Interface Effect Of Fiber And Matching With Bulk Matrix In Ultra High Performance Concrete

As the development of civil engineering technology,structures of high rising,long span and special functions spring up,leading to the developments of concrete to be stronger and more durable.These requirements promote the progresses of concrete technology.Amongst them,the inovation of ultra high performance concrete(UHPC)is the most significant one.UHPC is a new cementitious composite showing the advantages of utra high strength,toughness,and durability.Fibers are usual y used for reinforcing and toughening UHPC.Much research in recent years concerned on the mechanism of fiber toughening in cement-based materials,but mostly focousing on crack resistance and crack bridging.Actual y,fibers on one hand can resist crack propagation and bridge cracks,on the other hand could introduce interface defects into composite materials.Because the microstructure of UHPC is very dense,interfacial flaws between fibers and matrix have negative effects on the mechanical performance of UHPC and these effects increase with the matrix strength,also are related to the defect size which is correlation to the fiber size.Hence,there is a competition between the positive and negative effects of fibers on the performance of UHPC,varying with the matrix strength and fiber size,and so on.This il ustrates that the matching between fibers and matrix could be optimized.In this study,both positive effects(reinforcement and toughening)and negative effects(cause of cracks)of fibers with differnent types,shapes,sizes and directions on the UHPC performance were investigated.Based on this,a theory on the matching between fibers and matrices was developed according to the improvement and the corresponding mechanism of the effects of different fibers on the strength and toughness of UHPC.This study provides a theoretical foundation for the design and preparation of UHPC.The main results achieved are as follows:(1)The bonding property between fiber and cement-based matrix in UHPC was studied.Amongst steel fiber,polypropylene fiber and carbon fiber,the bond strength between steel fiber and matrix was the best,arriving at 15.8 MPa.The bonding prorperty between fiber and matrix can be improved by reducing the water to binder ratio,prolonging the curing age,adding the mineral admixture,optimizing the mix proportion,and changing the shape of fiber.Because the bond strength between irregular shape fiber and matrix was large,the fiber was broken more under tension force.The negative effect of matrix strength on fibers was more significant for irregular shaped fibers than that for straight fibers.(2)The microstructure of the interfacical transition zone(ITZ)between fiber and cementbased matrix in UHPC was studied.It was found that more aligned calcium hydroxide(CH)crystals formed in the ITZ than in the bulk paste,also the porosity of the ITZ was increased and the hardness was reduced.The back scattering(BSE)image analysis technique was used to quantify the porosity in the ITZ.The ITZ thickness between fibers and UHPC matrices with different strength grades was quantified by combining the BSE image analysis technique with the microhardness,nano-indentation,and energy dispersive analysis(EDS).It was observed that the ITZ between fiber and high strength UHPC matrix was around 40 ?m,while that between fiber and ultra high strength UHPC matrix decreased to approximate 10 ?m.This il ustrates that the incorporation of fibers brings a new weak zone in UHPC.The microstructure of the ITZ can be improved by reducing the water to binder ratio,adding the mineral admixture,optimizing the proportion of UHPC,and changing the type and property of fibers.With these methods,the weak zone in the ITZ can be reinforced,which could improve the performance of the interface between fiber and matrix.(3)The fracture properties of UHPC and their influencing factors were studied.Amongst steel fiber,polypropylene fiber and carbon fiber,steel fiber showed best improvement effect on the fracture properties of UHPC,while carbon fiber showed less improvement on the fracture properties of UHPC.By adding the same amount,for eample 0.5% by volume,of steel fiber,polypropylene fiber and carbon fiber,the fracture energy of UHPC can be increased by 773.4%,648.1% and 95.5%,respectively.As the addition of fibers was less than 2.0% by volume,the reinforcement effect of fiber can be increased by increasing the fiber amount.The fracture properties can be improved by reducing the water to binder ratio,prolonging the curing age,optimizing particle gradation and proportion of UHPC and hot water curing.However,dry curing at high temperature showed negative effects on the fracture propertes of UHPC.(4)The fiber orientation showed little effect on the strength of UHPC,while exhibited significant effect on the fracture properties of UHPC.One single fiber can increase the fracture energy of matrix by 119.7% and decrease by 59.4%.The interface effect had both positive and negative effects on the fracture properties of cement-based materials,respectively.As the fiber orientation was parallel to the stress direction of matrix,fibers mainly showed crack resistance effect.However,as the fiber orientation was perpendicular to the stess direction of matrix,fibers mainly showed crack initiation effect.The fibers could exhibit excel ent effect of crack resistance and reinforecement on the UHPC on the condition that the fiber number is adequate and the fiber distribution is random and uniformly.Hence,once the fiber orientation is difficult to be controlled,the fibers should be dispersed as evenly as possible.The positive effect of fibers can be enhanced by improving the interfacial bonding between fiber and cement-based matrix,while the negative effect of fibers can be weakened.Consequently,the toughness of UHPC can be improved.(5)The strengthening and toughening effects of different fibers on the UHPC with different strength grades were studied.Fibers with different properties showed different effects.Amongst three types of fibers,i.e.steel fiber,polypropylene fiber and carbon fiber,steel fiber showed best strengthening and toughening effects,wherein the improvement can be as high as 14.2% and 946.3%,respectively.The strengthening effect of polypropylene fiber was poor,while its toughening effect was good,arriving at 495.3%.The strengthening effect of carbon fiber was 12.7%,while its toughening effect was poor,only 37.8%.(6)It was found that the fibers with a large size had a better matching with the low strength grade UHPC,while the fibers with a small size had a better matching with the high strength grade UHPC.To be specific,in the UHPC matrix with a strength of approximate 200 MPa and below,the fibers with a larger aspect ratio showed a greater reinforcement to the UHPC matrix,while in the UHPC matrix with strength of about 270 MPa and above,fibers with a smal er aspect ratio,for example 75.0,show a greater reinforcement to the UHPC matrix.Based on the experimental study,the pulling force of the fiber in UHPC with different strength grades was calculated based on the meso-mechanical model,which was used to obtain the bridging stress of the fiber on the unit fracture surface.In terms of the reduction in fiber toughening effect caused by fiber interface defects,the meso-mechanical model was modified,and used to analyze the matching characteristics between fibers and UHPC matrices with different strength grades.