Abstract:
An experimental program was conducted to develop Glass Fiber Reinforced
Plastic (GFRP) reinforcing bars (rebars) for the first time in Pakistan using available local
resources, with tensile and bond strengths closely conforming to the international
standards. The average bond strength of locally developed GFRP rebars was evaluated
using normal strength concrete through direct pullout and beam bond tests by varying the
bonded length, rebar diameter, concrete cover, surface texture as well as the concrete
strength. Sequence and methodology of research work was divided into three distinct
phases, in which first two were related with the development of GFRP rebars.
The optimum composition of resin mixture was determined first of all basing on barcol
hardness criterion through hit and trial approach using standard pultrusion process. Fifty
trial productions of GFRP rebars with barcol hardness tests were executed for this
purpose, and the optimum composition of resin mixture was finalized.
The next stage of experimental program was to achieve the optimized combination of
three process parameters namely, fiber content, pull speed and heating die temperature for
9.5mm, 13mm, 16mm, 19mm, 22mm and 25mm diameter rebars. It was achieved initially
through hit and trial approach using the optimum composition of resin mixture for 9.5mm
and 25mm diameter rebars and production models were developed for these two rebar
diameters relating the tensile strength of rebar with fiber content, pull speed and heating
die temperature. These production models helped to reduce the trials for two comparable
diameter rebars of 13mm and 22mm respectively. Similarly optimum combinations of
process parameters were determined for remaining diameter rebars based on their
production models developed on same analogy thus reducing the time and cost of GFRP
rebars. Total 165 trial productions along with simple tension tests were executed for this
purpose. Finally a single and comprehensive model named as ‘unified production model’
was developed in which fiber content, pull speed, heating die temperature, rebar diameter
and its square were the main parameters. The experimental tensile strength results were
validated using the unified production model. The unified model is recommended as a
comprehensive guideline for the development of GFRP rebars in future where patent
details are not available.
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ABSTRACT
GFRP rebar surface texture was finalized through preliminary bond study with plain
GFRP rebars by conducting 16 direct pullout tests using four diameter, 9.5mm, 13mm,
19mm and 25mm rebars, two bonded lengths, 5.0 d b & 7.0 d b with concrete strength of
41.4 MPa, to check for comparable bond strength as per American reference GFRP
rebars, Aslan-100 TM , developed by Hughes Brothers Inc. USA. The bond stress of plain
rebars was found quite low, therefore, deformed uncoated rebars were next developed and
subjected to simple direct pullout tests. A set of 24 simple direct pullout tests (without
recording the stroke or slip values) was conducted using 27.0 MPa concrete by combining
four diameter rebars of 9.5mm, 13mm, 19mm, & 25mm and three bonded lengths of 3.0
d b , 5.0 d b and 7.0 d b . Two pullout specimen sizes, Ø150mm x 300mm and Ø100mm x
200mm, were used. These deformed rebars exhibited the bond stress well comparable
with the above reference GFRP rebars.
The final production of deformed uncoated and sand coated GFRP rebars was made in six
diameter rebars of 9.5mm, 13mm, 16mm, 19mm, 22mm and 25mm using optimum
composition of resin mixture and optimum combinations of process parameters. Each lot
of final production was tested for quality assurance tests including barcol hardness,
tensile strength, tensile modulus of elasticity and the average bond strength.
The average bond stress of locally developed deformed GFRP rebars was evaluated
through 48 direct pullout tests using 41.4 MPa concrete, four diameter rebars of 9.5mm,
13mm, 19mm, & 25mm and three bonded lengths of 3.5 d b , 5.0 d b & 7.0 d b . Two pullout
specimen sizes, Ø150mm x 300mm and Ø100mm x 200mm, were used for this purpose.
The bond study was carried out by varying the bonded length, rebar diameter, concrete
cover/confinement and surface texture of GFRP rebars. Average bond stress of locally
developed deformed GFRP rebars in flexure was evaluated through six beams using 41.4
MPa concrete, two diameter rebars of 13mm and 19mm with above three bonded lengths
by varying the bonded length as well as rebar diameter. The effect of joint action on
average bond stress of primary beams of junctions was also studied using the same
parameters as of individual beams. The bond evaluation studies were carried out to ensure
the bond performance of locally developed GFRP rebars for their effective composite
action in RC members.
A model for predicting the average bond stress was developed basing on the direct pullout
experimental results; half of which were used to calibrate the model and remaining half to
validate. The proposed pullout bond model was further validated using the published data
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ABSTRACT
of direct pullout results by several researchers for the rebars whose surface textures were
comparable to the developed rebars. The model prediction agreed closely with the
experimental results.
The beam bond experimental results were also in close agreement with the published
beam bond results by several other researchers.
Basing on the barcol hardness, tensile strength, tensile modulus of elasticity, bond
strength comparisons with the ACI/ASTM requirements, reference GFRP rebars as well
as experimental results of several researchers, it may safely be claimed that the successful
development of GFRP rebars in Pakistan has been achieved, which is a major
breakthrough considering the poor to moderate technological facilities available in
Pakistan. The indigenous development process will help the country to economically
develop and use the GFRP rebars in RC flexure members for special applications as well
as to maintain the safety and durability of theses members.