Influence of steam curing on the pore structures and mechanical properties of fly-ash high performance concrete prepared with recycled aggregates

In this research work, High Performance Concrete (HPC) was produced employing 30% of fly ash and 70% of Portland cement as binder materials. Three types of coarse recycled concrete aggregates (RCA) sourced from medium to high strength concretes were employed as 100% replacement of natural aggregates for recycled aggregate concrete (RAC) production. The specimens of four types of concretes (natural aggregate concrete (NAC) and three RACs) were subjected to initial steam curing besides the conventional curing process. The use of high quality RCA (>100 MPa) in HPC produced RAC with similar or improved pore structures, compressive and splitting tensile strengths, and modulus of elasticity to those of NAC. It was determined that the mechanical and physical behaviour of HPC decreased with the reduction of RCA quality. Nonetheless steam-cured RACs had greater reductions of porosity up to 90 days than NAC, which led to lower capillary pore volume.     

Fracture process and reliability of concrete made from high grade recycled aggregate using acoustic emission technique under compression

The increasing amount of waste concrete makes desirable collection of high quality of recycled aggregate from waste concrete to be reused for construction. This research used high grade recycled coarse aggregate (RCA) created using pulsed power technology to make concrete specimens. Concrete created from natural aggregate was also prepared to compare the properties of concrete made using pulsed power recycled aggregate. Established acoustic emission (AE) parameter analyses which are AE hit, relationship between RA value and average frequency, and b-value of AE amplitude distribution were applied to analyze the concrete fracture behavior. In addition, AE Weibull analysis was also proposed to evaluate the reliability of the concrete. A set of AE measurement testing was applied to the concrete specimens during compression loading. At the age of 28 days, compressive strength reaches 35.4 MPa and Young’s modulus is 23.6 GPa. The results indicate that the fracture process and reliability of concrete made using pulsed power RCA is similar to that of natural coarse aggregate concrete suggesting that both concrete have equivalent characteristic under compression. Furthermore, the good agreement results shared by AE Weibull analysis with those of other analyses suggesting this method can also be employed as one parameter to determine the condition of concrete.     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

Bond behaviour between recycled aggregate concrete and deformed steel bars

The results of thirty pullout tests carried out on 8 and 10 mm diameter deformed steel bars concentrically embedded in recycled aggregate concrete designed using equivalent mix proportions with coarse recycled concrete aggregate (RCA) replacement percentages of 0, 25, 50, 75 and 100 % are reported towards investigation of bond behaviour of RCA concrete. Bond strengths of the natural aggregate concrete and the RCA concrete was found to be comparable, particularly for the 10 mm rebars, and the RCA replacement percentage had an insignificant effect on peak bond stress values. However, for both the bar sizes, when the measured bond strengths were normalized with the respective compressive strengths, then the normalized bond strengths so obtained across all the RCA replacement percentages were higher for the RCA concrete compared to the natural coarse aggregate concrete. Further, higher normalized bond strength values were obtained for the 8 mm rebars compared to the 10 mm bars. An empirical bond stress versus slip relationship between RCA concrete and deformed steel bars has been proposed on the basis of regression analysis of the experimental data and it is conservatively suggested that anchorage lengths of 8 and 10 mm diameter deformed bars in RCA concrete may be taken the same as in natural aggregate concrete.     

Bond strength prediction for deformed steel rebar embedded in recycled coarse aggregate concrete

This paper summarizes the results of an experimental investigation into the bond behavior between recycled aggregate concrete (RAC) and deformed steel rebars, with the main variables being the recycled coarse aggregate replacement ratio (RCAr) and water-to-cement ratio of the concrete mixture. The investigation into splitting cracking strength indicates that the degradation of the bond splitting tensile stress of the cover concrete was affected by not only the roundness of the coarse aggregate particles but also the weak interfacial transition zone (ITZ) between the cement paste and the RCA that has a more porous structure in the ITZ than normal concrete. In this study, a linear relationship between the bond strength and the density of the RCA was found, but the high compressive strength reduced the effects of the parameters. To predict the bond strength of RAC using the main parameters, a multivariable model was developed using nonlinear regression analysis. It can be inferred from this study that the degradation characteristic of the bond strength of RAC can be predicted well, whereas other empirical equations and code provisions are very conservative.     

Shrinkage of recycled aggregate concrete: experimental database and application of <Emphasis Type="Italic">fib</Emphasis> Model Code 2010

This paper describes a meta-analysis of previously published studies on the shrinkage strain of recycled aggregate concrete (RAC). The study aims at providing an analytic expression for the shrinkage strain of RAC to be used in conjunction with the existing fib Model Code 2010 shrinkage prediction model. For this purpose, a database of experimental results on the shrinkage of RAC and companion natural aggregate concrete (NAC), produced with the same water-cement ratio, was compiled using strict selection criteria. Results from 19 studies entered into the database, consisting of 125 shrinkage curves (39 NAC and 86 RAC) with a total of 424 data points. A comparison of RAC and companion NAC revealed that, on average, RAC displays a larger shrinkage strain. This difference increases with increasing recycled concrete aggregate (RCA) content and with decreasing compressive strength. Applying the fib Model Code 2010 shrinkage prediction model revealed that, relative to its performance on NAC, the shrinkage strain of RAC is underestimated. Finally, a correction coefficient for the shrinkage strain of RAC, \(\xi _{{\mathrm{cs}},{\mathrm{RAC}}}\), to be used in conjunction with the fib Model Code 2010 model, was proposed in the form of a bivariate power function with RAC compressive strength and RCA replacement ratio as variables.     

再生粗骨料含量对钢管再生混凝土粘结强度及失效性态的影响研究

为了探究再生粗骨料取代率对钢管与再生混凝土界面粘结强度及破坏机理的影响, 设计15个圆钢管再生混凝土和9个方钢管再生混凝土短柱试件, 以混凝土强度等级和长径比为变化参数分组进行取代率的影响分析. 通过推出试验, 获取荷载-滑移曲线的特征点参数, 回归得到极限粘结强度的计算公式. 从界面耗能、粘结抗剪刚度、损伤等角度分析了取代率对其内在失效机理的影响. 研究结果表明：极限粘结强度拟合公式计算值与试验实测值吻合较好;再生粗骨料取代率变化对钢管再生混凝土接触界面粘结失效过程的耗能能力影响不大;而界面弹性粘结剪切刚度却随着取代率的增加而降低;剪切刚度退化速度则相反, 随着取代率的增加而加快;再生粗骨料粘附的水泥基和内部裂纹会加快钢管再生混凝土界面的粘结损伤过程.     

基于可靠度分析的再生混凝土材料分项系数

为研究再生混凝土强度变异性对再生混凝土材料分项系数的定量影响,以普通混凝土受弯梁为参照,保持目标可靠指标不变,分别设定再生混凝土的强度均值,标准值和设计值与普通混凝土相同,分析了三种情况下的再生混凝土的材料分项系数和强度设计值的取值。分析结果表明,上述三种情况下,当再生混凝土的强度变异系数大于普通混凝土强度变异系数时,为保证再生混凝土梁受弯时的可靠指标达到现行规范目标可靠指标,其所需配筋率依次降低,所需再生混凝土配合比设计强度依次增加。当再生混凝土的强度变异系数小于等于0.15时,取均值相同时的C30、C40再生混凝土的材料分项系数为1.41和1.35,标准值相同时为1.43和1.41,设计值相同时为1.45和1.41,都可以保证再生混凝土梁的受弯可靠指标达到现行规范的要求。     

再生混凝土梁时变挠度分析与预测

重点考虑收缩和徐变等因素,对再生混凝土梁的时变挠度进行了理论分析。假定收缩和徐变不会相互影响,通过截面时随分析,得到再生混凝土梁时变相对受压区高度表达式。依据现有文献的试验数据,对分析结果准确性进行了验证。结果表明：对于不同的再生粗骨料取代率,时变挠度的计算结果与试验结果相比,大多数误差可控制在19%以内。该文的工作较好揭示了再生混凝土梁时变挠度的演化特点,可以为今后再生混凝土梁的长期挠度预测提供理论基础。     

纳米改性再生骨料混凝土单轴受压疲劳性能

采用纳米二氧化硅对再生骨料混凝土(RAC)进行改性,开展了不同再生骨料取代率及纳米改性后的再生骨料混凝土单轴受压疲劳试验,研究其疲劳寿命、疲劳方程、疲劳变形规律及疲劳剩余强度,并利用纳米压痕试验从细观层面上分析探讨了纳米二氧化硅对再生骨料混凝土多重界面过渡区的影响。结果表明:再生骨料混凝土疲劳寿命均较好地服从两参数威布尔概率分布。50%保证率、0.75应力水平下,再生骨料混凝土的疲劳寿命比普通混凝土降低了25.8%,掺入纳米二氧化硅可显著提高再生骨料混凝土的疲劳寿命。对比应变演化曲线和剩余强度模型,纳米二氧化硅改性后的再生骨料混凝土,剩余强度衰减非线性规律明显。纳米二氧化硅的掺入提高了再生骨料混凝土多重界面过渡区的压痕模量,改善了再生骨料混凝土的疲劳性能。     

Influence of field recycled coarse aggregate on properties of concrete

This paper investigates the influence of different amounts of recycled coarse aggregates obtained from a demolished RCC culvert 15 years old on the properties of recycled aggregate concrete (RAC). A new term called “coarse aggregate replacement ratio (CRR)” is introduced and is defined as the ratio of weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix. To analyze the behaviour of concrete in both the fresh and hardened state, a coarse aggregate replacement ratio of 0, 0.25, 0.50 and 1.0 are adopted in the concrete mixes. The properties namely compressive and indirect tensile strengths, modulus of elasticity, water absorption, volume of voids, density of hardened concrete and depth of chloride penetration are studied. From the experimental results it is observed that the concrete cured in air after 7 days of wet curing shows better strength than concrete cured completely under water for 28 days for all coarse aggregate replacement ratios. The volume of voids and water absorption of recycled aggregate concrete are 2.61 and 1.82% higher than those of normal concrete due to the high absorption capacity of old mortar adhered to recycled aggregates. The relationships among compressive strength, tensile strengths and modulus of elasticity are developed and verified with the models reported in the literature for both normal and recycled aggregate concrete. In addition, the non-destructive testing parameters such as rebound number and UPV (Ultrasonic pulse velocity) are reported. The study demonstrates the potential use of field recycled coarse aggregates (RCA) in concrete.     

Impact of particle packing mix design method on fracture properties of natural and recycled aggregate concrete

The fracture properties of four types of concrete prepared using natural coarse aggregate and recycled coarse aggregate and conventional and particle packing method (PPM) of mix design approaches are studied. The three‐point bending (TPB) test is performed using three different sizes of single edge notched beam. The fracture energy is calculated from the load‐CMOD curve obtained in the TPB test, and in this process the load‐CMOD curve is curtailed at 2% of the depth of the beam. Based on CTOD_c and w₁ relationship, appropriate softening function is used to estimate the double‐K fracture parameters. The fracture energy and fracture toughness parameters of recycled aggregate concrete (RAC) is inferior to the natural aggregate concrete (NAC). The PPM mix design improves the fracture properties of concrete in comparison to the conventional mix design approach. The fracture properties of PPM mix designed RAC are comparable to that of NAC prepared using conventional method.     

型钢再生混凝土组合柱轴压性能试验研究

为了研究型钢再生混凝土(SRRAC)组合柱的轴压性能,设计了23个试件进行轴压试验,考虑了再生粗骨料取代率、箍筋体积配箍率和混凝土强度等级3个变化参数。通过试验观察了试件的破坏形态,获取了试件受力全过程曲线、极限承载力等重要数据,并分析各变化参数对SRRAC柱轴心抗压承载性能的影响,基于试验提出其强度计算公式。研究结果表明：SRRAC柱破坏时型钢受压屈服、再生混凝土压碎,具有良好的承载性能,各变化参数均对其承载性能有显著影响,建议再生粗骨料最优取代率为40%,该文建议强度计算公式计算值与试验结果吻合较好。研究结果可供再生混凝土组合结构的进一步科学研究和工程应用参考。     

持压荷载与干湿循环作用下再生混凝土氯盐侵蚀行为

采用干湿比为3∶1和质量分数为5wt%的NaCl溶液,开展了持压荷载与干湿循环共同作用下不同再生粗骨料取代率(r=0%、30%、50%、100%)混凝土的氯离子传输试验,分析了持压应力水平(λ_c=0.1、0.3、0.5)对氯盐侵蚀性能的影响。基于非饱和混凝土的氯离子对流-扩散模型,提出了考虑应力水平和再生骨料取代率影响的水分和氯离子扩散系数模型,并验证了该模型的有效性。结果表明：相同再生粗骨料取代率的混凝土内自由氯离子含量、氯离子扩散系数和表面氯离子浓度均随应力水平的增加呈先减小后增大的趋势,同一应力水平下与再生粗骨料取代率呈正相关,再生粗骨料取代率为100%的试件承受0.1f_c、0.3f_c、0.5f_c(f_c为再生混凝土(RAC)立方体抗压强度值)应力作用的氯离子扩散系数分别是无应力状态的0.97、0.88和1.48倍;所建立的持压荷载与干湿循环作用下RAC氯离子传输模型,为再生混凝土耐久性分析提供理论依据。     

Physio-chemical reactions in recycle aggregate concrete

Concrete waste constitutes the major proportion of construction waste at about 50% of the total waste generated. An effective way to reduce concrete waste is to reuse it as recycled aggregate (RA) for the production of recycled aggregate concrete (RAC). This paper studies the physio-chemical reactions of cement paste around aggregate for normal aggregate concrete (NAC) and RAC mixed with normal mixing approach (NMA) and two-stage mixing approach (TSMA) by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Four kinds of physio-chemical reactions have been recorded from the concrete samples, including the dehydration of C(3)S(2)H(3), iron-substituted ettringite, dehydroxylation of CH and development of C(6)S(3)H at about 90 degrees C, 135 degrees C, 441 degrees C and 570 degrees C, respectively. From the DSC results, it is confirmed that the concrete samples with RA substitution have generated less amount of strength enhancement chemical products when compared to those without RA substitution. However, the results from the TSMA are found improving the RAC quality. The pre-mix procedure of the TSMA can effectively develop some strength enhancing chemical products including, C(3)S(2)H(3), ettringite, CH and C(6)S(3)H, which shows that RAC made from the TSMA can improve the hydration processes.     

基于CT图像的再生混凝土细观破坏裂纹分形特征

为深入研究再生混凝土的破坏形态和内部裂纹扩展情况与普通混凝土之间的差异,以不同再生粗骨料(RCA)取代率的再生混凝土为研究对象,利用Phoenix v | tome | x s240微焦点工业CT获取再生混凝土加载到90%预估破坏荷载的二维扫描图像,借助Photoshop CS6图像处理软件,对材料内部破坏裂纹进行提取,进而基于分形几何理论,以分形维数及多重分形谱表征裂纹的分形扩展规律,建立分形维数和多重分形谱特征参数与RCA取代率和再生混凝土抗压强度的关系。结果表明:再生混凝土的细观受力破坏模式与普通混凝土不同,其受力破坏形态不仅取决于粗骨料与水泥浆体的界面黏结强度,还取决于RCA自身性能,当裂纹发展至天然粗骨料或强度较高的RCA时会绕过骨料表面继续发展,发展至强度较低的RCA时会贯穿骨料;分形维数可定量描述混凝土材料内部细观裂纹的整体扩展情况,即裂纹越丰富,分形维数越大;多重分形谱可反映从局部到整体不同层次的细观裂纹特征,裂纹分形维数和多重分形谱特征参数均与RCA取代率呈线性下降关系,与抗压强度呈线性增长关系;本研究可为再生混凝土在大型结构工程中的广泛应用奠定理论和实验基础。      

Micromechanical investigation on size effect of tensile strength for recycled aggregate concrete using BFEM

In this paper, the validity and performance of base force element method (BFEM) based on potential energy principle was studied by some numerical examples. And the BFEM on damage mechanics is used to analyze the size effect on tensile strength for recycled aggregate concrete (RAC) at meso-level. The recycled aggregate concrete is taken as five-phase composites consisting of natural coarse aggregate, new mortar, new interfacial transition zone (ITZ), old mortar and old ITZ on meso-level. The random aggregate model is used to simulate the meso-structure of recycled aggregate concrete. The size effects of mechanical properties of RAC under uniaxial tensile loading are simulated using the BFEM on damage mechanics. The simulation results agree with the test results. This analysis method is the new way for investigating fracture mechanism and numerical simulation of mechanical properties for RAC.     

钢管再生混凝土长柱偏压性能研究

对10个圆钢管再生混凝土和10个方钢管再生混凝土长柱进行偏压静力单调加载试验,考虑了再生粗骨料取代率、长细比、偏心距3个变化参数,观察了试件受力的全过程和试件破坏形态,绘制出荷载-变形、荷载-应变等一系列重要关系曲线,给出了截面应变沿高度分布情况,并分析了变化参数对试件极限承载力的影响规律,采用国内外常用的8部相关规程计算两种截面形式试件的极限承载力。研究结果表明：钢管再生混凝土偏压长柱受力过程均经历了弹性阶段、屈服阶段和破坏阶段,破坏形态主要为弹塑性失稳破坏;建议采用规程DL/T5085-1999和DBJ13-51-2003设计圆钢管再生混凝土偏压长柱试件的极限承载力,采用规程CECS159:2004、DBJ13-51-2003设计方钢管再生混凝土偏压长柱试件的极限承载力。     

Impact performances of steel tube-confined recycled aggregate concrete (STCRAC) after exposure to elevated temperatures

The impact behaviours of steel tube-confined recycled aggregate concrete (STCRAC) following exposure to elevated temperatures of 20 °C, 200 °C, 500 °C and 700 °C were experimentally investigated using a 100 mm-diameter split Hopkinson pressure bar (SHPB). The recycled coarse aggregate (RCA) replacement ratios were set as 0, 50% and 100%. The effect of RCA replacement ratio and exposure temperature on the impact properties of STCRAC were analysed in terms of failure modes, stress-strain time history curve and dynamic increase factor (DIF). The results show that the fire-damaged STCRAC can maintain its integrity during impact load. However, there were evident degradations in the dynamic behaviour of STCRAC after exposure to high temperatures of 500 °C and 700 °C. The ultimate impact strength, impact secant modulus and residual impact strength of STCRAC obviously decreased because of the damage due to high temperature exposure. But the degradations of both the ultimate impact strength and impact secant modulus of STCRAC under impact loading were less severe than those under quasi-static loading. The remaining strength factor and the DIF tended to increase with the raise of the elevated temperatures. Overall, during the impact loading, the fire-deteriorated STCRAC exhibited excellent impact behaviour.     

直剪作用下再生混凝土力学性能及强度指标换算

以再生粗骨料取代率为变化参数,通过75个再生混凝土(RAC)试件的直剪、抗压与劈裂抗拉试验,揭示了RAC的直剪破坏机制及不同强度指标之间的换算规律。结果表明:RAC在直剪作用下为明显的脆性破坏,粗骨料和水泥基体均被剪断;随着取代率的增加,RAC直剪强度较普通混凝土变化不大,总体上呈降低趋势,但50%取代率(按质量)时直剪强度有所增大;峰值剪切变形随取代率的增大,总体呈增大趋势,平均提高了18.85%;初始剪切变形模量随取代率的增大,总体呈降低的趋势,平均降低了8.97%;最后,基于试验数据提出了RAC剪切强度与抗压、劈裂抗拉强度的换算关系式,计算结果与试验值吻合较好。