Recycled aggregate concrete (RAC) mixture of hybrid fibers has been proven to be an effective type of hybrid fiber-reinforced recycled aggregate concrete (HyFRAC). To further investigate the engineering potential for HyFRAC, the orthogonal test method was used to do sensibility analysis on the compressive strength and splitting tensile strength of HyFRAC.
And a prediction model of compressive strength of HyFRAC based on Convolutional NeuralNetwork (CNN) was proposed. The results show that the ratio of recycled brick aggregate (RBA) to recycled concrete aggregate (RCA) has been proved the greatest influence on the compressive strength and splitting tensile strength of HyFRAC, followed by the water reducing agent content, finally the ratio of glass fiber (GF) to polypropylene fiber (PF). When the RBA/RCA ratio is 2/8, the GF/PF ratio is 7/3, and the water reducing agent content is 0%, the compressive strength and splitting tensile strength of HyFRAC are the highest. According to JGJ/T10-2011, when RBA/RCA ≤ 6/4 and water reducing agent content ≥ 0.4%, the HyFRAC slump meets the requirement of 50m pumping height; when RBA/RCA ≤ 4/6 and water reducing agent content ≥ 0.6%, the HyFRAC slump meets the requirement of 100m pumping height. The CNN model is efficient in estimating the compressive strength of HyFRAC, as compared to the back propagation (BP) neural network model and multiple linear regression model. The average relative errors and max relative errors of the CNN model are 1.98% and 4.12%, respectively.
There are essential differences between recycled brick aggregate (RBA) and recycled concrete aggregate (RCA). The weak area of RCA is the old mortar attached to the aggregate surface, while the weak area of RBA is its own larger porosity. Silane has a small molecular structure, which can penetrate into the concrete with water as the medium and react with the active functional groups in the concrete to form a network structure of silane polymer. In this paper, 8% silane solution was used to modify recycled brick aggregate concrete (RBAC) in two schemes. The first scheme was that RBA was soaked in 8% silane solution, and the second scheme was that 8% silane solution was added in the mixing process of RBAC instead of water. The results showed that compared with the unmodified RBA, after silane modification of RBA its apparent density increased by 5.48%, water absorption decreased by 61.12%, crushing index decreased by 11.09%. The average compressive strength of RBAC with RBA modified by silane was 11.72% higher than that of unmodified RBAC, and the average compressive strength of RBAC mixed with silane was 9.12% lower than that of unmodified RBAC. The pores and cracks on the surface of RBA were significantly reduced and the interface transition zone (ITZ) of RBA was compacted after silane modification of RBA. RBA contained Si, Fe, Al, Ca elements and a small amount of Na, K, Mg elements brought by mineral powder; the content of Ca, O, Si elements in ITZ and cement mortar were rich, which indicated that there were a large amount of calcium silicate hydrate (C‐S‐H) in ITZ and cement mortar; ITZ and cement mortar contained Al elements, which indicated that there were ettringite and monosulfate; the content of Ca element in RBA and ITZ increased after silane modification of RBA, which indicated that silane hydrolyzed and reacted with free calcium ions to form calcium silicate.
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