Section 4: Delivery of Concrete
4.1 Introduction
Concrete to be used in concrete pavement may be delivered
to the paving operations in several ways without segregation. The
low slump of the paving concrete makes it possible for the concrete to
be delivered in non-agitated dump trucks, concrete mixing trucks,
and agitator trucks.
4.2 Concrete Mixing Trucks
Concrete mixing trucks may be used for mixing or agitation.
Typically, a concrete mixing truck will have two capacity ratings,
as a mixer and as an agitator. The agitator rating is usually significantly larger.
For example, a mixer truck may be rated for 6 cu. yd. as a mixer
and for 8 cu. yd. as an agitator. This means that if the concrete
is centrally mixed and then loaded into the mixer truck, a larger
load can be transported to the project. Otherwise, the smaller mixer
rated capacity must be used.
True agitator trucks are sometimes used on TxDOT projects.
Agitator trucks can be used to deliver agitated concrete to the
project, but never to mix concrete. Typically, these trucks will
resemble a mixer truck, except that the mixer drum is open on the
top. Instead of a drum turning, a series of paddles will rotate
within the agitator drum. These paddles can be seen as they rotate
above the mix. Use of agitator trucks is more common when higher
slump concretes are used that may be more susceptible to segregation
during delivery.
4.3 Concrete Delivery Time
The concrete delivery time needs to be carefully monitored.
This is the time from when water is first added to the dry ingredients
of the mix until the truck is ready to discharge the concrete on
the project. This is particularly critical in hot weather when the
concrete may start to set prior to delivery. It should be regularly
monitored using the time stamp on the batch ticket from the plant.
4.4 Water Additions
Concrete mixer trucks typically have a water tank mounted
on the truck. This water should never be added to the concrete batch
unless approved by the engineer. Occasionally, the use of an absorptive aggregate,
such as recycled aggregate, will cause a drop in the slump of the
mix as water is withdrawn from the mortar into the aggregate. Even
then, the water should be measured carefully using a certified gauge
to ensure that the added water does not cause the concrete to exceed
the water content of the approved mix design. Water should never
be added to “re-temper” concrete that has begun to set. This can
occur in hot weather or when delivery times are extended. It is
also more pronounced when higher cement contents are used. The water
tank should be full when the truck leaves the plant and full when
it reaches the project.
4.5 Wash Water
Wash water from the concrete trucks should never be disposed
of in other than an environmentally sound method, as approved by
the Texas Commission on Environmental Quality (TCEQ). The preferred
method is to capture the wash water at the concrete plant and recycle
the wash water as concrete mix water into subsequent concrete batches.