Using a power mixer makes the job much easier when you have a concrete project that is too large to mix by hand, but still not quite large enough to order pre-mixed from a concrete/masonry company.
Step 1: Rent Mixer and Gather Tools
Most hardware retailers that rent power cement mixers offer instructional sessions on how to use and clean them, often at no additional charge. Take advantage of this in order to save a lot of guesswork later. Have all of your other tools ready in your work area and make sure they are in good condition. When storing bags of portland cement, it is important to keep them on top of plastic sheeting to protect them from any moisture from the ground. Any bag of cement that gets wet on the bottom will start to cure, leaving you with a wasted lump of unusable concrete at the bottom of the bag.
Step 2: Load the Mixer
Wet the inside drum of the power mixer with the garden hose; this will help make cleaning up easier. Use a bucket to combine the dry portland cement, aggregate, and sand according to the rule of three parts aggregate, two parts sand, and one part cement. Mixing this in the bucket is simpler than keeping track of how many shovelfuls of each component you scoop into the mixer drum.
Always wear a protective face mask to keep from breathing in dust from dry cement; it contains harmful silicates that can lead to lung damage. Also, wear work gloves and other protective clothing to keep the cement from skin contact because it has alkalies that can cause irritation.
Carefully pour the dry components into the power mixer drum. Add the water gradually to the dry mix until you get the concrete consistency you want; it should be roughly as thick as a cake batter. Too little water is easier to handle than too much; it is better to add more to a too-thick mixture than to measure out more dry cement ingredients to add to a too-thin mixture. Allow the mixer to turn the concrete until it is uniformly grey, which typically does not take more than five minutes.
Step 3: Pour Concrete
Once the water and cement combine and form their chemical reaction, you have a relatively short window to pour and trowel it before it starts to cure and become too difficult to work with properly. Only mix the amount you need with the power mixer, and make sure before pouring that your concrete is thick enough to cling to the sides of the drum but still fall off once it rotates to the top.
After pouring out all your concrete, immediately hose down the inside of the mixer drum to remove any clumps still stuck to the sides. It is a good idea to use a higher pressure setting on the hose nozzle for this; any wet concrete left on the side will cure quickly and become difficult to completely remove.