I am pouring a 10X20X6 patio pad. It has been 15 years since I mixed a mass amount of concrete and the cement that I got is a little vague on the instructions. What is the ratio for cement:sand:aggregate? Is it 1:2:1 or 1:2:2? I know the water is a relative thing and I know what consistency I need, but the mix ratios have me confused. A bit rusty. Thanks!
Concrete obtains its strength due to the cement-water ratio. This means water is not a "relative" thing, but critical, in fact.
The fine and coarse aggregate (sand and stone) are basically just fillers. The space in between the aggregate needs to be filled with cement paste (cement and water). The more paste, the more cement you need.
A concrete mix design is based on the principles mentioned above. Depending on the source of your aggregate, your cement paste requirements may vary.
The "rule of thumb" ratios are based on experience, but don't guarantee a particular strength concrete.
A nice source of standard mixes for various requirements can be found at the South African Concrete Institute website
According to Wiki-How, general purpose concrete is created using a 1:2:3 ratio of cement to sand to gravel by volume.