Effects Of Curing Regimes On Strength And Toughness Of Ultra-High Performance Concrete

Effects of curing regimes on strength and toughness of UHPC including reactive powder concrete and ultra-high performance concrete with coarse aggregate was studied in this thesis.In this study,curing regimes were employed as follows:standard curing,hot water curing,dry air heating and combined curing(hot water precuring and dry air heating).In order to investigate the effect of combined curing on different types of fiber reinforced concrete,RPC specimens were prepared with hooked-end steel fibers and straight steel fibers.Besides,age was regarded as a parameter to study the effect of different curing method at different ages.The results of the strength test reveal that combined curing can improve the compressive strength of RPC prepared with two types of fibers.The compressive strength of RPC increased from about 120 MPa(standard curing)to over 200 MPa(combined curing)and the relative increasement of improvement is more than 65%.With regard to UHPC(CA),the compressive strength can be improved by heat curing,especially by combined curing which can get the highest strength of 218.5 MPa at 28 day.What's worth noting is that the relative increasement of compressive strength of UHPC(CA)under combined curing is higher than the sum of the relative increasement under single curing regime.The compressive strength RPC and UHPC(CA)under standard curing increased with the age prolonged,while the a small reduction is present under combined curing.Moreover,the compressive strength of UHPC(CA)was decreased under dry air heating and the drop is more severe.The results of toughness show that the toughness of RPC with hooked-end fibers can be improved by combined curing,but there is a small drop with age prolonged.However,compared with standard curing,the toughness of RPC with straight fibers was decreased after combined curing.Besides,toughness of UHPC(CA)can be enhanced through combined curing and dry air heating,while hot water curing has little influence on the toughness.However,the toughness of UHPC(CA)under combined curing and dry air heating was decreased with age.Considering the fracture energy of different ages,combined curing has better effect in improving the toughness of UHPC(CA).In addition,the results of MIP test show that combined curing can decrease the most probable pore size and the harmful pore volume.The most probable pore size and harmful pore volume of UHPC at 28 day under standard curing is 17.0 nm and 0.015 mL/g respectively,while under combined curing the most probable pore size and harmful pore volume decrease to 4-0 nm and 0.012mL/g.However,the most probable pore size and harmful pore volume increased with the age prolonged.