Types of Ready Mix Concrete – A Guide
Ready mix concrete is a combination of cement, sand, aggregates and water mixed either in part or entirely at a central plant and delivered in batches to construction sites. Each batch is mixed according to the contractor’s specifications and sent to sites in special trucks equipped with rotating drums. It’s an ideal solution for jobs where space does not allow the storing and mixing of components. Based on the mixing method, there are three main types of ready mix concrete. These are:
- Central-mixed concrete;
- Transit-mixed concrete;
- Shrink-mixed concrete.
The first type is concrete mixed at what is sometimes called a central batching plant or a pre-mix plant. The components are thoroughly mixed before being loaded into a truck mixer’s drum. During the delivery run, the drum acts as an agitator to prevent the concrete mix from solidifying. If the construction site is close to the plant or if the workability requirements are low, regular dump trucks can be used for transportation purposes.
The second type is ready mix concrete that is batched at the plant before being poured into the truck’s drum. The mixing process starts in the truck en route to the construction site and is usually completed at the site, just before the concrete needs to be poured. For this reason, this type is known as transit-mixed concrete. Transit mixing prevents the cement and aggregates from mixing properly with the water and thus keeps the concrete from hardening prematurely or losing slump, two major problems experienced when transporting central-mixed concrete.
As a rule of thumb, concrete is held to be good for an hour and a half of travel or 700 revolutions of the drum. In this case, the drum revolves at slow or medium speeds during the trip to the job site. Once the truck reaches the site, the rotation speed is set to high (12 or 15 rotations per minute) to make sure the concrete is mixed properly before pouring it.
Reliable ready mix concrete suppliers send representatives to construction sites several days before their products are needed in order to check the job and to make sure that all formula and delivery details are in order. The contractor knows what concrete mixes are required for the job and will place the appropriate order with the suppliers. Always remember that truck mixers can carry less pre-mixed concrete compared to a dump truck and plan accordingly. If the job is big, you may need to have more than one delivery coming in, which in turn may require some planning.
The third type is called shrink-mixed concrete, meaning concrete that was partially mixed at the plant. The rest of the mixing is done en route to the construction site. This kind of mix requires some rather careful planning of the transportation phase in order to avoid arriving too early with a batch of insufficiently mixed concrete and losing time at the site while waiting for the mixing process to end.
All three types of ready mix concrete are usually given a good mixing at the site, before they are poured and placed, to make sure that the loss of consistency or fluidity (usually referred to as “slump”) had not exceeded the level required to make the batch viable. If the contractor needs a longer setting time, then the slump level needs to be high and, therefore, the mixer needs to lose as little slump as possible during delivery. At the same time, keep in mind that remixed concrete tends to set and harden quicker than concrete that was mixed only one time.