The advantages of prestressed concrete.
Premature deterioration of parking structures takes one or more familiar forms--surface scaling or chloride-induced corrosion that results in surface spalling, cracking, and faulty expansion joint systems. In these cases, the deterioration can be seen as leakage, leaching, deterioration of the surface, or structural damage. The problems are pervasive in northern climates where road salt usage is common and many freeze-thaw cycles occur in the course of a year.
Every owner of a parking structure knows that these problems exist. Traffic-bearing membranes, epoxy coating reinforcement, specialty concrete and asphalt toppings, and cathodic protection are all measures once considered uneconomical that are being employed today to prevent problems from occurring. Each approach represents a significant increase in the price of a new structure but is money wisely spent in light of huge restoration bills.
The three main structural types of precast, prestressed concrete construction--double tees with topping, untopping double tees, and hollow-core slabs with topping--are highly resistant to chloride-induced deterioration. That is not to say these types of structures do not have problems. They do, especially related to cracking, expansion joints, bearings, connections, and drainage. But the corrosion of reinforcement steel is not one of the problems.
Why precast/prestressed concrete?
* Low cost. The use of precast/prestressed concrete offers a more economical approach for new structures. High quality concrete and predictable structural behavior are key reasons precast/prestressed concrete resists deterioration.
Due to the inherent cost savings associated with mass production, the precast/prestressed concrete components can mean significant savings in both materials and interim financing costs. Further savings can be realized through the use of smaller, more efficient erection crews that offer lower insurance rates and minimal maintenance costs. Components can be erected from the truck to the structure in almost every instance, making it ideal for projects with limited site access.
* Quality controlled factory manufacturing. Precast, prestressed concrete components are uniformly mass produced under strictly controlled, inplant manufacturing conditions for reliable consistency, assured quality, and prompt, on-schedule delivery. With this standardized approach, components fit together and are readily positioned into place.
* Low maintenance. Because precast/prestressed concrete is highly resistant to abrasion, impact, corrosion, vandalism, and the ravages of time, it is an ideal solution for the demanding use experienced by a parking structure. And because painting and other exterior care is minimized, maintenance is reduced considerably for substantial, long-term savings.
* Clear spans. Traffic engineers and parking consultants generally agree that large, clear spans are the most important criterion for successful multilevel parking facilities. Thoughtful design planning, in conjunction with the expertise offered by a prestressed concrete producer, can result in clear spans of 60 feet or more to permit two-way traffic, wider aisles, easier parking and maneuvering, less congestion, faster parking turnover, easier interior maintenance, and increased parking capacity and revenues.
* Limited access advantages. Precast/prestressed concrete is ideal for limited access locations. Pretensioned parking structures have been erected sandwiched between buildings in a busy urban setting or over a main thoroughfare--without the need to block off the job-site area. The components can be lifted off a truck parked in an alley or even on the street without undue traffic congestion.
* Durability. The high strength concrete used in precast/prestressed concrete components is resistant to damage from freeze/thaw cycles, de-icing salts, and other severe weathering conditions. Precast/prestressed parking structures can be designed with the flexural strength to meet all anticipated loads, with reserve strength for overloading.
The connection details and structural design have been adapted to meet stringent seismic conditions. Prestressed concrete can also be designed to provide the degree of fire resistance required by local building codes.
* Rapid, all-weather construction. Construction is fast with the use of precast/prestressed concrete. As the components are being factory cast, site preparation work can proceed. Components arrive at the job site ready to install at rates of 8,000 to 10,000 square feet a day. This makes on-site storage virtually unnecessary and eliminates the usual job-site weather delays, scaffolding, and shoring associated with cast-in-place concrete. Deck slabs then become an immediate working platform for other trades. Also, construction loan periods are shortened, and income can start sooner.
* Structural design. The structural design and exterior appearance of a precast/prestressed concrete parking facility is limited only by the imagination of the architect or engineer. The structural components themselves can become strong visual elements of aesthetic design. Architectural spandrels or cladding can feature an almost infinite variety of patterns, aggregates, colors, textures, and shapes.
* Dual land use. Although dual land use is most commonly found with office building parking facilities, excellent results can be obtained by erecting structures over streets, flood control canals, rivers, railroad tracks, or under existing parks or structures. Precast/prestressed concrete is ideal for such situations due to its ability to take unusually heavy loading. In addition, construction time and congestion in surrounding areas is kept to a minimum over a shorter period of time when compared to most other construction materials.
Steps to success
A successful formula for preventing the common deterioration problems described above may be achieved by incorporating 10 recommendations into the construction process. These 10 recommendations will prevent leakage and corrosion-related deterioration for all types of parking structure construction but lend themselves most effectively and economically to precast/prestressed concrete structures.
* Use a mix design formulated for durable concrete. This means a low water/cement ratio of 0.45 or less, which translates into a low slump range of 2 to 3 inches (50-75 mm), much lower than is used in normal practice. Incorporate air entrainment in the 6-percent range with sufficient cement to develop at least 4,000-pound-per-square-inch compressive strengths.
These requirements are automatic in prestressed elements, which are usually far in excess of the minimum strengths. Obtaining them in field-placed toppings is tougher, however, and an emphasis on maintaining these standards often meets with resistance from field contractors.
* Insist on proper placement and finishing practice. Precast/prestressed concrete elements are manufactured under controlled conditions and surpass the concrete placement standard required. Placing low-slump concrete in the field requires special considerations. Mechanized screens must be used to obtain the desired vibration and compaction. Super plasticizers also may be used to aid placement but seem to create difficulties if proper procedures are not carefully followed.
Insist on minimal finish. The surface should be worked just enough to close it. Too much trowelling tends to drive out the air entrainment at the surface where it is most needed. Working it will raise the water/cement ratio at the surface and further reduce the durability of the concrete.
Finally, the surface finish should have texture to provide traction. This is best accomplished with a floated swirl finish or a light-to-medium broom finish.
* Insist on proper curing practice. The best cure for field-placed concrete is water, and the longer the better. Three days is a good practical minimum standard. During this period the concrete should be continually wet and covered. The curing process must begin as soon as possible. The logistics of providing a good water cure are difficult, and extra care should be taken to ensure success.
During spring and fall construction, cold weather may make the use of curing compounds more practical. Curing compounds should be compatible with any sealer or other additional surface treatment intended for use. Precast/prestressed concrete is cured at high temperatures in a closed, controlled environment.
* Be prepared in advance for bad weather. Be ready to cover the slab if it rains, erect windbreaks on a windy day, or cover the slab with insulated blankets if freezing conditions are likely.
* Provide adequate testing and inspection. Testing and inspection are protective procedures to ensure the project is constructed as it ought to be.
Certain procedures should always be included: test concrete strength by taking cylinders; test concrete for air content; record the concrete's temperature at time of placement; take slump tests.
Standards for these items should be established in advance, and concrete should be rejected if it does not work properly. The importance of the mix quality cannot be overstated. Testing is the best measure to ensure the intended quality.
* Provide proper clear cover over reinforcement. Highway research shows that the amount of clear cover of concrete over reinforcement is a very significant variable in the reduction or prevention of corrosion. A minimum 2-inch, clear cover or two-and-a-half times bar diameter are recommended as a practical guide. This standard is almost never achieved in cast-in-place concrete structures and is one of the causes of corrosion in parking structures. Precast/prestressed concrete has been virtually free of these problems.
In precast systems, significant reinforcement is in the webs of the tees or at the bottom of the hollow-core members, encased in very low slump, low water/cement ratio concrete, making it impermeable to salt penetration. Only mesh is used near the surface, and this element lacks the mass required to produce the expansion stresses resulting from ongoing corrosion required to delaminate the surrounding concrete.
* Provide a crack control system. The prime source of chloride intrusion into concrete is through cracks. It is cost effective to provide sealed control joints to reduce this problem. It is critical to provide the joints in locations where they will force cracking and relieve stresses so that detrimental cracks do not form randomly in other locations.
The joints can be made watertight by sealing with a high-quality urethane sealant. This task can be carried out in a precast structure by providing joints in the topping directly over all of the precast element joints below. When toppings are not used, the joints between the precast members are sealed directly.
For an effective crack control system, insist that joints be tooled, not sawed. Joints should be abraded prior to sealing to clean and reinforce weak edges, and primers should be used with a high-quality urethane sealant.
* Provide positive drainage and drain hardware that works. Stagnant water causes problems. It usually occurs because of a lack of positive drainage or faulty drain hardware. Precast concrete elements normally have some chamber that can be utilized to create drainage in one direction. A slope can be incorporated in the other direction to ensure a trough effect.
A good piece of drain hardware is one that is integrally sealed, has a sufficient opening to handle peak drainage conditions, and is easily cleaned to keep water flowing.
* Use a proven expansion-joint system. Expansion joints are often a source of ongoing problems. Because precast/prestressed concrete systems are segmental, they take movement in small increments at shorter intervals, allowing expansion joints to be spaced farther apart. Frequently spaced expansion joints are required in the more rigid frames of cast-in-place designs.
The best design eliminates them whenever possible. When they are required, however, proven products installed by the manufacturers with their warranties are the best course.
Expansion joint products are available for a variety of applications. High-wear areas, such as entrance ramps, snowplow exposures, and high-movement conditions, may warrant the expensive of a bridge-type strip seal expansion joint. For other areas, a factory-molded sealant strip is a more economical, yet effective solution.
* Provide a quality concrete surface sealer. During drying following curing, excess water from the concrete escapes, leaving pores in the surface by which waterborne chlorides and other contaminants may re-enter the concrete and seep down to the reinforcement. Precast/prestressed concrete has greater initial resistance to this problem because the concrete is denser and less permeable. A high-quality concrete sealer will complement good concreting practice--but not substitute for it--by filling these pores and blocking out re-entrant water.
There are many sealers available, so some criteria should be used to select an effective product. In recent years, the National Cooperative Highway Research Program (NCHRP) has been evaluating many products to determine their effectiveness.
Under this program, sealer products have been evaluated for effectiveness against water absorption, chloride absorption, and freeze-thaw deterioration. The samples were also submitted to accelerated weathering tests designed to simulate severe southern and northern weather exposures. The results showed polymer products based on urethanes, epoxies, acrylics, and salines provide the most effective protection.
The application methods, application rates, and solids content of the product used in the tests are important variables. The same procedures and application rates used in the testing program should be used in the field. Contractors should make sure that the manufacturer's recommendations meet these criteria.
David Monroe is the vice president of marketing for the Harry S. Peterson Company of Pontiac, Michigan, a firm specializing in the design, manufacture, and installation of Iso Flex sealant and waterproofing systems for concrete structures. Peterson is an operating division of Masterbuilders, Inc.
Mr. Monroe is active in the American Concrete Institute as a member of the Parking Structures Committee, and the Prestressed Concrete Institute as a member of the Marketing Committee and as a current director of the Institute. He has written a number of articles regarding the waterproofing of concrete structures and is a frequent lecturer on the subject.