Based on the validation of the generated ANSYS FEM for RC beams with various patterns of externally bonded CFRP laminates mentioned in the preceding sections, the verified FE models were utilized for further analysis and parametric study. Due to technical and durability issues that resulted in the detachment of externally bonded CFRP sheets, resulting in an abrupt decrease in the member’s flexural strength, the effect of increasing the contact surface between CFRP laminates and the beam’s exterior surface, mechanically anchored unbonded external CFRP strips, pre-tensioned unbonded external CFRP laminates, and concrete grades on the moment capacity of strengthened RC-beams have been investigated. The investigation’s results will be presented in the subsections that follow.
4.2. Effect of External Pre-Tensioned Unbonded Straight CFRP Sheets
The degradation of the adhesive hazard material over time was observed in several cases of strengthened beams utilizing exterior bonded CFRP strips. This deterioration in the adhesive material that caused the detachment of CFRP sheets may be attributed to variations in the thermal expansion coefficients of concrete, adhesive materials, and CFRP sheets, as well as the quality of adhesive material and the differential strains in CFRP sheets and the surrounding concrete layer. According to field observations, the detachment of CFRP sheets occurs abruptly, resulting in a sudden loss in the beam loading capacity, which may result in a catastrophic collapse.
As a solution to this issue, the current study proposes the use of external unbonded CFRP sheets that are mechanically fastened at the ends of the tension side of the strengthened beam. To improve the ultimate load and deflection of the strengthened beams, the exterior unbonded CFRP sheets were initially tensioned with varied degrees of pre-tension stresses as a ratio of CFRP ultimate stress (
fucf). These various levels of pre-tension forces in CFRP sheets were determined to generate controlled camber with no cracks in the compression zone of the beam when just the beam’s own weight was applied.
Figure 15 depicts the FE configurations of the strengthened RC beams utilizing 75 × 0.15 mm bonded and unbonded CFRP straight sheets with and without pre-tension forces.
Figure 16 presents the load–deflection relations obtained from the FEMs for strengthened beams using external CFRP straight laminates. The CFRP laminates have adhered to the concrete surface (B75), unbonded without pre-tension stress (U75 without pre-T), unbonded with pre-tension stress of 0.35
fucf (U75 with pre-T 35%), unbonded with pre-tension stress of 0.45
fucf (U75 with pre-T 45%), and unbonded with pre-tension stress of 0.65
fucf (U75 with pre-T 65%). The response of the strengthened beam using unbonded CFRP straight strips (U75 without pre-T) differs from that of the strengthened beam with bonded CFRP straight laminates (B75). The ultimate load of the beam (U75 without pre-T) is lowered by 17.6% when compared to the beam (B75). To enhance the flexural strength of the unbonded strengthened beam, pre-tension forces were applied to CFRP sheets with varied ratios of 35%, 45%, and 65% of
fucf. The initial tension forces were estimated to control the camber of the beam while keeping tensile stresses in the compressive side of the beam below the concrete modulus of rupture. As depicted in
Figure 17, the detected maximum camber was around 0.4 mm and the ultimate tensile stress in the compression side of the beam was 1.64 Mpa, utilizing only the self-weight of the beam and the maximum pre-tension force of 0.65
fucf. As shown in
Figure 16, the case of (U75 with pre-T 45%) was comparable to the case of (B75), with a minor improvement in load capacity of 1.57% and a 9.7% reduction in deflection. These modest discrepancies are regarded within the permitted tolerances, with which the behavior of externally bonded (B75) and unbonded strengthened RC beams utilizing pre-tensioned CFRP straight sheets (U75 with pre-T 45%) may be deemed equivalent.
4.3. Effect of Sheet Thickness of Bonded U-Wrapped CFRP
To explore the influence of increasing the thickness of bonded U-wrapped CFRP strips, five distinct thicknesses of 0.1, 0.15, 0.20, 0.25, and 0.30 mm were employed in the FEM.
Figure 18 illustrates the various configurations of the U-wrapped CFRP laminates used to strengthen the beams.
Generally, increasing the thickness of U-wrapped CFRP sheets enhances the overall flexural performance of the strengthened beams. The flexural load capacity was boosted by about 58%, while the deflection of the U-wrapped beam with 0.1 mm CFRP sheet thickness was reduced by 17.7% in comparison to the referenced beam. The maximum load increased by 17.7%, 18.1%, 17.5%, and 17.2% when the CFRP sheet thickness was increased from 0.1 mm to 0.15, 0.2, 0.25, and 0.3 mm, respectively. When the thickness of the CFRP laminates was raised from 0.15 mm to 0.2 mm, 0.25 mm, and 0.3 mm, the deflection was lowered by 15.6%, 28.7%, and 36.5%, respectively. As seen in
Figure 19a, raising the thickness of U-wrapped CFRP laminates from 0.15 mm to 0.2, 0.25, and 0.3 mm had almost no effect on improving load capacity, but had a substantial impact on reducing deflection. This might be attributable to the fact that all of these beams failed in tension, particularly owing to the failure of tensile steel bars, whereas the sole advantage of raising the thickness of the CFRP laminates was a reduction in deflection as a result of the reduction of the axial strains of the CFRP laminates. The maximum axial strains in the cases of T 0.15 mm and T 0.3 mm were dropped from 0.0109 to 0.0059, with a reduction ratio of 45.9%, as shown in
Figure 19b,c.
4.5. Effect of Using Exterior Unbonded CFRP Sheets with Different Concrete Grades
To determine the impact of concrete compressive strength on the flexural strength and deformation of strengthened beams using external unbonded CFRP straight-laminates, ANSYS software [
48] is used to simulate 12 distinct strengthened RC beams with varying concrete strengths (21, 50, 80, and 120 MPa) and CFRP cross-sectional areas (2.5, 5, and 7.5 mm
2).
Figure 22 displays the configuration and dimensions of the strengthened RC beams modeled using external unbonded CFRP sheets.
In this section, the analyzed beams were separated into two categories. The first category includes beam groups A, B, and C. Each group of beams has the same CFRP cross-sectional area as the concrete compressive strength increases. In the second category, the beams in each group (D, E, F, and G) have the same concrete compressive strength with an increase in CFRP cross-sectional area.
Figure 23 depicts the load–deflection relations for all modeled strengthened beams.
Table 5 shows the maximum values of concrete compression stress, CFRP tensile stress, SRFT tensile stress, ultimate load, ultimate load variance, and maximum midspan deflections. As seen in
Figure 23, increasing both the concrete compressive strength and the CFRP cross-sectional area improved the beam’s overall flexural behavior. Under the same load, the beam’s deflection was reduced in the elastic phase of all studied beams, which may allude to the rise in the beam’s overall modulus of elasticity induced by raising the concrete compressive grade.
Furthermore, all of the analyzed beams failed in tension when the tensile stress in the steel reinforcement exceeded the yield strength while the concrete strain in the compression zone was still under failure strain. As seen in
Figure 23 and numerically interpreted in
Table 5, for the beam groups with the same CFRP cross-sectional area and increased concrete compressive strengths (Group A: B1, B2, B3, and B4), (Group B: B5, B6, B7, and B8), and (Group C: B9, B10, B11, and B12), increasing the concrete compressive strength using the same CFRP cross-sectional area improves only the beam’s load capacity by (29.7%, 49.3%, and 60.8%), (22.3%, 43.5%, and 66.4%), and (30.7%, 61.2%, and 71.6%), comparable to B1, B5, and B9, respectively.
According to
Table 5, steel tensile stresses surpassed the steel yield strength at an increasing rate in beams with high concrete compressive strength, i.e., in beams with relatively high concrete tensile strength compared to that of beams with low concrete compressive strength. These beams with high compressive strength have a relatively high concrete tensile strength, which may assist in enhancing steel strain hardening and, as a result, delay the failure of these beams, indicating a significant increase in load capacity in these beams.
As depicted in
Figure 23 and quantified in
Table 6, in the beam groups with the same concrete compressive grade and increasing in CFRP cross-sectional areas (Group D: B1, B5, and B9), (Group E: B2, B6, and B10), (Group F: B3, B7, and B11), and (Group G: B4, B8, and B12), the load capacity increased by (6.7% and 11%), (0.7% and 11.9), (2.6% and 19.9%), and (10.4% and 18.5%), respectively, when compared to B1, B2, B3, and B4. However, the maximum deflection of these beam groups dropped by (10.6% and 26.4%), (20.4% and 31.6%), (12.8% and 20.4%), and (16.5% and 29.8%), respectively, when compared to B1, B2, B3, and B4.
It is revealed that the rate of increase in ultimate load caused by an increase in concrete compressive strength is greater than that caused by an increase in CFRP cross-sectional area. This finding may refer to that increasing the concrete grade has a substantial impact on improving the strain hardening of steel bars as a result of bond and concrete tensile strength improvement, whereas increasing the cross-sectional area of external CFRP sheets has no physical effect on the strain hardening of steel reinforcement bars.
According to the load variations shown in
Table 5 and
Table 6, increasing the CFRP cross-sectional area to strengthen RC beams with any constant compressive strength can enhance load capacity by up to 20% (As presented in
Table 6). Additionally, using CFRP sheets to strengthen RC beams constructed from high-strength concrete (HSC) is more efficient than using it to strengthen normal concrete (NC) beams, resulting in increasing the ultimate load by 22.3% in the case of NC and by 71.6% in the case of HSC, as shown in
Table 5.
The significant benefit that can be acquired by increasing the cross-sectional area of external unbonded CFRP sheets is by reducing the tensile stresses in CFRP sheets, which results in a reduction in the sheet’s axial strain, which in turn reduces the beam’s curvature and mid-span deflection by about 10% in the case of NC to about 30% in the case of HSC, as shown in
Table 6.