FRP bar properties - Reinforced Concrete with FRP Bars: Mechanics and Design - page 39

350

300

CFRP

250

200

150

100

GFRP

50

0

0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0%

Axial Strain, %

Figure 3.3 Typical tensile stress-strain curve of no. 3 GFRP and CFRP bars.

A typical stress-strain curve of no. 3 GFRP and CFRP bars is shown in
Figure 3.3.
Measuring mechanical properties of large FRP bars poses unique chal-
lenges when using ASTM D7205. For traditional wedge-type grips, the dif-
ficulty stems from the significant transverse stresses that must be applied
to the bar as a prerequisite to apply large enough longitudinal tensile loads.
The local stress triaxiality due to stress concentrations near the gripping
sections usually leads to premature failure at such locations, instead of the
desired test gauge section. For unidirectional FRP composites, whose trans-
verse strength is much less than the longitudinal strength, premature dam-
age in the gripping region occurs frequently and depends on the method
of gripping. The mechanisms of load transfer vary between different grip
designs but they can be categorized into two distinct groups. The first group
transfers load by applying a clamping force and relies on the interface inter-
action. If this clamping force is applied uniformly, then a high shear stress
will develop at the loaded end. The second group relies on the underlying
principle of the traditional wedge grip with various augmentations to miti-
gate the shear stress concentrations on the loaded ends.
An ASTM-recommended system is included in Annex A1 of ASTM
D7205 and involves the use of steel pipes filled with expansive grout or poly-
mer resin mixed with sand, as shown in Figure 3.4(c). The configuration