Concrete curing time

[gspren]

Today I had my pole building poured with 6" of concrete, 46'x64' plus a couple feet of apron in front of end doors and it took 58 yards. Stones on dirt then plastic vapor barrier and wire mesh. I asked the contractor about when I could park the RV on it and he said 2 weeks, I expected longer so I googled it and see recommendations from 10 to 28 days for heavy vehicles and 7 to 14 days for cars, what do the FF concrete gurus say? RV is a 5th wheel about 13,000 lbs. plus the F350 diesel truck.

[metalspinner]

I had the same size pad poured for my pole building. 
28 days is what my guy told me. Even then, there was lots of moisture in the building for a couple of months. Before I insulated it, it was literally raining in the building from condensation buildup on the metal roof then dripping down onto the slab. 

[Ljohnsaw]

Concrete cure time is dependent on water being available.  If you let the concrete dry out, curing stops.  I've always used 7 days IF it is kept wet all the time.  Something like 80-90% strength by the end of the week is what I remember.  But, if it was my slab and I spent that much money (58 yards!!!), I'd wait out at least 3 weeks IF you keep it wet!


Edit:  I stand corrected from the link below - 7 days approximately 70% strength.

[Southside]

Completely agree - put the water to it, and keep doing that.  You will be a lot happier down the road.  

[mike_belben]

A green slab achieving maximum hardness requires continued moisture. Dont let it get dry especially in the first week, but moisture plays a role in hardness the entire first year. 

Role of Concrete Curing

[Ianab]

The 2 weeks is probably "safe", it will have "most" of it's strength by then. 

Concrete is generally engineered with plenty of safety margin built in. If you expect the load to be 13,000 lb, you build it to take 26,000, just in case. 

Like the other say, keep it damp while it's curing. If it dries out before a week is up, you will have less than ~70% of the max strength. Maybe OK because of the margins built in, but better to go to 2 weeks and get to 90% of the theoretical max. If it fails for that last 10%, the design or mix was wrong. 

[HemlockKing]

I’m pretty sure concrete takes months to fully set, but it has most of its strength after 28 days/month 

[doc henderson]

my brother did road construction and he had a rule of 7s.  7 hours to walk on it, 7 days to drive on it.  7 years to fully cure and be at max strength.  we always keep it wet especially in this heat, and pour early in the morning.  thicker takes longer.  rumors are that the hoover dam is still putting off chemical heat.  



 


Monique watering in the sand around the clean out repair.    





 

 

6 inch slab we poured last week to park our camping trailer RV with sewer and power.  My guy Eddie wants it cut that evening.  what you see is the back half, and the front half will angle to the main drive so I can back in there.  the chemical reaction is exothermic, and heat catalyzes the reaction.  so wetting the slab cools it and slows the reaction.  the heaters in the MREs use a similar chemical reaction to heat food after mixing the chemical packs.

[Resonator]

I always went with 2 weeks to "full" strength, and covered my slabs with plastic. With the science of hydration you want to put the right amount of water in the mix to make it workable, (but not so wet as to weaken it), and then keep that moisture in by means of wetting, or covering it, as it sets. The cured hardness is achieved most in the first couple weeks, after that it is a diminishing return. So leaving it longer isn't necessary, but won't hurt. Also saw control joints, that way if it cracks it is in a predictable place.

[Sedgehammer]

Today I had my pole building poured with 6" of concrete, 46'x64' plus a couple feet of apron in front of end doors and it took 58 yards. Stones on dirt then plastic vapor barrier and wire mesh. I asked the contractor about when I could park the RV on it and he said 2 weeks, I expected longer so I googled it and see recommendations from 10 to 28 days for heavy vehicles and 7 to 14 days for cars, what do the FF concrete gurus say? RV is a 5th wheel about 13,000 lbs. plus the F350 diesel truck.

Wire mesh..... Hope it was on chairs and not "we'll just pull it up as we pour" and then walk it back down when they walk over it again

My engineer wants 1/2" rebar 12" OC both directions for that same depth and type of pour. I think that's over kill, might go 18" OC and use fiberglass. Friend that has a large crete company in minneapolis/st. paul area told me to never use wire mesh or fiber glass mesh. Says it's almost worthless as far as strengthening the slab goes

What was your yard price if you don't mind?

I called our plant we've been using 2 days ago. $175 per. Since we had an account, $117. Which is cheaper then nearly everyone else in the area, as i called around. I expected some difference between account vs non, but that's crazy

We add a product that slows cure time down and you can add some water and not weaken the crete

I like @Resenator using plastic to cover it. That should retain the moisture coming off and not need to add water to keep it wet

[Resonator]

If you pour with mesh you have to make sure the crew lifts it, it does no good on the bottom. Once it has some mix under it, it will stay suspended. Rebar has more structural strength, depends on what the slab will be used for. How hard the subbase is compacted will effect strength also, as well as if it is insulated underneath. All my training was from back in the 1900's, :D so I was taught to use a stiff mix for strength. Now watching video's of concrete mixed with water reducers, it looks like they are pouring soup.

[beenthere]

And more and more, we are seeing the use of post tension cables used along with reinforcing rod to keep tension on concrete slabs. 
Building a New Home: Post Tension Cable Foundation â€" Mondays with Mike - YouTube

Has been used in concrete structural bridges for years, but now seeing a lot of it used in concrete slabs. 

[alan gage]

Like others have said 28 days seems to be the default safety recommendation. You can probably get by sooner than that but how much sooner is the hard question to answer.

I have a friend who works for a commercial construction company. When they pour concrete they take multiple samples which they test for strength over the coming weeks to determine when it's safe to place certain loads on the main pour.

Alan

[reride82]

two weeks you should be good, but lets break it down by the numbers. 13,000 lbs divided by four tires(assuming a tandem) gets you 3,250 lbs per tire and 40 square inches of contact gets you around 82 psi of contact pressure. Assuming you poured 3500 psi concrete at 70% at two weeks your concrete will provide 2,450 lbs of support in a static situation. As long as you're not scrubbing tires or cranking the tires excessively you will be fine.

Levi

[jrsloan1]

Lots of good info given here.  Here is a few things I remember from when I was in Engineering school, and what numerous engineers have confirmed since, and the bajillion cubic yards I placed when working in construction:

1.  The concrete is designed for a particular strength at 28 days.  So, a 3500 psi mix is designed to have a breaking strength of 3500 at 28 days.  The concrete should reach 75% of the design strength in 7 days.  Those test cylinders made during the pour are generally tested first at 7days to confirm.  Usually a good mix/pour will meet or exceed the 75% at 7 days, and also the design strength at 28 days.  In US military work, we did not allow work to proceed until we had confirmation the design strength had been met.  Many times we had 100% strength at 7 or 14 days, confirmed by a cylinder break.

2.  Very important - the amount of water is calculated based on the desired slump of the concrete and strength design.  When you add water to the truck at the pour site, you will actually weaken the strength because the water/cement ratio is changed.  Always have the design made for the slump you want and have the batch plant mix it correctly.  The addition of a plasticizer will make the concrete easier to work, can add to the finish time and will not weaken the mix.  Wetting up a load to have an easier pour is not good for the finished product.

3.  The welded wire fabric does nothing for the strength nor does the fiber.  They are used to help prevent cracking.  One of the best things to prevent surface cracks and also to strength the concrete is KEEP IT WET while curing.  I mean wet.  Helped a neighbor pour a slab a couple years back and after it had "set" we actually put a sprinkler on it and watered it for a couple weeks.  no wire, did use fiber mix and rebar only in the footings.  To date, there are no cracks in the 30' x 42' slab!!  His dad thought we were crazy but has since changed his mind.

4.  Control joints help "control" where it cracks.

5.  Seems like I remember it actually takes MANY years for concrete to completely "cure".

6.  So much more that can be said, but three thinks you can bet on with concrete: it gets hard, it shrinks (might not see it, but it does), and it cracks!!

[gspren]

Thanks everyone! I did wet it today but I will be out of state for about 5 days so it wont get any extra water, just the way it is. Aside from the roof the building is shaded most of the morning by trees. I'll probably wait at least 3 weeks for the rv but will put some small stuff in sooner, nothing heavier than me for the first week.

[scsmith42]

Lots of good info given here.  Here is a few things I remember from when I was in Engineering school, and what numerous engineers have confirmed since, and the bajillion cubic yards I placed when working in construction:

1.  The concrete is designed for a particular strength at 28 days.  So, a 3500 psi mix is designed to have a breaking strength of 3500 at 28 days.  The concrete should reach 75% of the design strength in 7 days.  Those test cylinders made during the pour are generally tested first at 7days to confirm.  Usually a good mix/pour will meet or exceed the 75% at 7 days, and also the design strength at 28 days.  In US military work, we did not allow work to proceed until we had confirmation the design strength had been met.  Many times we had 100% strength at 7 or 14 days, confirmed by a cylinder break.

2.  Very important - the amount of water is calculated based on the desired slump of the concrete and strength design.  When you add water to the truck at the pour site, you will actually weaken the strength because the water/cement ratio is changed.  Always have the design made for the slump you want and have the batch plant mix it correctly.  The addition of a plasticizer will make the concrete easier to work, can add to the finish time and will not weaken the mix.  Wetting up a load to have an easier pour is not good for the finished product.

3.  The welded wire fabric does nothing for the strength nor does the fiber.  They are used to help prevent cracking.  One of the best things to prevent surface cracks and also to strength the concrete is KEEP IT WET while curing.  I mean wet.  Helped a neighbor pour a slab a couple years back and after it had "set" we actually put a sprinkler on it and watered it for a couple weeks.  no wire, did use fiber mix and rebar only in the footings.  To date, there are no cracks in the 30' x 42' slab!!  His dad thought we were crazy but has since changed his mind.

4.  Control joints help "control" where it cracks.

5.  Seems like I remember it actually takes MANY years for concrete to completely "cure".

6.  So much more that can be said, but three thinks you can bet on with concrete: it gets hard, it shrinks (might not see it, but it does), and it cracks!!

This is excellent information, especially the part about how critical the water ratio is.  If the guys that are placing the concrete are bitching about how thick it is, then that's just about the right consistency.
One thing that I have not seen mentioned is the use of a curing agent on the surface in lieu of keeping the slab wet.  This is a wax like substance that is applied immediately after the troweling is finished.  It takes the place of watering down the slab.
My last big pour was 128 yards when I built my wood shop.  12 years later and no cracks other than from an expansion joint to one of the poles.  4.5" slump, 4000 psi mix with an admixture for the concrete pump.  We did a November pour too so as to allow it to cure slowly over the winter.  This results in less cracking and greater strength too (as long as the slab does not freeze within a few days of the pour).

[Don P]

Seems like I remember it actually takes MANY years for concrete to completely "cure".

I've read that curing goes on for about 40 years but it is one of those sloping curves that is down to not much between 28 days and 40 years. At that point if you did everything right deterioration begins.

When they poured Hoover dam it was a special "cool" mix and they ran cooling water through pipes cast in the pour, I think that went on for decades.

Curing, hydration is the formation of long slender needle like crystals that knit the whole matrix of aggregates together. If it dries, freezes is mixed or troweled too long and probably dozens of other sins it disrupts or stops that crystal formation and you have dead cement in the concrete.

Roman concrete is still curing, natural pozzolans and slaked lime. CO2 is still hardening that lime.

[doc henderson]

the wire mesh I used was between 3/16ths, and 1/4 inch.  the steel is to keep a crack from separating or dropping down.  all concrete cracks as it is brittle.  so the cutting is to designate where the cracks will be and keep them straight, and they are hidden in the bottom of the kerf of the saw.  we pulled up the matts.  I have used chairs but they also crush as you walk on them.  we did spots of concrete and pulled up the mesh.  it is ideally in the center of the slab.  

[Sedgehammer]

the wire mesh I used was between 3/16ths, and 1/4 inch.  the steel is to keep a crack from separating or dropping down.  all concrete cracks as it is brittle.  so the cutting is to designate where the cracks will be and keep them straight, and they are hidden in the bottom of the kerf of the saw.  we pulled up the matts.  I have used chairs but they also crush as you walk on them.  we did spots of concrete and pulled up the mesh.  it is ideally in the center of the slab.  

I've been on gobs of demo jobs where wire mesh was at the bottom or nearly so and the concrete fell apart during tear out as it wasn't strong enough to hold it together. You could almost always tell what kind of job it was going to be when you saw what metal was in it. any metal in crete rusts, so the thinner the metal in it, the sooner it's not there. Drier locations will of course last longer, damper locations, not as long. smallest metal rebar on this place is #5. 

[doc henderson]

I think the selection is determined by the use intended and pounds per sq. feet.  loaded semi trucks parking would get 1/2 inch rebar.  when I've torn it out, you can break mesh or concrete fall off.  rebar tend to break less but still comes apart tearing it out.  the mesh is perfect for normal use and especially great with pex installation.  rebar costs more and takes more labor to place.  choices.  we sometime put rebar along the edge and of course in footings.  my opinion is the rebar/mesh is to keep the chunks of concrete aligned after cracking, it does not prevent cracking.

[Don P]

Except for pre or post tensioned, the steel doesn't "hook up" and go into load until the concrete has cracked. If it is in the middle it is in the neutral axis where the only thing it can really do is hold stuff together. If it is closer to the tension side it can do more work. Concrete is good in compression, steel in tension. 

[doc henderson]

true Don, great description!

[reride82]

Don beat me to it. rebar in the middle of a slab doesn't add any structural strength except for shear resistance. Ideally, rebar should be in the bottom third of a slab as your loading is from above and the bottom of the slab is put into tension. The problem with most driveways/slabs is they don't have adequate depth to have the steel in the bottom third and maintain the clear separation from the edge of slab to the rebar. In essence, the rebar has to be embedded far enough in the slab to keep from breaking away from the surface, be it the top or bottom of the slab. for #5 bar, you'll need at least 5.5" of slab depth to maintain your clear distance. I prefer #4 on a closer spacing set lower in the slab on anything less than 6" depth.

Levi