Capillary water absorption characteristics of recycled concrete under freeze-thaw environment
Recycled concrete refers to the new concrete prepared by crushing and screening construction waste,mixing according to a certain gradation,and replacing natural aggregate,which can effectively alleviate the problem of recycling of construction waste and shortage of natural sand and gravel materials in China. As a po-rous building material,the transfer of water is an important reason for the deterioration of concrete performance. The migration of water in concrete mainly includes three forms:capillary water absorption,infiltration and diffu-sion. In unsaturated concrete,capillary water absorption is the main way of water transmission,and it is also an important control factor affecting the frost resistance of concrete. In this paper,effects of the number of freeze-thaw cycles and coarse aggregate replacement rate of recycled concrete (0％,20％,40％,and 60％) on the capil-lary water absorption performance of recycled aggregate concrete (RAC) were studied by ASTM C1585-13 stan-dard test method. The prediction model of initial capillary water absorption of RAC under freeze-thaw cycles was established. Combined with the unsaturated capillary theory,the prediction model of relative water content distribution of RAC under different numbers of freeze-thaw cycles was established. The results show that the cu-mulative water absorption and initial capillary water absorption of RAC increase with increasing replacement rate of recycled coarse aggregate under the same freeze-thaw cycles. When the replacement rate is the same,the cu-mulative water absorption and initial capillary water absorption of RAC increase with increasing number of freeze-thaw cycles. When the content of recycled coarse aggregate is 20％ and 40％,the capillary water absorp-tion resistance is very similar. From an economic point of view,concrete containing 40％ recycled coarse aggre-gate has better resistance to frost. The capillary water absorption of RAC increases rapidly in the early stage,de-creases gradually in the middle stage,and tends to be stable in the later stage. It can be seen from the microscop-ic experiments that the freeze-thaw cycle gradually accumulates the internal damage of RAC,which in turn sig-nificantly increases the initial capillary water absorption. Therefore,the influence of frozen soil environment on the durability of RAC should be fully considered in cold regions. It is verified that the prediction model of initial capillary water absorption of RAC under freeze-thaw cycles based on regression analysis has high calculation ac-curacy and can be used to predict the capillary water absorption performance of RAC,which provides a theoreti-cal basis for evaluating frost resistance durability.
recycled concretecapillary water absorptionfreeze-thaw environmentcumulative water absorptionprediction modelmicrostructure
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