Identifying Problems with Post-Tension Concrete
Photos show debonded grout over an anchor pocket (top) as well as removed grout revealing corroded wedges and anchors due to water intrusion at the grout pocket (bottom).
Post-tensioning is a technique for reinforcing concrete with steel cables typically referred to as tendons. Post-tensioning tendons, which are high-strength steel cables inside plastic sheathing, are positioned in the forms before the concrete is placed. After the concrete has gained strength and before service loads are applied, the cables are tensioned with jacks and anchored against the outer edges of the concrete. The open pockets at the stressing anchors typically are filled with non-shrink grout.
Post-tensioning is commonly used in construction of buildings and parking garages, and its benefits include the following:
• Allows for longer spans than conventionally reinforced concrete
• Thinner slabs
• Fewer beams
• Fewer reinforcing steel bars
Similar to other structural building components in service and exposed to the elements, post-tensioned concrete slabs need to be maintained, inspected and repaired. There are several problems that can occur with post-tensioning components in balcony and garage slabs in service.
In typical post-tension buildings with balconies, the balcony slab is an extension of the concrete floor slab and usually cantilevered with a guardrail or parapet on the outer edge. The balcony slabs are exterior building components and therefore subjected to weather, temperature fluctuations, UV exposure, air pollutants and foot traffic.
Photos show a balcony slab edge with cracks and deteriorated coating, which permits water and oxygen to reach the embedded steel PT anchors and mild reinforcing steel (up), as well as a balcony slab edge after delaminated concrete was removed, revealing corroded PT anchors and mild reinforcing steel (bottom)
Post-tensioned concrete is very common for modern parking garages and can be constructed below or above grade. The system can be configured in short-span two-way slabs or long-span beam and slab systems. They’re typically supported on concrete columns and perimeter foundation walls. Protection of the slab surface can be provided by a traffic-bearing coating or penetrating sealer. Garage slabs are exterior building components and subjected to weather, temperature fluctuations, air pollutants, vehicular traffic, snow plows and deicing salts.
Maintenance and upkeep is vital for continued service and longevity of post-tensioned structures. Deterioration of post-tension elements results in loss of structural integrity and can create life-safety issues. Building owners should have post-tension elements examined periodically by a structural engineer qualified in post-tension repair. The condition survey will assess the condition of the post-tensioning and water-resisting elements, make recommendations for repair and maintenance, and help owners prepare budgets accordingly.