In an investigation in progress, reinforced concrete test specimens cut from two different sources of concrete culvert pipe were tested under cyclic saltwater pending and under constant exposure to seawater. The specimens were electrochemically monitored over 700 days (cyclic test) and 400 days constant exposure for evidence of corrosion initiation and measurement of resulting corrosion rates. Corrosion initiation took place after a few months of exposure in the cyclic pending tests. The corrosion was localized and developed at regions where the concrete cover was lowest (but still within a range consistent with manufacturing practice). Corrosion rates after initiation in the affected spots were estimated to be sufficient to cause concrete cover deterioration after a few years. No corrosion initiation took place after 400 days of continuous exposure to simulated seawater. Near-surface chloride concentrations were much higher in the concrete exposed to cyclic pending than in that continuously exposed to salt water. Within each exposure regime, the concrete containing fly ash developed a higher near-surface chloride content than the OPC. The apparent chloride diffusivity of the OPC concrete was greater than that of the concrete containing fly ash, even though the reported total binder content of the OPC concrete was higher. The average estimated threshold chloride concentration under cyclic pending conditions for fly ash containing concrete was - 0.6% of binder content. For OPC concrete it was - 0.34?% of binder content, but the latter value may be underestimated since stable corrosion initiation in the OPC concrete was not observed.
Design service life requirements for reinforced concrete highway culverts commonly approach 100 years. However, durability estimates made with current guidelines for culvert material selection are subject to uncertainty. To improve the present methods for performance prediction several reinforced concrete performance parameters need to be determined more accurately. Of particular interest is the case of culverts exposed to seawater or dilutions thereof, since under those conditions useable service life maybe significantly reduced.
Reinforced concrete deteriorates because of chemical degradation of the concrete itself (due to the presence of acidity and sulfates in the soil) and because of corrosion of the reinforcement. The latter process involves first an initiation period (b), in which the penetration of chloride ions through the concrete cover ending with depassivation of the steel surface takes place. A propagation stage follows in which the steel corrodes until the concrete cover is cracked or other externally evident damage occurs2. It is of importance to determine the ability of present-day concrete formulations used in culverts to resist chloride ion penetration.
Current methods for service lifetime prediction of chloride exposed reinforced concrete structures assume the existence of a critical or threshold chloride concentration (CCr)below which active corrosion is unlikely to occur. This threshold limit is generally expressed in terms of total chloride (i.e. ?bound?+ ?free?) where ?free chlorides? are dissolved in the pore water and ?bound chlorides? are either chemically or physically absorbed by the concrete. Any ~, prediction is based on the speed of chloride accumulation at the rebar surface, and computation of the time necessary to achieve a concentration equal to CCr. A simplified forecast method consists of assuming planar geometry with a concrete surface that has a constant chloride concentration (CJ. Chloride ion transport through the concrete cover is then assumed to take place by homogeneous diffusion with ti
