[jaykim]

I am going to pour concrete in small mold (5x12x8) for the wood post.

I have studied and researched on the web about this, and can I ask questions?

I am using 4000 psi concrete mix.


1. I made the mod out of 1/4" plywood and scrap particle boards with no shiny surface. If I paint lightly the inside the mold with the motor oil, will help in removing the mold after 3 - 4 days? Or is the motor oil painting not necessary? I may be wrong, but I want to leave the mold longer than needed(< 2 days) for moisture retention for better post base strength.

2. They say to mix concrete to 2 - 3" slump as shown in photo below. They also say to vibrate the concrete to remove void or bubbles in the concrete. But if the concrete is not soupy or a little watery, but rather chunky, is the vibrating or hitting the mold side with rubber mallet still good, or is tamping down the concrete with something like 2x2 wood stick better in removing the bubble? I think if the concrete is chunky and lumpy, there would not be any bubbles.

3. Do I need to cover the top of the post pier concrete with the wet cloth and/or vinyl sheet to retain moisture for better curing?

4. Is it better if I keep the mold in place for longer days for better concrete strength in the hot weather?

5. Is it generally better for the concrete strength to lay concrete in the cool temperature rather than hot temperature? Since I am ignorant in this, I stupidly thought the hot and dry southern California weather is better for making the concrete pier because the water dries faster.

6. If the anchor bracket is covered by 1/2" thick concrete in small portion of the pier, will that part of concrete chip off or crack later on? I know they say cover at least by 2" thick concrete.

Thank you.

 

[Nealtw]

The form rental places put vegetable oil on the forms. The post bracket wants to sit on top of the concrete.

I can't help with the mix and all that stuff.

 

[HotRodx10]

Even with 2"-3" slump, vibrating it will consolidate it.


Concrete needs to be kept moist to cure. It doesn't 'dry'; if it gets dry, it doesn't cure. concrete hardens as the water hydrates the cement, which forms a crystal lattice. The overlapping and interwoven crystals give it strength. High strength precast concrete is typically steam cured for several days. So yes, leave it in the mold, so the moisture doesn't evaporate, and cover the top, so that surface doesn't dry out either. You can even mist it with water after about an hour and pour water on it after 2-3 hours.


Concrete will actually cure underwater; drilled shaft foundations for bridges are often done that way.

 

[huesmann]

You can cover any exposed concrete while it's curing with wet burlap or plastic sheeting, or both, so the plastic sheeting helps keep the burlap from drying out.

 

[jaykim]

Thank you Nealtw, HotRodx10, huesmann very much for the comment.

Then, the corn oil, soybean oil, or canola oil, etc can be brushed on the form to prevent the form from sticking to the concrete.

I will mist and spray water the top of the mold and cover it with plastic sheeting. I will leave the mold in place at least a week.

I am guessing that 4000psi strength comes only if the concrete is cured in the ideal condition and duration like 28 days of curing.

Thank you.

 

[SeniorSitizen]

Using some throw away containers do some testing of your own. Without some hi-dollar vibrating equipment that - IDEAL - pictured in the samples you posted will look like the one sidewalk pic with voids galore. At the same time do test with a slump as in the pic samples you posted second from the right. A hammer vibration will work for that. I don't mix with less than a 8" slump and prefer a 10" even if i feel the need to add a little Portland.

If your mold has no taper i recommend lining with parchment paper. Or try oil first on your tests then go with the parchment.


There's been more that one pumper company tell engineers and ready mix companies to take their job and shove it when the mix won't flow down the ready mix chute and i'll guarantee that IDEAL mix wont flow.

 

[HotRodx10]

I am guessing that 4000psi strength comes only if the concrete is cured in the ideal condition and duration like 28 days of curing.

Even high strength precast only stays in the form for 1-2 days and is kept moist for 3-4 days more. That should be plenty for your purposes. If you do that for what's sold as a 4,000psi mix, you'll likely be in the 5,000 to 6,000 psi range.If you think you need anything close to that strength, consider adding a little reinforcing, which will do much more for the breaking strength than little higher compressive strength for the concrete. What's going to be surrounding this concrete, anyway? If it's embedded in soil, 2000 psi strength is far more than the loading it will be subjected to.

 

[huesmann]

Yup. You only need the formwork for a day until the concrete is solid. It's supposed to reach max compressive strength by 28 days, but you're about 70% of the way there by a week.

 

[jaykim]

Thank you SeniorSitizen, HotRodx10, huesmann very much for the comments

SeniorSitizen, because I am a know-nothing in this matter, I thought the IDEAL mix is rather too chunky to let out the bubble when vibrated, and that is why I asked. Your second photo concrete mix looks more familiar to me. I will try oil first to the mould.

May I ask what this mean?

I don't mix with less than a 8" slump and prefer a 10" even if i feel the need to add a little Portland.

HotRodx10, when the concrete mix says 4000psi, I thought that 4000 psi is the max attainable psi only in the ideal condition, meaning using the IDEAL mix shown in the photo above and the moist curing for 28 days. I do not need any high strength concrete or anything. The wood post pier will be on the concrete slab/floor. I first thought that concrete mixing was easy, not difficult, but as I read more on this, there is lot more science involved than I initially thought. That is why I asked. I just wanted to do my small toy project right by learning something.

huesmann, I will keep the mold in place for a day or two.

Thank you all.

 

[SeniorSitizen]

May I ask what this mean?

Google slump test and the pros will explain slump. I as a DIY'er don't bother with the engineer details but am usually mixing in a wheelbarrow and judge by the way it flows from my mixing shovel and when it pours out of the wheelbarrow like a Lava flow that's the way i like it. It sets up like a solid rock and no failures to this point.

 

[HotRodx10]

HotRodx10, when the concrete mix says 4000psi, I thought that 4000 psi is the max attainable psi only in the ideal condition, meaning using the IDEAL mix shown in the photo above and the moist curing for 28 days.

Actually, it's the minimum you should be able expect if you follow normal procedures for mixing, placing and curing.

 

[HotRodx10]

The slump is how far the concrete sinks when it's released from a 12" high cone that is full to the top. A really stiff mix that mostly keeps it's shape when you pull the cone off would be maybe a 1" slump (The freed mound is 11" tall). A really runny mix that spreads in out into a puddle 2" deep when the cone is pulled off, would be a 10" slump mix.

 

[Oso954]

I think you are suffering paralysis thru analysis.

You are using a bagged ready to mix labeled 4000 psi. The mfg knows damn well that it will probably be used by a DIYer who will over water it. So they add extra cement to make it a stronger mix than you would get from a batch plant. As someone else mentioned, it may be 5000 to 6000 psi if mixed to a low slump consistency.

Low slump concrete (2-3 inch) is a PITA to work with. It takes a lot of vibrating,tamping, or roding to drive out the air bubbles or voids. Banging on the forms a few times with a hammer isn't going to do much. Wetter concrete is easier to work with. It flows better, takes less vibration,etc. but the extra water reduces the strength. 2 extra gallons in a one yard mix will drop the strength by 400-500 psi. But, as mentioned previously, the mfg has allowed for some extra water in his ready to mix design.

A typical foundation footing and wall in my area is a 3000 psi mix. And those are often watered a bit for workability at the contractors request after it arrives at the jobsite. You are starting with a 4000 plus psi mix.

I don't think you have much worry, regardless of the slump you place it at. The main thing to avoid is having excess water on the top of the mix after mixing.

If you really want to measure slump, you need to get a slump test cone and do some real slump measurement. The cone,rod and base plate will probably set you back about $150. The cone is likely to hold as much or more concrete than one of your little piers will.

The cone is 8 in diameter at the bottom, 4 in diameter at the top, 12 inches high, I'll let you do the math.
But if that cone of concrete only slumps down 2-3 inches when you pull the cone, that is a dry stiff mix.

 

[jaykim]

Thank you SeniorSitizen, HotRodx10, Oso954 very much for the comments.

I am learning a lot from you experts and pros. I am a novice in this area, and I know that my common sense and thought are wrong in many cases. That is why I asked, and by learning something, I think I can do a better job by knowing the how and why.

Thank you very much.