3.1. Structural Behavior
The experimental results obtained from the tested beams are critically analyzed and discussed in terms of load–deflection curves.
Figure 6 and
Figure 7 depict the applied load (recorded by the load cell as the total load) and the corresponding midspan deflection (as the average of the two LVDTs). The structural curves are grouped for both A and B specimens. The graphs show the response during the first 12 load cycles overlapped with the structural performance under monotonic loading up to failure. The further 24 load cycles are not added to have a better resolution of the curves.
The behavior of the strengthened beams can be compared with the control beams (CB_A and CB_B) and the EB-strengthened monotonic beams (M/EB_SS_A and M/EB_SS_B) in terms of yield load, ultimate load, respective deflections and global/local ductility.
Table 5 collects the values of loads and midspan deflections corresponding to the yielding of the steel bars in tension (F
sy and δ
sy) and at failure (F
u and δ
u). Further, the same table summarizes the deflection ductility ratio (μ
δ = δ
u/δ
sy), the ratio between the failure load of the strengthened beams and the corresponding control beams (Δ
F = F
u/F
u,control) and the ratio of the μ
δ of each strengthened beam versus the corresponding control beam (Δ
μ = μ
δ/μ
δ,control). Finally, the failure modes observed during tests are labeled in the last column of the table and discussed in paragraph 3.3. For the specimens tested monotonically, the load and midspan deflection at the first crack formation were also reported (F
cr and δ
cr, respectively).
The yield load was identified graphically when a loss of stiffness occurred in correspondence with a change of slope in the load–deflection curve while the failure is accompanied/followed by a clear drop due to the sudden detachment or tensile rupture of the strengthening system.
As expected, the two monotonically tested beams (M/EB_SS_A and M/EB_SS_B) showed failure behavior governed by intermediate debonding of the external reinforcement. Bencardino and Condello [
35] investigated the flexural behavior of RC beams with the same dimensions and external reinforcement used in this study.
The results confirmed the effectiveness of the EB/SRG system and, in particular, improved performance of the IRS/SRG system in terms of ultimate loads and ductility. They showed that with the IRS technique, a considerably greater debonding strain is obtained. To summarize, the two monotonic beams A and B reach failure loads of 83.70 kN and 93.00 kN, with an increase of 19.22% and 31.30% with respect to the control beams, respectively. The corresponding deflection ductility was estimated at 1.93 and 2.73, respectively.
Starting from these experiences, the degradation in terms of flexural capacity and bond strength for the beams subjected to repeated load was evaluated. The beam reinforced with the SS fiber and EB technique of group B (EB_SS_B) showed an increase in the failure load of 9.27% and a ductility factor of 1.93. Therefore, reductions in the failure load and ductility factor were observed compared to the corresponding monotonically tested beam M/EB_SS_B. The beam EB_SS_A exhibited remarkable bond degradation with detachment of the composite strip by intermediate debonding at the end of the 12th cycle (
Figure 6c). In fact, both EB_SS beams were characterized by premature and sudden detachment of the external reinforcement. The weak flexural performance can be associated with poor matrix/steel strip impregnation. The cementitious matrix is not able to optimally saturate the space between two consecutive strands. As reported in paragraph 3.3, for both beams, the strengthening composite (steel strip embedded in the two layers of the cementitious matrix) was completely detached.
Conversely, the IRS technique considerably improves the structural performance in terms of ultimate load and ductility ratio despite the repeated loading. Indeed, the beams strengthened with the SS textile and IRS technique (IRS/SRG) showed good flexural behavior (
Figure 6d and
Figure 7d). Both beams IRS_SS_A and IRS_SS_B exhibited a good level of failure load (83.60 and 90.00 kN, respectively) and ductility factor (4.33 and 3.93, respectively). The experimental tests have highlighted that no premature debonding occurred during the load cycles. However, a very limited reduction in the failure load was observed compared to the two monotonically loaded beams (with reference to control beams equal to 19.08% and 27.06%), although the ductility was significantly increased.
On the other hand, the SRG system comprising the GLV steel strip exhibited very good behavior compared to the SS reinforcement. The failure occurred for all beams (EB_GLV_A/B and IRS_GLV_A/B) without debonding, exploiting the entire tensile strength of the composite material, independent of the strengthening technique used. All beams showed similar structural trends to each other as depicted in the load–deflection curves (
Figure 6e,f and
Figure 7e,f). For these specimens, the yield load of steel bars occurred at 51.70 kN, 56.20 kN, 63.20 kN and 59.80 kN and the steel strands ruptured at 75.70 kN, 75.50 kN, 78.30 kN and 77.70 kN, respectively, with the same percentual increase in the failure load (7.83%, 7.54%, 10.54% and 9.70%, respectively). The midspan ductility at failure was equivalent to 3.17, 3.24, 2.27 and 3.35, respectively. As denoted, the ductility ratios are also rather equal for the four beams. Specifically, it can be concluded that, with the low-density steel strip, the two strengthening techniques EB and IRS produce the same flexural improvement.
In the opinion of the authors, the proper impregnation of the GLV steel strip through the matrix layers allows a homogeneous and strong bond at the concrete/composite interface and an improvement of the interlocking phenomena around the steel strands. The different treatment of the external surface between the two strengthening techniques does not lead to significant changes in the structural behavior. In the case of the SS steel strip, the stress developed at the interface is interrupted by the formation of flexural cracks, and when the repeated loading is active, progressive detachments due to the degradation of the bond strength at the matrix/steel fiber interface occurred. In this regard, the use of matrices with high mechanical properties and structural performance is recommended. An example is represented by geopolymeric matrices. The effective performance of these materials in structural application has been proven/demonstrated in many studies as sustainable alternatives to the traditional epoxy resin and pure cementitious matrix [
32,
33].
Therefore, it can be concluded that the IRS technique makes the concrete surface much rougher, increasing the effectiveness of the high-density fibers and reducing premature detachment in SRG applications.
On the basis of the obtained results, it is possible to correlate at failure the mechanical properties of the steel strengthening systems with the stress in the composite systems. The quality of the bond of the composite system can be also associated with observed failure modes (detailed in paragraph 3.3). Specifically, the fiber with low mass per unit of area has the same behavior regardless of the strengthening technique with the exploitation of all of the strength. The failure is manifested by the total rupture of the strengthening fiber. On the other hand, the fiber with high mass per unit of area shows greater efficiency when applied with the IRS strengthening technique, which guarantees suitable protection against debonding phenomena. The failure mode involves the formation of a wider crack pattern with a progressive detachment of the external reinforcement.
3.2. Midspan Deflection Curves and Recorded Strains
The deflections along the effective length of the beams were plotted using the data recorded by the LVDTs arranged at midspan and at quarter span. The application of the external strengthening leads to a reduction in the midspan deflections under maximum load compared with the unstrengthened beams, as shown in
Figure 8a,b, respectively. In general, this reduction is directly proportional to the amount of area of the external strengthening. The experimental curves do not clearly highlight this behavior. Evidently, the beams strengthened with SS fibers undergo greater deflections due to the influence of load cycles and the local detachments of the steel strips. However, the IRS technique shows greater deflections than the EB technique, entailing an increase in ductility.
By focusing the attention on the monitored strains, at the end of each load cycle, nonzero values on internal steel reinforcement bars and the external reinforcement fibers were observed. In the tension zone, after the cracking, the concrete has negligible tension capacity, and the intact concrete between cracks increases the flexural stiffness of the RC beams (tension-stiffening effect). The tensile forces occurring in the crack are absorbed by the internal reinforcement and the external composite. By removing the load, the crack width does not return to its initial position and a part of the residual strain is recorded by the strain gauges. This strain does not represent a plastic accumulation because the cycles are carried out below the yield strength of the steel bars. Conversely, the compressed part of the beam (top concrete surface and top steel bars) shows no residual strain at zero loads. The schematization of the phenomenon is shown in
Figure 9, and for the sake of brevity, the load–strain curves of the EB_SS_B beam are shown in
Figure 10. The same phenomenon was observed for all tested beams under repeated loading.
The strain profiles at failure are depicted for both groups in
Figure 11a,b. The slope of the profile represents the curvature of the sections. The beams strengthened with IRS/SRG systems (IRS_GLV_A, IRS_GLV_B, IRS_SS_A and IRS_SS_B) showed greater local ductility than the EB/SRG systems (EB_GLV_A, EB_GLV_B and EB_SS_B). In detail, the GLV steel strip, thanks to the least amount of external reinforcement, reached the highest strain values than the corresponding SS steel strip. In general, the IRS technique allows beams to lose local and global ductility less (in terms of curvature and displacement).
Finally, the strains at failure load can be read in
Table 6 for concrete (ε
c), top steel bars (ε′
s), bottom steel bars (ε
s) and SS/GLV steel fiber (ε
f). The external steel strip strains are equal to 6.21‰, 8.55‰, 7.01‰ and 11.30‰ for M/EB_SS_A, IRS_SS_A, EB_GLV_A and IRS_GLV_A beams, respectively. The external steel strip strains for M/EB_SS_B, EB_SS_B, IRS_SS_B, EB_GLV_B and IRS_GLV_B beams achieved 8.33‰, 2.86‰, 7.75‰, 7.10‰ and 10.71‰, respectively.
3.3. Failure Modes and Crack Patterns
The crack patterns at failure for each strengthened beam are reported in
Figure 12 and
Figure 13. The failure modes are represented in the same figures. More precisely, the cracks formed during the load cycles are marked in red, whereas those during the monotonic loading are marked with black. All beams exhibited the classical flexural cracks in the region of constant moment (middle part of the span) with spread and growth in the clear span between the external supports. Near each end region, a few shear cracks developed at high values of the applied load. At the same time, the cracks across the cementitious matrix were observed and marked on the external composite.
The IRS/SRG reinforcement produces a more marked and distributed crack pattern in the lower part of the beam (microcracks at the SRG/concrete interface) compared to beams strengthened with the EB/SRG system. Furthermore, the cracks produced by monotonic loading are more visible, indicating a stronger bond after the repeated loading phase. The number of cracks increases in the beams strengthened with the GLV steel strip compared to the other ones. Finally, in the monotonically tested beams, a greater height of the neutral axis is highlighted, as can be seen from the figures.
For all beams strengthened with the stainless steel strip (SS), the failure mechanisms were controlled by intermediate debonding (
Figure 12a,b and
Figure 13a,b).
Specifically, the total delamination of the composite strip occurred in the externally bonded (EB) strengthened beams. It was related to the complexity, observed during the strengthening phase, to achieve proper impregnation between the stainless steel strip and the matrix. This drawback is partially overcome by using the IRS technique. In fact, the IRS-strengthened beams showed partial debonding in the middle section with detachment of part of the lower matrix layer, providing ductility behavior during the failure response, with damage developed through the matrix to steel strip interface. The specimen IRS_SS_A failed by concrete crushing with minimal debonding of the strengthening system (
Figure 12c).
As mentioned in subparagraph 3.1, all beams strengthened with low-density steel fibers (GLV) exploited the steel strand tensile rupture of the composite system with a wide flexural crack in the central span, as depicted in
Figure 12d,e and
Figure 13d,e. This type of failure is independent of the strengthening technique used. The crushing of the concrete occurred in the beams of group A (EB_GLV_A and IRS_GLV_A) (
Figure 12d,e).
Figure 14 reports a detailed summary of the main failure modes that occurred.