**4.1 Flexural performance of steel reinforced beams**

The load-deflection plot of steel-RC beams is broadly classified into three regions un-cracked elastic, cracked-elastic, and plastic zones (**Figures 4** and **5**). Initially, the applied loads as well as deflection are small and follow a linear relationship.

**Figure 4.** *Load v/s deflection plots for S-series beams.*

*Crack Classification in Steel-RC and GFRP-RC Beams with Varying Reinforcement Ratio Using… DOI: http://dx.doi.org/10.5772/intechopen.101305*

**Figure 5.** *Load v/s deflection plot for G-series beams.*

**Figure 6.** *Load v/s deflection plots for S-series beams (one beam per type).*

This zone I is named uncracked elastic zone as shown in **Figure 6**. With further increase in loading, a significant change and reduction in stiffness of the beam are observed with the development of hairline cracks at cracking load (Pcr) of (5.58, 7.59, and 9.54) kN with a deflection (*δ*cr) of (0.61, 0.60, and 0.43) mm for S-0.33-1, S-0.52-1, and S-1.11-1 beam, respectively (**Table 3**). These cracks progress along the sides of the beam at constant stiffness. The cracks initiate and start becoming visible at a load of (16, 20, and 30) kN for S-0.33-1, S-0.52-1, and S-1.11-1 beam, respectively, in the tensile zone of the beam. With further increase in loading, the cracks start propagating and appear in the form of distributed flexural and shear cracks leading to steel yielding at a load (Py) of (28.93, 44.18, and 75.09) kN, with a deflection (*δ*y) of (6.27, 6.85, and 4.94) mm in S-0.33-1, S-0.52-1, and S-1.11-1 RC beams, respectively as shown in **Table 3**. This part of the load-deflection plot from Pcr to Py is termed as cracked-elastic zone II.

In zone III named plastic zone, the concrete section is cracked and ineffective in resisting the loads and the entire load is taken by steel and yields. It is marked by an increase in the mid-span deflection 23.22, 14.47, and 12.48 mm with a minor


#### **Table 3.**

*Comparison of flexural parameters of steel and GFRP reinforced beams.*

increase in load up to a peak load (PPeak) of (35.18, 50.80, and 88.96) kN, pointing towards larger strain at the level of steel and increase in curvature of the cracked section with an increase in the percentage of steel. Further due to the strain hardening of steel, the beams fail at an ultimate load (Pu) of (33.43, 44.96, and 82.75) kN with a deflection (*δ*u) of (55.51, 43.1, and 30.9) mm in for S-0.33-1, S-0.52-1, and S-1.11-1 RC beams, respectively. In general, it is observed that with the increase in the reinforcement ratio, the ultimate load-carrying capacity increases by approximately 34% in S-0.52-1 and 42% in S-1.11-1 beams as compared to S-0.33-1 indicating higher load carrying capacity with an increase in tensile reinforcement. Another important observation is a significant increase in the area under the load-deflection plot with an increase in the reinforcement ratio. It is also observed that plastic zone III reduces drastically with an increase in steel. The failure takes place at the much lower strain in the S-1.11-1 RC beam. All S-series RC beams specimens failed by steel yielding and followed by concrete crushing.
