A wide variety of finishes are available in architectural precast concrete, ranging from a smooth form finish to deeply exposed aggregates. As a general rule, a textured surface provides more uniformity than a smooth surface because the natural variations in the aggregates will camouflage subtle differences in the texture and color of the concrete. A medium sandblast finish, for example, generally provides more uniformity and consistency than an acid wash finish. Dividing larger areas into smaller ones with reveals or rustications can also help lessen any variation in texture that might be visible.
It is more economical to maximize panel size and minimize the number of precast units on a project. This results in fewer erected pieces, fewer connections and fewer crane picks. However, the maximum size of a precast panel depends on a variety of factors. For example, the size may be limited by site conditions or the reach of the crane that will be used to set the pieces. A site with limited access, or one where the maximum panel weights are set by the crane capacity could be the overriding factor in determining panel dimensions. Similarly, the size or weight of precast panels may be limited by shipping or fabrication considerations specific to a region or individual precast supplier. Usually panels should not exceed a width of approximately 12'-0", without consideration for a special permit or escort. Also, panels that exceed 40' in length may require the use of prestressing to reduce handling stresses and minimize cracking. The maximum size of panels is also a function of the design loads and locations of building supports. In general, it is best to work with a MAPA precast supplier to determine the most economical sizes and dimensions for your project.
The recommended precast panel to panel joint width for architectural projects is 3/4". This is the minimum nominal joint width needed to adequately account for production and erection tolerances and still maintain an effective minimum joint width that can be caulked. A 30' long spandrel panel is allowed, per PCI tolerances up to a 1/4" variation in length. Keep in mind that of 3/8" is the minimum width that caulk suppliers will warrant. It is also important that the joint between precast panels and window frames also maintain the same nominal joint width.
Sealers are often specified to improve the weathering characteristics of precast panels, especially in urban areas where the building may be subjected to airborne industrial chemicals. Sealers can also help facilitate the cleaning and maintenance of the panels if they should become dirty. When sealers are used, they should be applied in the field, after all of the joints are caulked, repairs made and cleaning complete. Otherwise, it is likely that the panels will have to be recoated in spots, which could, in turn, lead to inconsistencies in color and finish.
The slab edge location should be clearly defined on the contract documents. It is recommended that a 1-1/2 inch dimension be allowed between the edge of slab and the precast panel to account for tolerance both in the slab as well as the precast. Pay particular attention to slab edge conditions along skewed or curved building edges as these areas are often the areas that cause the most difficulty during layout.
It is important to consider tolerances when designing the interior wall finishes and locations. For example, if inadequate space is left between the back of the precast panel and the inside face of the interior finish, connections may become exposed to view. Allowing at least an extra 1/2-inch between the back of the drywall and the theoretical back edge of the connection hardware is strongly recommended. When the distance between the back of the precast and the interior finish does not accommodate this, connections may have to be recessed. It is also a good practice for the engineer to specify the allowable locations for slab recesses and to provide reinforcing details to account for this.