Concrete Questions

[sandman2234]

Some people put a lot more stock in fibermesh concrete than the Tinius Olsen testing machine I ran gave credit to via actual test results.
David from jax

 

[RobJ]

. What's the theory on keeping a slab wet for days after it initially cures to where the water doesn't hurt the finish? Does it really help?

I've probably worked on building 50 homes. zero kept the slab or driveway wet. If we were ready to frame many times we started framing the next day after the pour. I've seen a few building kept covered but it gets real expensive to stop construction to keep a slab wet.

 

[RobJ]

My last garage (2 car) had a control joint down the center of the 2 bays.

That's weird. The average 2 bay garage down here is 20x20, never seen one down here, no where in the slab at all.

 

[Bird]

I don't know how you determine whether or not to keep concrete wet for awhile after a pour. Temperature? Thickness? Amount of cement in the mixture? I know I've seen lots of concrete poured that never had any water applied later to keep it wet, and I've seen it poured where it was kept wet. I was treasurer for our volunteer fire department when we had a fire station built, and the builder told us it would be much better if we could keep the concrete wet (I think for a week, if I remember right) after the foundation slab was poured. So three of us did that.

And I can't recall ever seeing an expansion joint in a garage floor myself. 20' x 22' is fairly common and when we built my brother's house, he made his garage 22' x 28' but no expansion joint.

I do have expansion joints in the driveways of my current home, but not inside the garage.

 

[EddieWalker]

Concrete moves from the outside tempatures. Heat and cold cause this to happen. It's the biggest issue with using it for roads, since they have to put those expansion joints in there, it's very difficult to make a road smooth.

Inside a building, shop, barn and garage, concrete remains at a fairly stable tempeture and moves very little. It's why home slabs are monolithic, they don't have the tempeture variations that an outdoor slab goes through.

A garage isn't a heated space, so there is more change and expansion. I've seen them poured as part of the house slabe without any problems, but I've also seen them poured indide the wall footings after the house was built. In those case, the perimeter of the garage floor was the expansion joint. It was also code the a garage that was attached to a house had to be lower the the floor of the house. Something to do with fumes being heavier then air and not being able to get into the living area. It's not done here, so I'm not sure if it's still code, or if that was a regional thing.

I've never heard or seen anybody put expansion joints, or cutting control joints into a garage. Concrete will crack where it wants to crack. If it's along the control joint, it's not so obvious, but other then that, I think most control joints are just cosmetic.

Only two things that you can garantee with concrete. It will get hard and it will crack.

Eddie

 

[patrick_g]

First I admit I have not read every post in this thread

WHY do people purposefully put control cracks (joints) in concrete????

Like the name implies, control joints are supposed to control cracking by giving a planned place to encourage a crack to form.

When concrete cures it shrinks. Over a large enough span (typically something like a 6-15 ft range )the shrinkage is more than the tensile strength of the concrete and VOILA!, a crack. Cracks are not always so neat and can just sort of meander around.

I sympathize with the insult to your aesthetic sensibilities and the real hassle of cleaning the joints. What most people do is fill the joints (or random cracks if they open up enough to accept material) with caulk made expressly for caulking cracks in concrete. There are foam plastic backers sold expressly for stuffing into gaps in concrete before topping the backer and filling the top of the gap with caulk.

The backing material (as I have seen it) is cylindrical lengths of medium density foam plastic. The application method is to shove the foam into the gap until it is below the surface level of the surrounding concrete and then putting concrete caulk on top.

This not only gives a fairly easy surface to sweep clean, it also prevents moisture penetration which with winter freezing can cause expensive damage over time.

The gap-filler backing comes in various diameters but isn't needed if the gap is too small to accept one. These elastomeric caulks are designed for filling concrete gaps and are even colored close to average concrete color so as to not stand out like a diamond in a goats backside. Some of the caulks are intended to be self leveling and do not require tooling to get a good finish/appearance.

Concrete cracks. With sufficient reinforcing steel and proper design you can reduce cracking, control it, and ameliorate it. You can try to control and manage the process or let it go like weeds in a yard and accept whatever happens.

Pat

 

[Bird]

Eddie, I think every house I've seen, built on a slab with an attached garage, had the garage floor poured at the same time and the garage floor is an inch or two lower than the floor of the rest of the house. I've always been told that was so you could wash out the garage floor without the water getting into the house. Now that makes sense to me, however, many of them have washer/dryer connections in the garage and maybe an electric water heater in the garage but the part of the garage those sit on is always up on the same level as the rest of the house floor. That way if you have a water heater or washing machine leak, it goes under the wall into the house, doing considerable damage, as a friend of mine learned. It always seemed to me that common sense would dictate leaving those two appliances on the lower level of the garage floor. I asked one home builder why he didn't have the water heater on that lower level and he said, "I guess we just never thought about it."

But if you have a gas powered water heater (either natural gas or LPG) in an attached garage, there are codes requiring those to be on some kind of platform to raise them above the floor. I've forgotten just how high they have to be, but think it's 18" to 24". The possibility of gasoline fumes from gas cans, lawn mowers, or leaking autos is supposedly the reason, since the fumes travel along the floor and the platform gets the water heater pilot light high enough to reduce the chances of it igniting those fumes. I know our daughter has two 50 gallon natural gas powered water heaters in her garage, so I'll have to try to remember and measure the height of the platform some day.

 

[Bird]

What most people do is fill the joints (or random cracks if they open up enough to accept material) with caulk made expressly for caulking cracks in concrete

Pat, that's something I ought to check into. When this house was built in 1991, they put 1 x 2 boards in the expansion joints of the driveway. Most of that wood has rotted out, and I even pulled some of it out. So I have a substantial gap that collects the cotton from the cottonwood tree next door, the blossoms from our own crepe myrtle, as well as some small leaves, grass clippings, etc.

 

[3RRL]

I had a 40' x 50' barn pad pured last Summer so I'll share some of the things the guys were talking about. I myself don't know a thing about pouring concrete, but my contractor sure did. My job was to prep the site by grading and compacting the decomposed granite soil. That makes a very good base for the pour since it is super hard and compacted. The slab is 6" to 8" thick since it will be used as a machine shop as well. I'm not sure, but I think it's 6,000psi.

Here are some photos which show the footings around the perimeter. There were extra deep 4' x 4' ones at each corner and also along the sides. Two extras on the 50' side and one on the 40' side. They put a vapor barrier down first and then several inches of sand on top of that. Then they used #4 rebar on 16" x 24" centers. Inside the deep footings they were like 8" apart.



They used that pumping machine to pump the concrete to all the footings first. Like the one Dargo shared in his pictures. There were 8 or 9 concrete trucks total for that pour, each with 9 yards in it. They poured the garage the day before with 2 trucks and the footings and basement for the house the week before with 6 trucks.



Including the guy who controlled the pumping machine, they had at least 8 guys all together. They started filling in the pad and dragging out the concrete. They used that tamper Rob was talking about to get the bubbles out and the stones pushed down.



After it was all dragged smooth over the form boards, they used that electric machine which has rotating blades to smooth the surface along with some hand work. then they cut those control joints into the slab. They cut control joints into every slab they poured including the basement for the home and the garage slab.



So there was a LOT of work done that day on this slab only.
One thing to keep in mind about the trucks coming in, if your drive is steep like mine, they needed to keep the trucks filled with only 9 yards or they would spill out on the drive on their way up to the homesite. Another thing about keeping the slab wet is that they rinsed it off the next day and did not put any more water on it after that for curing. The contractor said we were lucky though, since it rained softly off and on for the next couple days. But nothing after that.
George, I hope you don't mind me sharing those photos, but I thought it might help for some of the guys who had questions about what was involved or what they'd be getting into.

 

[6sunset6]

I hired out every slab I ever did. Rebar , wire , fiber , vapor barrier, foam it all works. My house is on a slab and I put radient heat pipes in it. My barn I never even thought about radient heat. With antifreeze and 4 panels and a photvoltaic sourced pump I could keep the floor above freezing and all the tools would be warmer. And not spend a dime. I have no idea why I never thought of it.