Concrete Questions

[CurlyDave]

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.

How many of those homes have you come back to 4 or 5 years later and inspected for concrete cracks? I bet that number is very low, approaching zero.

My point is that even though "everybody does it this way" and you have done it 50 times this way, that doesn't mean it isn't important to keep the concrete wet.

On my new house, the "professional" concrete guys did 90% of the flat work. Less than a year later there are way too many cracks for my liking. I did one slab, and I did three things differently.

1. more rebar.
2. I used a concrete vibrator (stinger).
3. I put a piece of sheet plastic over the slab after the pour and flooded the slab under the plastic twice a day for two weeks.

My slab has no cracks at all.

The average contractor would be out of business in a hurry if he took the care I did, but the result speaks for itself. Any owner-builder can do a better job on concrete by taking the extra time.

I did hire a professional finisher, which is well worth the cost.

 

[NuBota]

Will probably catch flak - but I agree with Pat g in terms of doing it all in increments - especially for your application.

In my first house when I was young and poor, I poured my 12x24 @ 4" patio by hand. Had a boss that would let me bring left over material home from our jobs. So at the end of each day I would shovel left over sand, gravel into my little S-10. Extra bags of portland I could take as well. After a few months I had enough material (had to buy a few bags of portland) - and an old 17cf mortar box and mixing hoe

Anyway, poured two slabs 12x12 a week apart - each took about hours - had my old man helping. Good solid base (had the area framed and prepped for months), 6mil plastic and wire mesh. Was in the house 13 more years with it - cold joint never opened up - no cracks. This was my first time ever doing it - I didn't lay mud on the job until AFTER I did this project. Was in the old neighborhood few months ago and it still looks the same - no cracks or shifting - 20+ years now.

Used a 12' 2x4 to screed/tamp and then just hand mag float and finish
trowel. Did a glass finish (just to see if I could) before brushing

I am doing the same thing as N80 - gonna build the barn with dirt floor - then decide how to handle floor later. Am thinking about doing it the same way since I don't like the pressure of getting it done fast with the truck on site. Buddy of mine got lime poisoning doing the footing for his garage - was rushing to keep up and mud went down in his boots - boy could barely walk for weeks.

Not negating all the good advice here - if I was building a garage or house - I would have the pros do it - I just need a level area to change implements. Over time could extend into a complete floor useing Pat's advice

 

[RobJ]

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.

How many of those homes have you come back to 4 or 5 years later and inspected for concrete cracks? I bet that number is very low, approaching zero.

My point is that even though "everybody does it this way" and you have done it 50 times this way, that doesn't mean it isn't important to keep the concrete wet.

On my new house, the "professional" concrete guys did 90% of the flat work. Less than a year later there are way too many cracks for my liking. I did one slab, and I did three things differently.

1. more rebar.
2. I used a concrete vibrator (stinger).
3. I put a piece of sheet plastic over the slab after the pour and flooded the slab under the plastic twice a day for two weeks.

My slab has no cracks at all.

The average contractor would be out of business in a hurry if he took the care I did, but the result speaks for itself. Any owner-builder can do a better job on concrete by taking the extra time.

I did hire a professional finisher, which is well worth the cost.

Dave I didn't do any of the concrete work, I was a teen and worked on a framing crew. This was just my observation. I'm sure there are some cracks in that concrete by now. But a small hairline crack doesn't mean the concrete failed because it wasn't done properly. You'll see cracks in concrete in a highway, thick concrete, 1/2" rebar on a 12" grid. I doubt if more rebar or bigger rebar would help much. Concrete just cracks!

The only way to be sure your concrete in your house slab isn't cracked is to pull up the carpet, wood, tile and see.

 

[tessiers]

I work for a large construction company (100 men) and we place probably 4 to 5 thousand yards per year. All concrete slabs over about 15 feet square crack. The steel keeps the cracks from opening and moving but they are still there. We use wire in most slabs although we do use rebar in some of the heavy use slabs. Cutting control joints where you want the cracks will help. We cut them into squares between 10 and 15 feet in each direction.

 

[Dargo]

Well, according to "ConcreteNetwork", I got really lucky with the approx 4 to 5 days of constant rain after my concrete was poured. Check out this link: Allow Proper Time to Water Cure - Increase the Strength of Concrete Slabs - The Concrete Network

That seems to backup what my concrete company said about keeping the slab wet after it's initial curing. It says "Concrete which is moist cured for 7 days is about 50% stronger than uncured concrete."

This water curing was purely by accident (luck?). I guess I'll see if it holds up better than other outside concrete I have that wasn't cured. I used a cure and seal product on all inside concrete poured as soon as I could walk on it without creating any damage. Most of my outside jobs were just way too big for me to do that. Maybe mother nature helped by water curing this last pour for me. Anyway, maybe this info and link may help someone with future pours. I'd not ever heard of water curing, but it does seem to be well documented online now that I've taken the time to look.

 

[bindian]

Split a 2x4 (or whatever size form board you use, lengthwise after drilling holes in it large enough for your rebar to stick through it (with the holes the distance apart you want rebar) on the side of the section to be added onto later (not the outer edge of the floor.) Place the rebar and then reassemble the form board with long deck screws (dry wall type work but break too easy if you are nearly as clumsy as me.) You want the rebar to stick out at least as much as your minimum overlap distance (over a foot for 1/2 inch rebar.) Pour, screed, and finish that section. As soon as the "mud" allows move the form over to be the form on the other side of the next section to be poured. If you haven't put down the rebar for that section do it now, overlap with the previous rebar, tie the rebar overlaps with wire using a pigtail tool.

Pat

Pat,
Splitting the form for rebar is a good idea. Would it work without splicing the rebar and run continious rebar and just slide your split form down the rebar to do you next section of concrete pouring? Or would the split form hang up on the little ridges of rebar?
hugs, Brandi

 

[JB4310]

My observations on concrete. I place concrete often but mostly just capping narrow trenches with bagged 5000 psi mix. I cut open all types of concrete, from 80 year old to 1 year old, from 1 inch to 8 inches. this is all done to repair or install sub-floor drainage systems.

Back in the day when more attention was given to concrete finishing there was much less cracking, keeping concrete wet was the standard.
When they did sidewalks I remember them keeping wet burlap on them for days. they really worked the concrete bringing up the cream and setting the aggregate down. That made for a very durable hard surface that lasted for decades.

Now the standard is pour it, strike it and broom finish it, all at once.
I've seen new sidewalks spall to crap after a couple of winters.

Inside the basements, the worse cases I've seen are the mcmansions, where they dump the concrete in there so wet it practically self levels, even at 4-6 inches and with control cuts, they shrink so much and relieve themselves in every direction.

I've also seen 50 year old slabs with no joints and no cracks. that's not the norm though and we tell people that it is not really a defect to have cracks in concrete, it's so common it should be expected.

I've heard that now they are installing formed slots in foundation walls to encourage straight line cracks, just like control joints in floors. Much easier to deal with a straight crack than one that goes on an angle.

Almost all cracks in basement floors and foundation walls occur within a short time after they are poured due to stress relief. not as many believe from settling years later. Slabs exposed to the elements can crack years later due to frost/ground movement.

JB

 

[patrick_g]

Pat,
Splitting the form for rebar is a good idea. Would it work without splicing the rebar and run continious rebar and just slide your split form down the rebar to do you next section of concrete pouring? Or would the split form hang up on the little ridges of rebar?
hugs, Brandi

Interesting coincidence... I just used a split form yesterday (and a few days ago) when pouring a sidewalk using a small electric mixer (3 1/2 cu ft.) and over two tons of 60 lb bags of redi-mix concrete.

Two things were apparent: 1. I made a good decision when I got pallet loads of 60 lb bags instead of 80 lb. and 2. you need to significantly oversize the drilled holes to make pulling the form easier.

When you are pouring over sand the removal is easier but I was pouring over hard clay with embedded shale (previous parking area) and the uneven surface makes taking out the split forms more difficult unless you "super-size" the drilled holes.

If you are pulling the forms before the mix sets up much the over sized holes are not a problem but if you wait till the crete is definitely hardened then the cured crete in the holes can make for removal woes. You can stuff clay into the space around the rebar in the holes to prevent concrete intrusion. You can stick a 2x2 inch patch of duct tape with a "starter" hole in the center over the rebar (rebar through center of tape) and then slide the tape into contact with the form. That or any other way to keep the mix out of the hole will make removal easy. A little wet dirt (clay is best) covering the holes after the form is assembled will do the job.

A pallet of redi-mix is about 3500 lbs as is a pallet of retaining wall blocks. I have used two of the three pallets of redi-mix and two of the four pallets of retaining wall block I bought. Yet another wise decision... I bought block that weigh less than 30 lbs each. The previous retaining wall I built (5 years ago) I used block that weigh 84 lbs each. Not so wise decision... I did this work in high 90's F temps.

Good luck on your project. I hope the split form works well for you as it has for me.

Pat

 

[Egon]

Doing "Concrete Work" all by hand at 90 degrees Farenheight may require a lot of determination or ???:thumbsup:

 

[patrick_g]

Doing "Concrete Work" all by hand at 90 degrees Farenheight may require a lot of determination or ???:thumbsup:

Probably mostly ??? and little else. It has been a while since anyone accused me of having good sense.

Maybe SOMEONE will take pity on me and come by for a visit and give me a good excuse to relax and take it down a notch or two, activity wise.

Pat