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I'm making concrete countertops, so sand-only is the way to go to reduce voids in the forms. (I've done a half dozen samples). I've looked everywhere for mix ratios, but it's a weird request, and I can only find either concrete with aggregate ratios, mortar ratios, or someone wants me to give them a bunch of money to tell me 3 numbers. My best guess is 0.5 water/1 cement/2 sand/ 0.0035 superplasticizer. The sample looks fine, and doesn't break when I hit it with a hammer... but I'm just worried that I'm doing it wrong, and it's going to crumble in 3 months or something.

  • I found this site reviewing another post here there was some free training here is a link concrete counter top training. I am sure they are trying to sell something but it may give you some pointers. I really liked an article that had some glow stones gave me an idea for the walk around my pool. – Ed Beal May 11 '16 at 20:43
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First I want to point out that the hardness has a lot to do with cure time and temperature. Longer cures (6-8 weeks) at cool temperature (say 50 °F) are harder. Short, hot cures are harder initially, and crumble later.

Generally, the 1pt cement : 2pt sand ratio is best and will have a 3500 psi compression stress. A 1:3 ratio will have less than 3000 psi. Gravel can actually strengthen the mix... up to 3 parts. The mix of choice (concrete) is 1pt cement :2pt sand : 3pt gravel, because it's the best/hardest (for the money).

  • What about water? – jgrant May 13 '16 at 14:50
  • If too much water is used, it will evaporate leaving air (entrained) which is weak. For cement, it's exactly 0.42pt water by weight per 1pt cement; so 0.42pt water, 1pt cement, 2pt sand and 3pt gravel. But the ideal amount (just enough to react with the cement) is actually less than what is used because gravel will absorb water. Slump is the term that masons use when "measuring" the water content. Slump is the best thing for you to read about I think. I think 1/2pt water would be close if the mix was bone dry. I would start with 1/3pt water, 1pt cement, and 2pt sand... and add water slowly. – Ben Welborn May 13 '16 at 15:44
  • I looked into slump, and it seems like it really is only about the workability of the mix, which is not very useful to me because for this application about 0.75 pt water makes the mix pourable to fill the forms, which drastically reduces the strength, from my understanding. That's why I'm using a superplasticizer. Your information is extremely helpful. Thank you. – jgrant May 13 '16 at 16:06
  • @jgrant slump is a measure of workability... and a measure of wetness. Coincidence? Meh. Slump is a good test because it's a reproducible test. Also, have you considered fiberglass (GFRC)? – Ben Welborn May 13 '16 at 16:18
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    Remember also less water stronger mix. Also fiberglass makes it extremely hard to work. You'll never have to worry about breaking, 3500psi with glass is jackhammer hard. Im sure u know this take a vibrating sander several if u have help and run it down all the edges, base, where ever u can get solid contact with the forms. – user60532 Sep 25 '16 at 17:27

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