Is Pyrite causing your garage floor to crack?
There are many reasons for a concrete garage slab to require repairs. Settlement cracks in a slab, frost heave, expansive soil damage and shrinkage cracks in a slab are just a couple of the most common reasons. In Quebec we have another serious issue in older homes located around Montreal, Laval and the South Shore which may have Pyrite in the backfill under their basement slab.
Pyrite or "Fool's Gold" is a type of mineral that can sometimes be found in the gravel used under garage floor concrete slabs that were created in and around the GTM (Greater Montreal Area). The problem occurred because there trace amounts of pyrite in the granular fill used as a base for the concrete slabs was used. When exposed to water, oxygen and a certain bacteria, the Iron Sulfide in the granular mix produces Ferric Iron that continues to be produced until one of the elements is exhausted. The concrete slab of your garage floor traps the elements in the Pyritic backfill and eventually the excess Ferric Iron heaves upward until the slab lifts and cracks.
The only way to repair cracks caused by pyrite under the garage slab is to 1st remove the concrete slab, then remove the existing gravel. Once the removal of the old concrete floor and gravel is completed, add new gravel and pour a new concrete garage or garage floor. If someone replaces the slab without replacing the gravel it is possible the slab could crack again.
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