Usually with a front end loader. Some people will build a tall ramp to fill their concrete plant directly with the front-end loader and other will use a feed system to feed the concrete plant.   Normally if a concrete plant has a feed system they will still have a front end loader filling the feed systems, but some plants use a drive over grizzlies and trucks delivered directly into a pit where the material can be conveyed to the concrete plant automatically without the need for a front end loader.