Before we go any deeper we should answer a question that a lot of people ask me - What is concrete? Concrete is NOT just water and cement. Water and cement is a simple mix that is weak and prone to cracking.
To make concrete you need cement, aggregate (sand with or without gravel) and water. Sounds simple right? Well, it’s not. Trust me on this one. Making concrete requires a basic chemical reaction to take place - hydration, the hydration of cement. The water is as important as the cement and the aggregate. You could even say that it is the most important ingredient because if you put too much water you will have an easy to work with mix, but it won’t be strong. And if you don’t put enough water you’ll end up with a mix that is impossible to work with. So, water amount is the most important part in making strong concrete, right? Wrong! There’s more.
The next part is your cement to aggregate ratio. This will only depend on the application of the concrete. It just makes sense - you won’t use the same mix you made a planter with to make a concrete countertop. An industry spread standard ratio is a “50/50” or equal parts of sand and (white) cement. Having said that, this should not be your go to mix as the high amount of cement creates the danger of high temperature and rapid setting, and eventual cracking of your cast. A “50/50” is best used with additional additives that we’re really not gonna go into now. But only FYI there are products on the market that are available - known as GFRC or Concrete admix, that will help you easily produce strong concrete. Or you can also choose to source and formulate the additives yourself - for Set-Retarding, Air-Entrainment, Water-Reducing, Accelerating, Shrinkage Reducing, Corrosion-Inhibiting, Superplasticizers etc. Great way to start in the industry is using ready made admixtures, but eventually you want to start making your own mix deigns. Why? Well, when you go to a restaurant and order Italian pasta do you expect the cook to make the pasta from scratch for you or to use a store bought one? That’s what I thought. Bespoke concrete doesn’t just mean bespoke size and shape, it also means bespoke mix for the unique application.
Next on the list is the type of cement and aggregate you’re using. Choosing those will depend on your budget, the application and your willingness to steal sand from the closest children’s sand pit - just joking about the last one, but sand there usually is of great quality. Every brand of cement is slightly different, every type of cement has different qualities (grey and white cement have different setting and curing times for example), sand varies on size of the grain and water content. Actually the water content in the sand is the main reason I can’t just give you a simple formula to follow. If I tell you to use 3 liters of water for a 10 kg (5 kg sand and 5 kg cement) mix and your sand is soaking wet your attempt of making strong concrete will fail.
The only way to learn to make strong concrete is to start mixing. The right amount of water, sand and cement will produce the strongest concrete on the planet. Ask the Romans, they never bought any admixes…