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Parking Garages

The conditions of a parking garage may seem like an afterthought to some property managers and investors. When asked about maintenance costs, one representative for a national cost estimating company replied: "Parking garages don't require maintenance. They're just steel and concrete." It is true that the mechanical systems generally pale in complexity to the main building(s). It is the parking garage, however, that suffers most the effects of weather and poor maintenance and often becomes the area of greatest deferred maintenance and cost.

Types of Structures

There are generally three types of parking structures:

  1. Precast Concrete (Prestressed) Structures. The advan­tage of precast is a high degree of quality control. When the precast units are prestressed, the poten­tial for cracking is reduced.

  2. Steel-Framed Structures typically consist of a structural steel superstructure with a cast-in-place slab. Steel beams can be left exposed or encased in concrete for added strength and/or fire protection.

  3. Cast-in-Place (CIP), Conventionally Reinforced Concrete Structures account for the largest percentage (approximately 36%) of structural systems found in the U.S. Because of their span limitations and susceptibility to crack­ing, CIP structures are not built as frequently today. One variant, Cast-in-Place Post-Tension Concrete Structures is among the most durable of all the types of parking structures.

Underground parking structures are typically found in urban areas because of space constraints. The structures are usually made of CIP concrete or, steel frame with poured concrete floors. As a rule, if habitable living space is found above a parking structure, the parking structure has to be fire-rated or equipped with an automatic sprinkler system.

Problems with Garage

All types of garages are exposed to moisture problems wherein water penetrates the concrete and causes rust on the rebar, which in turn expands and causes cracking and spalling of the concrete. Such corrosion is caused by two mechanisms. "Carbonation" is a natural process whereby carbon dioxide (CO2) in the air interacts with moisture in the hydrated cement minerals, causing gaps and voids. On a regional basis, chlorides (deicing salts) brought into the garage on tires and the undersides of cars permeate the concrete deck. Both these mechanisms lower the pH and lead to water entry and rust. While corrosion due to water infiltration is a major problem with all types of parking struc­tures, each parking structure type has specific problems.

Precast Concrete: The T-shaped units or planks used in precast structures are not monolithic; they contain many joints. Precast units are subject to flexure. A "rocking" effect can lead to cracks and separation in the connections and joints. As cracks develop, water has an opening to penetrate the slab, migrate down the sides of the beams, and collect on the ledger beams. If a topping is used on the precast concrete, the problem is magnified. Other concrete-related problems with precast parking structures include spalls to columns, beams, and corbels; cracks in precast membranes; and joint nosing failures at expansion joints. Operationally, plowing with steel blades can also cause great damage.

Steel-Framed: One of the problems associated with steel-framed structures is differential movement between the steel members and the concrete slab. When moisture and corrosion set in, the slab can heave from the beam, which causes cracking in the concrete. Cracks can also develop in areas subjected to negative bending moments, where tensile stresses are present in the top fibers of the concrete slab. When cracks devel­op in any slab, moisture is given a path of entry into the concrete. If the cracks are deep enough and the water ponds on top from insufficient drainage, moisture can pass completely through the slab. Water entry can cause corrosion to the rebar and also to the metal deck if it is still in place. If a corrugated metal deck is left in place below the concrete deck, the metal deck should be perforated so any moisture that permeates the deck has a way to escape.

Cast-in-Place: Random shrinkage cracks are very common in cast-in-place, conventionally rein­forced concrete structures. When cracks fully penetrate the slab, moisture can enter the top and pass completely through to the bottom of the slab. Over time, this causes deterioration to the top and bottom steel, which eventually spalls the concrete deck and the concrete soffit. Inadequate rebar cover (less than ¾ inch) is another source of problems for cast-in-place structures. It does not take much for the contaminants to permeate the con­crete and cause corrosion of the rebar. If the slab contains low points and has insufficient drainage, the water can collect and eventually cause problems.

Inspection & Repair

During a standard Property Condition Assessment, we inspect parking structures using a sounding device or chain drag (to identify subsurface delaminated areas) and a flashlight. With these simple tools, we look for evidence of corrosion, rust, spalling, cracking, and other visible defects. For a more detailed garage inspection, we may also use a masonry hammer and a pactometer to locate the rebar. Our report will then include a spall layout plan, a summary spreadsheet with quantity matrix, a repair cost esti­mate, and a projected future expenditure analysis. Additional testing may be required, including:

  • Photography (microscopic analysis to see interior cracking); this can be ac­complished by taking core samples.

  • Carbonation analysis to detect the presence and depth of carbonation, and chloride analysis to locate the sources of chloride.

Dealing with the symptoms without pinpointing the cause can result in inappropriate repair. Repairs to structures with delaminated concrete usually include removing all the delaminated con­crete, cleaning the corroded rebar, and installing a corrosion inhibitor on the bar in addition to placing a high early strength modified cementitious material in the patch. All deteriorated expansion joints and control joints should be repaired by completely replacing the sealant material.

Maintenance

All decks should be washed in the spring and fall of each year to remove chloride-containing salts, locate potential areas of water infiltration in and around drains and expansion and control joints, and identify delaminations. On a yearly basis, the deck of the parking garage should be sounded to determine if any delaminating concrete is present. If delaminated concrete or leaking control/expansion joints are encountered, they should be repaired immediately. Corrosion and/or water infiltration of the slab can greatly increase the repair cost. All caulked joints should be checked regularly and completely recaulked approximately every ten years.

Typical Costs

Because the procedures to repair parking garages are so repetitive, the more square footage or lineal footage of repair, the lower the price will be. This factor, along with the location, accessibility, and economic parameters, will determine the cost. Preventive maintenance is generally more cost-effective than waiting to make repairs. In our experience, maintenance costs of $35 to over $100 per space per year can be expected, depending on the type of structure and climate.

Volume 10, Number 2

September 1999

Copyright © 1999

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