Ultra-high Performance Concrete Members Research Articles

Comparative study on flexural performance of ultra‐high performance concrete beams reinforced with steel rebar and steel plate

AbstractTo enhance the mechanical and constructional performance of the ultra‐high performance concrete (UHPC) beams, the steel plate placed at the bottom surface of the UHPC beam is utilized to replace the ordinary steel rebars. In this paper, four 3.2 m UHPC T‐shaped beams with different reinforcement patterns of ordinary steel rebars and external steel plate were fabricated and comparatively tested under flexure loading. Their damage patterns, load versus deflection behavior, flexural capacity, load versus strain behavior, moment versus curvature behavior, stiffness, and crack development were investigated. The flexural experiments indicated that the external steel plate, positioned at the bottom surface of the UHPC members, could resolve the difficulty of installing ordinary steel rebars in slender UHPC components and improve its constructional performance. Compared with the inside steel rebar reinforcement, the employment of the steel plate at the bottom surface of the UHPC beam can effectively increase the distance from the neutral axis to the tensile reinforcement at the serviceability and ultimate states, thereby improving its flexural capacity and stiffness. Additionally, the configuration of the external steel plate was beneficial to reduce the tensile stress level of the tensile reinforcement and limit the opening of UHPC crack width, and thus their crack resistance can be effectively enhanced. Moreover, increasing the thickness of the steel plate or rebar ratio can also significantly improve the flexural capacity, stiffness, and cracking resistance of UHPC beams.

Continuous surface cap model for ultra-high-performance concrete under static and low strain rate loadings: Calibration, experiment, and simulation

The ultra-high compressive strength and ductile tensile behavior of ultra-high-performance concrete (UHPC) make it promising in the application of civil and crashworthy facilities. Although numerous experimental studies have been conducted to investigate the performance of UHPC, studies on the methods for simulating the behavior of UHPC are few. In this paper, an overview of widely used constitutive models of concrete-like materials in hydrocode was presented first, and then a comprehensive calibration of the continuous cap surface (CSC) model was performed to simulate the mechanical behavior of UHPC under static and low strain rate loadings. Four groups of parameters: (1) failure surface parameters; (2) cap and hardening parameters; (c) damage parameters; and (4) strain rate parameters were systematically calibrated via extensive available material test data of UHPC with uniaxial compressive strength ranging from 101 MPa to 238 MPa. A calculation method was proposed to address the inconsistency of existing conclusions regarding the fracture energy of UHPC, which significantly affects the modeling of UHPC damage. Moreover, a method was proposed to convert the engineering uniaxial tensile strain of UHPC to the true uniaxial tensile strain, and so does the tensile fracture energy of UHPC. To validate the accuracy of the calibrated CSC model in simulating the behavior of UHPC under static loading, uniaxial tension and compression tests of UHPC samples and three-point bending tests of steel-UHPC-steel composite beams were conducted and simulated. Existing triaxial compression tests of UHPC cylinders were simulated to validate the accuracy of the calibrated CSC under confinement. Moreover, low-velocity impact tests of UHPC columns were simulated to further validate the reliability of the calibrated CSC model to capture the structural behavior of UHPC members under low strain rate loading. It was found that the calibrated CSC model showed excellence in simulating the material properties of UHPC and the structural behavior of UHPC members under static and low strain rate loadings.

Experimental assessment on cyclic flexural behavior of UHPC beam reinforced with negative Poisson’s ratio (NPR) steel rebar

The objective of this study is to experimentally explore the implication of an innovative steel rebar with negative Poisson’s ratio effect (termed as NPR steel rebar) on ultra high performance concrete (UHPC) beam under cyclic loading. Distinctly from conventional steel rebar, NPR steel rebar employed in this research inherently owns high ultimate tensile strength of approximately 1100 MPa, high uniform elongation of around 40% and superior durability. Four reinforced UHPC beams are conducted and tested under monotonic/cyclic loading through the four-point bending method. The structural characteristics of all specimens are discussed via failure mode, hysteretic curve, envelop response, stiffness degradation, energy-dissipating capacity, and steel strain evolution and etc. The results show that reinforced UHPC beam longitudinally configured with NPR steel rebar is available to exhibit satisfactory and superior monotonic/cyclic flexural performance. Firstly, the final failure mode of reinforced UHPC beam with NPR steel rebar exhibits multiple (7–8) closely-spacing localized cracks distributing within the pure bending section whether under monotonic or cyclic loading. Additionally, the superior mechanical performance of NPR steel rebar endows reinforced UHPC beam with high flexural capacity, high deformability, excellent stiffness degradation control, and outstanding energy dissipation capacity under cyclic loading. In which, the flexural capacity of UHPC beams reinforced with NPR steel rebar have been upgraded approximately by 47.8%, 52.3% under monotonic and cyclic loading respectively, compared with that of UHPC beams reinforced with HRB 400 steel rebar, meanwhile, the comparison for the energy dissipation capacity is emphasized in stages. Finally, the deflection prediction method for UHPC beam reinforced with NPR steel rebar at normal service stage under monotonic/cyclic loading is proposed. The results of this study provide valuable information for attaining a preliminarily comprehensive understanding of the structural behavior of reinforced UHPC members with NPR steel rebar under monotonic/cyclic loading, and the proposed combination of UHPC and NPR steel rebar provides a new idea for seismic toughness design of anti-bending structural components as well.

Experimental and numerical study on shear behavior of shear pockets between ultra-high-performance and normal concrete for precast girder bridges

Ultra-high-performance concrete (UHPC) has been widely utilized in precast bridge girders as a new material to reduce structural self-weight and improve constructional efficiency. Typically, UHPC and normal concrete (NC) are implemented to construct precast girders and deck, respectively, and cast-in-place shear pockets are used to connect these two. However, these shear pockets are vulnerable components that affect the integrity of UHPC-NC bridge superstructure. Thus, to comprehensively and deeply investigate the shear performance of shear pockets, a push-off test on block specimens and a bending test on a scaled bridge superstructure are conducted in this study. In addition, A refined finite element model (FEM) is proposed for UHPC-NC interface, which incorporates the effects of cohesion, dowel action of reinforcement and frictional force. The accuracy of the proposed FEM is validated against experimental data. Besides, the code equations are evaluated by push-off test results. The push-off test results verify that reinforcement across the interface plays a critical role in enhancing the shear strength between UHPC and NC, and NC is a weak region for the UHPC-NC interface, which should be paid much attention when designing. The experimental and numerical results of the bending specimen indicate that NC deck and UHPC girder connected by shear pockets with shear reinforcement can satisfy the performance requirements of a bridge girder, showing satisfactory integrity for UHPC and NC members under flexure. Furthermore, design equations specified in ACI318R-19, AASHTO, MC-10 and Eurocode 4 can provide a satisfactory prediction of shear strength at UHPC-NC interface, as demonstrated by comparing with experimental results.

Fire Performance of Fiber-reinforced Ultra-High-Performance Concrete: A state-of-the-art review

Explosive spalling is a major problem for structures made of ultra-high-performance concrete (UHPC) after being exposed to fire, which hinders their practical applications. To overcome this obstacle, fibers have become an essential constituent of UHPC in past decades. This paper summarizes a state-of-the-art review related to the fire performance of UHPC using single and hybrid fibers. The spalling behavior, spalling mechanism, mechanical properties, and microstructure characteristics of fiber-reinforced UHPC are summarized, and structural performance of fiber-reinforced UHPC members at elevated temperatures is discussed. This review reveals that fire-induced spalling is a significant concern in UHPC, and it can be caused by various mechanisms. The addition of fibers can play a crucial role in preventing spalling, and different mechanisms for different types of fibers have been reported in the literature for UHPC with both single and hybrid fibers. Therefore, fiber hybridization is recommended to enhance the spalling resistance of UHPC. The future directions and challenges for UHPC at both the material and structural levels are discussed.

Experimental and numerical study on tensile performance of GFRP bar reinforced UHPC members

Glass fiber reinforced polymer (GFRP) bar reinforced ultra-high performance concrete (UHPC) (GFRP-UHPC) members’ tensile performance should be significantly different from that of steel-UHPC members due to differences between GFRP and steel bars in mechanical and bond properties. In this paper, six dog-bone shaped specimens with different types of reinforcement materials, rebar diameters, and quantities of rebars were fabricated and tested under axial tension, and their failure modes, load-displacement curves, and cracking behavior were recorded and investigated. Furthermore, an extensive nonlinear finite element analysis considering the bond behavior between rebars and UHPC was conducted. Finally, formulas were proposed to predict the tensile stiffness and ultimate tensile capacity of GFRP-UHPC members. Results confirmed that bond shear strength (τcs) and stiffness (K) of the GFRP and UHPC interface were the key factors that affected GFRP-UHPC members’ tensile performance: τcs mainly determined the failure mode; increasing K and the reinforcement ratio can improve the post-cracking stiffness and obtain tension hardening behavior; the wedging effect caused the post-cracking curve of the tension member in bond failure mode re-ascend periodically. The proposed formulas could produce accurate and safe predictions after comparing predicted results with experimental and simulation results. This study provides the underlying insights needed to guide the design of GFRP-UHPC members.

Ductility evaluation and flexural failure mode recognition of reinforced Ultra-High performance concrete flexural members

Although ultra-high performance concrete (UHPC) exhibits material tensile ductility far in excess of that expected from conventional or high-strength concretes, structural UHPC members typically fail in flexure due to reinforcement rupture after crack localization, which may result in a low structural ductility. In the flexural design of reinforced UHPC (R-UHPC) members, attention should be paid to addressing the issue related to the low structural ductility due to crack localization. This study firstly develops a two-dimensional numerical model to simulate the flexural response of R-UHPC beams, and the numerical results agree well with the experimental results of the load–deflection curves as well as the failure modes. Parametric analyses are then carried out to examine the effect of the flexural reinforcement ratio, tensile strength of steel reinforcement, tensile behavior of UHPC and compressive strength of UHPC on the flexural failure mode and ductility behavior of R-UHPC members. The results indicate that when the failure is controlled by fracture of tensile steel bars after crack localization, higher reinforcing ratio, higher steel post-yield hardening strength, and lower steel fiber volume lead to more ductile behavior. When the failure is controlled by crushing of UHPC, however, higher reinforcing ratio and lower UHPC compressive strength lead to a lower ductility. In addition, a flexural failure mode recognition method is proposed for R-UHPC flexural members, whose accuracy is verified based on the available experimental results of 105 R-UHPC flexural members. Finally, a minimum flexural reinforcement ratio is proposed to avoid low structural ductility in R-UHPC flexural members.

Flexural Tensile Behavior of Interface between Precast and Cast-in-Place UHPC Members Based on Four-Point Bending Test

This paper proposed a new method of exposing fibers using expandable polyethylene (EPE) foam to improve the interface performance. The flexural tensile behavior of the interface between precast steam-cured and cast-in-place non-steam-cured ultra-high performance concrete (UHPC) members was examined under four-point bending. The improvement effects of four non-destructive interface treatments—e.g., coating with epoxy glue and coating with a cement-based interface agent—were experimentally studied for comparison. The load-deflection curves of the specimens were measured, and the flexural tensile behavior of the interfaces was analyzed, including the failure mode, flexural tensile strength, ductility, fracture energy, and residual strength. The results indicate that the flexural tensile strength of an interface without any treatment is low and that coating with the cement epoxy glue or the cement-based interface agent has a limited effect on improving the flexural tensile strength. As a result of the bridging effect, the exposed steel fibers can transfer the tensile force across the interface. Therefore, exposing fibers using EPE foam can significantly improve the bond behavior of an interface. This enhancement effect depends on the number of exposed steel fibers and can be simulated based on the pullout load–slip relationship of the steel fiber embedded in the UHPC matrix. The proposed numerical model can be used to predict the bond behavior of an interface with the fiber-exposing treatment.

Experimental and theoretical study on bond behavior of helically wound FRP bars with different rib geometry embedded in ultra-high-performance concrete

Fiber reinforced polymer (FRP) bar reinforced ultra-high-performance concrete (UHPC) members have attracted wide interest due to their excellent durability and mechanical properties. Determining bond strength is of great importance to the design of FRP bar reinforced UHPC members. This study investigates the effects of bar type, concrete type and rib geometry on the bond behavior of helically wound FRP bars embedded within UHPC. Pull-out tests on a total of 48 specimens with three types of concrete (0%, 1% and 2% steel fiber content), three types of bar (steel, basalt FRP and glass FRP) and three types of surface condition (smooth, shallow ribbed and deep ribbed) have been conducted. The results show that the bond strength of FRP bars embedded within UHPC is greatly affected by rib height. Indeed, FRP bars with deep ribs can reach a bond strength comparable to that of ribbed steel bars. A novel bond strength model for FRP bars embedded within UHPC, considering the effect of rib geometry, has been proposed and validated by the experimental results and existing data in the literature.

TDOF model for UHPC members under lateral low-velocity impact

The dynamic behaviors of ultra-high performance concrete (UHPC) members under lateral low-velocity impact loading have been intensively studied by experiment and numerical simulation, while the corresponding structural design approach is still limited and mainly concerned in this study. Firstly, through comprehensively considering the compression arch action, strain rate effect, and unloading stiffness, the dynamic resistance function of UHPC members, e.g., reinforced UHPC (R-UHPC) and UHPC-filled steel tube (UHPC-FST) beams/columns, are built and verified. Then, a two degrees of freedom (TDOF) model of the drop hammer test on UHPC members is improved by considering the P-δ effect and stress wave propagation effect. Furthermore, the impact force- and mid-span deflection-time histories of the existing five series of drop hammer tests on UHPC members are applied to verify the validity and applicability of the established TDOF model. The influence of considering the stress wave propagation effect, strain rate effect, and steel fibers in UHPC for predictions of the TDOF model are discussed by parametric analyses. Finally, based on the energy balance, the explicit formulae are proposed to predict the maximum and residual mid-span deflections of the flexural critical UHPC members under the lateral low-velocity impact, which can be helpful for the design of UHPC structures under lateral impact loadings.

Shear strength of post-installed connectors in thin UHPC precast walls

Ultra-high-performance concrete (UHPC) is seeing increased application in the precast industry, as in insulated double wythe precast concrete (DWPC) panels for examples, where very thin profiles can be produced. This pauses a challenge with regard to connections, where conventional headed studs are not suitable. This paper presents an experimental campaign of 143 shear strength tests investigating the behavior of screw anchors and a newly designed threaded rod connector embedded in thin UHPC members. Parameters examined were slab thickness (tc), connector diameter (d), fiber percentage (pf) in UHPC, edge distance (c1), and support span length (ls). All specimens failed in shear concrete breakout with instances of splitting cracks, varying failure cone angles, and spalling. The threaded rod demonstrated better structural performance with ultimate shear loads (Vu) 29–93 % greater than the screw anchor. Increasing c1 had the most significant effect on Vu, where it was over two times greater when c1 was increased from 38 to 89 mm. Increasing tc from 25 to 38 mm resulted in Vu increasing by 24–43 % for the screw anchor. Meanwhile for the threaded rod, Vu increased by 1.3–3.3 times when tc increased from 15 to 25 mm. Results showed that d, pf, and ls did not have significant effect on Vu. Two existing models were evaluated and were found to overestimate the shear capacity of the connectors. A regression analysis was performed on the data and two design equations were developed to predict the shear capacity of each connector with a high degree of accuracy. A design case study is also presented.

One-dimensional fiber beam-column model and two-dimensional fixed crack model for numerical simulation of UHPC structures

Ultra-high performance concrete (UHPC), with much higher compressive strength, tensile strength and ultimate tensile strain than normal concrete, has been experimentally investigated and applied in engineering structures more and more widely. However, there are few numerical models appropriate for the accurate and efficient simulation of UHPC or reinforced UHPC (R/UHPC) flexural members including beams and columns as well as shear-critical members such as shear walls till now, which brings obstacles to the analysis and design of UHPC structures. In this study, the uniaxial skeleton and hysteretic rules for UHPC under tensile and compressive loads are first proposed and verified by material tests. The formulas to calculate all the basic parameters in the model are put forward for convenient use. Then the fiber beam-column model for flexure-dominated UHPC members is established in ABAQUS 2017 software and six R/UHPC column specimens subjected to cyclic load are simulated. Based on simulation results, the equations of transition curves in the uniaxial rules are determined, which have significant influence on the predicted hysteresis curves. On the other hand, a new two-dimensional fixed crack based constitutive model for shear-critical UHPC members is further proposed and implemented in ABAQUS 2017 software by considering the compressive softening due to principal tensile strain and shear stress–strain relationship along cracks of UHPC. Finally, ten R/UHPC shear wall specimens under cyclic loads as well as four R/UHPC deep beams under monotonic loads are simulated and the accuracy of the proposed 2D model for UHPC is validated.

Novel constitutive model of UHPC under impact and blast loadings considering compaction of shear dilation

Based on the framework of elastic-plastic-hydrocode concrete model, this study established a novel constitutive model of ultra-high performance concrete (UHPC) for impact and blast loadings considering the compaction of shear dilation. First, the established UHPC model is introduced, including five parts: (i) the dynamic failure strength surface, which is divided into tensile, compressive, and tensile-to-compressive regions and considers the Lode-dependence and strain rate effect; (ii) equation of state (EOS) that accounts for the nonlinear volumetric compaction; (iii) damage of strength, which contains independently described tensile and compressive strength hardening or softening formulae; (iv) damage of stiffness for depicting the compaction of shear dilation; (v) plastic flow rule, where the radial return algorithm is adopted for updating stress and an independent plastic potential function is employed to model the shear dilation. Then, parameters of the established UHPC model are systematically calibrated according to tests and empirical suggestions, and are further validated by conducting single element tests under varying loading paths. Through simulating the tests of reinforced UHPC beams/columns and UHPC filled steel tubes under drop hammer impact and reinforced UHPC panels under field explosion, it demonstrates that both the damage patterns and deflection-time histories of UHPC members are precisely reproduced by the established UHPC model. Furthermore, based on comparative simulations, it shows the compaction of shear dilation is critical for predicting the structural residual deflection of reinforced UHPC members. This study discloses a possibility to establish a constitutive model of concrete suitable for impact and blast scenarios with a wide range of strain rate and hydrostatic pressure.

Dynamic behavior of UHPC member under lateral low-velocity impact: Mesoscale analysis

Ultra-High Performance Concrete (UHPC), which mainly consists of the cementitious matrix and incorporated fibers, has superior impact-resistance compared with the normal strength concrete. Concerning the impact-resistant evaluation and performance-based design of UHPC structure under lateral low-velocity impact, a mesoscale modeling analysis approach considering the bond-slip behavior between random distributed fibers and cementitious matrix of UHPC is proposed. Firstly, the Continuous Surface Cap (CSC) model parameters for the matrix of UHPC were calibrated, and validated through a series of fundamental mechanical tests and low-velocity impact test. Secondly, by conducting a single steel fiber pull-out test, the fiber-matrix interaction was examined, and an appropriate interaction algorithm considering bond-slip effect was employed in the commercial program LS-DYNA. Then, the mesoscale model of UHPC was established through a self-coding program in MATLAB. The reliability of the mesoscale model and analysis approach were systematically verified at both material level (uniaxial compression, uniaxial tension, three- and four-point bending tests) and member level (total 15 low-velocity impact scenarios considering various member types, sectional shapes, sectional dimensions, and compressive strength). Finally, the influence of fiber volume fraction on the dynamic behavior of UHPC member was examined. It derives that the incorporated fibers can help reduce the lateral deflections and avoid the brittle punching-shear failure mode of UHPC members, and the optimal fiber volume fraction of 1.5% for the impact-resistant design is recommended. The present work provides a new way to accurately and conveniently evaluate the low-velocity impact-resistance and predict the dynamic behavior of the arbitrary configured UHPC structure.

Pull-out strength of post-installed connectors in thin UHPC members

Ultra-high-performance concrete (UHPC) has recently been introduced to the precast industry in the form of thin components such as the insulated double wythe precast concrete (DWPC) panels. Existing connectors for conventional DWPC panels may not be applicable for the very thin UHPC-DWPC panels. This paper presents an experimental campaign investigating the behavior and design of screw anchors and a newly designed threaded rod connector embedded in thin UHPC members under pull-out forces. The parameters examined are concrete thickness (tc), connector diameter (d), fiber percentage in UHPC (pf), embedment depth (hef), support ring diameter (ds) of the slab, and concrete compressive strength (f’c). The failure modes consisted of concrete breakout, radial cracking, or a combination of the two. The threaded rod demonstrated better structural performance, with ultimate pull-out loads (Pu) 20 to 50% greater than the screw anchor. Increasing the embedment depth from 0.75tc to 1.0tc (full embedment) increased Pu on average by 19% and 67% for the screw anchor in 25 and 38 mm thick slabs, respectively. Increasing slab thickness from 15 to 38 mm, with 1.0tc embedment, resulted in Pu to be on average over 3.5 and 4 times greater for the screw anchor and threaded rod, respectively. Results showed that connector diameter, fiber percentage, and support ring size did not have significant effect on Pu. Two existing models were evaluated and were found to over-estimate the pull-out capacity based on the parameters of the study. A regression analysis was performed on the data and two new design equations were developed to predict the pull-out capacity of each connector with a high degree of accuracy. A design case study is also presented.

A review of existing codes and standards on design factors for UHPC placement and fiber orientation

• Review different construction procedure and their effects on fiber orientation. • Review existing UHPC design codes and their consideration of fiber orientation. • Recommendation on following UHPC design codes regarding fiber orientation. • Identify future research on construction procedure to ensure fiber orientation. Ultra-high-performance concrete (UHPC) has been extensively researched and implemented in various infrastructure applications for its superior structural and durability performance. Despite their successful structural level demonstrations, fiber dispersion and orientation remain a concern in the UHPC structure design. The casting flow direction, formwork geometry, rebar arrangement, use of casting device, and rheology of the mixtures are all reported to affect the fiber orientation and ultimately the structural performance of the UHPC members. A number of UHPC design codes and guidelines have been published by professional organizations across different countries, and the effect of fiber orientation were considered in several distinct ways in these specifications. In this article, a comprehensive review was conducted on the influence of various factors on the fiber orientation in structural elements. Existing structural design guidelines on UHPC, and steel fiber reinforced concrete materials from France, Germany, Japan, South Korea, Canada, Switzerland, Australia, and the U.S. were reviewed with a focus on their respective methods to account for the effect of fiber dispersion and orientation. Several design codes introduced separate design reduction factors to compensate the effect of undesirable fiber orientation. The background work and underlying rationale of these design factors were also included in this paper. Recommendations to the design professionals and for future research directions were provided at the end of the paper.

Feasibility of UHPC shields in spent fuel vertical concrete cask to resist accidental drop impact

Ultra-high performance concrete (UHPC) has been widely utilized in military and civil protective structures to resist intensive loadings attributed to its excellent properties, e.g., high tensile/compressive strength, high dynamic toughness and impact resistance. At present, aiming to improve the defects of the traditional vertical concrete cask (VCC), i.e., the external storage facility of spent fuel, with normal strength concrete (NSC) shield, e.g., heavy weight and difficult to fabricate/transform, the feasibility of UHPC applied in the shield of VCC is numerically examined considering its high radiation and corrosion resistance. Firstly, the finite element (FE) analyses approach and material model parameters of NSC and UHPC are verified based on the 1/3 scaled VCC tip-over test and drop hammer test on UHPC members, respectively. Then, the refined FE model of prototypical VCC is established and utilized to examine its dynamic behaviors and damage distribution in accidental tip-over and end-drop events, in which the various influential factors, e.g., UHPC shield thickness, concrete ground thickness, and sealing methods of steel container are considered. In conclusion, by quantitatively evaluating the safety of VCC in terms of the shield damage and vibrations, it is found that adopting the 300 mm-thick UHPC shield instead of the conventional 650 mm-thick NSC shield can reduce about 1/3 of the total weight of VCC, i.e., about 50 t, and 37% floor space, as well as guarantee the structural integrity of VCC during the accidental drop simultaneously. Besides, based on the parametric analyses, the thickness of concrete ground in the VCC storage site is recommended as less than 500 mm, and the welded connection is recommended for the sealing method of steel containers.

Blast resistance of hybrid steel and polypropylene fibre reinforced ultra-high performance concrete after exposure to elevated temperatures

In this study, the blast resistance of fibre reinforced ultra-high performance concrete (UHPC) components after exposure to elevated temperatures was investigated. With a hybrid steel and polypropylene (PP) fibre reinforcement, this fire resistant UHPC maintained approximately 60% of its original compressive strength after exposed to 800 °C temperature. Uniaxial and triaxial material behaviour after exposure to high temperatures was studied experimentally and then incorporated into a plasticity concrete model, i.e. Karagozian & Case Concrete (KCC model) model for the blast induced structural response analysis. Material strength and failure surfaces, volumetric change with pressure, strain rate effect and material damage parameters were updated with consideration of fire hazards. The simulated UHPC uniaxial stress–strain curves after exposure to 200, 400, 600 and 800 °C elevated temperatures, together with the simulated post-fire blast tests results on UHPC members were compared with available experimental results. The reasonable agreement between the tests and simulation results validated the proposed model in both material and structural scopes. The numerical model was further applied to predict the blast response of reinforced UHPC components after exposed to thermal hazards.

Predicting corrosion in reinforced UHPC members through time-dependent multi-physics numerical simulation

Ultra-high-performance concrete (UHPC) has superior durability characteristics over traditional concrete. Improved durability behavior of UHPC has been attributed to the dense material property, which effectively constrains harmful material ingress and oxygen supply. However, the corrosion performance of reinforced UHPC under sustained mechanical loading, which may induce cracking, in addition to environmental conditioning is still not well characterized. This study investigates the electrochemical-mechanical coupling effects on deterioration mechanisms of reinforced UHPC beams subjected to service loading conditions and chloride attack through multi-physics simulation techniques. Finite element models were simulated considering the cracking induced by service loads and deformations caused by rust expansion on the cementitious matrix. Material transport properties such as chloride and oxygen penetration parameters were updated in each step of the time-dependent process based on cracking conditions. The simulation results show that there is substantial steel cross section and yielding load capacity loss, initial stiffness reduction, and damage area increase in reinforced concrete members while UHPC members showed relatively small cross section loss, negligible increase in UHPC damage, no observable reduction in initial stiffness, and minimal loss in yield load capacity. The effect of oxygen supply level on the corrosion performance is also explored showing that the corrosion rate of reinforced UHPC specimens were limited due to depleted oxygen at the steel-UHPC surface. Numerical simulation results were verified against experimental trends on corrosion performance of reinforced UHPC specimens. Further, the numerical framework herein provides an alternative way in predicting deterioration processes and evaluating service life performance of ductile reinforced concrete infrastructures in a computationally efficient manner.

A Comparative Study on the Shear Behavior of UHPC Beams with Macro Hooked-End Steel Fibers and PVA Fibers.

Structural members made of ultra-high-performance concrete (UHPC) have been attractive to engineers and researchers due to their superior mechanical properties and durability. However, existing studies were focused on the behavior of UHPC members reinforced with micro straight steel fibers at a volume fraction between 1 and 3%. There is a lack of studies on the influence of different types and amounts of fibers on the shear behavior of UHPC structural members. The objective of the study was to experimentally investigate the shear behavior of UHPC beams with macro hooked-end steel (MHS) fibers and polyvinyl alcohol (PVA) fibers, which are two of the most used fibers for high-performance fiber-reinforced cementitious composites. The shear behavior of ten large-scale non-prestressed UHPC beams was studied. The experimental parameters included the shear span-to-effective depth ratio, the fiber volume fraction, and the type of fibers. It was found that both MHS fibers and PVA fibers were effective in enhancing the shear performance of the UHPC beams whether the shear transfer mechanism was governed by arch action or beam action. Moreover, the measurement results of the average crack spacing imply the distinct difference in the fiber bridging effects of the MHS fibers and PVA fibers in the UHPC beams.