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Construction Techniques for Architectural Concrete

Source: | Updated: Jan 26, 2016

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.

Formwork Considerations

Formwork cannot avoid impacting the surface appearance of concrete. The impact can be controlled, however, by using formwork that has benign interactions with the concrete and has consistent contact surfaces. For satisfactory appearance, formwork has to be accurately placed, rigidly installed, and have tightly sealed joints. Forms should be clean, maintained, and used a consistent number of times. In some cases, forms may be limited to a single use if the conditions dictate.

Coatings and sealers should be tested to verify their visual impact and should be consistently applied. Forms should be sealed with non-vapor-transmitting coatings, and form release agents should be used to achieve consistent form removal. Consistent formwork in consistent conditions with benign interaction with the concrete is necessary for uniform-appearing architectural concrete.

Placing and Finishing

The placement of concrete has to be controlled to not damage the forms or segregate the concrete being placed. While concrete is being placed, avoid spattering on the forms to prevent irregular surface finishes.

Vibrating the concrete is important to achieve well-consolidated concrete. The concrete can be vibrated by immersed vibrators or exterior form vibration. Adequate vibration will contribute to achieving a uniform concrete surface. Under-vibrating the concrete can allow rock pockets and honeycombing. Over-vibrating concrete will result in segregation of the concrete components. Spading may also be used to improve concrete consolidation.

Curing Methods

The typical practices for curing structural concrete apply to architectural concrete, although there are some additional considerations for architectural concrete. A consistent curing process should begin as soon as possible and be consistently maintained among the different pours. Because it is easier to achieve consistent results with curing compounds, architectural concrete should not be wet-cured. But to make the curing more uniform, concrete should be consistently misted before applying curing compounds.

In accordance with the manufacturer’s specifications, apply additional curing compound for architectural concrete with rough surfaces. For concrete with a water-cement ratio below 0.40, use wet-curing due to the self-desiccating nature of concrete with a low w/c ratio.

Consistency of the temperature while concrete cures also helps maintain uniform color. Formwork should be removed in a consistent time interval, which may include weekend work. As much as is practical, cure concrete members in a balanced manner.

Planning for Repair Options

Although the design and placement considerations we have outlined decrease the likelihood of water intrusion through architectural concrete, it is important to discuss and budget for repair options during the design phase, should they be required. Typically, water intrusion through architectural concrete can be addressed in two ways: injection of cracks or application of a coating over the entire surface.

Applying a coating on architectural concrete changes the appearance of the concrete and is typically not desired from an aesthetic standpoint. Unfortunately, there is a reverse proportional relationship between the aesthetic impact and the ability of the coating to limit water intrusion. Penetrating sealers, which have the least visible impact, have little-to-no crack-bridging ability (typically less than 1/32 inch) and a limited service life, typically three to 10 years, based on wall elevation and the extent of weathering the wall receives.

With these properties, penetrating sealers alone will not prevent water intrusion as they do not seal the cracks in the concrete. On the other end of the spectrum, silicone-based elastomeric coatings typically have the greatest crack-bridging ability (up to 1/16 inch) and service life (10 to 20 years), but they drastically change the appearance of architectural concrete.

Due to the aesthetic impacts of coatings that address water intrusion, crack injection is often used to repair architectural concrete. Depending on the conditions, crack injection can be performed from either the interior or exterior.

Interior repairs require more access, but they are preferred, as injection ports can be drilled from the interior to allow the port to be installed intersecting the crack at a depth below the surface and still not damage the exterior finish. If exterior injection ports are used, they are installed within the polyurea paste to avoid having to drill ports in the exterior surface of the architectural concrete.

The type of injection grout specified also varies with the project conditions. Grouts used for this application are typically either epoxy or polyurethane. With either type of grout, a low viscosity material is desired to allow the material to travel within the crack. Epoxy grouts are a nonflexible material and, therefore, should not be used for dynamic (moving) cracks. If dynamic cracks are present within the architectural concrete, a highly flexible polyurethane grout should be specified. Polyurethane grouts are available both as hydrophobic and hydrophilic materials. The hydrophobic variety works by swelling within the crack, and the hydrophilic material works by bonding to the crack.

Due to the likelihood of water intrusion with architectural concrete, we recommend developing specifications in the design phase and assuming an allowance for grout injection in the base bid so the best cost will be gained should cracking in the architectural concrete occur.

Architectural concrete requires coordinating specifications and detailing during the design phase by the architect and structural engineer. Incorporate structural and waterproofing enhancements into the design and construction methods to improve the performance of architectural concrete from the start, which will decrease the likelihood of water intrusion later.

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