# Deep Embedment (DE) FRP Shear Strengthening of Concrete Bridge Slabs under Loads Close to Supports
## 4 Discussion of Test Results  
### Failure Mode and Ultimate Loads  
Figure 9a,b shows the propagation of crack patterns through the depth of the unstrengthened test slabs. For those two test specimens, it can be found that the first flexural cracks started to appear in the moment zone at an applied load of 25 kN. Those flexural cracks propagated upwards as the load was increased. Flexural cracks then formed over wider areas until a load of 100 kN, when diagonal shear cracks appeared in the shear zone. The unstrengthened test specimens both failed in brittle shear at around 130 kN. The loading procedure had to be terminated, as shown in Figure 10. Those two concrete slabs were separated to two components through this diagonal shear crack, see Figure 10a. In addition, the crack pattern of the test slabs with drilling holes (test specimen coded as S-D-1) was similar as that in the control model (S-Con). The ultimate capacities of those two models are also close, as shown in Table 1. This indicates that the drilling procedure does not cause significant structural damage for the concrete slabs.  
Figure 8: Measurement set-up in this test: (**a**) Displacement transducers (T1-T6: displacement transducers); (**b**) Strain gauges for concrete slabs; (**c**) Strain gauges for embedded bars and steel reinforcement (R1-R6: electrical resistant strain (ERS)).  
Figure 9: \(Cont.\)The crack patterns of all the strengthened test slabs are shown in Figure 9c-h. Similar to the unstrengthened slabs, multiple flexural cracks formed in the moment zone up to the appearance of shear cracks (the applied load reached 100 kN). As the applied load increased, the inclined shear crack appeared in the shear zone, particularly in the area between two embedment strengthening bars,  
Figure 10: Comparisons of failure modes in the strengthened test slabs and unstrengthened test slabs.  
(a) S-Con; (b) S-C-2-9; (c) S-C-1-9; (d) S-C-1-13.  
Figure 9: Crack patterns in test slabs.  
as shown in Figure 9c-f. Interestingly, these shear cracks are prevented from opening further by the deep embedment strengthening bars. In addition, no shear cracks formed in the test slabs strengthened with large embedment bars (test specimen S-C-1-13). This indicates that the embedment strengthening bars crossed by the cracks are successful in preventing shear failure. Thus, the strengthened specimens are forced to a ductile flexural failure, as shown in Figures 10 and 11. When the applied load reached the ultimate strength, the failure mode of all the test slabs with deep embedment bars was concrete crush at the loading positions. In all the tests, no debonding failure of embedment strengthening bars occurred. Due to this high bond-slip behaviour, the embedment strengthening materials do not influence the crack pattern and failure mode significantly.  
The ultimate loads of all the test slabs are shown in Table 1. The application of deep embedment strengthening methods results in larger ultimate capacity of test slabs. The loading carrying capacity was enhanced by more than 10% by this strengthening method. As discussed in the section on crack pattern and failure mode (see Figures 9 and 10), the test slabs with deep embedment strengthening methods failed in flexure tests. Because this strengthening method has no effect on flexural behaviour and flexural capacity of the untrengthened test slab is larger than the shear capacity, increasing the degree of the ultimate capacity (around 10%) is not as significant as that obtained in the test results of the embedment strengthened concrete beams (around 30-40%) [8; 10]. Additionally, the variation of strengthening materials and strengthening ratios did not have a significant effect on ultimate capacity of the strengthened test slabs.