Water Absorption And Desorption Behavior Of Recycled Aggregate And Its Effect On Concrete Shrinkage

With the rapid development of urbanization process of China,the amount of construction and demolition waste(CAD)is increasing significantly.The treatment methods such as open-air stacking and simple landfill not only pollute the environment,but also waste resources.The resource utilization of CAD,implementing the resource utilization of construction waste,and breaking the construction waste,is to crush the CAD to obtain recycled aggregate.The aggregate is then regenerated to prepare recycled aggregate concrete(RAC)and the like.It can not only solve the pollution,but also reduce the excessive exploitation of natural resources and generate huge social benefits.Many scholars believe that the high water absorption and low compactness of recycled aggregates affect the performance of RAC.Therefore,the recycled aggregate is usually pre-wetted in practical applications.However,the determination of the amount of water used for mixing after pre-wetting and the evaluation of the self-curing by recycled aggregates are still controversial.This paper focuses on the water absorption and desorption behavior of recycled aggregates.The water absorption and desorption characteristics of recycled waste brick aggregates in cement paste and concrete with different pre-wet modes were investigated.The water absorption and desorption behavior of recycled aggregates were studied for the apparent morphology and microhardness of the interface transition zone.By changing the type of recycled aggregate,replacement rate and pre-wetting method,the effects of water absorption and desorption behavior of recycled aggregate on concrete shrinkage and other properties were studied.Then the relationships between different shrinkage types,long-term shrinkage and mass loss of concrete were discussed.According to the foregoing research,the concrete mix design suitable for small-volume recycled aggregates is optimized based on the additional water consumption.The main conclusions are as follows:(1)The water absorption and desorption of the recycled brick aggregates show that the aggregate begins to absorb water after entering the slurry,and the aggregate begins to return to water when the capillary negative pressure rises rapidly.When the water of the aggregate and the slurry reaches a new equilibrium,the return ends.The water absorption and desorption characteristics are related to the degree of pre-wetting and the water to cement ratio(W/C).The interface between the recycled aggregate and the cement stone is more blurred than the interface between the natural aggregate and the cement stone,and the bond is denser.The microhardness of the transition zone of the recycled aggregate interface is higher than that of the aggregate and cement stone.(2)The water absorption and desorption behavior of recycled aggregate under different conditions have different effects on concrete shrinkage and other properties.When the total water consumption is the same,the higher the brick content of recycled aggregate,the smaller the drying shrinkage of RAC,and the greater the compressive strength.When the total water consumption of pre-wetting is constant and the replacement rate is within 50%,the compressive strength of concrete increases with the increase of the replacement rate.Whenthe replacement rate of recycled aggregate is within 20%,the shrinkage of concrete decreases as the replacement rate increases.The concrete produced from the dry recycled aggregate has the fastest slump loss.The early shrinkage of concrete is inhibited by the return behavior of the aggregate.The concrete with the same pre-wet total water consumption has the largest initial shrinkage and the smallest drying shrinkage.The pre-wet mixing water has the same initial shrinkage and the largest shrinkage.Under different pre-wet modes,the mass loss rate of surface-sealed concrete is much smaller than that of untreated concrete,but the sum of concrete dry shrinkage and moisture exchange isolated shrinkage is greater than total shrinkage.(3)In this paper,a method for designing the mix ratio of small-volume recycled aggregate concrete based on additional water consumption is proposed.On the one hand,the additional water consumption is calculated based on the water absorption rate of the aggregate.On the other hand,the additional water consumption is calculated according to the optimized Bolomey correction formula.On the basis of consistent total water consumption,the smaller value calculated by the two methods is used to formulate concrete with additional water consumption,which can reduce the amount of water reducing agent,and the compressive strength and drying shrinkage of the prepared concrete can meet the actual demand.