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Concrete In Australia : March 2011
Concrete in Australia Vol 37 No 1 41 in the FRP. e FRP stress limits as specified in ACI 440 and TR55 for this failure mode are plotted in Figure 8 for MBrace FRP's. For most cases, the strain limit is less than the ultimate strain. It means that using FRPs with higher tensile strength does not always lead to a more effective system. e TR55 method, in the author's opinion, is over simplified. It has been found that midspan debond failure loads are inversely proportional to FRP stiffness. is trend is indicated by the formulae in ACI 440. As the FRP laminate becomes thicker, the stress limit reduces significantly. As a result, the best method to improve the midspan debond load is to increase the bond area or the FRP width. It is also worth noting from Figure 8 that the FRP fabric systems tend to perform better in terms of midspan debond. However, fabrics are a great deal more difficult to install. Midspan debond can also be improved by some forms of anchorage. It has been found that U straps or jackets can improve midspan debond failure loads significantly (Pham, 2005). However, no method to quantify this improvement is yet available. 3.4 How to improve ductility? As noted above, bonding FRP to concrete can lead to different failure modes, which can be quite brittle. Beam ductility is dealt with in ACI 440 by examining the tensile steel strain at the ultimate limit state (ULS). e strength reduction factor can be reduced to as low as 0.65 from its maximum value of 0.9. e reduction is quite rapid once the prestress strain level is lower than 1.3%. It is therefore advisable to keep the tensile steel level at ULS above the limit (0.5% for reinforced concrete members, 1.3% for prestressed members). Ductility can be improved using anchorages as the strain limits can be increased. 3.5 How to improve shear strengthening effectiveness? e maximum strain level allowed in FRP used for shear strengthening is only 0.4%. is is only guaranteed if FRP is applied as full wrap and therefore fully anchored. is is rather conservative, in the author's opinion. To improve Vf components, the FRPs can be anchored or they can be orientated to intersect the shear crack at angles close to 90°. 4.0 CONTRUCTION ISSUES 4.1 Installation challenges In the author's experience, installation of CFRP proves to be the most challenging task even for experienced contractors. To prepare the concrete surface so that its soundness, levelling, and roughness are satisfactory can be a difficult task when dealing with very poor existing concrete. e required temperature and humidity ranges can also mean that the window for this application can be very limited. 4.2 Quality assurance Good quality assurance requires detailed documentation. For Figure 8. FRP stress limit for midspan debond for MBrace products on 40MPa concrete. (a) Original beam section (b) Strengthened beam section Figure 9. Example 5.1 -- Beam cross sections.