106
5 Projectile Penetrations into Coarse Aggregated UHPCC Targets
much better than that of the NSC, the impact resistance was not improved obviously
any more when the cylinder compressive strength was beyond 150 MPa. Liu et al.
(2002) experimentally investigated the impact resistance of SFRHSC targets, where
the fiber volumetric fraction ranged from 0% up to 12% and the projectile striking
velocities were 314 m/s and 414 m/s, respectively. It was found that, the strength and
toughness of the target increased tremendously with rising the fiber mixing ratio,
especially when the fiber volume fraction was beyond 4%. The DOP of the projectile penetrating into 100 MPa concrete target decreased about 33% when the fiber
volume fraction rised from 0 up to 10%. However, above-mentioned large addition
ratios of fibers destroy the workability of the concrete, which cannot be realized
in the field constructions. From the projectile penetration tests on concrete targets
with the compressive strengths of 45–235 MPa and the impact velocities around 610–
710 m/s, Zhang et al. (2005, 2007) discussed the effects of content, type and length of
the incorporated fibers as well as the compressive and flexural tensile strengths of the
concrete on the impact resistances. It indicated that, the compressive strength of the
target and the coarse granite aggregates are both beneficial in decreasing penetration
depth, while the incorporated fibers are mainly contributing to reducing crater diameter and prohibiting crack propagation. Zhang et al. (2005) further proposed that the
SFRHSC with a compressive strength of 100 MPa was the most efficient choice for
the anti-strike protective structures. From a series of comparative projectile impact
tests on NSC (40 MPa) and HSC (93–119 MPa) targets with the striking velocities
at 200–315 m/s, the enhancing effects of compressive strength of concrete as well
as the type and size of coarse aggregates on the impact resistance of the target were
also evaluated by Dancygier et al. (2007). Tai (2009) carried out the low-velocity
(27–104 m/s) projectile impact tests on plain or fiber reinforced NSC (~25 MPa) and
reactive powder concrete (RPC, 161.9–192.8 MPa) target, respectively. The results
indicated that, adding steel fibers slightly increased the compressive strength of RPC,
but remarkably increased the toughness and fracture energy of RPC, e.g. the scabbing
area decreased by approximately 50% when 1% steel fiber was mixed into RPC.
The main conclusions from above works can be drawn as follows: (1) the impact
resistance of the concrete target is enhanced with increasing the compressive strength,
while the brittleness of the target also rises, which is detrimental to prevent the target
from spalling and scabbing. (2) Mixing steel fibers in HSC can significantly improve
the toughness and fracture energy, restrain the initiation and propagation of cracking
due to the bridging effect, and thus reduce the damages of the impact crater as
well as the scabbing of the rear face. However, too many fibers would impair the
workability of the composite, and the allowable upper limit of the fiber volume ratio
for construction is nearly 4%. (3) The type and size of coarse aggregates, especially
the high-strength aggregates, are also very beneficial in decreasing DOP.
From the points of both engineering protection and weapon design, there are two
main shortcomings in the existing experimental investigations. Firstly, the SFRHSC
is recognized as the most efficient material until now in preventing projectile penetration and reducing the frontal and distal crater damages. However, the improvement
of the strength needs decreasing or even eliminating the ingredients sizes in order
to improve the homogeneity and density of the composite, and thus there are little
5 Projectile Penetrations into Coarse Aggregated UHPCC Targets
much better than that of the NSC, the impact resistance was not improved obviously
any more when the cylinder compressive strength was beyond 150 MPa. Liu et al.
(2002) experimentally investigated the impact resistance of SFRHSC targets, where
the fiber volumetric fraction ranged from 0% up to 12% and the projectile striking
velocities were 314 m/s and 414 m/s, respectively. It was found that, the strength and
toughness of the target increased tremendously with rising the fiber mixing ratio,
especially when the fiber volume fraction was beyond 4%. The DOP of the projectile penetrating into 100 MPa concrete target decreased about 33% when the fiber
volume fraction rised from 0 up to 10%. However, above-mentioned large addition
ratios of fibers destroy the workability of the concrete, which cannot be realized
in the field constructions. From the projectile penetration tests on concrete targets
with the compressive strengths of 45–235 MPa and the impact velocities around 610–
710 m/s, Zhang et al. (2005, 2007) discussed the effects of content, type and length of
the incorporated fibers as well as the compressive and flexural tensile strengths of the
concrete on the impact resistances. It indicated that, the compressive strength of the
target and the coarse granite aggregates are both beneficial in decreasing penetration
depth, while the incorporated fibers are mainly contributing to reducing crater diameter and prohibiting crack propagation. Zhang et al. (2005) further proposed that the
SFRHSC with a compressive strength of 100 MPa was the most efficient choice for
the anti-strike protective structures. From a series of comparative projectile impact
tests on NSC (40 MPa) and HSC (93–119 MPa) targets with the striking velocities
at 200–315 m/s, the enhancing effects of compressive strength of concrete as well
as the type and size of coarse aggregates on the impact resistance of the target were
also evaluated by Dancygier et al. (2007). Tai (2009) carried out the low-velocity
(27–104 m/s) projectile impact tests on plain or fiber reinforced NSC (~25 MPa) and
reactive powder concrete (RPC, 161.9–192.8 MPa) target, respectively. The results
indicated that, adding steel fibers slightly increased the compressive strength of RPC,
but remarkably increased the toughness and fracture energy of RPC, e.g. the scabbing
area decreased by approximately 50% when 1% steel fiber was mixed into RPC.
The main conclusions from above works can be drawn as follows: (1) the impact
resistance of the concrete target is enhanced with increasing the compressive strength,
while the brittleness of the target also rises, which is detrimental to prevent the target
from spalling and scabbing. (2) Mixing steel fibers in HSC can significantly improve
the toughness and fracture energy, restrain the initiation and propagation of cracking
due to the bridging effect, and thus reduce the damages of the impact crater as
well as the scabbing of the rear face. However, too many fibers would impair the
workability of the composite, and the allowable upper limit of the fiber volume ratio
for construction is nearly 4%. (3) The type and size of coarse aggregates, especially
the high-strength aggregates, are also very beneficial in decreasing DOP.
From the points of both engineering protection and weapon design, there are two
main shortcomings in the existing experimental investigations. Firstly, the SFRHSC
is recognized as the most efficient material until now in preventing projectile penetration and reducing the frontal and distal crater damages. However, the improvement
of the strength needs decreasing or even eliminating the ingredients sizes in order
to improve the homogeneity and density of the composite, and thus there are little