Wet footing trench before pour?

[Vintage Veloce]

Here in San Diego my dirt has a lot of clay, almost a solid layer under the topsoil. I hear this is called "expansive soil". The inspector advised wetting the ground so it was moist before the concrete pour.
Obviously there is a plastic membrane under the slab so wetting that would be useless. But I could wet the ground under the footings.
Not sure if the idea is to expand the soil a bit before the pour or to have the soil moist to slow down the concrete drying or what...
And I'm not sure this is even a good idea. But if I'm going to do it, tonight is the time, as the pour is scheduled for first thing in the morning.
If I do this, I won't wet it enough to make any real mud, just enough to wet it a bit. Certainly no puddles. I suspect it might completely dry by morning anyway. Low temperature overnight will be 60 degrees and the humidity now is about 60%.
So, should I wet the soil under the footings this evening or not?

 

[theoldwizard1]

I'm not sure if would really help, but, like chicken soup, "it couldn't hurt" !

Like watering concrete flat work.

 

[jbwilkins]

In Vegas when we had critically expansive soils we'd flood the slab for a couple of days.....I believe the idea was the soils would expand to their max and then you won't have issues later on with the soil expanding and causing the slab to fail.....

If the inspector recommends it due you your soil conditions then I'd probably do it...

 

[theoldwizard1]

There was a thread here about a week ago that had a link to a site that explained how concrete cures. The water does not "evaporate" (like the water in latex paint). It actually chemically combines with the dry cement and forms a new compound. The sand and aggregate are there mostly to fill up space. They do add some structure.

Anything that draws water OUT of the concrete before it is cured (28 days for a full cure) is not good. Large concrete drain pipes are steam cured in a few days.

 

[Vintage Veloce]

Thanks guys, I just wet it down. All done.

 

[Cyberbear]

Yes, wetting the soil helps expand it prior to the pour and helps slightly to retard the absorption of water from the concrete, and keeps the dust down. It can't hurt.

 

[Kaizen]

This is confusing to me. if you can't wet down the entire slab area and expand all of the soil wouldn't this leave the footings to have a void under it? is this less volatile then having the soil push up the whole structure?

 

[K'ledgeBldr]

The water is to stop the absorption/extraction of water from the concrete.

You certainly don't want to expand the soil BEFORE a pour/placement- guess what happens when the soil returns to its previous moisture content? It shrinks leaving voids- now the concrete isn't supported.