This is the most expensive mistake in Las Vegas concrete, because it is invisible for years. The slab looks flawless, then starts coming apart from the inside, and by then nothing fixes it.
Much of the valley soil carries sulfates. When water carries dissolved sulfate into hardened concrete, it reacts with compounds in ordinary Portland cement and forms new minerals that occupy more space than what they replaced.
That expansion happens inside the concrete. It pushes the material apart from within, and the slab spalls, delaminates and softens. The concrete was mixed correctly, placed correctly and finished correctly — the cement was simply the wrong type for the ground it was sitting on.
It is slow. A slab poured with ordinary cement on sulfate-bearing valley soil will look perfect for years. Then it begins to spall and delaminate, usually starting at edges and joints where water gets in.
By the time it is visible the reaction is throughout the slab. There is no coating, sealer or resurfacing that reverses it, because the damage is in the body of the concrete rather than on it. The fix is removal and replacement.
Type II cement has moderate sulfate resistance and Type V has high resistance. We specify 4,000 psi with Type II or Type V, selected from the exposure class of your soil rather than by default.
That last part is the point. The correct answer is not always Type V — it is whichever type the soil exposure calls for. Choosing by habit rather than by exposure is how both failures happen: the wrong cement on aggressive soil, or a needless premium on soil that never needed it.
By knowing the soil in your part of the valley, and where there is doubt, by testing. Sulfate content is measured in parts per million and the standards map ranges of concentration onto required cement types.
A contractor working the valley every week has a good working knowledge of which areas run aggressive. One who quotes the same mix everywhere is not making a decision, and on the wrong lot that costs you the whole slab in five years.
Ask which cement type is in the mix and why. A specific answer that mentions Type II or Type V and references your soil is a contractor who has thought about it. 'Standard mix' or 'the usual 4,000 psi' is an answer that does not know the question exists.
It is worth pushing on, because this is the one Las Vegas concrete decision that cannot be corrected later at any price.
A slab built for vehicle loads, not foot traffic.
Dissolved sulfate from the soil reacts with compounds in ordinary Portland cement and forms minerals that take up more volume, expanding inside the slab and causing it to spall and delaminate.
Much of the valley soil is sulfate-bearing, though concentration varies by area. That is why cement type should be chosen from the soil exposure rather than by default.
Type II for moderate exposure and Type V for high exposure, at 4,000 psi. Which one depends on the measured sulfate concentration in your soil, not on habit.
No. The damage is inside the concrete rather than on its surface, so sealing or resurfacing does not address it. The slab has to be removed and replaced, which is why the cement decision matters up front.
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