We are currently working on a job that requires integral color for a sculpted rock facing, but the plant close to the job does not supply color. The DOT we are working for has informed us they will not approve the use of retarder in the mixture so we can order the colored mixture from a plant that is 50 minutes away from the site because they are worried about long-term strength of the material decreasing. Do you know of any literature that we can provide to the DOT regarding effects of retarder on 28-day strength of shotcrete?

Shotcrete is concrete. PCA’s Design and Control of Concrete Mixtures, 16th Edition states: “In general, some reduction in strength at early ages (one to three days) accompanies the use of retarders. However, increased long-term strength may result from retarding the initial rate of hydration. Excessive addition rates of a retarding admixture may permanently inhibit the hydration of cement.” Thus, if you closely follow the admixture manufacturer’s recommendations for dosage, you may get higher long-term strength than non-retarded mixtures. You may also consider use of the newer hydration control admixtures that essentially stop hydration until activated, and can theoretically put the concrete “to sleep” for up to 3 days.