Rocks.  But, there are many types of rocks.  There are hard, durable rocks like granite.  There are soft rocks like sandstone.  There are rounded rocks like river gravel.  The rocks after processing, typically crushing and screening are called aggregates, and all of them can be used in asphalt.  The important aggregate qualities for your asphalt paving project are durability and angularity (fractured faces).  The aggregates shouldn’t crack or break under traffic loading or with freeze thaw cycles.  Angular aggregates(determined by the number of crushed faces) give the best and strongest structure (think stacking square blocks vs trying to stack round balls) and the best friction numbers and skid resistance.

For asphalt pavements, different size rocks are crushed and aggregates are blended into graduated sizes to provide the strongest, best packing structure while allowing enough extra space for the asphalt binder and some “breathing” space (air voids).  To get the strongest pavement structure, larger aggregates are used for the base, with successively smaller dating sites reviews aggregates used for upper layers in the pavement.  The hardest and most angular are used for the surface.

Fines.  Fines are the smallest sized aggregates, and they may be rounded or crushed angular particles.  When mixed with liquid asphalt, they form a mixture (mastic) which coats the aggregate.  This mastic will help improve the durability of your asphalt paving project, and will be more resistant to oxidative aging.  Bits of dirt, metals, glass and such can harm the performance of the asphalt pavement.  The most common contaminant (deleterious material) is clay.  Clay particles absorb moisture and work like talcum powder in preventing permanent bonding between the asphalt and aggregate.

First of all, it’s not “tar.”  Asphalt is a natural substance that has some amazing physical properties.  It’s sticky (adhesive) and it’s elastic, able to stretch, bend and flex without breaking (cohesive).  This material does an excellent job of waterproofing.  At air temperatures, asphalt cement is a very, very thick liquid (highly viscous).  When heated, it becomes thinner and easier to use.  Asphalt has been used since before Roman times as a glue and for water proofing.  In a few places in the world, it’s naturally occurring, such as in a lake on the island of Trinidad and in the LaBrea “tar pits” in downtown Los Angeles.  Almost all of the asphalt used today for paving comes from petroleum crude oil.  Liquid asphalt is the heaviest part of the crude—what’s left after all the volatile, light fractions are distilled off for products such as gasoline.  In Europe and Canada it is commonly called bitumen.

Asphalt is supplied in several different grades.  Generally softer asphalts are used in colder temperatures and harder asphalts in hotter climates.  The US government sponsored a multi-million dollar research project (Strategic Highway Research Program, SHRP) in the 1990’s which developed new standards for asphalt binders called performance grade (PG) binders.   For example, a PG 64-22 is meant for use where average surface (high) pavement temperatures in the sun reach 64°C (147°F) and lows reach -22°C (-8°F).  Premium grades usually have polymers or other modifiers for use in heavy duty applications such as intersections on city streets or airports or in extreme climates.

At normal temperatures, asphalt is too stiff to mix with the aggregates.  There are three ways to thin it enough to make it mixable—heat it, dilute it with a solvent (cutback) or emulsify it in water.

By the way, “tar” (or coal tar) is a product made from coal

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.

Related Links

• 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:

1. Select the materials
   Coarse and fine aggregates and liquid asphalt.
2. Test the aggregates
   For size, durability, angularity, and clay content.
3. 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.
4. 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.
5. 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).

Manufacturing Asphalt Mixes (Mixing)

Like any good recipe, the ingredients should be high quality, they should be carefully measured, and they should be cooked at exactly the right temperatures.  At the asphalt plant, the aggregates are precisely blended into a gradation and then dried and heated to the right temperature.  The mixture is then completely coated with liquid asphalt cement.  There are two types of asphalt plants, batch and drum.  Drum mix plants dry the aggregate and blend it with asphalt in a continuous process in the same piece of equipment.  With batch plants, the rocks are first dried and heated, then added to the pugmill and blended with the asphalt one batch at a time.

Transporting and Placing Asphalt Mixtures

Once mixed, the asphalt can be loaded into trucks and transported to the job site.  It is then placed on your asphalt paving project using a paver.  Once the paver lays the asphalt mix, it is compacted with rollers until it has just the right number of air voids, as determined in the mix design or recipe.

Hint:  The temperatures are very important.  The best temperatures for mixing, transporting, laying and compacting your asphalt pavement are determined based on how stiff (viscous) the asphalt is, how far and long it must be transported to your project site, and when it will be compacted.  If it is compacted too hot, the air voids will be too low and the mat will “shove” and rut.  If it is too cold, it will be difficult to compact and the air voids will be too high.

Related Information

Each state has an LTAP (local technical assistance program) center to help you with your asphalt paving project.  They have a wealth of printed, video and on-line materials as well as on-line discussion groups to give you the help you need for your asphalt paving project.

The hottest topic in asphalt pavements these days is warm—Warm Mix Asphalt (WMA), that is.  Introduced in Europe in 1997 and in the US in 2002, WMA is rapidly gaining acceptance.  WMA is a green product that lowers manufacture and lay down temperatures by 30° to 100°F.  By reducing the temperatures, WMA doesn’t need as much energy to produce.  It’s been documented to lower energy costs by 30-60%.  WMA reduces greenhouse gas and hydrocarbon emissions from the manufacture and construction of asphalt pavements.  And the lower temperatures make it safer for workers in the plant and at your asphalt paving project job site.  WMA has been especially successful in areas where air quality has been a problem (nonattainment areas).

WMA is manufactured, constructed and used with only minor adjustments.  European and American companies have developed several technologies that all increase workability at the lower temperatures.  There are chemical additives to improve workability and mechanical techniques to foam the asphalt binder.  Special equipment is attached to the asphalt plant to create the foaming process, which coats the aggregates at lower temps. Some of the additives can be easily pre-blended with the liquid asphalt or added at the asphalt mix plant. These additives will be specified in the job mix formula (JMF).

WMA Advantages

• The lower warm mix asphalt production temperatures reduce emissions of volatile organic compounds (VOC) at mix plants.
• Warm mix asphalt reduces fume exposure for workers at the job site.
• The lower construction temperatures reduce burn hazards.
• WMA is more easily compacted, which is beneficial during extreme weather conditions, for stiff mixes and mixes with reclaimed asphalt pavement (RAP), and for reducing the amount of roller passes.
• Warm  mix asphalt production and construction conserves fuel by requiring significantly less energy.
• The lower temperatures and longer workability allow longer transport, enabling construction at more remote locations as well as more flexibility in transportation schedules.
• Almost all of the WMA products and processes have been designed to use existing mix design technology, existing asphalt plants and existing transportation and construction equipment.
• Warm mix asphalt may be (and has been) used for base, binder and wearing courses.
• Because the pavement is constructed at lower temperatures, it may be opened to traffic sooner, which reduces user-delay costs.
• The lower temperatures mean that the asphalt binder is age hardened less during the mixing process, providing longer pavement service life.
• Some WMA additives can be pre-blended with the liquid asphalt at the refinery.
• The additives improve workability and constructability and may the improve moisture resistance of the mix.

WMA Disadvantages

• WMA may be more expensive than traditional asphalt mixes.
• Lower temperatures used for WMA can result in incomplete drying of the aggregates.  The resulting trapped water in the coated aggregates may cause moisture damage.  Care must be taken to monitor this.
• Performing handwork and luting with some WMA technologies can be difficult at the lower temperatures.
• WMA mixtures are still being reviewed for long term performance.  To date no known pavement failures or concerns have been observed.

For more information please visit National Clearing House for Warm Mix Asphalt.

• Asphalt pavements are the most recycled materials (80 million tons annually) in the United States—more than glass, metal, tires, paper or any other material.  We crush and recycle the reclaimed asphalt pavement (RAP) right back into our mixes every day, making asphalt pavements a truly sustainable product.
• Asphalt pavement recycling not only conserves our precious natural resources and speeds construction, it also saves American taxpayers over $2.6 billion a year.
• The Phoenicians caulked the seams of their merchant ships with liquid asphalt, the best waterproofing material available.
• A Federal Highway Administration (FHWA) study concluded that each $1 billion of federal spending on highway construction generates 28,000 jobs.
• The infant Moses’ basket was waterproofed with liquid asphalt.
• The FHWA estimates that each $1 spent on road, highway and bridge improvements results in a payback of $5.20 in reduced vehicle, road and bridge maintenance costs, reduced delays, reduced fuel consumption, improved safety and reduced emissions.
• Engineers estimate that a single vehicle stirs up 4½ pounds of dust into the air when traveling at 30 mph for every mile traveled on an unpaved road.
• Liquid asphalt was used by ancient Egyptians to embalm mummies, as they recognized the lasting qualities of asphalt.
• Asphalt pavement is approved by the Environmental Protection Agency and used as liners for both sanitary and hazardous waste landfills.
• Environmentally friendly asphalt is used as a mulching aid, it helps in the germination of seeds and to prevent soil erosion.
• The United States has 2.6 million miles of paved roads, and about 94% of them are surfaced with asphalt pavement.  Asphalt pavements are, and continue to be, America’s pavement of choice.