Concrete curing advice needed

[RickP]

I poured concrete for a lamp post base two days ago, and stripped off the forms tonight. It hasn't cured completely yet, but I don't know how long it should take?

Tonight, it was strong enough to be okay without the forms, but I can easily scrape the top layer off with a piece of plywood. And very light taps with a hammer will compress the ridges (from the form edges) pretty easily.

Our daytime temps have been 69 and 67 for the past two days, with overnight temps around 10-15 degrees cooler (pretty nice for December). I mixed the concrete slightly dry so I could get it to pack down hard in the forms (2 qts of water instead of 2.5 qts per 60 lb bag). I've sprayed it with water mist twice each day and kept it pretty well covered with plastic. But the surface is still soaking up water like a sponge tonight.

Here are some photos showing the prep work for the form:

For the bottom half, I poured in half a bag at a time and mixed water in the hole:

I couldn't find any wax so I coated the form with a little used motor oil:

For the top half, I poured it wet and packed it in tight, but I ran out of daylight and couldn't get a photo of the wet concrete.
Here's the cover I used for the night:

I'll post some photos of the stripped forms in the morning. Should I just keep misting it with water or do I need to wrap it in plastic?

Thanks,

 

[bczygan]

Give it 7 days. It should reach 60% of design strength by then.

In 28 days, under proper conditions, it should reach design strength.

 

[gpflepsen]

It concerns me that at two days you can scrape concrete off the surface. That says a very weak mix. You state the top pour was wet and you packed it in tight. How did you "pack"? Sounds like excessive rodding or vibration brought lots of water to the surface and resulted in compromised strength at the surface.

I'd soak it and wrap it in plastic. Walk away for 7 days the inspect again. Mount the pole and hope it's not too weak. If it's too weak knock it out and start over.

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[RickP]

It concerns me that at two days you can scrape concrete off the surface. That says a very weak mix. You state the top pour was wet and you packed it in tight. How did you "pack"? Sounds like excessive rodding or vibration brought lots of water to the surface and resulted in compromised strength at the surface.

Nothing that fancy -- I just packed it into the form with the blunt end of a wood stake. I used a piece of rebar to do a little rodding, but I didn't do any vibration. And no water came to the surface.

I'm actually concerned the mix was too dry, so I probably shouldn't have said I poured it wet. The surface still seems to be pretty dry, and it's soaking up a ton of water. It wasn't anywhere close to a slurry -- it was more like shoveling in clumps of semi-damp concrete mix. I'm not sure if I used enough water in the mix before I poured it into the forms.

Should I fill the hole with water or just continue spraying the surface with mist?

Thanks,

 

[Cyberbear]

There is a happy medium that must be achieved for a strong concrete. If you have a soupy mix with too much water the concrete will be weak. I get the sense that you may have put the dry concrete in the hole and simply added water, I don't believe this is a proper way to place concrete. It should be mixed to the proper consistency, then put in the forms and allowed to cure by leaving it alone and not adding water, which can be detrimental to strength. Some of the cheap redi-mix types of concrete bagged mix can have too little rock and cement which is needed for strength. If your pour is still soft after a couple days, there is something wrong.

 

[gpflepsen]

Get a good closeup shot of the top surface and post it. Your descriptions seem to be conflicting. Poured wet, too dry, soaks up a ton of water, etc. Is the surface honeycombed or smooth?

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[tcianci]

Stop worrying.

 

[RickP]

I will definitely get a photo in the morning -- I ran out of daylight after stripping the forms tonight.

Sorry about the confusion -- I mixed the bottom half and the top half of the hole differently:

Bottom half: I'm not worried about this at all -- I mixed water into dry concrete in the hole. The soil is wet enough that it will cure fine. (I've done post holes like this for years and never had a problem.)

Top half: I mixed water into the concrete in a wheelbarrow and shoveled it into the forms. I used 20% less water than called for in the directions, so I'm worried it's too dry. (I've never used forms before, so I don't really know what to expect with them.)

Thanks,

 

[RickP]

Some of the cheap redi-mix types of concrete bagged mix can have too little rock and cement which is needed for strength. If your pour is still soft after a couple days, there is something wrong.

I just bought Sakrete from Home Depot:

Link to product page: 60 lb Sakrete Bag

I'm not sure how good that mix is, but it's always worked fine for post holes. But I really don't know much about finishing concrete. I probably should have floated the top surface a bit before covering it.

 

[Rookie2]

How old was the bag of concrete mix ? older than 6 mo. throw it out !

 

[myredracer]

Any time I've used that Sakrete stuff, it seems to take forever to cure and I used to think there was something wrong with it. For some reason it's quite different than the stuff out of a concrete truck. It does eventually cure.

You should be okay in a few months...

 

[gpflepsen]

What kind/size of pole and how is it being mounted? If it's structural concrete you should use a high strength mix (4000 psi bag) or add cement to the regular 3000 psi sackrete. Setting a pole in concrete is entirely different from mounting to concrete.

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[bguerini]

by mixing the cocrete dry, you may not be getting the proper hydration in the concrete. you have to hydrate the cement for the concrete to reach design strength.

how are you going to mount the pole? i dont see any anchor bolts in your pictures.

 

[Falcon67]

I always buy Quikrete and it sets up pretty hard in a hurry - like hours. And for pole setting, I never do the dump and water - I always mix in a wheelbarrow and get the proper consistency before setting a pole or whatever.

2 days no set - I'd be pulling it out of the ground and starting over.

 

[Syberia]

I use the Quickrete too. It will dry hard enough that anything I set into it isn't coming out overnight. I'm usually slow to finish projects, though, so drilling into it doesn't happen for a few weeks to even a few months.

 

[RickP]

Thanks for all the replies -- I checked it this morning and it's finally hardening. The Quikrete suggestion got me thinking -- that's the only kind I've ever used, so the slow curing of this high strength mix really surprised me. Especially with overnight temps in the 50s and not quite enough water in the mix.

What kind/size of pole and how is it being mounted? If it's structural concrete you should use a high strength mix (4000 psi bag) or add cement to the regular 3000 psi sackrete. Setting a pole in concrete is entirely different from mounting to concrete.

I'm right with you -- I used 4,000 psi mix. Way high for a lightweight aluminum lamp post. But you're right about it needing to hold the anchor bolts, which are the kind with the expansion tabs at the bottom made to be inserted into drilled holes.

I always buy Quikrete and it sets up pretty hard in a hurry - like hours. And for pole setting, I never do the dump and water - I always mix in a wheelbarrow and get the proper consistency before setting a pole or whatever.

2 days no set - I'd be pulling it out of the ground and starting over.

Quikrete is all I've ever used before. I've mixed it both ways, in a wheelbarrow or in the hole, depending on how important the post is to me. Some of my posts have been temporary (1-2 years) so I didn't worry too much about mixing water in the hole.

With the cool temps, I'm going to wait a couple more days to see if it cures.

Any time I've used that Sakrete stuff, it seems to take forever to cure and I used to think there was something wrong with it. For some reason it's quite different than the stuff out of a concrete truck. It does eventually cure.

You should be okay in a few months...

You're right -- this stuff takes forever... But I'm hoping for a few days!

by mixing the cocrete dry, you may not be getting the proper hydration in the concrete. you have to hydrate the cement for the concrete to reach design strength.

how are you going to mount the pole? i dont see any anchor bolts in your pictures.

It was already dark when I set the anchor bolts, so I didn't get a photo of them that night. I just took some photos this morning -- see my post below.

I use the Quickrete too. It will dry hard enough that anything I set into it isn't coming out overnight. I'm usually slow to finish projects, though, so drilling into it doesn't happen for a few weeks to even a few months.

That's what I've always used before. But I wanted to use the high strength mix for this project. I guess I just learned how much of a difference there is between the two mixes.

Plus, I just realized the HD web site suggests using 3 qts of water in the mix, even though the bag says to use 2.5 qts. Since I only used 2 qts of water per bag, I guess I shouldn't be surprised the mix was a little too dry.

 

[RickP]

Here's the concrete base with the aluminum lamp post behind it:

Here's a close-up of the front, after curing 2.5 days:

Here's a close-up of the rear and the side:

It definitely ain't pretty! The surface is rough, but most of it won't be seen. The soil will cover the lower part, and the post will cover the top, so only the decorative edge will be visible behind plants.

But now I've learned to float it if I want a smooth surface -- this stuff is a lot harder to finish than I thought! Now that I see the finished surface, I'm thinking that I should have used a simple rounded edge instead of trying to get fancy... Something tells me I won't be making any decorative planters out of concrete anytime soon!

 

[gpflepsen]

I was envisioning a 10' pole subject to put much greater forces on the concrete than the short post you seem to have. I'd say no worries.

The sacked concrete mixes are usually very lean on cement content and rely on you adding the specified water to achieve rated strength. The lean mix then lacks cement paste content that makes surface finishing easier.

I think you'll be OK. Lower temps will retard strength development, just don't let it freeze for the next few weeks. Once it freezes, the hydration of the cement pretty much stops. Covering with plastic and a tarp if it gets below freezing should suffice.

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[matt_i]

I would not worry. Its a 1 bag lamp post, not a 17 yard foundation pour that has $5000 worth of labor and material in it and the prep.

Sack crete always is rougher surface than a truck. Not sure if they pack it with more gravel or more sand or ? Mixing in mixer or wheelbarrow would be preferable to a "soil pot"

I use mazola corn oil as my release agent. Works perfectly, no nasty stuff into the environment...it came from corn after all.

 

[jhelrey]

I mix concrete for fence posts, etc. in a 5 gallon bucket and a drill with a mixer. Add water to bucket, pour in half a bag, mix, add water or dry to get consistency. Dump into hole, repeat. I can do a dozen fence posts in about 30 minutes.

I've never liked the dry method.

Your concrete looks like it was poured way to dry.

 

[RickP]

I think you'll be OK. Lower temps will retard strength development, just don't let it freeze for the next few weeks. Once it freezes, the hydration of the cement pretty much stops. Covering with plastic and a tarp if it gets below freezing should suffice.

Thanks -- that really helps. I have it well covered with plastic now and I backfilled it tonight. I'll throw a tarp over it, since it's predicted to dip below freezing tomorrow night.

Give it 7 days. It should reach 60% of design strength by then.

In 28 days, under proper conditions, it should reach design strength.

Thanks -- I'll wait a week or so before mounting the post. The weight shouldn't be a problem, but I want to make sure the expansion bolts don't pull out of the base.