Experimental Research On Mechanical Properties Of Fine Recycled Aggregate Concrete After Exposure To Elevated Tempretures

Coarse and fine recycled concrete aggregates(RCA)are made from construction and demolition waste after crushing and classification and can be employed to replace natural aggregates(NA)partly and even totally in concrete preparation.This new kind concrete is so called recycled aggregates concrete(RAC).Nowadays,the application of RCA mainly concentrates on coarse RCA and fine RCA which accounts about 20% to 50% of the total RCA amount is generally ignored.Due to this situation,the reuse of waste concrete cannot be realized completely.The research object of this paper is RAC incorporating coarse and fine RCA at the same time and the replacement percentage of coarse RCA was 100% while that of fine RCA was 0%,50% and 100%,respectively.The RACs were firstly exposed to elevated temperatures(20?,200?,400?,600? and 800?)and then cooled to ambient temperature under two types of cooling regimes(natural cooling and water cooling).Then post-fire mechanical properties of RAC were investigated,which aims to provide reference for the damage assessment and repair of structures made of RAC after fire.The contents and results are listed as follows.(1)The basic material indexes of RCA including apparent density,crushing index and water absorption ratio were measured and the test results showed that the apparent density and crushing index of RCA were lower than that of NA while the water absorption ratio of RCA was much higher than that of NA.The relationships between the cubic compressive strength,splitting tensile strength,prism compressive strength and elastic modulus of RAC and the replacement percentage of fine RCA were studied.It was found that with the increasing of fine RCA replacement percentage,the cubic compressive strength,splitting tensile strength and prism compressive strength first increased and then decreased,but the values of elastic modulus kept declining.(2)The apparent characteristics,mass loss,cubic compressive strength and split tensile strength of a total of 180 standard cubic specimens after exposure to elevated temperature were studied.With the temperature increasing,the degree of specimen surface cracking and mass loss ratio of RAC increased while the cubic compressive strength and splitting tensile strength went through increasing first and then decreasing.The cubic strength was found to be the highest when the replacement percentage of fine RCA is 50%.Compared with the condition of nature cooling regime,the cubic compressive strength and splitting tensile strength of RAC were lower after water cooling.Recommend values for calculating the reduction factors of cubic compressive strength and tensile strength of RAC after high temperature are given based on the experimental data.(3)The axial compressive tests of 90 RAC prism specimens after high temperature were carried out and the failure modes were observed.Meanwhile,the influences of fine RCA replacement percentage,elevated temperature values and cooling regimes on peak stress,elastic moduli,peak strain and compressive stress-strain curve were investigated.The test results agreed very well with the stress-strain model proposed by our research group.