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Structural engineers love exposed structure. Our work is especially rewarding when our solutions and creativity are left visible to the public. Buildings that leave the structure uncovered allow us to learn about the practices of our fellow designers. Single components that serve a dual purpose — as both an architectural feature and a structural system — also reduce overall building materials, a tenet of sustainable design.
Structural elements that are visually exposed are often more expensive than those hidden by conventional architectural finishes. However, collaboration between the design team and contractors and awareness of constructability issues with significant architectural or budget impacts can reduce cost premiums. Ideally, contractors are actively involved in early design decisions.
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ACI 303 Committee Report
Cast-in-place concrete is used effectively in exposed structures because it can be formed to nearly any shape, is durable and resistant to weather and fire, and offers a variety of textures and colors. The ACI 303 Committee Report Guide to Cast-in-Place Architectural Concrete describes the planning, design, execution and inspection of exposed concrete from architectural, structural, and construction perspectives, including provisions with minimal budget impacts and those with very high cost premiums. Structural engineers should be familiar with this document and prepared to selectively reference sections that meet the client’s expectations for the project.
A structural engineer’s standard concrete specification is generally not adequate to fully describe the design intent of architecturally exposed concrete. A customized specification, written collaboratively with the architect, is necessary to combine typical structural requirements with the appropriate recommendations of ACI 303. Perhaps your client prefers the look of architectural concrete but is willing to accept a slightly less-refined appearance due to budget considerations, the relative importance of the element, or the proximity of that element to the viewer. Under these circumstances, structural engineers can bring great value by understanding the capabilities and limitations of concrete materials, the effective use of forming materials, and the construction means and methods required to produce an intended architectural result.
Materials and reinforcement
Concrete mixes can be designed for a desired appearance without compromising strength or adding significant cost by focusing on constituents that effect color, texture, durability, workability, and shrinkage resistance. Superior workability is critical to achieving clean lines, complex shapes, and well-consolidated surfaces with minimal voids or defects, while shrinkage resistance minimizes crack formation.
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Water content in exposed concrete should be constant from batch to batch, to provide uniformity in color and texture. Excessive water can increase shrinkage cracking. Wet, soupy mixes are more likely to stain, form bug holes, and encourage the separation of concrete ingredients, especially during vibration, with aggregates sinking to the bottom and cement paste rising to the top. Water added on site can also cause uneven surface coloring. Consider limiting water content and using superplasticizers to reduce water demand and increase workability. Shrinkage-compensating cements and admixtures are available, but are relatively expensive. Proper curing procedures that prevent rapid or non-uniform drying are critical to prevent color variations and plastic shrinkage cracks.
All cement, aggregates, and pozzolans should come from the same respective sources to minimize color variations. Unless an exposed aggregate finish is specified that requires a gap-graded coarse aggregate, a well-graded, combined mixture of coarse and fine aggregates will enhance workability and reduce shrinkage. Fly ash improves workability and reduces water demand, but can create unwanted color disparities. Slag cements have similar benefits, and have a lighter color than conventional cement, but can be subject to an initial green tint that typically fades as the concrete surface dries. Color concerns should be carefully explored with the architect and owner before these pozzolans are specified.
Self-consolidating concrete (SCC) has gained popularity among contractors and can be specified in some markets with minimal additional costs. SCC is a highly flowable concrete that fills formwork and encapsulates reinforcement with little or no vibration and without compromising durability or compressive strength. The capabilities of SCC are especially beneficial to exposed concrete elements because concerns about reinforcement congestion, poor consolidation, and surface imperfections are greatly reduced.
To better control cracking, smaller and more tightly spaced reinforcing bars should be used, and designers should consider increasing reinforcement to limit tensile stresses. The rigorous durability and crack control provisions of ACI 350 Committee Report Code Requirements for Environmental Engineering Concrete Structures can be referenced for guidance. Additional reinforcement should be specified around openings to replace interrupted reinforcing and prevent cracks induced at corners. Consider limiting reinforcement ratios to facilitate placement of concrete and ensure proper consolidation. Either self-consolidating concrete or mixes with smaller coarse aggregate will also ease placement in congested areas. Increased clear cover requirements, as recommended in ACI 303, will help prevent rust stains or other signs of corrosion, especially beneath architectural features and reveals. Support ties and chairs, consisting of stainless steel or plastic, minimize rust spots.
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Formwork
Forming typically accounts for more than half of the overall cost of architecturally exposed concrete and significantly impacts color and finish. Structural engineers must be aware of the characteristics of various forming materials and systems, and consider the relative availability, durability, and absorptive properties of each in specifying materials to accommodate a particular application.
Communication between the architect, structural engineer, and contractor is critical in achieving the desired appearance of a customized element built with repetitive forming materials. Establish clear expectations for formwork tolerances, surfaces, and form joints. Use non-absorptive forming material to avoid variations in surface water content, and consider the finish imparted on the concrete by the forming material. Specify non-yellowing, non-staining formwork coatings and release agents to avoid discolorations, and closely observe early pours to confirm visual acceptance.
Large concrete surfaces are often segmented using architectural reveals, or rustification joints, to accommodate repetitive formwork modules and diminish the importance of matching textures and colors from one pour to the next. Consider the formwork panel size in planning rustification joint locations, as form-facing material joints may remain visible on the concrete surface. Form joints, liners, coatings, and accessories can have a considerable impact on the final appearance of exposed concrete, and an extensive discussion of each is included in ACI 303.
Form joints and tie holes should be shown in the formwork shop drawings, and can be hidden by rustification joints or incorporated into the architectural design if they are effectively communicated and consistently installed. Formwork requirements should be increased only where necessary; where architectural concrete specification requirements are enforced on exposed portions of an element, structural concrete requirements might be enforced on unexposed portions of the same element.
Detailing
Structural engineers have the unique opportunity to advise both architects and contractors. We should collaborate openly with each throughout the design process and during pre-bid or pre-construction meetings regarding the range of possible details and finishes preferred by the contractor that are acceptable to the architect. The location of construction joints and the sequence of pours are generally determined by the concrete subcontractor, but for exposed concrete elements, the structural engineer should define critical construction joint locations on the structural drawings to preserve the architect’s intent, develop details that minimize the visibility of construction joints, and require that those joints be reproduced on the formwork shop drawings to confirm understanding.
Cracking in walls is minimized by providing contraction joints to relieve shrinkage stresses. Rustification joints can be conveniently used to conceal contraction and construction joints (see photo and detail below) or to channel water away from visible concrete faces. Structural engineers must coordinate the architects’ preferred rustification joint locations with crack-control measures and common forming practices. Uniformly spaced contraction joints provide greater flexibility in the reuse of formwork. Reinforcing that crosses joints should be reduced to encourage cracks to form at locations compatible with the designed surface presentation. Wherever rustification joints are used to obscure contraction and construction joints, they must maintain the width and depth of adjacent architectural joints, and be shown on the structural drawings. To meet ACI requirements for minimum joint depth, a deeper joint can be specified on the backside of walls where not exposed to view.
Sharp corners can be obtained with proper consolidation of exposed concrete, and provisions are listed in ACI 303. However, desired results often require self-consolidating concrete, specially fabricated formwork and extended stripping times. Chamfered corners, formed using a continuous chamfer strip attached to the inside corners of the formwork, are a less expensive solution commonly used to prevent broken or damaged corners and to reduce the formation of “fins” caused by formwork joint leaks.
Horizontal surfaces of exposed concrete should be sloped to prevent accumulation of water. The presence of standing water on exposed concrete for extended periods of time will increase the threat of corrosion and lead to surface stains. A “drip” on the underside of an exposed soffit is a specialized rustification joint that prohibits rainwater from traveling back along the bottom of the concrete member due to surface tension, effectively limiting stains and water damage (see photo and detail below). Drips should be shown on the structural drawings and duplicated on the formwork shop drawings.
Mock-ups
Acceptance criteria for architecturally exposed concrete, including color variation, finish, and suitable repair procedures are often established through the construction and review of mock-ups representative of the size of the element, placement and jointing techniques, lighting levels, and distance from the typical viewer to the element. To avoid the added cost of mock-ups, consider permitting the contractor to start work in less visually critical areas where mistakes can be tolerated to gain experience with particular concrete mixes and forming and repair procedures. The resulting “in-place mock-up,” if acceptable to the architect, can be used as the final structural element, allowing the architect to recognize potential defects and make necessary design adjustments. Furthermore, expectations can be clarified for contractors and bidders by identifying representative structural elements, applications, or products from completed projects that give a sense of acceptable levels of quality.
Conclusion
Supported by the emergence of sustainable design, exposed cast-in-place concrete is thought of as an architectural solution now more than ever. Structural engineers must look beyond the function of a visually-exposed element to consider how the material itself, and the manner of its construction, play critical roles in a successful project. Working closely with the architect and contractor to develop unique specifications and details that meet both the aesthetic and budgetary goals of the project will allow us to take full advantage of the versatility and beauty of exposed concrete.
Dennis Wittry, P.E., S.E., is a principal and Bart Miller, P.E., is a senior associate at Walter P Moore. They can be reached at dwittry@walterpmoore.com and bmiller@walterpmoore.com, respectively.
