Question: What causes aggregate shadowing or mottling on the form side of precast concrete? How can you minimize it?

Answer: The term "aggregate transparency" is sometimes used to describe this problem. The formed surface has dark areas similar in size and shape to the coarse aggregate particles. During vibration, fine particles, including cement, form a very thin layer over the aggregate in the narrow gap between the coarse-aggregate particles and the form face. Possible causes include:

  • Low sand content in the concrete
  • Forms that are too flexible or of nonuniform stiffness
  • Prolonged external vibration or use of vibrators with too much amplitude
  • Dry or highly absorptive coarse aggregate
  • Inadequate curing
This discoloration is most apparent on smooth, off-the-mold finishes. The Precast/Prestressed Concrete Institute's book Architectural Precast Concrete, second edition recommends using light-colored aggregates and white cement to reduce the effect.