Maintaining multilevel parking deck systems

by brittney_cutler | March 30, 2022 4:00 pm

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By Amir Hassan, MS, PE

It is hard to imagine any urban setting without parking garages. Given the limited availability of usable areas in busy cities, parking structures efficiently provide stalls for vehicles in airports, train stations, bus stations, commercial buildings, hospitals, and malls. They are commonly built using cast-in-place concrete, precast concrete, post-tensioned concrete, structural steel, engineered lumber, or a combination of these systems. Structural parking decks typically consist of reinforced concrete slabs over precast supporting units. The initial slopes of the slabs along with their structural deflections are important to direct water toward the drains as per the building’s water management plan.

Surface protection and insulation

Parking decks are costly to build, repair, and replace. Water is the major cause of concrete deterioration and steel rusting, so its management is vital to ensure proper durability. Designing, installing, and testing specialized systems are key steps to ensure water is diverted away from the deck’s structural elements. The waterproofing layer can be placed on top of the deck system where it is easy to inspect and repair; however, the layer is not secured against pedestrian and vehicular traffic. The preferable approach to parking garages is to use a protected waterproofing membrane. As the name implies, the membrane will be protected from traffic, which can extend the service life of the deck. Nonetheless, the limited accessibility will make the assembly hard and expensive to maintain. Loose-laid membrane can also be applied, but full adhesion is the most common application of deck membrane.

The adhered system can be achieved through cold, hot, or sheet application. However, challenges with maintaining thickness and quality through cold and hot adhesion makes rolling out premanufactured rubberized asphalt membrane the most common application. The sheet membrane has many joints which may leak water if they are not sealed tightly. Creating a continuous plane throughout the deck—especially around penetrations—is essential for watertightness. Flashing and pressure bars are used to maintain membrane continuity at terminations. Cracks plus expansion and construction joints in decks require special attention and consideration. Regular inspections and timely repairs are vital to maintain healthy buildings. The structural behavior of the floor slab, including deflection values and locations, affects the performance of the waterproofing system.

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The typical parking deck comprises a waterproofing membrane adhered to the top of the structural slab, drainage grid, and wearing course (Figure 1). This assembly works like a rainscreen exterior wall, as most of the water trenches over the wearing surface, while some finds its way onto the waterproofing membrane and moves toward drains through the drainage grid. Therefore, the topping acts as a membrane protector and water shedding layer—just like siding on walls. Other layers such as protection boards, insulation, and slip sheets can be added to improve overall performance and durability.

An insulation layer within a deck assembly is crucial when it separates two spaces with a wide difference in temperature, such as a parking floor and an air-conditioned/heated living space. Similarly, in cold climates, the lack of insulation (thermal barrier) below a plaza deck and above a heated parking space can cause condensation issues on the exposed bottom surface of the concrete deck. In contrast to a deck separating two parking floors, a plaza deck is a supported slab which provides green-scape and tree planters in addition to pedestrian vehicle loads over an occupied space. Special considerations should be given to the design and construction of waterproofing systems for plaza decks.

Unlike fully shielded buildings, parking garages are mainly open to the elements. Therefore, their concrete surfaces are more prone to deterioration, such as spalling, scaling, cracking, and delamination. These structures are also subjected to dynamic loading conditions and destructive chemicals from de-icing materials, as well as mechanical abrasions from cleaning and snow removal equipment. Functional maintenance and repair programs are important to extend the service life of the structure. In general, a full comprehensive condition review must be conducted by an experienced engineer at least every five years, or when there are signs of structural and performance issues.

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Keeping water drained away from the deck will increase its service life. Standing water rapidly deteriorates the deck components, such as the waterproofing membrane, the concrete slab, and the rebars. Water within the assembly can also cause extra damage via freezing and thawing cycles in cold climates. Maintaining a minimum slope of two percent (6.35 mm per m [0.25 in. per ft]) at the topping (for surface flow) and at the deck slab (for subsurface flow) is essential to direct water toward the highly recommended double-tiered drains (see Figure 2, page 50). Considerations should be given to the effects of structural deflections and building settlements on the magnitudes of slope.

Installation inspection and flood testing

The installation quality of the waterproofing membrane is as important as the selection and design of the waterproofing system, as the integrity and continuity of the membrane are paramount to the performance and durability of the parking deck system. Hiring a third-party reviewer to inspect installation and test the system is beneficial—especially before adding any additional layers and the wearing course. Visual reviews should be conducted frequently as part of the quality control program. Testing can be employed as a quality assurance measure.

Flood testing using ASTM D5957-98, Standard Guide for Flood Testing Horizontal Waterproofing Installations, is a good method to verify the continuity of the waterproofing membrane. This test suits parking decks since they mostly have a slope of no greater than two percent with liquid-applied, adhered, or loose-laid waterproofing membrane. Testing is best performed for a minimum of 24 hours and maximum of 72 hours, when rainfall is not expected. If a failure happens, the tested floor will be drained, and the breach will be mended. A retest will then be performed using the same conditions. Required preparation work includes installing containment assemblies and plugging the drains with inflatable devices consisting of rubber balls, draw chains, and rings, as well as other standard plumbing fixtures.

As an alternative to flood testing, utilizing electric leak detection (ELD) to test the integrity of the waterproofing membrane is safe, fast, accurate, and economical. ASTM D7877-14, Standard Guide for Electronic Methods for Detecting and Locating Leaks in Waterproof Membranes, describes standard procedures for using electrical conductance measurement methods to locate leaks in waterproof membranes. This method is suited for a typical parking deck, where the concrete acts as a conductor and the adhered membrane is the insulator. When there is a hole in the membrane, a breach will be detected to determine the hole’s location.

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High-voltage ELD is becoming more popular compared to low-voltage ELD, because the latter needs wet membrane and cannot test vertical surfaces. The presence of drains and an insulation layer under the membrane can cause a misreading of results. Those limitations can be overcome by applying methods some would consider impractical, such as installing electrically conductive primer or lightweight wire grid under the membrane. Some waterproofing membranes such as butyl and ethylene propylene diene monomer (EPDM) also behave as conductors, so ELD cannot be executed on those membranes.

Conclusion

Parking decks are harmfully affected by wear, abrasion, aggressive chemical agents, damaging environmental agents, extreme temperature variations, rough use, salt, and water intrusions. Therefore, they require comprehensive maintenance programs. Ignoring a small water leakage into the assembly may lead to serious and costly structural deficiencies, and catastrophic failures in extreme cases. Taking preventive measures would increase the service life of parking structures and reduce disruptive and expensive repair. Additionally, applying lessons learned from maintaining and repairing aging parking garages would help to design practical and durable structures. The Parking Facility Maintenance Manual by the National Parking Association (NPA) is a valuable guide to extending the lifespan of parking garages through site-specific maintenance and repairs.

Amir Hassan has more than 25 years of experience with building envelopes and forensic restoration in the Middle East, Europe, and North America. He has worked on complex projects for both existing buildings (including historical and heritage buildings) and new construction. His building science knowledge includes work with structural glass, curtain wall design and evaluation, frameless and skylight systems, structural assessment and repair, property condition assessment, building enclosure commissioning, roofing and waterproofing systems, weathertightness, energy modeling, thermal performance, and thermography. Hassan has a Bachelor of Science in civil engineering and a Master of Science in façade engineering. He is a frequent speaker and author covering roofing and the building envelope. He is the current president of Alberta Building Envelope Council North. He also holds the role of subject matter expert in the field of building science with the Association of Professional Engineers and Geoscientists of Alberta. Hassan can be reached at amir.hassan@exp.com[5].

Source URL: https://www.constructionspecifier.com/maintaining-multilevel-parking-deck-systems/

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