bar!, sacks, and super-p

[bob_mp]

Hi,
Getting ready to pour the slab -- 8" thick with two layers of bar over Stegowrap. The slab is floating. I have some questions I was hoping I could get some help on.

1. Location of the expansion joints as compared to the vapor barrier. It seems the best place to put the felt is between the vapor barrier and the foundation wall. Otherwise, it seems, that concrete could get under it and push up against the foundation. Is this correct? Should I place some plastic around the felt to avoid having it wick up water?

2. Sacks and Super-P. I have told the concrete contractor that I want high strength, dry concrete and that I am happy to pay for more cement and for the costs of the Super-P. Is it reasonable to suggest a max w/c ratio? Is 0.4 too low if he can dose it with plasticizer to his liking? I don't want to tell him how to do his job either.

3. Is more cement better? It seems a mixed set of tradeoffs. For the same w/c ratio, does the extra cement adds heat and shrinkage without adding strength? Beyond a certain point, does additional water, cement, and plasticizer only make the concrete easier to work with, not better (for the same w/c ratio)?

4. How firm should I be about them not adding water on the site, assuming they add the HRWR at the factory?

Thanks for the help!
Bob

 

[ConCretin]

Wow, that's a rugged slab you're building. Curious as to what you're putting on top of it. I've taken a shot at your questions below

1. Location of the expansion joints as compared to the vapor barrier. It seems the best place to put the felt is between the vapor barrier and the foundation wall. Otherwise, it seems, that concrete could get under it and push up against the foundation. Is this correct? Should I place some plastic around the felt to avoid having it wick up water?

We usually run the vapor barrier up the foundation wall and then attach the expansion material over it but you're fine either way. I like the closed-cell foam expansion material with the removable strip at the top for caulking.

2. Sacks and Super-P. I have told the concrete contractor that I want high strength, dry concrete and that I am happy to pay for more cement and for the costs of the Super-P. Is it reasonable to suggest a max w/c ratio? Is 0.4 too low if he can dose it with plasticizer to his liking? I don't want to tell him how to do his job either.

I wouldn't get too wrapped up in w/c ratios. Order a 4000 psi mix, which should be more than adequate. To protect the strength of the concrete and give your finishers a workable mix, I would use a mid range water reducer. A mid range is more manageable than SuperP.

3. Is more cement better? It seems a mixed set of tradeoffs. For the same w/c ratio, does the extra cement adds heat and shrinkage without adding strength? Beyond a certain point, does additional water, cement, and plasticizer only make the concrete easier to work with, not better (for the same w/c ratio)?

Generally yes but water is the bigger concern. Shrinkage is a factor or water not cement. The less water you add the less cement you need to reach a desired strength. The only real downsides to more cement are higher cost and potential finish-ability issues in warm weather. The primary advantage of a richer mix, other than compressive strength is surface durability.

4. How firm should I be about them not adding water on the site, assuming they add the HRWR at the factory?

There's nothing wrong with adding water at the site as long as doesn't exceed the mix design and it's not used to 'prolong the life' of the concrete. Ask them to bring the load to the desired slump (5"-6" with a MRWR) when the truck arrives but don't let them add water again. Tell them you expect them to have the men and equipment on site to place the concrete expeditiously so each truck is emptied in less than 30 minutes.

Hope that helps

 

[ForceFed70]

LLwillysfan has got you covered. If memory serves me correctly, he's in the concerete business and knows his stuff!

I just wanted to post in here to reflect his 1st comment "That's one heck of a slab you are planning!" You must be planning to park some very heavy equipment in there or something.

 

[bob_mp]

Thanks for the responses guys.

The heavy thing that I'm planning putting on the slab is a vertical machining center. Through what's likely a a long sequence of intellectual wrong turns, I concluded that it's a better idea to buy a used VMC than to upgrade my Bridgeport to CNC. For not a lot more money, it seems one can get rigidity, a much bigger working envelope, and a tool changer.

Also planning on adding a 2 post lift, but not too worried about that.

Cheers,
Bob

 

[pop pop]

LLW certainly gave you good advice. Just make sure to run the slump and strengh samples/tests. If the supplier knows your doing this, he'll be a bit more careful to give what you ordered.

 

[bob_mp]

LLW certainly gave you good advice. Just make sure to run the slump and strengh samples/tests. If the supplier knows your doing this, he'll be a bit more careful to give what you ordered.

Thanks for the responses. A couple of really basic questions on this.

1. If the water reducer is added at the concrete plant, the slump is going to be pretty high even for low water/cement ratios. What would I be looking for?

2. With test cylinders (say 6" x 12"), should I get 1 or 2 from every load, wait 28 days and then send them out for test? What is the role of the lid? Is this part of the test specification?

Best,
Bob