Durability of steel fibre reinforced concrete in corrosive environments

Steel Fibre Reinforced Concrete (SFRC) is increasingly used worldwide for the construction of civil infrastructure under aggressive exposure conditions. The use of carbon-steel fibres in concrete as partial or total replacement of conventional steel reinforcement bars is becoming an attractive solution to the industry, considering its simplified production processes, cost-effectiveness and overall durability in compressed elements subject to corrosive exposures.

However, the durability aspect is still under discussion when addressing the corrosion of carbon-steel fibres bridging cracks in concrete under some exposures, such as wet-dry conditions. Contradictions in design guidelines and in former studies regarding the durability of cracked SFRC under these exposures hampers further development of SFRC infrastructure.

In particular, the deterioration mechanisms affecting cracked SFRC under wet-dry cyclic exposures are not well-understood. This thesis presents an experimentally-focused multiscale investigation that aims at identifying the main variables influencing steel fibre corrosion in concrete subject to wet-dry cycles and quantifying the effect of fibre corrosion on the mechanical performance of SFRC.

The experimental campaign covered studies of SFRC exposed to wet-dry cycles under chloride and carbon-dioxide exposure, at the composite and single-fibre level, with focus on the deterioration inside cracks. Experiments at the composite level confirmed that corrosion damage in uncracked SFRC is negligible and does not entail damage to the mechanical performance of the uncracked material.

Corrosion damage of steel fibres inside cracks narrower than 0.3 mm occurred at the outermost 20 – 40 mm of the crack and had a limited impact on the mechanical performance of the cracked SFRC over a period of two years. Significant increases of the residual tensile strength of the cracked SFRC after exposure were observed at the composite and single fibre level.

A semi-empirical multiscale model based on the fibre bundle approach was developed to connect observations at the single-fibre and composite levels. The model confirmed that, under wet-dry cyclic exposure conditions, fibre corrosion may not be the dominant deterioration mechanism affecting the mechanical performance of cracked SFRC during exposure.

An increase of the fibre-matrix bond strength was responsible for a large share of the variations observed in the toughness of the cracked SFRC. An investigation of the damage mechanisms showed that concurring deterioration and recovery processes reduce the tensile capacity of the steel fibres and alter the cement matrix adjacent to the crack and surrounding the steel fibres.

Autogenous healing of mechanical damage at the cement matrix around the fibre hook was identified as the main recovery mechanism responsible of the increase in the fibre-matrix bond strength during the exposure. A conceptual deterioration model considering these processes was described and compared to experimental data.

The work presented in this thesis concluded that, under the exposure conditions and time-scales investigated, corrosion damage of steel fibres inside cracks below 0.3 mm has a subordinate impact on the alterations of the tensile toughness of cracked SFRC. Changes in the toughness of the cracked SFRC after exposure were related mainly to the alteration of the cement matrix surrounding the steel fibres.