Study On Crack And Shrinkage Resistance Of SAP Recycled Concrete

Because the anti-crack performance of recycled concretes is lower than that of ordinary concretes,in order to improve the anti-shrinkage cracking ability of recycled concretes,it is necessary to solve the problem from the root.The main reason for the shrinkage cracking of recycled concretes was that the humidity in the concrete decreased significantly in the process of cement setting and hardening,which made the relative pore pressure increase.Therefore,Super Absorbent Polymer(SAP)which can absorb and release water is used as internal curing agent to reduce the water loss in recycled concrete,and then the influence of SAP on the anti-crack resistance and shrinkage resistance of recycled concretes is studied.In this paper,the self-shrinkage and crack resistance of SAP recycled concretes were tested by considering the replacement rates of recycled aggregates,particle size and content of SAP.The mechanism of SAP on recycled concretes is further studied by using Scanning Electron Microscope(SEM).This paper is as follows:(1)In this paper,the initial setting time of concrete was defined as the measurement starting point of SAP recycled concretes.Therefore,the Vicat instrument method was used to measure the setting time of concretes,and the setting time of concretes was checked by the temperature change rate.It was found that the error of initial setting time was within ±1%,and the error of final setting time was within ±5%.The results showed that the content of SAP was the main factor affecting the setting time,especially the initial setting time.The starting point of concretes internal temperature change rate can be used as a reliable indicator of the starting point of concrete self-shrinkage measurement.(2)The self-shrinkage performance of 15 groups of specimens with different recycled aggregates replacement rate,content and particle size of SAP was studied.The results showed that under the same conditions,the higher the recycled aggregate replacement rate of concretes,the worse the anti-shrinkage performance of concretes,especially the RC-0-100 specimen had the worst effect on inhibiting self-shrinkage deformation;under the same recycled aggregate replacement rate and particle size of SAP,although the self-shrinkage strain of specimens with different content of SAP increased or decreased compared with the reference specimen.For example,the self-shrinkage strain of SRC-01-40-50 decreased by52.8%,while the self-shrinkage strain of SRC-01-65-50 increased by 64.1%.But in general,it was feasible to use SAP as internal curing agent to reduce the shrinkage deformation of recycled concretes.(3)The selection of particle size of SAP also had a great impact on the shrinkage deformation of recycled concretes under the same replacement rate of recycled aggregates and content of SAP.For the specimens with 50% replacement rate of recycled aggregates,when the particle size of SAP was 40 mesh,the self-shrinkage reduction rate was as high as50%,which significantly improved the shrinkage resistance of recycled concretes.However,for the concretes with 100% recycled aggregate replacement rate,different particle size of SAP had different effects on the shrinkage resistance of recycled concretes under different conditions,and there was no obvious pattern.(4)The crack resistance of six groups of specimens with different replacement ratio of recycled aggregates and SAP was studied by plate crack resistance tests.The increase of recycled aggregate replacement rate and content of SAP will affect the cracking sensitivity of concrete under the same conditions.Moreover,with the increase of the replacement rate of recycled aggregates,the crack resistance of specimens first increased and then decreased.The crack resistance of RC-0-50 specimen was the worst.With the gradual increase of content of SAP,the total crack area per unit area of concrete first decreased and then increased.The total crack area per unit area of SRC-02-40-50 specimen decreased by 71.2%,and the crack inhibition effect was the best.It can be seen that the appropriate amount of SAP can improve the crack resistance of recycled concrete.(5)The mechanism of crack and shrinkage resistance of SAP recycled concretes was analyzed by the SEM test.The results showed that the micro cracks of recycled aggregates and interface transition zone were the main reasons for the reduction of crack resistance of recycled concretes;appropriate SAP content can improve the internal humidity of recycled concretes,reduce the pore pressure and improve the crack and shrinkage resistance of specimens.However,if SAP was distributed unevenly or agglomerated,the crack resistance of concrete will be reduced.In addition,water released from SAP will form large pores and micro-cracks,which will reduce its crack and shrinkage resistance.Therefore,it is necessary to further study the influence of SAP optimal content on the crack and shrinkage resistance of recycled concretes.