The main objective of transporting a mixture from the asphalt plant to the laydown site is to deliver the mix without changing its properties and in a state that allows for appropriate placement and compaction. Minimizing heat loss and preventing seg­regation of the mixture constituents are two of the most important issues regarding transportation. Requirements for transporting the mixture to the work site are often defined in technical specifications. Typical requirements are shown, as examples, in the following:

• Specified maximum distance between the asphalt plant and laydown site (e. g., 40 km)

• Maximum travel time from the asphalt plant to the work site (e. g., 2 hours)

Specifying the distance between the asphalt plant and the construction site can be quite a misleading operation since traveling a distance of 40 km in a rural area does not take the same amount of time as driving the same distance in a large city. Driving time is the more accurate requirement; however, some differences may be noted even in that. Mix will not cool as much after 2 hours during the sum­mer as it will after the same amount of time in the late autumn. Probably the best way of setting the requirements for transportation is to establish temperature con­ditions for the mixture. This method of specifying requirements may be found in the European standards concerning bituminous mixtures (e. g., in EN 13108-5 [the European standard on SMA]). There are maximum production temperatures speci­fied for various types of bituminous binders and a minimum supply temperature for the delivered mixture. One should meet that temperature range when planning the production and transport of a mixture to a work site, taking into account such factors as the prevailing weather conditions, and the location of the asphalt plant. This is definitely the more common solution.

When transporting hot SMA, a crust of cooler mixture is formed on its surface. The higher the degree of cooling of the mixture, the thicker the crust, and the more problems it causes. As long as the mixture is well-protected thermally (by a tarp or insulation), the layer of cooler mixture will be thin and the chance to intermix it with the rest of the material during laydown will remain high. Significant cooling of the
mixture during transportation or while waiting for discharge leads to the build-up of a pretty thick crust. Large pieces of cool, unmixed material cause a nonuniform texture in a laid-down course (for details see Chapter 11). In that case, good mixing may be provided by a buffer like a material transfer vehicle (MTV) or Shuttle Buggy (Brown, 2002) (see Section 10.1.4).

There is an analysis concerning the heat losses of an asphalt mix during site transportation in the paper by Spuziak (2002). The analysis assumes that heat losses during mixture transportation may range from 5°C/hr (with good insula­tion) up to 48°C/hr (in exposed locations). The drop in the mixture temperature depends on the following:

• Weather conditions during transportation, including the air temperature, humidity, and wind velocity

• Air streamline speed (the speed of the truck loaded with mixture)

• Time of the mixture haulage

• Mass of the load

• Shape of the truck bed and its insulation properties

Concerning weather conditions, wind velocity is a decisive factor in the rate of cooling of the mixture during transportation. The mass, volume, and shape of the hauled material is also of great significance. Any reduction in the thickness of the transported layer of mixture induces quicker cooling, hence the shape of the truck bed is also of considerable importance. Taking this into account, beds with rounded or wedged edges are ideal (Figure 10.1) (Ulmgren, 2000, Spuziak, 2002).

In cool weather, the best way to retain the mixture temperature is to use an appro­priate means of transport. Trucks with insulated beds are the best solution, then the effect of the reduction of heat waste is perfectly obvious (Figures 10.2 and 10.3). In the case of such a truck being unavailable, tight and well-fitting tarps on trucks should be adopted as a minimum. Minimizing drops in the temperatures of SMA mixtures containing polymer modified binder (PMB) is particularly important.

FIGURE 10.1 Location of cooled spots of a mixture on a cross section of a truck bed (a) and (b). Truck bed shape prevents cold corners (c) and (d). (From Spuziak, W., Proceedings of the 6th International Conference Durable and Safe Road Pavements, Kielce, Poland, 2000. With permission.)

10.1.3 Discharging

The suitable discharge of a mixture from a truck to the paver hopper helps to avoid some problems. The dump truck with the mixture should pull in directly in front of the paver to allow for the following:

• The truck to be positioned exactly on its motion axis, enabling the push roll­ers on the paver to contact both rear wheels of the truck.

• The truck can avoid bumping into the paver; bumping the paver can cause a bump in the pavement.

As a general rule, the paver should approach the truck and not the other way round (then the paver can push the dump truck without bumping).

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.

Today almost all asphalt plants are computer controlled. It is worthwhile mentioning the necessity for caution during SMA production. This also applies to these modern asphalt plants. Usually such machines give personnel a feeling of confidence and a sense of complete control over the production process. However, many instances have proved that an excess of trust leads to problems (some of which are described in Chapter 11).

Particular attention should be paid to checking weight batchers, thermometers, and proportioning meters. All data produced by the computer system should be peri­odically checked. A good example of the type of problems that may occur is the clog­ging of batching devices for granulated stabilizers between the balance and the chute

Stabilizer

FIGURE 9.11 The SMA asphalt mixture with the stabilizer after extraction and before aggregate screening. (Photo courtesy of Krzysztof BlaZejowski.)

gate. The actual amount of stabilizer weighed out by the machine can be checked by disconnecting the chute pipe and measuring the amount of material batched during a production cycle (Figure 9.10). Another typical method is performing an extrac­tion and checking for the presence of the stabilizer in the mixture (Figure 9.11). Unfortunately this method does not allow for the determination of the exact amounts of fibers added to each batch.

Finally, a brief note about extraction. While performing an extraction of an SMA mixture, the rules for the correct preparation of the sample should be observed (hav­ing the stabilizer in mind as the mixture’s constituent—see Figure 9.11). The stabi­lizer fibers should be removed prior to starting the aggregate screening.

Issues concerning procedures for production of bituminous mixtures under EN 13108-21 and specified tolerances are discussed in Chapter 14.

9.5.2 Other Examples of Production Control

Some brief examples from around the world include

• In some countries, the accuracy of stabilizer dosage is ±10% of its mass as determined in a laboratory formula.

• If binder viscosity is the basis for establishing the mixing temperature of a binder and aggregate, some specifications may quote a permissible devia­tion from that temperature (e. g., ±10°C).

• In most countries, the permissible deviation of a mixture gradation and the content of binder depends on the number of tested samples.

• Some regulations provide clauses permitting a situation in which the grada­tion curve of a produced mix, considering given deviations, may deviate outside the area of the gradation envelope (between the so-called upper and lower limit curves).