Got my slab poured, but couldn't keep it wet enough?

[SickSilverado]

I just got my 24x36 slab poured Friday. When it dried enough to touch, I started watering it. But I didn't have a sprinkler, so I was going out and spraying it down with the hose every 4-6 hours. Sometimes when I would come out I could see the some of the surface looked a bit dry. Am I wasting my time continuing to water it? I read that once it's dry, it's too late to re-wet it.

 

[gpflepsen]

Just keep it damp. You could have put a curing compound on, and it may be too late for that now. If it gets dry on the surface, it is still beneficial to get it wet again.

A good curing alternative is to lay plastic over the slab, then flood under it with a hose. The film drastically reduces the evaporation. Is the slab level? If so, block off the doorways (dirt berm) and flood it.

 

[SickSilverado]

Yeah, I think the slab is level. I'll try flooding it right now.
The guy who poured it didn't know anything about curing compound, he said "it cures on it's own with the sun". Luckily I've been reading on here and tried to order it before the pour, but it won't get here for a few more days. As far as plastic, the only store that has it is 20mi away and closed until Monday.

 

[ConCretin]

The goal of curing concrete is to prevent the premature evaporation of the mix water in the concrete. Concrete gains strength through a chemical process called hydration, which requires mix water. If the water evaporates, your concrete stops getting stronger.

I mention all that because it's important to understand the purpose of curing. The water you are spraying on is just acting as a seal to keep mix water in. You can accomplish the same thing with poly, burlap or a spray on curing compound.

I'd wet down your slab and cover it with poly for seven days. Don't give up now. It's not too late

 

[gpflepsen]

A good goal should be to have a controlled environment to prevent drying in the first 7 days.

Curing compounds result in a film barrier against evaporation. The same with poly film. Burlap needs to be kept damp to be most effective. Failure to do this can contribute to surface wear, spalling and crazing (little cracks from shrinkage).

Have a wet substrate and maintain a damp surface after it is set. Don't let them "bless the concrete" when finishing either; that's way bad. Water is bad and good for concrete, it just depends when and where it is used.

 

[SickSilverado]

Ok, wife got me a good sprinkler. Just gonna leave it on for a few days and hopefully get a ride to the next town to buy some poly to cover it.

 

[SickSilverado]

By blessing the concrete do you mean putting their signature?

 

[CNGsaves]

So are all of the greenhorn replies about concrete curing from people who have never seen ANY concrete covered and kept wet before???

Ever driven where new concrete highway going in that gets covered with wet burlap??

Live in a country where there are concrete slabs and highways ???

Now might be good time to Update GJ Profile with City / State / Country. THEN use the Advanced Search of GJ and READ through the countless prior threads covering concrete slab curing.

 

[gpflepsen]

By blessing the concrete do you mean putting their signature?

Some finishing crews will throw water on the surface as they trowel. The problem is that the top thin layer of mortar paste ends up with a high water/cement ration (low w/c ratio gives the ultimate compressive strength). Surface shrinkage cracks develop, delaminations occur, and whoever lives with the end product gets a substandard concrete job.

This is usually an issue with good ol' boys on residential work. Commercial environments usually have some kind of quality control to prevent it, but it does happen.

 

[gpflepsen]

If you happen to see a bridge deck in the days after they are placed, they are typically covered in burlap, and sprinklers run 24/7 for about 5-7 days. It is required on the jobs I've been around.

 

[HOTFR8]

The expert speaks.

The goal of curing concrete is to prevent the premature evaporation of the mix water in the concrete. Concrete gains strength through a chemical process called hydration, which requires mix water. If the water evaporates, your concrete stops getting stronger.

I mention all that because it's important to understand the purpose of curing. The water you are spraying on is just acting as a seal to keep mix water in. You can accomplish the same thing with poly, burlap or a spray on curing compound.

I'd wet down your slab and cover it with poly for seven days. Don't give up now. It's not too late

 

[treimers]

Some finishing crews will throw water on the surface as they trowel. The problem is that the top thin layer of mortar paste ends up with a high water/cement ration (low w/c ratio gives the ultimate compressive strength). Surface shrinkage cracks develop, delaminations occur, and whoever lives with the end product gets a substandard concrete job.

This is usually an issue with good ol' boys on residential work. Commercial environments usually have some kind of quality control to prevent it, but it does happen.

Interesting -- that sounds exactly like what happened with our old driveway.
After many decades, it's now having real issues and I'm considering what to do when I have the money (which will probably never arrive, honestly)

But I recall my Dad having it resurfaced by some guy, and it delaminated
rather badly in just the next few years, which now has it looking like the surface of the moon, not to mention the bad cracks.

I suspect that 'blessing by a good old boy" is precisely what happened.....

 

[SickSilverado]

I was busy at the time they finished the concrete, but it didn't look overly wet when I checked it out for the first time. I gotta say though, they didn't smooth it out worth a darn. Looks like ***...got a huge wave/tsunami/glop of concrete in the middle(1/4" high about 6 feet wide). There's also a few elevated lines/peaks. I have to polish or grind those out when I can get my hands on a grinder.

 

[joes169]

A quick Google turns up this....

http://www.concreteconstruction.net/finishes-and-surfaces/wet-curing-floors.aspx

But in the first paragraph, it states different reasons than what most of this threads replies, try to state.

But it also says that it is "UNPROVEN," which typically makes "SnakeOil salesman disappear....

But it also leads to the science stated and proven, by using water to ease a Trowel.

Maybe in my next life, I'll be able to master Sprinklers and Concrete or "Mud hydration."

You're certainly a modern day Sherlock Holmes. And you're absolutely right, the folks producing and selling water are "snake oil salesman"................

 

[brickG-man]

Please read this simple explanation from the Portland Cement Asscociation. It should answer all of your questions.

http://www.cement.org/basics/concretebasics_concretebasics.asp

 

[Hephaestus29]

So is the process still the same if they are putting a real slick finish on the concrete to begin with by riding those trowels around on it ? Then after they are done with that start spraying it & cover it ?

 

[atfulldraw]

keep sprinkling....

 

[mds5951]

 

[Falcon67]

My 24x40 went down in 10x F weather. We poured on a Friday and I kept it wet all weekend. On Monday I told it "yer on your own" and went to work. Average surface temp on the exposed dry slab that summer ran 130F. I started building on it about 14 days after the pour. None the worse for wear here. There were some surface cracks during the cure, nothing unusual for around here. The cracks all got painted and have not changed at all in the last two years.

 

[TOOL_MONGER]

You are busy and concerned with keeping it wet... don't forget you should cut it as soon as possible... same day as pour...

Water bath with plastic or burlap is very common where a surface film cure is not allowed... as in a post tensioned tennis court that gets a court surface after cured.

 

[gpflepsen]

Concrete waits for no man! Really, there is such a science in the design of concrete mixes and how to get them to perform, then there's the art and skill of the placing crew that can also contribute to the quality of the end product. Experience takes time, and an experienced crew can get it right most of the times, though sometimes the variables conspire to make a nightmare of a pour. This is when experience can prevent a catastrophe, such as tearing out a job to redo it.

Laying down concrete may seem easy, until you do it! It takes a team working together, knowing what to do and what not to do. My experience comes mostly from concrete pumping, where concrete definitely won't wait for you!

 

[SickSilverado]

Well, I'm gonna stop watering tomorrow afternoon. So far I'm not too happy with the finish. Looks like they used their *** cheeks to smooth out the finish...in other words it looks like S**T. I knew this the day after they poured it, but it just hit me now. I'm gonna either rent a grinder/polisher or buy a wheel for my angle grinder and knock down the high spots myself. I would have done this myself if I would have known they were gonna do this bad of a job.

 

[Sureshot]

Well, I'm gonna stop watering tomorrow afternoon. So far I'm not too happy with the finish. Looks like they used their *** cheeks to smooth out the finish...in other words it looks like S**T. I knew this the day after they poured it, but it just hit me now. I'm gonna either rent a grinder/polisher or buy a wheel for my angle grinder and knock down the high spots myself. I would have done this myself if I would have known they were gonna do this bad of a job.

I hear you. I can mess things up for free and it ***** but paying someone to do a worse job double *****.

 

[Cyberbear]

I've used a sealant I sprayed on the floor surface to seal the concrete and retard evaporation. Each of the two large slabs I had poured using 6 and 7 bag mix, hard troweled and sealed have never cracked in many years. The water based sealent can be purchased at Home Depot or other builder supply centers and applied with a garden sprayer, I used two coats.

 

[Flounder_Pounder]

Some finishing crews will throw water on the surface as they trowel. The problem is that the top thin layer of mortar paste ends up with a high water/cement ration (low w/c ratio gives the ultimate compressive strength). Surface shrinkage cracks develop, delaminations occur, and whoever lives with the end product gets a substandard concrete job.

This is usually an issue with good ol' boys on residential work. Commercial environments usually have some kind of quality control to prevent it, but it does happen.

Yep that's what I call blessing.... Happens all the time

 

[e-tek]

Mine was poured, leveled, power-troweled (until 2am), and done. No watering, no plastic. A week later they cut it and I've never had so much as a crack - in 5 years.

 

[rockwithjason]

I was thinking the same thing etek. here in vegas when I was in construction not once was there any post watering and it was HOT.

 

[Angelfire]

The point is, you'll get your compressive strength without moist curing as you see with the many homebuilders etc.. that simply place it and walk away. But if you moist cure it, the compressive strength will be significantly stronger. Not saying curing it is a necessity but if you have time before framing, you might as well eek out as much strength out of it as you can.

 

[Flounder_Pounder]

Mine was poured, leveled, power-troweled (until 2am), and done. No watering, no plastic. A week later they cut it and I've never had so much as a crack - in 5 years.

Properly curing concrete is the right way to insure the best possible outcome… See below...

http://www.cement.org/basics/concretebasics_curing.asp
http://www.cement.org/basics/concretebasics_faqs.asp
See curing section
http://en.wikipedia.org/wiki/Concrete

And in the business there is a saying that goes around…

“There are only two kinds of Concrete… The kind that's Cracked and The kind that's gonna Crack…”

Travis T

 

[tomroblee]

The slab was poured over three months ago. I imagine that it is dry by now

 

[Flounder_Pounder]

The slab was poured over three months ago. I imagine that it is dry by now

Yep but if it's done right with the right mix it never stops getting stronger....

 

[6t7gto]

Pics of the project?

 

[Matt The Hammer]

I recall when I was in school and doing concrete inspections. The grizzled old finisher foreman put his arm around me and told me "son, I've been doing this before you were born" as he was making a lake of water on top of a slab floor.

I still rejected the pour and all the concrete from that huge floor. Hardest $8/hour decision of my life.

 

[BrerHair]

No watering, no plastic. A week later they cut it and I've never had so much as a crack - in 5 years.

here in vegas when I was in construction not once was there any post watering and it was HOT.

This is the world I live in. Been in luxury (million +) homebuilding for decades in the South. We do not use curing compounds and we do not make any effort to keep concrete wet.

Our concrete is guaranteed to crack, just like yours, but it generally cracks only in straight lines in the control joints.

The point is, you'll get your compressive strength without moist curing as you see with the many homebuilders etc.. that simply place it and walk away. But if you moist cure it, the compressive strength will be significantly stronger.

I think the question you need to ask is how strong does it need to be to adequately perform for a garage slab.
And what about other variables . . . is there gravel under the slab (how thick)? Was the substrate compacted? What mix strength did you pour? Is there wire mesh? Fiber in the mix?

 

[e-tek]

Well I'm glad to see all you later posters say what I was thinking. The first few posts got me thinking "Where have I been all this time - not knowing about all this concrete-curing hokus-pokus?"

Only time I've ever seen plastic draped over concrete was in Vancouver (construction footings or roadways), when it was either raining real hard or close to freezing. Seems to me if that's the case then watering it can't account for much...

 

[Daniel Dudley]

I was thinking the same thing etek. here in vegas when I was in construction not once was there any post watering and it was HOT.

There is a mud building in Arizona that is over 800 years old. Must have cured well...

It makes a much greater difference in areas where concrete gets wet and freezes on a regular basis. I have seen concrete spald into nothing in less than 10 years without proper curing. I have also tested out proper concrete at greater than 9000 PSI. SO hard you couldn't shoot a concrete nail into it.

The amount of ignorance about what concrete is and how it cures is staggering, and that would be just in the industry. What you got away with, and what is proper could be two different things.

 

[RobSmith]

In central QLD Australia they throw bags of Ice into the mixer to transport it....way too hot!

 

[ConCretin]

Well I'm glad to see all you later posters say what I was thinking. The first few posts got me thinking "Where have I been all this time - not knowing about all this concrete-curing hokus-pokus?"

With all due respect e-tec, referring to proper curing of concrete slabs as 'hocus-pocus' does a dis service to those who don't know any better.

Is it possible for a slab that only achieves 50% of it's design strength to perform adequately and resist cracks? Sure, but after spending all that money on prepping and placing a slab, why would you cheap out on a little poly or water at risk an inferior slab.

It's not just about cracks - proper construction procedures including curing maximize a slabs durability and help them stand up to the abuse we heap upon them. GJ should be a place people can come for good advice and some of the prior posts aren't providing it. Just sayin.

 

[Flounder_Pounder]

There is a mud building in Arizona that is over 800 years old. Must have cured well...

It makes a much greater difference in areas where concrete gets wet and freezes on a regular basis. I have seen concrete spald into nothing in less than 10 years without proper curing. I have also tested out proper concrete at greater than 9000 PSI. SO hard you couldn't shoot a concrete nail into it.

The amount of ignorance about what concrete is and how it cures is staggering, and that would be just in the industry. What you got away with, and what is proper could be two different things.

With all due respect e-tec, referring to proper curing of concrete slabs as 'hocus-pocus' does a dis service to those who don't know any better.

Is it possible for a slab that only achieves 50% of it's design strength to perform adequately and resist cracks? Sure, but after spending all that money on prepping and placing a slab, why would you cheap out on a little poly or water at risk an inferior slab.

It's not just about cracks - proper construction procedures including curing maximize a slabs durability and help them stand up to the abuse we heap upon them. GJ should be a place people can come for good advice and some of the prior posts aren't providing it. Just sayin.

Well said gentalmen (proper advice vs what happen to work out)
Also home builders and proper concrete placement don't go together in my book since I come from a structural concrete background...

 

[Sheriff245]

I happen to live in a place where the climate goes from very cold to very hot (-30C to +35C) throughout the year.

Moist cure is not necessary most of the time, but does help increase bearing capacity and crack resistance. I've worked on many projects and those that had the best quality concrete were those where it was poured in mid-late fall with temperatures around +10C. Lower temperatures = less evaporation = more hydration. When it's hot and sunny, keeping the surface wet for the first few days does help make the finished product better. However, with the right mix and proper pouring procedure (ie no on-site addition of water, proper vibration, etc.) your slab should do its job just fine, especially in the case of a slab on grade where crack resistance is not as critical.