The strengthening technique using advanced composite materials has been implemented worldwide into the retrofit of existing concrete structures for static and seismic loading. But, there is still a gap in knowledge regarding the effect of impact and explosion loading on concrete structures strengthened with composite materials. This situation is especially difficult since many serious problems exist in material modeling, load definition, and selecting effective approaches for addressing the behavior of complicated geometrical domains in a structure. A series of twenty-seven concrete beams were prepared and tested to investigate the behavior of concrete beams strengthened with composite materials. Twenty-five beams were tested with impact load induced by a steel cylinder drop weight; two beams were statically loaded as control specimens. Two of the twenty-seven beams were unretrofitted, one for impact test group and the other for static test. Two types composite materials were used in strengthening, Kevlar and carbon fabrics. Each type of fabric had two different weights. Five beams were reinforced with steel bars and the other beams without steel bars. Investigation during testing included monitoring reaction force at support of simply supported beam, history of strain vs. time in composite laminates, variation of beam stiffness and deflection, crack and failure modes and stress concentration at the cut-off end of composite laminates. The test results revealed that bonding composite laminates to concrete beam can significantly improve the performance of concrete structures to resist impact load, increase the cracking, flexural strength, and residual stiffness of beam and reduce number of cracks, crack width and maximum deflection. The residual stiffness of strengthened beam after first impact was two to three times of that of un-retrofitted beam; the maximum deflection decreased 30 to 40%. The gain depends on the type and weight of composite laminates. Comparing to the static test results, the ultimate deflection and crack width were smaller than those of beam under static load. But, the maximum reaction force was three to four times larger than that under static load. The residual stiffness of strengthened beam after first impact can be calculated using a regression equation. The impact force can be obtained with a semi-empirical equation, which is derived from Spring-Mass models and modified by test results. From flexural wave theory, an equation has been developed for predicting the deflection of beam caused by impact loading. Based on the test and analytical results, a retrofit technique and design guidelines have been proposed. To control failure modes of strengthened beam, the minimum and balanced ratio of composite plate reinforcement have been derived.
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