Page 139 - Riem-Vol6_nº2

310 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Influence of steel fibers on the reinforcement bond of straight steel bars
ed. These plates had dimensions of 7.5 cm x 7.5 cm for the test
specimens with bars of 20 mm diameter, and 10 cm x 10 cm for the
test specimens with bars of 10 mm diameter.
The area without bonding of the bars was created by enfolding
it with a plastic tube, its length varying in function of the bonding
length of the bar in the concrete (Figure 4 and Table 1). In order to
follow the deformation of the bar, a strain gage was glued on the
bar outside the concrete.
The concrete used in the research was characterized for its av-
erage compressive strength (fcm), following standard ABNT NBR
5739:2007 [28], modulus of elasticity (Ecm), following standard
ABNT NBR 8522:2008 [29, 30] and average splitting tensile
strength (fctm,sp ), or Brazilian Test, following standard ABNT NBR
7222:2010 [31]. All these parameters were obtained with tests on
cylindrical test specimens with dimensions of 150 mm x 300 mm.
Next to these, the three-point bending test, notched at mid-span,
in order to determine the fracture energy of the concrete (Gf) was
carried out conform to the test scheme recommended in [32]. All
these parameters were used in the computer modeling of the bar
pull-out tests.
3. Results and discussion
3.1 Bonding stress
Table 3 shows the results of the mechanical properties of the con-
crete, as well as the results of the pull-out tests, carried out on test
specimens with bars of 10 mm diameter. In this table, an increase
in compressive strength of up to 25% can be observed, when 1% of
steel fibers was added (78.5 kg/m3), confirming other authors who say
that the addition of up to 120 kg/m3 to the concrete increases with ap-
proximately 25% the concrete’s compressive strength [33]. However,
with the addition of 2% of steel fibers (or 157 kg/m3), a lower increase
of the compressive strength is observed, reaching a maximum of
19%. On the other hand, it is observed that the incorporation of steel
fibers caused a significant increase in splitting tensile strength of up to
80% for 1% fibers and 123% for 2% fibers. Yet, the modulus of elastic-
ity of concrete was little influenced by the addition of the steel fibers.
In the same way, the Poisson coefficient also was not influenced by
the addition of the steel fibers, presenting an average value of 0.20.
In all the pull-out tests with bonding length equal to 5φ, failure oc-
curred due to the loss of bonding between the bar and the con-
crete. The failure occurred due to the pull-out of the bar without
the splitting of the concrete cover and with the bar still in linear
elastic behavior. With the addition of 1% of steel fibers, the aver-
age bonding stress showed a small increase of 9%, going from
20.53 MPa to 22.28 MPa. Although the increase in fiber volume
improved the mechanical properties of the concrete, especially the
splitting tensile strength, this improvement did not reflect itself in
the increase of the average bonding stress between steel and con-
crete, calculated with Equation (1), once that with 2% of fibers the
average bonding stress remained almost the same as for the test
specimens without fibers (20.18 MPa).
(1)
 

d
max
b
F
f

tween 7.0, 5.5, 4.2, and 3.25 respectively for the bar of 10 mm,
12.5 mm, 16 mm, and 20 mm. The other variables were the bond-
ing length of the bar in the concrete (five and ten times the diam-
eter of the bar) and the added volume of steel fibers (1% and 2%,
which corresponds to a consummation of fibers of 78.5 kg/m3 and
157.0 kg/m3, respectively). The bonding length of 5φ was used in
order to determine the bonding stress of the bar, in line with what is
recommended in the literature [3], while the bonding length of 10φ
was used to confirm the basic anchorage length of the bar.
The pull-out tests were realized with displacement control in a uni-
versal test machine with load capacity of 300 kN. The test scheme
is shown in Figure 3. The load was applied perpendicularly to the
cast, since the specimens were cast in a horizontal position.
In order to allow fixing the test specimen to the test machine, two
bars of 25 mm were used (at 75 mm from the tested bar), attached
to a metallic device conform to the test adaptation shown in the
literature [27]. For the passage of the bars through the test speci-
men, two holes with 25 mm diameter (for test specimens with a
10 mm bar) or 32 mm diameter (for the others) were made. These
bars were screwed in steel plates on which the test specimen rest-
Figure 2 – Molding of test specimens
for the pull-out test