Behavior Of Ultra-high Performance Concrete Research Articles

Structural integrity of steel plate ultra high-performance concrete modules

Modular construction techniques allow for faster onsite construction, reduce cost and time, increase security, and facilitate flexible designs. In this study, steel plate concrete (SC) modular design benefits have been maximized by incorporating ultra high-performance concrete (UHPC). For a steel plate ultra high-performance concrete (S-UHPC) module to behave effectively, structural integrity of steel plate and UHPC is necessary. Different forms of construction for SC modules have been proposed in the past. Cross-ties method is considered to be one of the most efficient forms of construction of an SC module and has been proven to play an important role in ensuring the integrity of the module. This study investigates the structural integrity of S-UHPC modules. In addition, this paper focuses on finite element simulation (FEM) of the S-UHPC beams with emphasis on shear and bond slip behaviour. A new constitutive model is proposed to account for the bond slip behaviour of steel plates. The Cyclic Softened Membrane Model is utilized to simulate the shear behaviour of UHPC with embedded shear reinforcement. Both constitutive models are implemented into a finite element analysis programme based on the framework of OpenSees. The proposed FEM is able to capture the behaviour of the tested S-UHPC beams.

Combined effect of coarse aggregate and fiber on tensile behavior of ultra-high performance concrete

In this study, combined effect of coarse aggregate and fiber properties on tensile behavior of ultra-high performance concrete (UHPC) was investigated. Four replacement levels of coarse aggregates (0%, 15%, 25%, 35% by volume of mortar) and four types of steel fiber (three micro-fibers with a shape difference and one macro-fiber) were considered. Results showed that replacement level of coarse aggregate has a critical value of 25% and different fiber types act similarly in regard of compressive strength. Coarse aggregate brought impairment to bonding strength and utilization efficiency of fiber, especially for deformed ones. Furthermore, coarse aggregate could be successfully introduced into system of UHPC without impairing its tensile properties at a favorable replacement level (⩽25%). In addition, phenomenon of strain hardening behaviors of UHPC incorporating coarse aggregate could be triggered by further increasing fiber dosage to larger than 2.5%, however, it was independent of fiber type due to combined effect of coarse aggregate and fiber bridging.

Hybrid effects of steel fiber and microfiber on the tensile behavior of ultra-high performance concrete

Ultra-high performance concrete (UHPC) has ultra-high material performance including high strength and high flowability. However, its tensile strain capacity is generally lower than that of high ductile cementitious composite. This study experimentally investigated the effect of hybrid combinations of straight 0.2mm diameter steel fiber and various microfibers on the mechanical properties of UHPC. Four types of hybrid fiber reinforced UHPCs including steel, basalt fibers, polyvinyl-alcohol, and polyethylene fibers were designed and then compressive strength, density, and tensile behavior were investigated. Test results showed that combining a synthetic fiber with high strength, such as PE fiber, and steel fiber can improve the tensile behavior of UHPC and basalt fiber was effective for improving the tensile strength of UHPC.

Influences of nano-particles on dynamic strength of ultra-high performance concrete

Abstract Known for its high strength, ductility and durability, ultra-high performance concrete (UHPC) is a topic of interest in recent years for scientists and civil engineers. Great potential application of UHPC has driven increasingly more investments and research into this industry. In recent studies, taken advantage of the nanotechnology, novel UHPC material with nano material addition was developed. Great static performance improvement was observed when compared with normal strength concrete. To obtain full understanding of material properties, especially dynamic performance, Split-Hopkinson Pressure Bar (SHPB) tests were conducted on this new concrete material. For comparison purpose, static properties from uniaxial compression and split tensile tests are obtained and discussed. In this paper, effects of nanomaterials on behaviour of UHPC are assessed through group comparison. Dynamic increase factor (DIF) values for both the dynamic compressive strength and tensile strength are generated. It is found that the strength of UHPC increases with the rising of strain rates and dosage of nano material influences UHPC dynamic properties. However, UHPC is noted to be less rate sensitive comparing with normal strength concrete. Microscopy analysis including Scanning Electron Microscope (SEM) Analysis, X-Ray Diffraction (XRD) Analysis and X-Ray Fluorescence (XRF) Analysis are conducted to understand the macroscopic failure phenomenon, element composition and concrete hydration process.

Long-term Behavior of Ultra – High Performance Concrete (UHPC) Bended Beams

Unlike normal concrete (NC) the behavior of ultra-high performance concrete (UHPC) is different under long-term efforts, if we refer to creep, shrinkage or long-term deflections. It is well known that UHPC has special properties, like compressive strength higher than 150MPa and tensile strength higher than 20MPa - in case of UHPC reinforced with steel-fibers (Magureanu, 2010). Nevertheless, UHPC is not so elucidated regarding creep straining or serviceability behavior in case of structural elements. Some studies made on UHPC samples (Flietstra, 2011) (Burkart, 2009) shown that the creep is significantly reduced if the concrete is subjected to heat treatment and if it contains steel-fiber reinforcement. Relating thereto, it is important to know how does structural elements made of this type of concrete works in service life under long-term loadings. The results obtained on UHPC samples, regarding creep straining from tension or compression efforts may not be generalized in case of structural elements (e.g. beams, slabs, columns) subjected to bending (Spasojevic, 2008). Making this study it was proposed to understand the influence of the heat treatment and steel-fiber addition on the rheological phenomena of UHPC bended beams.

Effects of shrinkage reducing admixture and wollastonite microfiber on early-age behavior of ultra-high performance concrete

In this study, the effect of incorporating shrinkage reducing admixtures (SRA) and/or wollastonite microfibers on the early-age shrinkage behavior and cracking potential of ultra-high performance concrete (UHPC) was explored. Wollastonite microfibers were added at rates of 0%, 4% and 12% as partial volume replacement for cement, while SRA was added at 1% and 2% by cement weight. Results show that the reinforcing effect induced by wollastonite microfibers mitigated the reduction in compressive strength induced by SRA. Addition of wollastonite microfibers to SRA mixtures did not impart a significant change in the measured free shrinkage strain, while it enhanced the cracking resistance compared to that of mixtures incorporating SRA alone. Moreover, adding wollastonite microfibers reduced the leaching of SRA from concrete under submerged conditions, thus leading to higher efficiency of SRA in reducing shrinkage. Partially replacing cement with natural wollastonite microfibers also leads to a reduction in the cement factor, which represents economic and environmental benefits.

Torsional behavior of ultra-high performance concrete squared beams

This paper presents experimental results on the torsional behavior of ultra-high performance concrete (UHPC) beams. Thirteen beam specimens with 300×300-mm cross section were cast from UHPC with the compressive strengths greater than 150MPa. The experimental parameters were the specimens’ volume fraction of steel fibers, transverse reinforcement ratio, and longitudinal reinforcement ratio. The test results indicated that the beams’ initial cracking and ultimate torsional strength increased as the volume fraction of steel fibers increased. The ultimate torsional strength and torsional stiffness after initial cracking increased as the stirrup ratios increased, and ultimate torsional strength increased as the longitudinal rebar ratios increased. The effect of the quantity of transverse and longitudinal reinforcement on the angle of the diagonal compressive stresses was investigated. The results of this study provided valuable data that could be used in future studies to develop computational models of the torsional behavior of UHPC beams and predict their ultimate strength.

Estimation of Shear Behavior of Ultra High Performance Concrete I Girder without Shear Stirrups

Thinner and lighter members can be designed by utilizing the high stiffness and toughness, and high compressive strength of Ultra High Performance Concrete (UHPC), which reaches up to 180MPa. This high strength and ductile tensile behavior of UHPC makes it possible to design the web of the UHPC I Girder without conventional shear stirrups, which makes the UHPC I girder slender, light and economical. However, establishing shear design procedure for UHPC I girders without stirrups requires additional theoretical and experimental studies. This paper investigated shear behavior of UHPC I girder without shear stirrups. The test results show, in spite of no shear stirrups, test specimens have high ductility due to the bridging action of steel fibers against crack opening. UHPC I girders without shear stirrups tested show gradual increase of strength after initial cracking instead of brittle loss of strength as expected from the ordinary reinforced concrete I girders without stirrups. The decrease of the shear span-depth ratio increase the shear strength of the UHPC I girder without stirrups.

An Evaluation on the Restrained Shrinkage of Ultra-High Performance Concrete

Since ultra-high performance concrete (UHPC) is subject to large occurrence of shrinkage at early age due to its low water-to-cement ratio, the mixing of large quantities of powdered admixtures and the absence of coarse aggregates, UHPC presents large risks of shrinkage cracking caused by the restraints provided by the form and reinforcing bars. Accordingly, this study intends to evaluate the shrinkage behavior of UHPC under restrained state by performing restrained shrinkage test using ring-test. The test results reveal that increasing thickness of the inner ring increases the tensile creep at early age leading to the reduction of the average strain and residual stress of the inner ring.

Thermal Expansion Study and Microstructural Characterization of High-Performance Concretes

Ultra-high performance concrete (UHPC) is a family of emerging materials for building and construction applications. Behavior of UHPCs at high temperature is very important to their reliability and safety. In the current study, two UHPC materials were studied using the thermomechanical analysis (TMA) technique between room temperature and 800°C. Both reversible and irreversible phase transformations were observed from the TMA results, which were likely attributable to the α-β quartz transformation and the dehydroxylation transitions, respectively. Thermal expansion coefficients exhibited significant variations in different temperature regimes. Postmortem scanning electron microscopy (SEM) examinations revealed extensive cracking in the heated samples. In addition, microporosities were observed in the calcium-silicate-hydrate (C-S-H) phase as a result of phase changes during heating.

Early-Age Shrinkage of Ultra-High-Performance Concrete under Drying/Wetting Cycles and Submerged Conditions

Concrete is usually subjected to early-age variation in temperature, relative humidity (RH), wind, and other environmental loading that can alter its behavior. In this study, the early-age shrinkage behavior of ultra high-performance concrete (UHPC) is investigated under simulated field conditions. In this article, the effects of shrinkage mitigation methods, including a shrinkage-reducing admixture (SRA) and a super-absorbent polymer (SAP), were explored. The simulated field conditions included drying/wetting (DW) cycles and submerged (SM) conditions, and were compared to sealed and drying conditions under constant temperature and RH. The results in this article show the substantial influence of environmental conditions on the shrinkage behavior of UHPC. It was discovered that exposure to simulated field conditions dismisses the effectiveness of using SRA through a washing-out mechanism. Moreover, incorporating SRA and SAP simultaneously optimized their benefits and allowed for overcoming their deficiencies under various exposure conditions. The article concluded that adequate consideration of in-place conditions in testing protocols of UHPC should allow for gaining a better understanding of shrinkage mitigation mechanisms and developing suitable performance specifications before a wide implementation of UHPC in full-scale field construction.

Combined Influence of Expansive and Shrinkage Reducing Admixtures on the Shrinkage Behavior of Ultra-High Performance Concrete

Since ultra-high performance cementitious composite (UHPCC) exhibits remarkable compressive strength and ductility, its application for structural member provides significant reduction of the section. The use of low water-to-binder ratio and admixtures with high fineness induce large shrinkage strain, leading to the possibility of large occurrence of shrinkage cracks due to the restraints of the form and reinforcing bars. Therefore, this study admixes a combination of expansive and shrinkage reducing agents as a solution to reduce the shrinkage of UHPCC. The eventual appropriateness is computed to evaluate the free shrinkage and restrained shrinkage behaviors. From the test results, the admixing of a combination of expansive and shrinkage reducing agents is seen to achieve a reduction of the free shrinkage by 30% to 50% at 28 days and a reduction of the strain of the external steel by about 19%.