Just poured the slab for my new shop - monolithic pour, 3/4 - 3000psi concrete, 5" thick slab, 18" X 18" footers. What are the pros and cons of saw-cutting relief grooves? Is there a certain surface area beyond which the cuts are advisable? I could make one cut lengthwise and one widthwise, but am not sure if it is necessary. Also, what width is the cut typically, and how deep? Thanks for any inputs.
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Re: Garage slab questions
Everett, I have cut every shop (about 10 or so) that I have ever built. Concrete will crack no matter what you do. With cuts you control the direction of the cracks. My present shop is 60X72. I cut it twice lengthwise and twice width. I would cut it 1/2-3/4" deep. I usually rent a diamond blade concrete saw. Do not get a dry saw as it will wear the blade rapidly. I pay for the blade according to the wear. The width will be about 1/4"Dick Whittington -
Re: Garage slab questions
As stated above concrete in large areas will almost always crack, the relief joints will alow you to control where the cracks are. They are usually cut the day after the concrete is poured. As most of the cracks are caused by shrinkage of the concrete as it cures. The saw cut should be aproximatly 25% of the thickness of the concrete floor. In your case 1 1/4" deep. A standard wet concrete saw is typically used and cut about 3/16" wide. The smaller the area the less cracks. If you have a column line in the garage you should center you cut on the column line. The added weight at the column line makes that area more likely to crack. It takes 7 days for concrete to reach aprox. 75% of its design stregnth. So you may want to wait a couple of days before putting heavy loads on the floor.Comment
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Re: Garage slab questions
You really need to saw the slab as soon as it is hard enough to allow the sawing. Try to saw in almost a square grid. Concrete always cracks as Dick said. You just have to try to control the cracks. A garage slab out of the weather without the temperature extremes will not tend to crack as much as a driveway, but will likely experience cracking sooner or later. If you have a reinforced slab, you can go further between the joints, but for an unreinforced slab, joints at 12 to 15 feet would be advisable, but could go a little further to work with you plan dimensions. The depth of the joint should be 1/4 of the slab depth.Comment
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Re: Garage slab questions
I had my 5" (mesh-on-stands reinforced) slab cut in an 8-foot square pattern (3/4" deep) a week after it was poured five years ago, not a crack anywhere. It was steel shot-blasted and industrial epoxy-coated three months later.Comment
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Thank you all for the responses...
I knew there was a wealth of knowledge here, as always. It looks like we won't get around to cutting my slab for up to one week from the pour time - hopefully this won't be an issue. The weather has been quite cool (not too cool though), raining, high humidity, and on those days when it did not rain I sprayed it down several times a day. There ARE surface hairline cracks (which appeared during or immediately after they finished working the concrete) and the contractor told me that happens to all concrete. Is this true?
I am still undecided whether to epoxy finish the floor, but am leaning towards it. I might put in a small section of black and white tile as well...Comment
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Re: Garage slab questions
Everett,
I agree with all the posts above on this topic. Concrete is a rigid pavement and the least movement of the ground for whatever reason will cause a crack. If your slab is reinforced with just a #10 guage wire mesh the strength and resistance to cracking will be greatly increased. Some people also put expansion joint against the block but on a inside slab you probably don't need it. If you pour outside against another rigid surface always use expansion joint.
Depending on the size of your floor I would probably square cut so you end up with four sections. You can rent a small concrete saw on a cart with a wet kit and mark your line for a straight cut. Use a green diamond blade for uncured concrete (less than four weeks) and make your first cut about 1/2" deep so you get a straight line then go over it again to about 1 1/2" deep. After the water dries clean the area and use a concrete caulk to seal the area to keep the water out. Don't apply floor paint until well after the floor has cured.Comment
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Don't wait too long..
Everett,
Don't wait too long to cut the control cracks. Concrete, especially higher strength mixes, take on an initial shrinkaged as they cure. You want to control the shrinkage into control cracks as soon as possible after pouring and finishing. When you cut the control cracks, cut them in a square pattern 8'X 8' to 12'X 12' to fit your garage (the smaller squares the better).
After the initial cure shrinkage, cracking will continue with variation in temperature and that's what the control cracks are for. If the garage is heated hopefully a vapor barrier was put under. Heating will draw moisture from the subgrade up through the slab.
I think the hairline surface cracking right after finishing, is a poor finishing problem. Either they finished it too soon, or too late and added water during the finishing. A properly finished slab should not have that hairline cracking immediately after finishing. But there is no solution other then redoing the slab, or doing a polymer topping.
If you are going to put an epoxy sealer or coating on the slab, wait a while and let the moisture in the concrete and under it dry out. I have see slabs, even with vapor barriers, ruin the surface finish from moisture coming through after a year.
Hope I didn't ruin your day.
Jerry Fuccillo CE
#42179Jerry Fuccillo
1967 327/300 Convertible since 1968Comment
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Not that this will change your thinking...but
I wouldn't bother saw cutting the slab. I just finished putting up a 30 X 60 metal building and have a 4" floor. It's reinforced with 1/2" rebar on a 3ft grid center.
I told the guy who did my flat work "DO NOT saw cut it", but he did it anyway the day after they finished it (I wasn't there). He cut twice lenthwise and 4 times across the width. The cuts are about 3/4" deep and 3/16 wide.
It has now cracked between the saw cuts in 3 or 4 places. I'm sure there are more to come. The saw cuts didn't provide any "controlled" cracking that I can see. The saw cut just provides an unwanted collection point for dirt.
There are 2 things you can count on with concrete... It will get HARD and it will CRACK. I just wish I didn't have those blasted saw cuts to clean all the time.
tcComment
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Re: Garage slab questions
I just watched a garage and first floor slab pour down the block this week.
Part of the finishing process was using a tool to carve two grooves - about a half inch wide/inch deep in the garage slab while the concrete was still wet, but fairly stiff - one each along the longitudinal and lateral centerlines, so the slap is effectively divided into four equal size sections. A one-inch saw cut after the slab hardens does the same thing, but it's a lot easier to carve the grooves before the concrete sets up.
Either effectively causes a stress concentration at groove/cut, so the most likely crack locations (which will happen sooner or later, especially here at earthquake central) will be at the bottom of the grooves/cuts. It's a good idea for preventing visible cracks in the slap, but those grooves get full of debris and have to be vacuumed.
Another technique is to place pressure treated 2x4s or 2x6s along these lines (level with the top of the slab), which effectively yields four decoupled small slabs, and a smaller slab will have less tendency to crack.
Similar "breaks" and expansion joints are used for concrete raods.
Concrete has excellent compression load carrying characteristics, but is lousy in tension, unless heavily laced with rebar, especially pre-stressed rebar, so as the unreinforced concrete shrinks or gets rocked around by mother earth, tensile stresses develop that eventually cause it to crack.
DukeComment
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Re: Garage slab questions
Everett,
What's the size of your slab? I think Tracy has a good point especially if its a monolithic pour with perimeter footings, and if they put a rebar grid in and perimeter rebar in the footings. Maybe you don't need crack control scoring?
They do some wonderful things these days with special cement toppings.
Jerry Fuccillo
#42179Jerry Fuccillo
1967 327/300 Convertible since 1968Comment
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Re: Garage slab questions
Everett
If it was my garage floor,I would saw cut it an inch and a half.
Cement Mason Local 527
Tom HeffernanComment
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Saw cuts, cement, finishing etc.
Once again, thanks to everyone who has replied to my questions. My shop is not large compared to many - it is rectangular with an odd little triangle on one end, to fit an oblique angle on the rear of my property. I thought the added triangle would be where I locate my bench, cabinets etc., leaving the full rectangle for car space. The rectangle is 23' x 22', and the triangle is a right triangle with dimensions of 22' (hypotenuse) and legs of 14' and 17'. Total square footage is 640. I was considering one cut lengthwise and two cuts widthwise for the slab. The footers have rebar on chairs, and the slab has wire mesh.
I think the "mud" I received may not have been the best quality, but I may never know. I was not available to observe the pour. As many of you know, there is a nationwide concrete shortage, driven by a nationwide cement shortage (have you checked concrete prices lately...?). Anyway, the rumors all over my area are that concrete makers are cutting corners, watering down their product, etc. I have no way to verify this however, and must live with my slab (it is not too bad, and I believe it is indeed strong). If I cut, I assume the cuts run right off (through) the ends of the slab? Thanks again for all the information guys. Now, what bench, cabinets...Comment
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Re: Garage slab questions
Probably just blind luck: I poured my shop floor in 1984 - 24' x 32' with a pit in one bay. Pour was 5" thick with wire mesh reinforcement and no relief cuts. No cracking to this day.Comment
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Re: Garage slab questions
the spider cracks( surface cracks)..is usually caused by overworking the concrete's surface with too much water,,,he probably used a power trowel and added water while trowling to get a glass like finish...these cracks are usually on the surface only and should'nt pose any structual concerns....with my experience with concrete it will get hard and crack where it wants to!!!..but you can try to control it to some degrees....kentComment
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