For the occasional concrete job, like re-pouring a sidewalk slab or filling a post hole, it might not be financially feasible to rent a concrete mixer. There are homemade alternatives to mix concrete.
Homemade Cement Paddle
When mixing small batches of concrete, you can mix the concrete in a five-gallon bucket using a heavy duty electric drill and a cement paddle. Construct a cement paddle from a 2-foot piece of round bar, 7/16-inch in diameter and a 1-1/2 coils of a car spring. Weld a piece of 1/2-inch flat bar to one end of the rod and then weld the spring to the bar. The other end of the rod connects into the bit end of the electric drill.
Warnings About Drill Speed
If your electric drill's motor speed is too slow it will have to fight to move the heavy concrete and the motor will burn out. If you are using an industrial drill, keep the speed below 650 rpm to avoid air bubbles in the mixture.
Concrete Mixing Drum
If you will be mixing large batches of concrete, you can re-purpose a washer drum. Attach angle brackets to the center tumbler to create mixing blades. Fill the holes on the washer drum with auto body putty. Mount the washer drum at an angle on a steel pipe frame. Rotate the drum by hand so that the blades scoop the concrete and drop it from the top of the drum.