Ultra-high Performance Concrete Specimens Research Articles

Failure mechanism of steel fiber pullout in UHPC affected by alternating cryogenic and elevated variation

This paper investigates the pullout response of straight and hooked-end steel fiber embedded in Ultra-high Performance Concrete (UHPC) subjected to alternating cryogenic (−170 °C) and elevated (200 °C) temperature variation. The temporal evolution of Acoustic Emission (AE) parameters and failure behavior during the pullout process were monitored in situ using the AE test. Additionally, changes in the UHPC microstructure were also evaluated. Results showed that in addition to the UHPC matrices exposed to elevated temperature, the compressive and flexural strength of UHPC matrices subjected to single or multiple cycles were generally higher than those at ambient temperature. Whereas the bond strength/pullout energy of straight fiber increased with a single cycling exposure of UHPC specimens to either cryogenic or elevated temperature, the improvement in the pullout performance of the hooked-end fiber was obtained only after the post-cryogenic thawing of specimens. Moreover, the exposure to an increased number of temperature-variation cycles caused the pullout strength and energy of both fiber groups to gradually decrease. Nonetheless, the pullout performance of the hooked-end fiber was significantly and consistently superior to that of the straight fiber, regardless of the exposure temperature or cycles. The AE test confirmed that the pullout process of both fibers was mainly characterized by tensile failure, with a marginal occurrence of shear failure for the hooked-end fiber. When the UHPC was exposed to various temperatures, the fiber pullout behavior was predominantly influenced by the physical structure of the fibers, thermal deformation proclivity of the matrix, and the competition between mechanisms such as pore moisture phase change, secondary hydration, and the composition of hydrates.

Mechanical Properties and Durability of Sustainable UHPC Using Industrial Waste Residues and Sea/Manufactured Sand

Abstract Considering the continuous development of sustainable development, energy saving, and emission reduction concepts, it is very important to reduce concrete’s cement content in order to improve its environmental impact. Using a reactive admixture to replace part of the cement in ultra-high-performance concrete (UHPC) can effectively improve the overall performance of the concrete and reduce carbon dioxide emissions, which is an important aspect of environmental protection. Here, industrial waste residue (fly ash and slag), sea sand (SS), and manufactured sand (MS) were used to produce UHPC under standard curing conditions to reduce the material cost and make it more environmentally friendly and sustainable. The effects of water–binder ratio, contents of cementitious materials, types of sands, and content of steel fibers on the mechanical performance of UHPC under standard curing were investigated experimentally. In addition, evaluations of the impermeability, chloride, and freeze-thaw resistance of various UHPCs produced were conducted by investigating the effects of various factors on the depth under hydraulic pressure and electric flux of UHPC, as well as the mass loss, relative dynamic modulus of elasticity, flexural strength, and compressive strength of UHPC specimens after freeze-thaw cycles. The obtained experimental results show that the SS-UHPC and MS-UHPC prepared by standard curing exhibit high strength, excellent impermeability, and chloride resistance. The frost-resistant grade of all groups of UHPCs prepared by standard curing was greater than F500 and had excellent freeze–thaw resistance, including those produced with local tap water or artificial seawater. The investigation presented in this paper could contribute to the production of new low-cost and environmentally friendly UHPCs and accelerate the application of UHPC in engineering structures.

Behavior and modeling of FRP grid‐reinforced ultra‐high‐performance concrete under uniaxial tension

AbstractFiber reinforced polymer (FRP) reinforced ultra‐high‐performance concrete (UHPC) structural elements have become increasing popular in research and industry communities. In this paper, uniaxial tension tests were conducted to study the effects of fiber type, fiber length, and fiber content on tensile behavior of FRP grid reinforced UHPC plates (FGRUPs). Additionally, the microstructure analysis of specimens was conducted using scanning electron microscopy. Test results reveal that carbon FRP (CFRP) grid can substantially enhance the tensile performance of UHPC under tension: (1) the FGRUPs exhibit a post‐crack strain‐hardening behavior while the UHPC plates exhibit a post‐crack strain‐softening behavior; (2) the FRP grid enhances the cracking stress of UHPC specimens (except for plates with 1% basalt fibers) and the stiffness of the second segment of the stress–strain curve, and substantially enhances the ultimate tensile stress UHPC specimens. Also, the cost‐effectiveness analysis indicates that plates containing 1% of 12 mm length PE fibers have a highest cost efficiency index. Finally, a modified tensile stress–strain model for FGRUPs is proposed and verified.

Enhancing Sulfate Erosion Resistance in Ultra-High-Performance Concrete through Mix Design Optimization Using the Modified Andreasen and Andersen Method

This study presents an in-depth investigation into optimizing the mix design of ultra-high-performance concrete (UHPC) for enhanced sulfate erosion resistance, utilizing the modified Andreasen and Andersen (MAA) method. By testing the mechanical properties and slump flow of UHPC, it was determined that the optimal W/B = 0.2, and the best volume content of steel fibers is 2%. Through long-term tests lasting 360 days on three groups of UHPC specimens under different curing conditions, their mass loss, compressive strength corrosion resistance coefficient, surface appearance, and erosion layer thickness were tested. The results indicate that under sulfate attack, the mass and compressive strength corrosion resistance coefficients of UHPC specimens showed a trend of first increasing and then decreasing, due to the formation and expansion of ettringite and gypsum. The thickness of the erosion layer increases over time. By 360 days, the internal damage caused by sulfate attack is about twice as severe as it was after 60 days. However, the addition of steel fibers was found to effectively mitigate these effects, reducing mass loss and preserving the structural integrity of UHPC.

Dynamic damage constitutive model for UHPC with nanofillers at high strain rates based on viscoelastic dynamic constitutive model and damage evolution equation

This study established a dynamic damage constitutive model for ultra-high performance concrete (UHPC) with nanofillers, based on a viscoelastic dynamic constitutive model and a damage evolution equation. Ten types of nanofillers, including particle, tube and flake nanofillers, were incorporated to modify UHPC. The split Hopkinson pressure bar was used to obtain the relationship between stress and strain of UHPC specimens at a strain rate of 200/s-800/s. The experimental results indicated that the dynamic compressive strength of UHPC with nanofillers at strain rates of approximately 200/s, 500/s, and 800/s can reach 172.8 MPa, 219.6 MPa, and 275.9 MPa, respectively, reflecting an increase of 85.2 %, 76.5 %, and 53.9 % compared with the blank UHPC. The established dynamic damage constitutive model considered the damage accumulation with strains under dynamic loading. The fitting coefficients of the dynamic damage constitutive model, when compared against experimental results, range from 0.8796 to 0.9963, showing a higher accuracy compared with traditional Zhu-Wang-Tang (ZWT) viscoelastic model, especially at a strain rate of approximately 200/s.

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.

Study on mechanical properties and microstructure of green ultra-high performance concrete prepared by recycling waste glass powder

Ultra-high performance concrete (UHPC) encounters hindrance in pursing carbon neutrality and extended applications due to the dosage of cementitious materials and high energy consumption. To address this problem, this study utilized WGP to replace placement in the production of UHPC-WGP and invested its impact on workability, mechanical properties, and microstructure. The research results showed that incorporating WGP improved the flowability of UHPC and reduced its early-age mechanical properties but significantly enhanced the later-age mechanical properties. When the WGP content is 20 %, the sample's compressive strength after 28 days increases by 8.6 %. The specimens with 28 days exhibited the highest flexural strength and ultimate deflection, reaching 24.8 MPa and 0.809 mm, respectively, and the UHPC specimens (90 d) demonstrated the lowest total porosity, reduced by 14.6 % compared to the reference group. Furthermore, scanning electron microscopy (SEM) observations revealed that WGP had a good micro-aggregate filling effect and pozzolanic effect improved the microstructure. Finally, low cost and energy consumption characteristics further promote the sustainable application of WGP-based UHPC.

Performance recovery of high-temperature damaged ultra-high-performance concrete under different curing environments

The self-healing capability and susceptibility to salt solution erosion in ultra-high-performance concrete (UHPC) significantly influence the mechanical properties and long-term durability of concrete structures within coastal or underground environments. Acknowledging the intricacies of real-world environments, this study focuses on UHPC that has endured high-temperature damage at 800 °C. The research aims to analyze the property variations within 28 days following exposure to diverse dry and wet conditions, as well as salt solution environments. The findings demonstrate a substantial recovery in both mechanical and transport properties of the high-temperature-damaged UHPC, attributed to its inherent self-healing capability. Among the dry and wet conditions, the optimal performance recovery of UHPC specimens is observed when they are subjected to submerged and immersed environmental conditions. Furthermore, when immersed in a salt solution environment, the beneficial impacts attributed to the self-healing property tend to outweigh the detrimental effects of salt solution erosion. All scenarios exhibit a trend of enhanced properties, with the presence of chloride salts being particularly conducive to the recovery process. However, exposure to a sulfate-rich environment induces deterioration in transport properties and pore structure, resulting in a partial incongruity with the mechanical property results. Microscopic analysis techniques reveal that the primary contributors to enhanced properties encompass a range of hydration products formed due to the interaction between water, salt solutions, and the compromised UHPC. Nevertheless, owing to the intricate nature of the environment, disparities arise in both the type and quantity of hydration products, contributing to variations in the extent of property alterations.

Experimental study of bond behavior between concrete-filled steel tube and UHPC-encased

This paper examines the bond behavior between steel and ultra-high-performance concrete (UHPC) in members made of UHPC-encased concrete-filled steel tubes (CFST). For the push-out test, a total of ten UHPC-encased CFST specimens, eight normal concrete-encased CFST specimens, and two CFST specimens were designed. The specimen parameters considered for variation include steel fiber content, thickness of the steel tube, anchorage length, thickness of outer concrete, and concrete type. Through experimental observation and measured data, the force failure process and crack development pattern of the specimens were observed, and load-slip curves were obtained for the loading end and the free end. A comprehensive analysis was conducted to examine the influence and mechanism of different parameters on the bonding performance of the specimens. Furthermore, an equation was derived to calculate the bonding strength at the interface between the steel tube and the outer concrete. The findings reveal that both UHPC-encased CFST and normal concrete-encased CFST specimens exhibited bond-anchorage failure, with the load-slip curve exhibiting a sudden change at the peak value, indicating a high degree of brittleness. The bond strength between steel and outer UHPC was found to be twice that between steel and core UHPC, and 1.4 times that between steel and outer normal concrete. Increasing the steel fiber content and the thickness of outer concrete within a certain range effectively improved the bonding performance of the specimens. The thickness of the steel tube did not have a significant effect on the bonding performance of UHPC specimens. Additionally, when the anchorage length reached a certain value, further increases did not significantly enhance the bond strength. Moreover, the influence range of the steel tube's anchorage length and outer UHPC on the bonding performance was smaller compared to outer normal concrete.

Estimation of stress–strain constitutive model for ultra-high performance concrete after high temperature with an deep neural network based method

In order to better study and analyze the residual compressive behavior and stress–strain curve relationships of ultra-high performance concrete (UHPC) materials after high temperatures, high-temperature tests and uniaxial compression tests were carried out on 144 specimens in the temperature range of 20 °C-900 °C in this paper. The effects of different curing regimes and cooling regimes on the mechanical properties, apparent characteristic and stress–strain curves of UHPC after experiencing different temperatures were investigated respectively. As the target temperature increased, the stress–strain curves of all UHPC specimens gradually tended to be flatten, the peak point of the curve shifted to the right, the peak strain increased sharply, and the elastic modulus and proportional limits gradually decreased. The compressive strength and modulus of toughness of the UHPC specimens both increased and then decreased as the target temperature increased, with a critical temperature of 300 °C and 500 °C respectively. The Poisson's ratio of the UHPC specimens decreased and then increased as the target temperature increased, with a critical temperature of 500 °C. In addition, based on the experimental results, a Deep Neural Network (DNN) model with four hidden layers and 100 neurons in each layer was constructed to predict the complete residual stress–strain response of UHPC materials after high temperature. The training and validation of the DNN model was completed by pre-processing and segmenting the original dataset with the help of Pytorch deep learning libraries. The DNN model predicts stress–strain curves that are in excellent agreement with the UHPC test curves.

Influence of steel fiber types on residual mechanical properties and explosive spalling of hybrid fiber reinforced ultra-high performance concrete: Optimization and evaluations

To investigate the influence of steel fiber on improving the high-temperature resistance of hybrid fiber-reinforced ultra-high performance concrete (UHPC), four types of UHPC specimens were prepared. The residual strength and fracture energy of all UHPCs were determined after exposure to varying high temperatures, and internal temperature in the specimens were also measured. Additionally, explosive spalling tests were performed on specimens with different moisture contents. The results indicate that the residual mechanical properties of UHPC initially increase and subsequently decrease as the target temperature increases. Notable differences in thermal conductivity were observed among the tested UHPC specimens. As the moisture content of the specimens increased, the likelihood and severity of explosive spalling in the specimens increased. Corrugated high-strength steel fibers, measuring 35 mm in length and 1 mm in diameter, are highly effective at preventing explosive spalling of UHPC, and high-temperature treated recycled steel fiber from tires also exhibits better performance than other two types of steel fiber. High tensile strength of steel fiber and its distinctive corrugated appearance can be the primary contributing factors. Furthermore, two established computational models which demonstrated a significant level of congruence were employed for evaluating explosive spalling and anti-spalling properties of UHPC specimens. This suggests that cracking resistance provided by steel fiber plays an important role in preventing the explosive spalling of UHPC, emphasizing the importance of exploring specific steel fiber based on its appearance and tensile strength.

Performance of pre-cracked beams exposed to corrosion environment cast with ultra-high performance concrete

Ultra-high-performance concrete (UHPC) is a unique type of concrete that has a lot of advantages in the construction sector. There is no risk for reinforced UHPC elements in an aggressive environment or coastal region, unless cracking occurs due to extra-loading or inaccurate structural design. For that, there is a need for this study to be considered. The core concentration of this study is towards understanding the shear behavior of pre-cracked beams cast with UHPC as affected by durability. This program presents materials used to reach the main aim of the present study. 72 specimens were conducted to study UHPC physical and mechanical properties to be compared with traditional normal concrete (NC). Moreover, 10 RC beams were cast to investigate the shear behavior of RC-UHPC beams with dimensions of 100 mm, 150 mm, and 1300 mm. Five non-exposure beams and five exposure beams were corroded by an accelerated corrosion cell, and half the potential cell readings were recorded. Obtained results for specimens provide that; UHPC eliminates deflection with high modulus of elasticity, in chloride penetration test UHPC is 200 times less than NC, and the reduction in bond strength test after corrosion exposure is 66.24 % and 1.69 % for NC and UHPC specimens, respectively. For pre-carking UHPC beams at 80 % of experimental ultimate load; cracking resistance for un-corroded beams exceeds the corroded beams by 250 %. The residual flexural capacities for corroded beams pre-cracking up to 80 % of E.U.L. are 61.6 % and 119 % for NC and UHPC beams, respectively. UHPC is much better than NC in corrosive environments.

Self-healing capability of conventional, high-performance, and Ultra High-Performance Concrete with commercial bacteria characterized by means of water and chloride penetration

This study analyzes the self-healing capability of conventional, High-Performance, and Ultra High-Performance Concrete specimens, incorporating commercial bacteria products that are easily accessible, marketed, and affordable from different sectors. Bacteria were incorporated into different mediums: immobilized in diatomaceous earth and liquid. Specimens were pre-cracked to a range of 50–450 μm cracks and were left to heal for 28 days in 3 different conditions (1) water immersion, 2) one week of water immersion followed by three weeks in a humidity chamber, and 3) humidity chamber. To evaluate the self-healing enhancements of specimens using a bacteria-based healing agent, self-healing efficiency was quantified by optical assessment of crack closure, recovery of water tightness via water permeability testing, and chloride permeability via cracks and matrix. The results show that bacterial agents increased the protection against chloride penetration in cracked and healed specimens of conventional concrete, especially when the specimens were healed in water immersion. Owing to the dense matrix of HPC and UHPC, the chloride penetration in the presence of cracks up to 400 μm can be kept below 10 mm. Crack closure greater than 50% is required in UHPC samples to get a significant healing ratio. The penetration through the cracks is approximately twice that of the matrix penetration in conditions when healing is not enhanced.

Factors influencing pore pressure measurements in concrete during heating and its influence on fire-induced spalling

Measuring pore pressure build-up in concrete during heating is quite challenging due to a number of adverse factors that includes lack of instrumentation, test conditions, and interdependency of factors. This paper presents an experimental program undertaken to investigate different parameters that affect pore pressure measurements. A total of 21 specimens of varying shapes and sizes were made with conventional concrete and ultra-high-performance concrete (UHPC) and were tested under varying heating rates and exposure conditions. Measured results showed that the location of highest peak of pressure inside of the specimen is dependent on the type of concrete being tested and the heating rate used. The pore pressure development in UHPC specimens is fundamentally different than conventional concretes and is highly influenced by the presence of fibers. The complexities associated in measuring the pore pressure can be overcome by adopting certain key measures in the design of future experimental programs.

Experimental Study on Axial Stress–Strain Behaviour of Steel Fibre-Reinforced Steel Slag Micropowder UHPC

To investigate the toughening effect and stress–strain relationship of steel fibre-reinforced steel slag micropowder ultra-high-performance concrete (UHPC), nine sets of specimens with coarse aggregate and steel fibre contents were prepared for axial compression and elastic modulus tests. This study examines the variations in compressive strength and peak strain of the steel slag micropowder UHPC specimens to determine the corresponding characteristics of the stress–strain relationship. The results indicate that the experimental groups mixed with 1%, 1.5%, and 2% steel fibre increased the peak strain by about 20.3%, 25.3%, and 26.2%, respectively, compared to the non-steel fibre specimens. It can be seen that the toughening effect of UHPC with steel fibre and slag micro powder is good. With a fixed steel fibre content, the compressive strength and peak strain of steel slag micropowder UHPC initially increase and then decrease as the coarse aggregate content increases. The maximum compressive strength is achieved when the steel fibre content is 1.5% and the coarse aggregate content is 20%. A constitutive equation suitable for steel fibre-reinforced steel slag micropowder UHPC was derived through curve fitting based on the experimentally obtained stress–strain curves. The calculated values from the equation show deviations within 10% of the measured values, indicating a good fit. Nonlinear analysis of the entire compression process of prismatic specimens using the finite element method confirms the rationality of the constitutive equation, as the simulated curve closely aligns with the experimental curve. This research findings provide a reference for the engineering application of steel slag micropowder UHPC.

Sustainable ultra-high performance concrete with incorporating mineral admixtures: Workability, mechanical property and durability under freeze-thaw cycles

This paper evaluates the influence of mineral admixtures partially replacing cement, sea sand replacing quartz, sea water replacing fresh water on ultra-high performance concrete (UHPC). The fluidity and mechanical properties were studied. Besides, the impermeability, chloride resistance and freeze-thaw resistance were investigated. Failure modes, scanning electron microscope (SEM) analysis, mass loss, relative dynamic modulus of elasticity and mechanical properties of UHPCs after freeze-thaw cycles were conducted. The results showed the fluidity of UHPC paste gradually increases with the improvement of water-binder ratio. It is recommended that the water-binder ratio of UHPC be set at 0.19. The fluidity also increases with the improvement of the content of slag, fly ash and water reducer, but decreases with the improvement of silica fume content. The flexural and compressive strengths of UHPC enhance with the improvement of the content of silica fume, but reduce with the improvement of the content of fly ash and slag. The UHPCs made of quartz sand, river sand and sea sand, all, achieve a high strength. UHPCs prepared at standard curing conditions, with or without steel fibers, mixed by artificial seawater and made of sea sand, exhibited excellent impermeability and chloride resistance. The frost resistant grade of all UHPC specimens prepared by standard curing are greater than F500 exhibiting excellent freeze-thaw resistance and sustainability.

Physical, Mechanical, and Microstructure Characteristics of Ultra-High-Performance Concrete Containing Lightweight Aggregates.

This study explores and enhances the resistance of an ultra-high-performance concrete (UHPC) to explosive spalling under elevated temperatures. This study investigates the impact of lightweight aggregates (LWAs) on the mechanical and microstructural properties of the UHPC. Various UHPC specimens were created by replacing silica sand with LWAs in percentages ranging from 0% to 30%. The evaluation of these specimens involved assessing their compressive and flexural strengths, density, mass loss, shrinkage, porosity, and microstructural characteristics using scanning electron microscopy (SEM). This study provides valuable insights by analyzing the influence of lightweight aggregates on the strength, durability, and microstructure of UHPC. The results reveal that incorporating LWAs in the UHPC improved its flowability while decreasing its density, as the percentage of LWAs increased from 5% to 30%. Including 30% LWA resulted in a mass loss of 4.8% at 300 °C, which reduced the compressive and flexural strengths across all curing durations. However, the UHPC samples subjected to higher temperatures displayed higher strength than those exposed to ambient conditions. The microstructure analysis demonstrated that the UHPC specimens with 30% LWA exhibited increased density due to continuous hydration from the water in the lightweight aggregate. The pore size distribution graph indicated that incorporating more of the LWA increased porosity, although the returns diminished beyond a certain point. Overall, these findings offer valuable insights into the influence of lightweight aggregates on the physical and strength characteristics of UHPC. This research holds significant implications for developing high-performance, lightweight concrete materials.

Influence of red mud on the engineering and microstructural properties of sustainable ultra-high performance concrete

This paper introduces a new application of industrial solid waste red mud to use as a cementitious material in the development of ultra-high performance concrete (UHPC) performances. In addition, utilization of red mud can reduce the environmental impact of red mud landfill. For gaining this aims, UHPC specimens were prepared with different contents of red mud and analyzed the effects of different RM contents on the fluidity, setting time, mechanical properties, durability, drying shrinkage and microstructure of the red mud-based UHPC. The results indicated that fluidity was reduced to 310 mm at 20% red mud level, but it still belongs to enormous fluidity, which is convenient for construction. In addition, it is worth noting that using red mud instead of cement can significantly shorten the setting time, providing a new method for applying the UHPC rapid-repairing process. Red mud-based UHPC mortar prepared with appropriate red mud instead of cement exhibits higher mechanical properties, better durability and reduced drying shrinkage of UHPC during curing. After curing for 28d, the highest compressive strength of UHPC samples containing 20% red mud was 129.5 MPa, the compressive strength of the UHPC samples' sulfate resistance was 136.6 MPa, and the drying shrinkage value was around 414 µm. Moreover, observed information from scanning electron microscopy (SEM) noted that red mud can fill the micro-cracks between particles of cementitious materials, make the structure more compact and form new hydrates. Finally, the two characteristics of low cost and energy consumption further promote the sustainable application of red mud-based UHPC. Therefore, replacing OPC with 20% RM is meaningful to produce a sustainable UHPC mixture and field applications.

Fiber content optimization of UHPC and R-UHPC oriented to tensile behavior and cost reduction

The excellent performance of Ultra-high performance concrete (UHPC) is widely recognized, but its high cost remains a hindrance to its more comprehensive application. This paper focuses on the effects on the tensile cracking behavior and mechanical response of UHPC when its cost is reduced by decreasing the fiber content and adding steel rebars. Twenty-four UHPC specimens with different fiber contents (0.5%, 1.0%, 2.0%, and 3.0%) and reinforcement ratios (0% and 3.0%) were tested by direct tensile tests. The effects of varying fiber content and adding steel rebars on the cracking behavior and mechanical response of UHPC were assessed. The fiber efficiency in UHPC with different parameter combinations was quantitatively evaluated in terms of the ability to control the main crack opening, improve ductility, and enhance load-bearing capacity. The results show that reducing the fiber content leads to a substantial degradation in the capacity to restrain the maximum crack opening, ductility, and load-bearing capacity of UHPC. However, the addition of steel rebars weakens or eliminates the negative effects induced by lowering the fiber content and even obtains a better capacity to restrain the maximum crack opening. With tensile properties and fiber efficiency as evaluation indicators, a fiber content of 2% is appropriate for UHPC, and a fiber content as low as 0.5% is recommended for steel rebar-reinforced UHPC.

Strength, Ductility, and Collapse Response of UHPC Waffle Slabs

Waffle slabs made of ultra-high performance concrete (UHPC) are now being widely used in bridge deck construction. However, to date, no research has been conducted to study their failure behavior. Two one-way waffle slab strips made of UHPC were load-tested under four-point bending to investigate their strength, ductility, and failure patterns. An identical set made of high strength concrete (HSC) was also tested to provide a comparison to specimens made with traditional construction materials. The UHPC specimens exhibited multiple fine cracks prior to rebar yielding and failed by steel rebar rupture after a single crack localized. In contrast, the HSC slab strips exhibited several large cracks and substantially larger ductility than the UHPC slab strips, although their strength was about 40% less than the corresponding UHPC specimens. The experimental results were used to validate a three-dimensional finite element model, which was then employed to investigate the two-way response of UHPC waffle slabs. The simulation study revealed that the yield line design method is applicable to UHPC waffle slabs despite their markedly different behavior compared to concrete slabs. Like the HSC slabs, the UHPC slabs also exhibited displacements and load carrying capacities well in excess of those predicted by flexural theories, due to formation of membrane action.