3.4 Test Results and Discussions
63
fiber reinforced ones under the similar strain rates. With increasing the strain rate,
the UHPCC matrix is fractured into two or more parts, while those reinforced with
micro-straight or hooked steel fibers only have some small cracks. The UHPCC
specimens with 1.0% or 2.0% steel fibers basically maintain the integrity due to the
reinforcement of steel fibers. As well known, the matrix of UHPCC is a kind of brittle
material. After the tensile stress exceeds its dynamic tensile strength, the formation
and propagation of microcracks in brittle materials finally causes the concentration of
these microcracks into a narrow zone, inducing the formation of a visible macrocrack
which makes the fissure of specimens. Addition of steel fibers can effectively prevent
the formation and propagation of cracks.
3.4.2 Dynamic Spalling Strength
Figure 3.8 shows the typical examples to determine the dynamic spalling strengths
of UHPCC with different steel fiber contents and types, and the related test data are
listed in Table 3.2. Furthermore, the influences of strain rate, steel fiber content and
type on the dynamic spalling strength of UHPCC are shown in Fig. 3.9, respectively.
It can be seen that,
(i) As shown in Fig. 3.9, the dynamic spalling strength of UHPCC increases
from 18.7 MPa to 112.1 MPa when the strain rate increases from 14.3 s
−1
to about 110 s
−1 . Its significant sensitivity to the strain rate may result from
the insufficient time for the interior cracks to develop at high strain rates and
under the impact loadings. However, when the input compressive stress to
specimen exceeds the threshold required to trigger the compressive damage,
the evolution and cumulation of compressive damage in the specimens result
in the dynamic spalling strength decreases at the strain rate larger than 110 s
−1 .
(ii) The dynamic spalling strengths of UHPCC reinforced with steel fibers are
larger than those of the plain UHPCC at the strain rate higher than 60 s
−1 , as
shown in Fig. 3.9. Adding steel fibers could effectively improve the dynamic
spalling strength. For example, the dynamic spalling strength of UHPCC with
mixing 2.0% hooked steel fibers (112.1 MPa) is nearly two times larger than
that of the plain UHPCC (58.7 MPa) at the strain rate around 110 s
−1 . The
increase in the dynamic spalling strength may result from the fiber-bridging
effect which plays an important role to delay the formation and propagation
of the cracks under the dynamic tensile loadings.
(iii) With the 1.0% volume fraction of steel fiber, the micro-straight steel fibers have
slightly better effect on improving the dynamic spalling strength of UHPCC
than the hooked steel fibers at the strain rate higher than 60 s
−1 .
Besides, it is unreasonable that the dynamic spalling strengths of S-2 specimens
are smaller than S-1 specimens with the strain rate larger than 60 s
−1 . The reason may
be that the specimens with 2.0% micro-straight steel fibers fail to reach complete
fracture due to larger fiber-bridging stress than the other types of specimens under
63
fiber reinforced ones under the similar strain rates. With increasing the strain rate,
the UHPCC matrix is fractured into two or more parts, while those reinforced with
micro-straight or hooked steel fibers only have some small cracks. The UHPCC
specimens with 1.0% or 2.0% steel fibers basically maintain the integrity due to the
reinforcement of steel fibers. As well known, the matrix of UHPCC is a kind of brittle
material. After the tensile stress exceeds its dynamic tensile strength, the formation
and propagation of microcracks in brittle materials finally causes the concentration of
these microcracks into a narrow zone, inducing the formation of a visible macrocrack
which makes the fissure of specimens. Addition of steel fibers can effectively prevent
the formation and propagation of cracks.
3.4.2 Dynamic Spalling Strength
Figure 3.8 shows the typical examples to determine the dynamic spalling strengths
of UHPCC with different steel fiber contents and types, and the related test data are
listed in Table 3.2. Furthermore, the influences of strain rate, steel fiber content and
type on the dynamic spalling strength of UHPCC are shown in Fig. 3.9, respectively.
It can be seen that,
(i) As shown in Fig. 3.9, the dynamic spalling strength of UHPCC increases
from 18.7 MPa to 112.1 MPa when the strain rate increases from 14.3 s
−1
to about 110 s
−1 . Its significant sensitivity to the strain rate may result from
the insufficient time for the interior cracks to develop at high strain rates and
under the impact loadings. However, when the input compressive stress to
specimen exceeds the threshold required to trigger the compressive damage,
the evolution and cumulation of compressive damage in the specimens result
in the dynamic spalling strength decreases at the strain rate larger than 110 s
−1 .
(ii) The dynamic spalling strengths of UHPCC reinforced with steel fibers are
larger than those of the plain UHPCC at the strain rate higher than 60 s
−1 , as
shown in Fig. 3.9. Adding steel fibers could effectively improve the dynamic
spalling strength. For example, the dynamic spalling strength of UHPCC with
mixing 2.0% hooked steel fibers (112.1 MPa) is nearly two times larger than
that of the plain UHPCC (58.7 MPa) at the strain rate around 110 s
−1 . The
increase in the dynamic spalling strength may result from the fiber-bridging
effect which plays an important role to delay the formation and propagation
of the cracks under the dynamic tensile loadings.
(iii) With the 1.0% volume fraction of steel fiber, the micro-straight steel fibers have
slightly better effect on improving the dynamic spalling strength of UHPCC
than the hooked steel fibers at the strain rate higher than 60 s
−1 .
Besides, it is unreasonable that the dynamic spalling strengths of S-2 specimens
are smaller than S-1 specimens with the strain rate larger than 60 s
−1 . The reason may
be that the specimens with 2.0% micro-straight steel fibers fail to reach complete
fracture due to larger fiber-bridging stress than the other types of specimens under