<Li> Cracking and shrinkage . Concrete shrinks as it cures . It can shrink 1 / 16 inch (1.59 mm) over a 10 foot long area (3.05 meters). This causes stress internally on the concrete and must be accounted for by the engineers and finishers placing the concrete . </Li> <Li> Furthermore, access roads and site access have to be able to carry the greater weight of the ready - mix truck plus load . (Green concrete is approx. 3924 pounds per cubic yard .) This problem can be overcome by utilizing so - called "mini mix" trucks which use smaller 4m3 capacity mixers able to reach more weight restricted sites . </Li> <Li> Concrete has a limited life span between batching / mixing and curing . This means that ready - mix should be placed within 30 to 45 minutes of batching process to hold slump and mix design specifications . Modern admixtures and plasticizer and water reducers can modify that time span . However, it is limited in scope . The amount and type of admixture added to the mix is very important . </Li> <P> As an alternative to centralized batch plant system is the volumetric mobile mixer . This is often referred to as on - site concrete, site mixed concrete or mobile mix concrete . This is a mobile miniaturized version of the large stationary batch plant . They are used to provide ready mix concrete utilizing a continuous batching process or metered concrete system . The volumetric mobile mixer is a truck that holds sand, rock, cement, water, fiber, and some add mixtures and color depending on how the batch plant is outfitted . These trucks mix or batch the ready mix on the job site itself . This type of truck can mix as much or as little amount of concrete as needed . The on site mixing eliminates the travel time hydration that can cause the transit mixed concrete to become unusable, These trucks are just as precise as the centralized batch plant system, since the trucks are scaled and tested using the same ASTM (American standard test method) like all other ready mix manufactures . This is a hybrid approach between centralized batch plants and traditional on - site mixing . Each type of system has advantages and disadvantages, depending on the location, size of the job, and mix design set forth by the engineer . </P>

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