What to worry about when getting concrete done

[racecougar]

Ditto on the finish. I had my crew burnish my pad when it was poured in '18. It's easy to wipe up liquid spills without staining, easy to sweep/mop, doesn't get damaged by welding like a coating would, etc.

 

[bb29510]

cutting the slab, dont go over ten feet and cut it 1/4 the thickness, cut it as soon as you can walk on it. the same day, dont wait till next day

 

[C-S-H]

I would worry about every aspect of this job. The state-of-the-practice in residential construction is very poor when compared to the rest of civil construction. For little effort and money you can be buying a much better casting with improved performance and service life.

1. Drainage and structural design. Nothing was posted on this for us to comment. Water will kill your project. And there are important structural design implications on the mix design, and curing procedure. Take a look at this Garage Journal Forum post from November 2. thermal crack post
The colossal perimeter footing created a differential cooling issue. Both mix design and curing regiment (blankets) needed to address this. Instead he got a big unreinforced diagonal crack emanating from the corner of the slab. So 3 problems adding up to failure.

2. Mix design. Don't accept a residential crapcrete mix. Your mix design should be similar to a 4ksi DOT bridge deck mix, which will limit free shrinkage strain to <500 microstrain. Total mix water will be <270pcy, and max W/CM ratio <-=0.45. As a percentage of total cementitous weight, we typically use 20%-25% Type F flyash, or 40%-50% Grade 120 slag, or some other pozzolan. More for a mass mix, or a casting with dissimilar thickness of elements. My basic 4ksi mix uses 585pcy total cementitous, and is a minimum paste mix for use with #57 stone. 5" slump for a graded finish is achieved with Water Reducing Admixture not water (for last 35years).

3. Concreting. They should have 2 vibrators on-hand tested ready to use. They should have a backpack fogger tested with fuel, water, and a marked-up copy of the ACI water application nomograph that we have been using for many years. There is no bleed water from good concrete. Fog it. If the surface has standing water you are ruining it. If the surface frosts over dry you are ruining it.

4. Curing. Fog until time for power trowelling. Fog until initial set (about 4hrs at 70 degF). Then cover it until final set (about 12 hrs at 70 degF). Then keep the surface wet and covered for 7 days, then covered for a month after that. Don't put standing water on the concrete if it is going to freeze. You need curing blankets or just cover it. As an alternative you can apply a curing membrane after power trowelling. If of the resin type, it will allow a surface coating to be applied later.

 

[Smoker]

I would worry about every aspect of this job. The state-of-the-practice in residential construction is very poor when compared to the rest of civil construction. For little effort and money you can be buying a much better casting with improved performance and service life.

1. Drainage and structural design. Nothing was posted on this for us to comment. Water will kill your project. And there are important structural design implications on the mix design, and curing procedure. Take a look at this Garage Journal Forum post from November 2. thermal crack post
The colossal perimeter footing created a differential cooling issue. Both mix design and curing regiment (blankets) needed to address this. Instead he got a big unreinforced diagonal crack emanating from the corner of the slab. So 3 problems adding up to failure.

2. Mix design. Don't accept a residential crapcrete mix. Your mix design should be similar to a 4ksi DOT bridge deck mix, which will limit free shrinkage strain to <500 microstrain. Total mix water will be <270pcy, and max W/CM ratio <-=0.45. As a percentage of total cementitous weight, we typically use 20%-25% Type F flyash, or 40%-50% Grade 120 slag, or some other pozzolan. More for a mass mix, or a casting with dissimilar thickness of elements. My basic 4ksi mix uses 585pcy total cementitous, and is a minimum paste mix for use with #57 stone. 5" slump for a graded finish is achieved with Water Reducing Admixture not water (for last 35years).

3. Concreting. They should have 2 vibrators on-hand tested ready to use. They should have a backpack fogger tested with fuel, water, and a marked-up copy of the ACI water application nomograph that we have been using for many years. There is no bleed water from good concrete. Fog it. If the surface has standing water you are ruining it. If the surface frosts over dry you are ruining it.

4. Curing. Fog until time for power trowelling. Fog until initial set (about 4hrs at 70 degF). Then cover it until final set (about 12 hrs at 70 degF). Then keep the surface wet and covered for 7 days, then covered for a month after that. Don't put standing water on the concrete if it is going to freeze. You need curing blankets or just cover it. As an alternative you can apply a curing membrane after power trowelling. If of the resin type, it will allow a surface coating to be applied later.

He's building a shop, not a nuclear reactor. How many concrete guys do you think are going to follow these instructions?
Talk about overkill.

 

[Markromeo]

Take a day off of work and be there during the pour.

I can't even stress this enough! This was my terminal mistake. My concrete is usable...but not exactly how I wanted it, or even exactly what we talked about. I didn't have a written contract (also my mistake), but if I had been there, the small issues would have been alleviated.

 

[C-S-H]

He's building a shop, not a nuclear reactor.

Give me a thousand more pages and I will write you a spec for a slab at a nuclear reactor.

How many concrete guys do you think are going to follow these instructions?

All of them will already know to do it except in the residential crapcrete industry and the worst of light construction.

Talk about overkill.

You are stuck in the 1950's - 1980's dark age of concrete. Residential crapcrete industry is the legacy of that miserable time.

 

[Smoker]

Give me a thousand more pages and I will write you a spec for a slab at a nuclear reactor.

All of them will already know to do it except in the residential crapcrete industry and the worst of light construction.

You are stuck in the 1950's - 1980's dark age of concrete. Residential crapcrete industry is the legacy of that miserable time.

But are we not talking about residential work here? Residential electrical work/specs is "****" compared to commercial and downright Neanderthal compared to industrial. Doesn't mean we should be running explosion proof boxes and rigid conduit with sealoffs in our basements though.

 

[ConCretin]

We should all stop feeding the gibberish speaking trolls. With relatively rare exceptions, the residential concrete industry provides a serviceable product for a reasonable cost even it it doesn't seems so lately. The more outlandish the demands you try to place on your contractor, the higher the price will be and the less likely you'll even get a call back. Stick to the basics and protect your financial interests as best you can with smart payments terms and lien waivers.

 

[Joemctag]

Take a day off of work and be there during the pour.

Don’t consider not being there.

 

[Joemctag]

add a point, without vapor barrier, the soil will **** the water out of the bottom of the concrete white th etop is still wet, can we say cracks, you want it to set unformity and slow.. quick setting causes cracks.

But concrete contractors want to add water to work it easier and then want that water to go away. I don’t think they like to see vapor barriers. You gotta tell them your architect, brother who’s a contractor, wife whoever. So he knows the barriers staying. Probably the worst thing they do is add water when they’re power troweling. Ok if you’re going to add carpet or laminate, but with concrete floor this causes ‘dusting’. Maybe can’t be fixed. Next to my shop is a developm it of $350k homes. I observe all these sins. Oh yeah and they’ll e pouring or finishing concrete when it’s almost dusk. Sidewalks and driveways too. Never seen them forget the joints, but have seen sidewalks with no edge tooling. This a major nationwide homebuilder. The buy the material packages and the concrete subs work directly for them. A vapor barrier Is always done. Probably code. I’ve seen many of their slabs with dusting but it’s ok in a house.
I wish I knew what you could do to have do the finish right. Say you’re going to have epoxy, etc and there can be no dusting?

 

[Joemctag]

add a point, without vapor barrier, the soil will **** the water out of the bottom of the concrete white th etop is still wet, can we say cracks, you want it to set unformity and slow.. quick setting causes cracks.

I put down heavy clear poly a run it over the slab to the ground and weight it down and run water from an hose under it.Water’ll be there for daysForget the sprinklers.

 

[Joemctag]

But concrete contractors want to add water to work it easier and then want that water to go away. I don’t think they like to see vapor barriers. You gotta tell them your architect, brother who’s a contractor, wife whoever. So he knows the barriers staying. Probably the worst thing they do is add water when they’re power troweling. Ok if you’re going to add carpet or laminate, but with concrete floor this causes ‘dusting’. Maybe can’t be fixed. Next to my shop is a developm it of $350k homes. I observe all these sins. Oh yeah and they’ll e pouring or finishing concrete when it’s almost dusk. Sidewalks and driveways too. Never seen them forget the joints, but have seen sidewalks with no edge tooling. This a major nationwide homebuilder. The buy the material packages and the concrete subs work directly for them. A vapor barrier Is always done. Probably code. I’ve seen many of their slabs with dusting but it’s ok in a house.
I wish I knew what you could do to have do the finish right. Say you’re going to have epoxy, etc and there can be no dusting?

I’d like to point out that concrete work is hard work. They learn to do it the easiest way to get ACCEPTABLE results.

 

[Joemctag]

Honestly, it's more about picking the right guy than it is about the contract. I understand your desire to get the contract wording correct but the reality is that it's virtually impossible to monetize and recover damages related to a failed placement. Give my Guide to Floor Slabs a look for some thoughts on a successful slab placement.

With that said, if I had to pick one thing that determines the success or failure of a slab placement, it would be water. Mix water in the concrete must be minimized, bleed water must be managed, water must never be added to the surface and mix water must be preserved through proper curing. If you get the water part right, you are well along the way to a successful slab placement. It's hard to write all this into a contract but a truly competent concrete guy will understand it. Ya gotta find that guy.

Strongly second his advice . I’ve poured enough garage and addition slabs that know it’s true.

 

[Joemctag]

go buy you a cheap soaker hose and a roll of plastic, as soon as they finish where you can walk on it lay the hose on the slab and cover with plastic, every day turn the water on for a while to keep the slab wet for 28 days. in the old days we would have the form six inches high and when they got done, we cover with hay and then flood it like a pond, kept it underwater for 28 days

There IS the alternative, used in commercial and municipal slabs, sidewalks, drives , where they spray ‘hand-held’ with a parrafin-based compound with a solvent that soon evaporates. They spray when they can walk on it. Depends on the weather. Don’t think I’d do that if I was contemplating any kind of finish in the future. Also, ona different point, you need to tell them the finish. Troweled smooth? That’s what I’d go with even though I have a building don like that and it’s easy lip on dry hay thats scattered about. I think broom is out because it holds too much dirt. Maybe see what the finish guys say.

 

[Joemctag]

Shop/garage area for working, I want FLAT. No slope.

I'd want a smooth finish, easier to clean, push a broom and mop if need be.

Yes: Whether flat or sloped is a major decision for you! And VERIFY that formwork and intermediate grade stakes will give you that. Buy a cheap builders laser or use theirs. These subjects have been discussed MUCH on Garage Journal. Many begging with someone’s tale of woe.
Also, anything you don’t want concrete splashed on, should be covered with cheap painters plastic. Say three feet min. up nearby walls. Most residential concrete guys don’t do that. You can’t remove set-up concrete. Can’t wash it off when it happens without messing up your nice, fresh, but still-wet concrete. Even in commercial you’ll see this sometimes.