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1. Introduction
Precast elements are characterized by offering ease of manufac-
ture. On the other hand, the need to make connections between
these elements to form structures is one of the main problems to
be faced in the use of precast concrete. Thus, the connections are
the most important parts in the design of precast concrete struc-
tures and are of fundamental importance both in relation to their
production (manufacture of part of the adjacent elements to the
connections, mounting structure and complementary services in
the buildings) and to the performance of the finishing structure.
The transfer of compressive stresses on the connections between
components of precast concrete is usually made by direct contact
or by placing boards between the parties. Due to the low tensile
strength and brittle performance of concrete, the first method (di-
rect contact) is rarely used and is limited to cases where there
are very low compressive stresses.
The second method consists of placing a board between the ele-
ments, which is made of a material with good deformation capac-
ity to minimize the concentration of compressive stresses.
Usually, these boards are denominated bearing pads and are made
of elastomer, the most common polychloroprene pads. Being made
of a very soft material, these pads accommodate the irregularities
of the surface, promoting a more uniform stress distribution and al-
low certain movements of the structure. The movements allowed
are the rotation and horizontal displacement. The rotation causes the
performance of the connection to be similar to a pinned connection.
The horizontal displacement enables the relief of stress caused
by the variation in length of the supported elements, which would
be a great advantage of this type of material when remarkable loads
are introduced in the structure. The disadvantages of this type of
material are cost, less durability compared to concrete, low resis-
tance to fire and relatively low compressive strength.
This paper presents the development of bearing pads made of Port-
land cement mortar, modified to be more deformable and have a
higher toughness than the usual mortars. The pads made of this ma-
terial are intended to transfer compressive stress - uniform or not - to
avoid stress concentration and, if appropriate, allow small rotations
between the elements without introducing high stresses along the
edge of the contact area. These pads are made of Portland ce-
ment mortar adding the following components: a) lightweight aggre-
gate or air-entraining agents, b) latex, and c) short fibers.
The lightweight aggregate (expanded vermiculite-term) or air-en-
training agent significantly increases the deformation capacity of
the material in the hardened state. Due to the presence of surfac-
tants used in the production of latex, a significant amount of air
can be incorporated into the mix, while increasing the deformation
capacity of the material. The addition of fibers reduces the work-
ability of the mix and can incorporate air into the hardened mate-
rial, reducing its modulus of elasticity.
Several studies were performed to obtain mixtures with low mod-
ulus of elasticity but with acceptable compressive strengths:
Barboza et al [1], El Debs et al [2], El Debs et al [3], El Debs
et al [4], Montedor [5] and Siqueira [6].
As the mortar pads are less deformable than the corresponding
polychloroprene pads, they initially promote a more rigid structure.
In the case of beam-column connection with grouted dowel, the
replacement of elastomeric pads with this type of pad makes pos-
sible the partial transmission of the bending moment and there-
fore increases the rigidity of the structure. A comparison between
the pads of mortar and the pads of polychloroprene can be seen
in Montedor [5].
Thus, the performance of the pinned connections becomes semi-
rigid. Compared to the pinned connections, semi-rigid connec-
tions present a significant improvement in the structural behavior
of lateral loads, even for connections with a low degree of rigid-
ity (El Debs et al [7]). On the other hand, the effects of length
variation on structural elements must be carefully considered to
minimize the loads that may be introduced in the structure, since
the mortar pad is stiffer than the polychloroprene pad.
M. K. EL DEBS | E.K. BELLUCIO
39
IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 1
Figure 1 – Bearing pads manufacture
Bearing pads in curing process
B
Bearing pads casting
A