Study On Shrinkage Performance And Mechanism Of Recycled Brick Masonry Aggregate Concrete

In recent years,with the accelerated process of urbanization in China,about 1.5 billion tons of demolition waste were brought annually.The crushing processed into recycled aggregate,can absorb a great amount of city construction waste,as well as to achieve the construction waste recycling and harmless.Recycled aggregate,which were porous and has high porosity,high strength,low water absorption characteristics,resulting in recycled aggregate concrete has larger shrinkage.Especially in the constraint condition,concrete shrinkage cracking were caused by the large shrinkage stress,the safety and durability of the whole structure is extremely unfavorable.In this paper,the free shrinkage deformation and stress development under restraint conditions of Recycled Brick Masoney Aggregate Concrete(RBMAC)was studied,the main results are as follows:(1)The shrinkage and deformation law of RBMAC under unconstrained conditions.It was shown that the free shrinkage of RBMAC increases with the increase of the content of recycled brick aggregate,and when the recycled brick aggregate is completely(100%)replaced by the total amount of water used in this experiment.The shrinkage of concrete at 28 d increased by 46.2% compared with ordinary concrete,and the shrinkage of concrete increased by 54.2% at 180 d.With the increase of pre-wetting degree of recycled brick masonry,the free drying shrinkage of RBMAC increased,saturated pre-wet recycled brick aggregate aggregate preparation of concrete drying shrinkage to dry recycled aggregate preparation of concrete drying shrinkage of 165%.(2)The effect of fly ash incorporation on drying shrinkage of RBMAC.It was shown that the substitution of fly ash can effectively refine the internal pore structure of the sample and reduce the ratio of pores,thus inhibiting the dry shrinkage of RBMAC to a certain extent,and 50% fly ash instead of cement can make RBMAC dry shrinkage reduced by about 39%,which is about 8% less than the dry shrinkage of ordinary concrete without adding fly ash.Comparison of domestic and foreign related shrinkage test model,based on the dry shrinkage mechanism of hardened cement paste,a modified formula for drying shrinkage of fly ash RBMAC was established.(3)The mechanism of internal curing by recycled brick masonry.It was shown that the internal pressure of aggregate in the cement paste with different pre-wetness is the threestage law of rising-balance-decreasing in the cement paste with different pre-wetting degree.The pre-wetting treatment would make water absorption capacity of the recycled brick masonry aggregate in the cement paste decreases and the water return capacity is enhanced.When the replacement rate of the recycled brick masonry aggregate was increased,the peak value of the capillary pressure corresponding to the recycled concrete is decreased gradually,and time of peak appearing is delayed,and the recycled brick aggregate is pre-treated to make the recycled concrete The capillary negative pressure peak appears.(4)The stress development and creep deformation of RBMAC under the condition of uniaxial constraint.It was shown that the increase of aggregate content of recycled brick masonry has little effect on the temperature rise of RBMAC.When using 100% recycled aggregate instead of natural aggregate,the adiabatic temperature reduced about 2.36 ℃,cracking stress decreased about 0.33 MPa,stress cracking reserves reached 47%,it means has a higher safety factor.The influence of the water binder ratio change to the temperature rise is limited,but it will cause the restraint stress corresponding to the RBMAC specimen increases with the decrease of water binder ratio.The creep deformation of concrete is analyzed theoretically,and the feasibility of the theoretical calculation is verified by the results of temperature stress test.