Novel solutions for strengthening RC columns: an experimental study

This paper presents an experimental study undertaken to investigate the seismic behavior of full scale square (300x300 mm) RC columns strengthened with two alternative systems employing carbon FRP wraps. The first system (“type A”) consists of placing four steel threaded rods inside grooves cut into the concrete which were anchored into the foundation for a length of about 250mm; then the strengthening is completed by external CFRP confinement. The second system (“type B”), instead, differs from the first one for the additional use of steel profiles at the column corners. Tests were performed by subjecting the specimens to combined axial and lateral loads applied in displacement control. A value of the normalized compression load (NCL) equal to 0.40 was considered for all tests except one for which NCL = 0.14 was adopted. Table 1 summarizes the main data and results of the six performed tests. Each test is identified by a label providing the following information: the column number (from C1 to C5) with the addition of “NS” (i.e., “new series”) the longitudinal steel reinforcement type (“D” stands for deformed rebars); the strengthening system (type ‘‘A’’ or ‘‘B’’). It is noted that the test C1NS-D-R refers to the control specimen C1NS-D that, after being tested, was repaired and FRP retrofitted according to the layout “type A”. In order to better compare the effectiveness of the considered strengthening techniques, Table 1 also reports data and results of two tests performed in a previous experimental campaign, namely C24-D-A1 and C15-D-A1. These specimens were strengthened using a system “type A1”, characterized by the combined use of CFRP confinement and steel profiles connected to the foundation by using an L-shape steel system. Under high axial load (NCL=0.40) the strengthened specimens have exhibited increases of strength ranging from 48 to 60%. The improvement of the column’s confinement through the addition of steel profiles (system “type B”) avoids the rupture of the FRP wrapping and delays the crushing of the concrete; indeed, after the achievement of the conventional collapse, the strengthened specimens are still capable of undergoing significant lateral displacements. By focusing on the system “type A”, it is highlighted that using a double FRP layer (test C3NS-D-A) delays the FRP fracture, so that the displacements exhibited by the specimen are about 30% over those measured for the counterpart confined with a single FRP layer.