Expert repairs to the St. Louis County Government Center Parking Garage in Clayton, Missouri, will successfully restore it for 20 to 30 years more use.

Partial or complete closing of a multi - level parking deck produces headaches for everyone involved - the building owner, property manager and tenants alike.  However some facilities constructed in the 1960's and 1970's could be closer to the inconvenience of prolonged closure - or even catastrophic failure - than their appearance would convey.

The building boom of those decades coincided with the increased use of post-tensioned methods for parking decks that reduced the amount of cracking, thus reducing water leakage and damage in the structures.

In addition, cast-in-place post-tensioned methods required less concrete: 18" beams and 5" slabs instead of 30" beams and 10" slabs required for conventional construction.

Longer clear spans, eliminating many interior columns, were another benefit.  Using post-tensioning allowed parking designers to accommodate the maximum number of cars on expensive real estate parcels.

HIDDEN DISTRESS
Exposed to the elements, post-tensioned multi-level decks have suffered damage in the 20 to 30+ years since their construction.

What's alarming is the degree of damage that may not be visible, thus delaying much needed repairs.

Given the amount of time that has elapsed since construction, many owners and managers may not be aware of the construction methods used for their parking structures.  Here is a list of identifying characteristics to determine if your facility is a post-tensioned structure:
         -  Main beam span of 35 feet or greater in any direction;
         -  Beams are shallow with a thin slab;
         -  Pan joist design of over 45 feet.

After determining if the parking deck is post-tensioned, an expert should be called in to perform a visual inspection and evaluate the following factors:
         -  Structure design;
         -  Water flow patterns;
         -  Type of concrete;
         -  Maintenance program;
         -  Environmental aspects.

By judging the interaction of these conditions, a determination can be made on the possibility of hidden deterioration and whether diagnostic testing is necessary.

Button-headed post-tensioning was a system widely used during the 1960's and early 1970's.  A review of this technique will highlight how major degradation of a structure can remain hidden.

The system employs from four to 10 steel wires per tendon that are embedded in a beam or slab and then tensioned during construction to cause compressive forces.  By placing and tensioning the tendons, the compressive forces counteract the load applied to the concrete slab.

The tendons were first heavily greased and wrapped in reinforced kraft paper to enable movement during tensioning.  After the concrete was placed and allowed to cure for a specified time, the tendons were tensioned to between 35,000 to 74,000 pounds and anchored.  The anchor ends were covered with concrete.

Post-tensioning required substantially less concrete, making the structure lighter.  The technique also eliminated the need for large amounts of rebar, thus reducing the chance of spalling and cracking due to corrosion.  Post-tensioning compression also closed shrinkage cracks, thus limiting water-borne chloride penetration, a major cause of deterioration.

However, at the time of installation of the system, the potential for severe damage due to chloride penetration was not fully realized.  Air-borne contaminates and the amount of road salts and de-icers used had steadily increased.  Yet the grease and kraft paper gave only minimal protection from corrosion due to water and salt intrusion.  The anchor ends had no protection at all, nor was the concrete placed around them sealed to protect against water penetration.

Thus the system is vulnerable to corrosion along the tendons, where it may not be observable.  Damage that can occur at the anchorheads will often go unnoticed until the facility must be closed completely for repairs or catastrophic failure is a possibility.

 INSPECTION AND EVALUATION OF THE DAMAGE

In general, the condition of these decks can be categorized in the following manner:

Level One: Mild degradation, usually requiring surface maintenance and minimal tendon repair;

Level Two: Severe tendon damage, slab may be deteriorated but the structure itself has not sustained serious deterioration;

Level Three: Severe deterioration of the slab and structure, catastrophic failure is likely.

There are a variety of factors that can influence the condition of the structure, including valiance in the technique used and placement of the tensioning. Spalling concrete, for example can be a sign of major deterioration or minor damage, depending on these factors.

Again, it must be emphasized that expert evaluation should be undertaken to determine if damage has taken place; what tests should be performed to assess the damage; and the most effective way, both in terms of time and cost, to repair the structure.