Torsional retrofit of spandrel beams with composite laminates

This paper describes the experimental and analytical findings of a project focused on the structural strengthening of reinforced concrete (RC) spandrel beams using carbon fibre reinforced polymer (CFRP) composite laminates, and subjected to pure torsion. Many current torsional strengthening and repair methods are time and resource intensive and quite often very intrusive. The proposed method, however, uses composite laminates to increase the torsional capacity of concrete beams. In traditional cast-in-place construction, the slab adjacent to the spandrel beam does not allow a complete CFRP wrap around the beam. To simulate this condition, the CFRP laminate was provided only on three sides of the beam, and special anchors were used to allow a continuous torsional shear flow around the specimens. The experiments showed that this method increased the torsional strength of the spandrel beams by up to 77%. Excessive concrete cracking followed by composite delamination caused the specimen failures. An equation was developed that, for most of the specimens, accurately predicted the composite contribution. This analytical procedure was based on the existing information on the torsional resistance provided by closed stirrups, and on the equations developed to design the composite shear retrofit of RC beams.