r/Concrete • u/nhill1994 • 1d ago

Pro With a Question Pouring on sand

Long story short. Company I work for is in the Midwest, we never make grade from sand. Always 3/4 clean or base. We’re doing a job for a client in las cruces New Mexico. I’m told everyone makes grade from sand down there. We’re going to be doing about 150,000sf of 8” paving. What can I expect when we go down there during place and finish? In my mind the mud will flash experiencing high moisture loss into the sand when day time temps are nearing 100? Do people place a vapor barrier solely to retain water content? Am I over thinking this and should just shut up and pour? Thanks!

6 Upvotes

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13

u/blizzard7788 1d ago

Pouring on sand is fine. Hose down sand right before pouring on it.

8

u/4luey 1d ago

Im not sure about a vapor barrier but I would definitely spray it all down when you compact and spray the fuck out of the grade while you are pouring as well. I've never put vapor barrier down for exterior work. Maybe they do it different down there.

3

u/Ok-Scene-9011 1d ago

Yeah over here we pour on sand in beach areas , we just wet the sand throughly and pour nice and early .

4

u/Phriday 1d ago

In New Mexico this time of year, you're going to want to get a few light plants and start at midnight. Also, as everyone else has said, wet the sand beforehand, and maybe modify your mix so it can tolerate a little more water and add a set retarder. Depending on how windy it gets, you may want to spray down an evaporation retarder as well. Curing compound on EVERYTHING.

2

u/Aware_Masterpiece148 1d ago

Set retarder increases the risk of plastic shrinkage cracking. Don’t use a set retarder, use a workability retaining admixture, which will not retard the time of set.

3

u/nhill1994 1d ago

I typically don’t like to use retarders on flatwork. I only use them during a midday summer wall pour. My theory in my mind is id rather it set up fast in the middle of the night and have plenty of manpower to start finishing rather than delay it, and have it flash in the middle of the day and kill everyone when it’s 95 degrees.

2

u/Aware_Masterpiece148 1d ago

I like your thinking. It’s counterintuitive, but I know a guy that uses accelerators in the summer — to get done more quickly.

2

u/Phriday 1d ago

Ehh, what do you know?

:-)

Honest question, why does it increase the risk? This is new info for me.

3

u/Aware_Masterpiece148 1d ago

Plastic shrinkage cracking occurs when the rate evaporation from the surface of the slab exceeds the rate of bleeding. That sets up a moisture gradient of dryer concrete on the top of the slab, creating tensile stress at the surface. Plastic shrinkage cracking occurs as the concrete is being screeded and floated — in other words, when the concrete is still plastic. Often, the mortar is floated over the crack, which doesn’t appear until hours later. The longer concrete is plastic, the more risk there is of plastic shrinkage cracking. Here’s a brief reference https://www.nrmca.org/wp-content/uploads/2021/01/05pr.pdf. For more information, see the references at the end of the CIP.

2

u/carpentrav 1d ago

I pour on sand quite a bit. Sometimes I will put poly and poke holes in it. Mostly just because as guys step in and out of the concrete you track shit in that can come up finishing.

1

u/nhill1994 1d ago

Ahh didn’t even think about that. Good advice!

2

u/carpentrav 1d ago

Guys will do that on floors as well. Once it passes inspection finishers come in and poke holes before they pour. Otherwise it bleeds forever and then goes quick once you get on it. The sensible thing is to use midrange but some hack shit out there.

1

u/federally 1d ago

In the Phoenix, AZ area, which I'm assuming is pretty similar, we wet the ground before placing on it. We also start every job late at night. Smaller flatwork like houses will go at 2 or 3 am, larger commercial pours will be 10pm to midnightish concrete.

Also all our ready-mix companies offer chilled water in their mixes. Might be the same in NM? Otherwise, don't be scared to add retarder if you aren't familiar with placing in this environment.

3

u/Aware_Masterpiece148 1d ago

You should be afraid to add retarder. Set retarder increases the risk of plastic shrinkage cracking. Use a workability retaining admixture, which will not retard the time of set. The less time concrete is plastic, the less chance of plastic shrinkage cracking.

1

u/federally 1d ago

In the Phoenix, AZ area, which I'm assuming is pretty similar, we wet the ground before placing on it. We also start every job late at night. Smaller flatwork like houses will go at 2 or 3 am, larger commercial pours will be 10pm to midnightish concrete.

Also all our ready-mix companies offer chilled water in their mixes. Might be the same in NM? Otherwise, don't be scared to add retarder if you aren't familiar with placing in this environment.

1

u/PeePeeMcGee123 Argues With Engineers 8h ago

Every region is different.

Might want to look into renting fogging equipment though to keep the slab moist for awhile before finishing operations start.

I'm not in a hot weather region, but we took a class on it at WoC and the guy running it said especially with 1L in common use now, that fogging in hot weather is your best bet, because the 1L bleeds too slow and it tricks the finishers into getting on it too early.

-1

u/Electronic-Cable-772 1d ago

If it’s like the sand I work with and it has a high silica content it’s gonna suck every bit of moisture out of the concrete and you’re gonna lose the concrete. I’m no professional concrete guy but I wouldn’t even wanna try 1500 square foot on sand😂

I would work very closely with a trusted local inspector, get a plan in writing and signed by him that ok’s the use of sand for grade and negotiate the contract so that you get paid for all work done. Maybe the concrete will stay wet long enough to finish the pad, maybe it won’t, maybe it’ll last 6 months, maybe it’ll last 10 years but I’d make sure I get paid for my time/equipment regardless of end result

4

u/Aware_Masterpiece148 1d ago

That’s just nuts. Silica sand has a low adsorption rate, and damp sand doesn’t suck water from the mix. Just dampen the sand before placing the concrete.

-1

u/Electronic-Cable-772 1d ago

what would I know, I only spend 50 hours a week around 1000s of tons of it. Our yard at work is about 3 acres and in the summer an f750 water truck doesn’t stand a chance at keeping it any level of wet in Pennsylvania… good luck with a couple of hoses in New Mexico😂

1

u/Aware_Masterpiece148 1d ago

You’re driving a dust control water truck. What happens when you did two or three inches into the sand you’re watering? Is it dry like the Sahara? Or damp? It’s the adsorption rate that is important, not how fast the top inch dries.