Load deflection behaviour

The deflections of all beams were measured using linear variable differential transducer (LVDT) and were recorded using a high-speed data acquisition system as mentioned in section 3.5. Typical load versus deflection curves obtained from the three LVDTs are shown in Figure 2.22 (Beam BGH-A2-01). The deflections of all beams at mid-span are tabulated in Table 2.8. The reactions at the supports are equal to the applied load, which are one half of the actuator load. The shear force in a beam is equal to the applied load. Hence, in the discussion of the results, the term shear load is used instead of the applied load. The load versus deflection diagram that is shown in the figures contains three stages behaviour: before cracking, transition from un-cracked to cracked stage, and after cracking. Since the beams in this investigation failed shortly after the formation of diagonal cracks, the shear crack induced deformation was small and this was neglected.

Table 2.8: Experimental results

G

roup

Bar

Type Specimens ID

Beam properties Experimental observations

𝑓𝑓𝑐𝑐′ b h d a/d L Total ρ V cr-ops P exp V exp Deff.

Failure Modes (MPa) (mm) (mm) (mm) (mm) (%) (KN) (KN) (KN) (mm) A1 GFRP BGN-A1-01 33.50 150 150 119 2.5 1000 1.27 6.92 34.32 17.16 10.54 BF BGN-A1-02 33.50 150 150 117 2.5 1000 2.29 7.32 41.22 20.61 6.48 SC A2 GFRP BGN-A2-01 33.50 130 230 200 2.5 1500 0.91 7.12 40.62 20.31 13.22 DT BGN-A2-02 33.50 130 230 200 3.0 1800 0.91 6.05 32.36 16.18 13.12 DT Steel BSN-A2-03 33.50 130 230 200 2.5 1500 0.91 7.63 53.86 26.93 7.02 Y-DT

BSN-A2-04 33.50 130 230 200 3.0 1800 0.91 6.90 43.90 21.95 11.09 Y-DT A3 GFRP BGL-A3-01 28.50 200 250 219 3.1 2000 0.52 8.90 51.80 25.90 13.35 ST BGL-A3-02 28.50 200 250 217 3.1 2000 0.93 9.10 71.20 35.60 9.84 SC BGH-A3-03 49.10 200 250 219 3.1 2000 0.52 10.45 59.52 29.76 14.12 ST BGH-A3-04 49.10 200 250 217 3.1 2000 0.93 12.50 74.20 37.10 10.71 ST Steel BSL-A3-05 28.50 200 250 219 3.1 2000 0.52 10.30 62.50 31.25 6.08 ST BSL-A3-06 28.50 200 250 217 3.1 2000 0.93 14.80 85.60 42.80 6.88 SC BSH-A3-07 49.10 200 250 219 3.1 2000 0.52 11.80 71.36 35.68 5.74 ST BSH-A3-08 49.10 200 250 217 3.1 2000 0.93 16.95 89.60 44.80 6.12 SC SC = Shear-compression failure ST = Shear-tension failure DT = Diagonal tension failure

Y-DT = Diagonal tension failure after yielding (steel reinforcement) BF = Bond failure

Figure 2.22: Typical load versus deflection curves (Beam BGH-A2-01)

The load-deflection behaviour of 150 mm thick beams in Group A1 for different reinforcement ratios. In general, the load-deflection behaviour of the beams can be defined by three stages: before cracking, transition from un-cracked to cracked stage, and after cracking, as shown in Figure 2.23. It can be seen that all beams behaved linear elastic at the beginning. However, the load-deflection behaviour in the second stage, which is the transition zone from the uncracked to the cracked stage, where the existing cracks grow and new flexural cracks developed in the constant moment zone.

The behaviour of the beams after the second stage; for the same load level, and as expected, the deflection of the beam decreased as the axial stiffness of GFRP bars increased. Table 2.9 shows the axial stiffness of GFRP bars. This result is in good agreement with the other test results of FRP reinforced concrete beams without web reinforcements (Tureyen and Frosch 2002, EI-Sayed et al.2006a).

Table 2.9: Axial stiffness of the reinforcing bars in different beams

Group Bar Type Beam ID Ef (GPa) ρf Axial stiffness (GPa) A1 GFRP BGH-A1-01 52.32 1.27 0.66 BGH-A1-02 56.72 2.29 1.30 45

Figure 2.23: Load-deflection behaviour of 150 mm thick beams in Group A1

Figure 2.24 shows the applied shear load versus mid-span deflections for all of the beams in Group A2 with different shear span-to-depth ratio (a/d). Before flexural cracking occurred, the load-deflection behaviour was approximately linear. In this stage, the stiffness of the beams with the same shear span to depth ratio was approximately the same for different reinforcement types. This indicated that the deflections before cracking were not affected by the reinforcement type. The beam progressively changed from an un-cracked to fully cracked state, where the existing cracks grow and new flexural cracks developed in the constant moment zone. At the end of this stage, the behaviour of the beams became shortly linear, this linear behaviour continued until failure.

For the same shear span-to-depth ratio, and at a certain load level, the deflections of the GFRP reinforced beams were higher than those reinforced with steel. This could be attributed to the low axial stiffness of GFRP reinforcement. Nonetheless, it should be noted that GFRP reinforced members have greater tension stiffening than steel reinforced members (Biscboff and Paixao 2004).

(a) (b)

Figure 2.24: Load-deflection profile of beams in Group A2: (a) a/d = 2.5, and (b) a/d = 3.0

The concrete strength did not have a significant effect on the shape of the load- deflection behaviour for 250 mm thick beams. All beams in Group A3 showed similar load-deflection characteristics as shown in Figure 2.25. It can be seen that all beams behaved linear elastic at the beginning, and as the load increases, the beam starts to behave non-linear due to the development of cracks until failure occurred. However, beams reinforced with steel bar had higher load compared to GFRP bar reinforced concrete beams. This could be attributed to their low modulus of elasticity of GFRP bar.

(a) (b)

Figure 2.25: Load-deflection behaviour of 250 mm thick beams with different concrete strengths and same reinforcement types and ratios in Group 4:

(a) ρf = 0.52% (b) ρf = 0.93%

(c) (d)

(cont.) Figure 2.25: Load-deflection behaviour of 250 mm thick beams with different concrete strengths and same reinforcement types and ratios in Group 4:

(c) ρs= 0.52% and (d) ρs= 0.93%