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Heading: 1.4Transparent Aggregate
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Dear Technical People on

Our company manufactures chemicals for the cast concrete industry. One of our customers is experiencing a strange phenomenum. They cast pavers in polyurethane rubber moulds. Upon demoulding we find that there is a spot associated with each stone below the surface which is darker than the surrounding area. We thought that this was due to differential drying out of the surface and that once the whole tile was completely dry these marks would disappear. However the spots remain. Sometimes the spots are lighter than the surrounding area!

I have been told this is called "Transparent aggregate " but no explaination for its occurance has been given. We call it Leopard spot !

Would you be able to assist me in any way giving an explaination of this even which now seems to plague us more and more.

Best regards,

John Souter
Redec Chemicals
Cape Town
South Africa

Tania Hudson

Publication date: August 1, 1998

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.

Alan Kirby
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