As structural engineers, we’ve helped facilitate a trend in architectural design that takes advantage of the concrete structure in defining the architectural expression of the building. With more attention to reducing the environmental impact of buildings, finish materials are being reduced by leaving the concrete structure exposed. This is not a new idea, but is being integrated into architecture in a variety of creative and interesting ways. The results are durable finishes that are very efficient in terms of material use. The visible structure, coordinated with the spatial organization of the building, can provide a straightforward basis for the architectural expression.
However, there are critical aspects that must be considered in successful execution of an exposed structure. The exposed concrete may or may not meet the definition of “architectural concrete” as defined by the American Concrete Institute (ACI). Architectural concrete is defined as “concrete exposed as an exterior or interior surface in the completed structure, [that] contributes to its visual character, and is specifically designated as such in the contract documents.” We do not attempt to address all of the architecturally exposed concrete considerations; instead, we provide some guidance on the use of various forms of exposed cast-in-place concrete, whether “architecturally exposed” or not. The American Concrete Institute (ACI) has developed many excellent resources to guide in various aspects of exposed concrete construction. For the more high-profile architectural concrete elements in particular, ACI 304, Guide to Cast-in-Place Architectural Concrete Practice, provides a wealth of information.
It is important that the structural engineer understands the design goals of the architect and owner and participates as an active team member in the development of a successful design. The building module, organization of spaces, building image, and other building systems are important issues. In addition to considering the building as a whole, the engineer must have a good understanding of the various structural system options and the capabilities and limitations of each. Critical design, specification, and construction issues must be considered and addressed. Careful attention must be given to integrating the structure with the other building systems, particularly when the concrete structure is exposed. An exposed concrete structure can contribute a key component to the design composition.
From the outset, one critical element in a successful project is a clear understanding and buy-in of the key stakeholders. In particular, the architect and owner must have an understanding of what level of quality is required, acceptable, and included in the budget. Other stakeholders may include various constituencies, such as the owner’s representatives, facilities managers and users, and governing boards.The key decision-makers must be identified and educated to avoid painful and costly misunderstandings during and after construction. It is helpful to observe existing construction similar to that being considered to clarify key appearance issues. The structural engineer, contractor, and formwork subcontractor can facilitate and should attend site visits with the architect and owner whenever possible. Local practices and capabilities of the subcontractors also need to be considered. Several examples should be visited to reach consensus on key issues, and areas deemed acceptable as a standard of quality should be identified and documented. The degree to which this is necessary will vary depending on the experience of the architect and the owner, the extent of the exposed structure being considered, and the prominence of the exposed elements. The key is to reach consensus on expectations for the exposed concrete appearance that are compatible with the budget. Some level of imperfection, patches, and cracking are inherent in concrete construction and this should be communicated. Often, the goal is merely good quality structural concrete with a little extra attention to key details. Regardless, the level of quality must be clearly understood and defined in the planning process, then incorporated into the contract documents.
Once an acceptable standard is identified, the design team must develop the drawings and specifications to communicate the key issues to the contractor. Common construction manager and design-build processes help facilitate this. The design intent must be clearly communicated to the builder through the drawings and specifications. If the exposed concrete is to meet “architecturally exposed” concrete quality, it must be noted as such on the contract documents. This standard may not be necessary to meet the design intent, so it should be noted only when required.
From a structural perspective, some of the important issues, when cast-in-place structural concrete is exposed, include selection of a floor framing system compatible with the architectural goals, deflection control, crack control, detailing of reinforcing, and consistency of member sizes.
For structures where the bottom of some of the floor framing is to be left exposed, the framing must be coordinated with the architectural elements. Where a regular building module exists, beam and joist spacings should be laid out to work with repetitive wall locations. The structure should be organized, if possible, so the walls terminate against a soffit surface of uniform elevation to avoid the walls having to track up and down variable vertical faces. Consideration should be given to adding a little extra width to beams or joists, if needed, to module out with the walls. For some building types, a flat plate may be the best choice to present a uniform soffit elevation. Using a slightly thicker slab or stud rails for shear reinforcing should be considered to avoid drop panels. In all cases, regularity and repetition should be maximized for forming economy.
Where horizontal elements will be exposed, proper consideration should be given to deflection control. Architectural requirements may dictate that deflections be limited to smaller levels than typically permitted. Permissible visible deflections should be vetted with the architect. Deflections on the order of L/360 to L/480 are often acceptable from an appearance perspective. The viewing angle will have an influence upon the allowable amount of deflection. The addition of post-tensioning often is a good solution in limiting deflection when structural depth is limited, or in unique situations. Camber of the formwork is another possibility, though it is not as common. It should be clearly noted and specified, if used.
Crack control is another structural consideration. Concrete structures have unavoidable cracks by their very nature. This needs to be understood by the architect and owner. However, with careful attention, cracks can typically be limited to visually acceptable levels. For interior exposed beams and slab soffits, cracks due to flexure and shear typically are controlled adequately by adhering to the requirements of ACI 318, 10.6.4. In cases where visually exposed elements are exposed to weather, the flexural tension stress in the bars should be limited to reduce crack widths. Moisture can enter cracks when it rains. This seeping water causes the concrete to dry more slowly than the adjacent surfaces, leaving the crack highlighted as a temporary damp streak. The PCI Design Handbook, 5th Edition, provides guidance in 18.104.22.168, though this has been modified in later editions. Restrained shrinkage and temperature, abrupt changes in geometry, slab openings, and unaccounted structural restraint are more common sources of objectionable cracks. Expansion and delayed pour joints can be used to reduce cracking due to volume change.
In addition to provision of reinforcing to reduce cracking, attention should be given to the detailing of reinforcing to allow successful concrete placement. Adequate space between bars and layers should be provided to permit satisfactory placement and consolidation of concrete. ACI 304 recommends that in exposed walls and columns, at least 5 inches be provided between vertical mats of reinforcing, and that at least 4 inches be provided in walls with a single mat, between the mat and the form. This is necessary for placement and vibration of the concrete. Where “shadowing” of the reinforcing bars would be objectionable, it is recommended that at least a 2-inch cover be provided between the reinforcing and the forms on the exposed face. The minimum horizontal distance between bars should be increased to the larger of 2 inches – 1.25 times the bar diameter, or 1.75 times the largest aggregate dimension. Attention should be paid to areas where large amounts of reinforcing is spliced, such as at column bases and heavily reinforced beam bottoms over supports.
One of the larger influences upon the appearance and cost of exposed concrete is the formwork type and quality. ACI 347, Guide to Formwork for Concrete, and ACI 117, Standard Specifications for Tolerances for Concrete Construction and Materials, provide extensive information regarding the design, specification, and construction of formwork. Considerations include form materials, jointing of formwork, form tolerances, and treatment of joints and corners.
Form materials are typically wood, plywood, plywood with an overlay, or steel. Plywood without an overlay will transfer the wood grain, including the football-shaped veneer patches in the typical grade B veneer, to the concrete. This may be acceptable, or even desirable, depending upon the situation. Overlay plywood or steel will impart a shiny texture. The appearance aspects of the various form materials should be considered as the selection is made and specified. If appearance is not critical, the contractor should be given the maximum flexibility in selecting the most economical material.
Joints are common to all form materials and will be visible in the cast surface. Their location and treatment should be carefully considered. Where possible, establish member sizes to match the dimensional module of the form materials. Shop drawings should be required to show the joint layout for prominently exposed surfaces. Whether joints are sealed or not affects the appearance and cost significantly. Sealing of the joints is expensive and should be considered only in prominent areas. Additionally, the treatment of exposed corners must be specified. Chamfers, typically three-quarters of an inch, are most reliable and least troublesome but may not provide the desired appearance. Square corners are chipped easily during form removal and construction, so more care is required.
ACI 347 and 117 establish general standards for form offsets at joints. They are as follows:
Class A – one-eighth inch, either gradual or abrupt. Suggested for prominent architecturally exposed surfaces and exposed surfaces in generally finished spaces.
Class B – one-quarter inch, either gradual or abrupt. Suggested for surfaces receiving plaster or stucco.
Class C – one-half inch either gradual or abrupt. Suggested for exposed surfaces in generally unfinished spaces.
Class D – 1 inch, abrupt or gradual. Suggested for concealed surfaces where roughness is not objectionable.
These can be modified by the designer and must be clearly specified. We often modify class C to allow only one-quarter inch abrupt, but one-half-inch gradual offsets. This is often acceptable for exposed slab and beam soffits that are above lighting. For more visible soffits, class B is typically acceptable. Class A should be used where a smooth surface is required or in highly visible areas. The cost increases with smaller form offsets. Local concrete subcontractors can provide budget input for specific project conditions. Alternate bids for the various form classes can be considered to test the market value. Whatever choice is made, it must be clearly communicated. Soffit plans defining areas of higher quality formwork should be considered where there is variation. Critical exposed surfaces must be noted on the drawings.
Form tolerances that are different from standards must be specified. Generally, it is only practical to achieve formwork of about half the standard tolerances, regardless of methods. Concrete tolerances must be accounted for in the connection and interface with finishes.
The construction process should include a mock-up of all the important exposed surfaces. The mock-up should accurately reflect the specified construction in the formwork, joint treatment, concrete material, placing, and curing. The same personnel who will construct the building should build the mock-up. When a successful mock-up is built, it will establish the level of quality for the building. In addition, be sure to conduct a preconstruction meeting to review requirements and obtain the contractor’s input. Issues should be proactively addressed before they become problems cast in the concrete.
The construction process must include concrete placement methods that prevent segregation and provide adequate vibration for consistent consolidation. Even in the best of projects, some defects will occur. Repair methods should be devised and tested in advance by experienced personnel. It may be advisable that all visually significant repairs be inspected and approved by the architect before repairs commence.
Many of the practices required to obtain quality exposed concrete are the same as for any quality concrete work. With some extra attention to detail by the design and construction team, the concrete structure can become an important element in the design composition.
Robert Fry, P.E., is senior vice president for Datum Engineers and has 34 years of experience. Datum Engineers, founded in 1937, is a Texas-based structural engineering firm with offices in Dallas and Austin. For more information, visit www.datumengineers.com.