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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 con- nections.The preferredway to dealwith 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 aminimumof 5,000 psi in Durability Zone 3, where freezing and thawing and deicing chemicals are common. (SeeAmericanConcrete Institute 362.1R-97,Guide to theDesign ofDurable Parking Structures.) In cast-in-place concrete and precast concrete, cover for the


top surface should be 2 inches in areas where freezing and thaw- ing are frequent and deicing chemicals are used.A1½inch 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 reinforcementmay 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 fromthe flanges towalls, beams, columns and spandrels. (The variety of precast concrete connection sys- tems is detailed in the ConnectionsManual fromthe Precast/Pre- stressed Concrete Institute. See also the PCI JournalWinter 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 con- nection should be a minimum of ¾-inch below the top surface. Plate anchors should have 1½inches of cover.


Eight Steps to Prevent Cracks, Leaks, Failures of Precast Parking Garages


(1)Recognize regional differences. (2) Specifyhigh-strength, loww/c concrete. (3) Planfor 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 onpropermaintenance.


Figure 2 Floor and Tee Stems in a 23-Year-Old Garage in Vienna, VA FEBRUARY 2010 • PARKING TODAY • www.parkingtoday.com 25


In pretopped double-tee structures, flange-to-flange connec-


tions 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, galva- nized, ormade fromstainless steel.


Joint sealants 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 prepara- tion, incorrectly sized joints, and/or poor installation of backer rods and sealants. Surface preparation of double tees generally consists of


grinding the edges towhich the sealantwill bond.Once the bond- ing surfaces have been prepared, primer is applied if necessary, followed by the installation of joint backing


Insist on propermaintenance 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 ade- quate maintenance procedures are to increase reliability, prevent premature cracks and deterioration,maximize the life span of the parking structure, and reduce overall long-termoperating 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 refer- ences, log on to www.parkingtoday.com, click onMagazine, Cur- rent and Past Issues, and then February 2010. Find the article in the e-zine and click where noted at the bottom.


PT


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