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Old 08-25-2004, 11:35 AM
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Concrete

I am getting ready to do the concrete floor in my new 40x60 building and have a couple of questions. I've done driveways before, but never garage floors. I know I should saw lines about ever 10' for control joints, but do I need to put expansion joints too? I just can't ever remember seeing a garage or warehouse floor with expansion joints. Also has anyone used on of the power finisher machines? Are they very hard to use?

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Old 08-25-2004, 11:46 AM
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Alan,
I was always taught to use sawcuts that were making the concrete into squares, since that would allow equal expansion.
In the case of 40x60, make 20x20 squares. If you have drains, X cut to the corners, as that will be the area it cracks from. I seriously doubt you will need an expansion joint.
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Old 08-25-2004, 11:50 AM
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I just had 200 cu yds of concrete poured for the foundation of my new shop (80'x100') and there are no expansion joints, only saw cuts (every 20' on the 100' side and 1 down the middle at 40'). Concrete cracking is inevitable and the rebar/wire will help hold it but will not prevent it from cracking. The type of soil and prep is the key... where I'm at it is all adobe (very expansive/not good for concrete) and the best you can do outside of excavating down 20' is to have the slab sitting on sand. My slab (6" thk) has 9 gauge 6" wire screen, 5 sack mix with fiber and a 2' wide x 2' deep perimeter footing.
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Old 08-25-2004, 12:03 PM
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I think my base should be pretty good. Its packed clay and sat a few months in the heavy rain before I put the building up, so it should be settled good. then I put a couple of inches of clean 1/2 gravel.

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Old 08-26-2004, 11:18 AM
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You should have expansion joints on all sides of any columns that pass thru the floor slab. Standard practice sizes the boxed out area around a column at 6-12" away from the face of the column at the closest points. The boxed area should orient the box corners on the structural grid lines. Form a box around the column. Do not place concrete into the box at this time. Place the concrete for the floor slab and let it cure for a few days. Strip the formwork around the column. Place 1/2" thick expansion material against the fresh concrete. Now place concrete in the void around the column and finish it flush with the floor.

This method prevents transferring any horizontal loading caused by the slab expanding or contracting from reaching the column. A structural engineer's worst nightmare is when a force is applied to a structural member that is inappropriate. Expansion forces from a concrete slab can become very high and dangerous if not properly controlled. If you did not follow my description and explanation, call a structural engineer in your area. I'm sure he will be able to explain it again.
Good Luck!
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Old 08-26-2004, 11:54 AM
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I understand what you mean. That is great info... Thanks!
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Old 08-26-2004, 11:59 AM
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You can rely on it. I'm an architect and I do this kind of thing every day.
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Old 08-26-2004, 12:08 PM
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Would this go for the posts on a pole barn around the perimeter? just do it around the 3 sides?
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Old 08-26-2004, 12:47 PM
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Yes, the same principal applies. In a case like that, form a "V" at each post with the "V" pointing directly at the column on the opposite side.

You might also ask your concrete contractor how much extra it would be to ad fiber reinforcement in the concrete mix. It will reduce shrinkage cracking and makes the concrete even stronger. The fibers that stick out of the surface after final finishing will wear off in time and don't cause any harm. We typically spec the fiber when concrete is subjected to freeze/thaw cycles or when cracking is undersiable.
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Old 08-26-2004, 02:25 PM
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I was a terrazzo journeyman for many years and have poured a few concrete slabs. The only thing I have to add (All the posts are excellent advice) is to try the power trowel on a slab of cured concrete. Power trowels for concrete are usually running pretty fast so you'll need to get the hang of it and understand what happens whe you push up/down or pull right/left. Adjust the angle of the trowels at this point. A good start point is where the Front side of the trowel is about 3/8" above the back part. You will see if this is too much or too little angle when you start on the fresh slab.

Screed the wet concrete with a straight edge resting on guides or a large aluminum screed on a handle (you can make one out of wood, but renting an aluminum one might be a better choice). Probably by the time you finish empting out the truck, you can start power troweling. I can't give an exact time because it depends on how quickly the slab is curing and what kind of concrete you're using. Have someone start edging by hand walking and kneeling on pieces of plywood or styrofoam to distribute their weight. The more you trowel the more laitence comes to the surface and the harder and more mirror like the slab will get.

If it were my slab I would sawcut it after about 15-24 hours depending on the hardness, some people like to wait longer, but the weak points should be made before the slab finds it's own weak points. I would then cover it with burlap and soak it with water if it were outside. If it's indoors, you can omit the burlap (I wouldn't) and soak it or just cover it with poly and omit the soaking. Concrete needs water to hydrate. Very hard concrete is kept wet during the initial cure. Some of the jobsites I've been at kept the concrete soaking for up to three days after the pour. Many concrete guys omit this step, but I personally think it's very important. At the Toronto international airport (terminal new), the floor slab was covered with a hydrating mat, soaked and covered with poly. The mat wasn't removed for at least 14 days. when I pour terrazzo stairs I usually soak them the morning after the pour and then again before I go home for the day. Any concrete poured under water will be stronger than any concrete poured above water. Ultimately it's up to you, that's just what I would do.
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Old 08-26-2004, 06:29 PM
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I almost forgot. when you're troweling, try not to add water. If you have to then just a little sprinkle with a whitewash brush. Working the moisture to the surface will yeild a much better job then adding it to the surface. Also, too much water on the surface may cause spalling.
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Old 08-26-2004, 09:07 PM
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expansion joints

I am in the process of building a 40x60 steel frame building, and have just poured the individual footings for the structural steel frame. These footings are located in the corners and at 20' intervals front to back. From an earlier thread, mentioning expansion joints around columns, do these footings also require expansion joints? The finished floor will be in the same plane as the top of the footings, and tied into the floor with rebar every 24 inches. I am doing 90% of the work myself, so it will be some time next spring before I do the floor, so now is the time to do any changes to my initial plan!
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Old 08-27-2004, 05:15 PM
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All of your received answers to your concrete garage floor post sound like good tips. I had my 30' by 80' slab poured about a year and a half ago, and so far no cracks. The only thing I did differently was to specify #3 rebar at 16"o/c, doubled the required #4 rebar in the footings, and went with a six sack mix. I didn't want any saw cuts in my nice smooth 4" thick floor. I"ve always wanted my own in floor lift, and so had one installed at about the same time. Good luck!

Bill
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Old 08-28-2004, 08:05 AM
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Sawcut & Expansion Joints

Isolate any columns as described by jpd37. Flyfisher has some good advice on finishing and curing as well, although I disagree with his sawcutting timeframes.

Based on my experience with industrial/commerical construction, I would not install any sawcut control joint at an interval of more than 15'. Therefore, I would sawcut at 15' centers in the 60' dimension and either 10' or 13'-4" for the 40' dimension. Generally, it is recommended that a sawcut joint be 1/4 the depth of the slab. Therefore, if you have a six inch thick slab, sawcut 2" deep. Also, IMO, if you don't sawcut the same day as the pour, within a few hours after the finishing is completed, don't bother cutting them. The first few hours of crack control are the most critical since this is when the concrete is shrinking the most rapidly. This is called "soft-cutting" and it the norm, at least in my part of the country. Since concrete will contract (shrink) and expand, a sawcut joint is designed to induce "control" the inevitable cracking at a location that will be concealed by the sawcut joint.

You must have expansion joint filler around the perimeter if the concrete pours up against a masonry wall. If the edges are formed (i.e. a turned down slab edge), then obviously exp. joint filler is not required.

Do not forget to cure the slab properly. Curing compounds can be either sprayed or rolled on. You can wet cure as flyfisher describes, but we generally use poly and wet cure the slab for seven days. The intent is to keep the concrete from losing too much water while it is hydrating, especially at the top. When concrete cures, it will curl. Curling is the result of the concrete top shrinking more than the bottom. As the concrete curls, the perimeter edges lift off the subgrade. Proper curing will also help prevent surface stress cracking that can later lead to bigger cracks.

As far as finishing the concrete itself, I will defer my recommendation, other than to let someone else do it. Depending on the time of year (if too hot) you place the concrete, it can set up too quickly and you can lose the slab if you do not have enough manpower on hand.

Just remember, no matter what you do, concrete WILL still crack . A a general contractor, this is the most difficult thing to explain to an owner. The next time you go into a Lowes or Home Depot, make it a point to look at the slab. Rest assured, they spent extra money to make their slabs look good, yet they will still crack in undesirable places.


Quote:
Originally posted by swellwelder
I am in the process of building a 40x60 steel frame building, and have just poured the individual footings for the structural steel frame. These footings are located in the corners and at 20' intervals front to back. From an earlier thread, mentioning expansion joints around columns, do these footings also require expansion joints? The finished floor will be in the same plane as the top of the footings, and tied into the floor with rebar every 24 inches. I am doing 90% of the work myself, so it will be some time next spring before I do the floor, so now is the time to do any changes to my initial plan!
Since the top of your footings will be flush with the top of the slab, yes - you need to wrap the footing with expansion joint filler to whatever the depth of your slab will be.

But do NOT dowel into the footing. You want it to be isolated for two reasons.
1. If the column footing were to ever settle, you want it to slide past the slab; otherwise it will pull the slab down and induce a crack near the edge.
2. If you dowel into the footing and when the slab curls (and it will), the concrete may split horizontally.

Good luck with your projects, Ed www.edgesz28.com

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Last edited by edge; 08-28-2004 at 08:05 AM.
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Old 08-28-2004, 08:13 AM
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Ha! a lot of good advice for your slab,BUT! concrete is what it is & if your SUB_GRADE isn't tight, it will crack! I build Pole Buildings every day & pour my own slabs. I NEVER!!!!! cut lines in a perfectly good slab! NEVER! The best advise I read was the one about leaving the crete out around the poles & pouring that later.
That is the spot where a slab will crack in a pole barn, expansion is the devil! All I do is either trim the corners round on the poles at slab thickness or what I am about to start doing is to wrap the poles with expansion joint material & then pour.
If you go look at a 10 year old pole barn you will probably see any & all cracks going from pole to pole. Sounds like you have a good base so don't Pre-crack the sucker! Also, you will get NOTHING! out of a bunch of iron in it unless your street rods are made by catapillar! HA! 5 sack is fine 6 will yeild a better finish. & if you don't already have the sidewalls on it GREAT! you can do the edges from the outside! no knee boards!!!!!! Also, drive some 16 galvies into the bottom preasure treated board at mid slab about 3 or 4 per bay and about an inch 1/4 into the wood. this will keep the P.T. board from separating from your slab & getting little rocks stuck down between them. & DON"T FORGET!! to slope those rollup door entrances!!!! from the back of the pole!
If you don't rain will find it's way in for sure. As far as a power trowle is concerned, you got good advice already. Pull up on the handle, it will move left push down, go right. The trick to one of these suckers is not to get on the mud too fast!!! get the edges real good & wait till you can walk on it without sinking in at all. If you start & you are whipping up tons of shmang, back off for a couple more beers! He He. Good Luck! O yea! just above IDLE is all the faster you need to run a PT. flaten the blades a little when the shmang is all gone & you are ready to make your last couple of passes. Runs like a floor polisher! By!
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