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The effects of a new steel fiber in concrete
under small-caliber impact
M. Michal1, M. Keuser1 & C. Frey2
1Universtät der Bundeswehr, Germany
2Feel Fiber GmbH, Germany
Abstract
Military facilities, buildings such as embassies or nuclear power plants must be
protected against effects and impacts from terrorist attacks or accidents. In the
course of new building or upgrading existing structures the protective function
against gun fire or debris due to explosions should be fulfilled with small
dimensions. The main material property influencing the resistance against these
threats is the tensile strength. By adding steel fibers post-cracking behaviour and
ductility of components can be improved.
A new type of steel fiber has been tested for application in construction
elements under high loading rates. Protection shields have been produced and the
effect of small caliber gun fire has been investigated. The tests on concrete
shields showed a considerably reduced crater dimension and reduced raptures.
By using steel fibers the endangerment of persons and facilities by debris can be
reduced.
With the tests the positive influence and the fundamental suitability of the
new long fibers could be confirmed. A possible application would be the usage
for barriers or modular systems made of precast concrete or hardening of existing
structures.
Keywords: steel fiber, protection shield, small calibre gun fire.
1 Introduction
Impact on concrete structures is characterised by local damage. The concrete in
the area directly affected by the impactor is pushed aside with high velocity.
Fragments are thrown out of the front side of the target [1]. The reflected wave
can lead to scabbing on the rear side of the protection shield (Figure 1).
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doi:10.2495/SUSI140121
Resulting debris can cause harm on persons or equipment. In Kustermann et al.
[2] the influence of different concrete mixtures were investigated. While failure
on the front side is determined by high local pressure, the rear side fails by
tensile stresses due to the reflected wave. The choice of aggregates and
reinforcement has a great influence on the required properties of the protective
component.
Figure 1: Schematic drawing of projectile impact.
Compressive and tensile strength of concrete are increasing with increasing
strain rate. Some of the reasons for this strain-rate effect have been identified by
Curbach [3] as the crack velocity, stress distribution and failure of aggregates. In
drop tower tests with different slabs by Hummeltenberg et al. [4] the slabs with
steel fibers were particularly suitable to resist impact.
In different tests conducted by EMI/UniBw and from in Literature with fiber
reinforced concrete (FRC) under high strain rates no remarkable difference on
the dynamic increase of tensile strength compared to plain concrete (Figures 2
and 3) was found. The same formulas used to describe the DIF for plain concrete
(PC) can also be applied for FRC. The advantage of FRC over PC is the
post-cracking behaviour. The concrete behaves in a more ductile manner and
there´s still a residual load capacity left after cracking.
Figure 2: DIF for tensile strength of plain concrete [1].
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Figure 3: DIF for tensile strength of fiber reinforced concrete (FRC).
With the gun fire tests the effect of varying the steel fiber content on the
extent of debris and failure of the protection shields should be evaluated.
2 Test method
2.1 Test equipment
The gun fire tests could be performed at the laboratory of the Department of
Mechanical Engineering, Weapons Technology and Materials Science at the
University of the Bundeswehr Munich (UniBW). For model tests twelve plates
with dimensions of 50 cm x 50 cm x 8 cm were manufactured. With this
dimensions it was possible to make two hits on the fiber reinforced plates.
Without fibers the plates broke after the first hit (Figure 7). The plates were
mounted in a steel frame and placed into a backstop. To determine the residual
energy, steel sheets (St 37) have been fixed behind the concrete plate (Figure 4).
The plates were exposed to gun fire by ammunition for a Russian sniper rifle
Dragunov (armor-piercing bullet 7.62 x 54R, bullet weight ~ 10.4 g). The gun
fire tests have been performed from a distance of 12 m. The projectile velocity
was about 825 m/s.
Material properties for the panels were obtained from static tests at the
UniBW and tests with a Hopkinson-Bar at the Fraunhofer Ernst-Mach-Institute
in Efringen-Kirchen.
The measurement of projectile velocity was carried out via light barriers
which were placed after the gun and right before the target. The velocity after
penetration has been determined with high speed photos, taken at the rear side of
the plate (Figures 5 and 8).
[5]
[6]
[8]
[7]
[9]
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Figure 4: Shooting range (left), panel mounted in the backstop (center), panel
and steel sheets (right).
Figure 5: Measurement of projectile velocity by light barriers (left, center)
and optical (right).
2.2 Preparation of the specimens
The preparation of the specimens was carried out in the laboratory of the
Institute of Structural Engineering at the UniBW (Figure 6). Beside the plates for
the gun fire tests, specimens to determine static and dynamic material properties
were built. Cylinders for tests on a Hopkinson-Bar were drilled out of concrete
blocks [6].
Figure 6: Manufacturing of specimens in the laboratory of Structural
Engineering of the UniBW Munich.
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After removing the formwork, the specimens were stored under water until
the tests.
2.3 Evaluation
For the description of the damage in the plates the following points were
recorded:
Front and rear side of the plates were photographed
Crater diameter, depth and volume were measured
Debris was collected and weighed
The crater profile was recorded using a measuring rule
The damage was recorded in main points
Figure 7: Typical damage pattern of plate with 0% fiber content (left) and
plate with 0.5% fiber content.
Figure 8: Optical measurement of residual projectile velocity.
3 Test programme
3.1 Concrete mixes
For the tests a concrete mixture similar to the type HFB_s_qu in Bludau et al.
[10], where the influence of different concrete aggregates has been investigated,
was used [11]. Four types of plates were produced with different contents of
steel fibers within the range of 0.0% to 2.0%.
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Table 1: Composition of concrete, maximum grain size dg = 16 mm.
Cement CEM I 42.5 R-HS 297.00 kg/m³
w/b-ratio 0.34 [-]
Aggregates 0–2 mm 353.47 kg/m³
2–8 mm 708.80 kg/m³
8–16 mm 710.12 kg/m³
Fly ash 135.00 kg/m³
Silica fume 36.00 kg/m³
Flow improvers 13.37 kg/m³
Water (with dry aggregates) 121.64 kg/m³
Table 2: Hardened concrete properties with the herein used steel fiber content.
Fiber
content
(Feel Fiber)
Density Compressive
strength
Splitting
tensile
strength
Static
Young’s
modulus
Dynamic
Young’s
modulus
[%] [kg/dm³] [N/mm²] [N/mm²] [N/mm²] [N/mm²]
0.0 2.475 99 6.34 43300 47400
0.5 2.495 79 5.73 43300 42200
1.0 2.505 94 8.34 41500 42700
2.0 2.545 93 8.4 - -
3.2 Steel fibers
For the experiments fibers with the manufacturer’s name FF-50/72-SH-5.01
(length: 50 mm, diameter 0.72 mm, S = Steel, H = 1400 N/mm² tensile strength,
5 = 5 anchor nodes, .01 = variant of the anchor node size) produced by steel fiber
GmbH were used.
Figure 9: Feel fiber GmbH, type FF-50/72-SH-5.01.
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The fibers are manufactured in an innovative technique, where band steel is
grooved on both sides and then flex-leveled until the band fractures. This method
is consuming less energy and more cost-effective then previous manufacturing
processes. CO2-emissions can be reduced significantly.
Anchor Nodes at the fiber improve the bond between steel and concrete.
4 Results
The debris from the front and rear crater was collected and was weighed after
every gun shot. Afterwards the volumes of the front and rear crater were
determined separately by filling them with quartz sand. With the known bulk
density it was possible to calculate the respective volume. With both methods, a
remarkable decrease of debris with increasing fiber content was found. This
effect was most noticeable for the rear crater, where for the 2% FRC nearly no
crater volume was left.
Figure 10: Concrete debris and crater volume subject to fiber content.
Under consideration of the cracks visible in the cross section (Figure 11), the
failure-surface for all specimens showed a similar geometry. The diameter of the
front crater was in a range from 100 to 200 mm with an angle about 23 degrees.
For the rear crater a diameter about 150 to 250 mm with an angle about 20
degrees was determined. Especially for the 2% FRC very small crack widths
about 0.1 mm were measured. The initial crack formation starts when the
reflected tensile wave exceeds the tensile strength of concrete. Fragments and
debris are sewed together in the post-cracking stadium and additional energy is
dissipated by deformation of the steel fibers.
The resistance against gun fire and the protection against debris increased,
with increasing fiber content.
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Figure 11: Cross section.
5 Conclusions
The tests showed hardly any influence of the fibers on static or dynamic tensile
strength of concrete. However, in the gun fire tests a significant reduction of
visible crater diameter and debris for the plates with fibers provided by feel fiber
GmbH could be noted. Thus the risk for persons and furnishing by debris can be
reduced to a considerable extend.
In further steps fracture-mechanical methods should be used for a better
assessment of the dynamic post-cracking behaviour and ductility of FRC. The
concrete mix will be optimized to improve the workability with the steel fibers.
With the experiments the positive influence and the suitability of the new
long fibers for use in protection components could be shown. A possible
application would be for barriers and modular systems in prefabricated concrete
or for reinforcing existing structures.
Acknowledgement
The authors thank the Department of Mechanical Engineering at the UniBW and
the Fraunhofer Ernst-Mach-Institute (EMI) for providing their experimental
facilities for this research.
0.5% 0.0%
2.0% 1.0%
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