Due to the low water-cement ratio of ultra-high-performance concrete(UHPC),fluidity and shrinkage cracking are key aspects determining the performance and durability of this type of concrete.In this study,the effects ...Due to the low water-cement ratio of ultra-high-performance concrete(UHPC),fluidity and shrinkage cracking are key aspects determining the performance and durability of this type of concrete.In this study,the effects of different types of cementitious materials,chemical shrinkage-reducing agents(SRA)and steel fiber(SF)were assessed.Compared with M2-UHPC and M3-UHPC,M1-UHPC was found to have better fluidity and shrinkage cracking performance.Moreover,different SRA incorporation methods,dosage and different SF types and aspect ratios were implemented.The incorporation of SRA and SF led to a decrease in the fluidity of UHPC.SRA internal content of 1%(NSRA-1%),SRA external content of 1%(WSRA-1%),STS-0.22 and STE-0.7 decreased the fluidity of UHPC by 3.3%,8.3%,9.2%and 25%,respectively.However,SRA and SF improved the UHPC shrinkage cracking performance.NSRA-1%and STE-0.7 reduced the shrinkage value of UHPC by 40%and 60%,respectively,and increased the crack resistance by 338%and 175%,respectively.In addition,the addition of SF was observed to make the microstructure of UHPC more compact,and the compressive strength and flexural strength of 28 d were increased by 26.9%and 19.9%,respectively.展开更多
Ultra-high performance fiber reinforced concretes （UHPFRC） were prepared by replacing 60% of cement with ultra-fine industrial waste powder. The dynamic mechanical behaviour of UHPFRC with different fiber volume fra...Ultra-high performance fiber reinforced concretes （UHPFRC） were prepared by replacing 60% of cement with ultra-fine industrial waste powder. The dynamic mechanical behaviour of UHPFRC with different fiber volume fraction was researched on repeated compressive impact in four kinds of impact modes through split Hopkinson pressure bar （SHPB）. The experimental results show that the peak stress and elastic modulus decrease and the strain rate and peak strain increase gradually with the increasing of impact times. The initial material damage increases and the peak stress of the specimen decreases from the second impact with the increasing of the initial incident wave. Standard strength on repeated impact is defined to compare the ability of resistance against repeated impact among different materials. The rate of reduction of standard strength is decreased by fiber reinforcement under repeated impact. The material damage is reduced and the ability of repeated impact resistance of UHPFRC is improved with the increasing of fiber volume fraction.展开更多
This paper explores the shrinkage of reinforced UHPC under high-temperature steam curing and natural curing conditions.The results are compared with the existing shrinkage prediction models.The results show that the m...This paper explores the shrinkage of reinforced UHPC under high-temperature steam curing and natural curing conditions.The results are compared with the existing shrinkage prediction models.The results show that the maximum shrinkage strain of reinforced UHPC after steam curing is 164μεand gradually becomes zero.As for natural curing,the maximum shrinkage strain is 173μεand the value stabilizes on the 10th day after pouring.This indicated that steam curing can significantly reduce shrinkage time.Compared with the plain UHPC tested in the previous literature,the structural reinforcement can significantly inhibit the UHPC shrinkage and greatly reduce the risk of cracking due to shrinkage.By comparing the results in this paper with the existing models for predicting the shrinkage strain development,it is found that the formula recommended in the French UHPC structural and technical specification is suitable for the shrinkage curve in the present paper.展开更多
Traditional retrofit methods often focus on increasing the structure’s strength,stiffness,or both.This may in-crease seismic demand on the structure and could lead to irreparable damage during a seismic event.This pa...Traditional retrofit methods often focus on increasing the structure’s strength,stiffness,or both.This may in-crease seismic demand on the structure and could lead to irreparable damage during a seismic event.This paper presents a retrofit method,integrating concepts of selective weakening and self-centering(rocking)to achieve low seismic damage for non-code compliant reinforced concrete shear walls.The proposed method involves con-verting traditional cast-in-place concrete shear walls into rocking walls,which helps to lower the shear demand,while allowing re-centering.Two large-scale lateral load tests were performed to validate the retrofit concept on a concrete shear wall designed according to pre-1970s standards.The design parameters investigated were amount of energy dissipating reinforcements and confinement enhancement.Two different methods using Ultra High Performance Concrete(UHPC)were investigated to provide additional confinement to boundary elements of older shear walls.Observations from the tests showed minimized damage and enhanced recentering in the retrofitted wall specimens.Use of UHPC in the boundary elements of the retrofitted walls provided additional confinement and reduced damage in the rocking corners.展开更多
Ultra-high performance concrete(UHPC) developed rapidly in research and commercial use during the recent decade. Significant progress has been achieved in its material science and technology, including why and how to ...Ultra-high performance concrete(UHPC) developed rapidly in research and commercial use during the recent decade. Significant progress has been achieved in its material science and technology, including why and how to add discontinuous fiber reinforcement in it.This paper reviews the researches on understanding the effects of various fibers on the mechanical properties of UHPC, focus on the straight steel fibers but involving also deformed steel fibers, non-steel fibers as well as hybrid fibers. It also discusses the research methodology, prediction of mechanical properties by fiber factors, and the classification of UHPC mechanical properties related to this topic. It shows that(1) the experimental research is the main methodology for investigating the effect of the fibers on the mechanical properties of UHPC;the tensile performance of UHPC should be studied by uniaxial tensile tests and its representative indicators should include tensile strength, initial cracking strength, and peak tensile strain;(2) fiber plays an essential role in the reinforcement of the tensile strength, compressive strength, modulus of elasticity, and other material properties of UHPC, but in weakening the flowability of fresh UHPC. The positive and negative effects of fibers on the mechanical properties of UHPC should be considered,and the technology should be developed to maintain the flowability when high volume fraction of fibers is added in the UHPC;(3) the parameters of steel fibers affecting the mechanical properties of UHPC include volume fraction, size, shape, orientation and distribution, average bonding strength and minimum tensile strength, etc., which are mainly studied independently in the existing research. The studies on the combined effect of these parameters are limited but worthy of further investigation;(4) hybrid fibers could efficiently produce reinforcement effects for UHPC. It has great practical and research significance to conduct in-depth studies though the theoretical analysis and quantitative prediction are complex.展开更多
基金the Key Research and Development Program of Hubei Province(2022BCA082 and 2022BCA077).
文摘Due to the low water-cement ratio of ultra-high-performance concrete(UHPC),fluidity and shrinkage cracking are key aspects determining the performance and durability of this type of concrete.In this study,the effects of different types of cementitious materials,chemical shrinkage-reducing agents(SRA)and steel fiber(SF)were assessed.Compared with M2-UHPC and M3-UHPC,M1-UHPC was found to have better fluidity and shrinkage cracking performance.Moreover,different SRA incorporation methods,dosage and different SF types and aspect ratios were implemented.The incorporation of SRA and SF led to a decrease in the fluidity of UHPC.SRA internal content of 1%(NSRA-1%),SRA external content of 1%(WSRA-1%),STS-0.22 and STE-0.7 decreased the fluidity of UHPC by 3.3%,8.3%,9.2%and 25%,respectively.However,SRA and SF improved the UHPC shrinkage cracking performance.NSRA-1%and STE-0.7 reduced the shrinkage value of UHPC by 40%and 60%,respectively,and increased the crack resistance by 338%and 175%,respectively.In addition,the addition of SF was observed to make the microstructure of UHPC more compact,and the compressive strength and flexural strength of 28 d were increased by 26.9%and 19.9%,respectively.
基金Funded by the National Natural Science Foundation of China (No. 50808101)Jiangsu Provincial Program for Basic Research (Natural Science Foundation) (No.BK2008417)China Postdoctoral Science Foundation (No. 20080431100)
文摘Ultra-high performance fiber reinforced concretes （UHPFRC） were prepared by replacing 60% of cement with ultra-fine industrial waste powder. The dynamic mechanical behaviour of UHPFRC with different fiber volume fraction was researched on repeated compressive impact in four kinds of impact modes through split Hopkinson pressure bar （SHPB）. The experimental results show that the peak stress and elastic modulus decrease and the strain rate and peak strain increase gradually with the increasing of impact times. The initial material damage increases and the peak stress of the specimen decreases from the second impact with the increasing of the initial incident wave. Standard strength on repeated impact is defined to compare the ability of resistance against repeated impact among different materials. The rate of reduction of standard strength is decreased by fiber reinforcement under repeated impact. The material damage is reduced and the ability of repeated impact resistance of UHPFRC is improved with the increasing of fiber volume fraction.
基金financial support received from the National Natural Science Foundation of China(No.52108211)Hunan Provincial Department of Education(No.21B0188)+1 种基金Natural Science Foundation of Hunan Province(No.2022JJ40186)Water Resources Science and Technology Program of Hunan Province(No.XSKJ2023059-44).
文摘This paper explores the shrinkage of reinforced UHPC under high-temperature steam curing and natural curing conditions.The results are compared with the existing shrinkage prediction models.The results show that the maximum shrinkage strain of reinforced UHPC after steam curing is 164μεand gradually becomes zero.As for natural curing,the maximum shrinkage strain is 173μεand the value stabilizes on the 10th day after pouring.This indicated that steam curing can significantly reduce shrinkage time.Compared with the plain UHPC tested in the previous literature,the structural reinforcement can significantly inhibit the UHPC shrinkage and greatly reduce the risk of cracking due to shrinkage.By comparing the results in this paper with the existing models for predicting the shrinkage strain development,it is found that the formula recommended in the French UHPC structural and technical specification is suitable for the shrinkage curve in the present paper.
基金study described in this paper was made possible by a grant from the National Science Foundation’s Engineering for Natural Hazards(ENH)program,Grant#1662963.
文摘Traditional retrofit methods often focus on increasing the structure’s strength,stiffness,or both.This may in-crease seismic demand on the structure and could lead to irreparable damage during a seismic event.This paper presents a retrofit method,integrating concepts of selective weakening and self-centering(rocking)to achieve low seismic damage for non-code compliant reinforced concrete shear walls.The proposed method involves con-verting traditional cast-in-place concrete shear walls into rocking walls,which helps to lower the shear demand,while allowing re-centering.Two large-scale lateral load tests were performed to validate the retrofit concept on a concrete shear wall designed according to pre-1970s standards.The design parameters investigated were amount of energy dissipating reinforcements and confinement enhancement.Two different methods using Ultra High Performance Concrete(UHPC)were investigated to provide additional confinement to boundary elements of older shear walls.Observations from the tests showed minimized damage and enhanced recentering in the retrofitted wall specimens.Use of UHPC in the boundary elements of the retrofitted walls provided additional confinement and reduced damage in the rocking corners.
基金supported by the National Key Research and Development Program(2018YFC0705400)the National Natural Science Foundation of China(51878178)。
文摘Ultra-high performance concrete(UHPC) developed rapidly in research and commercial use during the recent decade. Significant progress has been achieved in its material science and technology, including why and how to add discontinuous fiber reinforcement in it.This paper reviews the researches on understanding the effects of various fibers on the mechanical properties of UHPC, focus on the straight steel fibers but involving also deformed steel fibers, non-steel fibers as well as hybrid fibers. It also discusses the research methodology, prediction of mechanical properties by fiber factors, and the classification of UHPC mechanical properties related to this topic. It shows that(1) the experimental research is the main methodology for investigating the effect of the fibers on the mechanical properties of UHPC;the tensile performance of UHPC should be studied by uniaxial tensile tests and its representative indicators should include tensile strength, initial cracking strength, and peak tensile strain;(2) fiber plays an essential role in the reinforcement of the tensile strength, compressive strength, modulus of elasticity, and other material properties of UHPC, but in weakening the flowability of fresh UHPC. The positive and negative effects of fibers on the mechanical properties of UHPC should be considered,and the technology should be developed to maintain the flowability when high volume fraction of fibers is added in the UHPC;(3) the parameters of steel fibers affecting the mechanical properties of UHPC include volume fraction, size, shape, orientation and distribution, average bonding strength and minimum tensile strength, etc., which are mainly studied independently in the existing research. The studies on the combined effect of these parameters are limited but worthy of further investigation;(4) hybrid fibers could efficiently produce reinforcement effects for UHPC. It has great practical and research significance to conduct in-depth studies though the theoretical analysis and quantitative prediction are complex.
