The results of Beam H-S50-2 are shown in Fig. 13. This beam was strengthened by 1 mm-thick developed hybrid fabric. Figure 13(a) shows the load-versus-deflection curve of the beam. The beam yielded at a load of I 13.9 kN (25.6 kips) due to the yielding of both the steel and the fabric. The increase in yield load gain was 38%. The beam failed at an ultimate load of 146.4 kN (32.9 kips) due to compression failure of the concrete. A ductility index of 2.25 occurred. Figure 13(b) shows the load versus the fabric strainat midspan. The recorded strain when the beam yielded was 0.35%, and the maximum recorded strain before beam failure was 1.2%. The beam at failure is shown in Fig. 13(c).  Figure 14 shows the results of Beam H-S75-2. This beam was also strengthened by the developed hybrid fabric, but with 1.5 mm thickness. Figure 14(a) shows that the beam yielded at a load of 127.3 kN (28.6 kips) with an increase in yield load of 55%> due to yielding of both the steel and the fabric. The beam finally failed at an ultimate load of 162.0 kN (36.4 kips) by compression failure of the concrete at midspan. The beam exhibited a ductility index of 1.89. Figure 14(b) shows the load versus the fabric strain at midspan. The maximum recorded strain before beam failure was 0.74%. The beam at failure is shown in Fig. 14(c).  Figure 15 shows a comparison between the results of beams of Group B. The following are the observations from their test results:  1.Beam H-S50-2 showed a higher yield load than Beam CS, although the hybrid fabric has a lower yield-equivalent load than the ultimate load of the carbon fiber sheet. This is because the hybrid fabric has a higher initial stiffness than that of the carbon fiber sheet; and  2.The beams strengthened with the developed hybrid fabric showed high yield loads with reasonable yielding plateaus.  

One of the advantages of the developed hybrid fabric is that it is easy to determine, by visual inspection, whether the fabric yielded or not, because any failed carbon fiber yarns can be seen. Also, the hybrid fabric is less expensive than the currently available carbon fiber materials, as more than 75% of the fibers used are glass fibers that are less costly than carbon fibers. 

CONCLUSIONS

Based on the research investigation presented in this study, the following can be concluded:  1.Currently available FRP materials used as flexural strengthening systems for concrete structures do not always provide yielding plateaus in the strengthened beams similar to those for unstrengthened beams. In some applications, the strengthening may result in a brittle failure or an insignificant increase in the yield load of the strengthened beam, or both;  2.The hybridization of selected types of libers was utilized to develop a pseudoductile fabric, which had a low strain value at yield (0.35%). The fabric was designed so that it had the potential to yield simultaneously with the reinforcing steel of the strengthened beam;  3.The beams strengthened using the developed hybrid fabric generally showed a higher increase in yield load than those strengthened with the carbon fiber strengthening systems. Some of the beams strengthened with the hybrid fabric showed a yield plateau similar to that of the unstrengthened beam. This is critically important to ensure an adequate warning before structural failure; and   4. The beams strengthened with the developed hybrid fabric system showed no significant loss in beam ductility. The beams strengthened with carbon fiber sheets showed also no significant loss in ductility but with relatively less yield loads.  ACKNOWLEDGMENTS  This research was conducted at the Structural Testing Center at Lawrence Technological University, Southfield, Mich., and was funded by the National Science Foundation under Grant No. CMS-9906404, awarded to the first author. The authors wish to thank J. Doyle, President of Diversified Composites Inc., Erlanger, Ky., and M. Bonne, Vice President of Baker Concrete Technology, Columbus, Ohio, for their support during this research work. The support of ACI-CRC Committee is greatly appreciated.