International Concrete Abstracts Portal

The role of concrete in the corrosion of steel in reinforced concrete has received a considerable amount of attention in recent years. This is due to the recognition of the strong relationship between the nature of the concrete and its ability to protect embedded steel. Therefore, in addition to some of the commonly used corrosion protection methods that focus on either coating the concrete, increasing the cover of the concrete, coating the reinforcing steel, or the use of inhibitors that change the nature of the surface of the reinforcing steel, other methods should be included that emphasize the role of the concrete mix design. Paper deals with the contribution of concrete to the corrosion of reinforcing steels in reinforced concrete. Twenty-six mix designs that represent concretes that could be used today were selected for study. Variables included cement content, water content, amount and type of fly ash, the addition of superplasticizers, and air entrainment. Strength and macrocell current were measured as a function of chloride exposure. The results of 1 year of cyclical exposure to 3.5 percent NaCl solution revealed that the concrete influences the corrosion process greatly. Furthermore, modification of concrete can become another method of corrosion protection through a better understanding of the relationship between the corrosion process and concrete mix design.

Explains the technology developed by Alberta Transportation and Utilities for field prestressing repairs to precast, prestressed concrete bridge girders. This repair technique restores structural integrity quickly and cost effectively. Applications regarding high load impact damage and corrosion of prestressing strand are discussed, based on Reid Crowther's experience and application of this repair technique.

In the early 1960s, the city of El Paso constructed the Texas, Piedras, Raynor Street Bridge, a mile-long, four-lane artery leading to the center of downtown, and a main road important to many businesses and merchants. Time and the effects of chloride penetration from road salts, plus carbonation, took its toll on this bridge. Corrosion of the reinforcing steel led to spalls on pier caps and corrosion of columns, in addition to massive spalling and delaminations of the bridge deck and parapets. Early in 1990, a complete structural repair of the bridge was conducted. Attempts to repair the bridge were unsuccessful. Cracks and delaminations in the substrate and in the areas that had been repaired were evident. Due to safety concerns, the bridge was closed for structural repairs. Repairs began again in November 1991. The first stage of repair consisted of removing the previous failed materials and all unsound concrete. Once all areas were prepared, the low-pressure spraying of a structural, one-component, high-strength fiber reinforced, shrinkage-compensated product began. Three men were employed in the batching and spraying of the material--two at the hopper/mixing unit, and one operating the spray nozzle. Using a pump for placement, the contractor was able to realize a maximum of 3 yd 3 per hr. Workers with trowels followed closely behind the nozzle man, and, using a finishing aid, were able to finish the placement quickly and efficiently. Circular bridge piers were finished with a template to maintain the design cross section. A water-based curing compound was applied to insure maximum moisture retention and full hydration of the mortar. All patches were covered with thermal protection for 7 days, to allow for cure. Because the sprayed material adheres well, it performed exceptionally on all of the vertical and overhead repairs required on this bridge. Its excellent bond strength, outstanding structural properties, and low permeability provided the needed characteristics for this job. The sprayability of the product meant less labor required on the job and faster turnaround. Since this was the second repair of this bridge, these characteristics were very important. The product performed exceptionally well and resulted in a hard and dense repair.

The service life of latex-modified concrete and low-slump dense concrete bridge deck overlays is estimated by extrapolating historical performance data obtained from the results of field research and investigations conducted in the U.S. and Canada. The data suggest that when concrete removal criteria are based on half-cell potential rather than actual damage, when removal of chloride-contaminated concrete is extended to below the reinforcing bar, and when the substrate is sandblasted to remove microcracking prior to cleaning, a mean service life of 30 to 50 years is likely.

On-site monitoring of the rate of corrosion of reinforcements is a priority task. There are various available devices for measuring I corr. Most of them entail a prior measurement of the {DELTA}E/{DELTA}I, which defines the polarization resistance (R p). However, direct estimations of R p from the {DELTA}E/{DELTA}I ratio are usually unfeasible with large structures because they provide an apparent polarization resistance (R p app) that differs to a greater or lesser extent from the true R p value depending on the experimental conditions. This paper analyzes the influence of some experimental factors (that is, the use of unconfined or guard ring-confined electric signals, the CE size, and the presence of active and passive areas in reinforcements) on the R p app accuracy of the I corr values derived from it. The more correct I corr values can be obtained by sensorized confinement of the electric signal applied with the aid of a guard ring and supplementary reference electrodes for monitoring the electric field confinement.