The devil’s in the details
Let’s start with the color of the concrete, and to make it easy, let’s make it gray. One popular building-supply store advertises paint in 195 shades of gray. Which “gray” is required, and how many variations in shades are acceptable? State- of-the-art concrete technology allows for color matching given a fixed set of input ingredients, but how will the color change as the raw materials change or as haul time, mix time, and curing vary? Since the color of portland cement paste generally gets lighter as the water-cement ratio increases, how will control of water content affect the color?
To stick with the paint-color example, if the owner could live with concrete color that matches any one of perhaps 10 shades of gray, would he be equally happy to have variations of these same 10 grays throughout the building? Tougher yet, will he be happy with those 10 shades of gray on a single wall? Color variation might be acceptable if the shades are randomly or artfully distributed, but if those same shades are separated by construction joints or lift-lines, it may be unacceptable.
To make concrete color even more complicated, human perception of color is strongly influenced by lighting and the color of the background or adjacent surfaces. A gray concrete wall appears darker against a bright sky and lighter against a dark sky. Gray concrete looks almost white when lit by the setting sun against dark clouds. The color of exposed interior concrete can change dramatically when viewed under temporary construction lighting than under the permanent lighting. Over that same time period the concrete surface has usually dried (and lightened), and contrasting colors (floor treatments, for example) have been installed.
On exterior concrete the color changes with age, for better and for worse. One advantage of white or gray concrete is that the “whitish” efflorescence that may begin to appear early in the life of the structure is not readily noticeable. But when the owner has opted for a darker concrete color, minor efflorescence sticks out like a sore thumb. For darker shades, concrete appearance can go from acceptable to unacceptable depending on the duration of the project and frequency of rain; and in a rare perversity of nature, the colder the rain, the more efflorescence.
But in areas affected by acid rain, long-term exposure can not only dissolve the efflorescence, but generally mutes and blends the concrete color. On one project in the Northeast, the owner withheld $1 million from the contractor on an architectural concrete building because of efflorescence and color variation in the specified earth-tone concrete. Nature and time have accomplished what concrete technology could not: a uniform (although lighter) color, and very little visible efflorescence. A beautiful building remains; the contractor is no longer in business.
It is similarly difficult to specify and evaluate the texture of concrete. An architect required that the contractor create six sample sidewalk panels for the broom finish for a pedestrian mall. After deliberating, one finish was selected. “Easy enough,” claimed the contractor. But the real challenge was achieving the same broom finish with the same straightness and depth of striations every day from July to December, with variable temperature, wind, sun, humidity, and setting time.
For anything other than an absolutely flat, polished surface, “texture” implies variable depth of the concrete in and out of the plane of view. The highway industry has multiple ways to put a number on the surface texture of pavements. The building industry has floor flatness and levelness criteria. But the owner of an architectural concrete work of art will often judge surface texture on appearance. This has all of the problems of evaluating color with the addition of the third dimension.