1350
0.5 in. rebar
1300
Rod 6
1250
Rod 5
1200
1150
23 Sep
3 Sep
14 Aug
25 Jul
5 Jul
15 Jun
26 May
6 May
16 Apr
Period of Test
mm- (0. 5-in.-) diameter rebars.
Analysis and discussion
Table 16, in which a very small trend of increas-
Findley's general theory of creep behavior of
ing strain could be observed, the values of m and
viscoelastic polymer (1960) is represented by
n were determined as p = 9.45 and q = 0.297. These
ε = ε0 + p (t/t0)q
(12)
values closely match Mosallam and Chamber's
(1995) published values for commercially avail-
where ε = the total strain
able pultruded FRP WF beams: p = 9.72 and q =
ε0 = stress dependent strain
0.298. Findley's equation, when plotted over the
p = the coefficient of time dependent term,
Table 16 data points is shown in Figure 64, but the
which is dependent on stress level
match is not very clear because of the scatter in
t = duration of loading (hours)
the data. If the tests had been continued over a
t0 = unit time (hour)
longer time, a more discernible creep strain might
q = a material constant, independent
have developed. The data at room temperature
of stress.
and low temperature had not shown any trend of
increasing; therefore, they were not analyzed with
Parameters p and q are known as creep param-
Findley's equation. It must be noted that Findley's
eters. To obtain the particular values of p and q,
theory applies very well to viscoelastic polymers,
eq 12 can be rearranged and written in the fol-
but in FRC composites rebars, when the stress is
lowing form:
applied in the fiber direction, the behavior is not
totally viscoelastic. In fact, with a higher volume
log (e e0 ) = log (p) + q log (t/t0)
(13)
fraction of glass fibers oriented in the loading
Equation 13 represents a straight line of slope
direction, creep in FRP composites is not expected
n and intercept m at unit time, if log (ε ε0) is
to be a problem.
1300
1280
1260
1240
1220
1200
1180
1160
Data Points
1140
Findley's Eq.
1120
1100
0
500
1000
1500
2000
2500
3000
3500
4000
Number of Hours
Figure 64. Comparison of high-temperature creep data with Findley's
equation.
46
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