One of the most serious and costly problems presently facing highway agencies is premature deterioration of concrete bridge decks. The cause has now been linked to deicing chemical "chloride" induced corrosion of the reinforcement. In some cases bridge decks designed to last 40 years have required major repairs after only 10 years of service due to corrosion related "chuckholing" or spalling.
Mn/DOT has become increasingly aware of this problem during the last 5 years and has initiated a comprehensive program to resolve it. The two basic approaches to solving the problem are to:
1. Keep the salt and moisture out of the decks with protective overlays and membranes, modified concretes or sealers.
2. Protect the steel from corrosion once the concrete's protective nature has been destroyed by chloride contamination, using epoxy and zinc coatings.
Bridge decks were constructed or reconstructed using protective membranes, modified concretes and coated bars. Testing and evaluation consists of visual observations, delamination detection, chloride content testing, depth of concrete cover over rebars and electrical potential measurement.
The objective of this project was to conduct a state-of-the-art or synthesis research study on bridge design methods used in areas where low bearing capacity or unstable soils exist.
Over the past 15 years, foundations of certain bridges in the Red River Valley region of Minnesota have experienced detrimental degrees of movement. Settlement or shifting of piers, abutments and approach fills are common and particularly difficult to correct. The result has been that maintenance operations to keep structures in service are frequent and costly.
Three courses of action were pursued in preparing the synthesis. First, a literature search was conducted and various reports dealing with the topic were reviewed. Second, a limited field inspection was undertaken and problem sites (failure areas) were visited. Lastly, several meetings were held with persons knowledgeable about the situation and possible methods for correcting it.
Through this process, the problem was more thoroughly analyzed and a better definition of the scope of the study was developed with limits established.
This report represents data obtained through a comprehensive field study encompassing some 70 bridge decks with a variety of protection systems in place and subject to varying traffic volumes and conditions of exposure. Decks repaired also exhibited varying initial condition and extents of concrete removal prior to rehabilitation.
The basic categories of protection systems under study are as follows:
1. Membrane and bituminous overlay.
2. Special concrete overlays.
3. Coated rebar systems.
Primary testing of these systems consisted of:
1. Delamination/debonding surveys.
2. Clear cover measurement.
3. Half cell potential testing.
4. Chloride penetration testing.
5. Visual surveys.
The criteria followed for evaluation of system performance were separated into two elements:
1. System effectiveness (how well i.t does its job).
2. System durability (how., long it performs under service conditions
In August of 1976 a special study was undertaken by the Mn/DOT Research and Development Section under the existing Investigation 639 program (Bridge Deck Deterioration and Restoration). The objective of the special study was to identify sealers which would reduce chloride penetration into concrete decks and hopefully slow the corrosion process. This would in turn slow the corrosion induced spalling of the deck surface.
