A Guide to Mixing Concrete

When concrete for your cement job of choice, several elements need to be taken into consideration. Figuring out the amount of concrete need for the outdoor project is the first thing and it is really not that difficult unless, like yours truly, you are a total math idiot.

Estimating Concrete Volume

Estimating how much concrete you need can be done with a simple formula that involves multiplying the length by height by width of the concrete forms you will need. Divide that figure by 27 to convert into the order standards for measuring concrete in cubic yards. It helps to add anywhere from 7 to 10 percent onto this total for mistakes in the measuring of the concrete you actually need.

Testing Concrete Consistency

Your typical concrete is a mix of 11% Portland cement, 26% sand, 41% crushed stone, 16% water and 6% air. Mixing the ingredients to achieve this perfection can be difficult unless you know what to look for regarding concrete testing out too wet or dry. When mixing the concrete you will know if it has gotten too wet by attempting to make ridges in the material. If the ridges you attempt to make with your trowel won’t hold shape, you will know the concrete is too wet for the project at hand. On the other hand, that cement trowel will not even be able to make a ridge if it is too dry. Testing concrete consistency for perfection means building ridges with the trowel that hold their shape. You should be able to see a minimum of water in the concrete mix.

Air Entrainment in Concrete

Ready-made concrete projects that require more than a yard of the stuff need to have air entrainment in the mix. Air entrainment is an additive put into the concrete that increases durability in colder regions. The proportions and percentage of air entrainment vary according to the sizes of the concrete project in question. Most patios or concrete slabs will need between 6 and 7 percent if the patio or slab is going to be laid down where it gets colds often during the year.

Mixing Sand and Gravel in the Concrete By Hand

When mixing concrete by hand, you want to measure out the sand and gravel together initially. Add the cement on top of the sand and gravel. Use either a sheet of plywood or a paved area that is clean enough and turn the pile with a shovel until you arrive at a uniform color and texture. Spread the mixed material around in a ring with a depression in the center into which you will be adding the water.

Adding Water to Sand and Gravel to Create Concrete

Add water to your concrete mix in small amounts and make sure to mix the dry ingredients into the center so that the water is absorbed. You want the concrete mix to reach a state where it begins to resemble crumbly cake. Add more water until you reach the correct consistency while being mindful that it is usually easier to add water to moisten the dry mix rather than trying to get something useful out of a soupy mix.

The Concrete’s Slump

The slump is the term used to describe your concrete’s consistency and it is refers exactly to the number of inches that 12-inch-high tower of concrete slumps when fresh. The concrete is considered stiff if there is a one-inch slump and soupy if there is a 10 inch slump or the tower does not hold its shape at all. The typical slump for residential concrete projects is 4 inches. If you can build a tower of concrete that slumps about halfway, you are ready to finish that outdoor project.