**4.4 Effect of longitudinal reinforcement ratio on experimental moment carrying capacities**

The experimental *moment* carrying *capacity* is a maximum *bending moment* that can be resisted by a beam or any other structural member before it fails in *bending*. The effect of experimental moment carrying capacities with the same concrete strength is essentially related to the longitudinal reinforcement ratio.

As the longitudinal reinforcement ratio changes from 0.33, 0.52, and 1.1% the experimental *moment* carrying *capacity* increases in both steel and well as GFRP reinforced concrete beams. For example in the case of steel-reinforced concrete beams, increasing the longitudinal reinforcement ratio changes from 0.33, 0.52, and 1.1% increases the experimental *moment* carrying *capacity* by 11.06, 16.39, and 28.36 kN-m. On the other hand in the case of GFRP reinforced concrete beams increases by 16.48, 19.41, and 27.23 kN-m as shown in **Figure 9** and **Table 4.** Overall, it is observed that with the increase in the longitudinal reinforcement ratio, the ultimate load-carrying capacity increases by approximately 34% in G-0.33-1 and 42% in G-0.52-1 beams in comparison to S-0.33-1 and S-0.52-1. It is pointing towards higher tensile strength of GFRP bars in comparison to steel reinforced beams. But as the reinforcement ratio increases from 0.52 to 1.1% there is a slightly 5.09% decrease in load-carrying capacity in G-1.11-1 beam as compared to S-1.11-1 beam due to increase brittleness with higher ρ. It points towards a lower modulus of elasticity of GFRP bars as compared to steel bars.

## **4.5 Modes of failure**

The design of both steel-RC and GFRP-RC is based on ACI Code [8, 34]. The GFRP-RC beams were designed as over-reinforced beams with a reinforcement ratio of 0.0052 [8] which was greater than the balanced reinforcement ratio of 0.00308 [34]. The steel-RC beams were designed as under-reinforced beams having a reinforcement ratio (0.0052) less than the balanced reinforcement ratio of 0.02. A compression failure for the GFRP-RC beams and a tension failure for the steel beams were expected during flexural testing. The observed modes of failure of

**Figure 9.** *Variation in an average experimental moment carrying capacities Vs 'ρ'.*

**Figure 10.**

*Failure modes in steel-RC and GFRP-RC beams. (a) S-0.33-1. (b) G-0.33-1. (c) S-0.52-1. (d) G-0.52-1. (e) S-1.11-1. (f) G-1.11-1.*

steel-RC and GFRP-RC beams are presented in **Figure 10.** Steel-RC beam failed by the crushing of concrete after the tension reinforcement yielded (**Figure 10a**, **c**, and **e**) whereas the GFRP-RC beam failed typically in shear followed by concrete crushing **(Figure 10b**, **d**, and **f**) since they are designed as overreinforced beams to prevent their failure by GFRP rupture. This indicates that even though both steel-RC and GFRP-RC beams have the same area of tension reinforcement (Ast) ratio, GFRP-RC beams experience a different mode of failure as compared to the steel-reinforced concrete beam. Therefore, some modification in the design has to be considered when the GFRP bar is to be used as reinforcement.

A comparison of theoretical and experimental moment carrying capacities of steel-RC and GFRP-RC beams is presented in **Table 4.** The ratio of Mth/Mexp is less than 1 for both steel-RC and GFRP-RC beams. Thus, for design purposes, the strength reduction factor (Ø) for all over reinforced GFRP reinforced beams is calculated by Eq. (1). The theoretical moment of resistance (Mth) in the case of the GFRP-RC beam is calculated by Eq. (1) [8].

$$\mathbf{M}\_{th} = \rho\_f f\_f \left[ \mathbf{1} - \mathbf{0}.59 \rho\_f \frac{f\_f}{f'c} \right] bd^2 \tag{1}$$

On the other hand, the theoretical moment of resistance (Mth) in the case of the steel-RC beam is calculated by the formula given in Eq. (2) [34]

$$\mathbf{M}\_{th} = \rho\_f f\_f \begin{bmatrix} \mathbf{1} - \mathbf{0}.59 \rho\_f \frac{f\_y}{f'c} \end{bmatrix} bd^2 \tag{2}$$

where Mth = theoretical bending moment resistance, *ρ<sup>f</sup>* = reinforcement ratio, b = width of the beam, d = effective depth of the beam, f'<sup>c</sup> = Design characteristic *Crack Classification in Steel-RC and GFRP-RC Beams with Varying Reinforcement Ratio Using… DOI: http://dx.doi.org/10.5772/intechopen.101305*

concrete compressive strength. Where, ρfb = balanced reinforcement ratio, ρ<sup>f</sup> = actual reinforcement ratio and Ø = 0.65 for ρ<sup>f</sup> ≥ 1.4 ρfb [8]. For underreinforced steel beams, the strength reduction factor is taken as 0.9 (ACI 319, (2019)). The fracture/failure and cracking pattern in the two differently reinforced beams along with a varying percentage of tension reinforcement ratio are further studied using acoustic emission in Section 5 respectively.

**Figure 11.**

*Typical crack patterns. (a) Damage level I. (b) Damage level II. (c) Damage level III.*

**Figure 12.** *Damage level classification.*
