View Full Version : OT making concrete spans for a deck
07-16-2009, 03:04 AM
I'm just tossing this idea around, I'm not stuck on it - yet. I'm considering replacing my front steps since they are starting to sink beyond what I'm comfortable with, plus all the support wood is pretty much rotted now. I would be replacing them with a deck which will be about 20 ft long by 7 ft or so wide, which would fit well in the area. I'm not looking to build a typical deck, just replacing the steps with its smallish landing with something appropriate to fit the corner area there. I'm considering making some concrete spans that would be about 7 ft long and about 3 ft wide- 7 of them would be needed in all. I will obviously need a support wall at the house, and another 7 ft away, with proper footings of course.
Each of the spans would have reinforcing mesh in the casting, so that becomes mostly an engineering problem to size the thickness of the concrete, plus the size of any ribs that might be cast into each span. Considering the cost of creating this structure using pressure treated decking material and underlying support structure, it appears it might be cost effective. Once done, the surface should be virtually maintainance free.
Any and all thoughts welcome.
07-16-2009, 04:35 AM
You do realise that at 4" thick, which is about the minimum which would work, these would weigh nearly 1/2 ton each?
07-16-2009, 05:20 AM
Why not replace it with the engineered plastic decking boards. Two of my friends have used the materials with excellent results. The installations were relatively easy, and they look really good.
One of them said the sawdust was a bit irritating, but that's easy enough to avoid with a mask and safety goggles.
Once done, the surface should be virtually maintenance free.
Probably not. With the freeze/thaw cycles you get it will undoubtedly begin to flake and break up in the first winter unless coated with something impervious to water. As soon as you have a coating you have a maintenance issue.
The idea to use precast concrete spans may seem attractive at first glance but to me it immediately starts my *This is maybe not a good idea and could turn into an expensive disaster* alarm ringing loudly.
Casting the pieces will require a very rigid form system that can be assembled and disassembled for each piece. You will need rebar, not just mesh to give enough strength to prevent cracking or even collapse. Concrete has next to zero tensile strength which is why span sections are made as prestressed sections that ensure the entire cross section is under compression by either internal cables of bars that are stretched and locked at the ends by cast in fixtures. You could do that using long sections of allthread rod which is then tightened via nuts and then welded and ground flush at the ends but it would be an engineering crap shoot.
I don't like the plastic wood decking products. The ones I have actually stood on are too flexible and limber. What I have done and still do is to use pressure treated wood for the supports only. The rest of the deck is treated before assembly using a 50/50 mixture of zinc napthanate mixed with oil based wood stain. This soaks in very well and will provide, in my direct experience, a decking that will last at least 20 years as long as the surface is restained with plain oil stain every few years. Zinc napthanate is particularly effective and deadly to the moss and fungi common in humid climates while not being as toxic as copper napthanate. As well, the zinc compound is water clear and imparts no colour to the wood which is why I mix it with stain to act as a marker so I don't miss any sections.
I use green epoxy coated deck screws for all fastenings. I do not build with nails. All joints between support structures and the deck are made using metal fixtures, angles and brackets that I make myself from substantial material and then paint with high zinc metal based primer followed by a colour coat.
07-16-2009, 07:00 AM
My wife the unemployed architec used to specialize in concrete post and beam construction. If you used pandecking and poured the span in place a 12" rebar mat it might work. Of course you would be using 6 sack mix and a 21 day wet cure
07-16-2009, 08:04 AM
For the amount of trouble and expense involved I would sooner pour a chain wall,back fill with rubble then form and pour a homogeneous slab for a deck.It would actually take 1/2 the amount of concrete and only 3500# mix.
Of course no matter what you do remember to slope the surface 1/4" per foot away from the house for drainage.
07-16-2009, 08:47 AM
Use ipe wood decking and framing. Also called ironwood, same family as purpleheart and a few others. The price scares you but will last for ever with no maintaince. Turns silvery after 1-2 years indircet sun. Do not use for a boat, sp.gr to close to 1.
07-16-2009, 09:09 AM
Hmm, I have a cistern that is 10'x20' with 10" thick walls and I poured a 5" thick cap with rebar in a 12" square pattern about 2" off the bottom. I used high strength concrete with fiberglass in the mix. I did that in the fall of 2004 and it is still in fine shape. I have considered using a sealer on it to seal the concrete from freezing problems.
It was a real trick putting the supports in to pore the concrete but the real problem was getting the wood down and out the manhole in the top.
07-16-2009, 09:10 AM
The concrete idea sounds "non-optimal"........ I have somewhat the same reaction as Evan... and noting the address in B.C., you will definitely have freeze-thaw issues.
Yes rebar, probably bent/welded in hooks at each end, yes sealant, and you still may get cracking and rust spalling.
The plastic wood substitutes SOUND nice, but have issues.
1) they get REALLY hot in the sun, and transfer heat to your feet really well. Not barefoot surfaces. Of course at your 55 degree or so latitude that may be less important than here at 35 deg.
2) The fire department tends to call most plastics "solid gasoline".... although obviously flame retardant helps that. I don't know that I'd want to have a potential escape route made of a material that will either burn, or at least melt, in case of fire.
07-16-2009, 12:21 PM
Wood is really really easy to build with and looks great.
Don't be discouraged about concrete lasting. Properly placed and finished, it can last for decades. I think the pan decking would be ideal. First, you place the concrete "in place", no moving. The pan deck is a system, between it and the concrete gives you a composite slab, used very often. A 7 foot span is quite doable. Rebar should be used, not mesh.
Use an air-entrained concrete mix, not too much slump, less than a 4". The key is to finish it properly. The concrete needs to be floated then troweled, when it is ready. The biggest cause of surface failure, is using water while troweling just to make it easier. It dilutes the concrete surface and that is what will spawl off. Proper curing is also a must, but easy to do. Just cover it with plastic sheeting to keep the moisture in the concrete so it hydrates properly, not too fast. 21 days is a bit much, but certainly won't hurt a thing.
Don't use salt on it to melt ice, or calcium. A sealer once thoroughly cured won't hurt a thing.
Oh, cut in a control joint half way the length. It should be at least 1/4 of the depth. That will control the shrinkage cracking. You can caulk that joint later, which will keep water out so it doesn't freeze inside the joint.
07-16-2009, 01:59 PM
Thanks for your thoughts. I'm going to look into some of these things.
I looked at the idea of using existing square concrete slabs and arranging them on top of joists to crease the deck surface, but they aren't made to span anything. Even a two foot square one needs full support below it, and they will still crack. Then there's the issue of water going between them and rotting out the support structure. I thought maybe lay a strip of that thin white foam over each joist before the slabs are placed, but I don't know how well that would help keep the wood from moisture damage.
I could make my own 2 ft square slabs with mesh embedded- maybe make a mold with a slight pattern to become the decorative surface-
Need to think more about all this.
Mesh doesn't stop concrete from cracking. It only stops it from falling apart when it cracks. You said it yourself, "but they aren't made to span anything."
The only way to span 7 feet with concrete is to use a system designed to deal with the tensile stress at the bottom of the spans. Concrete doesn't bend worth crap. I would move on to plan B...
07-16-2009, 10:53 PM
After checking out a few places today and pricing it out, I'm moving on to plan F for now. Forget it. I hope my existing steps don't collapse while someone besides me is on it.
Looks like it will be wood after all, and still more dollars than I want to spend right now. As who ever they are seem to be saying, it is what it is.
07-17-2009, 12:46 AM
I have been around concrete structures of varying design for years, and demoed thousands of tons of concrete- from bridges to buildings.
Having said that, I think it is wise to move to a different product. Having said that, if You do seriously ponder concrete construction I would look in the direction of either fiberglass rebar or stainless steel. I would also think about foam forms to remain for insulation and support.
If You were in the mood for adventure , You could fabricate the deck out of structural steel tube and trench plate. With enough effort it could look like a concrete slab on post and beam supports that would last forever.