Steel Fibers In Ultra-high Performance Concrete Research Articles

Dynamic pullout behavior of half-hooked and twisted steel fibers in ultra-high-performance concrete containing expansive agents

This study investigates the effects of matrix shrinkage, fiber geometry, and loading rate on the pullout behavior of steel fibers embedded in ultra-high-performance concrete (UHPC). For varying the matrix shrinkage, different quantities of calcium sulfoaluminate-based (CSA) expansive agent (EA) were adopted, ranging from 0% to 8%. In addition, moderately and highly deformed, i.e., half-hooked (HH) and twisted (T), steel fibers were used along with three different loading rates ranging from 0.018 (static) to 793 mm/s (impact). The test results indicated that the addition of the CSA EA is effective in reducing the shrinkage strains of UHPC only when its quantity is greater than 6%. The addition of the CSA EA improved the static average bond strengths of all the HH- and T-fibers; however, its effectiveness on the pullout energy was only valid when these fibers were aligned. The addition of the CSA EA enhanced the dynamic bond strengths of HH- and T-fibers, and the improvement was more obvious in the T-fiber specimens than in the HH-fiber specimens and in the aligned fibers than in the inclined ones. However, its implication on the dynamic pullout energies of the HH- and T-fibers was ambiguous. Both the addition of the CSA EA and a faster loading rate increased the probability of rupture failure of the deformed steel fibers in UHPC. The loading rate sensitivity was the highest in the straight steel fiber in UHPC, followed by the HH- and T-fibers, respectively. Consequently, straight or moderately deformed steel fibers are recommended for use in reinforcing UHPC under extreme loads, such as impact and blast.

Comparative pullout behavior of half-hooked and commercial steel fibers embedded in UHPC under static and impact loads

This study investigates the pullout behaviors of various steel fibers embedded in ultra-high-performance concrete (UHPC) under static and impact loading conditions. For this purpose, four types of steel fibers, i.e., straight, hooked, twisted, and half-hooked, and three different loading rates applied by static and impact pullout test machines were adopted. To examine the effect of inclination angle on the pullout behavior, four different inclination angles of 0°, 30°, 45°, and 60° were considered. Test results indicate that the highest average bond strengths were found for the hooked and twisted fibers for static and impact loads, respectively, whereas the straight fibers exhibited the lowest bond strength at all inclination angles. The effectiveness of using half-hooked fibers increased when they were inclined, and maximized at an inclination angle of 45° compared to straight and highly deformed fibers. The use of the twisted and half-hooked fibers was also more effective in static pullout energies than the hooked and straight fibers. The pullout resistance of all the steel fibers in UHPC was improved under impact loading conditions, and the order of the loading rate effectiveness regarding both the average bond strength and pullout energy was as follows: straight fibers > half-hooked fibers > twisted fibers > hooked fibers. The change in the failure mode from pullout to rupture, due to the increase in the loading rate, adversely affected the rate sensitivity of the bond strength and energy absorption capacity. Since the deformed steel fibers were easily ruptured under impact loads, their rupture failure needs to be prevented to achieve an excellent bond strength and energy absorption capacity.

Numerical simulation and visualization of motion and orientation of steel fibers in UHPC under controlling flow condition

To improve fiber orientation in ultra-high performance concrete (UHPC), one L-shape device was developed to control the flow of fresh mixture. A smoothed particle hydrodynamics (SPH) method was used to analyze and simulate the motion of steel fibers in UHPC with flow through this L-shape apparatus. SPH simulation indicated that steel fibers tended to rotate to be parallel with local streamlines and most fibers were aligned in the horizontal channel of this L-shape device, inducing an improvement of fiber orientation in hardened specimens. This modification effect of fiber orientation can be a little strengthened with the decreasing height of the horizontal channel. On the other hand, a colorless, transparent and viscous concentrated suspension was obtained by adding super absorbent polymer (SAP) into water for representing fresh UHPC matrix and the visualization of fiber motion was achieved. This visualization approach, coupled with image analysis of steel fibers in hardened UHPC specimens, well verified the SPH numerical simulation results. Therefore, this type of L-shape apparatus provides a potential method to manually improve the steel fiber orientation in UHPC for better mechanical properties.

Improvement effect of steel fiber orientation control on mechanical performance of UHPC

In order to improve mechanical properties of ultra-high performance concrete (UHPC), one L-shape device with a narrow horizontal channel was developed to control the flow of fresh mixture and then the orientation of steel fibers was significantly improved. Mixtures with water-to-binder ratios of 0.20, 0.22 and 0.24 were prepared by introducing steel fibers with 1%, 2%, and 2.5% of the total volume respectively. Three fiber orientation parameters (fiber orientation angle, fiber orientation number and fiber orientation coefficient) were quantitatively evaluated by the image analysis method. Based on experimental results, mechanical properties (compressive strength, flexural strength, toughness, deflection at modulus of rupture and maximum deflection) were notably increased with the higher dosage of steel fibers. Compared with specimens prepared by a direct cast, this flow control method improved fiber orientation number and fiber orientation coefficient from 0.6 to 0.7 and 0.4–0.6 to 0.7–0.9 and 0.6–0.8 respectively. The best improvement on flexural strength, toughness, deflection at modulus of rupture and maximum deflection reached 64.3%, 65.1%, 77.1% and 14.9% respectively. Based on experimental results, linear relationships were regressed for fiber orientation parameters and mechanical properties of UHPC. Therefore, a potential method was developed to improve the reinforcement effect of steel fibers in UHPC.

Nondestructive evaluation on dispersion of steel fibers in UHPC using THz electromagnetic waves

Ultra-high performance concrete (UHPC), which has better mechanical properties and durability than reinforced concrete, can greatly reduce the weight of structures due to its high strength and reliability. So, it is very effective for the construction of high-rise buildings and long-span bridges. The mix design for UHPC is not only composed of traditional materials (i.e. cement, sand, and aggregate) but also includes binder and admixture. Moreover, steel fiber is necessary to improve ductility. Since the tensile strength of UHPC depends greatly on the direction of the steel fiber, it is important to confirm the dispersibility and orientation of the steel fiber. However, verification of the dispersibility of the steel fiber after curing is indirectly performed through destructive methods like strength tests through coring. Thus, for a better method of dispersibility confirmation, a more effective dispersion analysis technique is required. In this study, the dispersibility of steel fiber in UHPC was evaluated through tomography of UHPC using THz electromagnetic waves. These waves have both infrared straightness and microwave penetrability. Three UHPC specimens were fabricated with steel fiber and were tested with THz electromagnetic wave spectroscopy by experimentally scanning the specimen. From the experimental results, it was possible to obtain a 3D image of the steel fiber arrangement inside the specimen through scanning of the specimen. Also, through image analysis, the dispersibility of the steel fiber in each specimen was numerically derived. For the future, it is expected that the dispersion analysis technology proposed in this study will be applied as an automated nondestructive testing method for enhanced quality control.

Investigation of Microstructural Damage in Ultrahigh‐Performance Concrete under Freezing‐Thawing Action

This work aims to investigate the damage in ultrahigh‐performance concrete (UHPC) caused by freezing‐thawing action. Freezing‐thawing tests were carried out on UHPCs with and without steel fibers. Mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and X‐ray computed tomography (X‐ray CT) were applied to detect the microstructure of the UHPC matrix before and after the freezing‐thawing tests. The results showed that UHPC possessed very excellent freezing‐thawing resistance due to its dense microstructure. After the freezing‐thawing action, cracks occurred and were prone to initiate at the sand‐paste interface in the UHPC matrix. MIP results also indicated that cracks appeared in the UHPC matrix after the freezing‐thawing action. The number of defects that can be seen by X‐ray CT increased in UHPC after the freezing‐thawing action as well. The mismatch of the thermal expansion coefficients of the aggregate and the paste is considered to be the reason for the cracking at the sand‐paste interface. The steel fibers in UHPC inhibited the propagation of cracks in the matrix and improved the freezing‐thawing performance of UHPC.

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.

Load Transfer Test of Post-Tensioned Anchorage Zone in Ultra High Performance Concrete

Researches on ultra-high performance concrete (UHPC) have been conducted worldwide owing to its outstanding durability and strength performances. The exploitation of the mechanical properties of UHPC will render it possible to achieve economic design through substantial reduction in the cross sectional dimensions and simplification in the reinforcement arrangement. This paper investigates experimentally the load transfer in the prestressed concrete anchorage zone. To provide distinctive features of UHPC compared to ordinary concrete, the cross sectional dimensions of the member were reduced and the stress distribution, deformation and cracking pattern of the PS anchorage zone were examined experimentally according to the degree of reinforcement of the members chosen. The distributions of the bursting stress, spalling stress and longitudinal edge stress in the specimens were observed according to the various types of reinforcement. All the specimens satisfied the load-bearing capacity criterion specified by the European ETAG-013 guidelines and their stress distributions were similar to those in the PS anchorages of post-tensioned members applying ordinary concrete. The cracks propagated longitudinally with lengths up to twice the cross sectional dimensions and their width was smaller than when applying ordinary concrete owing to the bridging effect of the steel fibers in UHPC. Accordingly, the exploitation of the high strength of UHPC enabled us to secure the resistance of the anchorage with no need for particular reinforcing devices.

Enhancement of Ultrahigh Performance Concrete Material Properties with Carbon Nanofiber

Ultrahigh performance concrete (UHPC) realized distinctly high mechanical, impermeability, and durability characteristics by reducing the size and content of capillary pore, refining the microstructure of cement hydrates, and effectively using fiber reinforcement. The dense and fine microstructure of UHPC favor its potential to effectively disperse and interact with nanomaterials, which could complement the reinforcing action of fibers in UHPC. An optimization experimental program was implemented in order to identify the optimum combination of steel fiber and relatively low-cost carbon nanofiber in UHPC. The optimum volume fractions of steel fiber and carbon nanofiber identified for balanced improvement of flexural strength, ductility, energy sorption capacity, impact, and abrasion resistance of UHPC were 1.1% and 0.04%, respectively. Desired complementary/synergistic actions of nanofibers and steel fibers in UHPC were detected, which were attributed to their reinforcing effects at different scales, and the potential benefits of nanofibers to interfacial bonding and pull-out behavior of fibers in UHPC. Modification techniques which enhanced the hydrophilicity and bonding potential of nanofibers to cement hydrates benefited their reinforcement efficiency in UHPC.