Early Age Shrinkage And Cracking Resistance Of PP Fiber Reinforced Expansive Self-Consolidating Concrete Under Internal Curing Condition

Self-consolidating concrete(SCC)is prone to produce shrinkage due to its low water binder ratio,high sand ratio and more mineral admixtures,resulting in concrete cracking and other durability issues.In order to solve these problems,several shrinkage reducing materials have been proposed and studied,such as polypropylene fiber,lightweight aggregate,and expansive admixture.Although many scholars have done some research on the incorporation of the above materials,few studies have considered the synergistic effect of the three materials when they are incorporated into concrete.This paper mainly studied the influence of polypropylene fiber(0.05%,0.1%,0.15% of the volume fraction of concrete),pre-wetted lightweight aggregate(10%,20%,40% of the volume fraction of fine aggregate),expansive agent(0%,8% of the weight content of cementitious material)on the basic mechanical properties(compressive strength,splitting tensile strength),early age shrinkage of concrete and the early age crack control.The working performance and basic mechanical properties of SCC(slump flow,J-ring test,cubic compressive strength,splitting tensile strength)were tested,and the results show that the addition of polypropylene fiber and lightweight aggregate can reduce the working performance of SCC,the effect of polypropylene fiber is the greatest,and the expansion agent can reduce the compressive strength of concrete in varying degrees.For the concrete mixed with 0%,0.05% and 0.1% PP fiber,the cube compressive strength of the ordinary concrete and the expansive concrete with the continue addition of lightweight aggregate decreases first,then increases,and then decreases.For the concrete mixed with 0.15%,the compressive strength and fluidity are greatly affected.A total of 32 groups of mix proportion were designed to study the shrinkage performance of SCC in early age.The main test method is contact method,which mainly collects the shrinkage and expansion changes within 7h-7d after pouring.Results show that it is more effective to mitigate the early age shrinkage by adding light aggregate alone than by adding polypropylene fiber alone.In the sealed cure(S-Cure)condition,adding 10% light aggregate can compensate the shrinkage and 0.1% polypropylene fiber content exhibits the highest shrinkage reduction.While for the top-surface exposure cure(TE-Cure)condition,20% polypropylene fiber can compensate the shrinkage and 0.15% polypropylene fiber shows the highest shrinkage mitigation.Regardless the addition of the expansive agent,at the low content of PP fiber(0.05%,0.1%),it shows that the shrinkage of concrete is decreased with the increase of lightweight aggregate content.A total of 18 mixture proportions were designed,and were quantified for their early age cracking control using slab test.The development of cracks in 2-,3-,6-and 24-hour periods after concrete placement were recorded and collected.The results demonstrated that the incorporation of expansion agent,polypropylene fiber and lightweight aggregate alone can improve the early age crack control to a certain extent,and among them,polypropylene fiber has the best effect.Particularly,when at low PP fiber content(i.e.,0.05% vol.),the cracking control of SCC was significantly improved to a certain extent with increase of LWA ratio.However,at higher PP fiber content of 0.1% vol.,LWA did not show significant effect on improving concrete cracking control.When a certain amount of LWA was added into SCC,the addition of PP fiber can improve the early age cracking control of SCC.With the increase of fiber content,however,the cracking control increased first and then decreased.When the EA was added,the cracking control enhanced with the increase of fiber content.