Study On Preparation And Properties Of Economical Ultra-high Performance Concrete

To solve the problems in the application of ultra-high performance concrete(UHPC)engineering,this paper carried out the research on the preparation and performance of steam-free economical ultra-high performance concrete(EUHPC).On the composition system of cementitious materials,The preparation and properties of UHPC with a high-volume mineral admixture were studied and the microstructure differences under standard curing and steam curing conditions were compared.On the aggregate system,four aggregate particle sizes EUHPC with quartz sand(<1.18mm),river sand(<4.75mm),small crushed stone(<9.5mm)and medium crushed stone(<19mm)were prepared.The effects of aggregate size,weight ratio of aggregate to binder(A/B),weight ratio of small crushed stone to river sand(SS/RS)and steel fiber content on EUHPC performance were studied.The relationship between aggregate reinforcement effect and steel fiber reinforced toughening effect was discussed.(1)Under standard curing conditions the SF20G20 and SF17.5FA10G17.5 in the composite mineral admixture UHPC have better performance.The compressive strength of the 28 d mortar is >140MPa,and the flexural strength is >40MPa.Under steaming curing conditions,the 7d compressive strength of SF20G20 can reach 172.5MPa,and the flexural strength reaches 50.9MPa.Under standard curing,the microstructure analysis shows that the porosity of UHPC is 7%~9%,and the pore size of 10~20nm is more.By the 28 th day,the degree of hydration of UHPC is gradually deepened,accompanied by the pozzolanic effect of mineral admixtures.Under steaming curing,The UHPC porosity is 3.5%~5%,mainly capillary pores less than 10 nm,but the hydration degree of UHPC in 28 d is almost stagnant compared with 7d.(2)Both the increase of A/B and the increase of SS/RS at the same A/B reduce the slump flow of EUHPC.With the increase of the A/B,the compressive strength and bending strength of EUHPC increase first and then decrease,and the elastic modulus shows a linear growth trend.When the SS/RS is 1/2,the reinforcement effect of crushed stone to compressive strength and axial compressive strength is most significant.The larger the SS/RS,the higher the EUHPC elastic modulus increases.When the A/B isn't more than 0.9 and the SS/RS isn't more than 1/2,the positive effect of the skeleton reinforcement deriving from the crushed stone is higher than its negative effect on reducing the reinforcing and toughening efficiency of the steel fiber,and it helps to improve the bending strength of EUHPC.(3)Among the four groups of EUHPC with different particle size aggregates,the compressive strength and elastic modulus of the UHPC with medium crushed stone(MS-UHPC)are the highest,followed by the UHPC with small crushed stone(SS-UHPC),and the UHPC with river sand(RS-UHPC)is slightly lower than the UHPC with quartz sand(QS-UHPC).With the increase of aggregate particle size,the bending strength and flexural toughness index of UHPC have different degrees of reduction.Compared with the RS-UHPC,SS-UHPC has a lower reduction in bending strength and bending toughness and still maintains good bending performance.(4)The increase of the volume fraction of steel fiber will seriously reduce the workability of EUHPC,and the larger the aggregate particle size,the greater the decrease.When the volume of steel fiber is ?2%,the compressive strength and bending strength of SS-UHPC and QS-UHPC are similar.After the steel fiber volume is >2%,the compressive strength and bending strength of QS-UHPC continue to increase linearly,while the growth of SS-UHPC slows down.With the increase of steel fiber content,the increase in compressive strength and bending strength of MS-UHPC is relatively small,but its elastic modulus was the highest among the three.The flexural toughness index of EUHPC increases with the increase of steel fiber content and the difference is that the increase of small particle size EUHPC is higher.(5)With the increase of aggregate particle size,the anti-chloride ion permeability of EUHPC decreases,but the EUHPC electric flux of all four particle sizes are less than 100 C,and still maintains excellent resistance to chloride ion permeability.With the increase of aggregate size and content,the dry shrinkage value of EUHPC decreases to varying degrees and the incorporation of crushed stone significantly reduces the shrinkage of EUHPC.The steel fiber and expansion agent can significantly improve the drying shrinkage of EUHPC,and the better the effect with the increase of the dosage.