Search

• Mechanical and damage mechanisms of RUHPC depends on the tensile performance of UHPC. • Reduction of the tensile strength of UHPC in RUHPC was observed. • AE analysis method shows further insight into the tensile damage evolution of RUHPCs. • Gini index can characterize the damage distribution nonuniformity of RUHPC. Understanding the tensile mechanism of reinforced ultra high performance concrete (RUHPC) is important for structural design and application of ultra high performance concrete (UHPC). In this study, the direct tensile test accompanied with crack width detection and acoustic emission (AE) source locating were conducted on RUHPC specimens with the longitudinal reinforcement ratios of 0%, 2.3% and 4.6%. Two types of RUHPCs were studied: high strain hardening RUHPC (HSH-RUHPC) and strain softening RUHPC (SS-RUHPC). Experimental results included the tensile stress (load)-strain curves, the crack width-strain curves and the AE source distribution maps under tensile loading. The tensile mechanisms were elaborated from two aspects. From the view of mechanical mechanism, during Stage I (the elastic stage) and Stage II (the elastic-plastic stage): (1) HSH-RUHPC had a globally uniform stress distribution while the steel rebar in SS-RUHPC changed from total elastic state to partial yielding and finally to partial strain hardening; (2) the reduction of the tensile strength of UHPC was observed in RUHPC, which was more obvious with the higher reinforcement ratio. From the view of damage evolution mechanism based on AE analysis method at the micro level: (1) HSH-RUHPC exhibited a homogeneous damage distribution owing to its multiple-micro-cracking mode before steel yielding; (2) SS-RUHPC showed several damage concentrations at crack positions after UHPC softening due to its several-macro-cracking mode. Meanwhile, Gini index was proved to be an effective parameter to evaluate the distribution nonuniformity of the internal damages of RUHPC. [ABSTRACT FROM AUTHOR]

• ES-UHPC exhibits significant tensile strain-hardening behavior at early age. • ES-UHPC obtain high ductility and bonding between steel fiber and matrix at 1 d. • Damage evolution was monitored by acoustic emission method. • AE analysis proves that the ES-UHPC exhibits multiple cracking behavior at 28 d. • Ettringite is the main hydrates and the content increases rapidly from 4 h to 1 d. The properties of early age strength ultra-high performance concrete (ES-UHPC) made with calcium sulfoaluminate (CSA) cement, including tensile performance, damage evolution monitored by acoustic emission (AE) and microstructure were evaluated in this work. Results show that the ultimate tensile strain ε Utu of ES-UHPC can reach to 0.199% at 1 d, which is 4.23 times larger than that of normal UHPC (N-UHPC) at the same testing age. AE testing results indicate that both types of UHPCs exhibit multiple cracking behavior at 28 d. With the tensile strain developing, the two UHPCs show a similar damage evolution trend and multiple microcracks with crack-width smaller than 0.01 mm generated after 0.046% strain reached. Microstructure tests demonstrate that ettringite presented after 4 h and the content increased significantly from 4 h to 3 d, which contributed to the early mechanical properties for ES-UHPCs. [ABSTRACT FROM AUTHOR]

Highlights • Providing quantitative information on recovery of UHPC by self-healing effect. • Tensile strength of new crystals after self-healing was lower than the intact part. • API shows a sensitive response to the tensile damage characterized by AE sources. • Steam re-curing method is more effective for self-healing than water re-curing method. Abstract High strain hardening UHPC was expected to be self-healing due to its multiple micro-cracks in tension and the low water-to-binder ratio. The direct tensile tests with the acoustic emission (AE) analysis method and air permeability tests were carried out on high strain hardening UHPC in order to investigate the self-healing behavior under three re-curing conditions (water, 80%RH and 45%RH). Moreover, the cyclic tensile loading was applied to create more severe damage and two re-curing schemes (water and steam) were used to evaluate the self-healing effects. Experimental results show that the direct tensile stress-strain curves of high strain hardening UHPC re-cured in water and 80%RH conditions have the recovery of first cracking strain. New crystals were observed in the micro-cracks after self-healing, whose strength was proved to be lower than that of the intact part. The high strain hardening UHPC can recover 40%-47% of the first cracking strain after 28 days of water re-curing, where new damages characterized by AE sources number were found to generate inside the UHPC matrix. Air permeability test shows a sensitive response to the damage created by the tensile deformation. Steam re-curing method shows more effective and less time consuming for self-healing than water re-curing method. [ABSTRACT FROM AUTHOR]

Ultra-lightweight cement composite (ULCC) has attracted extensive research interests in both civil and offshore engineering constructions due to its high specific strengths. In order to satisfy different engineering construction demands, four types novel steel fiber reinforced ultra-lightweight cement composite (ULCC) with different densities ranging from 1250 kg/m 3 to 1550 kg/m 3 were proposed. Extensive standard compressive and tensile tests were performed to obtain the mechanical properties of these ULCCs with different densities, which will offer useful information for the developments of design on engineering constructions with such types of ULCC. Based on these test results, constitutive laws were established to describe the compressive and tensile stress–strain behaviors of ULCC with varying densities. Regarding the applications of the ULCC in engineering constructions, a ULCC flat slab with density of 1550 kg/m 3 under concentrated loading was tested. The failure mechanisms and ultimate strength behaviors of this ULCC flat slab were reported. 3D FE model was also developed to simulate the structural behaviors of the novel ULCC flat slab, and its accuracy was confirmed by the reported test results. With the validated FE model and reported mechanical properties of ULCC with different densities, structural behaviors of ULCC flat slabs with these ULCCs were investigated. Analytical model based on the method of bending resistance in MCS-EPFL was proposed to predict the ultimate resistance of ULCC flat slab. [ABSTRACT FROM AUTHOR]

Ultra-lightweight cement composite (ULCC) has been developed for building and offshore constructions. This paper aims to apply the ULCC in the flat slabs of commercial buildings and double-skin-composite (DSC) ice-resistant walls in the Arctic offshore structures. After developing the structural concepts, quasi-static tests were performed on ULCC flat slabs, DSC slabs, and DSC shells to investigate their ultimate strength behaviors under concentrated loads. General behaviors of these specimens were reported, and the influences of flexural reinforcing ratio (or steel content), volume fraction of the fibers in ULCC, and depth of cross section on ultimate strength behaviors were studied and analyzed. A design approach with a series of equations was developed to predict the ultimate resistances of the reinforced flat slabs, DSC slabs and shells with ULCC. This design approach incorporated influences of fiber content, the resistance contributed by top steel skin, the influence of the curvature on the size of the punching cone in DSC shells, and the modifications on the punching shear resistance by the stud connectors in DSC structure. The accuracy of the design approach is checked against the reported 15 tests. Design recommendations were given for the flat slabs and DSC structures with ULCC. [ABSTRACT FROM AUTHOR]

Energy efficient building is defined as achieving satisfactory internal environment and service with minimum energy consumption. One of the most important parameters that affect the heat transfer through the building envelope is thermal conductivity. The thermal conductivity of lightweight concrete is generally lower than that of normal-weight concrete due to the lower thermal conductivity of air. Although introducing voids in concrete reduces its thermal conductivity and increases its insulation capacity, the mechanical properties are generally compromised. This study focuses on the development of ultra-lightweight cement composites (ULCCs) with low thermal conductivity but high specific strength so that they can be used for structural applications. The lightweight is achieved by incorporating hollow cenospheres from fly ash generated in thermal power plants. The ULCCs had 1-day densities ranged from 1154 to 1471 kg/m 3 and 28-day compressive strengths ranged from 33.0 to 69.4 MPa. The properties of the ULCCs were compared with those of cement pastes with comparable water/binder and those of a concrete with 28-day compressive strength of 67.6 MPa. Results indicate that the compressive strength, flexural tensile strength, and elastic modulus of the ULCCs were reduced with the decrease in density. However, compressive and flexural tensile strength of 69.4 and 7.3 MPa were achieved for the ULCC, respectively, similar to the cement paste with w / b of 0.35 and concrete. With similar 28-day compressive strength, the thermal conductivity of the ULCC was 54% and 80% lower than that of the cement paste and concrete, respectively. The low thermal conductivity of the ULCC is due to the incorporation of hollow cenospheres as micro-aggregate which effectively introduce voids and decrease density of the ULCC. The high specific strength of the ULCC may be attributed to (1) the presence of hard and stiff shell in the cenospheres, (2) the “control” of void sizes in the cenospheres, and (3) the creation of strong cement paste matrices that provide “three dimensional” confinement to the cenospheres. [ABSTRACT FROM AUTHOR]

• A meta steel rebar with both high strength and high uniform elongation is developed. • NPR steel rebar significantly improves flexural performance of UHPC beam. • NPR steel rebar reinforced UHPC beam exhibits multiple (6–8) close-spacing main cracks. • Presence of progressively increased strength, no obvious yielding plateau and high uniform elongation results in crack clustering. An innovative meta steel rebar with negative Poisson's ratio effect (termed as NPR steel rebar), which possesses both high strength (ultimate tensile strength is approximately 1100 MPa) and high uniform elongation (approximately 40%), is developed. This paper experimentally investigated the implications of NPR steel rebar on flexural behavior of ultra high performance concrete (UHPC) beams. Four UHPC beams reinforced with varying reinforcement ratio and longitudinal reinforcement type were prepared and tested. Traditional monotonic four-point bending test was performed. The test results showed that UHPC beams reinforced with NPR steel rebar could effectively improve the flexural strength by 47.8%-55.2%, and upgrade the deformability and ductility by 13.6%-180.9% respectively compared with UHPC beams reinforced with HRB 400 steel rebar under same reinforcement ratio, and the crack pattern of which exhibited multiple (6–8) uniformly main cracks located in pure bending region. Discussion included crack pattern, load-midspan deflection curve, ductility and load–strain response. The crack clustering mechanism of beams reinforced with NPR steel rebar was selectively emphasized in the discussion. [ABSTRACT FROM AUTHOR]

Abstract: Thermoplastic pipe confined concrete (TPCC) is a composite system that uses thermoplastic pipe to confine the concrete bridge pier columns. TPCC aims at piers exposed to the tidal zone or the air–earth zone in marine and saline environments, and protects them from salt damages due to isolate them from the aggressive substances and to eliminate the zones of the concrete structure. PVC pipes were chosen to make TPCC in this paper. Compressive behavior like ultimate strength, stress–strain curve, ductility and toughness of TPCC and effects of plastic pipe thickness, the cubic compressive strength of core concrete (f cu) on them were investigated in this paper. Results indicate: (1) TPCC overcomes the shortcomings of the common concrete greatly, such as high brittleness and bad ductility. The strain at peak stress of TPCC are 1.415–5.540 times higher than the common concrete, meanwhile, the energy absorption of TPCC are 14.8–38.8 times higher than the common concrete. (2) The pipe thickness and f cu have significant effects on onset behavior and post-peak behavior of the stress–strain curves and the ultimate strength of TPCC. (3) Effect of pipe thickness on ultimate strength of the specimens with same core concrete is not unique. In addition, ultimate strength of the specimens with same pipe increases with the increase of f cu. [Copyright &y& Elsevier]

• The permeability of chloride ions is found to increase with the increasing tensile deformation. • The relationship between free and bound chloride ions conforms to the Freundlich isothermal equation. • The protective layer of steel fiber is relatively thin, and it has a greater corrosion risk in marine environment. • Adjacent exposure surfaces accelerate the aggregation effect of chloride ions. Ultra-high performance concrete (UHPC) has excellent mechanical properties and durability. To determine the allowable tensile deformation range for good durability of these materials in the marine environment, the effects of tensile deformation and subsequent cracking upon the chloride ion permeability properties are investigated herein. The permeability law of chloride ions under tensile deformation is analyzed via marine exposure and indoor immersion testing, and the evolution process of tensile damage and the distribution of steel fibers are characterized by acoustic emission and X-ray computed tomography (X-ray CT) techniques, respectively. The results show that the permeability of chloride ions is larger in the marine environment than in a high chloride-concentration indoor environment. In addition, the permeability of chloride ions is found to increase with the increasing tensile deformation. The relationship between free and bound chloride ions conforms to the Freundlich isothermal equation. Finally, based on Fick's second law, the two-dimensional diffusion process of chloride ions in UHPC is fitted, and the relationship between the tensile deformation and the diffusion coefficient of chloride ions is established. [ABSTRACT FROM AUTHOR]

• Under the action of electric field, ions migration will occur in UHPC matrix. • The steel fiber network is at risk of electrochemical corrosion. • As the degree of corrosion increases, the crack control capability of the UHPC decreases. • A small amount of corrosion can increase the binding force between the steel fiber and the UHPC matrix. In this paper, the mechanisms by which the tensile mechanical properties of ultra-high performance concrete (UHPC) are influenced by the degree of steel fiber corrosion are investigated via accelerated electrochemical corrosion, direct tensile testing, and acoustic emission (AE) analysis. The elastic modulus is shown to decrease, while the ultimate tensile strength initially increases and subsequently decreases, with increasing corrosion time. Further, an increased degree of corrosion is shown to decrease the crack control capability of the UHPC such that crack widths rapidly increase and a large amount of AE energy is generated. Concurrently, the cracking behavior due to corrosion of the steel fiber network is examined by X-ray computed tomography (X-ray CT), revealing that corrosion cracking and cambium spalling are facilitated by concentration of the steel fibers. In the case of a small amount of corrosion, the chloride ions permeation is found to be small, and the corrosion only occurs on the surface of the specimens. With increased corrosion time, however, chloride ions gradually penetrate into the interior of the specimen, thus increasing the degree of corrosion. Finally, a formula is established for predicting the time of corrosion cracking in the concrete cover, and a parametric analysis is performed. [ABSTRACT FROM AUTHOR]

• AE analysis provides a good explanation for the evolution of mechanical properties of UHPC under cyclic tension. • The Kaiser effect can memorize the tensile strain more correctly than the tensile stress in the high strain hardening UHPC. • The reduced stiffness of UHPC is controlled by the nominal debonding length of the steel fiber. Understanding the cyclic tensile behavior of high strain hardening ultra high performance concrete (UHPC) is important in structural design and for predicting its damage state. In this study, acoustic emission (AE) analysis was applied to monitor the damage evolution process of high strain hardening UHPC under direct tensile cyclic loading at different objective tensile strain levels. The test results show that when the objective strain is in the elastic state, the tensile cyclic stress–strain curves show marginal change, whereas when it is in the strain hardening state, the residual strain and the stiffness of UHPC are reduced by increasing the number of cycles and finally tends toward stability, the maximum stress remains stable. The AE analysis results can help to explain the evolution of the mechanical properties of high strain hardening UHPC under cyclic tensile loading and proves the existence of the Kaiser effect, which can be used to quantify the deformation and the degree of damage of UHPC. The stiffness of UHPC under cyclic tension is controlled by the nominal debonding length of the steel fibers. Finally, the relationship between the total tensile strain and the loading stiffness degradation ratio under cyclic tension was proposed and could be employed in evaluate of stiffness of the high strain hardening UHPC under cyclic tension in some extent. [ABSTRACT FROM AUTHOR]

• Damage mechanism of UHPFRC is connected with different tensile stages of its stress–strain curve. • Tensile cracking mode and in-plane shear cracking mode in UHPFRC are identified via AE average frequency. • Fibre-matrix debonding is distinguished from fibre pullout in UHPFRC via AE energy. • Contributions of matrix cracking, fibre–matrix debonding and fibre pullout at different tensile stages are quantified. The two strain-hardening stages for ultra-high performance fibre reinforced concrete (UHPFRC) under direct tensile loading are the swift strain-hardening (stage II) and widely held strain-hardening (stage III) stages. A direct tensile test with acoustic emission (AE) registration and air permeability measurement was conducted on three UHPFRC types. Stage II displays more superior crack-width control, higher tensile stiffness, and better impermeability compared to stage III. Based on the analysis of three AE parameters, the damage mechanism of UHPFRC at different tensile stages was identified. The AE source location provides strong evidence of multiple randomly distributed micro-defects (<0.01 mm) at stage II. The AE average frequency and energy can distinguish three UHPFRC damage patterns (matrix cracking, fibre–matrix debonding, and fibre pullout) and evaluate their contributions at different tensile stages. Matrix cracking is dominant at stage II. Matrix cracking and fibre–matrix debonding is roughly balanced at stage III. The large-scale fibre–matrix debonding is generated accompanied by marginal fibre pullout at the strain-softening stage. The stress state of steel fibres at UHPFRC crack sections at stages II and III was also analysed. The configuration of the strain-hardening stage is determined by the relationship between the fibre–matrix bond strength and the tensile stress of the steel fibres at the crack sections. [ABSTRACT FROM AUTHOR]