Basement slab reinforcement
Not really a “green building” question per se, but too much knowledge and too good a response time for me to not ask here.
My basement slab will be poured in the near future. It is currently planned as a 4000 psi slab with expansion joints. No reinforcement is specified. I have read articles recommending fiber and no wire and articles recommending wire and no fiber…and obviously both would be strongest. The basement is not drive in. It will contain a weight-room, and I’d like to be able to drop 500lbs from ~3′ up onto rubber tiles and not have the floor crack under them. Should I have the floor reinforced? If yes, how?
There will be recycled foam under and around the perimeter of the slab…if that greens it up enough to be applicable…
Thank you!
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Replies
Jim,
It's common to include 6x6 welded-wire mesh in basement slabs, but reinforecement is not required by code. Rebar would be overkill.
And the concrete is likely to crack in any case. I also looked in to fiber reinforcement. If you are planning to finish your concrete floor, it might not be the best choice. The fibers apparently can create a layer of fuzz on top of the slab that might be a problem when staining and polishing.
Our polished concrete floors were placed about 18 months ago. We incorporated control joints into the design, rather than space them equally. We also placed them closer together than is typically recommended. We specified 4000 psi and peastone aggregate, 4" reclaimed XPS over well compacted 3/4 stone and 6x6 wire mesh. No cracks yet.
Thank you. Am I correct in understanding that the 6x6 welded-wire mesh does not necessarily prevent cracking, but is intended to prevent cracked portions of the slab from becoming vertically misaligned? Is this a concern with the slab poured over a drained bed of compacted stone over a solid substrate?
Edit: I should also ask, how effective is it? If I include 6x6 wire reinforcement, can it be counted on to hold a cracked slab on one plane to a degree that would allow installation of a finished floor, for example?
Oversimplifying somewhat: Rebar is needed if you require any significant tensile strength in your slab. Wire mesh will help hold cracks in alignment. Chopped fiber will reduce nuisance cracking. Nothing will totally prevent cracking, but expansion/control joints will allow you to place the big cracks where you want them.
Chopped fiber will make a mess of the slab top surface as poured/screeded. This is not a problem if you will be grinding or blasting the slab surface for a concrete finish, or covering the slab with underlayment for flooring.
Some types of polymer floor coatings (e.g., BASF methyl methacrylate) are flexible enough to prevent cracks (even cut control joints) from telegraphing through to the finished surface. However, having used such a coating in a machine shop, I will tell you that any impact which can spall the surface of naked concrete will damage the coating and also spall the concrete underneath.
You have a requirement which is a bit unusual for residential service: repeated impact resistance. I think I'd use a minimum 4" thick slab with chopped fiber, with a healthy underlayment under those rubber tiles. As a conservative, I'd also use the wire mesh, although if your concrete contractor tells you he/she would prefer to use rebar, you'd be advised to listen. Wire mesh is very, very often pushed to the bottom of the slab, where it's nearly useless, during the process of pouring a slab because it gets walked on.
The single biggest thing you can do to prevent cracking is to keep the concrete very damp for at least a week. More is better. It typically reaches 90% of its potential strength after 28 days. If it dries out, the chemical reaction stops and the concrete is left weak and prone to cracking.
Thank you guys for the input. My contractor did recommend rebar and I will follow his recommendation. I will keep the slab damp. They stripped my wall forms today; would it be beneficial to keep them damp as well? Would it be at all benneficial to hose the walls down a couple times a day, or would they need to be covered in plastic for it to be worth the effort?
Spray some water on there a bit. That should help, especially if the forecast ahead is for dry weather. As to rebar, I would put it just where you expect to have the weight room. The amount of weight you are planning to place on a basement slab is so minute that it is a little absurd to use steel of the basement is not the kind with a garage. After all, the Christmas bulbs, Halloween decorations, spare pantry goods, tool chest and couch (assuming you are going to build a finished portion) won't be exuding any force on the slab. If you compact the gravel well on virgin soil, your settleage will be very minimal. Fibermesh is similar. Wet your concrete after the pour. That is the best way to have good concrete in such a place. If you are using rebar under the weight room, raise it on stools, tie it off in a 12" grid. Make sure it is in the very center of the slab as if it is too close to the top or bottom it is also useless. Don't waste your money in steel where you don't need steel. Place it carefully. Spend your money on things you need. Unless you are Bill Gates using an alias. Then who cares?
Joe thanks for the advice. There is definitely not unlimited money; in fact, the opposite is what motivated me to add reinforcement. The foundation is by far my biggest single expense and the only thing I plan to use a contractor for, so I want a great product. The contractor did agree to let me install the rebar. Actually, he offered to sell it to me at cost, deliver it, throw in the ties, and let me use his tools for install - so for under $1,000, I get a grid on 2' centers under the whole 28 x 50 slab - which I feel is generous of my contractor and a good investment if only in peace-of-mind. Now I just need to hope my sons end up big enough to need the reinforced weight room floor and college scholarships make it a fantastic investment.
Jim, the more water the better. Contractors almost always underestimate the importance of keeping the concrete fully hydrated for as long as possible, then they say that cracking is inevitable, which is not true.
The traditional approach for a slab is to cover it with burlap and mist heavily a couple times a day, or more if it's hot/dry. Plastic would also work; you lose the ability to easily add more water, but capture evaporating moisture, so it probably equals out.
For the walls, the forms should have been left on for a week, but everyone likes to strip the forms after a day or two at most, because it's easier to strip when the concrete is still green and they are in a hurry to get building. Burlap would help but I might go with plastic, after a very heavy wetting.
As others have said, rebar is probably overkill for a basement slab, but it won't hurt anything so go for it.
(I first learned about concrete from my dad, who was a highway engineer and built long stretches of interstate with very large slabs of cast-in-place concrete. They have lasted 40+ years without cracking.)
Lot's of good advise has been provided to minimize cracking: compacted base, slow curing, control joints, etc. An important consideration that wasn't mentioned is the concrete mix itself. Though many factors in concrete mix design influence shrinkage, water/cement ratio is the most important and most easily controlled. Concrete hardens by a chemical process that requires water (hydrolysis) but only a small percentage of the mix water is used in hydrolysis. The majority of water in a concrete mix is for workability. Much of the water not chemically bound in the cured concrete will evaporate from curing concrete. That volume of displaced water will reduce the volume of the slab and result in shrinkage cracking, much like the cracking of clay soil as it dries following a rain. Thus the key to reduce shrinkage cracks is to minimize the amount of water to only that amount essential for proper placement. Watered down concrete increases shrinkage and reduces strength. Admixtures (plasticizers) can be added to the mix to reduce water/cement while maintaining workability.
A few rules of thumb for control joints: Space them 2 to 3 times in feet the thickness of the slab in inches (4" slab = 8' to 12' max); Joints should create nearly square sections; joints should be struck or sawn to a depth of 1/4 the slab thickness; Sawn joints need to be cut before shrinkage cracks occur, no more than 12 hours.