FRP bar properties - Reinforced Concrete with FRP Bars: Mechanics and Design - page 35

however, were obtained for specimens made for aerospace applications.
Results indicated that the plot of the ratio of the stress and the initial static
strength versus the logarithm of the number of cycles at failure shows a
continued downward trend (per decade of logarithmic life) of about 10%,
5%-6%, and 5%-8% for GFRP, AFRP, and CFRP, respectively [15-20].
The fatigue strength of CFRP bars encased in concrete was observed to
be dependent on the moisture, the environmental temperature, the cyclic
loading frequencies, and the ratio of maximum-to-minimum cyclic stress
[14,21,22]. A recent study [23] showed that full-scale GFRP reinforced
concrete bridge deck specimens exhibited better fatigue performance and
longer fatigue life compared to their steel counterparts.
Durability. The mechanical properties of FRP bars are influenced by
the environment. The presence of water, alkaline or acidic solutions, saline
solutions, ultraviolet exposure, and high temperature may affect the tensile
and bond properties of FRP bars.
Data from short-term experiments on bare bars (most of the time
unstressed) subjected to alkaline environments are available. However,
the extrapolation of these data to field conditions and expected lifetimes
is difficult [24-26]. High pH degrades the tensile strength and modulus
of GFRP bars [27]. The degradation is accelerated by high temperature
and long exposure time. The reduction in tensile strength and modu-
lus in GFRP bars (stressed or unstressed) ranges from 0% to 75% and
between 0% and 20%, respectively [28-33,35,36]. In the case of AFRP
bars (stressed or unstressed), tensile strength and stiffness reduce between
10% and 50%, and 0% and 20% of the initial value, respectively [37-39].
In the case of unstressed CFRP, strength and stiffness have been reported
to each decrease between 0% and 20% [39]. Bars embedded in concrete at
various temperatures and with good fiber-resin combinations show only
limited degradation, which, however, increases with temperature and stress
level [28-31,33-35,40,41]. Direct exposure of FRP bars to sunlight (UV
rays) and moisture has a detrimental effect on the tensile strength. Strength
reduces from 0% to 40% of its initial value in the case of GFRP bars, 0%
to 30% for AFRP bars, and 0% to 20% for CFRP bars [34,42]. Although
FRP bars embedded in concrete are not exposed to UV while in service, UV
light may cause degradation during storage.
A field study conducted by the Intelligent Sensing for Innovative
Structures (ISIS) Canada Research Network collected data with respect to
the durability of GFRP bars in concrete exposed to natural environments
[43,44]. Concrete cores containing GFRP bars were extracted from five
selected structures: a 5-year-old harbor wharf and four 6- to 8-year-old
reinforced concrete (RC) bridges. The GFRP bars were analyzed for their
physical and chemical composition at the microscopic level. The experi-
mental results were compared with the ones obtained from control GFRP
bars preserved under controlled laboratory conditions [45]. The results of