Research On The Shrinkage Performance Of Ultra High Performance Concrete Under Different Curing Condition

Ultra High Performance Concrete(UHPC) is a new cement-based composite materials,which have high strength, toughness, volume stability and better durability.In the research region of new concrete materials,the issue of shrinkage always belongs to the focus that all researchers pay attention to,and it is one of the factors restricting the wide application of UHPC in the engineering of structure.Shrinkage is the main reason leading to the cracking of concrete.Once concrete cracking emerge,it would decrease the durability of structure largely,even threaten the security of structure.The shrinkage performance of UHPC is different under the different curing condition.Some research results have showed that the dry shrinkage of UHPC decreased significantly after heat curing because of the pore structure refining and the matrix changing more dense.The paper studied the early age shrinkage and dry shrinkage of UHPC under different curing condition,and paid an emphasis on the shrinkage performance of UHPC during heating curing as well as the tendency of the shrinkage development after heat curing.Meanwhile,the experiment of mercury intrusion porosimetry,the measurement of scanning electron microscope and the thermal analysis were used for analyzing the microstructure of UHPC.Based on the research of this paper, we could conclude the following results:(1) Under the condition of standard curing,UHPC had large shrinkage value which was 1.5 times as much as dry shrinkage value.The shrinkage of UHPC was tending towards stability reaching to 1500με approximately while the age of UHPC was 65 days.(2) the temperature expansion coefficient of UHPC reach ed to11.76 με / °C approximately. The shrinkage of UHPC could not be observed obviously until the curing temperature reached 50°C during the heat curing.While the curing temperature was close to 70°C,the value of shrinkage increased rapidly and reached to 450με ultimately.(3) With the increasement of reinforcement ratio,the shrinkage of UHPC reduced during heat curing illustrating that reinforcement assembly could reduce the shrinkage ratio of UHPC during heat curing.The elastic strain of reinforcement UHPC during heat curing was very small and reached to 2.4 με~10.37με approximately leading to little residual stress.(4) The size of specimen had great impact on the shrinkage.The shrinkage of specimen with small size was usually less than that with big size because specimen with small size promoted the directional distribution of steel fiber.(5) By analysis on the microstructure of UHPC with different age, some changes emerged after heating curling including pore structure refining,producing more hydration products and reducing the volume of calcium hydroxide and ettringite.