纳米SiO2浸泡改性再生橡胶混凝土力学性能研究

本实验主要目的是研究经过纳米SiO2溶液浸泡的再生骨料橡胶混凝土坍落度、抗压强度和劈裂强度的变化规律、确定此操作对再生骨料混凝土的影响。实验以C30素混凝土为基准,通过掺入5%(等质量取代砂)的橡胶颗粒、纳米SiO2溶液（0%、1%、3%,相对水的质量）和再生骨料（Recycled concrete aggregate, RCA）（30%、50%、100%,等质量取代天然骨料）,制备纳米SiO2改性再生橡胶混凝土试件并进行坍落度、抗压和抗劈裂试验,分析不同浓度的纳米SiO2溶液和不同RCA取代率对再生橡胶混凝土力学性能的影响。结果显示：控制其他变量相同的RCA混凝土试块中,纳米SiO2溶液浓度的增加会降低拌合物的坍落度,而RCA取代率的增加会提高拌合物的坍落度;在30%RCA取代率和1%浓度的纳米SiO2溶液时再生橡胶混凝土的抗压强度和劈裂抗拉强度比基准组分别高出2.5%和6.7%;总体来看,经过纳米SiO2溶液浸泡后的再生橡胶混凝土呈现出早期强度和后期强度提升的特点。     

Behavior of structural polymer-modified concrete containing recycled aggregates

The use of recycled concrete aggregate (RCA) in structural applications is hindered by the inferior concrete mechanical performance and unpredictable behavior with embedded steel reinforcement. The main objective of this paper is to evaluate whether the addition of small amounts of styrene-butadiene rubber (SBR) polymeric latexes could mitigate the drop in RCA concrete properties. Two series of mixtures prepared with 320 or 440 kg/m³ cement were tested by direct bond and beam-end methods; the SBR addition rates varied from 1 to 3% of cement mass. Test results showed that SBR could remarkably improve RCA concrete workability as well as compressive and splitting tensile strengths. The initial stiffness of load vs. slip curves was considerably accentuated, reflecting increased interfacial shear stresses between the reinforcing bar and surrounding concrete. Also, the responses of ascending curves showed extended nonlinear regions together with higher ultimate bond strengths at failure. The experimental direct bond and beam-end test data are compared with the design bond strengths determined by ACI 318-14, European Code EC-2, and fib Model Code MC2010.     

Residual cube strength of coarse RCA concrete after exposure to elevated temperatures

An experimental investigation was carried on the residual cube strength of concrete made with coarse recycled concrete aggregate (RCA) after exposure to temperatures of 20°C to 800°C. A total of 360 cube specimens were made with 2 water/cement ratios (w/c = 0.31 and 0.45) and 5 replacement percentages (r = 0%, 25%, 50%, 75%, and 100%) of coarse RCA. Effects of different cooling regimes (natural cooling, water cooling) on the residual compressive strength of coarse RCA concrete after exposure were also investigated. Experimental results show that the cube compressive strength and splitting tensile strength of coarse RCA concrete diminish with increasing temperature, of which the splitting tensile strength declines quicker than the compressive strength. The effects of coarse RCA replacement percentage and w/c ratio on losses in relative strength after being exposed to high temperatures are found to be insignificant. The results also reveal that the relative compressive strength of coarse RCA concrete cooled in water after heating process is lower than that of specimens cooled naturally.     

Reactivity of reclaimed concrete aggregate produced from concrete affected by alkali-silica reaction

This paper presents results from a research program that focused on studying the reactivity of reclaimed concrete aggregate (RCA) produced from concrete affected by alkali-silica reaction (ASR). The results showed that RCA produced from ASR-affected concrete causes significant expansion when used in new concrete. The expansion was similar to that produced in concrete containing the reactive aggregate used originally in the old concrete. It is believed that crushing the old concrete exposed fresh faces of the reactive aggregate which causes renewed reaction and expansion in the new concrete. The alkalis contributed from the RCA are also believed to contribute to the expansion. The amount of supplementary cementing materials required to mitigate the expansion in new concrete containing ASR-affected RCA was higher than those normally needed in concrete containing the virgin reactive aggregate. The results showed a good agreement between the 14-day expansion of accelerated mortar bars and the expansion of concrete prisms.     

High-strength concrete with different fine aggregate

High-strength concrete (HSC) has undergone many developments based on the studies of influence of cement type, type and proportions of mineral admixtures, type of superplasticizer and the mineralogical composition of coarse aggregates. Most studies were carried out using natural sand with rounded and smooth grains. In practice, crushed sands from various sources are frequently used in concrete. In this paper, two aspects of the effect of crushed sands on HSCs are presented. First, the performance of crushed sands in relation to natural sand using a low water/cement (w/c) ratio and fixed coarse aggregate and cement content is analyzed. Results show that concrete with crushed sand requires an increase of superplasticizer to obtain the same slump. It also presents a higher strength than the corresponding natural sand concrete at all test ages, while its elastic modulus is lower at 28 days and is the same after that. Studies on the development of hydration and mortar phase of concrete show that the increase of strength can be attributed to the improvement of paste-fine aggregate transition zone. Second, the influence of the mineralogical source of the crushed sands was studied using three different types of crushed sands (granite, limestone and dolomite) with similar grading. Two mixtures containing 450 and 485 kg/m³ cement and low w/c ratio are analyzed. Results show the adverse effects of shape and texture on workability of concrete, but the compressive strength of concrete is improved. Granite crushed sand appears as the most advantageous sand for this purpose.     

Experimental study of the material and bond properties of frost-damaged concrete

In an extensive experimental investigation, several types of tests were conducted on a reference specimen and frost-damaged concrete. Two levels of internal frost damage were quantified by the relative dynamic modulus of elasticity and compressive strength. Test results showed a significant influence of freeze–thaw cycles on the compressive strength and even more influence on the modulus of elasticity and the compressive strain at peak stress. Reduced tensile strength and increased fracture energy were measured. From inverse analysis of wedge splitting test results, a significant effect of frost on the shape of the tensile stress–crack opening relationship was observed: tensile strength was reduced, while the post-peak behaviour was more ductile for the frost-damaged concrete. Pull-out tests showed the influence of freeze–thaw cycles on bond strength and slip. The pull-out test results are compared with similar tests available in the literature and the effect of frost on bond behaviour is discussed.     

Enhancement of properties of recycled coarse aggregate concrete using bacteria

Due to rapid construction, necessity for raw materials of concrete, especially coarse aggregate, tends to increase the danger of early exhaustion of the natural resources. An alternative source of raw materials would perhaps delay the advent of this early exhaustion. Recycled coarse aggregate (RCA) plays a great role as an alternative raw material that can replace the natural coarse aggregate (NCA) for concrete. Previous studies show that the properties of RCA concrete are inferior in quality compared to NCA concrete. This article attempts to study the improvement of properties of RCA concrete with the addition of bacteria named as Bacillus subtilis. The experimental investigation was carried out to evaluate the improvement of the compressive strength, capillary water absorption, and drying shrinkage of RCA concrete incorporating bacteria. The compressive strength of RCA concrete is found to be increased by about 20% when the cell concentration of B. subtilis is 10⁶ cells/ml. The capillary water absorption as well as drying shrinkage of RCA are reduced when bacteria is incorporated. The improvement of RCA concrete is confirmed to be due to the calcium carbonate precipitation as observed from the microstructure studies carried out on it such as EDX, SEM, and XRD.     

Shrinkage cracking of lightweight concrete made with cold-bonded fly ash aggregates

Shrinkage cracking performance of lightweight concrete (LWC) has been investigated experimentally on ring-type specimens. LWCs with and without silica fume were produced at water-cementitious material ratios (w/cm) of 0.32 to 0.55 with cold-bonded fly ash coarse aggregates and natural sand. Coarse aggregate volume ratios were 30%, 45%, and 60% of the total aggregate volume in the mixtures. A total of 12 lightweight aggregate concrete mixtures was cast and tested for compressive strength, static elastic modulus, split-tensile strength, free shrinkage, weight loss, creep, and restrained shrinkage. It was found that the crack opening on ring specimens was wider than 2 mm for all concretes. Free shrinkage, weight loss, and maximum crack width increased, while compressive and split-tensile strengths, static elastic modulus, and specific creep decreased with increasing coarse aggregate content. The use of silica fume improved the mechanical properties but negatively affected the shrinkage performance of LWCs. Shrinkage cracking performance of LWCs was significantly poorer than normal weight concrete (NWC).     

Bond strength of steel deformed rebars embedded in artificial lightweight aggregate concrete

The recycling of industrial waste such as bottom ash from furnaces is an important issue in construction industry, since it enables reduction in construction cost and has beneficial effect on the environment. In this study, we have investigated the bond characteristics of steel deformed bars embedded in artificial lightweight aggregate concrete which is manufactured from bottom ash. A pullout test was performed on 144 lightweight aggregate concrete specimens to measure the bond strengths. In this test, the parameters included the compressive strength of the concrete and embedment length of rebar. The pullout load vs. slip responses and modes of failure of the specimens were identified during the test. A bond strength equation for lightweight concrete is formulated by performing a regression analysis on the test results and compared with the predictions by the existing equations such as ACI 408, Orangun’s, and Darwin’s. The comparison shows that the existing bond strength equations cannot be directly applied to the design of lightweight concrete structures and the proposed equation is able to provide a more accurate estimation of the bond strength of lightweight concrete than the existing equations.     

An experimental study on the bond strength between reinforcement bars and concrete as a function of concrete cover,strength and corrosion level

The effect of corrosion on the bond strength between reinforcement bars and concrete was studied in a series of experiments. An accelerated corrosion method was used to corrode the reinforcement bars embedded in concrete specimens. Pullout tests were performed to develop an empirical model for the ultimate bond strength by evaluating bond strengths in two different concrete mixes, three concrete cover depths and different mass losses of reinforcement bars after corrosion. Bond-slip relationships for the different corrosion levels were compared. It was found that the relationship between bond strength and concrete strength in uncorroded specimens differed from that of corroded specimens set in high-strength concrete because of brittleness in the corroded specimens, which caused a sudden loss of bond strength. The results revealed that specimens with higher concrete strength levels and corroded reinforcements showed a higher percentage of bond strength degradation due to concrete cracking during the pullout tests.     

Triaxial strength and failure criterion of plain high-strength and high-performance concrete before and after high temperatures

Triaxial tests were performed on 100 mm × 100 mm × 100 mm cubic specimens of plain high-strength and high-performace concrete (HSHPC) at all kinds of stress ratios after exposure to normal and high temperatures of 20, 200, 300, 400, 500, and 600 °C, using a large static-dynamic true triaxial machine. Friction-reducing pads, using three layers of plastic membrane with glycerine were placed between the compressive loading plate and the specimens; the tensile loading planes of concrete samples were processed by an attrition machine, and then the samples were glued-up with the loading plate with structural glue. The failure mode characteristic of the specimens and the direction of the crack were observed and described. The three principally static strengths in the corresponding stress state were measured. The influence of the temperatures and stress ratios on the triaxial strengths of HSHPC after exposure to high temperatures was also analyzed. The experimental results showed that the uniaxial compressive strength of plain HSHPC after exposure to high temperatures does not decrease completely with the increase in temperature, the ratios of the triaxial to its uniaxial compressive strength are dependent on the brittleness-stiffness of HSHPC after different temperatures and the stress ratios. On this basis, a new failure criterion with the temperature parameters is proposed for plain HSHPC under multiaxial stress states. It provides the experimental and theoretical foundations for strength analysis of HSHPC structures subject to complex loads after subjected to a high temperature environment.     

High-strength lightweight concrete made with scoria aggregate containing mineral admixtures

This paper presents a part of the results of an ongoing laboratory work carried out to design a structural lightweight high strength concrete (SLWHSC) made with and without mineral admixtures. In the mixtures, basaltic-pumice (scoria) was used as lightweight aggregate.A control lightweight concrete mixture made with lightweight basaltic-pumice (scoria) containing normal Portland cement as the binder was prepared. The control lightweight concrete mixture was modified by replacing 20% of the cement with fly ash. The control lightweight concrete mixture was also modified by replacing 10% of the cement with silica fume. A ternary lightweight concrete mixture was also prepared modifying the control lightweight concrete by replacing 20% of cement with fly ash and 10% of cement with silica fume. Two normal weight concrete (NWC) were also prepared for comparison purpose.Fly ash and silica fume are used for economical and environmental concerns. Cylinder specimens with 150 mm diameter and 300 mm height and prismatic specimens with dimension 100×100×500 mm were cast from the fresh mixtures to measure compressive and flexural tensile strength. The concrete samples were cured at 65% relative humidity with 20 °C temperature. The density and slump workability of fresh concrete mixtures were also measured.Laboratory test results showed that structural lightweight concrete (SLWC) can be produced by the use of scoria. However, the use of mineral additives seems to be mandatory for production of SLWHSC. The use of ternary mixture was recommended due to its satisfactory strength development and environmental friendliness.     

Microstructural analysis of recycled concrete using X-ray microtomography

This paper reports an investigation of the recycled concrete (RC) microstructure using synchrotron microtomography (μCT) at the Advanced Light Source combined with Scanning Electron Microscopy analysis. The study evaluated the influence of 50% of recycled concrete aggregate (RCA) and its water absorption compensation on the RC microstructure. The following variables were studied: a) the compressive strength of the original concrete used to obtain the RCA (40 and 80 MPa) and b) the initial moisture condition of the RCA (Saturated Surface Dry and Oven Dry). The microtomographic images showed the mixtures cast with RCA in the dry condition developed an evident macropore network surrounding the RCA particle that was not observed in the mixtures using RCA in the SSD condition. SEM images confirmed the initial findings from μCT and showed that the thickness of the interfacial transition zone in RC is in the same order of magnitude as the reference concrete.     

Influence of the nature of aggregates on the behaviour of concrete subjected to elevated temperature

An experimental study is carried out on concretes composed of three different types of aggregates: semi crushed silico-calcareous, crushed calcareous and rolled siliceous. For each aggregate type, two water/cement ratios (W/C), 0.6 and 0.3 are studied. Aggregates and concrete specimens were subjected to 300, 600 and 750 °C heating–cooling cycles. We analyse the evolution of thermal, physical and mechanical properties of concrete in terms of behaviour and physical characteristic evolutions of aggregates with temperature. The study of thermal behaviour of aggregates showed the importance of initial moisture state for the flints. The crystallisation and microstructure of quartz play an important role in the thermal stability of siliceous aggregates. The residual mechanical behaviour of concrete varies depending on the aggregate and the influence of aggregates is also dependent on paste composition. This study allowed to better understand the influence of chemical and mineralogical characteristics of aggregates on the thermomechanical behaviour of concrete.     

Mechanical properties of recycled aggregate concrete under uniaxial loading

In this paper, the compressive strength and the stress-strain curve (SSC) of recycled aggregate concrete (RAC) with different replacement percentages of recycled coarse aggregate (RCA) are investigated experimentally. Concrete specimens were fabricated and tested with different RCA replacement percentages of 0%, 30%, 50%, 70% and 100%, respectively. Uniaxial compression loading is applied in the experiments. Special attention of the analysis is devoted to the failure behaviour and the influences of the RCA contents on the compressive strength, the elastic modulus, the peak and the ultimate strains of RAC. Analytical expressions for the peak strain and the stress-strain relationship of RAC are given, which can be directly used in theoretical and numerical analysis as well as practical engineering design of RAC structures.     

A new way to increase the long-term bond strength of new-to-old concrete by the use of fly ash

The short-term and long-term bond strengths of new-to-old concrete were experimentally investigated with an emphasis on the influence of new concretes and binders. These new concretes included ordinary Portland cement concrete, expansive concrete and high-volume fly ash concrete, while the binders included pure cement paste (C-binder), expansive binder (E-binder) and fly ash mortar (F-binder). The results showed that the short-term bond strength of all specimens with fly ash concrete was lower than that with ordinary Portland cement concrete, which in turn was lower than that with expansive concrete. The bond strength of the specimens with F-binder was the lowest at the age of 7 days. However, the long-term bond strength of all specimens with added fly ash was the highest and strength losses were observed in the specimens repaired with expansive concrete or E-binder at the age of 3 years. The microstructure of the transition zone with F-binder was also studied by using both scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) at the ages of 28 days and 1 year, respectively.     

Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete

The influence of moisture states of natural and recycled aggregates on the properties of fresh and hardened concretes was investigated. Concrete mixes were prepared with natural and recycled aggregates at different proportions. The moisture states of the aggregates were controlled at air-dried (AD), oven-dried (OD) and saturated surface-dried (SSD) states prior to use. The ratio of cement to free water was kept constant for all of the mixes. At the fresh state, the slump loss for various concrete mixtures was determined, while the compressive strength was determined after curing for 3, 7 and 28 days. The test results showed that the initial slump values of the concrete mixtures were dependent on the initial free water contents, and the slump loss values of the mixtures were related to the moisture states of the aggregates. Slump loss was significant when 100% AD or OD recycled aggregate was used. The effect of the moisture states of the aggregates on the strength of the concretes prepared with OD and SSD state aggregates at early age (i.e., 3 and 7 days) was noticeable. The concrete prepared with the AD aggregates achieved the highest average strength values at 3, 7 and 28 days. However, at 28 days, the concrete strengths prepared with different types of aggregates were similar. The results suggested that an AD aggregate that contains not more than 50% recycled aggregate is optimum for producing normal strength recycled aggregate concrete.     

Post-fire residual mechanical properties of concrete made with recycled concrete coarse aggregates

This paper presents results of an experimental study on the residual mechanical performance of concrete produced with recycled coarse aggregates, after being subjected to high temperatures. Four different concrete compositions were prepared: a reference concrete made with natural coarse aggregates and three concrete mixes with replacement rates of 20%, 50% and 100% of natural coarse aggregates by recycled concrete coarse aggregates. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the following basic mechanical properties were then evaluated and compared with reference values obtained prior to thermal exposure: (i) compressive strength; (ii) tensile splitting strength; and (iii) elasticity modulus. Results obtained show that there are no significant differences in the thermal response and post-fire mechanical behaviour of concrete made with recycled coarse aggregates, when compared to conventional concrete.     

Effect of crack width on chloride diffusion coefficients of concrete by steady-state migration tests

The purpose of the present study is to explore the diffusion characteristics of cracked concrete according to the width of cracks. Major test variables include crack width, concrete strength, fly ash addition, and maximum aggregate size. The diffusion characteristics have been measured by steady-state migration test. The present study indicates that the diffusion coefficients do not increase with increasing crack widths up to the so-called “threshold crack width.” The threshold crack width for diffusion is found to be around 55–80 μm. Above this threshold value, the diffusion coefficients start to increase with crack width. A composite model with the introduction of “crack geometry factor” was derived to identify the diffusion coefficient in cracked concrete. It was shown that the crack geometry factor ranges from 0.067 to 0.206. Finally, the effects of concrete strength, fly ash addition and maximum aggregate size on diffusion coefficients are also discussed.     

Influence of aggregate size and volume fraction on shrinkage induced micro-cracking of concrete and mortar

In this paper, the influence of aggregate size and volume fraction on shrinkage induced micro-cracking and permeability of concrete and mortar was investigated. Nonlinear finite element analyses of model concrete and mortar specimens with regular and random aggregate arrangements were performed. The aggregate diameter was varied between 2 and 16 mm. Furthermore, a range of volume fractions between 0.1 and 0.5 was studied. The nonlinear analyses were based on a 2D lattice approach in which aggregates were simplified as monosized cylindrical inclusions. The analysis results were interpreted by means of crack length, crack width and change of permeability. The results show that increasing aggregate diameter (at equal volume fraction) and decreasing volume fraction (at equal aggregate diameter) increase crack width and consequently greatly increases permeability.