Concrete Pad - Control/Expansion Joints?

[neilly2]

I'd like to respond to your post but one question first? Was it curing compound you applied or concrete sealer? I think its just a matter of terminology but I think of concrete sealer that goes on later as a floor finish in industrial areas.

 

[Junkman]

Don't remember the exact name of the product, but it came from the masonry supply store and was recommended by both the concrete installer and the concrete manufacturing facility. The instructions said to install it as soon as possible after the concrete was poured and set up. I might have a pail of it left in the garage. I will have to check tomorrow. Going out for the evening now....

 

[Currmudgeon]

</font><font color="blue" class="small">( I'd like to respond to your post but one question first? Was it curing compound you applied or concrete sealer? I think its just a matter of terminology but I think of concrete sealer that goes on later as a floor finish in industrial areas. )</font>

It's not just a matter of terminology. Curing compound and sealer have different functions.

Curing compound is usually a waxy stuff that is sprayed on the surface of the wet concrete to hold the water in and promote curing. It is washed off after curing. Sealer is a stuff you apply after the concrete is cured, to seal the surface against chemicals or water.

Concrete does not set by drying. It cures chemically by hydration of the portland cement. In order to properly cure, it must be kept moist. Concrete will cure under water. This is where the visqueen and curing compound come in. They trap the water in the concrete permitting the hydration to continue, and permitting the concrete to continue gaining strength. If the concrete dries out before the hydration is complete, it cannot be restarted, and the concrete stops gaining strength. Kept moist, the concrete keeps getting stronger, forever, although at a constantly reducing rate. Concrete reaches design strength at 28 days, and maybe 2/3 of that after 7 days.

I've never seen curing compound at the Big Box home centers. You need to buy it from the people who sell concrete products and formwork to contractors. I doubt it comes in anything smaller than 5 gallon pails. You can achieve pretty much the same effect by spreading visqueen on the troweled surface and keeping that watered. I usually spread straw or grass clippings on top of the visqueen to hold it down and retain moisture.

 

[tugwell]

I'll chime in with my 2 cents . . .

Four years ago I poured the foundation for my 32 x 40 shop with apartment over (my family calls it the "shouse"). /forums/images/graemlins/wink.gif This is a "depressed edge" monolithic slab, with footers 18" deep and 15" wide. This was the first big pour I had ever done, though I had poured and finished smaller slabs. I was also concerned about strength of my slab which was 4 inches thick. I used both rebar (18 inch centers, I think) and fibermesh. So far there are minimal expansion cracks, but nothing that has actually moved. I'm very pleased, and the building inspector thought it was overkill (he's probably right).

Without a big crew of experienced finishers, I did three pours over three days. I was concerned about the sections not being monolithic, but then I discovered a product called "Superscreed". This stuff is great. It is a combination screed and expansion joint made of galvanized steel, and it is key-shaped to lock adjoining sections together. Pre-punched for rebar, and it just stakes into place. It seems much better than the fiber expansion joints -- barely noticeable when in place and finished. I've used this twice more on slabs since, and it is the way to go! /forums/images/graemlins/wink.gif

I had minimal problems with balling of fiber during finishing. Make sure you don't float the slab too soon, and it will be fine. A trick I learned too late for my own slab is to get a bag of Portland cement (not premix) and sprinkle it across the slab to "enrich" the surface mix. (If you want a smoother finish and can afford it, ask for a 6 or 7 sack mix instead of the usual five sack mix -- more cement per cubic yard means more strenght and more cream when finishing. I went with six sack mix.) I've heard that you can burn off any fibers with a big propane torch after the slab is cured, but haven't tried this personally.

I don't think I have a digital pic of my slab prep, but if I can find a good picture of the superscreed, I'll post it. I can't get on the tractor 'cause of the rain, so I might as well look at construction pictures!

Oh -- and the words of wisdom from a contractor friend of mine . . . he says all you need to know about concrete is it's grey and it gets hard!! /forums/images/graemlins/grin.gif

Good luck with your project.

Tugwell

 

[zuiko]

Thanks for all the great tips. One other delima I have is that only one side of the slab will be accessible once the concrete is all layed out. How would I screed it if I can only get to one side?

I am planning on doing it in 8 foot wide sections so it wont be really far from one side to the other. I will have boards or some kind of expansion joint around the edges where it meets the existing foundation.

 

[tugwell]

By "only accessible on one side" I assume that is the door opening? It sounds as if you are pouring a slab inside an existing foundation (typical for a garage).

There are different ways to handle this. The first would be to use either removable screed guides (usually 2x4s) that are pulled as you move along. If you choose this method, after you pull the screed guide you will have to shovel wet mud into the resulting void before you float the slab. I used this method when I did a slab for a neighbor last month. It works, but requires careful planning and an extra set of hands on the day of the pour.

The other method is to "wet screed" around the stem walls of the foundation. This involves carefully marking the height of the slab top around the perimeter, and then at the start of the pour shoveling concrete around the edge and carefully troweling it off to the line. I did this once on a friend's basement, and found it a bit difficult to screed to -- the screed board wants to dig into the concrete, and it is more difficult to avoid slight depressions in the finished slab.

Have you finished slabs before? If not, starting with 24 x 40 is a lot to tackle. I've only finished by hand (read aching back and forearms!!). I have done 20 x 30 on my own (just help with screeding) but I'm not worth much the next day! /forums/images/graemlins/tongue.gif If you're going to do the entire slab on one day, you'll want an army of friends (with experience!) or a power trowel, or a professional.

Lots to think about! Hope this helps.

Tugwell

 

[zuiko]

This is just the back of a building so one whole side is open. I am planning on using a screed guide on the back, but I am not sure how feasible it is to screed it all from one side without a person on the other side. I've done some smaller stuff before but out in the open, not where there were some existing walls in place. I will have some people there on the day but I'm not sure they can do much to help with screeding if they cant get to the other side.

I figured I would buy a bull float and do the finishing by hand. It does not need to be a really smooth finish. It will be a for a machine shop area. I thought about renting one of those power trowels but im not sure if that is worthwhile.

The actual size will be end up more like 16.5x40 because I am increasing the thickness of the slab.

Thanks!

 

[Egon]

Rubberboots and stand in front of the screed. No law says you can't walk in concrete being poured. And make sure the fellow doesn't get concrete inside the boots. He will not be a happy man come evening.

Best screed in the world is a knowledgeable fellow on the chute. That why a bull float is all that is required.

Egon

 

[fishman]

Tugwell,

I did a search on "superscreed" and all I got were some hits from a powered machine that screeded concrete. Where did you find this product and how much did it cost? Do you feel that your slab is as resistent to bending stresses as a monolithic slab? That product sounds ideal for someone who can't muster an army of trained concrete guys without laying out some $ (like me /forums/images/graemlins/smirk.gif).

 

[tugwell]

Fishman --

When I used the Superscreed product, it was four years ago. A couple of years later when I did my friend's basement, they didn't have the same product, but a very similar product by a different manufacturer.

I couldn't find Superscreed, either. But after some serious detective work /forums/images/graemlins/cool.gif I found a link to a similar product. (The successful combination was "masonry supply galvanized metal expansion joint"!!) Try this link to the Masonry Supply . They have an image available there, but it was scanned from the catalog was fairly large to post.

I'm sure you could print this info and take it to a local masonry supply or maybe even home center and get the stuff. It is a specialty product, and not many people seem to be familiar with it -- my friend in Idaho tried to find it, and they had never heard of it.

As for my slab, yes I feel it is very stable, and there has been absolutely no movement at the control joints. /forums/images/graemlins/grin.gif Normally in a slab this size there would be either saw-cut expansion joints or fiber joints, but the metal key joint seems to really lock the sections together. I think the fact that the rebar runs through the joint has a lot to do with the strength.

So far as thickness goes, I was initially leaning toward a 6-inch slab, but a local logger has an unreinforced 4-inch slab which he parks trucks and equipment on. No steel in the slab, just fibermesh and zero cracks or movement. That convinced me that 4-inch was sufficiently thick.

It has been mentioned previously in this thread -- it is very important to allow concrete to hydrate completely as it cures. I covered my slab with plastic and kept it sprinkled for the first week. I used a vapor barrier underneath to help maintain moisture during the cure. I also think the substrate is critical. I have six inches of 5/8 minus that was watered down and compacted with a plate compactor.

This is my own experience, hope it helps.

Tugwell