Opinions on Large Concrete Decks?

[jimmy]

LOL! What kind of incompetent clowns did you hire?

[Mark and Gloria Hagwood]

There are actually two kinds of "joints" in flatwork. The one that is either sawed into the slab after curing, or troweled in during the pour controls where the slab will crack when it cures, or contracts and expands during temperature changes. The cuts or troweled joints make a deliberate weak point in the pour. The wire mesh embedded in the slab is there to hold the pieces together and (hopefully) keep any shifting to a minimum.

The other type of "joint" is to protect an adjacent structure from expansion of the slab, regardless of where it cracks. This is the asphaltic material that is inserted between the foundation and slab, for instance. As the slab expands, the joint can compress, saving the foundation or wall beyond.

Personally, I would be quite hesitant to pour a large slab against a foundation. . . even using the asphaltic expansion strips. As someone mentioned, a stone or concrete can get into voids in or below the strips and provide a direct pressure between the expanding slab and the foundation. In my own home, we had a large patio, but separated it from the foundation with a 12" strip of colored gravel. It looked decorative, and prevented any contact between the slab and foundation.

I also very strongly recommend that ANY slab poured against a foundation not be tied to it with rebar or wire if you live in frost country. The slab will get moisture under it, and when it freezes, it will heave upward. I had a neighbor who once told me his front door wouldn't open. When he and I took a closer look, his entire front area had been heaved about 2" upward by his front porch slab.

Mark

[Mark and Gloria Hagwood]

The material is typically purchased in pre-cut 3-1/2" wide strips, since a 4" slab is typically formed with a 3-1/2"

2X4. Screeding the pea gravel is usually done before the strips are inserted, since the concrete is usually what holds them in place. My guess would be that about 95% of the pours have areas where the pea gravel is lower than the 3-1/2" strips either because of sagging in the middle of the screed board, failure to insert the strips fully or having them pushed up as the concrete is being poured or floated. It doesn't take much concrete-to-concrete contact to transfer the pressure directly to the foundation. All a good contractor would have to do would be to take his foot and make sure that there was pea gravel under the strips to prevent concrete from getting under them.

You are right about it being caused by poor workmanship, but that's just what happens in real life. The best way is to use gravel to separate the pours, when it comes to patios.

Mark

[Norminn]

Have just read a lot of newsgroup stuff about problems people have either from DIY jobs or poor contractor work. Not that funny, IMO.

[Norminn]

The most recent concrete work I've seen was repairs to our seawall, and work done by a neighbor on their seawall. Expansion joints were a consideration in placing the new cap on the seawall.

Got a sidewalk out front with long spans between expansion joints, and cracks across the middle ... the expansion joints don't make a difference?

The OP mentions a new house, which suggests excavation and grading. If the ground has been dug and replaced, compacting the soil under the patio is not a consideration? Making sure there is a slight slope away from the house is not a consideration? Silly me :o)

[Jenniferlpaulsen]

I am indeed talking about a concrete DECK, not a patio. The deck will be off the ground about 12 feet. We are having it engineered so that the deck will be well supported, I'm just worried about the appearance.

How far apart should the expansion joints be?

Thanks for all the advice!

Jennifer

[Mark and Gloria Hagwood]

Well, if it's off the ground on three sides, then I'd not worry about expansion problems crushing a foundation. Frankly, I'd prefer a concrete deck to one of wood or wood substitute. To keep it looking consistent with the railing, consider banding the concrete with whatever you use for the railing, such as Trex or another wood substitute. Also, consider having diamond sawed joints, such as every 6', instead of troweled in ones. They look neater, IMHO. I'd also consider using the fiberglass reinforced concrete and also look into air entrained concrete.

Sorry for the misunderstanding. When someone says concrete and deck in the same sentence, my mind automatically says that they really mean a patio.

Mark

[Edwin Pawlowski]

Have you ever been to a city? Philadelphia, New York, Boston, Chicago? Ever look at how much concrete is right up against how many thousands of houses? Not one has fallen down from lack of an expansion joint that I'm aware of.

[Hogwild]

Oh.

That sounds like a very nice deck. With all the engineering that will be required - I'm sure thermal expansion will be fully incorporated in the design.

IMO I think it would be better than a wood deck for a few reasons.

It will provide a rainproof area below.

No splinters. Low maintenance.

Good luck.