Production, Transport and Placement of Asphalt Mixes
1.1 Description — The Asphalt Mixing Plant
1.1.1 Process Control Mandated by Quality Specifications and Environmental Protection
Today’s asphalt plant can be characterized as a modern facility belonging to a sophisticated process industry where emissions are low and well-controlled. Typically, it takes only three to five people to run an asphalt mixing plant. In every country, the asphalt industry must comply with stringent regulations and specifications with respect to materials used, process conditions, and pavement specifications. These regulations and specifications are designed to protect the environment as well as to ensure the quality, durability, smoothness, and safety of the roads.
1.1.2 The Asphalt Mixing Process
There are two types of asphalt plants: batch plants and drum plants. In both, the mineral aggregates are dried and heated in a rotating drum. In batch plants, aggregates are stored in hot bins prior to mixing with bitumen in discrete batches before being stored or loaded into trucks. In drum plants, the mixing of the aggregate and the bitumen takes place in the same drum, after which it is stored in a silo before being loaded into trucks for delivery. Today the predominant plant type in the U.S. and New Zealand is the drum-mix plant. Batch plants prevail in Europe, South Africa, and Australia.
The following diagrams (Figures 1.1a, 1.1b) show the batch plant design and the drum-mix plant design.
Process sketches and flow diagrams along with process description follow.
Various asphalt mix formulas are used for the various types of pavement materials. These formulas are engineered to meet the needs of the owner of the pavement. In the case of major roads, highways, and airport runways, the owners are typically governmental entities. In the case of parking areas, low-volume roads, and other facilities, many owners are from the private commercial market, but they often use specifications from government agencies.
Bitumen is stored in heated tanks on site between 150°C (302°F) and 180°C (356°F), which enables the viscous liquid to be pumped through insulated pipes to the mixing plant. The mineral aggregates – stone, sand, and gravel – are stored in stockpiles at ambient temperature. In addition to virgin aggregates, most facilities have stockpiles of reclaimed asphalt pavement (RAP). The aggregate stockpiles are neatly sorted by type and size.
Aggregate and reclaimed materials are taken from various stockpiles and loaded into specific bins. Each size of aggregate and reclaimed asphalt material is fed onto conveyor belts in proportions specified by the job mix formula and transported to be dried in a drum. At a batch plant, the aggregates are dried and heated in a rotating drum, where the aggregates tumble through a stream of hot air. After drying, the aggregates and any fillers are then mixed in batches with the exact proportions of bitumen and possibly RAP in a second machine called a pug mill.
In contrast, at a drum mix plant, the bitumen is added to the dried aggregates and continuously mixed in the same drum used for drying. Here, the RAP and bitumen are added to aggregate far downstream from the source of heat.
Every part of the plant has enclosures and/or control technologies. Most plants are fuelled by natural gas or fuel oil, and state-of-the-art scrubbers keep combustion related emissions very low.
Dust is controlled in the baghouse, where fines and dust are collected on the outside of filter bags, while clean air passes through the center of the bags. The fines are periodically subjected to bursts of air which force them to the floor of the baghouse, where they are collected for metering back into the paving mix. Clean air is vented out the top.
Most plants are on permanent sites, but even portable mixing plants have the advanced environmental controls that are seen on plants on permanent sites.
After the aggregates have been dried and thoroughly mixed with the bitumen at the required temperature, the asphalt pavement material may either be temporarily stored in silos on the plant site or discharged directly into a truck for transport to the paving site. The asphalt mix is transported from the plant site to the paving site in trucks. Transport distances vary, but are normally on the order of up to 30-80 km (18-50 mi). The distance of transport is limited, as asphalt must be delivered to the paving site while it is still warm enough to be placed and compacted on the road.
In the beginning of the 20th century, hot asphalt mixtures were spread manually by hand and shovel. Later, asphalt paving machines (mechanical spreaders) were introduced. Beginning in the late 1930s, these paving machines were provided with floating screeds for better leveling and pre-compaction of the asphalt paving mixture The earliest ones were mechanical; they were followed by hydraulic, and later electronic, leveling controls and vibratory screeds.
Today, paving machines incorporate the latest technology. Trucks discharge the hot asphalt mix into a hopper on the paving machine. The material then is conveyed through the paving machine where it is spread across the width of the machine by an auger at the rear of the machine. As the auger distributes the material along the screed, the paver continues to move forward, so that asphalt mix cools throughout this process and must be quickly compacted by a roller to the required pavement density and smoothness by one or more rollers following the paving machine. A paving crew typically consists of one or two paver operators, one or two screed operators, and two or three laborers with rakes and lutes. Each roller has its own operator.