Effects of Reinforcement and Coarse Aggregates on Chloride Ingress into Concrete and Time-to-Corrosion: Part 1—Spatial Chloride Distribution and Implications

Abstract

The chloride concentration in concrete required to compromise reinforcing steel passivity and initiate active corrosion, [Cl⁻]_th, is an important service life determinant for reinforced concrete structures. However, consensus regarding a value for [Cl⁻]_th is lacking, as evidenced by the fact that reported experimental determinations vary by more than an order of magnitude. A recently recognized factor that may contribute to the range in [Cl⁻]_th is the obstruction to Cl⁻ migration posed by reinforcement, such that ingress of this species must be treated as two- rather than one-dimensional. In the present research, reinforced concrete slabs were exposed to outdoor conditions while cyclically ponded with a sodium chloride (NaCl) solution. Subsequent to corrosion initiation, selected slabs were cored, dissected, and [Cl⁻] determined both from the cores and from shallow millings of concrete immediate to the reinforcement. The experiments were supplemented by analysis of Cl⁻ distribution in cores acquired from both laboratory specimens and from a marine bridge substructure; it is demonstrated that, in addition to the reinforcement obstruction effect, the spatial distribution of coarse aggregates immediate to the reinforcement, combined with Cl⁻ partitioning between aggregate and mortar or paste phases, influenced distribution of this species and its measured value upon sampling. The results are discussed within the context of understanding factors that effect measured values for [Cl⁻] and the relatively wide variation in [Cl⁻]_th that has been reported in the literature.