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.     

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.     

再生粗骨料硅烷浸渍处理对混凝土介质传输性能的影响

由于残余砂浆的存在,再生粗骨料的物理力学指标远不及天然骨料,致使再生混凝土力学和耐久性能较差;此外,水分及有害离子侵入混凝土内部是引起混凝土材料性能劣化的主要原因。本试验用质量分数为8wt%的硅烷乳液浸渍强化再生粗骨料,通过抗压强度、毛细吸水和抗氯离子侵蚀试验对硅烷浸渍前后不同骨料质量取代率(0%、30%、50%)的再生混凝土介质传输性能进行了研究,最后利用SEM对再生混凝土内部的微观结构进行分析。试验结果表明,硅烷浸渍处理再生粗骨料的吸水率显著降低,由其制备的混凝土强度稍有所下降;再生混凝土毛细累积吸水量明显减少,且抗氯盐侵蚀性能显著提高,其中骨料质量取代率为50%的再生混凝土浸渍处理后氯离子扩散系数降低了37.5%。研究表明,硅烷浸渍处理再生粗骨料是提高再生混凝土耐久性的有效途径。      

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.     

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.     

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。     

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。      

Effect of recycled coarse aggregate on damage of recycled concrete

This study evaluates the possibility of measuring the damage of the recycled concrete. In this way, two conventional concretes with a w/c ratio of 0.55 and 0.65 were designed. Based on them, six recycled concretes with different percentages of replacement of natural coarse aggregates with recycled coarse aggregate (20, 50 and 100%) were obtained. To take into account the high absorption capacity of the recycled aggregates, before using them they were pre-wetted for 10 min. The results concluded that scalar damage mechanics (based on the variations of the elastic modulus) and volumetric strains curves can be use to quantify the damage of the recycled concrete. The results from both approaches indicated that the damage to concrete depended on the percentage of replacement, increasing with higher replacement percentages. Additionally, values of the damage, that are quantified using the critical stress and according to the scalar damage mechanics, are given.     

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。     

混杂纤维增强再生砖骨料混凝土增强机制与抗压性能计算方法

通过钢-聚烯烃混杂纤维增强再生砖骨料混凝土（HF/RBAC）的抗压与弹性模量试验,研究了再生砖骨料（RBA）取代率、混杂纤维掺量、纤维种类对混凝土抗压强度和弹性模量的影响。根据RBA的XRD图谱、X-CT图像、RBA火山灰活性成分与水泥水化产物反应原理及能量平衡原理,分析了HF/RBAC的破坏机制和纤维增强机制。研究表明,当RBA全取代天然骨料（NA）时,HF/RBAC立方体抗压强度、轴心抗压强度和弹性模量分别降低了36.72%、24.95%和43.53%。当钢-聚烯烃混杂纤维体积掺量为1.5%时,HF/RBAC立方体抗压强度、轴心抗压强度和弹性模量分别增加了20.51%、30.33%和35.84%。最后,提出了考虑RBA压碎指标和取代率、纤维种类和掺量等因素影响的HF/RBAC抗压强度和弹性模量的计算方法。     

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。     

钢筋再生混凝土柱的承压性能及强度计算

为了研究钢筋再生混凝土柱的承压性能及其强度计算方法,以有限元软件ABAQUS分析为主要研究手段,结合6个试件的实测数据,并拓展了36个试件的数值模拟分析,综合探讨了再生粗骨料取代率、体积配箍率、柱截面纵筋配筋率、长细比以及相对偏心距等变化参数对钢筋再生混凝土柱承压性能的影响规律,分别研究了钢筋再生混凝土轴心受压柱和偏心受压柱的极限承载力计算方法,并提出实用设计公式。研究结果表明：随着再生粗骨料取代率的增大,钢筋再生混凝土柱的受压承载力呈现波动性变化,但其变化幅度在15%以内;随着体积配箍率的增大,其极限承载力有小幅提高;随着截面纵筋配筋率的增大而有明显提高;而随着相对偏心距的增大,其极限承载力则明显降低;在一定范围内,随着长细比的增大,极限承载力显著变小。提出考虑取代率影响系数的钢筋再生混凝土柱极限承载力计算方法,计算结果与试验结果吻合较好。     

Tension stiffening approach for interface characterization in recycled aggregate concrete

This study proposes a comprehensive analysis on the structural performance of reinforced Recycled Aggregate Concrete members. Particularly, it summarizes the results of an experimental investigation aimed at analyzing the tension stiffening behavior of normal and high strength class concretes produced with Recycled Concrete Aggregates (RCAs). The mixtures were proportioned in order to achieve 25 and 65 MPa of compressive strength and, moreover, several recycled-to-natural coarse aggregates replacement ratios were considered: 0%, 25% and 50%. The results derived from this type of test furnish a comprehensive analysis on both the steel-to-matrix interaction and the crack formation and propagation on concrete elements as well as distributed cracking mechanisms. Using a finite difference numerical model, the experimental results are used to back-calculate and identify the steel-to-concrete bond slip law. Also, it is an alternative mean of developing the stress-crack-width law for concrete in tension. The results showed that the use of recycled concrete aggregate does not affect the resulting concrete performance and, therefore, the RCAs can be successfully employed, up to the levels analyzed herein, for the production of structural elements made with normal and high strength class concrete mix.     

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.     

再生混凝土三轴受压力学性能试验研究

为了研究再生混凝土在三轴应力状态下的力学性能,以再生骨料取代率和围压值为变化参数,设计了36个圆柱体试件,采用位移控制的加载方式进行常规三轴试验。通过试验观察了试件的破坏形态,获取试件受力破坏全过程应力-应变曲线以及峰值应力、峰值应变等特征参数,分析了再生骨料取代率和围压值对再生混凝土三轴受压力学性能的影响,提出了侧向围压值与初始弹性模量、竖向峰值应力、峰值应变之间的关系公式。研究结果表明：取代率对三轴受力再生混凝土的峰值应力和峰值应变影响不大,与天然混凝土相比,其变化分别在5%和2%的范围内波动,而取代率对再生混凝土的初始弹性模量有较大的影响,与天然混凝土相比,三轴受力再生混凝土的弹性模量提高了20%左右;侧向围压值对三轴受力再生混凝土的各项性能指标影响显著,增大围压值能有效提高再生混凝土的弹性模量、承载能力和变形性能。研究结果为再生混凝土的进一步科学研究和推广应用提供参考。     

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。      

多因素对再生混凝土力学性能及抗冻性的影响

为研究不同因素、不同水平对再生混凝土力学性能的作用。该文通过正交试验研究钢纤维掺量、再生粗骨料掺量和粉煤灰掺量对再生混凝土力学性能(抗压强度、劈裂抗拉强度和抗折强度)的影响,确定各因素对再生混凝土力学性能的影响程度,并加以量化表征,并提出多因素共同作用下再生混凝土力学性能的多元非线性回归模型且进行验证。在此基础上,该文进一步研究再生混凝土的抗冻性。结果表明:再生混凝土的力学性能随钢纤维掺量的增加而提高;随粉煤灰掺量增加而降低;再生粗骨料掺量对再生混凝土的力学性能影响较小。钢纤维的掺入可提高再生粗骨料的掺量。再生混凝土力学性能的实测值与通过建立的回归模型得到的计算值的最大误差在6.5%以内。此外,钢纤维的掺入和减少再生粗骨料的掺量均可以提高再生混凝土的抗冻性。     

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。     

型钢再生混凝土偏压柱受力性能试验及承载力计算

为了研究型钢再生混凝土柱在偏心压力作用下的力学性能,设计9个试件进行偏心受压单调加载试验,考虑了再生骨料取代率、相对偏心距2个变化参数。通过试验,观察了该类新型构件的受力破坏过程及形态,获取了应力分布、变形情况、极限承载力、荷载-变形全过程曲线、再生混凝土受压破坏极限压应变值等重要数据;并分析了取代率和相对偏心距对试件极限承载力的影响规律。研究结果表明：型钢再生混凝土柱的偏压破坏过程及形态与一般型钢混凝土柱相似,根据偏心距的不同,表现为大偏心受压破坏和小偏心受压破坏两种形态,试件极限承载力随着相对偏心距的增大而减小,而再生骨料取代率的变化对承载力的影响作用不明显。基于修正平截面假定推导了型钢再生混凝土偏压柱的正截面极限承载力计算公式,计算值与试验值吻合较好。     

Bar-concrete bond in mixes containing calcium carbide residue,fly ash and recycled concrete aggregate

A mixture of calcium carbide residue and fly ash (CRFA) is an innovative new binder for concrete instead of using ordinary Portland cement (OPC). Therefore, this study aims at investigating the bond interaction between common steel reinforcing bars and the aforementioned concrete. To this end, both CRFA and OPC concretes using crushed limestone and recycled concrete aggregate (RCA) as a coarse aggregate were prepared to investigate the bond strength of smooth and deformed bars by pull-out tests. The bond stress−slip relationships were also identified to determine the effects of CRFA binder and RCA on the bond strength behavior. The results indicate that the values the of bond-slip behavior and bond strengths of steel bar in CRFA concretes are similar to those embedded in OPC concrete. Moreover, the bond strength was significantly affected by RCA and the types of steel bar. Although the concretes had the same compressive strengths, the deformed bar embedded in CRFA concrete with RCA had a lower bond strength than the one with crushed limestone. However, the reduction in bond strength of the CRFA concrete with RCA was still less than that of OPC concrete with RCA. For the CRFA concretes, the bond strengths of the deformed bars were approximately 1.7–3.6 times higher than that of smooth bars.