Experimental investigation on flexural capacity of reinforced concrete beams strengthened with carbon fiber reinforced polymer strips

Abstract

Externally bonded carbon fiber reinforced polymer (CFRP) composites are widely employed for enhancing the flexural capacity of reinforced concrete (RC) beams. However, the adhesion between CFRP and concrete substrate is an issue of concern. It generally controls the ultimate capacity of RC beams. Particularly, premature intermediate debonding phenomenon that begins from an intermediate point throughout the CFRP-concrete interface is one of the most common and peculiar failure modes observed in RC beams externally strengthened in flexure by using CFRP. To evaluate the flexural capacity of reinforced concrete beams strengthened with CFRP strips, a comprehensive experimental work done consists of testing 78 simply supported concrete beams under flexure. The beams had a rectangular cross-section of 150 mm width, 200 mm height, and 1524 mm in length. The specimens tested under 3rd point loading. The load-displacement histories obtained from the control and strengthened specimens are synthesized with the strain measurement results gathered using Digital Image Correlation Technique (DICT). To inspect debonding of CFRP from concrete, the clear cover varied in between 12.50 mm and 37.50 mm. The beams are over reinforced against shear failure. To examine the compression and tension zones of specimen, the longitudinal steel are provided in two ways. In one group, to obtain tension failure, sections are over reinforced at the compression zone. In other group, sections are over reinforced at the tension zone by providing CFRP strip in the tension face of beam to initiate compression mode failure. To understand the debonding phenomena, three types of anchorage system are employed (AT1, AT2, and AT3). In AT1, CFRP discontinued right before the support to keep the strengthened system free from anchorage. In AT2, CFRP strip extended beyond the end supports to generate the idealized support condition. In AT3, CFRP wrap used in different combinations to act as anchorage in the support locations (full wrap, L/4, L/2.5). Two types of coarse aggregates, crushed stone and crushed brick having same gradation are used in this research program. Two types of mounting technique of CFRP employed, namely surface mounting (SM) technique and near surface mounting (NSM) technique. No significant difference is observed in moment capacity enhancement when brick aggregate is used instead of stone aggregate. Slightly higher moment capacity is observed in SM technique compared to NSM technique. Beams strong in compression zone gives higher moment capacity compared to the beams weak in compression zone. The tested beams shows different local and global modes of failure including CFRP wrap rupture, concrete crushing, flexural cracks, shear cracks, debonding, delamination, or a combination of these. The stiffness increased significantly for the CFRP wrapped anchorage cases compared to the controlled cases. From the flexural strain versus depth curve it is observed that the neutral axis shifted upward during the increase in load. By using Anchorage Type 1 (AT1), 30% to 40% lower moment capacities are obtained compared to analytical capacity predicted by ACI code. In Anchorage Type 2 (AT2) better performances are observed with respect to analytical capacity. For Anchorage Type 3 (AT3), significant improvement (upto 60%) in moment capacity is observed compared to analytical capacity. The fundamental contribution of this work with these cardinal findings to the knowledgebase for future development of adequate analytical model based on anchorage type is obvious.