The short load concrete truck carries separate bins which hold gray powder cement, aggregate and water. As you need concrete, an auger on the concrete delivery truck mixes the components as they come out of the truck. The operator can control the amount of water. The operator has precise control of how much concrete is made.
In addition, the operator can add other ingredients to make the concrete harder, such as a fiber mesh. Some trucks have the ability to add color, to make colored concrete patios. By controlling the amount of water, the operator can control the "slump", which is important when the concrete is sloped. If the concrete is too runny, it will run down hill and out of the form. If the slump is just right, the concrete will stay on an incline.