Ready-mix concrete (RMC) is produced at batching plants as per specifications of concrete and then transferred to the project sites. Wet mix plants are more popular than dry mix plants. In wet mix plants, all ingredients of concrete including water are mixed in a central mixer and then transferred to the project sites by agitator trucks. During transit, the trucks continuously revolve at 2~5 rpm to avoid setting as well as segregation of concrete. The whole operation of the plant is controlled from a control room. The ingredients of concrete are loaded in the mixer as per the mix design. The mix design of concrete is a recipe for production of one cubic metre of concrete. The mix design is to be changed with the variation of specific gravities of cement, coarse aggregate, and fine aggregate; moisture states of aggregates, etc. For example, if specific gravity of coarse aggregate is increased, the weight of coarse aggregate is to be increased accordingly. If aggregate contains extra amount of water over saturated surface dry conditions, the amount of mixing water is to be reduced accordingly. At the RMC plant, the Quality Control Engineer should make a check-list to ensure quality of the product.
RMC has many advantages over on-site mixing. RMC (i) allows for quick construction, (ii) reduces cost associated with labour and supervision, (iii) has superior quality control through accurate and computerised control of ingredients of concrete, (iv) helps in minimising cement wastage, (v) is relatively pollution free, (vi) helps early completion of the project, (vii) ensures durability of concrete, (viii) helps in saving natural resources, and (ix) is an effective option for construction in a limited space.
On the other hand, RMC has some limitations also: (i) the transit time from the plant to the project site is a critical issue as concrete sets with time and cannot be used if concrete sets before pouring at site, (ii) agitator trucks generate additional road traffic, and (iii) the roads may become damaged due to the heavier load carried by the trucks. If a truck carries 9 cubic metres of concrete, the total weight of the truck would be about 30 tonnes. However, there are ways to minimise these problems. By using a chemical admixture, the setting time of cement can be prolonged. The roads can be designed considering the weight of agitator trucks. RMC can also be transferred by smaller trucks having carrying capacities of one to seven cubic metres of concrete. Considering the advantages of RMC over on-site mixing, RMC is popular worldwide. It can be noted that almost half of the total volume of concrete consumed globally is produced at RMC plants.
The ingredients of RMC are cement, coarse aggregate, fine aggregate, water, and chemical admixture. Under our cement standards, 27 kinds of cement are specified. CEM Type I is a purely clinker-based cement. In other types, a part of clinker is replaced by a mineral admixture, such as fly ash, slag, etc. Due to the slow rate of chemical reaction with water, the mineral based cements are better compared to the purely clinker cement. Mineral based cement delays setting and keeps concrete workable for a prolonged time. It also reduces heat accumulation in concrete due to slow reaction with water.
Post time: Jul-17-2020