The Marble Test

The marble test

The marble test consists of placing an ordinary marble on the floor or countertop and then allowing it to roll if it is so inclined. Some people place a great deal of meaning in this little “test.”

Reality

The reality is that the top surface of a slab-on-ground foundation is never level when it is first constructed. The way slab concrete is finished and the way concrete cures make a level slab a virtual impossibility. If you simply look at a slab that has just been placed, you will immediately see areas that have puddles of water and areas where there is no standing water.

How far out of level can be expected?

How far out of level can a newly placed be? According to the American Concrete Institute, the levelness construction tolerance for two points 10 feet apart is 1.25 inches. That is a slope of more than 1%. This may sound small, but slopes this size can be felt by most people and a marble would roll on a smooth concrete surface with this much slope.

Slab deflection

Consider also that if the concrete floor bent within the design criteria it could create a slope of 1-inch over 15-feet. that comes to a total floor slope of 1.54% assuming they add together which is a possibility. This much slope is likely to result in a rolling marble even if the surface is moderately rough.    

    

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