Modification And Mix Proportion Design Of Recycled Concrete

A growing number of construction waste, especially waste concrete, has plagued the city’s development. To reuse the waste concrete as concrete aggregate source can be an effective solution to land occupation and environmental pollution caused by waste disposal. And the recycling can relieve natural aggregate resource shortages and can also avoid natural ecological environment destruction caused by mining rocks. The first thing of applying recycled aggregate to concrete is to solve the mix design problem of recycled concrete, and the key is the design of mix proportion. In most cases, due to the low quality of recycled aggregate compared with natural aggregate in the same strength grade, the quality of prepared concrete is worse than ordinary concrete. Therefore, it is very important to optimize the performance of recycled aggregate and recycled concrete. This paper, based on the method of experimental study and theoretical analysis, probes into the factors influencing the strength of recycled concrete, mix design and modification. Meso-structure and composition of recycled concrete are analyzed. The compression failure process is simulated and its compressive failure mechanism and characteristics are discussed based on the numerical simulation technology. The main works and conclusions are as follows:The main factors affecting the strength of recycled concrete have been studied, including water-cement ratio, strength grade of cement, sand ratio, maximum aggregate sizes, aggregate gradation and curing age. The results show that the water-cement ratio should be controlled in 0.45±0.05. Below this value, workability would be decreased, while above this value, the strength of recycled concrete would decline sharply. When the sand rate is 35%, the compressive strength is the highest. When the maximum aggregate size is 20 mm, the compressive strength and splitting tensile strength are both the highest. If the concrete is made of recycled aggregate according to the maximum density curve, the compressive strength and splitting tensile strength of it are both the highest. With a longer curing period, the smaller the water-cement ratio is, the higher the compressive and splitting tensile strength growth rates are. The strength of 90 days has increased greatly than the 28 days’. The relationship between the compressive and splitting tensile strength and the cement-water ratio is linear. When age reaches 90 days, the strength of the recycled concrete is only lower about 10 percent than that of the ordinary concrete. Based on the 7d strength, the compressive strength prediction formula of recycled concrete is established.The relationship formula of recycled coarse aggregate is built between water absorption and(unit volume) additional water consumption with its quality, water absorption rate, water content. The relationship between compressive strength and cement-water ratio of the recycled coarse aggregate concrete and all recycled aggregate concrete has been determined, and the aggregate coefficients of these two kinds of concretes have been decided. Based on additional water consumption, the design steps and methods about mixture ratio design of recycled concrete are proposed. The concept of “effective water-cement ratio” and “nominal water-cement ratio” is put forward.To optimize the performance of recycled concrete, the preparation technology and strengthening methods were studied respectively. The research results of preparation technology show that the secondary mixing processes not only enhance compressive and splitting tensile strength of recycled concrete, but also improve its work performance. Because of its small strength discreteness, it is an optimal process for the preparation of recycled concrete, much better than the current widely used ordinary process. To improve the performance of recycled concrete, three ways are adopted. Firstly, steel fiber and fly ash(to increase the workability) are used to strengthen recycled concrete. Secondly, DSP and RPC slurry are utilized to strengthen recycled aggregate. Thirdly, DSP and RPC slurry are chosen to strengthen recycled concrete. The results show that: firstly, the above three methods can all improve the compressive and splitting tensile strength of recycled concrete. Secondly, for steel fiber(fly ash) to strengthen recycled concrete, the larger the amount of steel fiber is, the higher the strength is. But its workability is greatly reduced, and fly ash is needed to enhance its liquidity. However, the replacement ratio of fly ash should not be too high. The recommended value is less than 10%. Above this value, strength will decline dramatically. Thirdly, DSP and RPC slurry reinforcement recycled aggregate can improve its strength significantly. Fourthly, DSP and RPC slurry reinforcement recycled concrete can also improve its strength significantly.The macroscopic properties of concrete are determined by its microstructure. Through SEM, the mesoscopic composition, the internal structure of mortar and interface structure of the recycled concrete and the ordinary one were compared and analyzed. The ways and means to improve the performance of recycled concrete interface were put forward. By using the random algorithm, the mesoscopic model of recycled concrete was established, and the failure process under uniaxial compression was simulated. The effect of replacement rate of recycled coarse aggregate, new hardened cement mortar strength, old hardened cement mortar strength and interfacial transition zone strength on the compressive characteristics of recycled concrete was discussed. Under a 100% replacement rate of recycled coarse aggregate, the influence of relative strength of the new mortar and old mortar, the new interface and the old interface on the failure characteristics of recycled concrete under uniaxial compression was discussed. Results prove that compression failure all begins from the weakest interface. When the old interface strength is close to the minimum, the compressive properties of recycled concrete is the best. When the new interface strength is close to the minimum, the compressive properties of recycled concrete is the worst. Therefore, the strength of the new interface and new mortar should be improved in the preparation of recycled concrete.