Effect of Composite Impregnation on Properties of Recycled Coarse Aggregate and Recycled Aggregate Concrete

Influence of Pretreatment Methods on Compressive Performance Improvement and Failure Mechanism Analysis of Recycled Aggregate Concrete

The utilization of recycled aggregate can avert the squandering of resources and the destruction of the environment. Nevertheless, there exists a slew of old cement mortar and microcracks on the surface of recycled aggregate, which give rise to the poor performance of aggregates in concrete. In this study, for the sake of ameliorating this property of recycled aggregates, the surface of the recycled aggregates is covered with a layer of cement mortar to compensate for the microcracks on the surface and reinforce the bond between old cement mortar and aggregates. In order to demonstrate the influence of recycled aggregate by different cement mortar pretreatment methods, this study prepared natural aggregate concrete (NAC) and concretes with recycled aggregate after the wetting pretreatment (RAC-W) and cement mortar pretreatment (RAC-C), and conducted uniaxial compressive strength tests on different types of concrete at different curing ages. The test results indicated that the compressive strength of RAC-C at a 7 d curing age was higher than that of RAC-W and NAC, and the compressive strength of RAC-C at a 28 d curing age was higher than RAC-W but lower than NAC. The compressive strength of NAC and RAC-W at a 7 d curing age was about 70% of that at a 28 d curing age, and the compressive strength of RAC-C at a 7 d curing age was about 85-90% of that at a 28 d curing age. The compressive strength of RAC-C increased dramatically at the early stage, while the post-strength of the NAC and RAC-W groups increased rapidly. The fracture surface of RAC-W mainly occurred in the transition zone between the recycled aggregates and old cement mortar under the pressure of the uniaxial compressive load. However, the main failure of RAC-C was the crushing destruction of cement mortar. With changes in the amount of cement added beforehand, the proportion of aggregate damage and A-P interface damage of RAC-C also changed accordingly. Therefore, the recycled aggregate pretreated with cement mortar can significantly improve the compressive strength of recycled aggregate concrete. The optimal amount of pre-added cement was 25%, which is recommended for practical engineering.

Influence of Citric Acid-Assisted Impregnation of Recycled Aggregate on the Properties of the Resultant Concrete

The paper presents the results of tests on concrete with recycled aggregate impregnated with the use of citric acid. Impregnation was carried out in two stages, with a suspension of calcium hydroxide in water (so-called milk of lime) or diluted water glass used as the second impregnant. The mechanical properties of the concrete were carried out: compressive strength, tensile strength and resistance to cyclic freezing. In addition, concrete durability parameters such as water absorption, sorptivity and torrent air permeability were investigated. The tests showed that this type of impregnation did not improve most of the parameters of concrete with impregnated recycled aggregate. The mechanical parameters after 28 days were significantly lower compared to the reference concrete, although after a longer curing period, these differences decreased significantly for some series. The durability parameters of the concrete with impregnated recycled aggregate also deteriorated compared to the reference concrete with the exception of air permeability. The results of the tests carried out indicate that impregnation using water glass in combination with citric acid gives the best results in most cases and that the order in which the impregnation solutions are applied is very important. Tests also showed that the effectiveness of impregnation is very much influenced by the value of the w/c ratio.

Experimental Study and Analysis on Workability and Mechanical Performance of High Fluidity Recycled Concrete

In order to study the workability and mechanical performance of high fluidity recycled concrete, parallel tests were carried out to prepare high fluidity recycled concrete by changing the amount of recycled aggregate (20%, 30%, 40%, 50% and 60%) and the sand ratio (0.37, 0.38, 0.39 and 0.40). The fluidity, compression strength, tensile strength and flexural strength of each mix were tested. The results show that the slump of a recycled concrete mixture is 120 mm when the content of recycled aggregate is less than 30%, and the mechanical strength satisfies the requirement of the high fluidity ordinary concrete. As the ratio of sand increases, the fluidity of the recycled concrete also improved. The best sand ratio is not consistent with the strength index. When the sand ratio is between 0.37 and 0.40 and the content of recycled aggregate is not more than 40%, the concrete of C60 can be prepared. Both the fluidity and the strength index can meet the design requirements and can be used in the practical engineering.