Ultra-high Performance Concrete Material Research Articles

A unified tensile constitutive model for mono/hybrid fibre-reinforced ultra-high-performance concrete (UHPC)

Providing a representative and reliable tensile constitutive model for ultra-high-performance concrete (UHPC) has been challenging in the past due to the absence of standardised test methods. In this paper, the authors have proposed a unified constitutive model calibrated to the most commonly used types of steel fibres within the practical range of fibre content. While the novel model serves as the core, a comprehensive experimental program and results are pivotal for validating the model’s effectiveness. In pursuit of this, the recently approved AASHTO standard test method was employed to offer standardised and comprehensive experimental insights into the direct tensile behaviour of mono/hybrid fibre-reinforced UHPC. A set of predictive equations, derived either empirically or from simple mechanics, to predict the key parameters defining the mechanical constitutive law of UHPC material subjected to uniaxial tension are introduced based on experimental findings. The proposed predictive equations and constitutive model exhibited robust predictions of tensile behaviour, aligning well with both the test results and additional independently tested UHPC specimens.

Bending resistance mechanism of prestressed ultra-high performance concrete - reinforced concrete beam based on a full-scale experiment

Ultra-High Performance Concrete (UHPC) is a new type of engineering material with high compressive strength, high tensile strength, and high fracture toughness. Its bending failure mechanism is different from that of traditional concrete beams, which requires a new computational model to describe the bending failure phenomena of the prestressed ultra-high performance concrete - reinforced concrete (UHPC-RC) beam without web reinforcement. Therefore, this paper, through full-scale tests on a 30m prestressed UHPC-RC beam without web reinforcement, captures unique bending failure phenomena, including initial cracking, development of local cracks, and rupture of prestressed steel strands. Considering the tension-compression constitutive relationship of UHPC material, an innovative computational model for bending bearing capacity is proposed. Based on this model, a study on the minimum reinforcement ratio of full prestressed-ordinary steel bars is conducted. The results show that in the bending failure of the prestressed UHPC-RC beam without web reinforcement, excessive tensile strain of steel strands will occur at the local crack location. At this time, the structure does not satisfy the assumption of plane sections, and the introduction of the calculation model of the limit state of external prestressed tendons can effectively match this model, which is highly consistent with the experimental results. The minimum reinforcement ratio of full prestressed-ordinary steel bars is revised to the auxiliary reinforcement ratio of full prestressed-ordinary steel bars, quantifying the minimum reinforcement requirements of ordinary steel bars. The research results of this paper can provide reference for the next step of theoretical research.

Static and dynamic tensile properties of ultra-high performance concrete (UHPC) reinforced with hybrid sisal fibers

Ultra-high performance concrete (UHPC) is widely applied in concrete structures against impact and explosive loads. To achieve a more eco-friendly and cost-effective UHPC material, mixed plant fibers instead of conventional steel fibers are considered in the present study. UHPC with single and hybrid long and short sisal fibers (12 mm long fibers and 6 mm short fibers) reinforcement (2% fiber content) were prepared and compared against UHPC with no fiber reinforcement. The quasi-static tensile and bending properties of these six types of UHPC specimens were investigated. A split Hopkinson pressure bar test has also been adopted to evaluate the dynamic tensile performance of UHPC specimens under five strain rates, the effect of strain rate has been investigated, and the failure process has been recorded with a high-speed camera. The experimental results showed that the UHPC containing 1% long sisal fibers (12 mm) and 1% short sisal fibers (6 mm) has optimum dynamic mechanical properties. Hybrid sisal fibers are more effective in strengthening UHPC specimens in dynamic tests than in static tests. A synergy effect is also introduced to further characterize the strengthening effect of hybrid sisal fibers. Finally, to more accurately predict the dynamic tensile strength of the specimens under various strain rates, an improved prediction model of the dynamic increase factor for tensile strength is proposed by combining the results of this experiment and previous experimental data.

Time-dependent properties of steam cured non-proprietary ultra high-performance concretes

Steam curing of ultra-high-performance concrete (UHPC) has been a common practice to accelerate its material properties development in research and field applications. The time benefits of steam curing come with additional energy demands and CO2 emission. In this research, the effect of steam curing duration on development of UHPC material properties is systematically investigated on four non-proprietary UHPC matrices and two fiber reinforced UHPCs, and fills this current knowledge gap. Investigated material properties includes compressive strength, dynamic modulus of elasticity, electrical surface resistivity and shrinkage. Empirical relationships between these material properties and the duration of steam-curing were derived and compared to available results and relationships from other researchers. In addition, the results were compared to those from regular curing conditions. These research findings might be useful for the precast industry to tailor their energy input, fuel consumption and CO2 emission to the accelerated gain of material properties of non-proprietary UHPC developed using locally available materials. The results show that after 24 h of steam curing at 90 °C the compressive strength and dynamic modulus of elasticity of developed UHPC matrices and fiber reinforced UHPCs exceed 170 MPa and 60 GPa, respectively. In addition, the electrical surface resistivity exceeds 1000 K-ohm-cm and shrinkage is less than 0.04% after 24 h of steam curing. These properties surpass the properties when compared to 28 days of regular curing.

Self-sensing nano-engineered ultra-high performance concrete (UHPC) under tension

With the addition of electrically conductive steel or carbon fibers, Ultra-High-Performance Concrete (UHPC) possesses an intrinsic self-sensing capability. This opens up the possibility of combining the resilience and sustainability of UHPC with the development of self-sensing solutions for structural applications. In this study, the self-sensing behaviour of a proprietary nano-engineered UHPC material subjected to tension was investigated. To assess the self-sensing performance of the material, bulk resistivity measurements were used on direct tension and pure flexure tests, while a novel wireless approach that operates on was used on out-of-plane bending tests. The wireless approach used alternate current (AC) measurements while the bulk resistivity methods were performed through direct current (DC) and the four-probe method. In both methods, the fractional change in resistance was correlated to the state of deformation. The disposition of the actual strain field was evaluated using Digital Image Correlation (DIC). It was found that in the case of direct tension and pure flexure, the fractional change of resistance was initially decreased up to the onset of strain localization, while it gradually increased in the post-peak range, where the separation of the localized crack gradually increased. In the case of the wireless approach using AC, the onset of cracking was successfully predicted with an abrupt increase in resistivity. The wireless strain-sensing approach also captured the Poisson’s effect due to the loading.

Flexural response of UHPC beams post-tensioned with internal unbonded tendons: Experimental study

This paper presents the results of an experimental study that deals with the characterization of the complete flexural response of post-tensioned (PT) ultra-high-performance concrete (UHPC) beams with internal unbonded tendons. A total of 6 PT UHPC beams were monotonically loaded to failure. The variables investigated include volumetric fiber content, non-prestressed reinforcement ratio, and loading configuration. Flexural response is characterized in terms of crack pattern formation, propagation, and localization as well as failure mode identification. The influence of the investigated variables on change in: strand strain, UHPC compressive and tensile strains, strand stress, moment capacity, curvature ductility and member level ductility is presented including explanations of observed phenomena. The measured flexural response and capacity are compared with the predicted response using two mechanics-based methods and a numerical method. Prestress losses are quantified and used to obtain the predicted response. UHPC material characterization is conducted to obtain the necessary inputs for the prediction models. The measured to predicted ratio of flexural capacity and the corresponding COV for the considered prediction methods vary from 0.95 to 1.02, and from 6% to 11% suggesting good predictive capabilities. The use of nonprestressed flexural reinforcement in PT UHPC flexural members is recommended from the perspective of crack distribution, delay in the precipitation of flexural failure through crack localization and subsequent hinging, and member level ductility.

A multiscale optimization methodology for cyclic elastoplastic performance of beam-pile joint member in pile-supported wharf

A material-member-structure multiscale optimization methodology, which can be utilized for improving the elastoplastic behaviour of the beam-pile joint member in pile-supported wharf under cyclic loading, was developed on the base of the application of high-specific stiffness material. Based on the representative volume element (RVE) method, the parameterized relations between the mesoscale phases (e.g., steel fibers, interfacial transition zone, and matrix) and macroscale homogenized elastoplasticity of ultrahigh-performance concrete (UHPC) reinforced with steel fibers were obtained under periodic boundary conditions. The physical equivalence relation between material scale and member scale was established by utilizing the refined constitutive model with the macroscopic mechanical parameters and exponential decay damage evolution factors. UHPC material property tests and cyclic loading tests of small-scale beam-pile joint specimens were performed respectively, and the results were analyzed and utilized to validate the numerical model and multiscale optimization method. Using the computational platform ABAQUS, the cyclic elastoplastic performances of the novel beam-pile joint members were evaluated numerically by pseudo-static hysteretic loading. As an illustration, a multiscale modelling approach for the pile-supported wharf frame system was presented and employed to assess the seismic performance improvement through the optimization of the beam-pile joint members. The coupled solution of refined member damage and large-scale structural bearing capacity was realized by utilizing the deformation coordination condition and force balance equation, which bridges the member scale and structure scale. This study aims to provide a reference for future design and optimization of the joint members in marine frame structures, to enhance cyclic bearing capacities and reduce reinforcement consumptions.

Moment curvature response of composite UHPC filled hollow structural steel cross-sections

A mechanics based analytical method for obtaining the complete moment–curvature response of composite hollow structural steel (HSS) and ultra high performance concrete (UHPC) cross-sections is presented. Flexural response at the cross-sectional level is traced past the local buckling of the steel elements and up to the exhaustion of UHPC and steel material capacity. The proposed method is validated using experimental data from seven flexural tests on composite HSS-UHPC beams. Flexural failure mode is classified as either a UHPC compression-controlled failure, UHPC fiber tension-controlled failure, or a balanced failure. Behavioral differences between the cross-sections that fall into these three categories are discussed in terms of flexural capacity, curvature ductility, and relative contributions of HSS and UHPC to flexural capacity. Flexural response of the composite HSS-UHPC cross-section is compared to that of the bare HSS cross-section. The enhancement in flexural capacity and curvature ductility, for UHPC fiber tension-controlled sections compared to the bare HSS cross-section, was 52% and 79%, respectively. For composite cross-sections that feature a balanced failure, the increase in flexural capacity and curvature ductility, compared to the bare HSS cross-section, was 37% and 35%, respectively. Closed form formulations are proposed to identify the flexural failure mode and predict flexural capacity. The influence of various parameters, such as HSS material grade, size of cross-section, and UHPC material characteristics on the complete flexural response of the cross-section is investigated. The average ratio and corresponding coefficient of variation (COV) of tested to computed flexural strength obtained using the proposed procedure was 1.06 and 21.28%, respectively. Similarly, the average ratio and COV of tested to calculated flexural strength obtained using the proposed closed form formulations was 1.07 and 22.18%, respectively.

Damage evaluation of UHPC-GFRP hybrid beams based on acoustic emission technology under bending experiment

The ultra-high performance concrete (UHPC)-glass fiber reinforced polymers (GFRP) hybrid beam is a novel concrete structure. To evaluate effectively the damage status and crack propagation of the UHPC-GFRP hybrid beams under bending, the specimens UHPC-GFRP hybrid beam (UGCB) and GFRP hybrid beam (GNCB) were designed, and the damage process of the specimens was monitored under the bending of four-point based on the acoustic emission (AE) technology. Results indicated that UHPC material enhanced better the bearing capacity of specimen UGCB, and the signal characteristics of accumulated AE energy, events, and ringing count could characterize the damage development of hybrid beams. The AE Ib-value had a decreasing trend due to the continuous generation of the macro-cracks, and the damage of the specimen GNCB was more serious. The proportion of the amount of shear cracks in the specimens increased gradually in case of the bending, and the proportion of shear cracks of specimen UGCB was 65%, and the specimen GNCB was 70% when the specimens were damaged to failure, and the strengthening of the UHPC reduced effectively the generation of shear cracks. The boundary of the HI and Sr values of failure region corresponding to the failure of the hybrid beam was proposed, and the Sr value was more responsive to the development of microcracks. This study provides some reference for the evaluation of the damage development of the UHPC-GFRP hybrid beams.

Assessing the performance of palm oil fuel ash and Lytag on the development of ultra-high-performance self-compacting lightweight concrete with waste tire steel fibers

Recent advances in ultra-high-performance concrete (UHPC) materials have emphasized the necessity for continuous research. This imperative is particularly pronounced given the widespread demand for materials that facilitate the design and construction of long-span bridges, high-rise towers, and architecturally complex structures. The current study strives to address this need by developing a novel ultra-high-performance self-compacting lightweight concrete (UHPSCLWC). This research introduced Palm Oil Fuel Ash (POFA) and Lytag as replacements for specific conventional components. Specifically, 5%–25% POFA was utilized at 5% intervals as a partial substitute for cement to counter the heavy cement requirement typical of UHPC. Similarly, 5%–25% Lytag was used in 5% increments as a lightweight aggregate substitute for fine aggregate. To further optimize UHPC's production cost, waste tire steel fibers (WTSF) sourced from discarded truck tires were employed as primary reinforcement. This study comprehensively evaluates the rheological, physical, functional, and extreme durability properties of the UHPSCLWC and provides microstructural analysis. Test results suggest that samples utilizing 20% POFA and Lytag demonstrate optimal improvements across assessed characteristics, notably improved thermal and acoustic performance. At 90 days, the highest enhancement in compressive strength, modulus of elasticity, and modulus of rupture was 14.09%, 33.54%, and 17.44% for samples with 20% POFA and Lytag. Due to the unique properties and porous nature of Lytag, the hardened density of UHPSCLWC was reduced from 1993 kg/m3 to 1705 kg/m3. These modified samples also displayed enhanced durability, reduced mass loss and chloride penetration, and increased residual compressive strength when subjected to extreme conditions. Furthermore, the optimal inclusion of 20% POFA and Lytag in the mixture facilitated higher peaks of hydration phases, as observed in x-ray diffraction analysis. The findings of this study suggest the practical potential of POFA and Lytag in developing an innovative UHPSCLWC modified with waste tire steel fibers. This study adds valuable insights to the growing body of knowledge in the field of UHPC material development.

Study on hybrid fibre reinforced UHPC beams under single and repeated lateral impact loading

This paper investigates the dynamic response of hybrid fibre reinforced ultra-high performance concrete (UHPC) beams against single and repeated low-velocity impact loads. A brief description of the drop weight impact tests on the UHPC beams was presented first followed by the development of the material and structural model in finite element analysis. A plasticity-based concrete material model with validated compressive and tensile strength surface and damage algorithm was adopted for hybrid fibre reinforced UHPC material. Based on the test results, the bond-slip behaviour between steel rebar and UHPC matrix was developed in an empirical form and incorporated in the model. Compared to the model with the bond-slip definition, the model with perfect bonding was found to underestimate the maximum mid-span deflection, which highlighted the necessity of considering the bond-slip behaviour in dynamic analysis where large deflection occurs. The repeated impact tests were performed numerically, and the results were validated with experimental data. A parametric study was then performed to investigate the effect of key parameters, including different impact energy and the same impact energy but different impact numbers. The results indicated when the total energy increased, the repeated impact loads became more hazardous than the single impact load. With the validated model, the dynamic shear force and bending moment distribution diagrams were compared to study the failure mechanism in single and repeated impact loads.

Avaliação de danos e inovação de solução eficiente de reabilitação de lajes de concreto armado expostas a explosão de contato

abstract: Under contact explosions, the reinforced concrete structures can behave in a brittle manner with highly localized damage like concrete cratering, spalling, and reinforcement rupturing. High-speed fragmentation resulting from concrete spall may cause severe casualties and injuries. It is therefore important to restrained concrete fragments and improve collapse resistance of the slab. A new retrofitting technique is proposed in this paper which completely prevents fragmentation. To mitigate blast effects on civil structures, a new kind of concrete material named Ultra-High-Performance-Concrete (UHPC) is now widely studied and applied. UHPC material is known for its high compressive and tensile strength, large energy absorption capacity as well as good workability and anti-abrasion ability compared to normal strength concrete(NRC). All of recent experimental published work concerning blast performance of UHPC slabs under far or near explosion effect, on the other side, the contact explosion tests are relatively limited experimentally and nearly impossible because of security restrictions and costly in terms of both preparation and measurements. So, the real and accurate finite element models are needed to address this gap and understanding the real contact-explosion behavior of both NRC and UHPC slabs. The numerical analyses allow gaining insight into the complex failure mechanisms occurring in the slab and not directly observable. In this study, coupled smoothed particle hydrodynamics (SPH) method and finite element method is utilized to simulate the contact blast tests. Numerical results are compared with the experimental observations, and the feasibility and accuracy of the numerical model are validated. The validated numerical model provided a useful tool for designing potential blast-retrofitting solutions which can prevent the local material damage and fragmentations in both NRC & UHPC slabs subjected to contact explosion. This study introduced adequate and very efficient protection solution for both NRC & UHPC slabs exposed to contact explosion (1 kg of TNT) by utilizing the composite action generated between slabs & bonded steel plates. The 2 mm and 1 mm bonded steel plates at both faces of the NRC and UHPC slabs respectively attained a superior resistance to contact explosion.

Flexural behaviour and cost effectiveness of layered UHPC-NC composite beams

This paper presents a pilot numerical study to investigate the flexural performance of simply-supported layered ultra-high performance concrete (UHPC)-normal strength concrete (NC) beams consisting of two UHPC layers with an NC core layer sandwiched in between. A three-dimensional finite element model (FEM) is developed for the beam and is validated against the existing experimental results of UHPC-NC bi-layer composite beams. The effects of the bonding between UHPC and NC layers, the fraction of UHPC and NC in the beam, and the mechanical properties of UHPC material on the flexural performance of the beam are discussed in detail. Numerical results show that the flexural behavior of the layered UHPC-NC beam is significantly influenced by the interfacial bonding between UHPC and NC layers that is represented by the cohesive stiffness which should be higher than 3 to avoid possible delamination failure in the beam. The material cost effectiveness of the layered UHPC-NC beam is also evaluated. It is found that with properly designed UHPC layer thickness, the beam with a thick NC core layer can not only sustain the same loading as a pure UHPC beam with the same thickness but is also much less costly, indicating the excellent mechanical performance and great cost effectiveness of the proposed layered UHPC-NC beam.

Anchorage performance of straight rebars in UHPC with strain-hardening property

Bond and anchorage are the basis to conduct performance analysis on reinforced-concrete structures. The strain-hardening property makes the bond-slip behaviour of rebars in ultra-high-performance concrete (UHPC) different from that in ordinary concrete. This study investigates the anchorage performance of straight rebars in UHPC based on its material properties. First, the anchorage failure process is revealed using pull-out test combined with computed tomography scanning. Thereafter, a finite element model is established to analyze the effects of strain-hardening property on failure mechanism and anchorage performance. The regularity of bond stress distribution along the rebar is also obtained. Subsequently, parametric analyses are developed to analyze the effects on bond stress, including the embedded rebar length, concrete cover thickness, and tensile strength and ductility of UHPC. Expressions for the non-uniform distribution of bond stress are established. Finally, a calculation formula is proposed for the anchorage-bearing capacity considering the strain-hardening property of UHPC material.

Bond-slip behavior of steel reinforced UHPC under flexure: Experiment and prediction

Ultra-high performance concrete (UHPC) is an advanced class of concrete materials that show superior mechanical and durability performance. While several studies have investigated the bond-slip behavior of steel reinforced UHPC using pullout tests, very limited information is available on the bond-slip behavior of reinforced UHPC from beam-type tests, which produce flexural stress states commonly seen in structural members. This study experimentally examines the local bond-slip behavior of reinforced UHPC using beam-end tests. The test variables include UHPC material designs (one low-cement, green UHPC mix and one typical UHPC mix), fiber volumes (0%, 0.5%, and 1.0%), cover thickness (16 mm–51 mm), steel bar sizes (16 mm–32 mm), and test ages (3 days and 50 days). A total of thirty-two tests are conducted. While the non-fiber UHPC specimens fail in a brittle manner due to splitting failure, all fiber-reinforced UHPC specimens exhibit confined splitting failure and ductile bond-slip responses. The peak bond strength is found to exhibit a linear relationship with the cover-thickness-to-bar-diameter ratio and the square root of UHPC compressive strength. Furthermore, a local bond-slip model is developed, which includes a new bond-strength prediction method that predicts the experimental results with a mean absolute error of 7.6%. Finally, this local bond-slip model is combined with a partial interaction model to explore the global bond stress variations under different embedment lengths and flexural states. Equations are proposed to predict the development length that can lead to reinforcement yielding with minimized bonded length or minimized global slip.

Experimental, analytical, and numerical investigation on flexural behavior of hybrid beams consisting of ultra-high performance and normal-strength concrete

In this study, the flexural behavior of hybrid beams consisting of an ultra-high-performance concrete (UHPC) substrate and normal-strength concrete (NSC) overlay is investigated. From the experimental perspective, five UHPC-NSC hybrid beams were loaded to failure, and their failure patterns, load–deflection curves, ductility, strain distributions, and crack evolutions were compared. The UHPC substrate was found to significantly enhance the load-carrying capacity and suppress massive cracking of the hybrid beams. Interestingly, the increasing thickness of the UHPC substrate was observed to impair the beams’ overall ductility, leading to the apparent descending post-peak branch of the load–deflection curve. In other words, the agreed ductility of UHPC at the material level did not always benefit the deformation capacity of the beams at the structural level. Furthermore, an analytical derivation is proposed to capture the load–deflection curve of the hybrid beams. Specifically, sectional analysis in OpenSees was adopted to obtain the moment–curvature relation. This numerical strategy significantly alleviates the complicated derivation of the moment–curvature relation from conventional analytical formulas, owing to the significant tensile contribution of the UHPC material. Finally, finite element (FE) modeling of the hybrid beam specimens was performed. A nonlocal phase-field approach was implemented for UHPC and NSC materials. The numerical simulations are in good agreement with the experimental observations in terms of stiffness, load–deflection relation, and crack evolution.

Anti-collision performance of RC columns strengthened with a composite of FRP grid and UHPC

To improve the anti-collision performance of existing reinforced concrete (RC) columns subjected to vehicle impact loading, this paper proposes a composite strengthening method involving the use of fiber-reinforced plastic (FRP) grid and reinforced ultra-high performance concrete (UHPC). An RC column strengthened with the composite of FRP grid and UHPC and an ordinary RC column were prepared to investigate their anti-collision performance under impact loading. Using the methods of numerical analysis and tests, the effects of the vehicle impact velocity, reinforcement layer thickness, and material strength on the anti-collision performance were discussed. The test results showed that the lateral displacement of the RC column significantly decreased after strengthening with the composite. This indicated that the anti-collision performance of RC columns strengthened with the composite of FRP grid and UHPC can be effectively improved. The numerical results showed that the lateral displacement of the strengthened columns was increased by 37.8 % with an increase in the impact velocity from 3.69 m/s to 4.3 m/s; however, the impact velocity had no evident effect on the impact force acting on the surface of the RC columns. In addition, the anti-collision properties of the strengthened RC columns were significantly enhanced with the increase in the FRP grid–UHPC composite thickness and material strength. The lateral displacement of the strengthened RC columns reduced by 7.5 %–13.5 %, and its peak impact force increased by 14.9 %–33.2 % with an increase in the composite thickness from 20 mm to 40 mm. Moreover, the lateral displacement of the strengthened RC columns reduced by 14.5 %–36.1 %, and its peak impact force increased by 10.9 %–46.4 % with the increase in the compressive strength of concrete from 45 MPa to 210 MPa.

Impact of UHPC Tensile Behavior on Steel Reinforced UHPC Flexural Behavior

Ultrahigh-performance concrete (UHPC) typically contains short steel fibers with a fiber volume of 2% or larger. This relatively high fiber volume leads to high UHPC tensile strength, typically larger than 8 MPa, and constitutes 30%–40% of the UHPC material cost. Recent studies show that if the reinforcing ratio is low, the high tensile strength of UHPC results in a low structural drift capacity (e.g., less than 2.5%). To lower the cost and improve the ductility of steel-reinforced UHPC (R/UHPC), this study experimentally and numerically explores the relationship between R/UHPC flexural behavior and UHPC material tensile behavior, which is affected by the fiber volume. Experimental variables include reinforcing ratios (0.96% and 2.10%), fiber volumes (0.5%, 1%, and 2%), and two proprietary UHPC materials. Seven simply supported R/UHPC beams are tested. Experimental results show that (1) reducing the fiber volume has little impact on the maximum crack width at the service limit state and the crushing resistance at the ultimate limit state; and (2) with a low reinforcing ratio, reducing fiber volume increases the structural member ductility and provides more failure warnings at the peak load. To understand the relationship between R/UHPC flexural behavior and more possible variations of material properties, a two-level, five-factor factorial experiment was conducted numerically with different combinations of reinforcing ratios, reinforcing steel properties, and UHPC tensile properties. Numerical results show that higher reinforcing ratios, higher steel postyield hardening strength, and lower UHPC tensile strength lead to a more ductile flexural failure. Finally, a minimum reinforcing ratio is validated to mitigate the possibility of low structural drift capacity (e.g., less than 2.5%).

I-shaped ECC/UHPC composite beams reinforced with steel bars and BFRP sheets

This paper proposes a new type of small-sized I-shaped engineered cementitious composite (ECC)/ ultra-high performance concrete (UHPC) composite beam which has the potential to be suitable for corrosive environments. The lower tensile part of the beam was made of ECC material (2/3 of the height), and the top compressive part was made of UHPC material (1/3 of the height). Inner embedded steel bars and surface-bonded basalt fiber reinforced polymer (BFRP) sheets were adopted as the reinforcing materials in combination. A total of nine I-shaped beams were designed and tested under four-point bending test. The influence of parameters such as the ratio of the embedded tensile steel bars, the top UHPC flange, and the surface bonded tensile BFRP sheet on the behavior of the beams was investigated. The results showed that the I-shaped ECC/UHPC composite beams have excellent comprehensive performance, and thanks to the ultra-high durability of the component materials, they have ultra-high durability that ordinary I-steel beams do not have and thus have broad application prospects in corrosive environments. The shear resistance capacity of the thin-walled ECC web needs to be further improved, and UHPC is recommended for the web in the follow-up study.

Numerical investigation on early age performance of ultra-high-performance concrete shear keys between an adjacent prestressed concrete box beams

Adjacent precast prestressed concrete box beam bridges have been widely utilized for decades and have shown satisfactory performance. However, significant issues regarding to the longitudinal shear key cracking have been noted by bridge maintenance personnel. The cracks are typically initiated at beam-shear key interfaces due to shrinkage and temperature and propagate due to applied load. Recently, ultra-high-performance concrete (UHPC) was employed in the shear keys with the anticipation to prevent joint cracking. Although the field-collected data at early age from bridge utilizing UHPC shear keys indicated promising performance, the results only reflected the early age joint behavior at locations which were instrumented during the field test. In the current study, a 3D finite element (FE) model was developed to calculate the early age stresses due to shrinkage and temperature. The results indicated that the UHPC material associated with a specific shear key configuration created a “self-locked” phenomenon that generated compression on the upper level shear key. The early age tensile stress during the first couple of days near the end of the joint was relatively small compared to the tensile strength of UHPC material. Although the interface had sufficient capacity to resist the early age stresses, it is still a critical component and needs to be designed with sufficient capacity.