Novel closed-loop FRP reinforcement for concrete to enhance fire performance
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Date
10/07/2017Author
Kiari, Mohamed Ahmed Abubaker
Metadata
Abstract
The use of fibre-reinforced polymer (FRP) as an internal reinforcement for concrete
has many advantages over steel, most notably lack of corrosion which is considered to
be a major problem for structures incorporating steel. In Europe alone, it is estimated
that the annual repairing and maintenance costs associated with steel corrosion in
infrastructure are around £20 billion (Nadjai et al., 2005). Despite of its corrosion
resistance, the widespread use of FRP as an internal reinforcement for concrete was
hindered due to its relatively weak performance at elevated temperatures, such as in
the event of fire. Under heating, the polymer matrix in FRP softens, which causes bond
degrading between reinforcement and concrete. The softening of polymer matrices
occurs around their glass transition temperatures, which is typically in the range of 65–
150 °C. The sensitivity of FRP bond to temperature is recognised in design guidelines,
therefore many advise against utilising FRP as an internal reinforcement for concrete
in structures where fire performance is critical. On the other hand, fibres, the other
component of FRP, can tolerate temperatures much higher than polymer matrices.
This research investigates a new design for FRP internal reinforcement, which exploits
the fact that the FRP fibres in general and carbon fibres in particular are capable of
sustaining a large proportion of their original strength at high temperatures. Instead of
the traditional way of using separate bars, FRP reinforcement was made as closed
loops produced through the continuous winding of carbon fibre tows. When the surface
bond degrades at elevated temperatures, interaction with concrete can still be provided
through bearing at loop ends.
The concept of FRP loops was investigated through a series of experimental work.
Firstly, the performance of carbon FRP (CFRP) loops was evaluated through a series
of push-off tests in which specimens consisting of CFRP loops bridging two concrete
cubes were tested in pull-out using hydraulic jacks. Specimens with straight and
hooked reinforcement were produced as well for comparison. A total number of 18
specimens were tested at ambient temperature, glass transition temperature (Tg), and
above Tg. Results showed that while at ambient temperature there was no distinction
in performance. At elevated temperatures, CFRP loops developed strength about three
times higher than specimens with straight or hooked bars. Also, while failure mode
occurred due to de-bond in the case of straight and hooked reinforcement, rupture
failure occurred with CFRP loops.
For better demonstration of the concept in more realistic conditions, four-point
bending tests were conducted upon 28 beam specimens reinforced either with CFRP
loops or straight bars as flexural reinforcement. Beams were tested under monotonic
loading at ambient temperature, or under sustained loads with localised heating over
the midspan region that contained the reinforcement overlaps. The benefit of CFRP
loops became evident in the elevated temperature tests. Beam specimens with spliced
straight bars failed due to debonding after a short period (up to 15 minutes) of fire
exposure. Conversely, the fire endurance increased four to five times when CFRP loop
reinforcement was used. Unlike straight bars, debonding failure was avoided as failure
occurred due to reinforcement rupture. The overlap length of the CFRP loops was
found to be important in the order for the loop to develop full capacity. Premature
failure can occur with short overlap length due to shear off concrete within the overlap
zone. The presence of transverse reinforcement increases confinement levels for
reinforcement, so the bond failure of straight bars at ambient temperature testing was
eliminated when stirrups were provided. However, at elevated temperatures straight
bars failed by pull-out even in presence of transverse reinforcement.
To facilitate design with CFRP loops, a numerical analysis tool was developed to
calculate the bond stress-slip response of reinforcement at ambient and elevated
temperatures. A Matlab programme was designed based on a one-dimensional
analytical model for steel. The bond law was modified to be used for CFRP
reinforcement. Other analytical models from the literature to account for bond
degradation with temperature and tensile strength of curved FRP were also utilised.
The developed Matlab code has the capability of producing slip, axial stress, and bond
stress distribution along reinforcement.
The novel FRP loop reinforcement was demonstrated to be a promising solution for
enhancing the fire performance of CFRP internal reinforcement at elevated
temperatures. It contributes to removing a major obstacle preventing widespread use
of FRP-reinforced concrete, and paves the way for CFRP reinforcement to be used in
situations where fire performance is critical.