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How do I get my concrete to cure faster!
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08-18-2013, 08:21 PM #1
Avcraft Newbie *
Status: Offline Posts:2 Threads:1 Joined:Aug 2013
Hi guys, I had a 4" floor poured back on 30th May and need it coated asap. We're using 1 coat each Flowseal EPW, OP and UV. The company applying it want 75% RH. I've put in four Rapid RH readers last week. The first readings are 13'C and 99%, wait a couple minutes and they're 96%...?

I stare at the floor with hatred every day but it doesn't seem to cure any quicker!!

The floor is in a large hangar with no heat. Ambient conditions are around 13'C and 75% humidity

Any tricks to get it cured faster?

Thanks heaps for any advice!

08-20-2013, 11:43 AM #2
rapidrhrep Concrete Moisture Tutor **
Status: Offline Posts:85 Threads:3 Joined:Aug 2011

First thing, welcome to the forum.

You have a good(better) news bad news scenario. First, bad news....I doubt, very seriously, given the ambient conditions of the facility, that 75%RH, internally in the slab, will be achieved for many months, if not years. Curing the concrete is one thing, based on the time you have described since concrete placement, the concrete has done most of its hardening. Drying to an acceptable level for a coating is quite another issue. The combination of heat and low RH in the air will give the slab the appropriate conditions for drying. Unfortunately, your RH differential between the slab and the air is low, thus making any moisture movement from the slab into the air slow. Most people will say, that to dry concrete as effeciently as possible you need:

1) Higher ambient dry heat in conjunction with low ambient RH%.
2) Air movement across the concrete.
3) Dehumidifcation equipment.

The heat and low ambient RH% allow the moisture in the concrete to move more rapidly from an area of high RH to an area of low RH. The air movement helps the moisture move out of the capillaries, once it has moved to the surface of the slab, and into the air. The dehumidifier then captures this increased moisture from the air, in order to keep the ambient RH% conducive for the cycle to continue. In your situation though, you may go through this, only to find out that once you put the environment back to its natural ambient RH%, the moisture will be reabsorbed into the concrete.

Now the good news, or at least better news. I called Flowcreate in Texas after looking at the technical data sheets for the products you are using. The EPW is being promoted for "green" concrete. When I spoke with their technical person, he confirmed that 95%RH is the number they look for with this product because it is a water based epoxy. After looking further, it looks like the OP product for texture may be the biggest problem because it is a 100% solid epoxy. Is there any chance that something else can be done for texture? I definitely am not an expert on these products, but it might be something worth investigating if timing is critical.


08-20-2013, 04:37 PM #3
Avcraft Newbie *
Status: Offline Posts:2 Threads:1 Joined:Aug 2013
Hi Jason,

Thanks for your welcome and reply. Also appreciate your efforts contacting Flowcrete! All 4 sensors are now consistently showing 99%RH and a fairly consistent 12'C. I have had two sheets of polythene taped to the floor for a couple days now and getting no condensation or even signs of a damp patch on the floor? But I suspect perhaps the low air temps and highish humidity are slowing the evaporation?

The Rapid RH units are quite new to New Zealand, no one in my area has seen them before. The supplier is showing a great deal of interest in the project as well

I now have a large gas heater keeping the hangar temp around 12'C at night (ambient is dropping to 0'C at night) and also fans circulating air. Hopefully this will help...

Unfortunately my very limited budget means we have to stick with the products mentioned, they seem to be the best I can afford. Are there any alternates you could suggest?

Thanks again for your help

09-01-2013, 08:44 AM #4
CC Solutions Concrete Moisture Evangelist *******
Status: Offline Posts:1,067 Threads:69 Joined:Dec 2009
Polyethylene will show condensation only if the moisture coming from the slab is able to condense on a cool polyethylene surface. Calcium Chloride tests involve sealing a plastic dome on concrete and measuring the amount of water absorbed by a dish of CaCl crystals. Many times the concrete will not pass the CaCl test, but how many times have you seen moisture condense on the inside of the dome?

First be sure your test probes were properly installed, at the proper depth, hole vacuumed and cleaned correctly and the probes have acclimated at least 24 hours. After that is verified you can trust the results you are seeing.

Also verify that the slab is constructed properly for the planned flooring. If the flooring is moisture sensitive (such as the floor coating you are contemplating) a properly installed and functioning sub-slab moisture retarder is necessary. Without that, or if it has been compromised with punctures and cuts, slab moisture can be subject to extreme variation over time.

Insure no slab curing or sealing compounds have been applied.

Assuming all the above conditions have been met, Jason is spot-on with his recommendations for accelerating the drying of concrete. Concrete is a sponge, in your case a wet sponge. To dry it you will need low ambient relative humidity (dehumidifiers will help), air movement, and heat will help (warm air holds more moisture than cool air).

An open porous slab will dry much faster than a steel-troweled, hard, dense surface slab. If your concrete is burned black with a steel trowel it may never dry. You may consider opening the surface with a coarse grinder or light bead blast. To check the porosity of your concrete, drop a single drop of water on the slab (about the size of a dime) and time how long it takes to absorb into the concrete. If it takes over a minute, your slab is relatively non-absorptive.

Ultimately you may have to change your flooring finish and go with something that can be installed on wet concrete. There are many finishes that breathe and perform quite well.

As a last resort, the floor could be mitigated using an epoxy sealer that permanently locks in the moisture and alkalinity. These systems provide a warranty against failure for 15 years, but of course there is an additional cost to install them.

JD Grafton
Concrete Answers for Flooring Problems

11-06-2013, 04:33 PM #5
Ernesto Concrete Moisture Expert *****
Status: Offline Posts:606 Threads:33 Joined:Sep 2009
Hope those gas heaters are not propane. The exaust from propane has a high moisture content.

Stephen Perrera dba
Top Floor Installation Co.

12-04-2013, 08:46 AM #6
CC Solutions Concrete Moisture Evangelist *******
Status: Offline Posts:1,067 Threads:69 Joined:Dec 2009
(11-06-2013, 04:33 PM)Ernesto Wrote:  Hope those gas heaters are not propane. The exaust from propane has a high moisture content.

Yes it does! So does kerosene which you see as the white trails left in the sky by jets. It's moisture freezing in the cold dry air up there.

Temporary heaters also tend to contribute to elevated CO levels, increasing the carbonation at the concrete surface. I had a couple projects that had so much carbonated concrete due to unvented temporary heat that the concrete surface was soft and easily scratched with a pocket knife.

JD Grafton
Concrete Answers for Flooring Problems

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