Mix transport involves all actions and equipment required to convey HMA from a production facility to a paving site including truck loading, weighing and ticketing, hauling to the paving site, dumping of the mix into the paver or material transfer vehicle hopper, and truck return to the HMA production facility (Roberts et al., 1996). Ideally, the goal of mix transport should be to maintain mix characteristics between the production facility and the paving site. Transport practices can have a profound effect on mix temperature at the paving site, aggregate and/or temperature segregation of the mix and mat quality. This section will discuss the types of trucks used for mix transport and the various considerations involved with mix transport.
There are three basic truck types used for mix transport classified by their respective HMA discharge methods:
- End dump
- Bottom dump (or belly dump)
- Live bottom (or flo-boy)
Each truck type is capable of adequately delivering HMA from a production facility to a paving site. However, certain situations such as the ones listed in Table 1 below, may make one truck type advantageous over another.
Table 1. Truck Type Situations
|Situation||Possible Truck Type||Reason|
|Paving on congested city streets||End Dump||Better maneuverability because it has no trailer and is smaller than a bottom dump or live bottom truck.|
|Paving using a mix highly vulnerable to segregation||Live Bottom||Live bottom trucks deliver the HMA by conveyor, which minimizes segregation.|
|Paving on rural highways||Bottom Dump||Usually has a larger capacity than end dump trucks (therefore fewer trucks are needed) but requires space and equipment for windrows.|
There are several mix transport considerations, or best practices, that are essential to maintaining HMA characteristics between the production facility and the paving site. These considerations can generally be placed into four categories:
- Loading at the production facility
- Transport within the truck
- Unloading at the paving site
- Operation synchronization
Loading at the Production Facility
Loading at the production facility involves transferring HMA from the storage silo or batcher (for batch plants) to the transport truck. There are two potential issues with this transfer:
- Truck bed cleanliness and lubrication. Truck beds should be clean and lubricated to prevent the introduction of foreign substances into the HMA and to prevent the HMA from sticking to the truck bed. Non-petroleum based products should be used for lubrication such as lime water, soapy water or other suitable commercial products (Roberts et al., 1996). Petroleum based products, such as diesel fuel, should not be used because of environmental issues and because they tend to break down the asphalt binder.
- Aggregate segregation. HMA should be discharged into the truck bed so as to minimize segregation. Dropping HMA from the storage silo or batcher (for batch plants) in one large mass creates a single pile of HMA in the truck bed (see Figure 1 and Video 1). Large-sized aggregate tends to roll off this pile and collect around the base. Dropping HMA in several smaller masses (three is typical) at different points in the truck bed will largely prevent the collection of large aggregate in one area and thus minimize aggregate segregation.
Truck transport affects HMA characteristics through cooling. HMA is usually loaded into a truck at a fairly uniform temperature between 250°F to 350°F (see Figure 2). During transport, heat is transferred to the surrounding environment by convection and radiation and the HMA surface temperature drops. This cooler HMA surface insulates the interior mass and thus transported HMA tends to develop a cool thin crust on the surface that surrounds a much hotter core (see Figures 3 and 4 and Video 2). Things such as air temperature, rain, wind and length of haul can affect the characteristics and temperature of this crust. Several measures that can be taken to minimize HMA cooling during transport are:
- Minimize haul distance. This can be accomplished by choosing an HMA production facility as close as possible to the paving site. Closer production facilities create shorter haul times and result in less HMA cooling during transport. Unfortunately, many paving locations may not be near any existing production facilities and economics may prohibit the use of a mobile production facility.
- Insulate truck beds. This can decrease HMA heat loss during transport. Insulation as simple as a sheet of plywood has been used.
- Place a tarpaulin over the truck bed. A tarp over the truck bed (see Figure 5) provides additional insulation, protects the HMA from rain and decreases heat loss. A study by the Quality Improvement Committee of the National Asphalt Pavement Association (NAPA) studied truck tarping and found that the HMA surface temperatures of tarped loads dropped more slowly than untarped loads but temperatures 100 mm (4 inches) below the surface between tarped and untarped loads were not significantly different (Minor, 1980).
In most cases, truck transport appears to cool only the surface of the transported HMA mass, however this cool surface crust can have detrimental effects on overall mat quality if not properly dealt with. Actions such as reducing transport time, insulating truck beds or tarping trucks can decrease HMA surface cooling rate. Additionally, since the majority of the HMA mass is still at or near its original temperature at loading, mixing the crust and interior mass together at the paving site (“remixing”) will produce a uniform mix near the original temperature at loading.
Unloading at the Paving Site
HMA unloading involves those procedures discussed in End Dump Truck as well as a few other basic considerations such as:
- HMA should be unloaded quickly when it arrives at the paving site. This will minimize mix cooling before it is placed.
- Before HMA is loaded into the paver, the inspector and/or foreman should be certain it is the correct mix. Occasionally, paving jobs require several different mix designs (i.e., one for the leveling course and one for the wearing course) and these mixes should not be interchanged.
Ideally, HMA production at the plant, truck transport and placement at the paving machine should be synchronized to the same rate to minimize accumulation of excess HMA in any one of the three segments. Realistically, however, this synchronization can be quite difficult because of varying laydown rates, unpredictable truck travel times and variable plant production. Detailed information on operation synchronization can be found in:
- National Asphalt Pavement Association (NAPA). (1996). Balancing Production Rates in Hot Mix Asphalt Operations, IS 120. National Asphalt Pavement Association. Landham, MD.
Ideally, all operations are designed to meet optimal mat laydown rates. However, these rates can vary based on paving width and lift thickness. Also, complicated paving locations such as intersections or near manholes and utility vaults can temporarily increase or decrease the laydown rate.
Truck transport should be planned such that the HMA transport rate (expressed in tons/hr) closely matches plant production rate and laydown rate. Some factors to consider are:
- Number of trucks to be used.
- Truck type.
- Average truck hauling capacity.
- Production facility output rate.
- Availability and condition of storage silos at the production facility.
- Time to lubricate the truck bed before transport.
- Waiting time at the production facility.
- Loading, weighing and ticketing time at the production facility.
- Time to cover the load (when tarpaulins are used).
- Distance between the production facility and the paving site.
- Average truck speed.
Traffic plays a large role in HMA delivery rates because it affects truck speed. Especially in congested urban areas, heavy and/or unpredictable traffic may substantially increase, or at least vary, truck travel time. As truck travel time increases, more trucks are needed to provide a given HMA delivery rate. Therefore, as traffic gets worse, trucking costs increase. Additionally, the unpredictability of traffic may result in either long paver idle times as it waits for the next truckload of HMA or large truck backups as several trucks all reach the paving site or production facility at the same time.
Finally, production facility output is typically controlled to match haul or laydown rate. However, this can result in suboptimal plant efficiency or HMA uniformity, which may increase plant exhaust output, shorten emission control device lifetimes, and affect contractual payment if payment is tied to HMA uniformity. It may often be more economical to run the production facility at maximum rate and store excess material in storage silos for discharge into trucks as they arrive. Storage silo insulation has progressed to a state where dense-graded HMA can be stored in them for up to a week at a time without significantly affecting HMA characteristics. However, gap graded mixes such as SMA or OGFC should still not be stored for more than about 2 to 3 hours.
In sum, synchronization should be the goal but it is often difficult to achieve (based on varying laydown rates, haul time and traffic) and may result in plant inefficiency and HMA quality degradation. If a production facility has modern well-insulated, airtight storage silos and is producing a dense-graded HMA, it may be beneficial to run the plant at maximum production rate and store the mix until needed rather than try and match haul or laydown rate.
- Roberts, F.L., Kandhal, P.S., Brown, E.R., Lee, D.Y., and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. National Asphalt Paving Association Education Foundation. Lanham, MD. ↵
- Minor, C.E. (1980). Are Hot-Mix Tarps Effective? Information Series 77. National Asphalt Pavement Association. Landham, MD.↵