Ready-mix concrete (RMC) is concrete that is manufactured in a batch plant, according to each specific job requirement, then delivered to the job site "ready to use". There are two types with the first being the barrel truck or in–transit mixers. This type of truck delivers concrete in a plastic state to the site. The second is the volumetric concrete mixer. This delivers the ready mix in a dry state and then mixes the concrete on site. However, other sources divide the material into three types: Transit Mix, Central Mix or Shrink Mix concrete. Ready-mix concrete refers to concrete that is specifically manufactured for customers' construction projects, and supplied to the customer on site as a single product. It is a mixture of Portland or other cements, water and aggregates: sand, gravel, or crushed stone. All aggregates should be of a washed type material with limited amounts of fines or dirt and clay. An admixture is often added to improve workability of the concrete and/or increase setting time of concrete (using retarders) to factor in the time required for the transit mixer to reach the site. The global market size is disputed depending on the source. It was estimated at 650 billion dollars in 2019. However it was estimated at just under 500 billion dollars in 2018. History There is some dispute as to when the first ready-mix delivery was made and when the first factory was built. Some sources suggest as early as 1913 in Baltimore. By 1929 there were over 100 plants operating in the United States. The industry did not expand significantly until the 1960s, and has continued to grow since then. Design Batch plants combine a precise amount of gravel, sand, water and cement by weight (as per a mix design formulation for the grade of concrete recommended by the structural engineer or architect), allowing specialty concrete mixtures to be developed and implemented on construction sites. Ready-mix concrete is often used instead of other materials due to the cost and wide range of uses in building, particularly in large projects like high-rise buildings and bridges. It has a long life span when compared to other products of a similar use, like roadways. It has an average life span of 30 years under high traffic areas compared to the 10 to 12 year life of asphalt concrete with the same traffic. Ready-mixed concrete is used in construction