Long-Term MgSO4 Durability of CFRP-Strengthened NSC and UHPC Beams: Surface Preparation Effects on Bond-Dependent Flexural Performance

Shahad Majid  Madhlom; Hussam Ali  Mohammed

Authors

Shahad Majid Madhlom Al-Furat Al-Awsat Technical University, Al-Mussaib Technical College, Department of Building and Construction Engineering
Hussam Ali Mohammed Al-Furat Al-Awsat Technical University, Al-Mussaib Technical College, Department of Building and Construction Engineering

Keywords:

Carbon fiber-reinforced polymer, Ultra-high-performance concrete, Magnesium sulfate exposure, Surface preparation, CFRP–epoxy–concrete interface, Long-term durability, Flexural behavior

Abstract

The long-term effectiveness of externally bonded carbon fiber-reinforced polymer (CFRP) strengthening systems is strongly governed by the durability and stress-transfer capacity of the CFRP–epoxy–concrete interface. This study experimentally investigates the bond-dependent flexural performance of CFRP-strengthened normal-strength concrete (NSC) and ultra-high-performance concrete (UHPC) beams after exposure to magnesium sulfate solution. Twelve reinforced concrete beams were tested, including six NSC beams and six UHPC beams. For each concrete type, one unstrengthened control beam was used as a reference, while the remaining beams were strengthened with a single externally bonded CFRP sheet after applying mechanical grinding, vertical grooving, 45° inclined grooving, sandblasting, or diluted HCl treatment. After CFRP bonding and seven days of epoxy curing, all specimens were immersed in a 3.5% MgSO₄ solution for 120 days before flexural testing under two-point loading.

The results showed that CFRP strengthening remained effective after sulfate conditioning, although the improvement depended strongly on surface preparation and substrate type. In the NSC group, sandblasting produced the highest ultimate load of 145.70 kN, corresponding to a 20.55% increase over the control beam. In the UHPC group, sandblasting also achieved the highest ultimate load of 205.20 kN, representing a 28.12% increase. The 45° inclined grooving technique achieved a comparable ultimate load of 202.89 kN and provided the highest UHPC stiffness response, with a secant stiffness of 40.96 kN/mm and a 47.28% increase over the control specimen. Overall, sandblasting was most effective for load enhancement, whereas 45° inclined grooving provided a more balanced long-term response in terms of strength, stiffness, deformation behavior, and interface stability.

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