Efficiency and economy in precast concrete construction is driven by repetition, standardization and minimum number of pieces. Optimum bay sizes are most commonly based upon the width of the standard double tee module being used. A garage manufactured by a precaster utilizing 12' wide double tees would optimally use 36' or 48' bays while a garage constructed with 15' wide double tees would implement 30' or 45' bays. Precasters in the project region should be consulted to determine available modules.
PCI's Parking Structures: Recommended Practice for Design and Construction recommends a minimum 1% slope be maintained throughout a garage. Camber is generally not used as a drainage mechanism. Careful consideration should be given to product tolerances and camber of double tees when establishing drainage slopes
The best first line of defense against concrete deterioration and steel corrosion is use of a high quality concrete mix and attention to proper construction practice. A high quality mix normally includes a low water / cement ratio (< 0.40), hard-rock aggregates, and a minimum compressive strength of 5000 psi. Proper construction practice includes attention to correct finishing and curing methods, adequate concrete cover over reinforcing steel and sufficient drainage. Beyond these fundamental measures, other items may be considered for garages exposed to especially harsh environments (coastal areas rich in airborne salts, geographical areas prone to heavy use of deicing salts and geographical areas subject to frequent and severe freeze-thaw conditions). These supplemental measures include corrosion-inhibiting admixtures, concrete densifying admixtures, surface applied sealers and traffic-bearing membranes. Cost and benefit of each of these measures should be discussed with local precasters and design professionals.
Cracking in double tee flanges is occasionally encountered on newly erected parking garages. This is commonly the result of torsional stresses developed during loading, transporting or erection of the member. Similarly, this may occur when a double tee is "warped" in its final position to allow for proper drainage. This type of cracking does not jeopardize the structural integrity of the double tee. To avoid long-term durability problems, these cracks should be filled with an approved sealant or epoxy. Epoxy repairs should be covered with a cementitious material to limit UV exposure.