The effect of important parameters such as the FRP elastic modulus, FRP thickness, shear steel stirrups, concrete compressive strength, width ratio between the FRP to the concrete beam is investigated through a parametric study. This study is carried out for side-bonded shear-strengthened beams, and the variation of the axial strain in the FRPs is the focus. The results show the shear force versus axial strain in the FRP composites at mid-depth of the sheet at the centre of the shear span. The governing failure mode is debonding of FRP sheets. The principal objective of this study is to examine and assess the parameters that most influence the behaviour of shear-strengthened beams.
7.2.2.1 Steel Stirrups
The effect of steel stirrups is studied by increasing the amount of steel stirrups along the shear span. The spacing between the shear steel stirrups is assumed to be 300, 200 and 100 mm, while the cross section of steel stirrups was kept identical. The study included an additional beam without shear steel stirrups. The interaction between the shear steel stirrups and externally bonded FRP composite is confirmed. Figure 7.6 shows that the presence of steel stirrups in shear-strengthened beams is found to reduce the contribution of the FRPs to the shear capacity of the section. The figure illustrated that the gain in the axial strain of the FRPs absolutely decreases as the amount of steel stirrups increased.
Additionally, the increase of the amount of shear steel stirrups increase the load carrying capacity of the beam, and the tendency for the debonding.
7.2.2.2 Concrete Compressive Strength
The influence of the concrete strength to the axial strain of the FRPs was only studied by [Deniaud and Cheng, 2001a]. It is well established that the bond conditions depends on the tensile strength of concrete, where such value is proportional to the concrete compressive strength. So one may argue that the axial strain in the FRP composites depends on the concrete compressive strength. In this study, five quantities of concrete compressive strength are taken, i.e., 25, 31, 35, 40 and 45 MPa. We can report that from Figure 7.7 the
7.2. PARAMETRIC STUDIES
400 -
Z 3 0° -
M
%—' •O
£ 200 -a u
"S.
9* ioo -
o
0.0 0.5 1.0 1.5 2.0
Axial strain /10"3
Figure 7.6: Effect of shear steel stirrups on the applied load-FRP axial strain relationship
axial strain in the FRPs is observed to increase proportional to the increase of the concrete compressive strength. Furthermore, the gain the ultimate load carrying capacity tends to increase by delaying the debonding. This is explained by the increase of compressive strength proportionally increases the tensile strength of concrete, which increases the bond between the FRPs and concrete. These observations coincide with those of Deniaud and Cheng [2001a].
7.2.2.3 Effect of F R P Elastic Modulus
Various values of FRP elastic modulus were used to investigate the influence in the FRP axial strain. These values ranged from 233 GPa to 102 GPa. Figure 7.8 shows that the axial strain in the FRP composites is roughly proportional to the FRP elastic modulus, thereby confirming the results reported by other researchers (Triantafillou 2000). The measured shear carrying capacity gain increases as the value of the FRP composites in- creases. The implication of this argument is that as the FRP composites become stiffer, debonding of FRPs dominates over rupture, and the axial strain in the FRPs is reduced.
Therefore, with decreases of the FRP elastic modulus, the axial strain reaches higher values when the failure mode by rupture of FRPs.
No steel stirrups s=300mm s=200mm s=100mm
500
Axial strain/10"
Figure 7.7: Effect of concrete compressive strength on the applied load-FRP axial strain relationship
400
300
o
E=102GPa
0.0 0.5 1.0 1.5 2.0
Axial strain /10"
Figure 7.8: Effect of FRP elastic modulus on the applied load-FRP axial strain relationship
7.2. PARAMETRIC STUDIES
400
t=0.75mm
0.0 0.5
Axial strain/10
1.0
3
1.5
Figure 7.9: Effect of FRP thickness on the applied load-FRP axial strain relationship
7.2.2.4 Effect of F R P T h i c k n e s s
The considerable effect of the plate thickness on the axial strain in the F R P s is verified in this study. The variation of the axial strain versus the load carrying capacity on a shear-strengthened beam for four different plate thicknesses: 0.165, 0.3, 0.5 and 0.7 mm are shown in Figure 7.9. Similar to the effect of the F R P elastic modulus, the increase of the plate thickness is found to reduce the axial strain in the F R P composites. It is also interesting to note t h a t the shear capacity of the beam increases with the increases of the composite plate thickness. By observing t h a t the increase of the plate thickness accelerates the debonding.
7.2.2.5 Effect of W i d t h R a t i o B e t w e e n t h e B o n d e d F R P P l a t e t o t h e C o n c r e t e M e m b e r
Although the width ratio between the bonded F R P plate to the concrete member was shown to have a significant effect on the ultimate bond strength, this parameter has not been studied by any of the researchers. This study is perhaps the first to investigate this ratio. Only one sheet was attached at the centre along the shear span with various sheet widths. These are 50, 100, 350 mm, while the shear span length was 750 mm. The effect of the width ratio between the bonded F R P plate to the concrete member on the F R P axial strain and the load carrying capacity is drawn in Figure 7.10. It is observed t h a t the
400
300
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0 ^
bf=350mm
1
Axial strain/10":
Figure 7.10: Effect of width ratio between the FRP sheets to the concrete beam on the applied load-FRP axial strain relationship
contribution of the F R P s to the ultimate capacity of the beam increased with increasing the ratio, while the axial strain in the F R P s decreased with increasing the ratio. The results for the F R P s axial strain might be misleading, because those results were taken in a point and the sum of strain along the sheet width might increase with increasing sheet width.
7.2.2.6 Effect of S h e a r S p a n t o D e p t h R a t i o
Figure 7.11 presents the measured applied load carrying capacity versus the axial strain in The F R P s . Four different shear span to depth ratios are considered: 2.6, 3.0, 3.4 and 4.0; this is obtained by changing the shear span length. Two zones can be identified in the figure: (a) the zone corresponding to shear span to depth ratio is between 2.6 to 3.4 (2.6 < a/d < 3.4), where the gain in shear carrying capacity and the axial strain in the F R P s tend to decrease with increasing the ratio; (b) the zone corresponding to shear span to depth ratio greater t h a n 3.4, where the contribution of the composite material (load carrying capacity and F R P axial strain) appears to increase with the increase of the a / d ratio.
T h e controversial aspect of the results is t h a t the shear span to depth ratio appears to be less effective as far as the contribution of the F R P s axial strain is concerned. It can