The durability of structural strengthening systems remains a critical concern for civil engineers and researchers focused on extending the service life of concrete infrastructure worldwide. A new study published in Construction and Building Materials examines the performance of near-surface-mounted (NSM) carbon fiber reinforced polymer (CFRP) bonds to concrete following six years of exposure under both controlled laboratory conditions and natural outdoor environments. The research, led by authors Luís Correia, Aloys Dushimimana, Sayed Adeel Jawed, João Miguel Pereira, Susana Cabral-Fonseca, and José Sena-Cruz, provides one of the longest-term datasets available on this topic.

NSM CFRP strengthening involves embedding CFRP strips into grooves cut into the concrete surface and bonding them with epoxy adhesive. This technique offers advantages over externally bonded reinforcement (EBR) methods, including better protection from environmental factors and improved bond performance. The six-year investigation tracked bond strength, stiffness, and failure modes while also monitoring the constituent materials—CFRP laminates and epoxy adhesives—under accelerated laboratory ageing and real-world outdoor exposure.

Understanding NSM CFRP Technology and Its Applications

Near-surface-mounted systems have gained prominence in the rehabilitation of bridges, buildings, and other concrete structures. Engineers install CFRP strips by cutting shallow grooves, cleaning the surface, applying epoxy, and inserting the strips. The method transfers loads effectively while minimizing aesthetic impact and providing some inherent protection. Over the past two decades, NSM CFRP has been used in projects ranging from seismic retrofitting in earthquake-prone regions to corrosion repair in marine environments.

Long-term durability data is essential because many strengthened structures must perform for 50 years or more. Short-term laboratory tests often fail to capture the combined effects of moisture, temperature cycles, ultraviolet radiation, and chemical exposure that occur outdoors. The current study addresses this gap by maintaining specimens for six full years, with periodic testing of bond performance and material properties.

Research Methodology and Exposure Conditions

The experimental program included multiple series of concrete prisms strengthened with NSM CFRP strips. Laboratory specimens underwent controlled cycles of wetting and drying, temperature variations, and humidity exposure designed to simulate accelerated ageing. Outdoor specimens were placed at an exposure site in Portugal, experiencing natural seasonal changes, rainfall, and solar radiation. Researchers performed pull-out tests at regular intervals to measure bond strength and stiffness, alongside tensile tests on CFRP laminates and characterization of the epoxy adhesive.

Constituent material testing revealed that CFRP laminates retained their tensile strength and modulus throughout the six-year period. The epoxy adhesive, however, showed greater sensitivity to prolonged exposure, with changes in glass transition temperature and some reduction in mechanical properties under the most aggressive conditions.

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Key Findings on Bond Performance

After six years, the NSM CFRP-to-concrete bonds demonstrated remarkable resilience. Bond strength degradation remained limited, with the highest observed reductions around 12 percent in certain laboratory series and approximately 9 percent in others. Outdoor exposure produced comparable or slightly lower levels of degradation despite the uncontrolled variables. Failure modes stayed consistent with those observed in unaged specimens, indicating that the bond integrity was largely preserved.

These results align with earlier shorter-term studies by the same research group at the University of Minho’s ISISE institute, which reported minimal degradation after four years. The six-year data reinforces the conclusion that well-designed NSM CFRP systems can maintain structural effectiveness over extended periods, even when exposed to realistic environmental stressors.

Implications for Infrastructure Maintenance and Sustainability

The findings carry significant weight for asset managers and transportation authorities responsible for ageing concrete infrastructure. Bridges and buildings strengthened with NSM CFRP may require less frequent intervention than previously assumed, supporting more sustainable maintenance strategies. Reduced need for replacement or additional strengthening translates into lower material consumption and embodied carbon over the structure’s lifecycle.

In regions facing increasing climate variability, the study’s outdoor exposure results provide reassurance that NSM CFRP bonds can withstand combined moisture, temperature, and UV challenges. Engineers can incorporate these long-term performance data into design guidelines and service-life predictions, improving confidence in the technique for critical applications.

Challenges in Conducting Long-Term Durability Research

Long-duration studies like this one face practical hurdles, including specimen storage, funding continuity, and the difficulty of replicating real-world conditions in the laboratory. The research team maintained consistent testing protocols across yearly assessments, allowing direct comparison of degradation trends. Such continuity is rare and valuable for validating predictive models used in structural engineering software.

Future work could extend monitoring beyond six years or incorporate additional variables such as sustained loading or combined mechanical and environmental stresses. Collaborative efforts between universities, government agencies, and industry partners will be essential to build larger datasets and refine durability models.

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Opportunities for Researchers and Academics in Structural Engineering

Studies of this nature highlight growing demand for expertise in advanced composite materials, durability testing, and infrastructure resilience. Universities and research institutes worldwide seek candidates with experience in experimental structural engineering, materials characterization, and long-term performance assessment. PhD programs and postdoctoral positions in civil engineering departments often focus on topics directly related to CFRP strengthening and environmental durability.

Professionals interested in these areas can explore roles at institutions conducting similar work, including the Institute for Sustainability and Innovation in Structural Engineering (ISISE) at the University of Minho. Research positions frequently involve both laboratory experimentation and field monitoring, offering pathways to academic careers or specialized consulting in structural rehabilitation.

Future Outlook and Recommendations

As infrastructure ages and sustainability targets tighten, long-term durability data will become increasingly important for regulatory frameworks and design standards. The six-year NSM CFRP study contributes concrete evidence that these systems can deliver reliable performance, supporting their wider adoption in new construction and retrofit projects.

Practitioners are encouraged to consult the full paper for detailed test results, statistical analysis, and recommendations on adhesive selection and installation practices. Continued investment in extended-duration research will further strengthen the evidence base and help optimize strengthening strategies for diverse climates and loading conditions.

The original publication is available at https://www.sciencedirect.com/science/article/pii/S0950061826020088. The work was conducted by Luís Correia, Aloys Dushimimana, Sayed Adeel Jawed, João Miguel Pereira, Susana Cabral-Fonseca, and José Sena-Cruz.