Precast 8"x8" Post - Mix Recipe

[DDame]

I am building a 3 rail (8”x8’x2” rough sawn cedar) fence that will have a 8”x8”x102” precast concrete post. The rails will be bolted (counter sunk) to the outside of the 8”x8” concrete post. My question is this; I am going to build my own steal forms to precast the concrete post, I will pour in-place the necessary machine screw concrete anchors that will be used to bolt on the 3 rail post (2 per board so 12 total). Does anyone have suggestions on the concrete mix? I don’t want to use any rebar just for cost reasons but I was thinking of something like Quikrete Fiber Reinforced mix but I want to mix my own if I can save money. Does anyone have a recipe of Portland cement, sand, aggregate and fiber to use? I want to have a smooth finish, fiber and small holes ok but I want it to be as smooth as possible.

Do you see any problems with my plan?

 

[concretemasonry]

How many posts do you need? That will determine the choice of form materials, casting method, stripping methods and form oiling compounds if you do it youself.

Doing it yourself is a bad route if you have no experience in precast concrete. - It is not like ready-mix.

First, fiber mesh is no substitute for rebar.

You need steel (wire or rebar) to handle and pick up the post at the right locations. The post wil weigh 500 to 600 pounds stripped and several hundred more including the form.

As far as finish, you need the proper mix, placement and vibration. A precaster understands the placement,vibration (concrete, form or vibrating table) and orientation when casting (vertical or horizontal). You can get by if you cover the surface defects by rubbing the freshly poured and stripped posts with a cement/sand/water mix and the proper rubbing tool.

Dick

 

[DDame]

I need a total of 650 posts. The form I'm making will lay flat, horizontal like on a table and I plan to fit the pour table with a good high frequency vibrator as well.

I understand the weight... are you saing I should use both fiber and rebar or just rebar?

This will be like a big parking curb but standing upright, I would like to get the same finish as you see on a precast parking curb you would see in a parking lot.

 

[dik]

Conc curbs have a nice finish, because you are viewing the formed surface. They are cast upside down and the unfinished side is to the parking surface. You might consider adding chamfer strips in the bottom of the form. This will prevent a 'sharp' corner on the post and you will have to finish the top surface. Check with Goldblatt to see what type of edgers they have.

8x8 will only permit a couple of bars. You should have 2" cover to the soil and that leaves a 4"x4" cage; it's going to get a little crowded for fabricating and for concrete placement. You might consider using a single rebar 15M (#5) down the middle. The size of rebar depends on your local wind loads and post spacing. You also have to consider how the posts are going to be lifted and you have to have sufficient concrete strength to lift the post. You may have to let the concrete cure for a week. As concretemasonry noted, polypropylene fibres shouldn't be relied on.

Depending on the soil type, you may require sulphate resistant cement; check with your local contractors about what they use for foundations.

Dik

 

[concretemasonry]

Depending on your location, you may need air entrainment. Your vibrating table may upset the entrainment percentage, depending on the frequency.

I have seen them cast vertically with moveable form faces for stripping. These were also on a vibrating table with adjustable frequency and amplitude. - Probably way beyond what you will be able to spend on just 650 posts.

Dick

 

[cvg]

since the fence will be subjected to wind loading, it may place a bending moment on the posts. Un-reinforced concrete doesn't do well in bending. You may need to reinforce the posts with rebar to handle this. Have you done any structural calcs? I'm not sure your fiber will provide any significant strength to resist the bending so is fiber really necessary?

 

[DRC1]

For cast in place mix that could be used for precast:

752 # cement
1200# Sand
900 # 3/4 stone
430 # 1/2 stone
430 # 3/8 Stone
38 Gal water
3 oz Air Entrinment

5,00 psi mix. Mix also uses 70 oz of midrane water reducer, but you may simply want to add a LITLE more water. Some strength drop, but should still be fine for what you are doing. Air will be good as it will prevent long term cracking & spalling due to frost. You may not want to use the 3/4" stone as it may be difficult to finsh. If so, divide among 1/2 & 3/8. Keep some of the same cement for rubbing. You may be able to by small quantities of concrete from some suppliers who will mix what you need on site in a Zinn Mixer. This may be better for what you are doing.
I would definately use rebar. Cost in terms of overall project are not high and it will give you a much better product. 3,s or 4's may surfice for your needs, but check first.
Finish on precast comes from steel lined forms. Wood forms will not give you the same finish. You can rent steel forms from some contractor supply outfits. Try A.H.Harris.
To achive finsh you will need to vibrate. Use a small vibrator and vibrate well but do not overvibrate such that segergation occurs.
Finally, I would ask a couple of local precasters for a price. This is certianly enough of an order that they would be interested, and by the time you added up your costs, you might think it worth the cost to have it done at a plant.

Good Luck

 

[sdz]

650 posts! You'd be better getting them professionally precast.

 

[concretemasonry]

This looks like a large DIY project.

When you are talking about substituting mesh for rebar on 8'-8" long precaste elements using home made "steal" forms to save money, that is another reason to suggest finding a precaster to supply the posts.

You cannot use mesh in a hand batched/mixed concrete with any reliability even if it is an acceptable material.

The cost to have the raw materials delivered will be as much as having the precast delivered.

When you are talking about 650 pieces of anything in the ground, you should make sure you have the right materials and some experience to rely on.

Dick

 

[DDame]

Dick (concretemasonry)… you sure sound VERY knowledgeable, do care to share any of your knowledge with me? I understand you don’t think I’m capable of doing this (you have made that clear), but I am AND I will make this happen.

I have made a test form from steel, troff style so it will lie horizontal on a vibrating 12" channel table consisting of a variable speed Vibco US-450 high frequency vibrator. I will be using 2 x #3 rebar on 4" diagonal offset.

DRC1 is the only one to suggest a mix (THANK YOU DRC1) I have checked with our local ready mix and they say sulfate stabilizers are not necessary.

752 # cement
1200# Sand
900 # 3/4 stone
430 # 1/2 stone
430 # 3/8 Stone
38 Gal water
3 oz Air Entrainment
70 oz of mid-range water reducer

 

[civilperson]

8 sack mix is 150% over kill. Try a 500 lb/cy cement with less than 226 lbs of water. High Range water reducer can still give slumps of 3-5". 4000 psi concrete is plenty of concrete strength but the two #3 bars is not enough reinforcement for handling horizontal posts. Self weight with end supports will break the post, (67 lb/foot length).

 

[DDame]

So what would you suggest for rebar size, placement and count?

 

[civilperson]

Free engineering is worth just what you pay for it. (I get 150/hour with a four hour minimum.) Make some trial posts with varying rebars and test the survivability of handling and flexure loading. I already saved you over 500 dollars in cement costs with the 500 lb /CY mix design.

 

[DDame]

Thanks civilperson... How many hours do you need?

 

[civilperson]

The minimum fee will be plenty. I will give you a series of four different reinforcement schemes and the maximum flexural loading for each. Try rho minimum of 0.00333, or 0.213 inches square for area of reinforcement, (one each #5 bar in the center is rho of 0.0048).

 

[concretemasonry]

For 650 posts, you should have a lot of forms or use a quicker curing method or both. The name of the game to get the concrete out of the mold and into a curing area as soon as possible. The size and shape you have compounds the problem.

The amount of steel is not a structural problem, but a manufacturing problem. Once you can develop the bond and enough compressive strength to handle and remove the posts to a longer term curing area. As an example, twenty forms requires 32 cylcles of production. Depending on your stripping time and handling methods, you will have a highly variable period. Add to that, the curing time required for practical movement and construction handling. - In other words, you have two different curing cycles (cure to strip and handle and then cure until they can be used for construction).

High early cement is a common method or using heated forms to accelerate the initial curing to develop minimal handling strength. Insulated blankets and possible a source of heat could shorten the cycle time to construction.

If you have all the time in the work, one form will work. Numerous forms will be more practical and realistic for batching, concrete placement and construction.

Good luck on your endeavor. - Dick

 

[cvg]

you will need enough forms and temporary additional labor to handle 1/2 of a standard readymix load. If you ask for a smaller delivery, they may say no or add on a surcharge. So plan on enough forms and material for 5 cy of concrete per cycle.

 

[concretemasonry]

DDame -

cvg brings up another factor in the manufacture of your post.

If you are limited to a 5 yard load as a economic minimum, you have 135 cubic feet of concrete, which will fill 35 molds. If you plan to use ready-mix, you will then need 35 molds.

If you plan to mix it yourself, then the number of molds needed goes down, but the over-all time to make all the posts goes up.


if you are mixing it yourself, I would forget about the 3/4". Keep in mind the water weights are only approximate since the moisture in the sand can be anywhere between 1 -1/2% and 8%. Your real concern should be filling the mold completely and properly with a minimum of water in the concrete.

For handling the posts, make sure you have a good pick-up system with a spreader bar. I doubt your bolts will be where you want them.

You did no say where you are located, but in some areas, there are manufacturers that make precast lintels, which are very, very similar (8" x 8" x whatever reasonable length is needed) to your posts. They use a zero slump mix with programmable vibration and have the ability to make whatever rebar cages necessary. Because they use automated equipment, zero slump concrete, have all the handling and delivery equiment for a continuous production and there is no need for form cleaning and oiling after every casting, the production rates are very high (100-200 per day). The only problem might be a way to get the bolt configuration into the mold configuration in a way that is compatible with the euipment. In Europe they would would use a tensioning bed with curing heat and cut the 200 or 300 length pour to the desired length.

Dick

 

[cvg]

not only will you need 35 molds, but enough help from your friends to pour, vibrate, finish and cure all 35 of the molds within about 60 - 90 minutes. This would not be a one man operation as you could probably only do a handful by yourself, you will probably need as many helpers as you can find.