Concrete Today May 2010 - the Irish Concrete Federation

concrete today - fibre reinforced polymer
FRP reinforced laterally restrained slabs for
sustainable structures
G Tharmarajah, S.E Taylor, D.J Robinson and D.J Cleland
School of Planning, Architecture and Civil Engineering, Queen’s University Belfast
Fibre reinforced polymer (FRP)
reinforcement is a corrosion free
alternative to steel in reinforced concrete
structures to meet the structural and
safety requirements. FRP is a light weight
material with higher strength capacities,
but the brittle behaviour and low modulus
of elasticity were the perceived drawbacks
of this composite material. Compressive
membrane action (CMA) or arching
action has a beneficial influence on the
flexural strength of laterally restrained
slabs. It has been recognised for some time
that restrained slabs behave differently
to simply supported slabs and thus FRP
reinforcement may perform well in
structure which develop CMA.
Introduction
Concrete can be a very durable
material. However, the corrosion of
steel reinforcement can cause severe
deterioration to reinforced concrete
structures which can result in spalling
and cracking of concrete (see Figure 1).
Extreme environmental conditions cause
chloride intrusion and carbonation in
concrete structures which subsequently
lead to expansive corrosion of steel.
Expansive corrosion in steel reinforcement
significantly reduces the design life and
durability of concrete structures. In some
cases repair and maintenance costs, as a
direct result of deterioration caused by steel
corrosion exceed the original cost of the
structure(1).
Fibre reinforced polymer (FRP)
reinforcement, stainless steel and epoxy
coated steel reinforcement are alternative
reinforcement materials to replace
high yield steel in reinforced concrete
structures(2). Among the alternatives,
glass fibre reinforced polymer (GFRP)
reinforcement has good strength, is cost
effective, durable and widely available.
Laterally restrained slabs are inherent
in much of bridge deck construction. To
date, the benefits of arching action have
not been fully realised to produce highly
durable FRP reinforced concretes slabs in
Ireland and the rest of Europe. Therefore,
this research investigates how the benefits
of arching action can be incorporated
to effectively use GFRP reinforcement
Figure 1: Chloride induced corrosion damage
(Courtesy: http://cce.oregonstate.edu)
to replace conventional steel without
compromising the strength, serviceability
and safety of reinforced concrete slabs.
Previous research has outlined preliminary
findings(3, 4, 5) and this paper gives an
overview of recent research at Queen’s
University Belfast.
Research Programme
The research programme is aimed at
investigating the effect of GFRP in laterally
restrained concrete slab strips typical of
bridge deck slabs in Y beam bridges in
Ireland and the UK. Several parameters
were investigated including bar size,
different reinforcement percentage,
spacing, position and size. The slabs
were loaded with a knife edge line load
representing local wheel loading on a
bridge deck slab at the mid-span of the slab
using an accurately calibrated hydraulic
actuator (see Figure 2 & 3). A steel rig was
used to represent the vertical restraint of
Restraint,
K
b=475mm
h=150mm
d=effective
Figure 2 – Model Test Slab Set-up
Load, PkN
the supporting Y beams and the horizontal
restraint of the surrounding slab.
Given concerns by some researchers and
practitioners over the service behaviour
of GFRP reinforced concrete slabs, the
deflection and crack width and pattern
were fully investigated within the service
load range. Deflection was observed
directly below the loading line using two
50mm displacement transducers. A steel
rig was used to represent the supporting Y
beams and surrounding area of unloaded
slab. The displacement of the rig was
monitored using two 25mm transducers
placed at both ends of the rig to check
for any lateral expansion. The strain
development on GFRP bars was monitored
using embedded Fibre Optic Sensors (FOS)
and Electronic Resistant Strain (ERS)
gauges. Crack width development was
recorded using vibrating wire gauges placed
perpendicular to the primary cracks formed
during the test.
1425mm clear span
h
concrete today
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