Page 41 - Riem-Vol6_nº2

212 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Numerical analysis of reinforced concrete beams strengthened with high strength cement-based
composite material
1. Introduction
Composite materials based on high-strength synthetic fibers have
been widely employed to strengthen and rehabilitate reinforced
concrete structures in the last few years due to their excellent
properties, including low weight, high mechanical strength, high re-
sistance to corrosion, etc. Fiber-reinforced polymers are available
as sheets or laminates bonded to reinforced concrete structures
by epoxy-based bonding agents, improving their structural perfor-
mance both under service conditions and ultimate loads.
Although epoxy-based bonding agents may present excellent
concrete bonding and mechanical strength, their use also poses
some problems. Epoxy resins have low permeability, low thermal
compatibility with concrete, low fire resistance, and high suscepti-
bility to ultraviolet radiation. Some of these problems may be pre-
vented using a composite system based in synthetic fiber fabric or
laminate bonded to the concrete surface by cement mortar. These
systems can have different solutions, such as textile reinforced
concrete (TRC), textile reinforced mortar (TRM), fiber reinforced
concrete (FRC), and fiber reinforced cementitious mortar (FRCM).
Di Tommaso et al. [2] and Aiello et al. [3] analyzed the behav-
ior of reinforced concrete beams externally strengthened with
FRCM consisting of carbon-fiber fabric in a cement matrix. The
results showed that the composite system was efficient in terms
of strength, stiffness, and ductility. The FRCM system was re-
cently improved by the use of polypara-phenylene-benzo-bisthia-
zole (PBO) fiber fabric. The mechanical properties of PBO fibers
are much better than those of the most resistant carbon fibers
(Ombres [1]). In addition, they have high tolerance to impact, bet-
ter energy absorption capacity than other fibers, as well as high
resistance to fire and are chemically compatible with cement mor-
tars (Wu et al. [4]).
The use of PBO fiber fabrics (see Figure 1) in FRCM systems is
still being investigated. Tests with concrete beams reinforced with
PBO fabric bonded to concrete using cement mortar (PBO-FRCM)
were recently carried out. Experimental analyses with CFRP (car-
bon fiber reinforced polymers) bonded with epoxy resin and PBO-
FRCM to reinforce concrete beams were performed by Di Tomma-
so et al. [5]. The following results were obtained: (i) flexure failure
of the beams strengthened with PBO-FRCM was more ductile
than CFRP-strengthened beams due to a gradual loss in compos-
ite action caused by the slipping of fibers/cementitious mortar; (ii)
in PBO-FRCM strengthened beams, failure mechanisms related
to the loss of strengthening action (debonding) are determined
by the concrete/cementitious mortar interface, whereas in CFRP
strengthened beams, failure due to debonding is determined by
shearing of the concrete cover layer; and (iii) PBO-FRCM has also
shown efficient as strengthening against shear of reinforced con-
crete beams.
The objective of the present study was to perform a numerical
analysis of concrete beams strengthened with this new material
(PBO-FRCM) relative to flexural strength. The numerical model is
based on the finite element method, and it may follow up the re-
sponse of the evaluated structure from initial loading to failure load.
The model can also predict failures modes, including ductile – due
to excessive elongation of the tension reinforcement or to rupture
of the strengthening system – and fragile – due to concrete crush-
ing or strengthening system debonding – failures. The numerical
data were compared with the experimental results obtained by
Ombres [1], demonstrating both the efficacy of the strengthening
material and the potentials of the numerical model.
2. Tested beam characteristics
Flexural strength of the simply supported beams was tested
with two loads concentrated at 90cm from the bearing supports.
Beam span was 270cm between supports, and beams presented
15x25cm rectangular cross section (Figure 2). Two beam series,
designated as S1 and S2, were tested. In series S1, the tensioned
reinforcement, As, consisted of three 12mm bars (3ø12mm),
whereas the compressed reinforcement, A’s, consisted of two
10mm bars (2ø10mm). In series S2, the tensioned reinforcement
consisted of two 10mm bars (2ø10mm), whereas two 8mm bars
(2ø8mm) where used in the compressed reinforcement. In order
to prevent beam shear failures, 8mm stirrups placed every 17cm
were used (ø8c.17). The beams were strengthened with one, two,
Figure 1 – PBO fabric mesh
Figure 2 – Longitudinal and cross section
of the beams tested by Ombres [1]