Mechanical Behavior Based on Aggregates Microstructure of Ultra-high Performance Concrete

We developed ultra-high performance concrete(UHPC)incorporating mullite sand and brown corundum sand(BCS),and the quartz sand UHPC was utilized to prepare for comparison.The properties of compressive strength,elastic modulus,ultrasonic pulse velocity,flexural strength,and toughness were investigated.Scanning electron microscopy and nanoindentation were also conducted to reveal the underlying mechanisms affecting macroscopic performance.Due to the superior interface bonding properties between mullite sand and matrix,the compressive strength and flexural toughness of UHPC have been significantly improved.Mullite sand and BCS aggregates have higher stiffness than quartz sand,contributing to the excellent elastic modulus exhibited by UHPC.The stiffness and volume of aggregates have a more significant impact on the elastic modulus of UHPC than interface performance,and the latter contributes more to the strength of UHPC.This study will provide a reference for developing UHPC with superior elastic modulus for structural engineering.

The Influence of Chemical Admixtures on the Fluidity,Viscosity and Rheological Properties of Ultra-High Performance Concrete

To achieve higher strength and better durability,ultra-high performance concrete(UHPC)typically employs a relatively small water-binder ratio.However,this generally leads to an undesired increase in the paste viscosity.In this study,the effects of liquid and powder polycarboxylate superplasticizers(PCE)on UHPC are compared and critically discussed.Moreover,the following influential factors are considered:air-entraining agents(AE),slump retaining agents(SA),and defoaming agents(DF)and the resulting flow characteristics,mechanical properties,and hydration properties are evaluated assuming UHPC containing 8‰powder PCE(PCE-based UHPC).It is found that the spread diameter of powder PCE is 5%higher than that of liquid PCE.Among the chemical admixtures studied,AEs have the best effect on improving UHPC workability,while DFs have the worst effect.When the addition of AE and SA is 1.25‰and 14.7%of PCE,paste viscosity reduces by 35%and 19%,respectively compared to the paste with only 8‰PCE.A low AE dosage(1.25‰)decreases compressive strength by 4.1%,while SA(8.1%)increases UHPC compressive strength by 9.1%.Both AE and SA significantly delay the UHPC hydration process,reducing the hydration heat release peaks by 76%and 27%,respectively.

Effect of Shrinkage Reducing Agent and Steel Fiber on the Fluidity and Cracking Performance of Ultra-High Performance Concrete

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.

Design of Eco-friendly Ultra-high Performance Concrete with Supplementary Cementitious Materials and Coarse Aggregate

Aiming to investigate the mix design of eco-friendly UHPC with supplementary cementitious materials and coarser aggregates, we comprehensively studied the workability, microstructure, porosity, compressive strength, flexural strength, and Young’s modulus of UHPC. Relationship between compressive strength and Young’s modulus was obtained eventually. It is found that the compressive strength, flexural strength, and Young’s modulus of UHPC increase by 19.01%, 10.81%, and 5.99%, respectively, when 40 wt% cement is replaced with supplementary cementitious materials. The relationship between compressive strength and Young’s modulus of UHPC is an exponential form.

Experimental research on refractory performance of reinforced concrete beams strengthened with CFRP sheets bonded with an inorganic adhesive

To meet the requirement of fire endurance for concrete structures strengthened with CFRP sheets, this study develops an inorganic adhesive whose strength at 600 ℃ is not lower than that at normal room temperature. The inorganic adhesive is then used to bond CFRP sheets on reinforced concrete beams in order to strengthen them. The fire protection of the CFRP sheets is done using the thick-type fireproofing coatings for tunnel ( TFCT) and steel structure ( TFCSS) respectively. Four specimens are tested in the furnace together. Specimens are exposed to fire for 1. 5 h in according to the ISO834 standard fire curve,and then naturally cooled for 1 h. In the tests,the largest displacements at the mid-span positions of specimens are only from 1 /1400 to 1 /318 of actual span corresponding to the highest temperatures from 300 ℃ to 470 ℃. After the specimens are naturally cooled to the normal temperature and the fireproofing coatings are then removed,it can be seen that the CFRP sheets keep in a good state,which indicates that CFRP sheets can be tightly bonded on the concrete and work well together with the concrete beams during and after fire. Besides,the tests also verify that the fire performance of TFCT is superior to TFCSS for the strengthened beams.

Preparation of Heavyweight Ultra-high Performance Concrete Using Barite Sand and Titanium-rich Heavy Slag Sand

The heavyweight ultra-high performance concrete(HUHPC)was prepared with barite sand partially replaced by titanium-rich heavy slag sand(THS)at replacement proportion of 0%,30%,50%,70%and 100%in this work.The results show that THS incorporation can effectively improve the mechanical properties and reduce the volume shrinkage of HUHPC.The HUHPC with 50%THS replacement reaches an apparent density of 2890 kg/m^(3)(for fresh HUHPC),28 d compressive strength of 129 MPa,28 d flexural strength of 23 MPa,28 d flexural toughness of 28.4,56 d volume shrinkage of 359×10^(-4) and,as expected,excellent durability.Microstructural investigation demonstrates that the internal curing of pre-wetted THS promotes the hydration of the surrounding cement paste thereby strengthening the interfacial transition zone,resulting in the“hard shell”formation around aggregate to“protect”the aggregate.Additionally,the“pin structure”significantly improves the cement paste-aggregate interfacial connection.The combination of“hard shell protection”and“pin structure”remarkably improve the mechanical properties of HUHPC produced with porous THS aggregate.

Dynamic Mechanical Behaviour of Ultra-high Performance Fiber Reinforced Concretes

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.

Influence Mechanism of Curing Regimes on Interfacial Transition Zone of Lightweight Ultra-High Performance Concrete

This study aims to clarify the effects of curing regimes and lightweight aggregate(LWA)on the morphology, width and mechanical properties of the interfacial transition zone(ITZ) of ultra-high performance concrete(UHPC), and provide reference for the selection of lightweight ultra-high performance concrete(L-UHPC) curing regimes and the pre-wetting degree LWA. The results show that, under the three curing regimes(standard curing, steam curing and autoclaved curing), LWA is tightly bound to the matrix without obvious boundaries. ITZ width increases with the water absorption of LWA and decreases with increasing curing temperature. The ITZ microhardness is the highest when water absorption is 3%, and the microhardness value is more stable with the distance from LWA. Steam and autoclaved curing increase ITZ microhardness compared to standard curing. As LWA pre-wetting and curing temperatures increase, the degree of hydration at the ITZ increases, generating high-density CSH(HD CSH) and ultra-high-density CSH(UHD CSH), and reducing unhydrated particles in ITZ. ITZ micro-mechanical properties are optimized due to hydration products being denser.

Different Curing Systems on Mechanical Properties of Ultra-High Performance Concrete with Coarse Aggregate

High durability and high tensile strength makes ultra-high performance concrete( UHPC) an ideal material for bridges,while its early shrinkage in the construction of cast-in-situ mass concrete leads structure crack-easily,which restricts the application of UHPC in deck system. Whether reasonable amount of coarse aggregate can influence the strength of UHPC and improve the shrinkage performance or reduce the cost is still in doubt. Besides,in order to improve its constructability and workability, whether autoclaved curing system of UHPC can be changed remains to be further researched. In response to these circumstances, a systematic experimental study on the strength of UHPC mixed with coarse aggregate in different ratios has been presented in this paper. The three curing systems,namely standard curing,180-200 ℃/1. 1 MPa autoclaved curing,and hot water curing were tested to reveal the relationship between UHPC's properties and curing systems,and the UHPC ' s microstructure was also preliminarily studied by scanning electron microscope( SEM). The experimental research can draw the following conclusions. Under the condition of the same mix ratio, autoclaved curing guarantees the highest compressive strength,followed by hot water curing and standard curing. The compressive strength of concrete increases with the temperature in the range of 25 to 90 ℃ hot water curing,and high temperature in precuring period can speed up the strength development of UHPC,but the sequence of precuring period does not obviously affect the results. In 90 ℃ hot water and autoclaved curing,the strength is over 150 MPa,and it has little relation with gravel ratio. While the value increases first and then decreases in a lower temperature curing with the increasing of gravel amount,even only about 80 MPa at room temperature. The strength increases moderately along with the increase of the curing age by standard curing,especially in the initial stage.

Tests and Simulation Analysis on Fracture Performance of Concrete

To obtain the fracture parameters of concrete, fracture tests were conducted with three-point bending beam method aiming at 30 concrete beams with different sizes and different intensity. The concrete specimen with prefabricated crack to determine the fracture parameters of concrete were conducted and the fracture performance of the specimen was analyzed. The test results show that, initial fracture toughness is unrelated to the size of specimens; while unstable fracture toughness is related to the size of specimens. As for specimens of bastard size, when concrete intensity is relatively low, unstable fracture toughness increases along with the increase of intensity; when concrete intensity is relatively high, unstable fracture toughness will decrease; when concrete intensity increases continuously, unstable fracture toughness will further increase somewhat. As for specimens of standard size, unstable fracture toughness will increase along with the increase of intensity. Aiming at concrete beam specimens, we conducted two-dimensional non-linear finite element analysis, obtained the stress intensity factor, and carried out contrastive analysis with the experimental results.

Experimental Study on the Static and Fatigue Performance of Corroded RC Beams Strengthened with AFRP Sheets

In order to research the strengthening effects of aramid fiber reinforced polymer （AFRP） sheets on the flexural performance of corroded reinforced concrete （RC） beams, the static and fatigue performance of corroded RC beams strengthened with non-prestressed AFRP sheets under different degrees of corrosion （minor： reinforcement mass loss is 2.0%, medium： reinforcement mass loss is 6.0%） have been researched experimentally in this paper, to compare with that of the control beams （un-strengthened and un-corroded） and un-strengthened corroded beams, and additionally compare with fatigue performance of those medium corroded RC beams strengthened with prestressed AFRP sheets with permanent anchors. The results show that, （1） compared with un-strengthened corroded beams under the same degrees of corrosion, the cracking, yield and ultimate monotonic loads of the minor corroded RC beam strengthened with non-prestressed AFRP sheets is respectively increased by 20%, 27% and 60%, whereas for the medium corroded RC beam strengthened with non-prestressed AFRP sheets increased by 15%, 36% and 83% respectively. The ultimate deflection of the medium corroded beam strengthened with non-prestressed AFRP sheets is 166% larger than that of the corroded un-strengthened beam under the monotonic load. （2） The fatigue life of the non-prestressed AFRP strengthened medium corroded beam is 10.4 times more than that of the un-strengthened corroded beam, but lower than that of the unstrengthened-uncorroded （virgin） beam. （3） Fatigue experiments of the beams strengthened with prestressed AFRP sheets and with those non-prestressed AFRP sheets show that the fatigue life of the retrofit RC beams increase with increasing prestress level of AFRP sheets.（4） The use of AFRP sheets for strengthening corroded RC beams is an efficient technique that can maintain the structural integrity and enhance the structural behavior of such beams.

Automatic detection and assessment of crack development in ultra-high performance concrete in the spatial and Fourier domains

Automatic detection and assessment of surface cracks are beneficial for understanding the mechanical performance of ultra-high performance concrete(UHPC).This study detects crack evolution using a novel dynamic mode decomposition(DMD)method.In this method,the sparse matrix‘determined’from images is used to reconstruct the foreground that contains cracks,and the global threshold method is adopted to extract the crack patterns.The application of the DMD method to the three-point bending test demonstrates the efficiency in inspecting cracks with high accuracy.Accordingly,the geometric features,including the area and its projection in two major directions,are evaluated over time.The relationship between the geometric properties of cracks and load-displacement curves of UHPC is discussed.Due to the irregular shape of cracks in the spatial domain,the cracks are then transformed into the Fourier domain to assess their development.Results indicate that crack patterns in the Fourier domain exhibit a distinct concentration around a central position.Moreover,the power spectral density of cracks exhibits an increasing trend over time.The investigation into crack evolution in both the spatial and Fourier domains contributes significantly to elucidating the mechanical behavior of UHPC.

Modifying effects and mechanisms of superfine stainless wires on microstructures and mechanical properties of ultra-high performance seawater sea-sand concrete

Ultra-high-performance seawater sea-sand concrete(UHPSSC)presents a prospective solution to address the natural resource shortage in marine infrastructure construction.To eliminate the corrosion risk of steel fibers and broaden the applicability of UHPSSC,this study investigates the mechanical properties and free chloride ion content as well as microstructures of UHPSSC reinforced with superfine stainless wires(SSWs)under natural curing.The results indicate that 1.5%SSWs can remarkably improve the flexural strength and toughness of UHPSSC by 127%and 1724%,respectively,and mitigate the long-term strength degradation of UHPSSC.The strong interfacial bond between SSW and UHPSSC improves the compactness of UHPSSC,thus reducing the growth space for Ca(OH)_(2) crystals and swelling hydration products generated by sulfate and magnesium ions.This can be supported by the observed reduction in the Ca/Si ratio of C–S–H gels,CH crystal orientation index,and porosity.Moreover,through mechanisms such as pull-out,rupture,overlapping network,and internal anchor interface,SSWs effectively prevent microcrack growth and propagation,transforming single long-connected microcracks into multiple-emission microcracks centered on SSW.Additionally,the free chloride ion content of the composites at 28 and 180 d meets the ACI 318-19 standard requirements for concrete exposed to seawater.This compliance is attributed to the chloride immobilization facilitated by Friedel’s salt and C–S–H gels within the interfaces around SSWs and sea-sand.Consequently,SSWs-reinforced UHPSSC exhibits considerable potential for applications in sustainable marine infrastructures,demanding long-term mechanical properties and high durability.

Experimental testing of RC shear wall seismic retrofit using selective weakening,self-centering and Ultra High performance concrete

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.

A comparative study for the impact performance of shaped charge JET on UHPC targets

With the development of two-stage munitions(a precursor shaped charge(SC)and a following kinetic energy projectile)to attack the hard concrete targets,as well as the increasing applications of ultra-high performance concrete(UHPC)in both civil and military protective structures,a comparative study on the impact performance of SC formed jet on UHPC target is performed experimentally and numerically at present.Firstly,a series of jet penetration/perforation test on the UHPC,45# steel and UHPC/45# steel composite targets are conducted.By assessing the penetration depth and borehole(crater and tunnel)diameter,the influences of target material and configuration as well as the standoff distance of SC on the impact performance of jet are experimentally discussed.Then,by adopting the 2 D multi-material Arbitrary Lagrange-Euler(ALE)algorithm,Fluid-Structure Interaction(FSI)method and erosion algorithm implemented in the finite element code LS-DYNA,the formation and impact performance of jet in the present test are well reproduced.Finally,based on the validated numerical algorithms,constitutive models and the corresponding parameters,the influences of target material(UHPC,NSC and 45# steel),standoff distance,target configuration(stacked and spaced)and weight efficiency on the impact performance of jet are further discussed.The derived conclusions could provide helpful references for evaluating the ballistic performance of jet and designing the protective structures.

A Study on the Flexural Performance of UHPC Precast Deck-Joint Interface by the Exposure of Steel Fiber

As a solution against the serviceability problem caused by the cracks occurring at the UHPC precast deck-joint interface, this study proposes a method exposing the steel fiber at the interface and evaluates the corresponding flexural performance of the lap spliced construction joint. After having slowed down the strength development of the concrete placed in the joint of the precast deck by means of a curing retardant, the concrete at the interface is crushed so as to expose the steel fibers and the change in the flexural performance is observed experimentally according to the exposure of the steel fibers. The results show that, even if the ultimate strength and stiffness of the UHPC precast deck including the joint are mostly determined by the arrangement details of the rebar lap splice, the exposure of the steel fibers can secure stable ductile behavior and reduce the width of the cracks generated at the precast deck-joint interface under service load.

Behavior and strength of headed stud shear connectors in ultra-high performance concrete of composite bridges

This study presents an experimental and numerical investigation on the static behavior of headed stud shear connectors in ultra-high performance concrete (UHPC) of composite bridges. Four push-out specimens were tested. It was found that no cracking, crushing or splitting was observed on the concrete slab, indicating that UHPC slab exhibited good performance and could resist the high force transferred from the headed studs. The numerical and experimental results indicated that the shear capacity is supposed to be composed of two parts stud shank shear contribution and concrete wedge block shear contribution. The stiffness increment of a stud in UHPC was at least 60% higher than that in normal strength concrete. Even if the stud height was reduced from 6d to 2d, there was no reduction in the shear strength of a stud. Short stud shear connectors with an aspect ratio as small as 2 could develop full strength in UHPC slabs. An empirical load-slip equation taking into account stud diameter was proposed to predict the load-slip response of a stud. The reliability and accuracy of the proposed load-slip equation was verified by the experimental and numerical load-slip curves.

Analysis of Compressive Strength Development of Ultra-high Performance Concrete

A numerical procedure was presented for evaluating the compressive strength development of ultra-high performance concrete(UHPC) with cement-silica fume-slag binder.This numerical procedure started with initial packing behavior of designed UHPC using a random sequential packing method.Furthermore,synergistic effect of combined mineral admixtures was addressed with respect to hydration heat.Accordingly,hydration degree of cement and reaction degrees of mineral admixtures were determined based on a blended cement hydration model.Finally,a compressive strength evolution model was proposed and the evolution of compressive strength of three mixes with different binder recipes was compared.The results showed that the both initial packing behavior of UHPC mixes and synergistic effect of mineral admixtures are critical for predicting the properties of UHPC.A remarkable void fraction of 0.2042 was observed for UHPC mix designed by optimization algorithms under random packing.Furthermore,a negative synergistic effect of the combination of silica fume and slag was obtained with regarding to compressive strength.In addition,for a given mineral admixture replacement of 20%,the formulation of mineral admixture should be carefully tailored where the UHPC incorporating 5%silica fume and 15%slag shows the highest compressive strength.

部分包覆钢-混凝土组合梁柱节点抗震性能试验研究

为研究不同连接构造的部分包覆钢-混凝土组合梁柱节点(PEC梁柱节点)的抗震性能,对2个PEC梁柱节点试件进行了拟静力加载试验,研究了低周往复荷载作用下PEC梁柱节点试件的破坏现象、滞回曲线、骨架曲线、延性、耗能能力和刚度退化等抗震性能。结果表明:强轴连接PEC梁柱节点的滞回曲线呈梭形和弓形,在达到极限承载力后仍能保持一定的延性和耗能能力;弱轴连接PEC梁柱节点牛腿与梁间的焊缝处发生破坏,未展现出预期的耗能能力,PEC梁仍在弹塑性状态,没有达到极限状态;PEC梁柱节点核心区混凝土替换为加劲肋板后,试件仍具有较好的承载力、延性和耗能能力,刚度退化规律无明显变化,且强轴连接节点与弱轴连接节点刚度变化规律基本一致;PEC柱牛腿设计过短会导致焊缝连接处断裂,试件延性和耗能能力得不到发挥,剩余刚度较大。

不同类型钢纤维UHPC无腹筋梁受剪性能试验

基于单点加载下4根超高性能混凝土无腹筋梁构件受剪破坏试验结果,研究其受剪性能,以钢纤维类型为主要研究参数,分析钢纤维类型对无腹筋梁的受剪承载力、裂缝发展形态、挠度和韧性等发展规律。结果表明:端钩型钢纤维对提高超高性能混凝土无腹筋梁的受剪承载力、阻滞裂缝发展及控制试件变形能力作用优于光滑平直型和波浪型钢纤维,而端钩型Ⅱ钢纤维优于端钩型Ⅰ钢纤维;端钩型Ⅰ和光滑平直型钢纤维增韧效果较好,但分担纵筋拉力较少;波浪型钢纤维分担纵筋拉力较多,提高超高性能混凝土无腹筋梁抗裂性的效果较好,但增韧效果较差。经对比分析,陈璇公式计算值精确度较高,离散程度小。