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IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
D. L. ARAÚJO | A. R. DANIN
|
M. B. MELO
|
P. F. RODRIGUES
obtained from the tests is also compared to the recommended
bonding stress in references [8] and [21] for concretes reinforced
with steel fibers. Equations (2) and (3) show the expressions to
calculate the recommended bonding stress from these two refer-
ences, respectively:
(2)
62,25
c
f
c
f
) RIA 50( 22,3 f
37,0
70,0
cube
4,0
4,0
cube
2
b
(3)
5,0
cm
b
f5,2 f
Here, f
b
is the average bonding stress, in MPa; f
cm
is the compres-
sive strength of the concrete obtained from cylindrical test speci-
mens, in MPa; f
cube
is the compressive strength of the concrete
obtained from prismatic test specimens (assumed to be equal to
f
cm
/0.8); RIA is the relation between the height of the ribs (h
r
=0.64
mm) and the distance between the ribs (s
r
=6.1 mm) of the steel
bar;
φ
is the diameter of the steel bar (10 mm); and c is the con-
crete cover (70 mm).
When comparing the values obtained with Equation 2 with the ex-
perimental values, it is noted that this expression overestimates
the bonding stress by 85% on average. On the other hand, the
values obtained with Equation 3 differ from the experimental val-
ues by only 3% on average. This shows that this last expression is
adequate for the evaluation of the bonding stress of bars immersed
in concrete, with and without steel fibers.
3.2 Basic anchorage length
Table 3 shows the results of pull-out tests of the bars with 10 mm
diameter and bonding length equal to 10 cm, i.e. ten times the bar
diameter (10
φ
). For the large majority, the bars were pulled out of
the concrete after reaching the steel’s yield stress (Figure 6), inde-
pendent of the presence of steel fibers. Due to the high concrete
cover around the bar (c/
φ
=7), in none of the tests splitting of the
concrete cover was observed.
Although in these tests the bar as pulled out after the steel’s yield
limit, the average bonding stress between bar and concrete was
still calculated. This is justified by the fact that from these tests one
wishes to obtain the basic anchorage length, defined as the straight
length of a reinforcement bar necessary to anchor the yield load A
s
f
y
in this bar, assuming a uniform bonding strength along this length
[34]. Furthermore, other researchers confirm that the bonding stress
obtained from the pull-out test without yielding of the reinforcement
gives bonding strength values well above the normative values [8].
From the test specimens with fibers, it was noted that the addition
of 1% fibers increased the average bonding strength at the mo-
ment of pull-out of the bar with only 10%, while the addition of 2%
of fibers increased this same bonding strength with only 7%. For
the test specimens with 2% of fibers, those that were pulled out be-
fore the yield of the steel (CP10.10.2.A1 and CP10.10.2.A2) were
disregarded. The small increase that was observed can be related
to the residual resisting capacity of the steel after having reached
its yield strength, i.e. the increase is due to the higher strength than
yield stress of the steel at the maximum load applied in the pull out
test. Furthermore, the closeness of the values suggests that the
volume of added fibers did not have any influence on the average
bonding strength.
Taking into account that in the majority of the tests, the bare reached
the yield stress before being pulled out, it can be concluded that
the bonding length of 10φ was higher than the reinforcement’s
basic anchorage length, independent of the fibers’ presence and
volume. It should be emphasized that in two test specimen with 2%
fibers (CP10.10.2.A3 and CP10.10.2.A4), the bars were not pulled
out from the concrete block, as failure occurred in the part without
bonding (Figure 7). This suggests that the steel fibers might have
improved the bonding strength of these test specimens enough to
cause failure of the steel bar. However, this residual increase of
Figure 5 – (A) Bonding stress normalized by the concrete's splitting
tensile strength; (B) Comparison with empirical equations
A
B
