1. Fiber pulling behavior
In order to analyze, design, and optimize damage-resistant tunnel linings made of SFRC, it is first necessary to understand the mechanical behavior of the material at the structural level. It is well known that the mechanical properties of SFRC are related to the bridging effect across the crack opening. The bridging effect is determined by the total contribution of all fractured fibers. Since the bridging efficiency of each individual fiber strongly depends on the fiber-concrete matrix bonding properties, the pull-out behavior of individual steel fibers embedded in a concrete matrix was systematically studied through laboratory experiments and analytical models. The research results provide a basis for the numerical simulation of the mechanical process of SFRC. On the other hand, they also provide a basis for further research on the application of SFRC in lining segments, such as performance research under local regional loads.
(1) Single steel fiber drawing experiment
The bonding mechanism of steel fibers in the concrete matrix was studied by conducting pull-out tests on single steel fibers. The experimental program analyzes the influence of parameters such as fiber shape, size, tensile strength, tilt angle, and concrete strength by changing the relevant parameters influencing the pull-out behavior.
To conduct pull-out tests, cylindrical specimens with dimensions of 60mmx60 mm were prepared, with steel fibers fixed and embedded in concrete.
(2) Fiber shape and size
Effect of different fiber shapes (straight, corrugated, hook-ended, double-tapered) on the pull-out response of fibers with embedded length=20 mm and tilt angle=0 in a high-strength concrete matrix (=84 MPa). As shown on the left, the pullout response differs significantly between fibers of different shapes. For straight-grained fibers, the bond between the fibers and the matrix is provided only by interfacial friction. Therefore, at almost very low pullout loads, complete debonding occurs, accompanied by a sudden drop in load. In contrast, the pullout load continued to increase after peeling due to the mechanically anchored deformed fibers. Therefore, textured fibers provide significantly higher pullout resistance relative to straight-grained fibers. Due to their strong anchorage in the high-strength concrete matrix, corrugated and bi-tapered fibers fail by fracture soon after exceeding their ultimate load. However, to achieve ductile material behavior, fiber fracture at small pullout displacements should be avoided. Beneficial pull-out behavior was observed for hook-end fibers, for which the hook-ends tended to gradually displace and straighten, which resulted in a decrease in pull-out force and a further increase in pull-out displacement.
By comparing the effects of hook-end fibers of 60/0.75 (mm/mm) and 35/0.55 (mm/mm) on fiber size under the same test conditions. It can be seen that the load-displacement curves of both fibers are similar and almost parallel. However, fibers with larger dimensions exhibited significantly higher ultimate pullout loads ( plus 77 percent ). As the diameter and hook size increase, the bending stiffness of the fiber and the contact area with the matrix increase, which results in an increase in the energy required for the plastic deformation of the hook. However, by comparing the ratio of the ultimate pullout load of the two fibers to the fiber strength, the results showed that the efficiency of the two fibers differed tiny (60.1 percent and 61.0 percent ).
In concrete with the same fiber content, the number of short/fine fibers is several times that of long/coarse fibers, and accordingly, the number of fibers that intercept potential cracks is relatively higher for smaller/fine fibers. So although the results show that long/coarse fibers have higher pull-out resistance compared to smaller/thin fibers, it should not generally be assumed that smaller/thin fibers have poorer load-bearing behavior, on the contrary, it can be concluded from the results Positive synergy. In fiber blends, it is possible to combine the advantages of using fibers with different sizes (longer/thicker and shorter/thinner fibers). In particular, the use of such fiber mixtures in the edge areas of tube segments may have a positive effect on preventing cracking and spalling.
(3) Strength of fiber and concrete
The influence of fiber strength (ft=1225 MPa, 2600 MPa) and concrete strength (fc=44 MPa, 84 MPa). For both concrete strengths, the pullout resistance of high-strength fibers before the friction-slip phase was almost twice that of normal-strength fibers. As expected, fibers embedded in high-strength concrete had higher pullout resistance than fibers embedded in normal-strength concrete. However, this effect is more pronounced for high-strength fibers.
Although there are significant differences in the load-displacement curves of the high-strength fibers in the two concretes, the load-displacement behavior of the normal-strength fibers is similar regardless of the concrete strength. The curves for high-strength fibers tested in normal-strength concrete decrease relatively slowly after reaching the ultimate pullout load. This shows that due to the low strength of the concrete, the mechanical anchoring effect of the hook (hook end) is not effectively exerted. Therefore, the efficiency of high-strength fibers tested in normal-strength concrete was relatively low (42.7 percent ). However, it is important to note that these fibers are significantly more efficient (61.6 percent ) in high-strength concrete. Therefore, to achieve optimal fiber efficiency, the tensile strength of steel fibers should be adjusted to the strength of the concrete.
(4) Fiber tilt angle
Pull-out tests of normal and high-strength hook-end fibers embedded in high-strength concrete at different tilt angles (0 degree , 15 degree , 30 degree , 45 degree and 60 degree ). As expected, the ultimate pullout load of high-strength fibers is significantly higher than that of normal-strength fibers for all tilt angles considered. When ordinary strength fibers are pulled out without failure, the ultimate pullout loads are very similar regardless of the tilt angle. In contrast, high-strength fibers showed a significant increase in ultimate pullout load between tilt angles of 30 degree and 45 degree .
Generally speaking, as the inclination angle increases, the pullout displacement under ultimate load increases and the slope of the front peak branch decreases, especially for inclination angles greater than 30 degree . An increase in matrix crushing and spalling at the fiber exit was also observed with increasing tilt angle. This phenomenon is more obvious in the case of high-strength fibers. Furthermore, as the tilt angle increases, fiber breakage occurs more frequently, especially for ordinary-strength fibers.
