How well the liquid asphalt works (its physical properties) depends upon the chemistry of the crude oil source and the method used to refine it, as well as any additives used. How well the aggregate material works (its physical properties) depends upon its chemistry, size and shape. How well the asphalt mixture used on your paving project works (its physical properties) depends upon the relative proportions of the components as well as the physical properties of each of the components.
The strength and quality of the pavement depend on 1) the physical properties of the mix, 2) the design of the pavement structure, and 3) the quality of the construction.
Using materials with inferior physical properties (not enough strength, stickiness or elasticity) results in poor quality pavements (rutting, cracking, potholes). A pavement structure design that doesn’t allow water to quickly get away or that isn’t strong enough to support the traffic results in poor quality pavements (rutting, cracking, potholes). And poor quality construction…..well, you get the idea.
To provide the best quality pavements at the lowest costs, engineers have identified the most important physical properties, and they have developed standard lab and field test methods and sampling procedures to make sure they will give the expected performance. These engineers have set upper and/or lower limits for those test results, and these limits are the specifications.
There are several national organizations that develop specifications. States, counties and cities usually adopt these specifications, but may adapt them with some changes based on the particular local conditions. The two most common groups who write standard specifications for asphalt are:
- ASTM (American Society for Testing and Materials), made up of users (government engineers) and producers (suppliers)
- AASHTO (American Association of State Highway and Transportation Officials), made up of representatives from 50 states as well as the Federal Highway Administration
Hint: For your paving project to be successful, the materials, structural design and construction should meet standard specifications.
- National Highway Specifications
This FHWA site is an electronic library of the most current specifications, construction manuals, and drawings submitted by State Departments of Transportation and other agencies.
- Federal Aviation Authority (FAA) specifications
The Mix Design
Hint: This might sound very complicated for your asphalt paving project, but most suppliers have developed a good mix design that they use over and over again. Most mix designs are certified by mix designers who have passed rigorous tests and are state DOT certified. Ask your asphalt pavement contractor for a mix that meets your needs.
Aggregates and asphalt can vary greatly. The asphalt mix manufactured for your paving project will follow a good job mix formula (JMF). The JMF is determined by a mix design process that has been standardized by those agencies responsible for specifications. The steps in a mix design are:
- Select the materials
Coarse and fine aggregates and liquid asphalt.
- Test the aggregates
For size, durability, angularity, and clay content.
- Select a good aggregate structure
By finding a good blend of sizes (gradation). The coarse and fine aggregates should fit together with a strong stone-on-stone structure, but there should be enough empty spaces for asphalt (binder and mastic) and air voids.
- Pick the percent of liquid asphalt (binder content) and air voids
First we choose the appropriate PG binder for the project. The volumetrics include such things as air voids, voids in mineral aggregate (VMA), and voids filled with asphalt (VFA). Some aggregates are more porous than others and therefore require more asphalt binder to fill those pores. Based on the volumetrics, the lab technicians blend the job aggregate and asphalt at several different percentages and then compact and test the blends using standard test methods (see Specifications). The test results are used to determine the best job mix formula to meet the specified mix volumetrics, typically 4% for dense graded mixes. Mixes designed to drain away surface water called porous mixes are designed to have 16% to 20% air voids. Too much liquid asphalt, and the HMA might rut because the rocks swim or float in the liquid asphalt when it gets hot. Too little asphalt, and the HMA might crack, because there isn’t enough binder to hold the rocks together. Technicians also determine the percentage of air voids that gives the most strength. If the air voids are less than about 3%, we get that “swimming in asphalt” unstable mix. If the air voids are above about 8%, the air voids become interconnected, which lets water into the mix. If water penetrates through the mix, the pavement is no longer waterproofed and long term durability and strength is compromised.
- Check for adhesion, cohesion and sensitivity to water
The lab technicians blend the proposed job mix formula and compact samples (specimens). They test the samples for water sensitivity by submerging the samples in water for a specified time at a specified temperature, and then they retest the samples for retained strength. This test is comparing strength of unconditioned mixtures versus samples that are conditioned under water. The asphalt should be adhesive enough not to “strip” from the aggregate and to give a good retained strength. If mix design testing indicates the mix may have a moisture problem, other tests are conducted to determine the proper additives such as liquid anti-strips (to the asphalt binder) or hydrated lime (to the aggregate).