# What's the best ratio for sand-only concrete?

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. Commented May 11, 2016 at 20:43

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? Commented May 13, 2016 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. Commented May 13, 2016 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. Commented May 13, 2016 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)? Commented May 13, 2016 at 16:18
• 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
Commented Sep 25, 2016 at 17:27

I use 10 parts sand and 4 parts portland cement and it seems quite suitable for most home projects ie trash can pads , picnic table pad , sidewalk etc

• Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Commented Feb 20, 2020 at 16:48

You are overthinking. Concrete holds up huge buildings, your cup of tea or your wife's bottom will be just fine sat on any old mix you throw together. Just don't go thinner than 2". 3-4" is better.

High-strength concrete has a compressive strength greater than 40 MPa (5800 psi). In the UK, BS EN 206-1 defines High strength concrete as concrete with a compressive strength class higher than C50/60. High-strength concrete is made by lowering the water-cement (W/C) ratio to 0.35 or lower.

Bob Vila is looking at 6000 PSI (40 MPa) concrete for countertops.

Three days after pouring a counter with Countertop Mix, the compressive strength of the slab will achieve resistance equal to 2,000 pounds per square inch (PSI). At seven days, it reaches 4,000 PSI, and at full cure (28 days), the countertop slab will reach 6,000 PSI. All-purpose concrete at full cure reaches only 4,000 PSI. Choosing the mix with the higher PSI will provide better defense against cracks (and limit crack repair) down the line.

Concrete Mix Design as per IS 10262-2009 – Procedure and Calculations. If that math isn't beyond you as it is me, then you could figure it out. The important part is a water-cement ratio of 0.35 or lower.