Most people probably have an inate belief that concrete, being the hard, stone-like material that they see, is free from all problems. Concrete is an excellent building material, but it’s not perfect. The good news is that problems with concrete are avoidable, or at least can usually be fixed.
Dusting, that fine, powdery, material that easily rubs off from the surface of a hardened concrete slab, is one of those problems. It’s called laitance in the conctruction field, and it’s the result of a thin, weak layer, made up of water, cement, and fine particles.
Newly poured concrete is a fairly uniform mass of cement, aggregate, and water. After a period of time, the water and cement react to form the hardened concrete. During this time period, the particles of aggregate and cement are suspended in the water. But the aggregate and cement particles, being heavier, begin to sink, displacing the lighter water which moves to the surface. The result is more water at the surface and the fine “dusts” that the water carried with it. Proper curing and finishing can usually prevent the dusting or laitance.
So what is the problem with it? Laitance a very thin film or cement paste that rubs off. And because it rubs off, any coatings that might be applied to the hardened concrete, would be bonded to the laitance, which . . . rubs off!
But there are cures and shotblasting is one of them. Shotblasting is a very common method that uses steel shot blasted onto the concrete surface at a very high velocity, fracturing off the surface layer of the concrete along with any dirt, coatings, paint, contaminants, and . . . laitance. At the same time shotblasting profiles the concrete surface giving it more bonding surface for coatings or overlays.
Specialized Industrial Cleaning, Concrete Coatings, and Concrete Repair www.blackwells-inc.com
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