Uniaxial Strength Research Articles

Integrated static and dynamic geophysical and geomechanical data for characterization of transport properties

The observatory of transfers in the vadose zone (OZNS-France) offers a unique support for characterizing the Beauce limestone aquifer at various spatial and temporal scales. This observatory consists of a large well associated with external boreholes dedicated to imaging, monitoring, and understanding mass and heat transfers through the vadose zone. An initial geological characterization of core samples gave valuable information on the facies encountered within the vadose zone. It is composed of highly incoherent limestones down to 7 m-depth and a massive and altered limestone rock layer from 7 to 20 m-depth. The latter presents fracturation and karstification increasing with depth. This study focuses on the hard limestone rock units and aims to quantify the level of heterogeneity through static and dynamic laboratory tests performed on core samples. Along 20 m of three boreholes, representative samples are tested through uniaxial or triaxial conditions. Elastic wave velocities are also measured. The obtained Young’s modulus, compression strength, and P- and S-wave velocities highlight strong variations with depth. For example, at 14 m-depth, uniaxial maximal strength ranges from 13 to 123 MPa. In addition, at 16 m-depth, P-wave velocities are distributed from 3,650 to 5,700 m.s −1 . These large scatterings confirm the strong heterogeneity of the geological formations. It is interpreted in terms of a high porosity, degree of fracturation and/or initiate active karstification. The double porosity is discussed from the difference between the static and dynamic Young’s modulus. Their variations are interpreted in terms of connected porosity and important crack density. This last parameter is estimated from the P- and S- wave velocities, and ranges from 0 to 0.43, with an extremum value of 1.21 around 19 m-depth. These values and variations are in good correlation with log imaging and independent permeability laboratory measurements made on the hard limestone rock units. • Coupled methods allow to characterize transport property of heterogeneous limestones. • Comparison of static and dynamic data shows a strong variation in double porosity. • High heterogeneities are in good correlation with permeability measurements.

Dynamic uniaxial compression testing of veined rocks under high strain rates

The occurrence of strainbursts is of major concern in underground deep mining operations. Strainbursts are complex events characterized by a spontaneous strain-energy liberation at the boundaries of underground excavations, putting both the long-term safety of operations and exposing workers to risk. In this study, we reported a series of uniaxial compression tests by using a split Hopkinson pressure bar (SHPB) and a high-speed camera, to understand the role of dynamic loads and high strain rates (10–102 s-1) in the mechanical behavior and fracture patterns of veined rocks. The samples are representative of major deep underground mine districts in Chile: Chuquicamata (QS), Andina (RB), and El Teniente (CMET). Results show a clear increase in the dynamical uniaxial strength concerning quasi-static values. The increase was up to 2,2 times in CMET, 1,8 in RB, and 2,6 in QS. Additionally, the dynamic increase factor (DIF) indicates a strong strain rate dependency of the QS samples. This effect on RB and CMET samples is less evident. The data obtained with dynamic compressional testing indicates that both the increase in stiffness and dynamical compressive strength depends largely on the rock strain rate dependency. Further, we report a direct relationship between fracture patterns and the presence of veinlets. We suggest that fractures propagate through the matrix when the mechanical properties show a strong strain rate dependency, whereas are restricted to veinlets and veins when this dependence is less evident. Thus, as stress pulses enhance the stiffness, the energy dissipation increases, and the proneness of block formation is higher. These are conditions favoring strainbursts occurrence and might increase the severity of this phenomenon in underground excavations.

Fatigue failure mechanisms for AlSi10Mg manufactured by L-PBF under axial and torsional loads: The role of defects and residual stresses

Additive Manufacturing (AM) is with no doubt the most revolutionary manufacturing process developed in the last two decades. Despite the indisputable advantages of this technology, the poor surface quality of net-shape components, the presence of internal defects and the development of process induced residual stresses still represent the main problems for the fatigue strength of critical stressed components. In previous works investigating the same alloy, the uniaxial fatigue strength of both machined and net-shape specimens was correlated with the defect size through a Kitagawa diagram, allowing to describe the problem from the threshold perspective. The aim of this work is to extend this approach by investigating the failure mechanisms under torsion in presence of manufacturing defects, both volumetric and superficial anomalies. Specimens manufactured with laser powder bed fusion (L-PBF) technique out of AlSi10Mg, featuring both machined and net-shape surface state, were tested and analysed. The two experimental campaigns allow to investigate the competition between internal defects and superficial features and their effect on the fatigue performances. Tests were performed under two loading conditions, namely fully reverse torsion (RT=−1) and positive torque ratio (RT=0.1). It was found that for the net-shape specimens manufacturing residual stresses have a key role in influencing fatigue strength of this material, making the fatigue limit in torsion of the two considered loading conditions comparable. All the tested specimens failed onto a maximum principal stress plane, which is in line with multiaxial tests performed on a cast A356-T6 aluminium alloy. In some relatively high shear stresses there is a competition between Mode I and Mode II crack propagation, whose threshold condition is controlled by ΔKth,I.

Prediction of uniaxial and biaxial flexural strengths of resin-based composites using the Weibull model

The aim of the study was to assess the applicability of the Weibull model for resin-based composites (RBC) to predict the outcome of different flexural tests based on one another, while identifying the minimal sample number for a precise Weibull representation. Four RBCs underwent 3-point, 4-point and biaxial flexural testing (n=480). Fracture surfaces of all specimens were assessed under optical microscope, while fracture modes of the uniaxial specimens were documented. Representative specimens for each fracture mode were further analyzed under scanning electron microscope. Since fracture predominantly originated from a surface flaw, the effective surface was used in the Weibull model. The analysis was performed on 20, then 30 and finally 40 specimens per group to assess the effect of the specimen size. Further statistical analysis was performed through uni- and multivariate ANOVA, Tukey's post hoc test (α=0.05), and Pearson's correlation. The Weibull model could predict the results of uniaxial tests within the standard deviation, with the correlation between calculated and measured values reaching values of R2=0.985 and higher. Predictions for or based on the biaxial tests misestimated the measured values, with a weaker correlation (R2=0.875) between measured and calculated flexural strength (FS). The fit of the data to the Weibull distribution improved with rising sample size resulting in better predictions of the FS when n=40. The applicability of the Weibull model on RBCs allows accurate comparison between bending tests and their FS under consideration of the effective surface.

Analysis of dynamic fracture of granite after uniaxial recompression predamaged by high confining pressure cyclic loading based on acoustic emission

As the mining industry continues to mine deep, the safety and stability of deep rock masses (such as pillars) in a complex geostress environment become increasingly important. By selecting granite as the object, a uniaxial compression experiment was designed for predamaged specimens prepared under high confining pressure cyclic loading. The porosity, secondary uniaxial peak strength, and the dynamic fracture characteristics of saturated granite before and after predamage test in the whole process were obtained by nuclear magnetic resonance (NMR) and acoustic emission (AE) analyses. The porosity increment of granite before and after high confining pressure cyclic loading test indicates that the granite has different degrees of mesoscopic damage. The high confining pressure cyclic loading in predamage experiment makes the granite release a stronger energy signal when it ruptures, but the higher the number of cycles, the weaker this energy signal. The stress-strain curve can be divided into four stages: I, II, III, and IV, the fracture mode of natural saturated granite is tensile crack-tensile crack-tensile crack-mixed crack, and the fracture mode of saturated granite after predamage changes into tensile crack-shear crack-shear crack-mixed crack. The distribution range and rise and fall of CV(r) have a certain symmetry with the b-value. The reference value of predamaged saturated granite is CV(r) = 1.2.

Impact of tension stiffening on the tensile and flexural behavior of ECC ferrocement

Ferrocement is a type of thin walls constructed of hydraulic cement mortar reinforced with closely spaced layers of wire mesh. Although the applications of ferrocement have been common, there has been a limited number of studies on the development of new ferrocement composites. Particularly, premature failure could occur to ferrocement due to the vulnerability of spalling and delamination. Engineered cementitious composite (ECC) is characterized by its ductile strain-hardening behavior and crack width control under direct tension. To enhance the mechanical properties and crack width control of ferrocement, the potential for using ECC to replace cement mortar in conventional ferrocement, termed ECC ferrocement, was explored. The influences of the volume fraction, wire size, and wire spacing of the steel mesh on the mechanical properties of ECC ferrocement under tension and flexure were studied. It was found that while ECC ferrocement had substantially enhanced strength and crack width control compared to pure ECC and conventional ferrocement, there was a relatively limited improvement in the deformation capacity. The test results also showed that the interaction between the embedded steel mesh and the surrounding ECC led to a pronounced tension stiffening effect, which significantly enhanced the uniaxial and flexural strengths of ECC ferrocement. In addition to the experimental study, three flexural models, including a rigorous implicit model and two simplified explicit models, were proposed to evaluate the flexural strength of ECC ferrocement. The comparisons between the experimental results and predicted strengths demonstrated that the proposed three models can reasonably predict the flexural strength of ECC ferrocement with an average error of less than 6% when the tension stiffening of ECC was accounted for. The results of parametric studies indicated that a variation in the ultimate ECC compressive strain ranging between 0.003 and 0.005 for the flexural models led to a negligible influence on the predicted flexural strength.

Design parameter and method for airport asphalt mixture based on high-temperature performance

The objective of this study is to establish a design system oriented towards the rutting resistance of paving asphalt mixture. To achieve this goal, a series of experiments and analyses were conducted, including the selection of design parameter, the establishment of design method and the verification of design system. Test results indicated that in comparison with uniaxial penetration strength (UPS) and Marshall stability (MS), uniaxial compression strength (UCS) had a higher sensitivity to asphalt content and was thus suitable as a design parameter. Correlation analysis showed that linear correlation between UCS and dynamic stability was more significant with Pearson correlation coefficient (PCC) exceeding 0.8 for various types of asphalt mix, exhibiting an accurate characterization to the anti-rutting ability of asphalt mix. Moreover, 2.94 MPa was proposed as the design critical value based on fitting equation. Compared to conventional design method, the asphalt mixture from proposed method possessed a better high-temperature performance, especially under severe in-service condition where flow number increased by 27.5% to 37.1% in repeated load permanent deformation (RLPD) test and impression depth decreased by 18.3% to 19.7% in Hamburg wheel tracking (HWT) test. This research provided a new idea for anti-rutting technology innovation of asphalt pavement in civil airport field from the aspect of paving materials design.

Experimental Study on Carbonation Durability of Kaolin Strengthened with Slag Portland Cement.

Slag Portland cement is an environmentally friendly and energy-saving product, which is widely used in cement-reinforced soil. This study used slag Portland cement-reinforced soil as the research object and P.O 42.5 + kaolin (POK) as the reference group. The carbonation depth and strength of P.S.A 42.5 + kaolin (PSK) at different curing times were analyzed using carbonation depth, uniaxial ground pressure strength, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The test results show the following: (1) The active substances in PSK samples can react with calcium hydroxide produced during cement hydration and can reduce the content of OH−. The PSK samples react with OH− and CO2 in the carbonation environment. Both processes considerably reduce the content of OH−. (2) Due to the decrease in OH− content, the carbonation durability of slag Portland cement-reinforced soil is significantly less than that of ordinary Portland cement. (3) The carbonation of slag Portland cement-reinforced soil improves its strength. (4) The results of SEM + EDS and XRD confirm the carbonation depth and strength of the POK and PSK samples. The results show that PSK has important applications in subgrade or building grouting materials and in cement-soil mixing piles (walls).

Adapted multiaxial fatigue models based on the critical plane approach to consider the presence of small defects in steel

Non-metallic inclusions can often be seen in the fatigue crack initiation site in high-strength steels. These inclusions are known to be detrimental to the fatigue strength, as they behave as stress concentration elements. The same is valid for small defects such as notches, holes and scratches. The goal of this research is to investigate not only the effect of non-metallic inclusions but also of small artificial defects on the fatigue strength of the AISI 4140 steel under multiaxial loading conditions. In order to do so, two critical plane based multiaxial fatigue models are coupled with the √area parameter. This is an empirical parameter, which is used to estimate the uniaxial fatigue strength of metals containing small defects or inclusions. One of the major merits of √area parameter is the fact that the material fatigue limits (under push-pull or torsion) can be estimated by considering only the material hardness and the geometry (area) of the small defect, i.e no costly and lengthy fatigue tests are required. To validate the analysis, experiments under pure and combined push-pull and torsional loading in and out-of-phase have been conducted in smooth and also in specimens containing an artificially produced micro hole. Comparison between the estimates provided by the coupled critical plane-√area parameter multiaxial criterion and data provided very good results, with errors around 20%.

Prediction of Uniaxial and Biaxial Flexural Strengths of Resin-Based Composites Using the Weibull Model

Prediction of Uniaxial and Biaxial Flexural Strengths of Resin-Based Composites Using the Weibull Model

A Novel Approach for the Consolidation of Sand by MICP Single Treatment

Saving natural resources has become increasingly important. In construction, research has been done on alternative methods to replace conventional building materials. One of those novel methods is MICP (microbial induced calcium carbonate precipitation). In this process, calcium carbonate crystals are precipitated with the help of ureolytic bacteria. A cementation solution consisting of urea and calcium salt is used. This precipitation can be used for solidification. In the field of MICP research, there exist multiple publications with several kinds of tests, but no verifiable compressive strength test. However, most researchers are concerned with soil improvement or self-healing methods, to fill cracks in concrete. Similarly, column tests are mainly conducted to investigate the strength. This study presents, a new method of strength assay that uses hardened sand samples of 3 cm edge length. This allows for an accurate compressive strength verification and thus the effect of the biocementation treatment. In addition, this method applies a single treatment method with a novel type of formulation of the MICP components. The results show that a single MICP treatment is sufficient for the consolidation of various sands. Compressive strength of up to 1.8 N/mm2 was achieved in the process tests in the uniaxial strength test.

Micro-mechanism of brittle creep in saturated sandstone and its mechanical behavior after creep damage

Understanding the micro-mechanism of brittle creep in saturated rocks and studying their mechanical behaviors after creep damage are of great significance to the design of rock engineering and the prediction of the long-term evolution of the Earth's crust. In this study, we first performed a series of uniaxial creep tests with different creep stage cut-offs on saturated intact sandstones. The brittle creep mechanism of the specimens was explored microscopically by analyzing P-wave velocity, acoustic emission (AE), and nuclear magnetic resonance (NMR) measurements. Then, specimens with different degrees of creep damage were subjected to a secondary short-term loading test or creep test, and their short- and long-term mechanical behaviors, together with AE monitoring, were systematically investigated. The experimental results show that (1) the anisotropy of the specimens increases significantly during creep, with the axial and lateral P-wave velocities exhibiting a difference of 30% at the onset of tertiary creep. (2) The tensile cracks dominate the transient creep stage and the early part of the steady creep stage, while the shear cracks dominate thereafter. (3) The porosities of the specimens increase with the loading stages, and the porosity increments corresponding to small pores are greater than those corresponding to large pores, for all creep stages. (4) Specimens with different degrees of creep damage show greater deformability, and their short-term and creep mechanical parameters, except for uniaxial strength, change dramatically. (5) The AE characteristics of creep-damaged specimens differ significantly from those of intact specimens, and the Kaiser stress memory effect was observed during reloading, with the memory stress being the previous creep stress.

Experimental evaluation of uniaxial strength and creep behavior of frozen gravel

ABSTRACT A Water gravel stratum is commonly encountered during tunnel construction. Due to the high permeability associated with the gravel stratum, artificial ground freezing (AGF) technology is typically adopted to freeze the soil surrounding the tunnel. The strength and creep behavior of gravel vary significantly due to the subfreezing temperature. The influence of temperature and strain rate on the strength of frozen gravel and the creep behavior under different stress levels during loading and unloading conditions were experimentally explored in this study. Test results revealed that uniaxial compressive strength decreased linearly with temperature and increased exponentially with strain rate. Brittle failure was often encountered for the frozen gravel during uniaxial test, which was quite different from frozen sand or silty clay. The material deformation before the occurrence of brittle failure was usually insignificant. In other words, a structural damage due to the failure of frozen gravel may not be associated with any early warning sign, which means the safety of a structure deserves special attention when frozen gravel is involved. A new model was specially developed to predict the creep of frozen gravel under different loading conditions and proved to be more accurate when compared with the creep predicted using other models.

Mechanical Properties and Anchorage Angle Optimization of Rock Mass with a Weak Intercalated Layer

A weak intercalated layer has gradually become the key problem in roadway surrounding rock control in deep rock stress environment, and the selection of anchorage angle has a great influence on the stability of the intercalated rock. This study aimed to prepare weak intercalated layer-composited specimens with different dip angles and anchorage angles for the uniaxial compression test. The results showed that the strength curve of the composite specimens exhibited a single-peak variation trend of first decreasing and then increasing from 0° to 90° inclination variation, and the strength and strain reached the lowest value at 60° inclination angle. The failure modes of composite specimens with different dip angles could be divided into single splitting failure, coupled shear failure, and single shear failure. The initial strength of the unanchored specimen was basically the same as that of the anchored specimen, and with the increase in anchorage angle, the uniaxial strength first increased and then decreased. The appropriate anchorage angle range was 0° to 30°. Mainly slip deformation of the specimen occurred when the anchorage angle was smaller than 30°, and the deformation of the specimen mainly depended on the material of the bolt when the anchorage angle was larger than 30°. The strength variation model of the composite anchorage specimen was proposed, taking into account the stress of the bolt, the influence of the dip angle of the intercalated layer, and the anchorage angle of the structural plane.

Strength and rupture geometry of un-irradiated C26M FeCrAl under LOCA burst testing conditions

Ferritic iron-chromium-aluminum (FeCrAl) alloys are an accident tolerant fuel candidate to replace the incumbent Zr-based claddings. Nuclear grade FeCrAl alloys are marked by superior high temperature mechanical behavior and exceptional steam oxidation resistance , both of which increase safety margins during accident scenarios. In the present study, the loss of coolant accident (LOCA) burst behaviors of three un-irradiated commercially fabricated cladding materials in a simulated LOCA environment were compared: (1) T35Y2, a 1 st generation nuclear grade FeCrAl, (2) C26M, a 2 nd generation nuclear grade FeCrAl, and (3) Zircaloy-2. Both FeCrAl alloys showed improved mechanical strength and steam oxidation resistance compared to Zircaloy-2. C26M claddings burst at significantly higher temperatures for all tested engineering hoop stresses , had limited ballooning, and demonstrated preferential fuel retention behavior in terms of burst opening area and length. High temperature tensile data for C26M is also presented. For both un-irradiated FeCrAl alloys, it was found that a distinct “threshold” burst stress signified the transition between small and large openings. Higher threshold hoop stresses were associated with higher uniaxial strength for the FeCrAl alloys, indicating that tensile data, rather than creep data, could be useful for predicting rupture size and assessing fuel dispersal concerns.

Study on the Optimum Steel Slag Content of SMA-13 Asphalt Mixes Based on Road Performance

To reduce the use of aggregates such as limestone and basalt, this paper used steel slag to replace some of the limestone aggregates in the production of SMA-13 asphalt mixes. The optimum content of steel slag in the SMA-13 asphalt mixes was investigated, and the performance of these mixes was evaluated. Five SMA-13 asphalt mixes with varying steel slag content (0%, 25%, 50%, 75%, and 100%) were designed and prepared experimentally. The high-temperature stability, low-temperature crack resistance, water stability, dynamic modulus, shear resistance, and volumetric stability of the mixes were investigated using the wheel tracking, Hamburg wheel tracking, three-point bending, freeze–thaw splitting, dynamic modulus, uniaxial penetration, and asphalt mix expansion tests. The results showed that compared to normal SMA-13 asphalt mixes, the high-temperature stability, water stability, and shear resistance of the SMA-13 asphalt mixes increased and then decreased as the steel slag content increased. All three performance indicators peaked at 75% steel slag content, and the dynamic stability, freeze–thaw splitting ratio, and uniaxial penetration strength increased by 90.48%, 7.39%, and 88.08%, respectively; however, the maximum bending tensile strain, which represents the low-temperature crack resistance of the asphalt mix, decreased by 5.98%. The dynamic modulus of the SMA-13 asphalt mixes increased with increasing steel slag content, but the volume expansion at a 75% steel slag content was 0.446% higher than at a 0% steel slag content. Based on the experimental results, the optimum content of steel slag for SMA-13 asphalt mixes was determined to be 75%.

Evaluation of the Behavior of Carbon Short Fiber Reinforced Concrete (CSFRC) Based on a Multi-Sensory Experimental Investigation and a Numerical Multiscale Approach.

Carbon fiber reinforcement used in concrete has become a remarkable alternative to steel fibers. Admixing short fibers to fresh concrete and processing the material with a 3D printer leads to an orientation of fibers and a material with high uniaxial strength properties, which offers an economic use of fibers. To investigate its mechanical behavior, the material is subjected to flexural and tensional tests, combining several measuring techniques. Numerical analysis complements this research. Computed tomography is used with several post-processing algorithms for separating matrix and fibers. This helps to validate fiber alignment and serves as input data for numerical analysis with representative volume elements concatenating real fiber position and orientation with the three-dimensional stress tensor. Flexural and uniaxial tensional tests are performed combining multiple measuring techniques. Next to conventional displacement and strain measuring methods, sound emission analysis, in terms of quantitative event analysis and amplitude appraisal, and also high-resolution digital image correlation accompany the tests. Due to the electrical conductibility of carbon fibers, the material’s resistivity could be measured during testing. All sensors detect the material’s degradation behavior comparably, showing a strain-hardening effect, which results from multiple, yet locally restricted and distributed, microcracks arising in combination with plastic deformation.

Experimental investigation on the triaxial behavior of lightweight concrete

In this research, active true triaxial tests with different stress ratios were conducted on cubic specimens made from structural lightweight aggregate concrete (LWAC) to study their mechanical behavior. Different combinations of normal stresses were applied to the specimens to obtain various load paths and load angles. Force and displacement values were recorded during the tests, and stress–strain curves were plotted at each principal direction. Furthermore, the obtained peak stresses at failure points were used to draw the compressive and tensile meridians and failure surfaces in the deviatoric plane. Results show that different combinations of lateral confining stresses have a remarkable effect on the strength and deformation of LWAC under triaxial stresses. Results of the tests intended to capture the compressive meridian show that the variation of lateral confining stresses in a range of (0 – 0.98)fcu yields in peak axial stresses, which vary in a range of (1.00–4.38)fcu. In contrast, for the tests aimed to obtain the tensile meridian, the change in the lateral confining stress in a range of (0–0.25)fcu leads to the peak axial stress variations in the range of (1.14–1.92)fcu. Comparison of different types of LWAC shows that increasing the uniaxial strength of LWAC leads to improvement in their triaxial compressive characteristics. Finally, the model parameters of the well-known Willam-Warnke five-parameter failure criterion for normal concrete were modified for LWAC based on the test results. The experimental test data of the current study can be used for the development of other constitutive models for LWAC.

Pozzolanic Effect on the Hydration Heat of Cements Incorporating Fly Ash, Obsidian, and Slag Additives

Made up of an engineered mix of ordinary Portland cement (OPC) with artificial pozzolans such as trass, fly ash, and slag, the blended cements have been intensely employed within cementitious materials. The main reasons behind this intensive use can be clarified by enhanced workability/strength, the high resistance to chloride/sulfate, reduced permeability/alkali-silica reaction, and a drop in the heat generated by cement’s hydration. The use of cementitious blends within concrete not only offers durable products but also cuts climate impact by energy saving and falling CO2 emissions. This study presents pozzolanic effect on the hydration heat of cements incorporating fly ash, obsidian, and slag additives. The blended cements were manufactured by three different replacement ratios of 20%, 30%, and 50%. The change in the hydration heat of obsidian-, fly ash-, and slag-based cements was observed by several Turkish standards (TS EN 196-8 and TS EN 196-9). Mortars were used for determining the uniaxial strengths of obsidian-, fly ash-, and slag-based cements. The results show that cement’s hydration heat decreases as the rate of additives (e.g., obsidian) increases from 20% to 50%. The cement’s fineness greatly affects its hydration heat. Increasing the refinement of pozzolanic material to a certain level (30%) leads to an increase in the hydration temperature. After reaching this level, there is no clear relation between the fineness and the replacement rate of pozzolans. As a result, the findings of this work will provide a good understanding of artificial pozzolans on performance and quality of obsidian-, fly ash-, and slag-based cements.

Effects of fiber type and specimen thickness on flexural behavior of ultra-high-performance fiber-reinforced concrete subjected to uniaxial and biaxial stresses

In this study, we investigated the effects of steel fiber type and specimen thickness on the uniaxial and biaxial flexural behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC). For this purpose, three types of steel fibers (straight, three-times twisted, and six-times twisted) and three thicknesses of specimen (24, 48, and 72 mm) were used. The test results indicated that, owing to the larger perimeter of the triangular shape and mechanical anchorage effect, the twisted steel fibers exhibited better pullout resistance than the straight steel fiber with a circular shape, and its effectiveness increased with the number of ribs. In contrast, the best flexural behavior of UHPFRC was observed when the straight steel fiber was used under both uniaxial and biaxial stress states, and the six-times twisted steel fiber exhibited the worst flexural performance owing to the excessive bond strength of the composites. The uniaxial and biaxial flexural strengths of UHPFRC were insignificantly influenced by the sample thickness; however, the normalized toughness decreased with an increase in the thickness. A higher flexural strength, normalized toughness up to the peak, and deformability were observed under the biaxial flexural stress state than those under the uniaxial flexural stress state. The use of twisted steel fibers was more effective for slabs subjected to biaxial flexural stress than that for uniaxial beams.