concrete recipes and preventing cracks.

[EddieWalker]

Ground preperation is very important. Pouring on sand is the best, but gravel is good and dirt is the worst. If the soil under the pad isn't compacted, it will settle and move, which caused cracks.

Since you said it's 8 feet, I'm assuming it's 8ft by 8ft?????

That's small enough not to need any expansion joints. Where it meet the house, be sure to have either a piece of PT 1x6 or other flexible board used for concrete. You should also have rebar into the house foundation that ties into the new pad. If you don't already have them in place, be sure to drill the holes and epoxy the rebar in.

If it's hot out, pour early morning, or late afternoon. Pouring concrete in the heat of the day will cause it to dry too fast and not gain it's full potential in strength.

Be sure to have it as dry as possible. Too much water will make it weaker. The less water in the mix, the more strength it will have.

Use chairs for your rebar. There are allot of guys in my area that don't use chairs and say that they lift the rebar when they pour the mud. They try to lift it while working it, but it's never consistant and most of the rebar ends up at the bottom of the pad, or not even in the pad. This is especially true with wire.

Eddie

 

[crbr]

I'm getting ready to pour a concrete porch. It will be eight feet wide and five inches inches thick at the house and four inches thick at its end. My main concern is cracking. I will use steel re bar in a grid on two foot centers. I plan to use the concrete with the fiber mixed in the recipe. Are there any other additives that will help with cracks? I guess my question is this: what is the best recipe to help prevent cracking, especially those annoying hairline cracks? Thanks in advance.

Guaranteed fact.
Concrete will get hard and it WILL crack. No matter what you do. What you are in need of is how to control the cracks and where you want them to be.

Rebar is for tensil reinforcement. it will not stop a crack.

Fiber additive is for surface shrinkage (the annoying hairline cracks). It will not stop a stress crack. Allow me to say this again.. Fiber additive (fibermesh) is not a primary source of reinforcement nor will it stop a stress crack. It merely directs shrinkage cracks to a minimum surface control. I would use a fiber additive but do not consider it a crack preventer type application.

Place expansion joints where you want your cracks. Don't rely on myths and legends of those that say they do it this way or that way. Place expansion joints often and in two directions (90 deg angle to each other). Also place joints on edges where the placement allows for a change in directional use (an edge or curvature).

Your concrete will then crack underneath the surface, running to the joint (acting as a stress riser point, relieving the stress under the joint and not at the surface). Subsurface prep is also important. Subsurface prep is also important.... Subsurface prep is.......

I could go on and on... But I'll hush my mouth.

Will has told you correctly, however I tend to emphasize that you place the joints into the concrete placement, and not do a saw cut. Much better results as the concrete hydrates through the curing process. A saw cut is a guess on your part if the placement has hydrated enough to cut, or maybe too much to divert a stress crack. Put the expansion joint in as you finish the placement.

Air entrainment is needed only if you have repeative freeze / thaw cycles. The entrained air pockets allow water room to expand and contract during the freeze / thaw. Other than that, it's not necessary. It only makes your concrete more porous. However most common 4000 LB concrete in the souther tier states comes with air entrainment chemicals as part of the mix.

In addition, use a vapor barrier on top of the soil prior to rebar placement. Make sure you use rebar placement pegs. Rebar lying on the ground is useless. Get the bar in the center of the placement. They make gagets for the very thing.

Eight ft X 8' is certainly worthy of at least two joints (one perpindicular to the other) or more, depending on the pad. Place the joints, allow the concrete to cure, then in a few weeks, fill the joints with self-leveling crack filler. Done deal. You then will not see a crack at the surface (no guaranteed ). If you don't fill back in around the edges for a few months, you'll see the subsurface cracks go right where the joint allowed (or where you controlled that crack to go).


If you are "pouring" you concrete, it will have no slump. .. Place it with the proper slump....

 

[Piperflyer]

You don't want to finish a porch floor, especially outside like glass. When it gets wet it will be just like a ice rink. Good float finish is all you want not a steel trowel...

 

[Builder]

This thread is kinda funny. Everyone just keeps repeating the same thing over & over again.

 

[AlanB]

Guaranteed fact.

Will has told you correctly, however I tend to emphasize that you place the joints into the concrete placement, and not do a saw cut. Much better results as the concrete hydrates through the curing process. A saw cut is a guess on your part if the placement has hydrated enough to cut, or maybe too much to divert a stress crack. Put the expansion joint in as you finish the placement.

Could you elaborate on this please, are you saying while it is wet cut it in with one of those expansion joint groovers? Or are you saying to place what I call "expansion joint" (commercially made product that looks like creosote soaked cardboard) after a pour? (I have only seen it installed prior)

Oh, and Eddie, I always laugh about the guys telling me how they are going to "pull it up" as they go..... then they walk back across what they have "pulled up" and their boots go just as far down..

And yep, there is certainly a myriad of the little chairs and holders and what not's out there to hold the stuff in the right place.

Why I ask by the way is I am getting set (at the rate I move it may be a while ) to pour a driveway / work pad behind my house, to practice for my "shop" pour one day.

I have poured some small pads, 12 / 25 and helped on lots of big pads, but never been the lead on a big pad pour.

 

[Builder]

Could you elaborate on this please, are you saying while it is wet cut it in with one of those expansion joint groovers? Or are you saying to place what I call "expansion joint" (commercially made product that looks like creosote soaked cardboard) after a pour? (I have only seen it installed prior)

Oh, and Eddie, I always laugh about the guys telling me how they are going to "pull it up" as they go..... then they walk back across what they have "pulled up" and their boots go just as far down..

And yep, there is certainly a myriad of the little chairs and holders and what not's out there to hold the stuff in the right place.

Why I ask by the way is I am getting set (at the rate I move it may be a while ) to pour a driveway / work pad behind my house, to practice for my "shop" pour one day.

I have poured some small pads, 12 / 25 and helped on lots of big pads, but never been the lead on a big pad pour.

Black felt expansion joint is butt-ugly and will make your porch look like crap. Crack filler/sealer will be obvious and just spall-out later. I couldn't sell a house or addition with anything like that showing with today's customers. I saw cut soon after the pour and have never been back to fix a serious crack.

You don't need the chairs or holders. What you do is buy 12-18" 1/2" rebar pins. You hammer them into the ground level with your outside forms at 2' spacing (or the same spacing as your re-bar) and the line on your foundation wall. You buy a $5 dollar tool called a "twist tie tool" and a bag of 100 6" twist ties. You then use the twist tie tool to spin the wire around the vertical pin and your rods. Hold the rebar half way up your pour off the stone (~2"). The small chairs suck because when you dump loads of 'crete, they just blow over and everything gets all screwed up. The pins are nice because they also serve as level locators for the top of your concrete. After you screed, you just tap them down an inch with a hammer so the rebar doesn't go down too far and they disappear.

Thing ya gotta remember is there's more than one way to do it, just do it one of the "right" ways and you'll be OK.

 

[tydp]

A good concrete contractor told me once that there were two kinds of concrete: concrete that is cracked and concrete that is going to crack.

 

[tuolumne]

Plastic shrinkage cracks will be worse with too much cement. 3000 psi should be fine. Use poly to prevent the concrete from adhering to the subgrade, and it will be able to shrink back on itself with less cracking. Keep rebar in the top third of the slab. Keep the slump as low as possible...don't let the driver add water just so he gets home on time.

 

[Dargo]

Guaranteed fact.
Concrete will get hard and it WILL crack. No matter what you do. What you are in need of is how to control the cracks and where you want them to be.

Rebar is for tensil reinforcement. it will not stop a crack.

Yessir, that is correct. I have just over 800 yards of concrete so far at my house. Yup, over 800 yards! For a residence, I'm tellin' ya, that's an awful lot of concrete. The only way the top concrete construction companies will guarantee your concrete will not crack is if you buy prestressed concrete building slabs. Because of the extreme cost in this, I only see this in huge commercial applications. Other than that, the key is to control where it cracks and control the cracks with rebar and heavy gauge wire matting. The heavy gauge wire matting comes in large sheets, not in a roll. You can't roll 6 gauge wire with rebar ribbing in it.

Other than that, I'd suggest filling the cuts for control joints with a high quality concrete crack filler; not silicone caulk. That will keep water from getting into the cracks that will occur. Doing the correct caulk job is a PIA, but really preserves the job. I have a lot of custom brick work and concrete around my pool. I spent many, many hours on my knees filling all voids and seams with caulk. Several years later it looks just like it did when it was installed and I have no damage.

 

[Harvey7]

What are chairs ?