- Q. Many years ago, a concrete contractor gave me a recipe for mixing concrete by hand or machine. It is the only mix I have ever used. Is there any way to know how strong the mix is? I use 1 part Portland cement, 2 parts clean sand, and 3 parts clean crushed stone (7/16 inch or 3/4 inch, depending on the job).
A.Bob Shuldes responds: The strength of your 1-2-3 mix should be very good, but will vary somewhat depending on the water content (water/cement ratio). If you use 4.5 gallons of water per bag of cement, the strength will be 5,800 psi; at 5 gal. per bag, 5,300 psi; at 5.5 gal. per bag, 4,800 psi; at 6 gal. per bag, 4,300 psi; and at 6.5 gal. per bag, 3,900 psi.
Bob Shuldes, P.E., is a consulting engineer at the Portland Cement Association in Skokie, Ill.