It's cracking because it's only 3.5" thick!

Paolo Benedetti founder of Aquatic Technology Pool & Spa
Vanishing Edge Pool, Infinity Edge Pool, Perimeter Overflow Pool, Slot Overflow pool, Knife Edge Pool, & Disappearing Edge Pool expert

I recently was involved in a construction defect case, where among the many flaws was a substandard pool deck. The flagstone veneer was coming loose all over the place & the grout was cracking.

When I dug back a part of the lawn at the edge of the patio - the truth revealed itself...

The concrete slab under the stone work was only 3.5" thick! To make matters worse, the concrete contractor used 4-5" of crushed dolomite rock for a sub-base. Dolomite rock is very soft crystalline rock that has virtually no load bearing value. And, placing such thin layer of such a fine sub-base allowed the underlying expansive soils to transfer their energy directly into the masonry above.

To add insult to injury, there was a geo-technical investigation (soils report) available to the concrete contractor. But he didn't bother asking if one was available. Now that it has been thrown back into his face, he wish he had asked for it! The soils report made recommendations as to how much of the expansive top soils needed to be removed, what was to be imported, and how the sub-base was to be installed. As it turned out there was supposed to be "24" of compacted 3/4" drain rock" placed under the slab. The report specified "a minimum 6" thick slab with #4 rebar on 12" centers, topped with #10 wire mesh, tied to the surface of the placed reinforcing steel. Reinforcing steel to be supported on the sub-base with 3" concrete blocks or plastic chairs."

In his infinite wisdom, the concrete contractor used #10 wire mesh as his reinforcing steel. And because it is so flimsy and cannot be supported on dobbies (concrete blocks), they left it laying on the sub-base. Lots of good it does there! He stated that as they poured the concrete, he "hooked the wire & lifted it into the center of the slab." I made him a gentleman's wager, that the wire was consistently in the bottom 1/4 of the slab, and would be found mostly on the bottom. He did not realize that once the workers begin to tamp & finish the concrete, the wire settles back down to the bottom. At best, a cross section of the wire in the slab looks like a "wave" throughout the slab.

Then he violated the building codes that specify the clearances from the reinforcing steel to earth and reinforcing steel to the surface of the concrete. The minimum clearance from earth to the BOTTOM SIDE of the reinforcing steel when the concrete is cast against earth is 3 inches! The minimum clearance of the TOP SIDE of the reinforcing steel to the surface of the concrete is 2 inches! So, had he used #4 sized (1/2" diameter) reinforcing steel, the slab would have had to be 6" thick! Since the #4 bars would be laid out in a checkerboard fashion, they cross over each other. This means that the steel is in some places 1" thick (1/2" + 1/2" = 1" thickness of steel at the intersections). 3" of concrete below the steel + 1" steel thickness + 2" of concrete above the steel = a 6" thick slab.

So, any concrete contractor who forms concrete that is to be poured against the earth (that includes the sub-base material) with 2x4 limber is in violation of the building codes (IBC, UBC, or CBC). And any client that contracts for a 4" thick slab is being cheated, as a 2x4 is actually only 3.5" high not 4"!

The stone veneer was mortared directly on top of the concrete slab. As the slab cracked, the cracks migrated through the stonework. The grout cracked & came loose, as did the stones.

Decks of this type of construction need to be built to protect the stonework from failure. The concrete slab should have had thickened edges and turn down footings around the perimeter. There should have been a "crack control" or "cleavage" membrane installed on top of the concrete slab. This membrane prevents cracks in the concrete from migrating through to the stone work - manifesting themselves on the surface.

If ground water or poor site drainage was present, a vapor barrier should have been placed beneath the concrete slab. The crack control membrane could have also been upgraded to a waterproof membrane, adding a layer of protection against efflorescence on the surface of the stone (from water beneath).

The final mistake, which actually caused the ground to start moving in the first place - was that the concrete contractor did not install any surface drainage. Instead, he talked the homeowner into sloping the decks back to the planters & allow the water to run off into the landscaping - saving the homeowner money on a drainage system. Not a good idea, when expansive soils are present. The water migrated back under the concrete slab through the finely crushed dolomite, where it soaked into the soils. Being expansive, the soils swelled, causing movement & cracking of the concrete slab, and cracking & separation of the stone veneer.

The turn-down footing at the edge of a concrete slab, is designed to strengthen the edges of a slab, prevent "slab curl" as it cures, and prevent water from migrating beneath the slab. Surface drainage should always be installed to direct water off of the decks. If the water is being directed to the side of the deck, then a trench drain or french drain needs to be installed to redirect the water away from the sub-base.

Since he failed to inquire or follow the soils report recommendations and relied on his inferior construction practices (e.g. wire mesh, hooking the wire mesh, 3.5" forms), he will bear ALL OF the expenses of jack-hammering & hauling away the current decks, excavating the site 30+ inches, installing a drainage system, importing the proper base rock, properly re-pouring the concrete decks, and reinstalling the stone veneer.

Again, having (and using) the recommendations in a soils report would have saved this contractor a lot of time & money. Had he followed the recommendations and still experienced these same failures, he would not have been liable for their correction.

Do it right, or don't do it!

Paolo Benedetti - Aquatic Artist
"Creating water as art."™
Aquatic Technology Pool & Spa
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