There are two main types of mixing plants, the ready mix plant and the central mix plant. A ready mix plant is designed to take all of the ingredients except water and combine them for loading into a ready mix (or concrete transport) truck. Water is then added to the dry concrete and mixed en route to the job site. A central mix plant on the other hand mixes all the ingredients, including the water, for transport to the job site. The advantage of this is the fact that all the mixing is done in a single location and can be computer controlled for very precise mixes and excellent uniformity from batch to batch. Temporary batch plants are a sub-type of central mix plant. They can be transported to a site allowing the mixing of concrete to take place right on the site of construction. At the center of a batching plant, no matter what the type, is a mixer. There are three main types of mixers; the tilt, pan, and twin shaft mixer. They differ slightly. The twin shaft, for instance, is best for evenly mixing large amounts of material, while the tilt mixer offers very consistent mixing with lower operating cost and maintenance. No matter which type of mixer a plant is equipped with, the mixer is the part of the plant that takes the individual ingredients and mixes them into concrete. In the past concrete plants were known for their impact on the environment. The large amounts of dust and particulate as well as dirty water runoff that came as a byproduct of the mixing process had a huge impact on their surroundings. Modern plants, however, often come equipped with dust collectors and water reclamation apparatus. These modern plants can also be automated to various degrees. Self-loading plants are available for projects that require a 24 hour stream of concrete; as well as programmable plants with sensors for measuring the water content of aggregates and adjusting the amount of water added to achieve optimal performance of the final concrete mixture. There are two major trade shows for those interested in learning more about concrete batching and mixing plants and what types of equipment are currently available. The World of Concrete expo occurs every year, while the ConnExpo-ConnAgg comes around only once every four years, but is the largest exhibition in the concrete industry. Also, the Concrete Plant Manufacturers Bureau is a non-profit association of all the main concrete plant manufacturers whose primary goal is 'to establish minimum standards for rating various components of concrete plants for the protection and assurance to the user that the plated components of the plants conform to these standards.'