Study On Mechanical And Shrinkage Properties Of Coarse Aggregate Ultra High Performance Concrete

Ultra-high performance concrete（UHPC）is a kind of cement-based composites with excellent properties,but it has the disadvantages of high cost and large shrinkage.It has been proved that adding coarse aggregate to UHPC can effectively reduce the cost and shrinkage cracking,while maintaining high mechanical properties.In this paper,taking the type and content of coarse aggregate and the shape of steel fiber as experimental variables,the effects of coarse aggregate and steel fiber on the working properties,mechanical properties and shrinkage of UHPC-CA are analyzed.The main research contents and achievements of this paper are as follows:（1）Based on slump and expansion tests,it was found that adding coarse aggregate and changing the shape of steel fibers decreased the flowability of UHPC.Increasing the coarse aggregate will significantly reduce the fluidity of UHPC-CA.The content of coarse aggregate starts from 10%,and for every 5%increase,the slump decreases by 26%and the expansion decreases by 11%;under the same content of coarse aggregate,changing the type of coarse aggregate can improve the fluidity of UHPC-CA.When the coarse aggregate changes from crushed stone to pebbles,the fluidity of UHPC-CA increases by 11%;and when the shape of steel fiber changes from straight to end From the hook type to the corrugated type,the fluidity of UHPC-CA will decrease by about 6%on average;when the content of coarse aggregate exceeds 20%,the slump of UHPC-CA is less than 100mm,the expansion degree is less than200mm,and the fluidity is very high.Poor,not conducive to construction,it is recommended that the content of coarse aggregate should not exceed 20%.（2）Through basic mechanical tests,it was found that increasing the content of coarse aggregate increased the compressive strength and splitting tensile strength of UHPC-CA,but decreased the flexural strength.When the content of coarse aggregate is less than 15%,the compressive strength and splitting tensile strength of UHPC-CA increase with the increase of content,while when the content of coarse aggregate exceeds 15%,the The compressive strength and splitting tensile strength both decrease with the increase of the content.When the content of coarse aggregate is 15%,UHPC-CA obtains the highest compressive strength and splitting tensile strength,which are 156.67 respectively.MPa and 11.62MPa;the flexural strength of UHPC-CA decreases with the increase of the content of coarse aggregate,and the maximum value is 17.05MPa when the content of coarse aggregate is 0%;under the same content of coarse aggregate of 15%,the replacement of coarse aggregate from crushed stone to pebble can improve the splitting tensile strength of UHPC-CA by 6.20%,but reduce the compressive strength by 3.10%and the flexural strength by 2.30%;Taking CA as an example,the use of end-hook and corrugated steel fibers can increase the splitting tensile strength by3.80%and the flexural strength by 10.26%compared with straight steel fibers,but reduce the compressive strength by 2.99%.By fitting the test results,it is found that the skeleton reinforcement effect of coarse aggregate mainly affects the compressive strength,while the toughening effect of steel fiber mainly affects the flexural strength.（3）The content of coarse aggregate has a significant effect on inhibiting the shrinkage of UHPC-CA.Compared with UHPC without coarse aggregate,when the content of UHPC-CA is 20%,the self-shrinkage rate decreases by 21.75%,and the total shrinkage rate decreases by 28.12%.%,and it was also found that increasing the content of coarse aggregate from 0%to 20%would increase the proportion of UHPC-CA autogenous shrinkage in the total shrinkage from 70%to 90%;changing the type of coarse aggregate could reduce UHPC-CA4.47%of the self-shrinkage rate;changing the shape of the steel fiber can reduce the self-shrinkage rate of about 4%and the drying shrinkage rate of 39.91%.（4）By comparing the measured values of UHPC-CA autogenous shrinkage strain with different coarse aggregate content with the theoretical values calculated by six kinds of concrete shrinkage prediction models commonly used at home and abroad,it is found that the GL 2000 shrinkage prediction model is in good agreement with the measured data of UHPC-CA autogenous shrinkage.Then,according to the UHPC-CA self-shrinkage test results of different coarse aggregate content,the GL 2000 shrinkage prediction model is modified,and the influence coefficientω=-1.2356+1.3237 of coarse aggregate content on the autogenous shrinkage strain is introduced to establish the theoretical calculation model of autogenous shrinkage strainε_sh-ω=ωε_shωε_shuβ（RH）β（t）for UHPC-CA.Based on the above research results,the coarse aggregate ultra-high performance concrete with expansibility not less than 250mm,compressive strength not less than 150MPa,flexural strength not less than 15 MPA and total shrinkage not more than 800μεfor 120 days was prepared.Among them,15%of the UHPC-CA content,compared with the UHPC without coarse aggregate,the compressive strength increased by 14.5%,the splitting strength increased by 34.7%,the self-shrinkage decreased by 13.69%,and the total shrinkage decreased by 15.58%.