Exposed architectural concrete is a highly desired architectural finish throughout the world, but a lack of understanding of the design and construction requirements often allows water intrusion. For successful buildings, structural engineers, architects, and contractors must work in tandem to provide the required structural and waterproofing performance while still achieving the desired aesthetic finish. In Part 1 of this article (March 2015, visit go.hw.net/arch looksgreat), we focused on design and specification of architectural concrete. In Part 2, we examine the placement of architectural concrete in the field and why it is critical to success.
Control and Construction Joints
Cracks that undermine the aesthetic appearance and watertightness of concrete can be controlled by limiting the aspect ratios of the dimensions of the concrete elements. The aspect ratios of length, width, and thickness of a concrete slab are limited by control joints and construction joints. Slab-on-ground control joint spacing is typically limited to between 24 and 30 times the slab thickness; the length-to-width ratio of the panel formed by the surrounding control joints should be limited to 1.5.
For improved performance for architectural concrete, these ratios can be reduced even further. An example of the interrelated nature of concrete is that a mix design with larger aggregate, which is less susceptible to shrinkage, can be placed with larger joint spacing than a mix with smaller aggregate.
To effectively control cracking, control joints must be cut deeply enough — about one-fourth the depth of the wall or slab. The intent is to create a plane that is weak enough to focus cracking at that plane without the expense of deeper cuts or the impact of creating a discontinuity in the concrete wall. Water intrusion at control joints can be controlled by sealing the joints, but remember that shrinkage can continue for several months, so sealers should have the ability to accommodate the ultimate joint width or be replaced if cracked.
Construction joints are full-depth joints created by separate concrete pours to accommodate efficient construction and to achieve architecturally desirable lines. To prevent offsets between sides of the joint, use keys or reinforcing dowels across the joint. At all construction joints, waterstops should be specified and installed. Although there are many waterstop products on the market, most of them are intended for below-grade applications where there is a greater exposure to water. In particular, bentonite waterstops should not be used above grade since they require water to activate and can be affected by wetting and drying cycles of intermittent water exposure. For this reason, we recommend specifying nonswelling waterstops above grade.
Importance of Mockups
Because of the complexity of all the determinants that may affect the outcome of architectural concrete, using mockups is an efficient method to verify the adequacy of the design and construction practices. We do not have the tools to predict the outcome of mix designs, batching, placing, curing, and exposure to in situ conditions. Mockups of architectural concrete are also critical to obtain approval for the aesthetics from the design team and owner and to establish control samples as a baseline for what will be acceptable on the project.
The key to valid mockups is ensuring that the actual components are used and implementing all relevant aspects of the concrete application and curing. Carefully consider any compromises of dimensions from the final configuration before being included in a mockup. Mockup requirements should be clearly specified in the construction documents so the contractor can properly bid and assign adequate resources to the effort. If necessary, use the mockup to examine the aesthetic impacts of any proposed repair methods, should they be required.