Design Issues - Strengthening Design of Reinforced Concrete with FRP - page 74

4.6 LOADING, ENVIRONMENTAL, AND DURABILITY

FACTORS IN SELECTING FRP

4.6.1 C reep -r upture anD F fatigue
CFRP systems are highly tolerant to fatigue under cyclic loading and to creep rup-
ture under sustained loading. GFRP systems are more vulnerable to these loading
conditions. Accordingly, research has yielded that glass can sustain 0.3 times its
ultimate strength, while aramid can sustain 0.5 of its ultimate strength and carbon
can sustain 0.9 of its ultimate strength without a creep-rupture problem (Yamaguchi
et al. 1997; Malvar 1998).
The stress level in FRP is computed under a total moment consisting of all sustained
loading plus the maximum fatigue loading, as shown in Figure 4.2, using a linear elas-
tic analysis. Values of sustained and cyclic stress levels are given in equation (4.3) by
using a 0.6 reduction factor for the actual creep-rupture limits mentioned previously.

0.2 f (forGFRP)

fu

f

, =

0.3 f (forAFRP)

(4.3)

fs fu

0.55 f (forCFRP)

fu

4.6.2 i mpaCt r eSiStanCe
GFRP and AFRP systems have higher impact resistance than CFRP systems. This
may be attributed to the higher strain to failure they can sustain.

4.6.3 a CiDity anD a lkalinity
Dry carbon fiber is known to be resistant to alkalinity and acidity, while dry glass
fiber is susceptible to degradation when subjected to these environments. On the

FRP Stress

Fatigue Load

f ff,s

Sustained Load

Time

FIGURE 4.2 Schematic of the level of service FRP stress in terms of sustained and fatigue

loading.