Design Techniques to Prevent Cracks and Connection Failures
By Ned M. Cleland
Precast/prestressed concrete double-tee parking structures are remarkably durable. I base that assessment on more than 30 years’ experience in parking garage design and structural analysis and on my participation in the design of more than 250 parking decks.
But there’s a caveat.
In order to build highly durable and reliable precast structures, you must design, construct and maintain them according to principles of good practice. Proper design, construction and maintenance are vital in maximizing the life of double-tee parking decks and in slowing deterioration by preventing connection failures and tee-flange cracking.
Recognize regional differences
Deterioration of parking structures is caused by structural volume changes due to shrinkage, creep and annual temperature shifts, as well as by damage from freeze-thaw cycles and corrosion caused by chloride exposure.
Freeze-thaw conditions, the use of deicing salts and the effects of airborne salt in coastal areas vary greatly by region – as should the design details of individual parking garages. Northern regions require design for snow loads and the use of deicing salts. Other areas must include design for seismic conditions.
Specify quality concrete
Given that consideration, the first step for parking garage engineers, designers and owners in creating durable, moisture- and chloride-resistant parking structures is to specify the use of durable concrete. These conditions can easily be obtained with precast/prestressed components manufactured and heat-cured in a factory, but they can be difficult to achieve in the field.
The use of high-strength or high-performance concrete (with high cementitious content, lower water/cement ratio and strengths of 6,000 psi), often containing fly ash or microsilica, can further lower a structure’s permeability and increase its durability. Epoxy-coated steel or high-strength carbon-fiber composites can be used in the flanges to add to this protection of the concrete mix.
To prevent water ponding, it’s extremely important to design for adequate drainage. Floors must have sufficient slope in two directions to bring water to drains at low points. This can be accomplished by tilting the floors both on the tee spans and across the beam spans. To maintain level on a garage perimeter, some warping of the floor is permissible.
Plan for volume change
In a cast-in-place concrete structure, restraint forces tend to cause cracking, particularly around stair/elevator cores or shear walls. In a precast structure, such forces flow through the connections. The preferred way to deal with these forces is by the use of flexible or ductile connections that allow the structure to “breathe” and prevent cracking
Demand adequate topping
Pretopped precast members generally have concrete strengths of 5,000 psi or greater. Minimum strength for field-placed toppings should be 4,000 psi, but should be a minimum of 5,000 psi in Durability Zone 3, where freezing and thawing and deicing chemicals are common. (See American Concrete Institute 362.1R-97, Guide to the Design of Durable Parking Structures.)
In cast-in-place concrete and precast concrete, cover for the top surface should be 2 inches in areas where freezing and thawing are frequent and deicing chemicals are used. A 1½ inch cover is recommended for other areas and locations.
Structural concrete topping should be 2 inches or greater at all locations, including mid-span of the cambered tees, and 3 inches or greater at supports in high salt environments. Cover requirements for topping reinforcement may require thicker slabs.
To enhance durability, minimize finish work. Specify a medium broom finish for pretopped tees. Don’t finish field-topped tees until the bleed water has disappeared.
Carefully detail connections
In parking structures, joint connections are needed between double-tee flanges and from the flanges to walls, beams, columns and spandrels. (The variety of precast concrete connection systems is detailed in the Connections Manual from the Precast/Prestressed Concrete Institute. See also the PCI Journal Winter 2009 issue on Connections.)
With field-topped double tees, connection spacing can be up to 8 to 10 feet. With pretopped double tees, spacing should be limited to every 4 to 6 feet, although wider spaces may be used near tee ends that are not in the traffic lanes. The top of the connection should be a minimum of ¾-inch below the top surface. Plate anchors should have 1½ inches of cover.
In pretopped double-tee structures, flange-to-flange connections are key because these need to handle diaphragm shear and distribute wheel loads. Microcracking can develop behind these connections unless properly detailed, installed and welded.
The PCI recommends that bearing plates and other exposed plates be coated with rust-inhibitive paint, epoxy painted, galvanized, or made from stainless steel.
Joint sealant failures are probably the most prevalent repair issue with precast concrete parking decks. Most joint sealant problems are the result of improper substrate preparation, incorrectly sized joints, and/or poor installation of backer rods and sealants.
Surface preparation of double tees generally consists of grinding the edges to which the sealant will bond. Once the bonding surfaces have been prepared, primer is applied if necessary, followed by the installation of joint backing
Insist on proper maintenance
Another key element in reducing cracking and connection failures is to insist on a proper maintenance and repair program that includes weekly cleaning; regular inspection and patching; periodic replacement of sealers, sealants and membranes; and periodic condition audits.
The goals of insisting on quality concrete standards, proper design, effective connection details, quality construction and adequate maintenance procedures are to increase reliability, prevent premature cracks and deterioration, maximize the life span of the parking structure, and reduce overall long-term operating costs.
Ned M. Cleland, Ph.D., P.E., FACI, FPCI, is a consulting engineer and President of Blue Ridge Design Inc. in Winchester, VA.
Editor’s note: This article is an excerpt of a longer piece by the author. To read the complete article, with hot links to references, log on to www.parkingtoday.com, click on Magazine, Current and Past Issues, and then February 2010. Find the article in the e-zine and click where noted at the bottom.
Eight Steps to Prevent Cracks, Leaks, Failures of Precast Parking Garages
(1) Recognize regional differences.
(2) Specify high-strength, low w/c concrete.
(3) Plan for volume change.
(4) Demand adequate topping be applied.
(5) Carefully detail connections.
(6) Protect the connections.
(7) Use sealants to stop joint leaks.
(8) Insist on proper maintenance.
Article Abstract from February, 2010