Ready-mix concrete (RMC)
Concrete that is manufactured in a batch plant according to a predetermined mix design is known as ready-mix concrete (RMC). Ready-mix concrete is typically delivered in two ways.
The first is the barrel truck or the in-transit mixer. It delivers concrete that is in a plastic state to the job site.
The second type is the volumetric concrete mixer. The ready mix is delivered in a dry state and then mixed on site.
Batch plants precisely combine gravel, sand, water, and cement (as per the mix design formulation for the grade of concrete recommended by the structural consultant), allowing specialty concrete mixtures to be developed and implemented on construction sites. In the 1930s, the first ready-mix factory was built. Before the 1960s, the industry did not expand significantly. Since then, it has continued to grow.
It has a long life span when compared to other products of a similar use, such as roadways, and is cheaper than other materials. Ready-mix concrete is often used instead of other materials when building, particularly in large projects, such as high-rise buildings and bridges. Under high traffic areas, it has an average life span of 30 years, compared to the 10 to 12 years for asphalt concrete.
Ready-mix concrete refers to concrete that is specifically manufactured for customers' construction projects and supplied on site as a single product. It is a mix of Portland or other cements, water, and aggregates, such as sand, gravel, or crushed stone. A washed type of aggregate with minimal fines or dirt should be used. Concrete is also treated with an admixture to improve workability and/or increase setting time (using retarders) to account for the time it takes for the transit mixer to reach the site.
Ready-mixed concrete is used in construction projects where the site is unwilling or unable to mix concrete on site. Ready-mixed concrete can be delivered finished, on demand, in the specific quantity required, in the specific mix design required. Compared to the cost of ready-mixed concrete, where the customer pays for what they use, and lets others handle the rest, the hiring of mixing equipment, labour, and purchasing and storing the ingredients, may not make sense for small- to medium-sized projects. Outsourcing concrete production to ready-mixed concrete suppliers means handing over quality control and testing, material logistics, and mix design, to specialists who are already established in those areas, versus introducing another ordeal to the project, and losing some control and immediateness of the mixing process.
Ready-mix concrete is bought and sold by volume - usually in cubic meters (cubic yards in the United States). Batching and mixing are controlled processes. Ready-mixed concrete is specified either informally based on the constituent weight or volume (1-2-4 or 1-3-6 are common mixes) or formally by the European standard EN 206+ A1, which is supplemented in the UK by BS 8500. The customer can specify what the concrete needs to be able to cope with in terms of ground conditions, exposure, and strength, and the concrete manufacturer can design a mix that meets those requirements by using materials that are available locally. Laboratory tests are performed to verify compression strength[3] as well as field tests to verify the plasticity of the mix, such as slump tests conducted on site.
Admixtures can alter the performance of concrete mixes. Admixtures can be used to reduce water requirements, to introduce air to an existing mixture, to improve durability, or to superplasticise concrete to make it self-leveling, but using them without a batch plant or the necessary laboratory equipment means they cannot easily be used.
Batching / mixing and curing of concrete have a limited lifespan. This means that ready-mixed concrete should be placed within 30 to 45 minutes of the batching process to hold slump and mix design specifications in the USA,[5] though in the UK, environmental and material factors, plus in-transit mixing, allow for up two hours to elapse.[6] Modern admixtures and water reducers can modify that time span to some degree.[7]
Several methods can be used to transport and place ready-mixed concrete on site. One of the most common and simplest is the chute fitted to the back of transit mixer trucks (as in the picture), which is suitable for placing concrete near places where a truck can back in. Concrete can be placed from trucks using dump trucks, crane hoppers, truck-mounted conveyors, and in extreme cases, wheelbarrows. Some concrete mixes can be pumped using concrete pumps.
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