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Trap rock and other materials such as quartz, emery, and metallic aggregates are often used in dry shakes to improve floor wear resistance. The term trap rock doesn't describe a specific class of rock. It's a general term for various dark-colored, fine-grained, igneous rocks such as basalts and diabases. These materials are exceptionally hard and dense, with a low absorption.


Hard, dense aggregate resists abrasion better than cement paste, so increasing the amount of aggregate at the surface improves wear resistance. That's the reason for using trap rock, either in a dry shake hardener or topping mix.

In a dry shake application, well-graded trap rock aggregate blended with cement is floated into the surface during the early finishing process. This increases the aggregate-cement ratio at the wearing surface and lowers the water-cement ratio.

Toppings containing trap rock aggregates normally cost more than conventional toppings using local aggregates. But they provide better resistance. Finishers screed the base concrete to a level 3/4 to 1 inch below finish grade, then give it a rough-textured finish. After the concrete has cured, workers mix the topping at the jobsite. Finishers use rollers to compact the topping before floating it.