Hot mix asphalt for agricultural applications

by John Davis

The goal of a successful stock or dairy farm is to maximize production under the most sanitary conditions at the least cost. Paving cattle pens, feed lots, silage platforms, and bunker and stack silos with hot mix asphalt (HMA) has proven to be a good way to achieve this goal—because HMA pavements are economical, durable and easily constructed.

Surfaces paved with HMA keep livestock out of the mud and muck, keeping the animals cleaner, reducing disease and improving hygiene. Farmers, as well, benefit from improved working conditions around paved silage bunkers. Cleaner equipment equates to less wear and tear and maintenance.

The floors of silage bunkers and feeding areas are commonly paved with either concrete or hot mix asphalt and the two materials react differently to their environment. Silage goes through a fermentation process, which creates acid and alcohol. These fluids can attack the concrete and cause it to deteriorate. Deteriorated concrete flakes can cause digestion problems for livestock when ingested, as well as contributing to silage waste and clean-up problems for farmers.

Unlike concrete, HMA is resistant to silage acids and animal wastes and does not readily deteriorate. The HMA surface stays smooth and structurally sound. And asphalt pavements are easy to clean—offering a practical solution to mud, dust and concrete deterioration.

HMA is a versatile product that can be used for many farm applications. It can be used for farm equipment yards, farm driveways, and equipment staging areas. HMA silage floors can be constructed in stages, with the final surface course being laid a year or more after the base course.

For farm applications, the HMA materials consist of a quality aggregate blend and a liquid asphalt binder that is appropriate for the local climate. The specific materials used and the thickness of each layer of HMA depend on the amount of heavy vehicle traffic and soil conditions.

The HMA should be purchased from a knowledgeable, local supplier. One way of getting a high quality HMA mix is to specify that it meet state highway department requirements for the area of installation. The Superpave method of materials selection can be applied to HMA for silos, bunkers and livestock feeding areas. For assistance in your geographic area, consult a local highway engineer, paving contractor or paving consultant.

Good practice requires paving the floor or feeding area with a relatively non-porous (low voids) mix. Crushed stone, gravel or slag is needed for the aggregate base course. These materials should be well-graded from coarse to fine with not more than 8 percent passing through the No. 200 sieve. Aggregates for the HMA surface course should be acid resistant. Siliceous aggregates such as granite, trap-rock and some gravels perform well.

Thickness Design
The pavement structure of a paved barnyard or feeding area typically consists of a prepared subgrade, an aggregate base, an HMA base, and an HMA surface. The aggregate base may be comprised of either bank-run gravel or crushed stone. The thickness of the aggregate base and HMA base depend on the strength of the subgrade and the anticipated vehicle use. When the aggregate base is placed over a heavy clay or plastic soil, some designers recommend placing a 3-inch insulation blanket of sand or fine aggregate between the subgrade and the aggregate base as a construction platform.

The following thicknesses are generally recommended:

For good soils—well drained gravelly or sandy soils—use a 2-inch HMA base course and a 2-inch HMA surface course. An aggregate base is not needed.

For fair to poor soils—average clay loams, plastic (swelling) base—use a 2-inch HMA base and a 2-inch HMA surface course. Also, use a 4-inch aggregate base.

For very poor soils—heavy clay, plastic when wet—use a 4-inch HMA base course and 2-inch HMA surface course. Also, use a 6- to 8-inch aggregate base.

Paving and Compaction
Before beginning the work, choose a contractor who has experience paving barnyards, bunkers and feed areas so the quality of the work can be observed. If a specialty contractor is not available, then choose a road or street contractor with a reputation for doing quality work.

The equipment for barnyard paving is usually the same as that used for road construction, and most asphalt contractors will be able to do this type of work.

The aggregate base should be compacted until it is stable. Then the HMA base and surface courses are placed. If the surface course is not placed immediately after constructing the HMA base, the existing course must be cleaned and tacked. Each layer should be no more than 4 inches thick after compaction. An 8-ton or heavier roller should be used to compact the HMA layers.

To prevent tracking after compaction, a sand cover of two to five pounds per square yard may be applied.

It is a good practice to provide for surface water drainage of all paved areas. A slope of at least 1.5 to 2 inches in 10 feet is recommended for surface drainage. The contractor can obtain proper drainage by sloping the pavement toward the center or placing a crown in the center with the pavement sloping to the outer edges. For poor to very poor soils, underdrains may be needed.

Resurfacing Concrete Silo Floors with HMA
Resurfacing a concrete floor with HMA is slightly different from building a new floor. As with a new floor, preparation is important. The old surface must be swept or power-brushed and be completely free of silage debris and old aggregate. Then, spray a tack coat onto the old concrete floor to improve the HMA’s adhesion to the concrete.

Before paving begins, clean the surface of the area to be paved with a high pressure wash or steam-cleaning equipment to remove all dirt. If the surface of the feeding area is excessively cracked, the entire floor must be removed, or proof-rolled to identify any failed base areas. If the floor has subsided in a few local areas, break out and replace the concrete. Cut out any shrinkage cracks in the existing silo floor and repair them.

All joints and cracks in the existing floor must be cleaned, enlarged where necessary, and filled with hot asphalt binder or suitable asphalt sealer.

Three inches is a common thickness for resurfacing a badly worn concrete silo floor. Typically the HMA is placed in two lifts, using both HMA base and surface. Concrete and HMA expand and contract at different rates, so any resurfacing job must be a complete edge-to-edge job, and not just patchwork.

Successful Applications
Many Michigan farmers are using HMA for paved areas on their farms. On Charles Pinkerton’s Chamabi Ranch, located near Prescott, Michigan, the floors of the bunker feed stalls and cattle barn are paved with HMA. A 4-inch HMA base was placed the first year, then a 1.5-inch surface course the next.

Another example of a farm successfully using HMA is the C. V. Wilder ranch in Bellington, Washington. On his 980-acre stock ranch, HMA was used to pave a large feed lot and loading area. Wilder also used HMA floors for a large barn, a calf feeding shed, a horse barn, and an equipment shed. The thickness of the HMA pavement on the Wilder ranch is 2 inches on a 6-inch gravel base.

For Wilder’s ranch, paving the floors of the calf shed, barn and equipment shed before the buildings were erected made the construction of each easier. Without walls or posts to interfere, the floors were paved with a minimum of cost and time.

HMA paved areas allow both efficiency and economy. By selecting appropriate materials and following recommended design and construction practices, HMA farm installations will provide many years of service.

John Davis is a contributing writer for Asphalt Magazine.