CHAPTER 5: CREEP STUDY OF FRP COMPOSITE REBARS FOR CONCRETE
provide a helically convex surface made with a
Background
strand spirally wound and cured on the surface.
FRP reinforcing bars are receiving increased
Other designs suggest the use of a sand or grit
attention as the tension element in reinforced con-
coating on the rebars. A recent design includes a
crete (Roll 1991). This is primarily because the cor-
pultruded ribbed surface. A comparative survey
rosion of steel reinforcement in concrete by chlo-
of the bond quality of these surface modifications
ride ions has been determined to be the major
is still not available. The bond strength of com-
cause of premature deterioration of concrete struc-
posite rebars and the bending response for carry-
tures (ACI Committee 208 1958). Available as long
ing concrete strengths have been investigated by
rods in the market, these rebars are made of very
many, including GangaRao and Faza (1992),
fine continuous glass fiber strands which are
Pleimann (1991), Daniali (1992), Larralde and Siva
bound together with a thermosetting polymer. Wu
(1990), Iyer and Anigol (1991), Tao et al. (1992),
et al. (1990) reported that E-glass reinforced com-
Challal and Benmokrane (1993), Challal and
posite rods, from which these rebars are made,
Benmokrane (1992), and Malavar (1994).
may have a tensile strength in excess of 689 MPa
(100 103 psi) and a longitudinal elastic modulus
There are several major barriers to FRP rebar
of about 51.7 GPa (7.5 106 psi). In tensile tests,
applications. These include a lack of sufficient data
on: durability or performance under extreme
the bars fail without any significant yield (brittle
environments (Dutta 1995b, GangaRao et al. 1995),
failure). The rods are produced by a pultrusion
creep, fatigue, and corrosion from the alkaline
process. Since glass is commonly used as the rein-
environment of concrete. Unlike steel, the FRP
forcing fibers in these rebars, they are also desig-
rebar is viewed as a viscoelastic material. As such,
nated as GFRP (G for glass). Currently, there are
many of its properties are suspected to be time-
several FRP rebar companies actively marketing
dependent. Creep refers to the slow deformation
their products in the U.S. Most FRP rebars con-
with time under a constant stress that is less than
tain about 55% E-glass fiber by volume and about
the yield stress. When a constant load is applied
45% thermoset resin. The sizes (diameter) of the
(except for a short initial duration when the strain
rebars follow the size designations of steel rebars
may increase quite rapidly) to a viscoelastic
(e.g., no. 3, 4, or 7 rebars). Faza (1995) reported a
material, the strain increases steadily. This increase
number of successful applications of rebars in the
of strain is creep. If creep increases beyond a cer-
USA, including applications in sea walls, hospital
tain limit, the effective stress owing to a decrease
MRIs, reactor pads, compass calibration pads, mill
in the cross-sectional area increases. The increased
roofs, laser test facilities, highway barriers, resi-
stress results in further deformation, which in turn
dential foundations, and bridge decks. Table 14
increases the stress even more. Thus, the defor-
gives a comparison of the mechanical properties
mation suddenly accelerates, leading to the fail-
of steel rebars and FRP rebars.
ure of the material.
The light-weight, corrosion-resistant and non-
At the microstructural level, creep occurs due
magnetic properties make FRP rebars an improved
to the presence of mobile defects, such as disloca-
alternative to steel. One of the most critical prob-
tions that move (enlarge) primarily at increased
lems to be overcome in large-scale applications of
stress and temperatures. Thus, the general math-
FRP rebars is the development of improved bond
ematical formulation of creep rate takes the form
strength with the concrete. Some available designs
Table 14. Comparison of mechanical properties of steel and FRP
rebars (Faza 1995).
Properties
Steel rebar
FRP rebar
Specific gravity
7.9
1.5 2.0
Tensile strength, MPa (psi 103)
483690 (70100)
5171207 (75175)
Yield strength, MPa (psi 103)
276414 (4060)
--
Compressive strength, MPa (psi 103)
276414 (4060)
310482 (4570)
Tensile modulus, GPa (psi 106)
200 (29)
4155 (5.98.0)
Coeff. of thermal expansion 106/C (F)
11.7 (6.5)
9.9 (5.5)
41
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