Section 4: Delivery of Concrete
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.
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 deliver.
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.
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the use of an absorptive aggregate, such as recycled aggregate will cause a drop in the slump of the mix, as water is withdrawn from the mortar into the aggregate.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.
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.