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Color problem with last of multiple pours- why?

Question:

What caused this last pour of integrally colored concrete to be off-color?

Answer:

Asking a few basic questions turned up the following. This was the last of multiple pours, which ran over many weeks. All were tightly controlled for slump and color. A clear water-based curing compound was used on all the slabs. The last slab was rushed a bit since the deadline for opening this area had already passed.

Looking more closely at the other areas of the slab revealed a few blotchy gray areas like the large area in question. This led me to believe that possibly the curing compound was covering the true color. In an effort to save time and money from costly rip out and replacement of the slab (which was the general contractors' first and only recommendation), a simple chemical stripper was used to remove any surface coatings. Sure enough, once a small test area was stripped, the correct color was revealed.

The curing compound had been applied too early and at too high a coverage rate in an effort to save time and cure the concrete faster. This actually backfired by trapping too much moisture, causing the curing compound to haze and turn white. The use of a colored curing compound would have helped in this situation, even at the higher application rate. More information on proper curing of concrete is available from your local ready mix association or from the American Society of Concrete Contractors.

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Author Chris Sullivan, ConcreteNetwork.com technical expert and vice president of sales and marketing for ChemSystems Inc.

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