Page 149 - Riem-Vol6_nº2

320 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Influence of steel fibers on the reinforcement bond of straight steel bars
bonding stress in this case can be considered constant. In the model
with bonding length equal to 10φ, there is an accentuated loss of
stress in the bar from the beginning of the bonding part and then
an approximately linear decay in the remainder of the bonding part.
This shows the change in the bonding stress distribution when the
anchorage length is higher than the basic anchorage length. The
same behavior can be observed in Figure 13, which shows the prin-
cipal tensile stresses for both models at the moment of maximum
numerical strength. A higher concentration of the tensile stress at the
beginning of the bonding part can be noted for the model with bond-
ing length equal to 10φ, while in the model with bonding length equal
to 5φ, the principal tensile stress was distributed along the bonding
part. This proves that the bonding length equal to 10φ was in fact
higher than the bar’s basic anchorage length.
Table 6 also shows the shear stress measured in the interface ele-
ment between the bar and the concrete for the maximum numerical
strength (fbm). This stress was, on average, 36% higher than the aver-
age numerical strength (fb,n) of the model with bonding length equal to
5φ. This shows that in fact, in the tests where the bar is pulled out, in
localized regions of the bar, the bonding stress can reach values high-
er than its average bonding stress. In the models with bonding length
equal to 10φ, the bonding stress at the interface (fbm) was almost equal
to the average numerical bonding stress (fb,n), which means there is a
lower stress concentration at the interface with the bar when the bond-
ing length is higher than the basic anchorage length.
4. Conclusions
This article treated the influence of steel fibers on the bonding
stress between straight steel bars and concrete, as well as the
influence of these same fibers on the splitting strength of the con-
crete cover. The main conclusions obtained are:
n From the test with specimens with a 10 mm bar and length
equal to 5 cm (5φ ), it can be concluded that the fibers do not
influence the bonding stress between the steel bar and the con-
crete. Upon analysis of the normalized bonding stress, obtained
through the relation between the bonding stress and the split-
ting tensile strength of the concrete, it can be concluded that it
was reduced by up to 45% due to the addition of the steel fibers.
n One of the objectives of this article was to evaluate the minimum
anchorage length of the bars when they are embedded in the
fibrous concrete, for which were executed the tests with bond-
ing length equal to 10φ . From these tests, it can be concluded
that an anchorage length of only 10φ was sufficient to improve
the bar’s anchorage, taking into account that they reached the
steel’s yield stress before being pulled out. Due to the higher
bonding length, in this test the steel fibers showed a positive
influence, taking into account that with 2% fibers the bars fail
without being pulled out from the concrete.
n In general, the expressions used to evaluate the basic anchor-
age length of the steel bars that are present in the Brazilian and
European standards for reinforced concrete structures, proved
to be useful to determine the anchorage length of the bars em-
bedded in the fibrous concrete, if in these expressions the ten-
sile strength of the fibrous concrete is used.
n The fibers had significant influence on the increase of the split-
ting strength of the concrete cover. With the addition of 2% fi-
bers, the bar of 20 mm diameter and concrete cover equal to
3.25 φ managed to reach the yield stress of the steel before the
failure by splitting. This represented an increase of 157% in the
splitting strength of the concrete when compared to the same
bar embedded in concrete without fibers.
n The average bonding stress determined by means of the com-
puter model was approximately 30% lower than the average
bonding stress determined in the test, due to the excessive
cracking of the concrete at the interface with the bar. However,
the modeling of the bars with bonding length equal to 5 cm (5φ )
indicated a positive influence of the steel fibers on the bond-
ing stress between the steel bar and the concrete. This shows
that the fibers can improve the steel-concrete bonding if a good
Figure 13 – Principal tensile stress in the concrete
2 Bonding length equal to 5 – stresses in N/m 2 Bonding length equal to 10 – stresses in N/m
(representation of bonding part only)
A B