Tech-Notes on Architectural Precast
There are several factors that affect the ultimate color of any colored concrete product. Variations in raw materials, mixing, curing, and finishing can all result in inconsistent color in the finished product. This tech–note will cover cur-ing and finishing processes in precast and poured concrete. Many procedures that help produce consistent and uni-form colored products seem to be common knowledge. It is, however, important to be aware of them when working with color.
Prior to discussion of curing, there are three important factors about placing that need to be addressed. When placing colored concrete, absolutely no retempering may be done. This would completely eliminate all control of the water/cement ratio and would change the end color. Secondly, two different colored mixes can not be cast into a single panel without waiting for the first to set completely before pouring the second. Even with demarcation features, it is extremely difficult to keep the areas totally separated. Finally, the temperature of the concrete and the forms must remain as consistent as possible throughout the job. The temperature of the concrete will affect the speed of curing and the finished color.
Consistency during the curing process is crucial when producing colored units. It is very important that the rate at which the water leaves the concrete is constant from piece to piece and within one piece. Once a piece has been cast, and after an initial preset, the piece should be covered with plastic or insulated tarps to insure gradual water exit. Generally, these covers are left on until strip time and then the piece can be put in the yard to continue curing if it is warm enough outside (approximately 50°F). When heat is necessary to promote curing, radiant fin heat works well under the beds. After the piece is stripped, it should remain at room temperature at least an additional twenty-four hours, and then can be transferred outside. The longer the piece is inside, the more curing will take place, and the more uniform the color will appear. One particular thing to watch with colored concrete is leaving pieces in the forms for an extended period of time. It is important to strip all pieces after approximately the same cure time. This means that leaving a piece in the forms over a weekend or holiday may cause a problem.
Once a piece has been stripped, the sooner it is patched or sacked the better. It is much easier to match the patching material to the original when the piece is not completely dry. Patching should be done with the same pigment, but the addition rate should be altered to compensate for the fact that a patch made with plain gray cement will cure dif-ferently than a gray precast piece. With dark pigments, a lesser quantity of color should be used in the patching mate-rial. With lighter pigments, experimentation will be necessary because there is a possibility that the pigment addition may actually need to be increased to overcome the grayness of the cement. With white portland, less pigment will almost always be necessary for patching. Typically the addition rate for patching materials would be no less than 60% and no more than 140% of the pigment added to the original precast piece.
The last element of production that needs to be controlled is the finishing. There are some finishes that do not work well with coloring and others that work well, but have some peculiarities that are important to be aware of. Any diffi-culty that occurs in producing a consistent finish with gray panels will be magnified with color because more attention is paid to the color and the color consistency. In this tech–note we will examine four different finishes; smooth, sand blasted, acid etched, and exposed aggregate.
An off the form finish is extremely difficult to produce consistently. Even with sacking, it is nearly impossible to cover all the color variations that occur on the surface of any piece. Any type of finish that produces texture will appear more uniform than a smooth finish. If a smooth finish is necessary, some sacking will be almost unavoidable.
Sandblasting is a good finish to use with coloring. Sandblasting will take the sheen off the aggregates because some of them are actually being fractured, and will tend to even out the color. Because any color variation is usually in the cement paste, which only occurs on the surface, any depth of sandblasting will improve the color consistency.
Acid etching is one finish that can produce peculiar results. Sometimes the color may change during etching. Some colors will not change at all. Therefore, it is extremely important to run a sample with the exact color and the exact acid that will be used in production. It is also important to remember that all patching must be completed before the etching process has begun. If the color is going to change, it will normally change consistently. Therefore, once the relationship between the initial color and the end color is established, production can proceed.
An exposed aggregate finish is very compatible with colored pieces. If the color used is similar to the predominant color of the aggregate, producing good exposed aggregate panels is actually easier when coloring is used. This is true because if there are bug holes or other bald spots on the panel, they are not as noticeable because they tend to blend with the background. Also, because most of the panel is aggregate, color variation is always less noticeable.