Concrete - To mist or not to mist?

[Dazzy50]

Hi all,

I've finally poured my overdue concrete slab!

I've read different information on what to do with it now it's in the process of curing.

PavingExpert suggests misting and covering can assist the cure, but also can cause problems such as weakening the slab and trapping evaporated moisture.

http://www.pavingexpert.com/concrete.htm

I have a dpm underneath the concrete slab so I think this should aid in moisture retention and assist the cure

Does anyone think I should mist/cover my slab?

Thanks

Daz

 

[Newboy]

Unless your expecting a significant change in the bank holiday weekend weather I wouldn't worry about misting it!

 

[Dazzy50]

Thanks newboy. I forgot to say its actually covered with tarp (but not touching the slab) because of the rain forecast this weekend. I didn't want it raining on the slab as I didn't think that would do it much good. Do you think I should leave it uncovered then if rain is imminent ?

 

[Newboy]

When was it finished and are you planning to leave the surface finish as it is now - not polishing etc?

 

[Dazzy50]

Finished yesterday evening and yes wasn't planning to polish/finish as will just be covered with a shed.

 

[Newboy]

Assuming you had a decent mix then I'd leave it alone and enjoy the rest of your weekend.

 

[Dazzy50]

Thanks. Was fairly happy with the mix and consistency - certainly don't think it was too dry.

With regards to taking off the tarp, I was planning to leave at least 3 days till the worst of the weather has passed. Is that enough time for the cure to establish and not be susceptible to rain ?

 

[JohnD]

I don't understand your bit about trapped water weakening it. Unless you mean overwetting the mix.

Once it is laid and set, the wetter you keep, it, and the longer you keep it wet, the stronger and harder it will grow. Once it dries out, it will stop hardening and gaining strength.

Cloud and drizzle are ideal, but covering it to prevent drying out, and especially protecting it from sun or drying winds, is good. A few days is too short, but a couple of weeks of being damp and it will be near enough maximum strength.

Underground concrete often never dries out and can continue gaining strength for 50 years (though it gets slower and slower over time).

 

[r896neo]

I assume the bit about the tarp is in reference to having polythene over it but directly in contact with it and it sweats and condenses on the sheet and could lead to a weak/ dusty crust.

After its been trowelled or finished however you choose It only needs about 5/6 hours until even heavy rain wont even affect it. I would leave it uncovered and relax.

 

[Dazzy50]

Hi John, about the comment of trapped water weakening it, I was just going by what I'd seen written on the PavingExpert page. Half way down the section on "Curing" says:

The simplest way to prevent the concrete drying out too quickly is to keep it damp. This can be achieved in various ways, but the most commonly used are:-

• continuous fine misting - can be difficult to set up and may weaken the surface
• covering with plastic sheeting - cheap and effective but can trap evaporated water between the slab and the sheeting
• spraying with a plastic membrane - relatively expensive, but becoming more popular as it produces near-ideal conditions. The membrane can be peeled off after a few days.

So i'm just trying to understand what the best thing to do is.

Are you suggesting I remove the tarp that is currently covering it in order to expose it to the elements and keep it wet? How do I then avoid exposing it to too much moisture?

 

[Dazzy50]

Hi r896neo,

Ok thanks this is starting to make more sense now. I might remove it today if it seems that no further heavy rain is likely to affect the curing process.