Behavior Of Ultra-high Performance Concrete Research Articles

Mechanical properties and early-age shrinkage of ultra-high performance concrete mortar with recycled fine aggregate

This study focuses on the shrinkage behavior of ultra-high performance concrete (UHPC) incorporating recycled fine aggregates (RFA). The effects of RFA content (0, 25, 50, 75, and 100%) and different moisture states of RFA were investigated through mechanical properties, autogenous shrinkage, drying shrinkage, and restrained shrinkage. The results showed that the incorporation of RFA generally reduced the mechanical properties of UHPC, although marginal improvements were seen with the use of oven-dried or air-dried RFA. Saturated surface-dry (SSD) RFA effectively reduced autogenous shrinkage of UHPC through internal curing. However, the autogenous shrinkage in UHPC increased with the increase of RFA content. RFA also increased the drying shrinkage. Microstructural analysis revealed that the observed changes were due to heightened porosity, particularly the increase in pores smaller than 50 nm, and a decrease in micromechanical properties resulting from the incorporation of RFA. In terms of restrained shrinkage, the circumferential strain, basic tensile creep, and cracking potential of UHPC were related to autogenous shrinkage. Reducing the moisture content of RFA led to an increase in autogenous shrinkage while concurrently decreasing drying shrinkage.

A piece-wise linear model for modelling the tension-stiffening effect in steel-reinforced UHPC composites

This study examines the tensile behaviour of Ultra-High Performance Concrete (UHPC) specimens reinforced with varying steel fibre content and rebar diameters. An experimental campaign involving 18 reinforced UHPC specimens was conducted, following individual tensile response characterizations of UHPC and reinforcing steel. These specimens were designed and fabricated with threaded welded steel cages to transfer loads to the central measurement area. Force–displacement curves were measured to derive stress–strain curves for the UHPC component, evaluating the impact of reinforcement rebar and steel fibre content on UHPC’s mechanical response. The analysis revealed that steel fibre volume content significantly influences the tensile response of UHPC reinforcement more than the steel bar diameter and yield strength. Comparing the strain in steel bars to the average strain in reinforced UHPC specimens indicated that post-cracking deformation concentrates at the crack. At the same time, the bond between reinforcement and UHPC remains intact in uncracked sections, allowing effective load transfer. UHPC specimens reinforced with 14 mm diameter steel bars displayed a significant horizontal segment in the descending phase of the stress–strain curve. In contrast, specimens with 18 mm or 22 mm diameter bars exhibit a progressive stress reduction at a nearly constant rate. Stress–strain curves of UHPC were fitted with a five-fold piecewise empirical model applicable to UHPC with a reinforcement ratio higher than 0.09%. The resulting model can be used to model the axial behaviour in UHPC, considering the effects of steel reinforcement.

Assessing the setting behavior of ultra-high performance concrete

The setting behavior of ultra-high performance concrete (UHPC) is demonstrably different from that of conventional concrete; thus, tools and guidance extending beyond common test methods such as Vicat and penetration are needed. While UHPC is known for its enhanced mechanical and durability properties, due to the low water and high cementitious contents, UHPC-class materials are prone to early-age autogenous shrinkage. Recognizing that UHPCs are commonly supplied to construction sites as prebagged, proprietary mixes with unknown constituents, and that accurate determination of setting time is crucial in determining the early-age autogenous shrinkage of UHPC-class materials as well as for scheduling construction operations and quality control actions, this study explores alternate test methods such as isothermal calorimetry (ASTM C1679), semi-adiabatic calorimetry (ASTM C1753), autogenous shrinkage (ASTM C1698), chemical shrinkage (ASTM C1608), and dual ring test (American Association of State Highway and Transportation Officials (AASHTO T 363) to evaluate the setting behavior of UHPCs. Setting times obtained using the alternate test methods aligned well with each other and were found to be different than the setting times indicated through standard test methods. Discussion and guidance on the applicability and the use of alternate test methods to determine the setting time of UHPCs for various laboratory and field applications are provided.

A damage constitutive model for consistent description of static and fatigue behaviors of ultra-high performance concrete containing coarse aggregate

A suitable constitutive model for describing the mechanical behaviors of ultra-high performance concrete (UHPC) under various loading histories plays a vital role in analyzing its structural performance. In this work, a consistent elastoplastic damage model is developed for UHPC containing coarse aggregate (UHPC-CA) subjected to static and fatigue loads, in which the driving and alleviating effects induced by the inclusions of coarse aggregate and steel fiber are considered. For damage evolution, based on the acting mechanism of the non-uniformity of stress wave propagation determined by the loading rate on the mechanical responses of UHPC-CA, the loading rate is skillfully integrated into the damage evolution rule to achieve the consistent description of mechanical behaviors of UHPC-CA under static and fatigue loading conditions. Regarding plasticity growth, an empirical plastic deformation model is adopted to improve the computational efficiency of structural nonlinear analysis. To verify the applicability of the proposed model, a user-defined UMAT subroutine is further developed for the subsequent numerical implementation. The comprehensive comparisons between the numerical predictions and independent experimental results at both the material level and structural level solidly demonstrate the capacity of the consistent model to capture the main features concerning the mechanical performance of UHPC-CA subjected to different loading paths.

Flexural fatigue behavior of ultra-high performance concrete under low temperatures

Ultra-high performance concrete (UHPC) shows superior mechanical performance, which leads to increasing applications in infrastructure constructions that are subjected to different loading (i.e., flexure, tension, compression, etc.) and environmental conditions (ambient, freeze, etc.). Among them, the flexural performance of UHPC under low temperatures (i.e., sub-zero temperature) is still little understood especially under fatigue loading. To investigate the flexural fatigue properties of UHPC under low temperatures, eleven UHPC prisms are subjected to cyclic bending at different stress levels from 0.40 to 0.80 (the ratio between applied maximum fatigue stress and static flexural strength) and temperatures of 20 °C, -10 °C, and -20 °C. Results indicate that the fracture interfaces for specimens under static and fatigue loading exhibit distinct differences. No fiber buckling appears for the monotonically-loaded specimens, while for specimens under fatigue loading, fibers buckled and fractured. Low temperature improves the mechanical properties of UHPC under monotonic loading due to enhanced matrix strength. Conversely, low temperature adversely affects the flexural fatigue performance of UHPC due to the cold brittleness nature of matrix and steel fibers, leading to the accumulation of matrix deterioration and the fracture of steel fiber. When the temperature drops from 20 °C to -10 °C and -20 °C, there is a 15.0 % and 12.7 % decline in the flexural fatigue strength of UHPC, respectively. In addition, low temperature accelerates the degradation of UHPC, resulting in a larger and faster accumulation of fatigue deformation, including tensile strain, mid-span deflection, and fatigue deformation modulus.

Effects of macro basalt fibers on the tensile behavior of ultra-high performance concrete

Macro basalt fibers (MBF), which is also named as basalt fiber reinforced polymer (BFRP) minibar, is a novel kind of fiber manufactured with BFRP. This work studied the effects of MBF on the tensile behavior of ultra-high performance concrete (UHPC) and proposed some MBF optimization recommendations based on the experimental results. Nine kinds of UHPC with MBFs and two kinds of UHPC with steel fibers or polypropylene (PP) fibers were designed for the tests of fiber distribution, density, compressive and tensile behavior. According to test results, UHPC with MBFs showed a more ideal fiber distribution compared to UHPC with steel fibers and PP fibers. For the samples with MBFs, UHPC with 3 % twisted MBFs had the highest compressive strength, tensile strength and ultimate tensile strain, which were 138.9 MPa, 8.14 MPa and 1714 με, respectively. Compared to UHPC with steel fibers, UHPC with MBFs had slightly lower density and mechanical properties, but a similar stain-stress relationship. Thereby, the linear-exponential softening model was adopted to show the tensile stain-stress relationship of UHPC with MBFs. Based on the test results, three recommendations for optimizing MBF, which contained reducing fiber diameter for larger aspect ratio, optimizing fiber shape for higher fiber-UHPC bonding behavior and increasing the fiber modulus, were proposed to improve the mechanical properties of UHPC with MBFs.

Uniaxial behavior of UHPC under cyclic compression: Experimental investigation and constitutive model

The compressive behavior of ultra-high performance concrete (UHPC) is experimentally investigated in this paper for three different micro steel fiber volume fractions (Vf= 1.0%, 1.5% and 2.0%) under uniaxial monotonic and cyclic compression. The cube specimens are loaded and fully unloaded twice per displacement increment to track the evolution of damage. Cycles are included till the peak load and after attaining the peak load, all the way till the load carrying capacity becomes marginal. A one dimensional (1D) elastoplastic damage constitutive model is proposed to simulate the damage of UHPC under uniaxial cyclic compression, incorporating the damage parameters within a damage function deduced from the tests. Damage is incorporated as a nonlinear function and thus the hysteresis behavior (distinguishable unloading and reloading paths) is adequately captured in the model. The experimental results from this study and the accompanying model demonstrate that the strength and stiffness degradation in UHPC due to low cycle fatigue stabilize at higher fiber dosage levels.

Flexural behavior of ultra-high performance concrete (UHPC) shaped thin-plate ribbed staircases: an experimental and numerical study

Flexural behavior of ultra-high performance concrete (UHPC) shaped thin-plate ribbed staircases: an experimental and numerical study

Behavior of ultra-high performance concrete under true tri-axial compression

Confinements have been adopted to alleviate the brittleness of ultra-high performance concrete (UHPC). However, as the foundation of confinement mechanism, the research on behavior of UHPC under tri-axial compression is rather limited. Furthermore, the confinement mechanism of PE fiber-strengthened UHPC (PE-UHPC) has not been reported yet. To this end, conventional and true tri-axial compression tests were performed for 22 UHPC cubes with different PE fiber lengths and various lateral confining stresses. The typical axial stress-strain curve has a long elastic ascending stage, a relatively short plastic ascending stage and a steep post-peak decline stage. Increasing the lateral confining stresses could extend the elastic ascending stage, and make the plastic ascending stage longer and clearer. Furthermore, the reinforcement effect of longer PE fibers became more pronounced with increasing lateral confining stresses. However, only increasing intermediate principal stresses with minor principal stresses unchanged had minor effect on the axial strain-volumetric strain behavior.

Characterization of the tension softening behavior of UHPC

Fracture simulation of concrete structures, which entails generating rational and accurate crack growth through the material, requires the tension softening curve as a constitutive property. Unlike regular concrete, the tension softening behavior of ultra-high performance concrete (UHPC) is highly pronounced and extends to large deformation levels. However, data about this property and, especially, how it is affected by key variables (i.e., fiber dosage and type of fiber used) is lacking. In this paper, a set of direct tension tests (DTT) are conducted to formulate an empirical relationship for UHPC in the tension softening range. The experimental variables are fiber type (two kinds of straight fiber and hooked), fiber length (lf = 13, 19 or 30 mm), and fiber volume fraction (Vf = 1.0 %, 1.5 %, and 2.0 %). The energy absorption capacity of UHPC is found to be different for different type of fibers, due to a difference in strain hardening as well as the pronounced softening phase. A dimensionless tension-softening curve for UHPC that is a function of fiber type, length and volume fraction is proposed as a generalized softening expression (expressed using two parameter a and b) and then implemented into a hybrid rotating/fixed crack smeared crack model. Four-point bending test (4PBT) specimens that correspond to the DTT specimens are made and tested to provide validation data. These validations confirm the model's adeptness at accurately emulating all facets of the 4PBT specimen's response. Following the validation, a response surface for parameters 'a' and 'b' is developed, enabling the generation of the softening curve across the entire spectrum of lf, Vf, and fiber type, without having to monitor the entire range of the softening response.

Exploring the piezoresistive sensing behaviour of ultra-high performance concrete: Strategies for multiphase and multiscale functional additives and influence of electrical percolation

Self-sensing ultra-high performance concrete (UHPC) features superior mechanical capacity, excellent erosion resistance, long life cycle, and broad range of stress sensing, and is expected to be a pathway towards next-generation smart cementitious composites. This study presents a novel strategy to achieve electrical percolation and elevate the piezoresistive sensing capability of UHPC through the incorporation of multiphase and multiscale functional additives including graphene (G) and carbon nanotube (CNT). The workability, compressive strength, microstructural characteristics, and alternating current (AC) impedance response are investigated. The effect of electrical percolation on the piezoresistive behaviour is studied and discussed using various ratios of G/CNT. Results indicate that as the G/CNT ratio decreases from 4:0 to 0:4, the compressive strength is reduced from 194.7 MPa to 166.3 MPa due to the certain increase in the plastic viscosity of fresh mixture and the proportion of harmful pores, although the basic requirement of 150 MPa is met. Moreover, the AC impedance response significantly moves left and the radius of high-frequency arc reduces from 72910 Ω to 1295 Ω, suggesting electrical percolation. Bounded by percolation threshold, the fractional change of resistance curves can be divided into a two-stage pattern (i.e., linear and nonlinear stages) and a three-stage mode (i.e., linear decrease, balance, and abrupt increase stages). An underlying mechanism is proposed to explain the tremendous change in piezoresistive behaviour of self-sensing UHPC, considering the tunneling-percolation theory and electromechanical coupling behaviour. Additionally, the gauge factors range from 11 to 28, which is higher than most of the existing reported values, demonstrating the great potential of using hybrid functional nano-additives in self-sensing UHPC.

The very early-age behaviour of Ultra-High Performance Concrete containing ground granulated blast furnace slag

Ultra-High Performance Concrete (UHPC), despite being a relatively new kind of advanced cement-based material (featuring outstanding mechanical performance and excellent durability), has been deemed unsustainable due to its high cement content, thus increasing its carbon footprint. The large amounts of cement and silica fume (SF) can also raise both production costs and cracking risks due to the high early-age autogenous shrinkage. This paper addresses an innovative approach for developing a more sustainable UHPC involving an efficient application of ground granulated blast furnace slag (GGBS). Slags of two fineness levels are used, displaying a Blaine fineness of 420 m2/kg (SL1) and 700 m2/kg (SL2), respectively. These slags have been incorporated as volume replacements for 30% and 50% cement.The results analysis shows that it is indeed possible to produce SL2-based UHPC with a behavior similar to that of classical silica fume-based materials. The partial replacement of ordinary Portland cement (OPC) by 30% of GGBS (SL1 or SL2) improves workability, promotes the hydration reaction of cement and accelerates setting. In this case, the nucleation site effect of GGBS particles dominates; it seems to be directly correlated with the fineness of the slag used. Thus, the finer additions result in a significant acceleration of setting and hydration. The dilution effect is more sensitive at 50% of SL1 substitution. This prevailing effect delays the hydration reaction, prolongs setting times and decreases the peak of heat flow. In contrast however, this delay effect was not observed for the mixture containing 50% of SL2. Shrinkage measurements indicated that 30% of SL1 induces a slight reduction in early-age chemical and autogenous types of shrinkage. The most severe shrinkage is clearly found in mixtures incorporating 30% of SL2. This phenomenon may be due to the distribution of refined pores, which increases capillary tension. A more sustainable UHPC containing a high level of superfine slag (50%) with less autogenous shrinkage has been successfully produced.

Experimental and numerical investigation of fracture behavior of ultra high performance concrete

The fracture behavior of ultra-high performance concrete (UHPC) with different depths of notches and curing ages were investigated using a three-point bending test. The ratio of total fracture energy to initial fracture energy raises from 5.9 to 8.3, and then drops to 7.8 with the increases of depth of notch. The fracture energy of UHPC beams with different depths of notches has size effect based on comprehensive analysis. Furthermore, the ratio of total fracture energy to initial fracture energy diminishes from 17.9 to 8.2. The two types of fracture energy of UHPC grow rapidly as the curing age increases, and the influence of curing ages on the fracture energy is mainly reflected in the energy growth within 14 days. In addition, a more suitable improved softening curve model for UHPC is proposed based on the above experimental results and analysis of fracture process. The model could not only better predict the peak load in the rising section of the crack mouth open displacement (CMOD) curve, but also simulate the failure process in the descending section curve.

Cyclic tensile behavior of ultra-high performance concrete with hybrid steel-polypropylene fiber: Experimental study and analytical model

The present study deals with the effect of steel-polypropylene hybrid fiber on the tensile stress–strain behavior of ultra-high performance concrete (UHPC) subjected to monotonic and cyclic loading. Eight batches of UHPC samples with hybrid fibers were tested, with the failure process monitored by acoustic emission (AE) technique synchronously. The failure mechanism of UHPC and reinforcing mechanism of hybrid fibers were analyzed based on the combination of AE source location and scanning election microscope (SEM) observation results. The results indicated that the inclusion of hybrid steel-polypropylene fibers in UHPC matrix contributes to the enhancement of tensile mechanical performance with respect to the tensile strength, peak strain and especially the tensile toughness. Moreover, the relationship between plastic strain accumulation and envelope unloading strain during cyclic tension presents a linear pattern and appears insensitive to the variation of fiber parameters. However, the hybrid fibers significantly contribute to alleviation of the elastic stiffness degradation of UHPC during the cyclic tension. Based on the stiffness degradation process, a damage evolution law characterized by an exponential equation associated with fiber reinforcement indexes is proposed. Notably, the failure of UHPC specimen stems from concurrent macro-crack propagation and multiple cracking behavior throughout the whole loading process, which is improved due to the multi-scale reinforcement effect of hybrid fibers. Finally, in consideration of the effect of hybrid fibers, an analytical constitutive model is developed to generalize the tensile behavior of UHPC, and the comparisons yield a close estimation with test results in current research and literature.

Basic and drying creep of ultra-high performance concrete

ABSTRACT Experimental observations of the creep behaviour of ultra-high performance concrete (UHPC) are limited, with existing studies all performed on material designs including fibres. This limits the understanding of the underlying behaviour of the cementitious material, including the relative contributions of basic and drying creep. An experimental study of the basic and drying creep was performed on UHPC samples subjected to two different loads for the duration of a year, with two different sample sizes considered. It is demonstrated that there is little difference in magnitude between basic and drying creep, indicating that basic creep is the governing mechanism. The existing creep model in the fib model code 2010 is examined and recalibrated for UHPC.

Flexural Behavior of Ultra-High Performance Concrete (UHPC) Deck with Joints subjected to an Improved Steel Wire Mesh (SWM) Treatment

Prefabricated Ultra-high performance concrete (UHPC) decks are often connected using UHPC wet joints to create the overall deck structure. The UHPC deck is, nevertheless, made weak by the fiber discontinuity at the contact interface. In this study, the fiber at the interface was made to serve as a bridge using an embedded steel wire mesh, and experiments were utilized to determine how the contact between the fiber and the steel bar affects the flexural behavior. The results show that the fiber significantly affects the ductility and flexural strength without reinforcement. The combined action of the fiber and reinforcement of the reinforced specimen somewhat increases the flexural capacity and ductility. The flexural capacity of decks with joints was finally calculated using three different ways, and it was found that the method that simplifies the tension and compression zones into rectangles is the most appropriate.

Behavior of UHPC columns confined by high-strength transverse reinforcement under eccentric compression

This paper presents the experimental and analytical studies on the compressive behavior of ultra-high performance concrete (UHPC) columns confined by high-strength transverse reinforcement under eccentric compression. Nine columns including test variables of spacing, strength and arrangement type of transverse reinforcements, steel fiber content and eccentricity were tested to analyze the load-lateral deflection behavior, failure modes and carrying capacity. The experiment results showed that the deformation capacity, peak load and residual carrying capacity of UHPC columns confined by high-strength transverse reinforcement under eccentric loading were significantly better than those of ordinary-strength transverse reinforcement. Reducing the spacing of transverse reinforcement and increasing the volume fraction of steel fiber could also improve the ductility and carrying capacity of UHPC columns. Then, a mesoscale model was proposed to characterize the tensile behavior of UHPC which considered the fiber inclination, fiber-matrix interface bonding and fiber embedded length. Based on the proposed tensile model, a simplified method for calculating the carrying capacity of UHPC columns under large eccentric compression was developed, where the tensile behavior of UHPC and the confinement effect of transverse reinforcements were considered. The errors between the theoretical and test values were within 10%, indicating that this method can be used to guide the design of UHPC columns.

Stress-strain models for ultra-high performance concrete (UHPC) and ultra-high performance fiber-reinforced concrete (UHPFRC) under triaxial compression

The constitutive behavior of ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHPFRC) under multiaxial stresses, which has not been well understood, needs to be urgently investigated in order to meet an increasing demand for use of UHPC/UHPFRC in construction. This paper therefore presents an experimental study on the triaxial compressive behavior of UHPC and UHPFRC under triaxial compression. The compressive strengths of UHPC and UHPFRC in present study were up to 126.9 and 151.5 MPa, respectively. The test variables included the level of lateral hydraulic pressure, steel fiber volume fraction, and uniaxial compressive strength of UHPC and UHPFRC. The present experimental study provides the much-needed systematic test data on the triaxial compressive behavior of UHPC/UHPFRC. The test results showed that the lateral hydraulic pressure significantly enhanced both the strength and ductility of UHPC and UHPFRC. The presence of steel fibers had significant effects on the axial stress-axial strain behavior and the dilation behavior of UHPC and UHPFRC. Finally, new axial stress-axial strain models as well as a new equation for the axial strain-lateral strain relationship for UHPC and UHPFRC were first proposed.

Monotonic and cyclic compressive behavior of ultra-high performance concrete with coarse aggregate: Experimental investigation and constitutive model

This paper presents a systematic investigation on the compressive stress-strain behavior of ultra-high performance concrete with coarse aggregate (UHPC-CA) subjected to monotonic and cyclic loading. A total of eight groups of UHPC specimens considering different coarse aggregate (CA) and steel fiber (SF) contents were tested, with a synchronous monitoring and acquisition of acoustic emission (AE) signal releasing process. The mechanical properties and performance degradation of UHPC were analyzed. The damage mechanism of UHPC under compression and synergy of CA and SF were discussed. The results show that the inclusion of CA and SF has notable positive synergetic effect on the compressive behavior of UHPC, in spite of deformability. Moreover, obvious elastic stiffness degradation of UHPC with loading cycles is observed, which is alleviated by the incorporation of CA and SF. The failure pattern of UHPC is changed from tensile cracks to shear cracks with the increase of SF and CA contents. In addition, SEM observations demonstrate that CA in matrix acts as the ‘skeletons’ of UHPC, although the microstructures of UHPC are broken because of more pores caused among the inclusions and ‘wall effect’ of CA. Finally, a semi-empirical elastoplastic damage constitutive model is developed to generate the compressive response of UHPC -CA under monotonic and cyclic loading.

Impact of Failure-surface Parameters of Concrete Damage Plasticity Model on the Behavior of Reinforced Ultra-high Performance Concrete Beams

The failure surface of elements can numerically represent by a concrete damage plasticity (CDP) model in Abaqus software which depends on five parameters. The parameters are eccentricity, shape, biaxial to uniaxial compressive strength ratio, dilation angle, and viscosity. This paper studies the effect of changing the values of the failure surface parameters on the bearing capacity, deflection, and overall structural behavior of ultra-high performance concrete (UHPC) beams. The parameters' changes include reasonable values higher or lower than the values adopted by Abaqus. An experimentally performed reinforced UHPC beam is simulated by Abaqus, and the five parameters are calibrated to coincide with the practical results. Then, one of the parameters is changed while others remain constant for each alteration to study its effect on the UHPC beam behavior. The numerical analysis results show that all five parameters do not affect the loading capacity and the corresponding deflection at the first cracking state. At peak state, the eccentricity does not affect the load and deflection. The influence of shape and the biaxial to uniaxial compressive strength ratio are confined to the deflection at peak load only without affecting the bearing capacity. The dilation angle effect appears when it is greater than 30 degrees to 56 degrees, while when it is less than 30, it does not affect the load and deflection. Rising the dilation angle and viscosity value promotes the peak load and the deflection.