concrete, by far the most common form of concrete, accounts for
nearly three-fourths of all concrete. Ready mixed refers to concrete
that is batched for delivery from a central plant instead of being
mixed on the job site. Each batch of ready-mixed concrete is tailor-made
according to the specifics of the contractor and is delivered to
the contractor in a plastic condition, usually in the cylindrical
trucks often known as "cement mixers."
As early as 1909, concrete was delivered by a horse-drawn mixer
that used paddles turned by the cart's wheels to mix concrete en
route to the jobsite. In 1916, Stephen Stepanian of Columbus, Ohio,
developed a self-discharging motorized transit mixer that was the
predecessor of the modern ready-mixed concrete truck. Development
of improved ready-mixed trucks was hindered by the poor quality
of motor trucks in the 1920s. During the 1940s, the availability
of heavier trucks and better engines allowed mixing drum capacities
to increase, which in turn allowed ready-mixed concrete producers
to meet the high demand for concrete that developed as a result
of World War II.
Ideal for Many Jobs
Ready-mixed concrete is particularly advantageous when small quantities
of concrete or intermittent placing of concrete are required. Ready-mixed
concrete is also ideal for large jobs where space is limited and
there is little room for a mixing plant and aggregate stockpiles.
There are three principal categories of ready mixed concrete:
- Central-mixed concrete is completely mixed at the plant then
transported in a truck-mixer or agitator truck. Freshly mixed
concrete may be transported in a open dump truck if the jobsite
is near the plant. Slight agitation of the concrete during transit
prevents segregation of the materials and reduces the amount of
- Transit-mixed (also known as truck-mixed) concrete, materials
are batched at a central plant and are completely mixed in the
truck in transit. Frequently, the concrete is partially mixed
in transit and mixing is completed at the jobsite. Transit-mixing
keeps the water separate from the cement and aggregates and allows
the concrete to be mixed immediately before placement at the construction
site. This method avoids the problems of premature hardening and
slump loss that result from potential delays in transportation
or placement of central-mixed concrete. Additionally, transit-mixing
allows concrete to be hauled to construction sites further away
from the plant. A disadvantage to transit-mixed concrete, however,
is that the truck capacity is smaller than that of the same truck
containing central-mixed concrete.
- Shrink-mixed concrete is used to increase the truck's load capacity
and retain the advantages of transit-mixed concrete. In shrink-mixed
concrete, concrete is partially mixed at the plant to reduce or
shrink the volume of the mixture and mixing is completed in transit
or at the jobsite.
concrete is often remixed once it arrives at the jobsite to ensure
that the proper slump is obtained. However, concrete that has been
remixed tends to set more rapidly than concrete mixed only once.
Materials, such as water and some varieties of admixtures, are often
added to the concrete at the jobsite after it has been batched to
ensure that the specified properties are attained before placement.
More information at the
National Ready-Mix Concrete Association Web page.