Wet Cure Concrete 3

[RFreund]

I received a question from a contractor asking to wet cure a slab on grade. However, they wanted to use Visqueen in lieu of burlap. They propose to soak the slab and cover it in visqueen for 5 days. There are no cure and seal products being used for this slab.

I had a few questions:

1. Does this seem like a possible option? My first questions is how/when would they "soak" the slab.
2. When/how are control joints made when you wet cure a slab?
3. Any reference material you could point me towards? Normally I would expect burlap to be wetted for about 7-14 days but the control joints need to be placed within a day.

Thanks!

EIT

http://www.HowToEngineer.com

 

[oldestguy]

Visqueen works great. Only question is sometimes the uneven contact with concrete leaves visible "marks" that later don't show.
1. To add water just lift the edge and add as needed. Evaporation is darn near zero.
2. Saw joints early on, preferably the day of the pour. Shrinkage starts only hours after placement. There are tools to do this even before you can walk on it.
3. Burlap seems out of date to me. How abut curing compound instead?

http://www.cement.org/cement-concrete-basics/working-with-concrete/curing

 

[hokie66]

Plastic sheeting works well, as long as it stays in place. You have to weigh it down to prevent the wind from getting under it.

Softcut sawing is the way to go, if you are sawing control joints. Saw, then apply the sheeting.

Most commonly used curing compounds are of limited value. The acrylic ones are useless. Chlorinated rubber does a pretty good job, but is toxic. Wax emulsion also works. But with the last two types, adhesion of finishes is problematic unless the compounds are removed first.

 

[XR250]

I have heard that using visqueen outside can cause the slab to heat up too much if the sun shines on it - like a greenhouse.

 

[DamsInc]

3. Burlap seems out of date to me. How abut curing compound instead?

Curing compounds aren't great. They are often not applied correctly by the contractors. You need to seal the entire surface without any gaps, and even then, the best you can achieve is stopping the evaporation of moisture. With wet curing you add additional moisture to continue the hydration reaction as the internal water is used up.

 

[RFreund]

Got it, thanks for the replies.

Does 5 days seem to be enough time?

EIT

http://www.HowToEngineer.com

 

[jike]

ACI says to wet cure 5 days at 70 deg F or 7 days at 50 deg F is minimum. I hope that I am quoting that (from memory) correctly.

I hope this helps!

 

[oldestguy]

Time for covering is not fixed as long as you know that hardening takes place at a reduced rate with time. Early covering is most important. Some specs look at the 28 day strength.

 

[RFreund]

I suppose a better question is - is there a spec or somewhere in ACI that I can reference?
I think ACI 5.11 is what Jike was quoting and that does sound correct. I suppose this is what I need.
Is there a cold weather provision in there? It's currently freezing here! I should probably do a little more homework at this point.

EIT

http://www.HowToEngineer.com

 

[RFreund]

I suppose a better question is - is there a spec or somewhere in ACI that I can reference?
I think ACI 5.11 is what Jike was quoting and that does sound correct. I suppose this is what I need.
Is there a cold weather provision in there? It's currently freezing here! I should probably do a little more homework at this point.

EIT

http://www.HowToEngineer.com

 

[oldestguy]

Considering the questions and cold weather, I'd go to the PCA web site.

Here is a link for cold weather concreting

http://www.cement.org/for-concrete-...concrete-construction/cold-weather-concreting

 

[SlideRuleEra]

ACI 306R-10 is the only construction industry document referenced by specifications, standards, and construction documents for cold weather practice for concrete...

Search the magazine's archives for other info: Link

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[cvg]

1. ACI-305 Hot Weather Concreting
2. ACI-306 Cold Weather Concreting
3. ACI-308 Recommended Practices for Curing Concrete

 

[RFreund]

Thank you!

EIT

http://www.HowToEngineer.com

 

[hokie66]

I think the statement above by DamsInc that wet curing can add moisture to the hydration process is fundamentally incorrect. Concrete already contains moisture in excess of the requirement for hydration. The purpose of curing is to retain moisture in the concrete, not introduce more.

 

[Ron]

hokie66 is correct. Cement hydrates with only about 50 percent of the water that is put in the mix....the rest is for workability (convenience).

 

[DamsInc]

I would argue it isn't "fundamentally incorrect", but perhaps requires a bit more justification. Looking at it from a durability standpoint...

My understanding is that to continue hydration (the cement never 100% hydrates), you need 80% relative humidity in the pores. You want to provide a favourable environment for the hydration to continue as much as possible to achieve durable concrete by reducing the size and continuity of the pores. Adding water at early ages will help keep the pores in the cover full to allow the reaction to continue and improve the degree of hydration. At lower w/c ratios, the need for this is even greater. This is also why some high performance concretes will utilize porous saturated aggregates to provide "internal curing" to replenish the pores.

Really, my point was that when it comes to curing, more water = better.

 

[hokie66]

I wouldn't disagree with that. Flooding of slabs is one method, not done much today. But I don't think absorption of the external water is a factor in strength gain.

 

[DamsInc]

Further hydration beyond that cause by the internal water isn't always necessary to get your design strength. This is really only for durability, since curing only affects up to 50mm or so.

Below w/c 0.5, concrete does use external water to hydrate. From "Properties of Concrete" by Neville,

 

[RWW0002]

Relative humidity definitely has an effect on strength gain. It is true that water in excess of that required for hydration is generally used for concrete mixtures, however for flat-work much of that is lost in the form of bleed water evaporation. The "sealed sample" referenced above is probably not a slab that has been allowed to bleed, finished, and then "sealed" with a curing compound.

Keep in mind that the strength of a mixture is established by moist-cured cylinders... See attached for plots of strength vs. moist cure time for concrete.

The strength loss at low relative humidities is due to a combination of microcracking due to shrinkage, and slowed or stopped hydration. Maintaining free water at the concrete surface addresses both.