The desirable appearance of concrete texture as imparted by form liners, rustications, chamfers, reveals, and exposed aggregate, and the undesirable texture resulting from tie holes, leaking form joints, fins, or bug-holes, are highly sensitive to lighting conditions. Under harsh light directed straight at a concrete wall, most of these textural features are washed out. The same wall will have a very different appearance when the light comes directly from the top or sides, parallel to the wall surface. Side light highlights and magnifies any projection out of the plane of the wall and puts shadows behind those projections and into any depressions. As a result, acceptability of texture can depend on time of day, season (sun-angle), and construction lighting versus permanent lighting. There is nothing like a low-angle spotlight at night to reveal concrete’s flatness and surface texture.
Clarifying owner expectations
When your significant other says, “Let’s paint the living room to match the dining room,” she/he is using the very helpful and clever device of an architectural reference standard. Before you agree to perform the work (as if you had a choice), you know exactly what you are getting into.
Likewise, one of the most effective ways for the design team to clearly define expectations is to point to one or more existing structures and say, “Give me one exactly like that.” Under the heading of “Architectural Concrete Reference Standard,” Section 6 of ACI 301-10 states that “When specified in Contract Documents, surface quality and appearance shall match a reference sample or portions of existing structure designated Architectural Concrete Reference Standard by Owner.” This goes a long way toward clarifying the owner’s intent, but be sure to inspect the designated reference standard carefully, and under several different lighting conditions. Look for not only color and texture, but also for the variation in both.
A close cousin of the owner’s reference standard is the field mockup that is prepared after the contract price has been established. The mockup is the opportunity to demonstrate your interpretation of the owner’s expectations—the architect and owner inspect this unblemished sacrificial offering and declare it worthy or not. But keep in mind the approved mockup will be the standard by which all the rest of the concrete will be judged. Depending on site location and season, it could be very difficult to accurately match the appearance of all portions of the finished structure to the mockup regardless of their actual physical similarity.
Kenneth C. Hover is a professor of civil and environmental engineering and Stephen Weiss Presidential Fellow at Cornell University, Ithaca, N.Y. He is a former president of the American Concrete Institute. This article originally appeared in Concrete Construction, TCP’s sister publication.
Mockups: Everything you need to Know
ACI 301 allows the reference standard to be “a sample” rather than portions of a real structure. The sample is provided by the owner as part of the contract documents and helps the bidder understand expectations. A close cousin of the owner’s reference standard is the field mockup that is prepared after the contract price has been established.
The mockup is the opportunity to demonstrate your interpretation of the owner’s expectations. The architect and owner inspect this unblemished sacrificial offering and declare it worthy or not. After what may be a number of iterations, the accepted mockup stands as a clear statement (cast in concrete) of what it will take to make the owner happy. If the contract is design-build or time and materials this process makes perfect sense, but if the contract was based on a hard-bid, post-contract can be an uncomfortable time to finally figure out what the owner wants.
When it comes to building the field mockup, ACI 301 has helpful and practical requirements that should be reviewed by the designers, contractors, and materials suppliers. But there are a few hidden dangers in the mockup preparation and approval process.
First, even though ACA 301 wisely requires multiple lifts, it would be rare for a single mockup panel to be large enough to reflect the normal batch-to-batch changes in color of concrete-making materials, or shift-to-shift changes in personnel and technique that will show up in the real building.
Next, the mockup is normally fixed in position on the jobsite, with one particular exposure to sunlight and the elements. Depending on site location and season, it could be very difficult to accurately match the appearance of all portions of the finished structure to the mockup regardless of their actual physical similarity. Remember the million-dollar efflorescence story? (See main article.) The approved mockup was actually shielded from the weather (in a chapel-like enclosure) while the real rained-on concrete started to effloresce. Of course the exterior production concrete did not match the mockup.
Finally, beware the temptation to overdo the mockup. Early in the project you may be rightfully eager to demonstrate skill and to justify the owner’s confidence, and in so doing the mockup is sometimes a piece of art in itself. You may assign the most skilled craftsmen to build the form; install the rebar; and to place, consolidate, and finish the concrete. But keep in mind the approved mockup will be the standard by which all the rest of the concrete will be judged.
On a recent project the architect and owner were disappointed with the color variation in interior basement walls. Upon investigation of this dissatisfaction it was discovered that no concrete was designated as architectural, and there were no specifications for color or color variation. But there was a required mockup and the basement walls did not match the absolutely beautiful mockup. The rest is history.