The Effect Of Strengthing And Toughening Of Steel Fiber On Ultra High Performance Concrete

Ultra high perfor mance concrete(UHPC) is a new type of cement- based mater ial wit h ultra high strength, high toughness and exce llent durability b y adding ultrafine auxiliar y cement it ious materia ls and superplast ic izer to reduce the water- to- binder ratio, decrease the porosit y and increase the densit y o n the pre mise of meet ing the work perfor mance. These excelle nt properties of UHPC will lead to reduct ion in size and se lf- we ight of str uctura l e le ments, and increase in t he ir resistance to earthquake loading a nd seawater attack. However, a lar ge amount of ce ment it ious materia ls and use of specia l c ur ing regime can result in low effic ie ncy o f productio n and high energy consumpt io n. Moreover, the cost is greatly increased wit h the addit ion o f steel fibers. All these restrict the large-scale app licatio n of UHPC in practice.This paper invest igates the effect of s ingle and hybrid steel fibers on static and dyna mic mecha nical propert ies of UHPC through the flowabilit y, compress ive strengt h, fle xura l strength a nd flexura l toughness, uniaxia l co mpression and impact compress ion test. The results are stated as fo llows:Wit h t he increase of steel fibers volume fractio n, the flowability o f U HPC decreases linear ly and the compressive strength increases but the growth reduces while the flexura l strength increases w ith a big d ifference in growth, t he init ia l strengt h and init ia l deflectio n change litt le, but the peak strengt h and peak deflect ion increase cont inuos ly and the toughness increases significa nt ly. When 2% straight- shape, waved or hooked steel fibers added, the compressive strength, init ia l strengt h and init ia l deflectio n of UHPC increase litt le, but the fle x ural strength, peak strengt h and peak deflect ion increase larger and t he toughness increases gradua lly. According to the characterist ics o f load- deflect ion cur ve, the load-deflect ion c urve model of UHPC is proposed, and comb ined wit h t he least square fitt ing met hod, the tested load- deflect ion cur ves of UHPC are fitted and the fitt ing results are well.When the total steel fibers vo lume fractio n rema ins uncha nged at 2%, the fluid ity, compress ive strengt h and fle xura l strengt h of UHPC first increase and then decrease wit h the increase of short fibers vo lume fract ion, whe n 0.5% lo ng fibers and 1.5% short fibers mixed, the flowabilit y is the largest while when 1.5% lo ng fibers and 0.5% short fibers mixed, the compressive strength and flexura l strength are the largest. The effect of hybrid fibers on t he init ia l strength and init ia l deflectio n of UHPC is litt le, but a greater impact on the peak strength and peak deflect ion, w hen 1.5% lo ng fibers and 0.5% short fibers mixed, toughness index η5 is the largest while when only 2% long fibers added, toughness index η10 and η20 are the largest, after that the toughness index decreases gradually w it h the increase of short fibers volume fract ion and the toughness decreases constant ly.The effect of hybr id fibers on the unia xia l co mpression stress- stra in cur ve of UHPC before the peak stress is litt le, but a greater impact on the uniaxia l compress ion stress- strain cur ve a fter the peak stress. When 2% long fibers added, the compress ive toughness index is the largest and the compressio n toughness of UHPC is the best while whe n 1.5% long fibers and 0.5% short fibers mixed, the peak stress, peak strain and elast ic modulus of UHPC are the largest, after that the peak stress, peak strain, elast ic modulus and compressive toughness index of U HPC both decrease constant ly w it h the increase of short fibers volume fract ion and the compress ive toughness decreases gradua lly. The unia xia l compression fa ilure pattern of UHPC may be shear fa ilure or splitt ing fa ilure, the unia xia l compression fa ilure pattern of UHPC wit hout t he incorporation of steel fibers is splitt ing fa ilure, whe n the total steel fibers volume fractio n is 2% and the long fibers volume fractio n is higher than 1%, the uniaxia l co mpression fa ilure pattern is s hear fa ilure w hile w hen the total stee l fibers volume fract ion is 2% and the long fibers vo lume fract ion is less than or equa l to 1%, the uniaxia l compressio n fa ilure pattern is splitt ing fa ilure.When the tota l steel fibers volume fract ion re ma ins unc hanged at 2%, the impact compress ion damage degree of UHPC increases gradually and the fragments gradually become more and sma ller wit h the increase of short fibers volume fract ion. The peak stress and dyna mic increase factors of UHPC both increase gradua lly w ith the increase of strain rate but the enhance ment effect gradua lly weakens. The dyna mic compress ive strength of UHPC wit h hybr id fibers is higher tha n that of UHPC wit h single fibers, whe n the impact ve locit ies are 8.9, 11.7 and 13.9 m/s, the dyna mic compress ive strengt h of UHPC w ith 1.5% long fibers and 0.5% short fibers is the greatest and when the impact velocit ies are 8.9 m/s and 13.9 m/s, the impact compress ion toughness of UHPC wit h 1.5% lo ng fibers and 0.5% short fibers is the greatest, after t hat t he dynamic co mpressive strength and impact co mpression toughness both decrease gradually w it h the increase of short fibers volume fract ion.