The previous sections discussed on enhanced concrete properties and structural
performances of fiber-reinforced concrete and FRLWC. Most of the studies attributed
the enhancement to the crack bridging effects (Abu-Lebdeh et al., 2011; Bischoff, 2007;
Campione et al., 2001; Erdem et al., 2011; Gao et al., 1997; Hamoush et al., 2010;
Singh et al., 2004; Song et al., 2005; Sun & Xu, 2009; Yang et al., 2013; Zīle & Zīle,
interfacial bond serves to improve the ultimate strength of concrete. The fibers are
randomly distributed in the concrete matrix and the fibers bridge across the potential
cracks. Once the cement matrix fractures, multiple microcracks will form and propagate
until the microcracks join up to form primary cracking. When a primary crack
approaches the fibers, the crack tip stress is significantly reduced by the fiber-matrix
interfacial bond. The fibers eventually blunt, stop or change the crack propagation. The
blunting effect reduces the crack tip stress and hence permits the concrete to sustain
further loading and to undergo higher deformation before the ultimate strength is
achieved. The primary crack width is significantly reduced and multiple finer secondary
cracks are form in this stage, as shown in Figure 2.5(c). This explains for the
enhancement of the ultimate strength capacity and its corresponding strain in fiber-
reinforced concrete. It is generally known that the steel fibers produced the highest
improvement effect on the ultimate strength than other fibers. This might be attributed
to the high tensile strength and modulus of elasticity of the fibers. When the crack is
approaching the stiff steel fibers, the blunting effect and crack tip stress reduction are
higher compared to other fibers.
For the enhancement effect on the post-cracking behaviors, after the cement
matrix fractures upon reaching the ultimate strength, the bridging of fibers across the
crack aids to hold the cracked cement matrix together (Mode 3 in Figure 2.6).
Additional energy is required for the fiber pullout/debonding/fracture from the matrix
for the cracks to further open up and propagate. This allows the concrete to sustain a
higher energy capacity after the cracking and hence the residual strength/post-cracking
toughness of fiber-reinforced is greatly improved by the incorporation of fibers. Similar
to the pre-cracking behaviors, the steel fibers always outperform other fibers such as
do not fracture as of Mode 1 in Figure 2.7, but most of the steel fibers failed by fiber-
debonding and fiber pullout which is evident from Figure 2.5(b). Both fiber-debonding
and fiber pullout consumes higher energy to overcome the fiber-matrix interfacial bond,
and eventually results in more significant crack closure and reduction in stress intensity
factor at the crack tip. While in the case of fibers of lower stiffness such as
polypropylene and nylon fibers, the fibers might fracture before the fibers are either
pullout or debond from the matrix. Hence it explains the reason for the higher increment
of concrete properties is observed in the fibrillated polymeric fibers than the
monofilament fibers.
Figure 2.6 Toughening mechanisms of fiber in crack bridging effect (Singh et al., 2004)
The crack bridging effect of fibers can be further explained in microstructural
scale. Figure 2.7 showed the crack bridging effect of polypropylene fibers in the
microstructural scale presented by Sun and Xu (2009). Figure 2.7(c) shows the plain
concrete mixture contains many crystalline with the leaf-like, needle or fiber shape
which interweave together among CSH, along with many other villiform gels. However,
microscopic image with the magnifying ratio of 5000 shows that the CH crystalline
exists in a layered structure with high orientation (Figure 2.7(a)). Meanwhile, many
voids are noticeable among these crystalline structures (Figure 2.7(c)). In contrast,
polypropylene fiber-reinforced concrete mixture contains many gel components and has
much less voids than the plain concrete (Figure 2.7(d)). Its crystalline size is obviously
smaller than that of plain concrete, at which the size of the needle-shaped crystalline
decreases from 8–10μm (plain concrete) to 2–4μm (mixture with fiber). The
polypropylene fiber-reinforced concrete mixture contains very few voids among
Figure 2.7 Microscopic images of plain and polypropylene fiber-reinforced concrete (Sun & Xu, 2009)
Furthermore, Figure 2.8 shows the microstructures of the aggregate-cement
interfacial transition zone for the plain and polypropylene fiber-reinforced concretes
with a magnifying ratio of 3000. The microscopic image from Figure 2.8(a) shows that
plain concrete contains many morifolium-shaped CSH gel and hexagonal sill CH
crystalline with relatively large particle sizes, and it has noticeable voids and
microcracking at the interfacial transition zone. However, the microstructures at the
interfacial transition zone of the polypropylene fiber-reinforced concrete mixture looks
much denser with more uniform particle sizes than that of C1, the voids at its interfacial
transition zone are very minimal (Figure 2.8(b)). The observations from Figure 2.7 and
concrete by fiber-matrix interfacial bond, but the fiber reinforcement also densifies the
interfacial transition zone and the CSH gel and reduces the formation of CH crystallines.
Figure 2.8 Microscopic images of interfacial transition zones of (a) plain and (b) polypropylene fiber-reinforced concrete (Sun & Xu, 2009)