Corrosion of Metallic Fasteners in Timber–Concrete Composite Structures

Significance 

The rapid increase in the multi-storey timber buildings and the need to develop an eco-friendly construction technique have led to great interest in the application of timber concrete composite structures. The composite system comprises of a concrete layer in the compressive zone, timber members in the tensile zone, and a shear connector between them. These structures can increase the stiffness and load-bearing capacity of existing timber slabs. Metallic fasteners such as screws and nail plates are examples of shear connectors that have been developed and implemented.

As concrete and wood materials have different pH values and chemical environments that affect the performance of shear connectors in many ways, there is a great concern about the risk of corrosion of steel fasteners in structural engineering practice. Despite the current use of coating technique increasing the durability of steel fasteners, corrosion of coated steel fasteners within timber concrete composite structures has not been fully explored.

Recently, ETH Zurich scientists: Dr. Wei Shi, Professor Ueli Angst, Deniz Yilmaz, Kaja Wenk and Professor Andrea Frangi investigated the corrosion behavior and galvanic effects of fasteners used in timber concrete composite structures. Their main objective was to explore and confirm the existence and effects of galvanic corrosion on the steel fasteners as initially proposed. Their work is currently published in the journal, Materials and Structures.

In brief, the research team started by fabricating timber concrete composite specimens for electrochemical testing in two different environments: one containing 95% relative humidity and the other involving immersion in water. Next, a comparative study between the galvanized steel fasteners and the browned fasters was conducted in these two exposure conditions to determine their corrosion behaviors and galvanic effects. To enhance results accuracy, the moisture content in the wood, the galvanic current and the difference in electrochemical potential were monitored for more than two months.

Water saturation in wood plays a significant role in the durability, integrity and general performance of the fasteners within timber concrete composite structures. Whereas significant galvanic currents were observed only upon immersion in water, the fasteners in the wooden part of the specimens were noted to act as anode while those in concrete act as cathode. This was attributed to the differences in the chemical environments of the concrete and wood that were noted to be strongly alkaline and slightly acidic respectively. Furthermore, the rise in the galvanic currents was as a result of an increase of the potential differences upon contact with liquid water and a decrease in ohmic resistance between cathodic and anodic sites. The magnitude of the galvanic current was influenced by the moisture content of the wood, hence its rise in immersed conditions.

The practical application of the metallic fasteners was assessed in structural engineering, according to the service classes in EN 1995-1-1. Even though the studied fasteners reported a mass loss of metal, the metal loss rates exhibited negligible impact on structures and especially those with medium building design life criteria. For instance, both the fasteners can be used in timber concrete composite structures in constructing buildings that are not expected to be exposed to water saturation conditions for long times. Therefore, the ETH Zurich study by Wei Shi and her colleagues provides a promising solution for supervising metallic fasteners corrosion in timber concrete composite structural applications.

Reference

Shi, W., Angst, U., Yilmaz, D., Wenk, K., & Frangi, A. (2019). Corrosion of Metallic Fasteners in Timber–Concrete Composite Structures. Materials and Structures, 52(3).

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