Post-cracking Strength Research Articles

Synergistic response of blending fibers in ultra-high-performance concrete under high rate tensile loads

This study investigated the synergistic tensile response of blending 1% long and 0.5% short steel fibers in ultra-high-performance concrete (UHPC) at high strain rates of 16–37 s−1. Three ultra-high-performance hybrid-fiber-reinforced concretes (UHP-HFRCs) containing twisted, hooked, or smooth long (30 mm) fibers blended with short (13 mm) smooth fibers, as well as one sample (LS10MS05) blending long and medium (19 mm) smooth fibers, were examined. The blending of long and shorter steel fibers in UHPC generated high synergy in the tensile responses of the UHP-HFRCs, especially at high strain rates. Synergies were significant for strain capacity and peak toughness, but not for post-cracking strength and softening fracture energy. Among the long fibers, the hooked fibers generated the highest synergy at high strain rates, but smooth fibers produced the highest rate sensitivity in UHPC. Consequently, the LS10MS05 sample demonstrated the highest tensile resistance at high strain rates.

Matrix-strength-dependent strain-rate sensitivity of strain-hardening fiber-reinforced cementitious composites under tensile impact

The effects of matrix strength on the rate-sensitive tensile responses of strain-hardening fiber-reinforced cementitious composites (SH-FRCCs) at high strain rates were investigated using an improved strain energy frame impact machine (I-SEFIM) overcoming the limitations of previous strain energy frame impact machine (SEFIM). The strengths of matrix were 56, 81, and 180MPa, while 1% long (30mm) hooked and 1% short (13mm) smooth steel fibers were blended in all matrices. Dynamic increase factors (DIFs) for the tensile parameters of the SH-FRCCs, as the strain rate increased, were clearly dependent on the matrix strength although they generally increased: a lower strength matrix produced higher DIFs for both the strain capacity and peak toughness, whereas a higher strength matrix generated higher DIFs for the post-cracking tensile strength. Besides, DIF equations were newly proposed for predicting the post-cracking strength.

Comparative flexural behavior of ultra-high-performance concrete reinforced with hybrid straight steel fibers

This study investigates the implications of fiber hybridization on the flexural behavior of ultra-high-performance concrete (UHPC). To do this, we considered three straight steel fibers with different lengths, lf, of 13mm (short), 19.5mm (medium-length), and 30mm (long) at various volume fractions. Test results indicated that the hybrid use of long and medium-length fibers effectively improved the flexural performance in terms of post-cracking strength, deflection capacity, toughness, and cracking behavior, whereas the hybrid use of long and short fibers generally decreased the performance. To verify this observation, a micromechanical analysis was performed for obtaining the fiber bridging curve, which most significantly influences the post-cracking properties. The hybrid use of long and short (or medium-length) fibers provided negative synergy values in toughness due to several detrimental effects. The complementary energy in the fiber bridging curve increased with an increase in the fiber length, so that an increase in the proportion of short (or medium-length) fiber clearly decreased the complementary energy. Based on a proposed equation for the relationship between the normalized modulus of rupture and the fiber reinforcing index, we found that it is possible to produce deflection-hardening behavior of UHPC with straight steel fibers when the fiber reinforcing index is higher than 0.5.

Investigating the impact resistance of ultra-high-performance fiber-reinforced concrete using an improved strain energy impact test machine

The direct tensile stress versus strain responses of ultra-high-performance fiber-reinforced concretes (UHPFRCs) at high strain rates (10–150s−1) were investigated using an improved strain energy frame impact machine. The high strain rates (>100s−1) were achieved by increasing the capacity of the hydraulic jack and coupler and changing the diameter and material of the energy frame. The use of a titanium energy frame (diameter of 30mm) increased the impact speed. As the strain rate was increased from 0.000333 to 170s−1, the post-cracking strength of the UHPFRCs containing 2% straight steel fibers increased from 16.5 to 47.1MPa.

Direct tensile behavior of ultra high performance fiber reinforced concrete (UHP-FRC) at high strain rates

This experimental study investigates the direct tensile behavior of ultra-high performance fiber reinforced concrete (UHP-FRC) at strain rates ranging from 90 to 146/s. The tests are conducted using a recently developed impact testing system that uses suddenly released strain energy to generate an impact pulse. Three fiber types were considered, a twisted fiber and two other types of straight fibers. Specimen impact response was evaluated in terms of first cracking strength, post-cracking strength, energy absorption capacity and strain capacity. The test results indicate that specimens with twisted fibers generally exhibit somewhat better mechanical properties than specimens with straight fibers for the range of strain rates considered. All UHP-FRC series tested showed exceptional rate sensitivities in energy absorption capacity, generally becoming much more energy dissipative under increasing strain rates. This characteristic highlights the potential of UHP-FRC as a promising cement based material for impact- and blast-resistant applications.

Fracture behaviour of a fibre reinforced earthen material under static and impact flexural loading

The study investigates the enhancements in the load carrying capacity, crack resistance and energy absorption properties provided by the addition of hemp fibres in an earthen material. Notched earthen samples reinforced with two fibre contents (2% and 3% in weight) and three fibre lengths (10, 20, and 30mm) were manufactured and tested under static and impact bending to investigate and compare the influence of the reinforcement on the fracture resistance of the soil material at low and high strain rates. The results of the experimental analyses show that the incorporation of fibres greatly improves the peak load, the post-crack strength, the ductility and the energy dissipation of soil under both static and impact bending. The mechanical response of both unreinforced and reinforced samples is significantly affected by the rate of loading, with samples exhibiting higher values of strength and absorbed energy under impact than under static bending. For both static and impact loading, the post-crack response of the material at large deformations is clearly improved by increasing the fibre content and, at the same fibre content, by increasing the fibre length.

Behavior Comparison of Prestressed Channel Girders from High-Performance and Ultrahigh-Performance Concrete

In response to the demand for sustainable and improved bridge design practices, the development of emerging materials like ultrahigh-performance concrete (UHPC) is at the forefront of structural innovation. UHPC offers significant advantages to bridge superstructure design as it provides advanced mechanical and durability properties, including high compressive strength and increased tensile capacity. With the introduction of high-strength steel fibers into mixture proportions, postcracking tensile and flexural tensile capacities are increased. These increased tensile capacities provide greater ductility and reduce or possibly eliminate the need for mild steel reinforcement. The present research investigated the behavioral response of full-scale prestressed bridge girders subjected to four-point flexural loading. Two channel-shaped girders were designed to provide equal design moment capacities to facilitate comparative analyses of performance. The first of these girders was designed with high-performance concrete [HPC; 9.5 kips per square inch (ksi; 66 MPa)] and mild steel reinforcement typical of that used in New Mexico bridge designs. The second girder used nonproprietary UHPC [20 ksi (138 MPa)] mixture proportions consisting primarily of local materials, local mixing procedures, and a local curing regimen, all of which were developed at New Mexico State University, Las Cruces. Digital image correlation was used to capture deformations throughout the testing. This information creates a full field of displacements that captures tensile and compressive strain behaviors in the pure moment region. This investigation demonstrated the advantages and improved performance of UHPC and the contribution of steel fiber reinforcement to postcracking strength and flexural capacity.

Corrosion resistance of strain-hardening steel-fiber-reinforced cementitious composites

This study investigated the corrosion resistance of strain-hardening steel-fiber-reinforced cementitious composites (SH-SFRCs) in a chloride environment. Two types of steel fibers, hooked and twisted, were added (2% by volume) to a high-strength mortar matrix (90 MPa). All the specimens were exposed to cyclic wetting in a 3.5% chloride solution followed by drying. The corrosion resistance of SH-SFRCs was then evaluated by measuring the direct tensile resistance after the chloride cycles. The strain capacity and toughness of all the SH-SFRCs decreased significantly after 105 chloride cycles, whereas a slight reduction was observed in their post-cracking strength. The corrosion resistance of SH-SFRCs after the chloride cycles was strongly dependent on the width of multiple microcracks when the SH-SFRCs were pre-cracked by tensioning until 0.1% tensile strain. The addition of calcium nitrite (CNI) was successful in improving the corrosion resistance of the pre-cracked SH-SFRCs in the chloride environment.

The effect of synthetic polyethylene fiber on the strain hardening behavior of engineered cementitious composite (ECC)

This research investigated the effects of polyethylene (PE) fibers on the toughness and compressive and flexural strengths of engineered cementitious composite (ECC) cubes and slabs. In particular, this study discussed the reinforcing index (R.I.) as the main parameter. Tests were conducted in direct tension to evaluate the strain-hardening behavior of ECC with different PE fiber contents. Flexural toughness was also assessed following the ASTM C1018 procedure and post-cracking strength technique (PCSm). Results showed that the compressive strength linearly decreased with the increase of the reinforcing index, which in turn decreased the first crack load and significantly increased the ultimate load and failure deflections, and the ultimate strength of slabs. The toughness indices I20 to I100 significantly increased with the increase in reinforcing index and even exceeded the considered limitations. Based on the observed results, a new definition for the ECC PE was proposed as an extension to the definition given in ASTM C1018. All the residual strength factors increased when the reinforcing index increased, indicating a higher amount of the retained strength. Similarly, the retained strength had a higher amount when the PCS24 values increased with increase in the reinforcing indices.

The effect of hybridization and geometry of polypropylene fibers on engineered cementitious composites reinforced by polyvinyl alcohol fibers

In the present study, the hybridization effect of polypropylene (PP) with polyvinyl alcohol (PVA) fibers in engineered cementitious composites (ECCs) was investigated. For this purpose, PVA reinforcing fiber was partially replaced by PP fibers with different circular, triangular, and trilobal cross-sectional shapes. The flexural behaviors of the ECCs containing hybrid fibers including first-crack strength, post-crack strength, and toughness were studied under three-point bending test. It was found that hybridization with nonround cross-sectional shape PP fibers had positive effect on the ductility but decreased the flexural strength of resultant ECCs. The greater reduction in toughness and post-peak strength was observed for composite containing circular PP fibers. The results indicated that partially replacement of PVA fibers with nonround cross-sectional shape PP fibers can be considered to be promising method to reduce the costs of ECC production along with attaining improved deformability.

Correlating plastic shrinkage cracking potential of fiber reinforced cement composites with its early-age constitutive response in tension

It is well understood that the early-age properties of cementitious materials influences its long-term performance. Cementitious materials experience high moisture loss at early age resulting in volumetric shrinkage. When this shrinkage is restrained, tensile stresses develop resulting in cracking of the material. This issue is more pronounced at early-age when the tensile strength of cementitious materials is not fully developed. There are many methods and models available to predict the drying shrinkage of concrete. However, the relationship between early age uniaxial tensile strength and restrained shrinkage cracking characteristics at early age is not well understood. Uniaxial tests were conducted on dog-bone specimens made using various fiber types including glass, cellulose and three types of synthetic fibers. Tensile characteristics including stress versus strain plots for all the specimens along with stress at peak, post crack strength at 2 mm deflection along with elongation at peak are calculated and presented in this paper. Plastic shrinkage cracking results obtained using an innovative test developed by the authors were correlated to these tensile strength results. Results indicate that post crack residual tensile strength at early-age is inversely proportional to the total crack area resulting from restrained shrinkage. The effectiveness of various fiber types is also discussed in the paper.

Soil-Structure Interaction of Steel Fiber Reinforced Concrete Slab Strips on a Geogrid Reinforced Subgrade

Performance of slabs on grade is governed mainly by the mutual coupled performance of the slab stiffness and the subgrade support. The use of reinforcement geogrids to increase the stiffness and structural capacity of the subgrade support can be potentially advantageous, but is currently poorly understood, especially under frequent transient loading conditions. The objective of this experimental study is to investigate the beneficial use of geogrids as a reinforcing material in a loose subgrade. This paper presents the results of large scale laboratory tests on slab strips (300 mm wide × 150 mm thick × 2,500 mm long) supported on a reinforced subgrade. The slab strips contained either steel fibers or were reinforced with welded wire fabric (WWF), and are compared with earlier tests on an unreinforced subgrade. All slab strips were tested under a central point load and were restrained from uplift at the ends. Both monotonic and cyclic loads are considered. The geogrid increases the bottom surface cracking load of the slab strip, does not affect the top surface cracking load, and increases the ultimate load carrying capacity. The use of steel fiber reinforced concrete (SFRC) was successful in providing post-cracking strength to maintain slab integrity when compared to the WWF reinforced slab strip used in this investigation. A simplified analysis of beams on grade is carried out for the elastic and post-cracking regions, and predicted values for the SFRC slab strip under monotonic load are compared to the measured load-deformation response. The comparison indicates that the theoretical response underestimates the capacity of the SFRC slab strip.

High rate response of ultra-high-performance fiber-reinforced concretes under direct tension

The tensile response of ultra-high-performance fiber-reinforced concretes (UHPFRCs) at high strain rates (5–24s−1) was investigated. Three types of steel fibers, including twisted, long and short smooth steel fibers, were added by 1.5% volume content in an ultra high performance concrete (UHPC) with a compressive strength of 180MPa. Two different cross sections, 25×25 and 25×50mm2, of tensile specimens were used to investigate the effect of the cross section area on the measured tensile response of UHPFRCs. Although all the three fibers generated strain hardening behavior even at high strain rates, long smooth fibers produced the highest tensile resistance at high rates whereas twisted fiber did at static rate. The breakages of twisted fibers were observed from the specimens tested at high strain rates unlike smooth steel fibers. The tensile behavior of UHPFRCs at high strain rates was clearly influenced by the specimen size, especially in post-cracking strength.

Structural performance of steel fibre reinforced concrete — Part IV. Toughness properties

The present paper of a series deals with the experimental characterisation of flexural toughness properties of structural concrete containing different volume of hooked-end steel fibre reinforcement (75 kg/m3, 150 kg/m3). Third-point flexural tests were carried out on steel fibre reinforced concrete beams having a cross-section of 80 mm × 85 mm with the span of 765 mm, hence the shear span to depth ratio was 3. Beams were sawn out of steel fibre reinforced slab elements (see Part I) in order to take into consideration the introduced privilege fibre orientation (I and II) and the position of the beam (Ba-a, Ba-b, Ba-c) before sawing (see Part I). Flexural toughness properties were determined considering different standard specifications, namely the method of the ASTM (American Standards for Testing Materials), the process of the JSCE (Japan Society of Civil Engineering), and the final proposal of Banthia and Trottier for the post cracking strength. Consequently, behaviour of steel fibre reinforced concrete was examined in bending taking into consideration different experimental parameters such as fibre content, concrete mix proportions, fibre orientation, positions of test specimens in the formwork, while experimental constants were the size of specimens, the type of fibre used and the test set-up and test arrangement.

Flexural behavior of engineered cementitious composite (ECC) slabs with polyvinyl alcohol fibers

This paper investigates the effects of polyvinyl alcohol (PVA) fibers on the toughness, compressive and flexural strength of engineered cementitious composite (ECC) cubes and slabs. The key parameter discussed in this study is the reinforcing index. To evaluate the strain-hardening behavior of ECC with different PVA fiber contents, tests were conducted in direct tension. Flexural toughness was also evaluated following ASTM C 1018 procedure and post-cracking strength technique (PCSm). Results showed that the compressive strength decreases as the reinforcing index increases in a nonlinear trend. By increasing the reinforcing index, the first crack load decreases and ultimate strength slightly increases. Furthermore, a significant increase in the first crack strength was obtained by an excess value 1000 of the reinforcing index. There is a significant increase in the deflection at ultimate load and the deflection at failure as the reinforcing index increases in a linear manner. The strain-hardening and multiple cracking behavior were observed for slabs with reinforcing indices higher than 316 whereas the softening behavior was observed for lesser values. The ECC PVA slabs did not attain the desired ductility due to the rupture of PVA fibers. A significant increase has occurred to the toughness indices I20, I30 and I40 with the increase in reinforcing index. Moreover, the indices exceed the limitations considered. A new definition as an extension to the definition given in ASTM C 1018 was proposed for ECC PVA material according to the observed results. All the residual strength factors increased as the reinforcing index increases which indicates a higher amount of strength retained. The PCS60 values increased with increase in the reinforcing indices. Thus, the increase in PCS60 values indicates higher flexural performance, better ductility and energy absorption capacity for slabs.

Strain rate dependent properties of ultra high performance fiber reinforced concrete (UHP-FRC) under tension

The results of an experimental investigation of UHP-FRC tensile response under a range of low strain rates are presented. The strain rate dependent tests are conducted on dogbone specimens using a hydraulic servo-controlled testing machine. The experimental variables are strain rate, which ranges from 0.00011/s to 0.11/s, fiber type, and fiber volume fraction. Five different types of fibers are considered including straight and twisted fibers with different geometric properties. The rate sensitivity of the composite material in tension is evaluated in terms of its first cracking strength, post-cracking strength, energy absorption capacity, strain capacity, elastic modulus, fiber tensile stress and number of cracks. The test results show pronounced rate effects on post-cracking strength and energy absorption capacity. Further, post cracking strength varies linearly with the fiber reinforcing index and energy absorption capacity varies linearly with the product of the fiber length and the reinforcing index, as predicted from the theory for fiber reinforced concrete.

Modeling of fiber-reinforced cement composites: Discrete representation of fiber pullout

The discrete modeling of individual fibers in cement-based materials provides several advantages, including the ability to simulate the effects of fiber dispersion on pre- and post-cracking composite performance. Recent efforts in this direction have sought a balance between accurate representation of fiber behavior and computational expense. This paper describes a computationally efficient approach to representing individual fibers, and their composite behavior, within lattice models of cement-based materials. Distinguishing features of this semi-discrete approach include: (1) fibers can be positioned freely in the computational domain, irrespective of the background lattice representing the matrix phase; (2) the pre- and post-cracking actions of the fibers are simulated with little computational expense, since the number of system degrees of freedom is independent of fiber count. Simulated pullouts of single fibers are compared with theory and test results for the cases of perfectly-plastic and slip-hardening behavior of the fiber–matrix interface. To achieve objective results with respect to discretization of the matrix, pullout forces are distributed along the embedded lengths of fibers that bridge a developing crack. This is in contrast to models that lump the pullout force at the crack surfaces, which can lead to spurious break-off of matrix particles as the discretization of the matrix is refined. With respect to fracture in multi-fiber composites, the proposed model matches theoretical predictions of post-cracking strength and pullout displacement corresponding to the load-free condition. The work presented herein is a significant step toward the modeling of strain-hardening composites that exhibit multiple cracking.

Flexural performance of fibrous concrete with cement additions

The paper presents the ongoing experimental work on flexural behaviour of steel fibre reinforced concrete containing cement additions such as fly ash, limestone powder, silica fume and metakaolin, added in binary and ternary blends. The mechanical properties, namely, compressive strength, flexural strength and flexural toughness were evaluated for all mix combinations. The flexural toughness parameters were determined for comparative evaluation of different concrete mixes by different methods viz. ASTM C1018, Japanese Concrete Institute (JCI) method, ASTM C 1609/C 1609M, post crack strength method. The results have witnessed the enhanced toughness performance of metakaolin based concretes as compared to silica fume based concretes. Additionally, preliminary results examining the probability distributions of fatigue life of limestone powder based mixes have also been presented at the end.

Composite Strain Hardening Properties of High Performance Hybrid Fibre Reinforced Concrete

Hybrid fibres addition in concrete proved to be a promising method to improve the composite mechanical properties of the cementitious system. Fibre combinations involving different fibre lengths and moduli were added in high strength slag based concrete to evaluate the strain hardening properties. Influence of hybrid fibres consisting of steel and polypropylene fibres added in slag based cementitious system (50% CRL) was explored. Effects of hybrid fibre addition at optimum volume fraction of 2% of steel fibres and 0.5% of PP fibres (long and short steel fibre combinations) were observed in improving the postcrack strength properties of concrete. Test results also indicated that the hybrid steel fibre additions in slag based concrete consisting of short steel and polypropylene (PP) fibres exhibited a the highest compressive strength of 48.56 MPa. Comparative analysis on the performance of monofibre concrete consisting of steel and PP fibres had shown lower residual strength compared to hybrid fibre combinations. Hybrid fibres consisting of long steel-PP fibres potentially improved the absolute and residual toughness properties of concrete composite up to a maximum of 94.38% compared to monofibre concrete. In addition, the relative performance levels of different hybrid fibres in improving the matrix strain hardening, postcrack toughness, and residual strength capacity of slag based concretes were evaluated systematically.

Size and geometry dependent tensile behavior of ultra-high-performance fiber-reinforced concrete

This research investigated direct tensile stress versus strain response of ultra-high-performance fiber-reinforced concrete (UHPFRC) with various sizes and geometries. The UHPFRC in this research contained 1% macro twisted and 1% micro smooth steel fibers by volume. The effects of gauge length, section area, volume and thickness of the specimens on the measured tensile response of the UHPFRC were experimentally discovered. The different sizes and geometries of specimens did not generate significant influence on the post cracking strength of UHPFRC whereas they produced clear effects on the strain capacity, energy absorption capacity and multiple cracking behavior of UHPFRC. The strain capacity, energy absorption capacity and the number of multiple micro cracks within unit length obviously decreased as the gauge length, section area and volume of UHPFRC specimens increased. In contrast, as the thickness of the specimen increased, different tendency was observed.