Criteria for Use of Thin Polymer Overlays
Corrosion of embedded reinforcing steel is the leading cause of deterioration in bridge structures. The mechanism of deterioration is well known. Chloride ions, originating from deicing chemicals used to maintain pavements in bare, dry conditions, penetrate through the concrete, either through cracks or through the fabric of the concrete itself . Once the chloride ion content becomes high enough at the level of the reinforcing ...more »
Corrosion of embedded reinforcing steel is the leading cause of deterioration in bridge structures. The mechanism of deterioration is well known. Chloride ions, originating from deicing chemicals used to maintain pavements in bare, dry conditions, penetrate through the concrete, either through cracks or through the fabric of the concrete itself . Once the chloride ion content becomes high enough at the level of the reinforcing steel, corrosion is initiated as the electronegative chlorides depassivate the steel. The resulting corrosion products occupy many times the volume of the steel from which they originated, resulting in internal tensile stresses, cracking delamination and eventually spalling .
The use of epoxy coated reinforcing steel has become widespread. Recent work has shown that this is not a panacea because the polymer coating does not sufficiently encapsulate the steel to prevent all corrosion.. Rather, it extends the time to corrosion by acting as a barrier,
Post construction treatment of bridge decks to prevent the ingress of chloride ions has been undertaken by most highway authorities in North America in cold and wet environments. Mn/DOT has performed this type of work, specifically in the application of polymer concrete overlays which both prevent further penetration of chloride ions specifically through the cracks which may occur in the low-slump overlay on the deck and improve friction properties of the pavement. .
Currently, there is not a consistent criteria for selecting the timing for efficient application of thin polymer concrete overlays by Mn/DOT. This results in a in a haphazard decision to coat a particular structure, perhaps for friction, aesthetic for other reasons, as well as for protection from chloride ion penetration.
A worthwhile exercise would be to determine the timing for placement of these overlays in the interest of determining when they are most effectively placed. Placement of the polymer overlay at the time of construction adds additional cost to the original construction, and may not be effective at that time due to the potential for cracking due to thermal expansion and contraction in the first year of service. Placement of the overlay after a great deal of chloride ion penetration has occurred does not prevent corrosion of the embedded reinforcing steel or extend the time to repair as chloride ions which are already in the structure will continue to move by diffusion to the elevation of the steel . Studies performed by Weyers et al as a part of the SHRP program identified that it may take more than 20 years to build up the chloride ion in the pavement surface to the saturation point.
The proposed study has three components:
The first is to evaluate the diffusion coefficient typical of concrete bridge decks constructed in Minnesota using standard concrete mixtures.
The second is to determine the diffusion rate of chloride ions through the thin polymer concrete overlays. Laboratory work would be performed to determine the rate at which chloride ions move through the concrete, described by the diffusion coefficient. A relationship between the electrical indication of this diffusivity and its actual properties would be determined as described by MacDonald
Finally, the data described above would be used to develop a process whereby the design engineer can select the timing of an overlay for the maximum and most cost efficient protection of the embedded reinforcing steel. This selection is dependent on the chloride diffusivity of the stacked system of structural concrete, low-slump overlay, polymer overlay and the existing condition of the overlay, as well as the propensity for reflective cracking.
This work would be dependent on the thermal expansion properties of the overlays, as well as their adhesion. It is understood that a project currently underway at the University of Minnesota Duluth is looking at these very issues. This work would not be repetitive but rather would reference the results of this work
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