I’ve been looking into mix designs for this little footing of mine. The Taunton concrete book talks about w/c ratios in the range from 0.45 to 0.62. But on Twining’s web site:
www.twininglabs.com/hs_conc.htm
they talk about keeping w/c in the 0.26 to 0.30 range. Has anyone worked with mud that dry, especially site-mixed in small batches? Any advice or stories?
I know I really don’t need 13,000 psi, but it’s interesting to try to make it as strong as is possible. Then again, maybe it’s just that I want to make trouble for whoever decides to demo my house in the distant future…. 😉
Thanks —
— J.S.
Replies
It will vary depending on the strenght and if the concrete is air entrained. For 2500 psi the w/c should be .67 for non-air entrained and .54 for air entrained. For 3000 psi the w/c should be .58 for non-air entrained and .46 for air entrained. Exposure to conditions also has a bearing on the w/c. The figures above are good for footings and are maximums. You only really need to pay attention to these figures on controlled commercial jobs when strenght data from field experience or trial mixtures are not available. Holding residential concrete subs to strict ACI standards may not be that realistic. Paying attention to soilbearing capacity, strenght of concrete and slump should be enough.
Edited 5/31/2002 4:47:10 PM ET by SLASH
Edited 5/31/2002 4:51:03 PM ET by SLASH
Whoa, 13 ksi concrete at home, like the previous post said, the soil is likely the limiting factor. Last time I was working missile silo hardness in '91 (all those programs were cancelled in Sept '91 at end of cold war), the best concrete guys in the business were concerned that 10 ksi on a production basis was unachievable. The quantity and type of cement is also important, as are the raw ingredients that go into making the cement for the super strength stuff.
Have seen Army corps has done 14 ksi to 20 ksi in lab type mixes, but that's with fiberglass and carbon fiber in the mix.
My at home method is to mix it as stiff as I can comfortable work it, which equate to a >45 degree slope of the concrete as it turns in my mixer for 4 sack mix (I'm cheap).
I work for a concrete testing lab that deals with construction practice, let me offer my two cents. There is no way to test for water cement ratio. This is a recipe item only. The mix was design on a theory of a water cement ratio. Then the mix is produce. At this time the driver washes down and screwed the w/c ratio out the window.
Now slump is only an indicator of the water cement ratio, is is not a test only an indicator.
back to w/c ratio. I have notice that hot mixes will always crack due to shrinkage. i have had better luck with weak mixes poured on a 3-4 slump. because it take forever to get strengh it will not crack. My slab at 120 days was only 1800 psi. at 240 days 2800 psi.
Think about psi, this is per square inch so 3000 psi would be a full size truck on one square inch. If you can put that many people walking on you slab at once then you might have problems.
In conclusion what I did on my slab, footing was 4000 mix, block fill was 4000 mix, floating slab was 2500 mix for cracking preventing.