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Aggregate transparency can occur in formed concrete when there is segregation of fines and associated water between the form-face and the coarse aggregate. It appears as mottled dark areas similar in shape and size to coarse aggregate.
To reduce the liklihood of its occurence, stiffen the formwork; possibly increase the sand content and use internal ´poker´ vibrators rather than external ´clamp-on´ ones.
Acknowledgement: Concrete Society
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