Once the SMA has been produced at a hot mix plant, it must be transported to the job site where it will be placed and compacted. This chapter describes issues that need to be considered during this stage of the construction process.
10.1 TRANSPORT OF THE MIXTURE TO A LAYDOWN SITE
While it may seem that hauling SMA to the job site is straightforward, there are things that can go wrong that can affect the rest of the construction process and ultimately the performance of the pavement. This section will describe potential problems and how to prevent them.
10.1.1 Loading a Truck
The hot SMA mixture, at first stored in a silo, must be transported to the work site. The way the mixture is loaded into the bed of the truck affects its subsequent quality. The following rules should be observed to prevent segregation of the mix:
• The discharge into the bed of the truck should be performed in defined drops; generally the first drop should be placed in the front part of the load bed, the second one toward the rear, and the rest evenly distributed in the middle (Hensley, undated; Roberts et al., 1996).
• The addition of small amounts of mixture to reach the full weight limit of a loaded truck may bring about the separation of larger particles of the aggregate from the mixture (segregation) and should be avoided.
• When using trucks with large beds, the truck should be loaded in four or more separate drops, overlapping the individual dropped piles to help remix the material.
Detailed instructions for loading mixtures out of silos may be found in the USACE Handbook (2000).
The draindown effect (i. e., binder or mastic draindown followed by its collection on the bottom) may happen under the following circumstances (Ulmgren, 2000): [60]
Also, remember to suitably prepare the truck bed prior to loading the mixture. The bed should be clean and free from dirt (see requirements according to EN 13108-21). Sideboards and bottoms of the beds of the delivery trucks should be covered with a special release agent (antiadhesive fluid) to avoid adhesion; diesel oil and other fluxing agents that may degrade the bituminous binder are not permitted to be used in this capacity. The truck bed should be free of major dents in which substantial amounts of release agents could gather. The boards should be evenly coated, and any excess release agent that may collect in deformations of the bed should be removed prior to loading the truck.
