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Old 09-15-2014, 10:55 AM   #1
hillbillypolack OP
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Best time / temp to pour concrete driveway

In the spirit of keeping things simple, I've decided to opt 'only' for a new garage floor and driveway. Through asking two reputable contractors, both have slightly different methods of the mix & prep.

One is traditional, compacted site and rebar, 4" standard thickness. Doing a vapor barrier (also possibly foam) in the garage. He suggests not to do a vapor barrier on the drive as this could affect surface hardness since water will only have one direction to evaporate (up).

Second guy uses fiberglass mix, no rebar. Also compacted site, 4" standard thickness. He has poured over vapor barrier on another clients drive, says this aids in keeping that guys drive snow free (per the clients request).

It's a coin toss which guy to use, as I will probably be home either way. Both look like they're booked until 3 weeks, putting me into mid October. In Michigan that starts to be iffy depending on rain & temp. I could wait until spring but was hoping to set up my shop when weather starts getting cold.

So, anyone in the upper Midwest, what's the best time to pour concrete? Do I wait? Or try to get on the books and hope for mild weather?

And, has anyone poured over foam in areas like a driveway? What about a vapor barrier in these areas not enclosed like a basement or garage?

Thanks for any input.
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Old 09-15-2014, 11:01 AM   #2
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When contractor A says rebar is he talking about rebar or just wire mesh? Big diff. Although I think the wire mesh is all you need for a driveway unless you are gong to park and or drive semis on it.

The merits of fiber mesh has nothing to do with the strength of the concrete. Either way they will both hold up and achieve the same compressive strength. The issue is if you want "hairy" concrete. The mesh tends to be exposed initially in most concrete placement applications. Eventually they will shear off and the fuzz goes away, but it can be an issue for some at first.

With regard to the vapor barrier it is not needed in exterior concrete applications and I have never seen it done.

I did stay in a Holiday Inn Express last night!
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Old 09-15-2014, 11:10 AM   #3
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I think you can find three consecutive decent weather days in October. I mean, I did when I poured a 6" pad for a job in Jackson, MI in the middle of October.
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Old 09-15-2014, 11:14 AM   #4
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Quote:
Originally Posted by Steverino View Post
When contractor A says rebar is he talking about rebar or just wire mesh? Big diff. Although I think the wire mesh is all you need for a driveway unless you are gong to park and or drive semis on it.

The merits of fiber mesh has nothing to do with the strength of the concrete. Either way they will both hold up and achieve the same compressive strength. The issue is if you want "hairy" concrete. The mesh tends to be exposed initially in most concrete placement applications. Eventually they will shear off and the fuzz goes away, but it can be an issue for some at first.

With regard to the vapor barrier it is not needed in exterior concrete applications and I have never seen it done.

I did stay in a Holiday Inn Express last night!
I think contractor A meant wire mesh- I understand what you mean about the difference. Rebar being the heavy gauge freeway prep bar.

As I understand it, fiberglass helps in the drying process to prevent or minimize cracks. The mesh keeps concrete in position if a crack occurs.
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Old 09-15-2014, 11:17 AM   #5
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I think you can find three consecutive decent weather days in October. I mean, I did when I poured a 6" pad for a job in Jackson, MI in the middle of October.
True.

Kind of hoping for an Indian Summer kind of thing this year. As I understand it, the temp shouldn't dip below 32 for the first 24 hours while it's initially setting up.

Keeping leaves off might be tricky though.
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Old 09-15-2014, 11:21 AM   #6
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Originally Posted by hillbillypolack View Post

Keeping leaves off might be tricky though.
I'm guessing you're going to have a broom finish? I think a few leaves will be no problem. They're just not heavy enough to leave impressions in the finish.
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Old 09-15-2014, 11:37 AM   #7
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Quote:
Originally Posted by hillbillypolack View Post
I think contractor A meant wire mesh- I understand what you mean about the difference. Rebar being the heavy gauge freeway prep bar.

As I understand it, fiberglass helps in the drying process to prevent or minimize cracks. The mesh keeps concrete in position if a crack occurs.
Both do the same thing. Prevent differential settlement. You use fiber in lieu of wire mesh. The only thing that prevents or minimizes cracking is control joints. Make sure no panel of concrete exceeds 3000sf. When we really get concerned about we reduce the specification for maximum panel size down to 1500sf. Joints can either be cut with a saw while still green (within 24 hours) or tooled into the slab at the time of placement.

Remember the 2 rules of construction:

1. Shit flows down hill
2. Concrete cracks
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Old 09-15-2014, 11:40 AM   #8
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Regardless of when you place the concrete it should be covered to slow the rate of drying. If it dries too quickly you get cracks. All our slabs are required to be covered during the curing process. About a week...

I do Construction Management for an Architect. My projects are usually large ($40-80 million) with very demanding clients. We put a lot of effort to make sure any exposed concrete is protected to make sure the finish is as good as we can get it and to minimize cracking. Nothing worse than an end user not familiar with construction moves into their new building and see concrete cracking.
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Old 09-15-2014, 11:53 AM   #9
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Quote:
Originally Posted by Steverino View Post
Regardless of when you place the concrete it should be covered to slow the rate of drying. If it dries too quickly you get cracks. All our slabs are required to be covered during the curing process. About a week...

I do Construction Management for an Architect. My projects are usually large ($40-80 million) with very demanding clients. We put a lot of effort to make sure any exposed concrete is protected to make sure the finish is as good as we can get it and to minimize cracking. Nothing worse than an end user not familiar with construction moves into their new building and see concrete cracking.
What are your thoughts on wetting the surface with a water mist during the drying process instead of covering? I've seen that method applied, but we're talkin' driveway here, not superstructure stuff.
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Old 09-15-2014, 12:02 PM   #10
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What are your thoughts on wetting the surface with a water mist during the drying process instead of covering? I've seen that method applied, but we're talkin' driveway here, not superstructure stuff.
We do a lot of drives and sidewalks as part of the projects we do. Wetting only allows for the concrete to not dry out too fast and not being able to get a good finish on it. For the most part we do not permit adding water to either the batch before it is placed or during the placement process. All added water reduces the over all compressive strength which is something we put a lot of effort into to make sure we get the strength we need. Even for sidewalks a s drives.

Covering to reduce the drying rate is the best way to get maximum strength and reduce the amount of cracking.
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Old 09-15-2014, 12:34 PM   #11
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Fall should be better than mid summer for slow drying and thus ultimate strength and crack avoidance. Are you thinking of putting radiant heat into the garage/workshop floor? Then you definitely want an insulated (from below) pad.


Somewhat related: Has anybody used flexi-pave in this (upper midwest) climate? On paper it sounds very nice for drainage and frost survival.
See e.g. http://www.environmentalpavingsoluti...lexi-pave.html
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Old 09-15-2014, 06:22 PM   #12
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I was always told the slower the cure, the better the slab. My last four pours have all been over plastic (both interior and exterior). Forcing the water to leave one way thru the surface, slowing the rate of drying. Knock on wood, everything is holding up. Most our pours have been on compacted clay sand, which acts like a sponge. It pulls water out of the mud too fast in my opinion.

I've used a lot of fiberglas. But the concrete plant I've been using now offers Buckeye fiber. Strong as fiberglas, but no whiskers. Same price upgrade as the fiberglas they used to offer. Trowels out smooth with no need to burn off the stray whiskers.
http://www.avrconcrete.com/buckeye.aspx#Overview
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Old 09-15-2014, 07:13 PM   #13
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I am also into structural concrete construction like Steverino, currently working on a 175 million concrete building project as primary concrete/rebar special inspector this one has 250,000 cyds concrete 500,000 lbs rebar with 140 post tension pours, $ floors undergound parking, 22 floors concrete above ground been on this one since last september
Resume' aside,
the contractor poured all year around, Pacific Northwest, rain , snow, ice, never held schedule for weather. Batch plant mixed with hot water at batch plant. Engineer targeted for 4000 psi concrete so the spec was at 5000 psi concrete in slabs and 8000 in columns
I had to fight with them to do cold weather curing, they thought acclerating admixtures would be enough. I had them melt frost from forms before pouring. They had to cover the slabs and columns with insulated curing blankets overnight and put propane heaters below shrouded off with plastic.
initial curing temp is most important, should be over 50 F but 40 will work if there is no frost on the ground
.... You could get high quality by having the ready mix plant add hot water at batch and covering with insulation for three days, for example after finish is set enough to walk on, cover with plastic and a 1-2 ' thick layer of leaves then another layer of tarps , (of course the contractor will think this is crazy)
The weak link in a slab on grade is the sub base (soil compaction), reinforcing in a slab on grade is primarily for crack control. The reinforcing should be mid slab up on dobies BEFORE pouring not laying on the ground, you cant pick it up later no matter what they say
A concrete mix has a specified optimum water content determined in the field by "slump" (measure of wetness), typical is 4" (dry) with a 1" plus or minus
contractors like to get it wet (around 6) to make it easy to work with
one inch of slump over tolerance weakens by 2-400 psi, 3 inches over slump can weaken by 1000 psi or more
adding water after 90 minutes (or point of max hydration 90 to 3 hrs) destroys the concrete and turns it into junk
Hot water allows initial hydration to occurr but the finishers whine like little bitches and want to add water to the top to make it easier to trowel
Adding water to the top when finishing is unprofessional and lazy but a professional finisher will argue to the point of trading blows about that , they all do it

there is a lot more to it but I probably bored you already
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Old 09-15-2014, 07:29 PM   #14
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Quote:
Originally Posted by Dustodust View Post
I am also into structural concrete construction like Steverino, currently working on a 175 million concrete building project as primary concrete/rebar special inspector this one has 250,000 cyds concrete 500,000 lbs rebar with 140 post tension pours, $ floors undergound parking, 22 floors concrete above ground been on this one since last september
Resume' aside,
the contractor poured all year around, Pacific Northwest, rain , snow, ice, never held schedule for weather. Batch plant mixed with hot water at batch plant. Engineer targeted for 4000 psi concrete so the spec was at 5000 psi concrete in slabs and 8000 in columns
I had to fight with them to do cold weather curing, they thought acclerating admixtures would be enough. I had them melt frost from forms before pouring. They had to cover the slabs and columns with insulated curing blankets overnight and put propane heaters below shrouded off with plastic.
initial curing temp is most important, should be over 50 F but 40 will work if there is no frost on the ground
.... You could get high quality by having the ready mix plant add hot water at batch and covering with insulation for three days, for example after finish is set enough to walk on, cover with plastic and a 1-2 ' thick layer of leaves then another layer of tarps , (of course the contractor will think this is crazy)
The weak link in a slab on grade is the sub base (soil compaction), reinforcing in a slab on grade is primarily for crack control. The reinforcing should be mid slab up on dobies BEFORE pouring not laying on the ground, you cant pick it up later no matter what they say
A concrete mix has a specified optimum water content determined in the field by "slump" (measure of wetness), typical is 4" (dry) with a 1" plus or minus
contractors like to get it wet (around 6) to make it easy to work with
one inch of slump over tolerance weakens by 2-400 psi, 3 inches over slump can weaken by 1000 psi or more
adding water after 90 minutes (or point of max hydration 90 to 3 hrs) destroys the concrete and turns it into junk
Hot water allows initial hydration to occurr but the finishers whine like little bitches and want to add water to the top to make it easier to trowel
Adding water to the top when finishing is unprofessional and lazy but a professional finisher will argue to the point of trading blows about that , they all do it

there is a lot more to it but I probably bored you already

That's a great post. Lots of info.

Oddly enough I spoke to Contractor A about the slump water this morning as I was considering a vapor barrier below the slab on the driveway. He explained the water has to go somewhere when curing. If it only is allowed up (as with vapor barrier), the surface can weaken during the cure. I suppose that makes sense, similar to ceramic or plaster slip where the top layer is often wetter than the body.

Decided to go with Contractor A due to his communication, a quick written estimate and willingness to talk over details. He's old school, still uses reinforcement mesh.

Regarding prep, should I insist on a particular material to be compacted? He's using compacted gravel on the drive, and while I'm planning on 2" foam in the garage, should I insist he run compacted gravel there as well? He would have to remove additional material, to accommodate the foam plus gravel in that case.
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Old 09-17-2014, 06:33 PM   #15
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Also considering a 'flat' floor in the garage instead of the pitched floor currently in there. It'll be a hobbyist garage, no real need for drainage.

Looking to have it semi-polished afterward with a densifier applied.

The drive will be cut (pads & control cuts), can anyone recommend a caulk for these grooves? Thx.
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