Q.: We are slipforming some 80-foot-high lime silos for a sewage treatment plant and the sewage treatment authority wants the appearance of the concrete to blend with the nearby natural rock of the environment. We want to do this by painting but the owner insists on integral color in the concrete.
We don't think we would be able to keep the concrete color uniform. The sliding is to be done during summer at about 120-degree-F daytime and 100-degree-F nighttime temperatures. The mix is designed for 4-inch slump but specifications permit slump up to 6 inches. We expect that as the job progresses we will probably have to change the amount of retarder, ice and water between day and night conditions and we think this will add to our problems with color and make it impossible to keep it uniform. Our concrete supplier agrees. Do you have any advice?
A.: The variables inherent in the slipforming operation--changes in water-cement ratio, temperature and quantity of retarder--combined with inevitable changes in mix proportions and rate of slide, all foretell nonuniformity of color. Furthermore, there is no way to compensate, from hour to hour or shift to shift, for these differences, because the final color won't be known until the concrete has been cured and dried--a matter of weeks, at least. If you can't get the owner to give up the use of integral color, you might ask the color manufacturer if he is willing to make a statement for the owner about the limitations of color in such an application.