Effect of geogrid reinforcement on high-strength concrete T-beams under pure torsion

The experimental program of this study involved testing ten beams subjected to pure torsional moment. The parametric study includes; efficiency of internal strengthening by geogrid reinforcement (GDR), the effect of opening size and shape, repairing the pre-tested beams using carbon fiber reinforced polymer (CFRP) technique. The results show that the GDR effectively enhance the cracking and ultimate torque, decrease the twisting angle, lower the strain in the transverse reinforcement (stirrups), and enable the specimen to recover the twisting angle by more than 75% of its maximum value after the load is terminated at the end of the testing process and the specimen had reached its ultimate capacity. The percentage of cracking torsional capacity to ultimate torsional capacity is ranging from 37% to 50 %. GDR effectively limited the loss in strength which occurs due to web openings. Circular openings (100mm diameter) with different locations (Lc/2 and Lc/3) reduced the ultimate torsional capacity by about (23% and 30%) respectively compared with the torsional capacity of GDR solid beam, and about (1 % and 11 %) respectively compared with torsional strength of the solid beam without GDR, while for beams with square openings of dimension (88mm x88mm) with different locations (Lc/2 and Lc/3 ) the decrease in ultimate torsional capacity is about (28 % and 32 %) respectively as compared with the torsional capacity of GDR solid beam, and by about (8% and 13%) respectively compared with the torsional capacity of a not GDR solid beam. Specimens with circular openings of diameter (150mm) with different locations (Lc/2 and Lc/3) have the lower ultimate torsional capacity by about (56 % and 61%) respectively as compared with GDR solid beam, and about (43 % and 51 %) respectively as compared with solid beam without GDR, while for beams with square openings of dimension (132mm x132mm) with different locations (Lc/2 and Lc/3 ) the decrease in ultimate torsional capacity is about (58 % and 66 %) respectively as compared with GDR solid beam, and about (46 % and 57 %) respectively as compared with a solid beam without GDR. CFRP sheets used to repair the tested specimens. The repaired control solid beams with GDR and without GDR have an ultimate strength after repairing reached (111% to 124%) respectively from its original strength respectively. Specimens with an opening (circular and square) have an ultimate strength after repairing reached (119% to 112%) from its original st