Mathematical models for properties of mortars with admixtures and recycled fine aggregates from demolished concretes

In order to expand the engineering application of recycle aggregate mortars （RAM） with aggregates from demolished concretes, the models for the properties of RAM and the replacement rate of these recycled fine aggregates were proposed. First, different kinds of mathematical models for the basic properties （compressive strength, water retention rate, and consistency loss） of RAM with two kinds of admixtures, thickening powders （TP） and self-made powdery admixtures （SSCT） designed for RAM, and the replacement rates were established, while the average relative errors and relative standard errors of these models were calculated. Additionally, the models and their error analyses for the curves of drying shrinkage and curing time of RAM ＋ SSCT at different replacement rates were put forward. The results show that polynomial functions should be used to calculate the basic properties of RAM ＋ TP and RAM ＋ SSCT at different replacement rates. In addition, polynonfial functions are the most optimal models for the sharp shrinkage sections in the curves of drying shrinkage-curing time of RAM ＋ SSCT, while exponential functions should be used as the models for the slow shrinkage sections and steady shrinkage sections.

Effects of admixture on properties of recycled aggregate mortar

Compared with strengthening the recycled fine aggregate(RFA)from construction and demolition waste,which is time-consuming and complex,adding admixture into the mixtures directly is more efficient and effective to expand the application of recycled aggregate mortar(RAM).The admixture(named as SSC),mixed with sodium hexametaphosphate,sodium ligninsulfonate and citric acid,was directly added into the RAM.First,the compositions and physical properties of the RFA and reference aggregate were studied,respectively.The properties of fresh and hardened mortars were then investigated.The results show that there is a clear difference between the RFA and reference aggregate,and the properties of RAM without SSC are not as good as those of normal mortar.However,the consistency value,water retention rate,compressive strength and setting time values of RAM increase by 5%,7%,66%and 67%,and its consistency loss and density values decrease by 42%and 4%after the SSC is added into the RAM.Therefore,improving the properties of RAM through adding admixture is an effective and efficient approach to expanding its application.

C60全珊瑚海水钢筋混凝土梁的抗弯性能研究

通过对不同种类钢筋的全珊瑚海水钢筋混凝土梁(CARCB)进行正截面抗弯性能试验,研究了CARCB的裂缝发展、破坏形态、跨中变形及抗弯承载能力,建立了弯矩跨中挠度、荷载纵向受拉钢筋应变、荷载最大裂缝宽度的关系,提出了适用于CARCB抗弯承载力的计算模型.结果表明:随着钢筋强度的提高,CARCB的开裂弯矩和极限弯矩均逐渐增大,裂缝宽度扩展逐渐减慢,能有效抑制裂缝的发展.CARCB的裂缝宽度均随着荷载的增长而增大,在加载初期,裂缝宽度增长缓慢,当接近极限荷载时,裂缝宽度迅速增大,最终导致梁破坏.采用有机新涂层钢筋,能有效抑制珊瑚混凝土结构中钢筋发生锈蚀,从而延长其有效服役寿命.综合考虑钢筋锈蚀引起钢筋截面损失和钢筋屈服强度降低的影响,提出了更加合理的CARCB抗弯承载力的计算模型.

再生混凝土抗碳化性能研究进展

通过分析已有文献,本文评述了再生混凝土(RAC)抗碳化性能的研究进展。阐述了混凝土的碳化机理与碳化分区理论。分析了影响RAC抗碳化性能的因素,并列举了RAC碳化深度预测模型。结果表明:已有文献研究了不同因素对RAC抗碳化性能的影响,并根据实验数据拟合得出了碳化深度预测模型。但是,加速碳化作用下不同因素对RAC微结构影响的研究报道较少,且已有的RAC碳化模型中,均属于经验模型或基于扩散理论与试验结果的模型。因此,建议未来系统地研究加速碳化作用下不同因素对RAC碳化区及其微结构的影响,并建立基于微结构演变的理论碳化模型。

混凝土强度对珊瑚混凝土梁抗弯性能的影响

通过对不同混凝土强度的钢筋珊瑚混凝土(CAC)梁进行正截面抗弯性能试验,研究了钢筋CAC梁的变形性能及承载能力,建立了弯矩-跨中挠度、相对弯矩-最大裂缝宽度(w_(s))等关系,提出了钢筋CAC梁的相对界限受压区高度(ξ_(b))、极限弯矩(M_(u))和w_(s)的计算模型.结果表明:随着混凝土强度的提高,钢筋CAC梁的开裂弯矩(M_(cr))和M_(u)均逐渐增大,裂缝宽度扩展逐渐减慢,能有效抑制裂缝的发展.同时,通过提高混凝土强度,可以增大M_(cr)/M_(u),从而延缓钢筋CAC梁的开裂时间.建议CAC的强度至少为C60,同时采用有机新涂层钢筋,这样能有效延长其服役寿命.最后,考虑钢筋锈蚀的影响,提出了更加合理钢筋CAC梁的ξ_(b)、M_(u)和w_(s)计算模型.

配筋率对全珊瑚海水钢筋混凝土梁受弯性能的影响

通过对全珊瑚海水钢筋混凝土梁(CARCB)和普通钢筋混凝土梁(OARCB)进行受弯性能试验,研究了混凝土种类和配筋率对CARCB的变形性能、承载能力的影响。结果表明:随着配筋率的提高,CARCB的开裂弯矩和极限弯矩均逐渐增大,裂缝宽度扩展逐渐减慢,能有效抑制裂缝的发展。同时,可以通过提高配筋率,增大开裂弯矩与极限弯矩的比值,从而延后CARCB的开裂时间。CARCB的裂缝宽度均随着荷载的增长而增大,在加载初期,裂缝宽度增长缓慢,接近极限荷载时,裂缝宽度迅速增大,最终导致梁破坏。此外,综合考虑钢筋锈蚀引起钢筋截面损失和钢筋屈服强度降低的影响,提出了CARCB的极限弯矩、纵向受拉钢筋应力、平均裂缝间距和平均裂缝宽度的计算模型。