PSI - Issue 84

Anna Bontempi et al. / Procedia Structural Integrity 84 (2026) 1039–1046

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3. Experimental Results Figure 6 shows the load–midspan deflection curves of the unstrengthened test (black), conducted over the yielding of the existing longitudinal reinforcement, and of the strengthened specimen (light blue). A significant increase, of about 33%, in ultimate load is observed. The response of the strengthened beam exhibits the typical initial uncracked branch followed by the cracked stage. After reaching the load corresponding to yielding of the existing steel reinforcement, no load plateau is observed; instead, the final portion of the curve remains approximately linear, with a residual stiffness. This behavior results from the combined contributions of yield of existing steel reinforcement and the CFRP bars, which remain in the elastic range. Indeed, the CFRP material exhibits a linear elastic response up to rupture. Despite the elastic–brittle behavior of CFRP, its combination with steel reinforcement provides an overall ductile response of the structural element, resulting in a ductility of approximately 6 in terms of displacement. Finally, two load drops leading to collapse can be observed, one associated with the rupture of each CFRP bar. After failure of the first bar, a limited load increase was recorded before the rupture of the second bar. The load– deflection diagram shows that, following rupture of both strengthening bars, the applied load returns to the value corresponding to the plateau observed in the unstrengthened test. This indicates that the CFRP bars are primarily responsible for the entire increase in load-bearing capacity. In this case, the mortar application was mainly required to protect the cross-section against the progression of corrosion and to enhance the durability of the structural element. In actual cases, the overlay could also make a significant contribution to shear strength; however, this potential was not exploited here because the specimen’s shear demand was not critical The crack pattern is shown in Figure 8. The critical crack developed in the proximity of loading point 4, reaching a width of approximately 15 mm. Another significant crack also formed near loading point 2; however, rupture of the CFRP bars occurred at the location of the former. Consistently, the load-point displacement diagram (Figure 7) indicates that loading point 4 exhibited higher deflections.

Figure 7: Load - Midspan deflection Curve of the unstrengthened and strengthened tests (a); Load – deflection curves of the two load points (b).

As mentioned in Paragraph 2, four potentiometric transducers were installed near the support, where the highest slip was expected, to measure possible slip between the existing section and the overlay. At the flange side, the transducer was anchored to the existing section by means of an L-shaped steel profile fixed with a mechanical anchor screwed in the concrete. The L-shaped profile was perforated, and one of the holes was used to connect the transducer through a threaded screw. At the opposite end, the transducer was connected by a screw to a small steel block, which was bonded to the surface of the overlay using hot-melt adhesive (Figure 6a). It should be noted that stress transfer between the two materials, with regard to the web connections, relies solely on surface roughness, as no mechanical connectors were employed in the strengthening intervention. In fact, the dowels are employed just to connect the overlay to the existing flange. In light of this, the instrumentation results indicate that the maximum measured slip was approximately 1,0 ∙ 10 -2 mm. When referred to the gauge length between the fixed points, this corresponds to a maximum strain of about 4,3 ∙ 10 -5 . Therefore, a perfect bond condition between the existing section and the overlay was assumed in the numerical analysis.