Small Concrete Engine Base

[veggie]

Hi All,

I am building a form to pour a small concrete engine base for a Changfa 175.
The finished base will be approx. 225 lbs (twice that of the engine).
My question is about the type of concrete to use.
I was planning to use Quik-Crete, the stuff used to pour into holes to support fence posts.
Will this work ?
I seem recall a thread where someone recommended adding something to the mix. Maybe Portland cement powder ?

Any suggestions concerns about using Quik-Crete ?

veggie

[Tom Reed]

Quickcrete, or any fence post concrete is low strength stuff. If you buy a a small bag of portland cement and add about 5-10% by weight to the fence post mix you'll have a really hard concrete. I did my own mix for the Listeroid base and slab in the engine room and after 5 years of pounding abuse still no cracks.

It wouldn't be a bad idea to put some wire or short pieces of rebar in the block too. That way even if it does crack it will still stay together.

[BruceM]

+1 on adding rebar or steel mesh reinforcing, more important that adding portland in my opinion, though you should avoid post hole mix; in my area the post hole mix is a different, weaker mix. Quickcrete should be OK, though adding a dash of portland gives you more strength.  Wet curing the slab (cover with plastic and seal edges well) for a few days is also more important that adding a bit of Portland cement.  Wet curing concrete is something we have to work at here in AZ. 

[Ronmar]

Yep "wet" curing makes for the strongest cure.  Do not add too much water.  A 60# bag of quickrete needs if I recall correctly 1 quart of water to mix properly. Added cement will increase the water required slightly, if you can come up with a total weight, it will be easier to calculate the added water required.  Too much water is a common mistake as it seems the speced ammount takes too long to mix and appears to not be enough water to wet the mix throughly, but it is. You may have to mix it quite a bit to get it evenly distributed but use a pitcher to measure the specified ammount of water for the product you are using, and it will all mix in. Of course a mixer is way easier than doing this with a hoe in a wheelbarrow:)  Also try not to agitate the conrete after the pour.  The more you work it, the more it settles which causes the little bit of water that is there to be squeezed to the top, making for an uneven cure from bottom to top...

Good Luck.

[PaperPatched]

You can buy High Strength concrete mix at most building supply stores and at most True Value Hardware stores (Yes, I do work for one.) There are also additives that can take the place of some of the water in the mix.  Be careful of how much water goes into the powdered mix; watery mix makes weaker concrete.  The additives can make the concrete less brittle, and thus less prone to cracking. Concrete continues to harden over a long period.  The more time you can give it before stressing the stronger it will be. Cover the top of the block with polyethylene for a few days to slow water loss and promote stronger concrete. If it is warm and/or dry you may have to mist the block with water for a few days to allow a longer and stronger cure.

[BruceM]

Weldbond or White Elmer's Glue (polyvinyl acetate or PVA) can be used as an additive to strengthen concrete. It's mixed with the water prior to mixing the cement/concrete.  10% glue is a good starting point.  It allows you to use less water, as it improves the flow and workability of the material, and this is good for strength.  It also improves the flexibility and crack resistance of the mix.

I've used it in homemade thinset as an additive instead of acrylic, also as an additive for grout repairs where plain sand and cement can't be properly wet cured, and for surface repairs on concrete as a bonding agent and patch (sand cement) additive.  See the Weldbond website for detailed engineering data on it's use.  I've used both and didn't find any difference between Weldbond and Elmer's on my thinset tests for bond strength and flexibility.  It's amazing to see a thin layer of sand and cement bend without cracking!

[mike90045]

Also, plan where your mounting bolts will go.  If installing J bolts or all-thread, make a plywood template first, before you mix the concrete, and have the bolts hanging in it to drop in when ready.    If you are going to drill bolt holes afterwards, be sure rebar is not crossing a hole site, or you will have a heck of a time drilling it out.

Reinforcing, what has been said, not too much water, and also polypropylene rope fibers (chop rope into 1" lengths and separate strands) added to the mix will add strength (fiber reinforced concrete) http://en.wikipedia.org/wiki/Fiber-reinforced_concrete

[veggie]

Very good advice and comments from everyone.
Thanks fellas's.

and yes... my intension is to add re-bar to the structure.
I am also hanging the engine mount rails into the mix during the pour.
J-bolts will anchor the engine mount rails.

Will post some pictures when done.

Cheers,
Veggie

[veggie]

Ok...the Form box is built and I have a couple more questions.

1] What is a good release agent for the form ? (I have lots of waste vegetable oil  )

2] I need to make some J-bolts to hold the engine mount rails.
    Can this be some threaded ready-rod bent into a "J" ?
    Or maybe some grade 8 bolts with a T welded on the end.?
cheers
Veggie

>>>Click picture to enlarge<<<

[BruceM]

J bolts available at any HD or Lowes or building supply in a variety of lengths.  1/2" is the most common size for construction.

You were going to secure them to the rails, then set the rails in the wet concrete, right? 

As I recall, some folks sleeve the top 2/3 of the bolt  and use the softer std. bolt- then the steel shaft of the bolt has some length to go into tension as the nut is tightened, and less chance of shearing off at the surface.  If only the bit at the surface can stretch, it's quite a stress riser and may fail. 

Heating and bending the head/ shaft of a long carriage bolt would work, or welding as you suggest, too.

[Tom Reed]

Concrete usually shrinks a bit during cure so it should fall out of the box when done.

[Number21]

They also sell little packets of fiberglass you can mix into your concrete to add strength. Not sure it would be needed, but it's cheap.

[BruceM]

Vegetable oil is just fine to coat on forms.  DON'T USE motor oil or diesel, which "kills" the curing of the surface cement which will later spall.

[mobile_bob]

+1 for added portland and a strong +1 for using as little water as possible.

when i spec'd the concrete for  my shop floor, i spec'd out 5500psi mix, and it comes out very dry and lumpy indeed. 

i called for that much strength as my floor is in places only about 2" thick. 

it didn't help that it was 108F when we started the pour, and went up to 114F that day just as we finished.

no i didn't get the nice glassy smooth surface i would have liked, but i have a floor that has no cracks save for the grooves i cut after the thing setup... i used 1/2" rebar on 4ft centers btw.

that floor rings like a bell when hit or something is dropped on it.

keeping the pour wet or damp to slow the rapid setting is a good idea too in my book, we spread straw over the surface and put out lawn sprinklers to keep the slab damp for the first 3 or 4 days too.

i am really a proponent of high strength mix!  the difference in cost at the time was about 8% over the more common 3500psi mix that contractor use around here (to which they add more water to make their job easier) so the effective strength is probably on par with rammed earth.

i suppose it is no wonder that they want to pour 6 inch shop floors and use tons of iron, with wire mesh.

bob g

[Derb]

Hi Fellas. When pouring footings for our pumps/compressors and motors, we use a mix of 3 parts builders mix to 1 part cement as apposed to post footings/footpaths/driveways ratio of 5 parts builders mix to one part cement. We also always have a length of reo bar bent up to just curve around the outside edge of the anchor-bolts and 100mm in from edge of shutter-work and top faces. Don't be scared to use a bit of steel - more steel is good. A handful of fibreglass mix doesn't go amiss either. One of our reference books states that after 32 days, concrete is as strong as it is ever likely to get although it never stops curing - go figure. Cheers.