Bending performance of RC slabs strengthened by CFRP sheets using sprayed UHTCC as bonding layer: Experimental study and theoretical analysis

Abstract

Debonding failure is one of the main drawbacks of reinforced concrete (RC) structures strengthened with externally bonded (EB) fiber reinforced polymer (FRP) systems. Due to the huge application potential of sprayed ultra-high toughness cementitious composite (UHTCC) in strengthening and retrofit, this study, with the aim of delaying or preventing intermediate crack-induced debonding of EB FRP sheets, experimentally and analytically investigated the flexural behavior of RC slabs strengthened by EB CFRP sheets with sprayed UHTCC as bonding layers, which has been rarely reported in previous research works. For this purpose, five RC slabs with low strength concrete and low reinforcement ratio, including one control specimen and four strengthened specimens repaired by EB CFRP sheets with sprayed UHTCC as bonding layers, were manufactured and tested under four-point bending load. Variable parameters in the four strengthened specimens included the thickness (10 mm, 20 mm, and 30 mm) and length (600 mm and 900 mm) of the sprayed UHTCC bonding layer. The experimental results indicated that the specimens strengthened by EB FRP sheets with sprayed UHTCC as bonding layers experienced higher load-bearing capacity (24.43 %–120.92 %) and toughness (43.15 %–166.14 %), compared to the control specimen. The thickness of the UHTCC bonding layer had a significant effect on the structural performance. By increasing the thickness from 10 mm to 20 mm and from 20 mm to 30 mm, the ultimate load was enhanced by 32.69 % and 33.81 %, while the ultimate deflection was decreased by 14.99 % and 41 %, respectively. The test findings also showed that sprayed UHTCC had a strong cracking-control ability and that no debonding failure occurred between the UHTCC layer and the CFRP sheet, while interface slip was observed between the concrete substrate and the UHTCC layer. The sectional analysis method was then used to propose an analytical model for analyzing the composite action between the concrete substrate and the strengthening material, as well as a model for predicting the load-bearing capacity of strengthened specimens. Comparing the analytical results to the experimental ones revealed that the proposed models can properly predict the flexural behavior of RC slabs strengthened by EB CFRP sheets with sprayed UHTCC as bonding layers under four-point bending load.