Questions on concret mixes

[StoneMaker]

Hopefully I am not violating the rules, I am not an engineer, however every forum other than this that discusses concrete was either inactive or filled with spam. I make and sell atlas stones which are the concrete balls seen in strongman contests and am trying to figure some stuff out so I can get away from using pre-mixed concrete mixes from Home Depot.

These stones are regularly dropped from 4 to 5 feet high and I make them from 8" diameter to 20" and can go as big as 24" diameter although that is not likely. The 8" stone weighs about 18 pounds and the 20" stones weight about 335 pounds. I want to make these as strong as possible as one poor quality stone can cost me potential customers, and I have no control over the surfaces the customers drop them to. The mix I am looking into is this,
1 Cubic yard
Portland Cement Type 1 423 pounds
Slag or Portland Cement 423 pounds
C-33 Sand 1100 pounds
#57 Stone 1700 pounds
Water 267 pounds
Air 3%
HRWR (as needed)42 ounces
Retarder (as needed)21 ounces
W/C Ratio 0.32
PVA macrofibers 96 ounces
PVA microfibers 48 ounces

Why would I choose to select gravel or crushed limestone for a coarse aggregate?
In a mix that is 50/50 Portland cement and slag cement, what effects could I expect to see by dropping that to 45/45 and adding 10% silicia fume, besides a stronger concrete?
What are the benefits/risks of eliminating air from the mix besides a denser concrete?
From what I read about the need for air entrainment, it seems to be that freeze/thaw damage is even worse with the presence of deicing compounds. How big of a threat is freeze/thaw damage if there isn't any deicing compounds coming in contact with the stones? What would be an appropriate air content percentage in this situation?
How serious is ASR?
Can concrete be so strong that it becomes brittle?
Is pursuing the highest possible (within reason) PSI foolish?
What are your opinions on shrinkage reducers? Can you go overboard with admixtures?
Also, would Design and Control of Concrete Mixtures from 1998 outdated? It's very cheap on Amazon.

Thank in advance!

 

[Ron]

You have obviously put some effort into this mix design. Here are my comments...

Overall, it is a good, high strength mix design for pre-casting such as your application. Would not be very useful for site cast building construction!

I would leave the air in. This helps to mitigate strain (the little bubbles are microscopically compressible, but because there are so many of them, you get some internal "cushioning"). You don't need it for freeze-thaw durability, but air entrainment gives other positive properties to the concrete that will help in your case.

ASR is serious if you are in an area that has known reactive aggregates. Otherwise, not an issue. With your cement content, it you have marginal aggregates, your potential for ASR will be higher, since it is the alkali content of the cementitious material that reacts with certain aggregates and causes expansion.

Essentially all concrete of reasonable strength is brittle. Yours is certainly "brittle" when compared to other materials. Ductility in concrete is gained from reinforcement, not the concrete mixture itself (there are some minor exceptions).

Your mix is a very high strength mix, so I wouldn't be concerned about trying to get it higher.

Yes, you can go overboard with admixtures. Some of them counter the effects of others. Since your sections are relatively small and do not have a prismatic cross section, I would not worry about shrinkage.

Design and Control of Concrete Mixtures is excellent in any edition. You are probably looking at the 6th or 7th edition and the current is the 15th edition. Yes, new information is added, but the basics are still there in older editions.

Now for a couple of recommendations:

1. Silica Fume, in my opinion, would be a waste of money for your application.
2. Using slag cement slows the strength gain of your product, so make sure you allow adequate curing time before selling.
3. Consider using a polymer additive to replace some of the water. This provides an increase in the ductility of the concrete.
4. Do some experiments with putting a styrofoam ball in the middle of your product. This might provide a bit more ductility, but you'll end up with a larger ball for the same weight.

 

[cvg]

you should use crushed rock, not gravel (particles should be angular to sub angular, not rounded).

you need to be concerned with constructability. since you are creating a round ball, I am unsure what your process is to form this shape. typically it would be two half spheres joined together. this would produce a construction joint in the middle which would be a weak point tending to split on impact. you might want to find another method

You need good vibration to consolidate the concrete in the form and remove air voids.

Maintain adequate cover over steel reinforcement otherwise it will just spall off on impact, regardless of the strength.

And proper curing will greatly increase the strength and durability of your end product.

there are free (and good) concrete design guides on the internet, you should not need to pay anything. For example, try the following:

http://140.194.76.129/publications/eng-manuals/EM_1110-2-2000_pflsec/toc.htm

 

[StoneMaker]

The mold is a sphere that has been cut in half. I put the mold together to form a complete sphere, tape it up and cover it in shrink wrap and pour concrete into a hole drilled in the top. I don't create two half spheres that I try to join together.

Is water bath curing the best way to cure? I realize certain situations can change what is "best" but I was looking into curing coatings and realized I could set up a small pool pretty easily to let the stones cure in.

"3. Consider using a polymer additive to replace some of the water. This provides an increase in the ductility of the concrete."
Are you referring to a high rate water reducer?

If anyone is from the around Cleveland/Akron/Columbus Ohio, where can I get bagged slag cement? Or even someplace where I could fill my own container? The few places I have found only sell as part of a mix in a mixing truck. I checked out slagcement.org already.

Thank you greatly for your help, it is appreciated.

 

[Ron]

No. A polymer additive is not a high range water reducing admixture. One common one is the liquid used for tile grout that you can buy at Home Depot or any tile supplier.

Yes, water curing is best. Make sure you add lime to the water so that lime is not extracted from your concrete as it cures.

I have not seen slag cement in bag form, only bulk. Go to one of the ready mix suppliers in your area and see if they will sell you a few hundred pounds.

 

[TXStructural]

Soak the aggregate and let it absorb water before use, to reach what is called "saturated, surface dry" condition. This will keep the aggregate from robbing the paste of moisture.
The small plastic fiber will not really help, and I might use alkali-resistant glass fiber, as it provides good tensile improvement. I'm not sure if the plastic microfiber would help resilience, but I have my doubts.
ASR is probably only an issue if you plan to let the stones be repeatedly subjected to wetting. Most crushed limestone you will get is not subject to ASR, but the sand might be. Also, the high SCM content will mitigate this somewhat.
Silica fume may make the concrete brittle and could cause some shrinkage issues.
You will prevent shrinkage problems by maximizing large aggregates, reducing cementitious content, reducing water, and consolidating properly.
Use of a polycarboxylate will reduce water requirements AND help keep the aggregate suspended, rather than having it segregate, (and don't overconsolidate.)

I might use steel fibers or a prepared product called Ductal, if dropping is really causing breakage or you need higher density.

 

[concretemasonry]

The steel fibers jumped into my mind.

Although they are nor recognized as "reinforcement", they do minimize micro-cracks. They could help to minimize micro-cracking that may be starting points of future cracks.

If you use any type of fibers, the mixing time is critical. Too little mixing will not disperse the fibers, while excessive mixing will cause "balling" and render them ineffective. Fibers work well in production of concrete products, but each producer usually has extensive access to making samples and testing facilities. The actual combined aggregate (sand and rock) gradation and the particle shape have an effect on the mixing time.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.