Evaluation of the durability of concretes produced with recycled concrete aggregates using the two stage mixing approach

ABSTRACT The recycling of the mineral fraction of civil construction residues is seen as a source potential of generation of aggregates. The Recycled Concrete Aggregates (RCA) are generally indicated for using in the production of new concretes. However, due to the old mortar adhered to RCA, these have smaller specific mass and higher porosity when compared to the natural aggregate. In the seek of improving the quality of RCA, various techniques have being developed, being one of the alternatives through of surface modification. This study objectified to verify the effect of the substitution of 20% of natural coarse aggregate by RCA and to perform the surface modification of the RCA through the two stage mixing approach. The replacement of aggregates was performed maintaining the particle size distribution of the natural coarse aggregate. The RCA treatment was performed in the pre-wetting period with contents of 25 and 100% of Portland cement. It were performed tests to evaluate the behavior of concretes in relation to mechanical properties and durability parameters. It was evident that the utilization of 20% of RCA in the production of a new concrete produces negative effects on the evaluated properties when compared to conventional concrete. The treatment in the RCA though the two stage mixing approach with 25% of Portland cement provided improvement in the concrete when compared to the normal mixing approach. On the other hand, the two stage mixing approach with 100% of Portland cement impaired the concrete behavior. It was concluded that the two stage mixing approach with 25% of Portland cement showed as a viable alternative to provide improvements in the RCA. Besides resulting in a concrete with good characteristics of mechanical properties and durability, it also enables the utilization of recycled aggregates and it does not generates additional cost in the process of concrete production.摘要：建筑废弃物矿物组分的回收利用，被视作骨料制备的潜在原料来源。再生混凝土骨料（Recycled Concrete Aggregates, RCA）通常被推荐用于制备新型混凝土。然而，由于再生混凝土骨料表面附着旧砂浆，其表观密度低于天然骨料，孔隙率则更高。为提升再生混凝土骨料的品质，学界已开发出多种改性技术，其中表面改性是可行方案之一。本研究旨在验证采用再生混凝土骨料替代20%天然粗骨料的效果，并通过两阶段拌合工艺对再生混凝土骨料进行表面改性。骨料替换过程中，保持天然粗骨料的级配不变。再生混凝土骨料的改性处理在预湿阶段进行，改性剂分别采用占比25%和100%的硅酸盐水泥。本研究开展了多项试验，以评估混凝土的力学性能与耐久性能表现。研究结果表明，相较于基准混凝土，在新型混凝土制备中掺入20%再生混凝土骨料会对其各项被测性能产生负面影响。采用25%硅酸盐水泥的两阶段拌合工艺对再生混凝土骨料进行改性后，制备得到的混凝土性能优于采用常规拌合工艺的对照组；而采用100%硅酸盐水泥的两阶段拌合工艺，则会劣化混凝土的性能表现。综上，采用25%硅酸盐水泥的两阶段拌合工艺可有效改善再生混凝土骨料的性能，是一种可行的改性方案。该方案不仅可制备出力学性能与耐久性能优良的混凝土，还能实现再生骨料的规模化利用，且不会在混凝土生产过程中产生额外成本。