Page 137 - Riem-Vol6_nº2

308 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Influence of steel fibers on the reinforcement bond of straight steel bars
1. Introduction
The bond between steel bars and the adjacent concrete allows
both materials to be compatible, enabling the use of reinforced
concrete as a construction material. If the bond is good, there will
be fewer cracks, and the reinforcing bar will be better protected by
the surrounded concrete.
Adherence is usually subdivided in three parts: adhesion, friction
and mechanical. This subdivision is based on the bond stress-slip
relationship, as shown in Figure 1. In this figure, s1, s 2 and su rep-
resent the slip relative to the bond stress due to adhesion (τ1), to
friction (τ2), and to mechanical anchorage (τu), respectively.
Adhesion bonding, also called chemical adhesion, corresponds
to the initial part (rather inclined) of the curve and consists of the
resistance to the shear stress between the concrete and steel par-
ticles. It occurs due to the physic-chemical connections between
the bar and the cement paste formed during the bonding. In com-
parison with the other parts of the bonding, adhesion is rather
small, being destructed as soon as the first slip between steel and
concrete occurs.
Friction bonding occurs when a material tends to slip in relation to
another one. However, it depends on the friction coefficient of the
steel-concrete interface and on the surface roughness of the steel
bar. The mechanical bonding is represented by the last upward slop-
ing part of the curve shown in Figure 1. This part is due to the ex-
istence of irregularities at the bar’s surface that function as support
points. This means that the more irregular the bar’s surface struc-
ture, the higher the mechanical bonding, since a so-called ‘wedging
effect’ will take place [1]. The part related to mechanical bonding is
the main reason for the anchoring of ribbed steel bars in concrete,
providing a certain post-peak resistance, and varying in function of
the inclination, height and the distance between the ribs.
There are two main forms of bonding failure: pull-out and splitting.
Failure by pull-out of the steel bar occurs when the shear stress
at the steel-concrete interface is higher than the bonding strength.
In this case, the bar slides without there being failure by splitting
of the adjacent concrete. This normally occurs for small bonding
lengths together with an external confinement pressure, provided,
for example, by a high cover/bar diameter ratio (c/φ).
On the other hand, when the concrete cover is small or there are
no confinement stresses on the anchorage part, failure of the con-
crete by splitting may occur, due to radial tensile stresses coming
from the bar’s ribs. This failure is brittle and usually occurs without
notice. The factors that most influence the resistance to splitting
are the rib’s geometry, the concrete’s strength, the concrete’s con-
finement stress and the relation between the concrete’s cover and
the bar’s diameter [2].
For high values of the adhering length and sufficient concrete cov-
er, the shear stress at the steel-concrete interface is smaller than
the bonding strength and the steel bar can break without being
pulled out from the concrete. In case the pull-out of the bar occurs
at the moment it reaches its yield stress, this bond length is called
basic anchorage length.
In literature, there exist several tests that allow determining the
bond stress-slip relationship. These tests can be basically subdi-
vided in bar pull-out tests [3-10], beam bending tests [6, 11-13],
and direct traction tests on rods (or concentric pull-out test) [14].
This last test consists of concreting two bars at the extremities of
a concrete prism, one of those being pulled-out due to its shorter
anchorage length. Traditionally, the bonding stress is determined
by means of the standard pull-out test, due to the easy executing
process of this test. However, it only provides the average bond-
ing stress along the anchorage length. Next to this, in this test the
concrete is confined at the prism’s support base, thus allowing it
neither to split nor to expand. Another negative aspect of this test
is the difficulty to guarantee the bar’s position during the concreting
of the test specimen, which requires a precise manufacturing pro-
cess of the test specimens such as to guarantee the bar’s linearity.
Fibers can be added to the concrete to improve its post-crack be-
havior, since these act as bridges to transfer stresses between
cracks, controlling the opening of the crack or its propagation [15].
In this sense, lots of research treats the influence of the steel fibers
on the steel-concrete bonding [8, 9, 16-18]. They commonly con-
clude that the steel fibers improve the bonding of the concrete with
the reinforcement, when this is corrugated, inhibiting splitting in the
forces transfer region between the reinforcement and the concrete.
However, some researchers observe that the fibers contribute in a
positive way only in concrete with a higher strength (65 MPa) and
bars with higher diameter (20 mm), and that in some cases, they
can also lead to a reduction of up to 30% in bonding strength [8, 19].
From the point of view of computer modeling of reinforced concrete
structures, various simple constitutive models to represent the lo-
cal bond stress-slip relationship of reinforcement bars immersed in
concrete are available in literature and are already included in proj-
ect codes [20]. These codes, however, do not consider the pres-
ence of the steel fibers and their influence on the bond stress-slip
relationship. In this case, constitutive models reported in literature
can be used [21].
2. Experimental program
For this research, thirty-three prismatic test specimens with di-
mensions of 300 mm x 200 mm x 150 mm were cast, as listed in
Table 1. The concrete was produced with 3% of its mass substitut-
ed by silica fume and another 13% of the cement mass substituted
by fly ash. The substitution by fly ash was done with the objective
to reduce the consummation of cement, and consequently of the
generated heat during the hydration process of the cement. The
Figure 1 –Bond stress-slip relationship