الفهرس 
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Abstract
Confinement of concrete is an effective way for strengthening of concrete menmbers, especially columns. The behaviour of strengthened concrete columns wrapped by fiber reinforced ploymer composite FRP sheets opens the way to new powerful strengthening technique. Square and rectangular-section columns were found to experience less increase in strength and ductility than their circular counterparts when wrapped with FRP because the distribution of lateral confining pressure varies from a maximum at the corners to a minimum in between. Most of the recent researchers on non-circular concrete columns concluded that increasing the stiffness of the FRP wrapping is a promising way to increase the wrapping efficiency.
To improve the effectiveness of wrapping, a sandwich-wrapping method is used in this research. sandwich wrapping consisted of wrapping the column with two layers of FRP, inner and outer, separated by an incompressible material as filling material.
A crylic plates were used as filling material between FRP sheets. Twelve square RC columns with dimensions 200*200*1500 mm were tested. Two columns were considered as control specimens while the remaining ten columns were stregthened with different wrapping schemes. The effectiveness of the sandwich wrapping system was investigated either in straps or full wrapping. The variables of the study were the method of wrapping; percentage of wrapping; thichness of acrylic plates and type of wraps. The number of layers was kept constant as two layers, except for one column where another layer was added to examine its effect on the column performance. The use of sandwich wrapping greatly enhanced the maximum axial load capacity of the columns. The ductility of the sandwich wrapped specimens also was higher than regular wrapped specimens; thus proves that using sandwich technique improves the behaviour of rectangular RC columns under axial loads. A model was proposed to describ the stress-strain behaviour of confied square or rectangular RC columns. The proposed model is suitable for diiferent wrapping configurations including regular and sandwich wrapping. The results of the model showed good agreement with experimental tests results. The model also showed good agreement with several other available test results considering different columns aspect ratios, straps or full wrapping, different coilumns aspect ratioa, straps or full wrapping, different types of FRP types.