Generally, substantial researches on the pull-off bond strength
tests were carried out to evaluate the behavior and performance
of retrofitting and rehabilitation systems, especially for FRP composites
such as CFRP and GFRP sheet bonded to concrete surfaces
[13–15]. This would include evaluating the performance of the
FRP composites subjected to various environmental factors such
as chemical attack, high humidity and freeze-and-thaw cycles [13–
15]. Maerz et al. conducted experimental tests to investigate the
performance of the ‘Dallas county bridge’ in Missouri, USA
strengthened with FRP composites prior to testing [13]. In order
to evaluate the long term performance of the strengthened bridge
with FRP composites, the pull-off tests were carried out for every
6 months up to date. The results showed that all samples obtained
from the pull-off tests to date failed at the FRP sheet and coated
epoxy interface. In addition, the average failure loads for the samples
were around 43 kN. Winters et al. investigated an enhancement
in the bond strength of FRP composite bonded to concrete
structures with partially submerged pillars [14]. Although this
study focused on the method itself, they carried out 151 pull-out
tests in accordance with ASTM D 4541 [26] by utilizing an adhesion
tester. A 0.1 m 0.1 mgrid was drawn and 83 pulled-out tests
were carried out above the waterline where the rest of the 68 pullout
tests were conducted below the waterline, resulting in lower
failure strength on average. In addition, they reported that pressure
bagging method had much higher improvement in the bond
strength of the FRP-bonded concrete than vacuum bagging method.
It was concluded that different methods of attaching the FRP
on the concrete surface had significant impact on the bond behavior
and performance.
Al-Salloum et al. carried out a series of experiments including
concrete specimens wrapped by CFRP and GFRP systems exposed
to high temperature for several hours [15]. They evaluated the
pull-off bond strength of the FRP-strengthened concrete subjected
to elevated temperature environment for possible bond deficiency
based on ASTM D 4541 [26]. The concrete specimens with the designed
compressive strength of 25 MPa at 28 days were prepared
for the pull out test with single layer of CFRP and GFRP systems
bonded to the specimens. The results showed that significant loss
in the bond strength was observed at a temperature of 200 C for
the case of the specimen strengthened with GFRP system compared
to the specimen strengthened with CFRP system.