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	Raw Materials Sand, gravel, and cement are transported to the concrete plant by truck. Certain materials, such as inert aggregates, are typically stored outdoors in stockpiles. Moisture-sensitive materials, such as cement and flyash, may be stored in high-capacity silos. As the materials are needed, they are transported by conveyor to large storage bins at the top of the block plant. Batching + Mixing At the start of production, dry materials from the upper storage bins are discharged into the plant's stationary central mixer. The proportion of materials in the mix is custom-designed to meet the specifications for each project. Proportioning is controlled by computer to ensure quality control. The customer typically works with the ready-mixed concrete producer to determine characteristics such as aggregate size, slump, air content, and strength based on the intended use. Typical composition by volume is about 10-15% cement, 60-75% aggregates, and 15-20% water. Entrained air bubbles may also account for 5-8%. Using less water generally results in a higher quality concrete. Transport After mixing is complete, the mixture is discharged into a truck-mounted, rotating drum mixer. Rotating-drum truck mixers typically have a capacity of 9-11 cubic yards and discharge the concrete from the rear. Because slump loss can occur during transit, it is required that the concrete be discharged on the job site within 90 minutes or before 300 revolutions after the addition of water to the cement. Admixtures may be added to the concrete mix in situations of extreme temperature or long delivery times. Types of admixtures include air-entraining agents, water-reducing agents, and set-retarding agents. Vehicle maintenance is extremely important for ensuring safe and efficient transport of concrete products. Concrete plants typically include a full service garage to maintain their fleet of trucks. slide show main