ASPHALT AND ASPHALT PAVING MATERIALS - 2.1

ASPHALT DEFINED

  

The black cementing agent known as asphalt has been used for road construction for centuries. Although there are natural deposits of asphalt, or rock asphalt, most used today is produced by the oil refining industry. Asphalt is a constituent of most petroleum and is isolated through the refining process of distillation. (See Figure 2.1)

Asphalt is called a bituminous material because it contains bitumen, a hydrocarbon material soluble in carbon disulphate. The tar obtained from the destructive distillation of Soft coal also contains bitumen. Both petroleum asphalt and coal tar are referred to as bituminous materials. However, because their properties differ greatly, petroleum asphalt should not be confused with coal tar. Whereas petroleum asphalt is composed almost entirely of bitumen, the bitumen content in coal tar is relatively low. The two materials should be treated as separate entities.

One of the characteristics and advantages of asphalt as an engineering construction and maintenance material is its great versatility. Although a semi-solid at ordinary temperatures, asphalt may be liquefied by applying heat, dissolving it in solvents, or emulsifying it. Asphalt is strong cement that is readily adhesive and highly waterproof and durable, making it particularly useful in road building. It is also highly resistive to the actions of most acids, alkalis, and salts. 

Covering more than 90 percent of the nations paved highways, Asphalt Concrete is the most widely used paving material in the world. For versatility, durability, and ease of construction, it has no equal.

AGGREGATES

Aggregates (or mineral aggregates) are hard, inert materials such as sand, gravel, crushed stone, slag, or rock dust. Properly selected and graded aggregates are mixed with the cementing medium asphalt to form pavements. Aggregates are the principal load-supporting components of an Asphalt Concrete pavement. They total 90 to 95 % of the mixture by weight and 75 to 85 Percent by volume.

Classifications

Asphalt Concrete paving aggregates are classified according to source or means of preparation. A brief description of the classifications follows.

Pit or Bank-Run Aggregates

Both gravel and sand are typically pit or bank-run natural aggregates. They usually are screened to proper size and washed to remove dirt before being used for Asphalt Concrete paving purposes.

Processed Aggregates

When natural pit or bank-run aggregate has been crushed and screened to make it suitable for Asphalt Concrete pavements, it is considered a processed aggregate. Crushing typically improves the particle shape by making the rounded particles more angular. Crushing also improves the size distribution and range.

Crushed stone is also a processed aggregate. It is created when the fragments of bedrock and large stones are crushed so that all particle faces are fractured. Variation in size of particles is achieved by screening. Aggregates that have received little or no screening are known as crusher run. These aggregates are generally more economical than screened aggregates and can be used in Asphalt Concrete pavements in many instances.

In the processing of crushed limestone, the rock dust produced is separated from the other crushed aggregate and may be used as crushed sand or as mineral filler in Asphalt Concrete pavements.

Synthetic Aggregates

Aggregates produced by altering both physical and chemical properties of a parent material are called synthetic or artificial aggregates. Some are produced and processed specifically for use as aggregates; others are the by-product of manufacturing and a final burning process. Blast furnace slag is an example of a synthetic aggregate.

Desirable Properties of Aggregates

Selection of an aggregate material for use in an Asphalt Concrete pavement depends on the availability, cost, and quality of the material, as well as the type of construction for which it is intended. To determine if an aggregate material is suitable for use in asphalt construction, evaluate it in terms of the following properties:

1. Size and grading. The maximum size of an aggregate is the smallest sieve through which 100 percent of the material will pass. How the Asphalt Concrete is to be used determines not only the maximum aggregate size, but also the desired gradation (distribution of sizes smaller than the maximum)

2. Cleanliness. Foreign or deleterious substances make some materials unsuitable for paving mixtures.

3. Toughness. Toughness or hardness is the ability of the aggregate to resist crushing or disintegration during mixing, placing, and compacting; or under traffic loading.

4. Soundness. Although similar to toughness, soundness is the aggregate’s ability to resist deterioration caused by natural elements such as the weather.

5. Particle shape. The shapes of aggregate particles influence the asphalt mixture’s overall strength and workability as well as the density achieved during compaction. When compacted, irregular particles such as crushed stone tend to “lock” together and resist displacement.

6. Surface texture. Workability and pavement strength are influenced by surface texture. A rough, sandpapery texture results in a higher strength than a smooth texture. Although smooth-faced aggregates are easy to coat with an asphalt film, they are generally not as good as rough surfaces. It is harder for the asphalt to 'grip' the smooth surface.

7. Absorption. The porosity of an aggregate permits the aggregate to absorb asphalt and form a bond between the particle and the asphalt. A degree of porosity is desired, but aggregates that are highly absorbant are generally not used.

8. Stripping. When the asphalt film separates from the aggregate because of the action of water, it is called stripping. Aggregates coated with too much dust also can cause poor bonding which results in stripping. Aggregates readily susceptible to stripping action usually are not suitable for asphalt paving mixes unless an anti-stripping agent is used.