Post Tension Slabs
. Many concrete subterranean parking structures serving condominium or apartment developments are constructed using “post tensioning” technology. This technology reinforces the thin concrete slabs with highly stressed wire cables (tendons) that pre-compress the concrete slabs for structural integrity. The design and construction of these parking structure slabs often incorporate “pour strips” internally and around the perimeter of the garages to allow tensioning of the tendons. The structural integrity of the concrete slabs is highly dependent upon the integrity of the anchorage devices for the tendons placed at each edge of the “pour strip”.
Poor Ventilation Deterioration
. These underground parking structures are often poorly ventilated, allowing a build-up of carbon dioxide in the trapped air. This initiates an increase of acidity (reduction of pH) in the concrete resulting from high levels of carbon dioxide from auto exhausts mixed with high ambient temperatures. This process, known as carbonation, results from atmospheric carbon dioxide reacting with the moisture within the concrete and converting the high pH calcium hydroxide to calcium carbonate which has a more neutral pH. When carbonation in the concrete slabs progresses to the depth of the reinforcing steel and/or post-tension tendon anchors, the protective passive oxide layer on the steel is no longer stable. As the pH drops below 9.5, the natural corrosion inhibition property of the cement is lost and the potential for corrosion of the reinforcing steel, including tendon anchors increases.
Path of Damage
. The construction joints at each edge of the pour strips provide a penetration path for carbon dioxide (present in high quantities from auto exhausts in the garage) and chlorides (present in water seeping from the perimeter walls), These elements change the properties of the concrete (lowering the pH from 12 /13 to as low as 7) and reduce the natural protection of the concrete for steel reinforcements. The consequences of tendon anchorage failure can be sudden and catastrophic.
. The carbonation and corrosion problems discussed herein can best be controlled through cleaning and preparing the pour strip and adjacent surfaces and applying anti-corrosive solutions to the top and bottom surfaces of the pour strips that will migrate and react with the cement and steel to reverse and inhibit carbonation, steel corrosion and reduce concrete porosity.
The detailed process and products can best be selection by your structural engineer and his/her chemists after they survey your property to determine the presence, extent and progress of the carbonation and resultant damage.
Information submitted by Garry D. Myers, S.E.with MHP, Inc. Structural Engineers.
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