Use of Fiber-Reinforced Polymers in Slab-Column Connection Upgrades

Use of Fiber-Reinforced Polymers in Slab-Column Connection Upgrades. Paper by Baris Binici and Oguzhan Bayrak

The discussers would like to extend their appreciation to the authors for presenting their work on the use of FRP for strengthening two-way slabs in shear. This discussion is written to bring notice to the work that was carried out on this topic at the University of Toronto in 2000 and 200220,21 and has not been referred to in the paper.

In a pilot test series, three slab specimens retrofitted with CFRP were tested and compared with a control specimen20 in an effort to validate the concept of stitching the slabs with FRP to enhance their punching shear capacity. All the slab specimens were 1500 × 1500 mm (59 × 59 in.) in plan and 150 mm thick (5.9 in.), and tested under displacementcontrolled concentric load applied at the center of the specimen on an area of 200 × 200 m (7.9 × 7.9 in.). Figure A shows details of the specimen and the test machine with a specimen simply supported on all four sides similar to those shown in Fig. 1 of the paper. Slab specimens in the pilot test series were reinforced with short strands of CFRP laminate that were passed once through the holes cast in the slab and had their ends adhered to the top and bottom surface of the slab for anchorage (Fig. B(a)). Large sheets of CFRP laminate were later installed on the top and bottom surface to ensure anchorage to the concrete (Fig. B(b)). The results showed a substantial increase in the shear capacity and ductility of the slabs due to FRP retrofitting and validated the hypothesis of upgrading the slabs with FRP for punching shear (Fig. C). The experiments also found partial separation of the CFRP laminates from the concrete surface during testing and considerable increases in flexural stiffness and strength due to the added CFRP laminates on the top and bottom surface of the slab specimens. An increase in the flexural capacity was not considered to be desirable for this application, especially if the upgrade is to enhance seismic resistance of slabs. In addition, the bond between FRP and concrete was a major concern. These issues were addressed in the next series of tests.21

Further work was carried out on 28 slab specimens of the same dimensions as those of the pilot series.21 Twenty-four of these specimens were retrofitted with CFRP and tested under concentric load. All the slab specimens were cast with one of the four patterns of holes shown in Fig. D. The number of concentric shear-reinforcing perimeters parallel to the loading plate periphery varied between 3 and 6. The major improvement from the pilot series was the change in the retrofit procedure. The slabs were effectively stitched with continuous strings of carbon FRP passing through paired holes and anchored at the ends (Fig. E). The solid rings of CFRP reinforcement minimized the dependence on bond between the concrete and the FRP laminate and avoided increases in flexural strength and stiffness. Figure F compares the behaviors of four slab specimens with different reinforcing configurations each retrofitted with CFRP in six shear reinforcing perimeters and a control specimen in which no shear reinforcement was used. The load has been normalized with respect to b^sub o^d[radical]f'^sub c^ to compare specimens with different concrete strengths in the entire test program and corresponds to the cumulative shear stress at a distance of d/2 from the bearing plate periphery. The retrofitted slab specimens demonstrated increases in shear strength and ductility of up to 82% and 768%, respectively, over that of their respective control specimens.

An increase of 57.5% in the load-carrying capacity of slabs as a result of similar FRP retrofitting observed by the authors in their tests (Fig. 5 of the paper) is comparable to that observed in the Toronto tests.

AUTHORS' CLOSURE

The authors thank the discussers for their contribution and for highlighting the oversight on the authors' part for failing to refer to their work.20,21 The authors are also glad to have this opportunity to clarify the differences between their work and the results reported in our original paper. Although the differences are not limited to the issues discussed as follows, the authors consider the following to be significant:

1. Geometric properties-The discussers indicate that their specimens were 59 × 59 × 6 in. whereas the specimens tested in our research measured 84 × 84 × 6 in. These dimensions indicate a difference of approximately 45% in the shear span-depth ratio. This significant difference is also reflected in the moment-shear ratios calculated at the critical perimeter for both bending axes. The different loading plate sizes used by the discussers (8 in.) and the authors (12 in.) result in significantly different b^sub o^/d ratios in the two test series. An examination of Reference 21 indicates that the discussers used two flexural reinforcement ratios: 1.49% and 2.23%. The flexural reinforcement ratio used in our tests was kept constant at 1.76%. CEB-FIP MC 9022 and BS 81109723 consider the punching shear resistance as a function of longitudinal reinforcement ratio. Hence, it is well established that punching shear strength of flat plates is significantly influenced by the flexural reinforcement ratio.