the results of an experimental investigation on the water and chloride transport properties of unsaturated concrete under combined short-term sustained compressive loading and chloride attack. Aiming for the coupled effect of sustained compressive loading and chloride penetration, an improved test apparatus, which is compared with the traditional gravimetric measurement, was designed to real-timely and continuously measure the amount of water solution absorbed by the cylindrical hollow concrete specimen under sustained loading. A series of chloride transport experiments on water/chloride diffusivity and penetration profiles of chloride were conducted on the saturated, half-saturated and fully dried concrete respectively subjected to several compressive stress levels (ranging in 0–50% of the corresponding compression loading capacity). The experimental results showed that the saturation level and sustained compressive loadings have significant influences on water and chloride transport properties of the unsaturated concrete. The quantitative relationship between water/chloride diffusivity and compressive stress level was developed by fitting the obtained experimental data. Finally, on the basis of convection-diffusion equation, the prediction model of chloride transport in unsaturated concrete under sustained compressive loading was proposed and validated by the experimental results. The comparison of chloride profiles indicates that the numerical results were in good agreement with the experimental measurements. [ABSTRACT FROM AUTHOR]

Knowledge of capillary imbibition of water in distributed microcracks within building materials plays a critical role in assessing and predicting the durability of reinforced concrete structures. This paper focuses on a comprehensive investigation of capillary imbibition of water in discrete planar cracks, involving both theoretical and experimental aspects. In view of dynamic capillary flow of liquid in discrete cracks, which is usually described by Lucas-Washburn (L-W) equation, the correlation between capillary rise height and crack width is theoretically established. A benchmark study to investigate capillary flow in a series of discrete granite cracks, artificially fabricated by means of ultrathin steel disc with various thicknesses, is reported, and this provides data to validate the theoretical model developed in the paper. The experimental results give out the measurement of discrete crack width with different sizes (23.64–240.38 μm) as well as the mass of absorbed water obtained by the traditional gravimetric method. The average cumulative water mass of specimen generally increases with an increase of crack width for the ranges studied. Moreover, the cumulative water mass rapidly increases at the initial stage of water absorption test while at later stage the variation of absorbed water mass is relatively less. The analysis shows that the influence of gravity on the variation of capillary rise height is remarkable when crack width is beyond a critical range. Finally, the comparison between experimental and theoretical results indicates that the L-W capillary imbibition model of porous capillary tube after taking into account the gravity effect can be applied to the case of discrete crack with a certain range of width. [ABSTRACT FROM AUTHOR]

A comparative study on the dynamic properties of dam and sieved concrete was conducted based on dynamic biaxial tensile-compressive tests. Specimens were designed to be prismatic shape with a size of 250 × 250 × 400 mm for dam concrete and 150 × 150 × 300 mm for sieved concrete, respectively. Each specimen was biaxially loaded by constant tensile-compressive (T-C) stress ratios (0:−1, 0.05:−1, 0.1:−1, 0.25:−1, 0.5:−1, and ∞) at the strain rate of 10 −5 s −1 , 10 −4 s −1 , 10 −3 s −1 , and 10 −2 s −1 by a servo-hydraulic multi-axial testing machine. The failure modes of specimens have been compared under various biaxial T-C loading conditions at each strain rate, thus the fracture surface of sieved concrete specimen is smoother than dam concrete. The dynamic strength of dam concrete is lower than sieved concrete under the same loading condition, and the ratio of them is insensitive to strain rate. The growth rates of dynamic strength for dam concrete is consistent with sieved concrete, whereas much larger than ordinary concrete. By regression of the test results, the dynamic T-C failure criterions for dam and sieved concrete were proposed after considering the combined effects of strain rate and stress ratio. [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]

Dynamic multiaxial strength experiments on both dam and wet-screened concretes were carried out by using a large static and dynamic triaxial electro-hydraulic servo testing machine. The dynamic strength under uniaxial compression (C), uniaxial tension (T), biaxial compression-compression (C-C), biaxial compression-tension (C-T), triaxial compression-compression-tension (C-C-T) and triaxial compression-compression-compression (C-C-C) are presented in the paper. The influence of strain rate (ranging from 10−5/s to 10−2/s) and stress ratio on the failure modes and dynamic strength of dam and wet-screened concretes is analyzed. It is found that the failure modes of both dam and wet-screened concretes were mainly dependent on the stress ratio, but independent of the strain rate. The ultimate strengths of dam and wet-screened concretes are found to increase with the increase of strain rate. Particularly, the dynamic increasing factor (DIF) increases obviously when the tensile stress is included. It is noted that the maximum DIF existed in the triaxial C-C-T stress state and the minimum DIF corresponded to the triaxial C-C-C state. The octahedral normal and shear stresses are increased but the similar angle is kept constant with the increase of strain rate in the octahedral stress space. The dynamic failure criterion is developed after analyzing the effect of octahedral normal stress and similar angle on the dynamic strength. [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]

• 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]

• A parameter of effective area ratio is proposed to describe the fatigue damage. • The single-cycle dissipated energy per unit volume is closely linked to stress level. • A fatigue life prediction model was developed by means of energy dissipation method. Fatigue failure is one of the most common failure modes for concrete structures. When subjected to fatigue loading, which may be induced either by vehicle load or by wind load, the damage of concrete would develop gradually due to the evolution of internal micro-cracks. Once the damage value exceeds a critical threshold, the concrete will lose its loading capacity. Therefore, knowledge about the fatigue damage evolution of concrete is crucial for evaluating its time-dependent deterioration as well as establishing the fatigue life prediction model of reinforced concrete structures. This paper dealt with an experimental research of stress level on fatigue performance of plain concrete based on the energy dissipation method. The developments of strain, stiffness degradation and energy dissipation were discussed in accordance with the obtained experimental data. Moreover, the parameter of effective area ratio, e.g. the ratio between the area of matrix and the area of concrete at the same cross section, was proposed to describe the fatigue damage as well as its evolution during the cyclic loading process. The results show that the developments of strain, stiffness degradation and fatigue damage can all be divided into three stages: (i) an instable and relatively brief stage due to the initiation of material original micro-cracks; (ii) a linear developing stage because of the propagation of internal micro-cracks; (iii) a rapid and unstable stage as a result of the growth and coalescence of cracks. In the linear developing stage, the value of strain rate remains constant approximately, irrespective of the stress level. In addition, the fatigue damage and stiffness degradation of concrete are closely linked to the stress level. With the increase of stress level, the fatigue damage enhances. But on the contrary, the stiffness degradation reduces when the stress level rises. The average value of single-cycle energy dissipation per unit volume in the second stage E d,V increases exponentially due to the increasing of the stress level, and a fatigue life prediction model was developed by means of energy dissipation method. It is demonstrated that the fatigue life decreases with the absorbed energy of concrete and there is a linear relationship between the logarithmic fatigue life and E d,V. [ABSTRACT FROM AUTHOR]

Summary: The results of a research program aimed at investigating the effectiveness of simultaneous application of carbon fiber-reinforced polymer (CFRP) sheets and steel jacket to upgrade corrosion-damaged reinforced concrete (RC) columns was presented. A total of 14 RC columns were tested under combined lateral cyclic displacement excursions and constant axial load. The variables studied in this program included effectiveness of different strengthening techniques, as well as effects of degree of rebar corrosion, axial load, CFRP sheets and steel jacket. It was showed that strengthening corroded RC columns with combined CFRP sheets and steel jacket was effective in enhancing the seismic performance of the columns and resulted in more stable hysteresis curves with lower strength degradations as compared with the un-strengthened ones. Additionally, it was also found that the corroded RC columns strengthened with combined CFRP sheets and steel jacket behaved better than those strengthened only with the single material. Analytical study was conducted and compared with the experimental results. A good agreement was observed. [Copyright &y& Elsevier]

To alleviate the consumption of resources and promote the further application of geopolymer concrete in practical engineering, the mechanical performance of steel tubed geopolymer concrete (STGC) columns under axial compression was first explored in this study. The impact of experimental parameters including concrete types, configurations of steel tubes, diameter-to-thickness ratios (D / t), and slag contents were analyzed in detail. The test results illustrate that after grooving at the two ends of columns, the axial deformation of the external steel tube becomes smaller compared with the core concrete and the confining pressure of the external steel tube is improved significantly, resulting in a remarkable improvement of the compressive strength and corresponding strain at both peak and residual state of the specimens. Although little influence of concrete types on the steel stress evaluation is detected, the ductility of the STGC columns is inferior to that of the specimens with ordinary concrete due to the obvious brittle property of geopolymer concrete. Moreover, the ratio of axial stress to yield strength of the external steel tube at the peak load of STGC columns is increased with the decrease of diameter-to-thickness ratio and concrete strength. Finally, a new model for predicting the steel axial stress in steel tubed concrete (STC) columns at the peak state is established and a load capacity model incorporating the steel axial contribution is also developed for STC columns. Compared with extensive existing test results, the developed model can provide satisfactory accuracy. • Axial compressive behavior of steel tubed geopolymer concrete columns was investigated. • Effects of key test parameters on the interaction mechanism of the composite columns were analyzed. • The axial stress model of steel tube at the peak state of STC columns is proposed. • The load capacity model incorporating the axial contribution of the steel tube is developed for STC columns. [ABSTRACT FROM AUTHOR]

• A valid FE model was established to simulate the axial compressive behavior of preloaded FRP-strengthened concrete columns. • The specific influence of preload was explored in detail and an extensive parametric analysis was carried out. • Two new models were proposed to estimate the dilation behavior and axial stress–strain response of the preloaded FCC respectively. A finite element (FE) analysis is performed in this paper to investigate the compressive behavior of FRP-strengthened concrete columns under preload by using ABAQUS software. First, the hardening/softening rule in Concrete Damaged Plasticity Model (CDPM) is modified based on the existing experimental data to introduce the preload effect on FRP confinement, and the FE model for FRP confined concrete under preload is established by adopting the operation of element deactivation and reactivation in ABAQUS. Then, the specific influence of preload on the mechanical performance of FRP confined concrete (FCC) during the entire loading process is explored in detail. The numerical results show that preload significantly reduces the increasing rate and amplitude of confinement stiffness for confined concrete in the plastic stage, leading to a relatively low improvement of axial stress of concrete. An extensive parametric analysis is also conducted based on the validated FE model to explore the effect of concrete compressive strength, confinement stiffness of FRP, and preloading rate on the behavior of preloaded FCC. Finally, two new models are proposed to evaluate the dilation behavior and axial stress–strain response of FCC under preload respectively, and the comparisons between theoretical and test results reveal that satisfactory accuracy can be achieved for the two developed models. [ABSTRACT FROM AUTHOR]