40
2 Dynamic Compressive Mechanical Properties of UHPCC
Correspondingly, by comparing of the failure patterns of UHPCC specimens
at a similar strain rate, the damage degree of UHPCC specimens weakened with
increasing the steel fiber content, which indicates that adding steel fibers can reduce
crack development and the micro-straight steel fiber has better crack confinement
ability. For example, at the strain rate around 150 s
−1 , UHPCC specimens with 2.0%
micro-straight steel fibers broke into few larger parts (Fig. 2.10c), while those with
1.0% micro-straight steel fibers broke into much smaller pieces (Fig. 2.10b). Also,
for the volume fraction of 2.0%, the UHPCC specimens with micro-straight steel
fibers almost kept unseparated under the strain rate around 100 s
−1 , but those with
hooked steel fibers were fragmented into pieces.
2.4.4 Dynamic Stress–Strain Curve
Figure 2.11 shows the average stress–strain curves of the present UHPCC specimens
at different strain rates. It can be seen that, the shapes of above curves are similar,
which consist of the ascending and descending stages. In the ascending stage, the
stress–strain relationship is initially linear elasticity and then progresses to the strainhardening behavior, where micro-cracks begin to develop as the strain increases.
After the stress reaches the peak value, the stress–strain relationship represents the
strain-softening behavior with the continuous cracks further propagation.
Table 2.2 lists the corresponding dynamic compressive properties, where the 1
~ 6 in the test No. represent the six different strain rates. It can be seen that, the
dynamic compressive behaviors of UHPCC are significantly sensitive to the strain
rate, the peak stress and the corresponding strain as well as the dynamic elastic
modulus of UHPCC increase obviously with the increase of strain rate. As stated by
Mindess et al. (2003), the strain rate sensitivity of concrete is usually caused by the
time-dependent movement of free water in the matrix through voids and pores and
the time-dependent nature of crack growth relative to the loading rate. Generally,
the influence of moisture is not always considered. The crack propagating speed
increases with increasing the strain rate, while it remains much lower than that of
stress wave in concrete, which leads to the enhancement of dynamic compressive
properties of concrete.
Figure 2.12 shows the effects of steel fiber content and type on the dynamic
compressive properties of UHPCC. It should be noted that, based on Fig. 2.12, the
average peak stress of “H-1–2” specimens (153.9 MPa) and average peak strain of
“S-2–3” specimens (6.71 × 10
–3 ) are discarded due to the large deviation. It can be
concluded that,
(i) The steel fiber content and type have little influence on the dynamic elastic
modulus and peak strain of UHPCC at different strain rates.
(ii) Adding steel fibers could slightly improve the dynamic compressive strength
of UHPCC, while there is an obvious effect of steel fiber reinforcement on
the static compressive strength and it increases with increasing the volumetric
2 Dynamic Compressive Mechanical Properties of UHPCC
Correspondingly, by comparing of the failure patterns of UHPCC specimens
at a similar strain rate, the damage degree of UHPCC specimens weakened with
increasing the steel fiber content, which indicates that adding steel fibers can reduce
crack development and the micro-straight steel fiber has better crack confinement
ability. For example, at the strain rate around 150 s
−1 , UHPCC specimens with 2.0%
micro-straight steel fibers broke into few larger parts (Fig. 2.10c), while those with
1.0% micro-straight steel fibers broke into much smaller pieces (Fig. 2.10b). Also,
for the volume fraction of 2.0%, the UHPCC specimens with micro-straight steel
fibers almost kept unseparated under the strain rate around 100 s
−1 , but those with
hooked steel fibers were fragmented into pieces.
2.4.4 Dynamic Stress–Strain Curve
Figure 2.11 shows the average stress–strain curves of the present UHPCC specimens
at different strain rates. It can be seen that, the shapes of above curves are similar,
which consist of the ascending and descending stages. In the ascending stage, the
stress–strain relationship is initially linear elasticity and then progresses to the strainhardening behavior, where micro-cracks begin to develop as the strain increases.
After the stress reaches the peak value, the stress–strain relationship represents the
strain-softening behavior with the continuous cracks further propagation.
Table 2.2 lists the corresponding dynamic compressive properties, where the 1
~ 6 in the test No. represent the six different strain rates. It can be seen that, the
dynamic compressive behaviors of UHPCC are significantly sensitive to the strain
rate, the peak stress and the corresponding strain as well as the dynamic elastic
modulus of UHPCC increase obviously with the increase of strain rate. As stated by
Mindess et al. (2003), the strain rate sensitivity of concrete is usually caused by the
time-dependent movement of free water in the matrix through voids and pores and
the time-dependent nature of crack growth relative to the loading rate. Generally,
the influence of moisture is not always considered. The crack propagating speed
increases with increasing the strain rate, while it remains much lower than that of
stress wave in concrete, which leads to the enhancement of dynamic compressive
properties of concrete.
Figure 2.12 shows the effects of steel fiber content and type on the dynamic
compressive properties of UHPCC. It should be noted that, based on Fig. 2.12, the
average peak stress of “H-1–2” specimens (153.9 MPa) and average peak strain of
“S-2–3” specimens (6.71 × 10
–3 ) are discarded due to the large deviation. It can be
concluded that,
(i) The steel fiber content and type have little influence on the dynamic elastic
modulus and peak strain of UHPCC at different strain rates.
(ii) Adding steel fibers could slightly improve the dynamic compressive strength
of UHPCC, while there is an obvious effect of steel fiber reinforcement on
the static compressive strength and it increases with increasing the volumetric