July/August 2010 Parking - 58

and Restoration
of Post-Tensioned Waffle Slabs
By Gabriel A. Jimenez, Ph.D, P S.E. .E.,

Assessment
joints and cracks results in spalling of concrete on beams (see Figure 2). The walls and columns suffering the distress from leakage are also continually exposed to the splashing and spray of salt-contaminated water. Concrete degradation, particularly in its advanced stages, is seldom due to a single mechanism. In harsh climates, significant deterioration can result from the combined effects of freezing and thawing along with the heavy use of deicing products.

Un-bonded post-tensioned (PT) reinforcement in waffle slabs has been popularly used in parking structures for many decades. Its use greatly reduces the amount of cracking and possible leakage in the structure and also allows economical long-span construction. The boom of the construction of waffle slabs in the United States began in the 1970s. However, many of the early PT structures have since been encountered in a deteriorated condition with varying degrees of damage to the PT systems. This article explains the deterioration mechanisms that affect post-tensioned waffle slabs and, more specifically, addresses sheathing strand systems. A case study illustrates the detrimental effects of deterioration mechanisms on PT structures, providing specific repair recommendations in order to mitigate and prevent structural related problems for unbonded post-tensioned waffle slabs.

major portion of the design load. In such cases, it is economical to rib the slab in both directions to provide greater stiffness with less weight. This structural slab system is referred to as “waffle slab construction.” Waffle slab construction consists of rows of concrete ribs at right angles to each other with solid heads at the columns. The ribs are built using dome forms to create the void between the ribs. The domes are omitted around the columns to form the solid heads. The top slab over the dome forms is usually reinforced with welded wire fabric or conventional reinforcement. The waffle slab systems have been improved with the use of PT reinforcement in the ribs, allowing for heavier loads and longer spans than those using conventionally reinforcement.

Deterioration Mechanisms
The most frequently encountered types of deterioration in PT parking waffle structures include the corrosion of the reinforcement, freeze-thaw damage, and material-related degradation. These conditions frequently occur in combinations and result in scaling of the driving surface, spalls, and delaminations of both the driving surface (see Figure 1) and slab soffit. In addition, water leakage through

Background
In the construction of conventional two-way flat slabs, the need of longer spans and/or the necessity for heavier loads requires increasing the slab thickness in order to limit deflections. This increase causes the dead load of the slab to become the

Corrosion-induced deterioration. Deicing products, either tracked into or directly applied to the surface of the structure during winter months, tend to accumulate and accelerate the infiltration of the chloride ions into the concrete floor surfaces. Similar conditions occur in coastal areas due to the salt spray. This exposure to chlorides may be worsened by poor drainage layouts, standing water or ponding, and clogged drains. Spalling and delamination of concrete are the most typical forms of deterioration encountered. Freeze-thaw cycles, vehicular action, and additional corrosion tend to further accelerate the rate of spall development. Crosssectional loss of prestressing tendons due to corrosion has a significant impact on the capacity and integrity of the parking structure itself. Release of the post-tensioning force due to tendon

National Parking Association PARKING July/August 2010

July/August 2010 Parking

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