PREPARING ASPHALT FOR CONSTRUCTION OPERATIONS

Paving grade asphalt (asphalt cement), which at normal atmospheric temperatures is semisolid and highly viscous, must be made temporarily fluid (liquefied) for handling during construction operations, such as pumping through pipes, transporting in tanks, spraying through nozzles, and mixing with aggregate. When pavement construction operations are finished, the asphalt cement reverts to its normal condition and functions as the cementing (or binding) and waterproofing agent that makes the pavement stable and durable.

Asphalt cement can be made temporarily fluid (liquefied) for construction operations in three ways:

1. By heating the asphalt. After construction operations, the hot liquid asphalt cement cools and changes from a fluid to its normal, semisolid condition.

2. By dissolving the asphalt in selected petroleum solvents. This process is called cutting back; the diluted asphalt is called cutback asphalt. After construction the solvent evaporates, leaving the asphalt cement in place.

NOTE: The use of cutback asphalt in the United States has declined because of the petroleum shortage and government environmental regulations. It is being superseded by emulsified asphalt, which contains little or no solvent, and can be used for almost any purpose that cutbacks can.

3. By emulsifying the asphalt with an emulsifying agent and water. While asphalt and water ordinarily do not mix, they can be made to mix by churning asphalt in a colloid mill. The resulting product, called emulsified asphalt, is a fluid and is ready for construction operations. During construction the water and asphalt separate. The asphalt particles merge into a continuous film that cements the aggregate particles together as the water evaporates. When the water and asphalt separate, it is said that the emulsion breaks or sets.

A hot-mixed or hot-load paving mixture is the best type of pavement; the aggregate and binder should be heated to approximately 310F and laid no colder than 250F. Determining the exact temperature(s) to use will depend upon the weather and the distance that the material is hauled. Some clues that indicate the condition of the asphalt are as follows: . Overheated asphalt loses some of its binding qualities. Blue smoke, rising from the spreader hopper, is sometimes an indicator this condition exists.

. A generally stiff appearance and improper coating of aggregate indicates the mix is too cold. l Material laying flat in the bed of the truck with a shiny appearance means the mix is too rich in asphalt cement.

. When it is too lean, the mix will look dry and dull.

Prime Coat

Priming consists of the initial treatment on a granular base before surfacing with a bituminous material or pavement. The purpose of a prime coat is to penetrate the base (about 1/4-inch minimum penetration is desired), fill most of the voids, promote adhesion between the base and the bituminous applications placed on top of it, and waterproof the base. Surfaces must be as clean as possible, and where and conditions exist (dried-out surfaces), a light fog spray with water should be considered before priming actually begins.

The priming material may be either a lowviscosity tar, a low-viscosity asphalt, or a diluted asphalt emulsion. The bituminous materials, used for the prime coat, should be applied in quantities known as rate of application (ROA) of not less than 0.2 gallon or more than 0.5 gallon per square yard. Normally, the construction project specifications denote the ROA for the prime coat application; however, when the ROA is not included in the project specifications, the NCF uses an ROA of .3 for planning purposes. When the base absorbs all of the prime material within 1 to 3 hours or when penetration is too shallow, the base is underpriced. Underpriming may be corrected by applying a second coating of the prime material.

An overprimed base may fail to cure or set and may contribute to failure of the pavement or bleed up through the asphalt mat. A free film of prime material remaining on the base after a 45-hour curing period indicates that the base is overpriced. This condition may be corrected by spreading a light, uniform layer of clean, dry sand over the prime coat to absorb the excess material. Application of the sand is usually followed by light rolling and brooming. Excess prime, held in minor depressions, should be corrected by an application of _{clean, dry sand.} Any loose sand should be lightly broomed from the primed surface before the wearing surface is laid.

The primed base should be adequately cured before the wearing surface is laid. In general, a minimum of 48 hours should be allowed for complete curing. Ordinarily, proper surface condition is indicated by a slight change in the shiny black appearance to a slightly brown color.

When a soil base is to be covered by a bituminous wearing surface, the area should be barricaded to prevent traffic from carrying dust or mud onto the surface both before and after priming. If it is necessary to open the primed base course to traffic before it has completely cured, a fine sand may be used; when you are ready to place the wearing surface, lightly broom the sand from the primed base course.

To estimate the amount of bitumen required for the prime coat, multiply the area to be treated by the rate of application (ROA).

NOTE: Under certain conditions, the estimate should include sufficient bitumen for an additional width of 1 foot on each side of the surface course to be constructed on the primed base.

The formula for a prime coat estimate is as follows:

Example: the specification and other data for a prime coat application are as follows:

Always round your answer to the next higher number. In this case, 7761.6 is rounded to 7762 gallons.