This study aims to investigate the effect of hybrid use of macro and micro steel fibers on the pullout and tensile behaviors of ultra-high-performance concrete (UHPC). To this end, three different ...macro steel fibers (i.e., straight, hooked, and twisted fibers) and a single micro straight steel fiber were used, whereby a portion of the macro fibers was replaced with the micro fibers at the constant total volume fraction of 2%. In order to fabricate ultra-high-performance hybrid fiber-reinforced concrete (UHP–HFRC), five different replacement ratios of the macro fibers with the micro fiber were employed, including 0, 0.5, 1.0, 1.5, and 2.0%. The pullout behaviors of multiple aligned fibers embedded in UHPC were also compared with the tensile behaviors of UHP–HFRC to analyze their correlations. The test results indicated that the average bond strength and normalized pullout energy of the macro straight fibers in UHPC were improved after their replacement with micro fibers, whereas those of the hooked and twisted macro fibers were reduced according to the replacement ratio. Lower fiber efficiency ratios were obtained after the replacement of the macro fibers with the micro fibers in the hooked and twisted fiber cases, while similar ratios of approximately 0.3–0.4 were obtained in the cases of the macro straight fibers. In comparison, the post cracking tensile strength and energy absorption capacity of UHPC reinforced with macro straight fibers decreased, whereas those of UHPCs with hooked and twisted fibers increased when they were replaced by the micro fibers. The negative correlation between fiber pullout and tensile behaviors of all UHP–HFRCs occurred because the actual fiber orientation and bonding area in the composites could not be considered in the fiber pullout tests. Thus, it is concluded that the effects of the use of hybrid steel fibers on the tensile performance of the UHPC composites could not be predicted based on the pullout test results of the aligned fibers.
This study evaluated steel fiber corrosion and tensile behaviors of plain and self-healed ultra-high-performance fiber-reinforced concrete (UHPFRC) exposed to 3.5% sodium chloride (NaCl) solution. ...The degree of steel fiber corrosion was quantitatively evaluated via energy dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM) image analyses. Test results indicate that, even after a 20-week immersion in the NaCl solution, only few steel fibers located near the surface of the non-cracked UHPFRC samples were slightly corroded, and they insignificantly affected the tensile behavior. A slightly better tensile performance was achieved by self-healing process, and it was further improved after exposure to the NaCl solution for a longer duration due to the moderately corroded steel fibers through the partially self-healed cracks. The surface roughness of the pulled-out steel fibers from the composites increased due to the self-healing and corrosion processes, relevant to the enhanced tensile performance, and by increasing the immersion duration.
This study investigates the effect of surface corrosion on the pullout behavior of straight steel fibers embedded in ultra-high-performance concrete (UHPC). To this aim, straight steel fibers, either ...with or without surface corrosion, were utilized, and various corrosion degrees from 2% to 15% by weight were considered. To evaluate the implication of rust layer on the pullout behavior of corroded steel fibers from UHPC, both washed and unwashed conditions were considered. The surface roughness of plain and corroded steel fibers was analyzed by means of scanning electron microscope and atomic force microscope (AFM) images. The test results indicated that surface corrosion is effective in enhancing the pullout resistance of straight steel fibers in UHPC when the fibers are completely pulled out from the matrix without breakage. The maximum average bond strength and pullout energy of moderately corroded fibers in UHPC were found to be 18.5 MPa and 715.7 N·mm, approximately 2.7 and 1.8 times higher than those of plain fibers in the same matrix at the aligned condition. The benefits of moderate surface corrosion on improving the pullout resistance were mitigated by inclining the fibers. A higher corrosion degree led to a better pullout resistance up to a certain value (2 or 5%); however, beyond such value, the resistance decreased significantly due to the rupture of fibers. A threshold value of 2% for the corrosion degree was thus suggested to achieve an excellent fiber bridging capability. The washed corroded fibers exhibited higher bond strength and pullout energy than the unwashed ones with the same degree of corrosion at aligned condition; however, the benefits of washing vanished when the fibers were inclined and ruptured prematurely. An obvious correlation between the bond strength and the surface roughness was observed from the AFM images.
The direct tensile behavior of ultra-high-performance surface-modified steel fiber-reinforced concrete was evaluated in this study. Various chemical modifications were applied to steel fibers, ...including acetone washing, hydrochloric acid washing, zinc phosphating, silica coating, and chelation using an ethylenediaminetetraacetic acid (EDTA) electrolyte solution, which improved the tensile strength, tensile strain, and g-value to a maximum of 17.76 MPa, 1.22%, and 144.51 kJ/m3, respectively. Acetone washing and EDTA chelating were the most effective methods for improving the tensile strength, whereas silica coating was the best for improving the strain capacity and energy absorption capacity. The optimal treatment time for EDTA chelation treatment was approximately 6 h, and the tensile performance decreased considerably after 12 h of treatment. Comparing these results with those of pullout experiments revealed that a high shear stress should be maintained after the fiber has fully debonded to effectively enhance the post-cracking tensile performance.
•Steel fibers with chemically treated surfaces had a significant effect of UHPC.•Acetone washing increases the tensile strength most effectively by 19%.•Silica coating improves strain and energy absorption capacity the most at 36% and 37%.•EDTA chelating improves the tensile strength by roughening the surface up to 6 h.•High shear stress of single fiber enhances the post-cracking behavior of composites.
This study investigated the effect of substituting nano-SiO2 for silica fume on the fiber-matrix interfacial bond performance of ultra-high-performance concrete (UHPC). In this study, silica fume was ...substituted by nano-SiO2 in the weight range of 0–50%. The degree of pozzolanic reaction of binder materials was evaluated using the thermogravimetric analysis (TGA) and compressive strength measurement. The single fiber pull-out test was conducted along with a measurement of autogenous shrinkage to evaluate the interfacial bond. The degree of pozzolanic reaction of nano-SiO2 was found to be higher than that of other binder materials. Although the packing density was predicted to increase continuously up to a substitution ratio of 50%, the highest compressive strength was obtained when 10% of silica fume was replaced by nano-SiO2, which improved the compressive strength by 5.9% compared to that of the plain sample. The autogenous shrinkage increased with an increasing content of nano-SiO2 up to 30%; however, it remained similar beyond the nano-SiO2 content of 30%. The best pull-out performance was obtained when 20% of silica fume was replaced by nano-SiO2, in which the average bond strength and pull-out energy were improved by approximately 21 and 68%, respectively. Therefore, substitution of 10–20% of silica fume by nano-SiO2 was recommended as an optimal amount considering the improvements of the compressive strength and fiber-matrix bond performance of UHPC.
Surface-treated steel fibers were developed for enhancing the dynamic pullout performance from ultra-high-performance cement composites (UHPCC). To this end, three types of straight steel fibers with ...a smooth surface (plain) and longitudinal and transverse abrasions were prepared and tested in impact loading conditions. Sandpapers with various grits were used to abrade the fiber surface; hence, various surface roughness parameters could be achieved. Test results indicated that the surface of smooth steel fiber became much rougher upon abrading it using the sandpapers. The pullout resistance of the abraded steel fibers from UHPCC was better than that of the smooth fiber from the same matrix under the static and impact loads. Some of the transversely abraded steel fibers demonstrated a slip-hardening response, which has been rarely observed in commercial smooth, straight steel fiber products. Considering the pullout resistance and rate sensitivity, the transversely abraded steel fiber was the most effective reinforcement for UHPCC subjected to high loading rates, and these fibers could achieve approximately three times greater equivalent bond strength than the plain fiber. The static and dynamic bond strengths increased almost linearly with the surface roughness of the fiber, whereas the pullout energy had no apparent relation with the roughness.
This study aims to develop a robust strain-hardening ultra-rapid-hardening mortar (URHM) with high-volume cementitious materials and polyethylene (PE) fibers. To achieve this, the combined effect of ...cement kiln dust (CKD) and silica fume (SF) on the initial hydration process of ultra-rapid-hardening cement and the tensile performance of URHM was analyzed. Optimum amounts of CKD and SF of 0.15 and 0.2, respectively, by weight ratios to cement, were determined to develop the strain-hardening URHM containing 2% PE fibers. As a result, the tensile strength of 7.3 MPa, strain capacity of 5.12%, and energy absorption capacity prior to tension softening of 297.5 kJ/m3, respectively, were achieved at a very early age (4 h) of air-drying curing. The tensile performance of URHM deteriorated when the CKD content was 0.4 or greater, regardless of the SF content. A lower SF content of 0.2 was effective in terms of the tensile performance enhancement compared with the higher content of 0.4 up to the CKD content of 0.2, but they became similarly lower at higher CKD contents due to insufficient initial hydration.
This study investigated the influence of ordinary Portland cement (OPC) and reactive and non-reactive mineral additives on the characteristic microstructure and mechanical performance of ...ultra-high-performance, strain-hardening cementitious composites (UHP–SHCCs). Nine mixes of cementitious composites were considered composed of reactive and non-reactive materials, such as ground granulated blast furnace slag (GGBS), silica fume (SF), cement kiln dust (CKD), and silica flour. Compressive strength and direct tensile tests were performed on the nine mixes cured for 7 d and 28 d. The test result was analyzed based on microstructural inspections, including thermogravimetry and scanning electron microscopy. The test result and analysis showed that the microstructural property of the UHP–SHCC impacted the compressive strength and the tensile behavior and also influenced the fiber-matrix interaction. Although most of the 7 d cured specimens did not exhibit notable strain-hardening behaviors, the specimen containing the CKD exhibited a tensile strength of 11.6 MPa and a very high strain capacity of 7.5%. All the specimens with OPC, silica flour, GGBS, or SF exhibited considerably improved tensile behavior at 28 d. The specimen with only OPC as a binder could achieve the tensile strength of 11.6 MPa and strain capacity of 6.2%.
This study evaluated the effects of volume fraction, aspect ratio, and shape of steel fibers on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) and the ...structural behavior of reinforced (R-) UHPFRC beams. The tensile strength and energy absorption capacity of ultra-high-performance concrete (UHPC) are improved by adding steel fibers and increasing its volume contents by up to 3.0 %. Compared with short straight steel fiber, medium-length straight and twisted fibers at a volume fraction of 2.0 % result in twice higher energy absorption capacity and higher flexural strength of R–UHPFRC beams. The flexural strength of R–UHPC beams increases by increasing the fiber content up to 3.0 %. However, the strain-hardening characteristics of UHPFRC negatively influence the cracking behavior and stress redistribution in structural beams, causing 48.2–54.1 % lower ultimate ductility indices. The small amounts of steel fibers with volume fraction of ≤1.0 % that exhibit strain-softening behavior only improve the peak ductility.
•Higher flexural strength of R–UHPFRC beams is achieved by using MS and TU fibers than SS fibers.•Strain energy density is predicted based on the quantity Vf(lf2/df) with an R2 higher than 0.98.•Strain-hardening characteristic negatively affects the cracking behavior and stress redistribution in R–UHPFRC beams.•Ultimate ductility of R–UHPC beams decreases by 48.2–54.1 % with the inclusion of steel fibers.•Small amounts of SS fibers (Vf of ≤1.0 %) with strain-softening behavior improves the peak ductility of R–UHPC beams.
This study develops a slip-hardening straight steel fiber in ultra-high-performance concrete (UHPC). For this, a surface treatment using sandpaper with various grits was adopted in both parallel and ...perpendicular directions to the fiber axis. Surface roughness was assessed by atomic force microscopy (AFM), and its correlation with the pullout resistance was evaluated. Test results indicate that although the surface treatment using sandpapers in both parallel and perpendicular directions is effective for enhancing the pullout resistance of straight fiber in UHPC, the latter is recommended since it exhibits better slip-hardening characteristics. The benefits resulting from inclination were more significant in the sanded fiber case and increased with increasing the grit. The higher surface roughness resulted in better pullout resistance, and the effectiveness was higher when the fibers were aligned. The sanded fiber achieved superior resistance and excellent slip-hardening response but exhibited smaller magnitude of frictional shear stress than the twisted fiber.
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