Study On Size Effect Of Basic Mechanical Properties Of Ultra High Performance Concrete

Ultra-high-performance-concrete(UHPC)has superior mechanical properties and durability,and it is widely used in huge structures such as bridges.By adding steel fibers,the compression and flexural behavior are improved greatly,but the practical application of UHPC has been limited owing to its high price.To overcome this limitation,hybrid fibers can be used to reduce costs and promote properties.Although the mechanical properties of UHPC are superb,they often exhibit different mechanical properties in different structural sizes.Thus,in order to accurately describe the size dependent mechanical properties,the study of size effect is particularly important.Based on the experimental study of compressive strength and flexural toughness of UHPC,the following conclusions are drawn:With the increasing volume of steel fiber,the flowability of UHPC gradually decreases and the compressive strength increases significantly.When 2% steel fiber added,UHPC with long hook fiber results in the greatest increase in compressive strength,while the increasing of compressive strength of UHPC with short hook fiber and straight fiber reduced.The size effect of UHPC with short hook fiber is the most obvious,straight fiber follows,and the long hook fibers contribute least to the size effect.As the water to binder ratio increases,the flowability increases significantly,compressive strength gradually decreases,and the size effect of UHPC gradually decreases.With the increasing volume of steel fiber,the initial cracking load and deflection change little,the size effect of equivalent initial cracking stress and deflection tends to be similar,while the peak load and deflection increase greatly,and the size effect of peak load and peak deflection be more obvious.When 2% steel fiber added,the type of steel fiber has similar influence on the initial cracking load and deflection,and the size effect is similar,too.The long hook fiber has the greatest effect on the peak load and deflection,the short hook fiber follows,and the long hook fibers contribute least.The type of steel fiber have similar effects on size effect of initial cracking deflection and load.Straight fiber and hook fiber with same aspect ratio have similar effects on the size effect of equivalent peak stress and deflection,and long hook fiber has minimal effect.When hybrid fibers were added,the initial crack load and initial deflection change little with the increasing volume of long hook fiber,but the peak load and peak deflection increase first,reached the maximum at S0.5H1.5,and then gradually decrease.The size effect of UHPC shows the opposite rule.With the increasing of long hook fiber,the size effect gradually decreases and then increases,the size effect of S0.5H1.5 is the least obvious.