2.4 Test Results and Discussions
45
with strain rate rapidly and the material changes from low strain rate sensitivity to high
strain rate sensitivity. As shown in Fig. 2.13, the DIFs for UHPCC material rapidly
increase with the strain rate rising, and the highest DIF of 2.27 is obtained for the
plain UHPCC at the strain rate of 328.4 s
−1 . Hao et al. (2016) thought that the specimen inevitably undergoes lateral expansion under dynamic loading, which results
in lateral stresses that will act as a form of confinement. Li and Meng (2003) derived
that the influence of lateral inertia confinement on the apparent dynamic strength
enchantment is significant and cannot be neglected for the strain rates exceeding
100 s
−1 for mortar material. Thus, the DIF may not be a material property, but rather
a physical result of lateral inertia confinement and many other influencing factors
(e.g., specimen size, moisture) with increasing strain rate.
Besides, for the influences of the steel fiber content and type, it indicates that,
(i) the plain UHPCC has constantly higher DIF values than UHPCC with different
steel fiber content and type, which suggests that UHPCC with various steel fiber
reinforcement is less sensitive to high strain rate than the plain UHPCC; (ii) for both
fiber types, the values of DIF for UHPCC material decrease with the increase of steel
fiber content; (iii) with the identical fiber mixing ratio, the DIFs for UHPCC with
hooked steel fiber are relatively higher compared to those with micro-straight steel
fiber within the experimental strain rate range. The reason may lie in that, addition of
steel fibers could improve the concrete quality and the micro-straight steel fiber has
relatively better effect on that, which leads to lower DIFs. The similar conclusion was
summarized by Bischoff and Perry (1991). They discussed the influence of concrete
quality on DIF by summarizing a large number of experimental data, and concluded
that the poorer concrete exhibits a larger DIF for compressive strength.
For the concrete-like materials, several empirical formulae have been proposed
to estimate the strain-rate effect on the compressive strength. The fib Model Code
2010 (2013) recommended
DIF =
(˙ ε/˙ ε ts )
0.014
for ˙
ε ≤ 30s
−1
0.012(˙ ε/˙ ε ts )
1/3 for ˙
ε > 30s
−1
(2.3)
where ˙
ε is the strain rate in the range of 30 × 10
–6 to 300 s
−1 and ˙
ε ts =30 × 10
−6 s
−1
is the static strain rate.
Based on a series of SHPB test conducted by Ross et al. (1989, 1995, 1996),
Tedesco and Ross (1998) further proposed
DIF =
0.00965log ˙
ε + 1.058 ≥ 1.0 for ˙
ε ≤ 63.1s
−1
0.758log ˙
ε − 0.289 ≤ 2.5 for ˙
ε > 63.1s
−1
(2.4)
Grote et al. (2001) conducted the SHPB test on mortar with the strain rates between
250 s
−1 and 1700s
−1 and established
45
with strain rate rapidly and the material changes from low strain rate sensitivity to high
strain rate sensitivity. As shown in Fig. 2.13, the DIFs for UHPCC material rapidly
increase with the strain rate rising, and the highest DIF of 2.27 is obtained for the
plain UHPCC at the strain rate of 328.4 s
−1 . Hao et al. (2016) thought that the specimen inevitably undergoes lateral expansion under dynamic loading, which results
in lateral stresses that will act as a form of confinement. Li and Meng (2003) derived
that the influence of lateral inertia confinement on the apparent dynamic strength
enchantment is significant and cannot be neglected for the strain rates exceeding
100 s
−1 for mortar material. Thus, the DIF may not be a material property, but rather
a physical result of lateral inertia confinement and many other influencing factors
(e.g., specimen size, moisture) with increasing strain rate.
Besides, for the influences of the steel fiber content and type, it indicates that,
(i) the plain UHPCC has constantly higher DIF values than UHPCC with different
steel fiber content and type, which suggests that UHPCC with various steel fiber
reinforcement is less sensitive to high strain rate than the plain UHPCC; (ii) for both
fiber types, the values of DIF for UHPCC material decrease with the increase of steel
fiber content; (iii) with the identical fiber mixing ratio, the DIFs for UHPCC with
hooked steel fiber are relatively higher compared to those with micro-straight steel
fiber within the experimental strain rate range. The reason may lie in that, addition of
steel fibers could improve the concrete quality and the micro-straight steel fiber has
relatively better effect on that, which leads to lower DIFs. The similar conclusion was
summarized by Bischoff and Perry (1991). They discussed the influence of concrete
quality on DIF by summarizing a large number of experimental data, and concluded
that the poorer concrete exhibits a larger DIF for compressive strength.
For the concrete-like materials, several empirical formulae have been proposed
to estimate the strain-rate effect on the compressive strength. The fib Model Code
2010 (2013) recommended
DIF =
(˙ ε/˙ ε ts )
0.014
for ˙
ε ≤ 30s
−1
0.012(˙ ε/˙ ε ts )
1/3 for ˙
ε > 30s
−1
(2.3)
where ˙
ε is the strain rate in the range of 30 × 10
–6 to 300 s
−1 and ˙
ε ts =30 × 10
−6 s
−1
is the static strain rate.
Based on a series of SHPB test conducted by Ross et al. (1989, 1995, 1996),
Tedesco and Ross (1998) further proposed
DIF =
0.00965log ˙
ε + 1.058 ≥ 1.0 for ˙
ε ≤ 63.1s
−1
0.758log ˙
ε − 0.289 ≤ 2.5 for ˙
ε > 63.1s
−1
(2.4)
Grote et al. (2001) conducted the SHPB test on mortar with the strain rates between
250 s
−1 and 1700s
−1 and established