Aggregate Materials for Highway Construction
Aggregates form the largest component of highway construction materials by volume. Crushed stone, gravel, and sand are used in base courses, subbase layers, and as components of asphalt and concrete mixtures. Aggregate properties including gradation, shape, texture, durability, and cleanliness significantly affect pavement performance. Los Angeles abrasion testing measures aggregate resistance to wear and impact.
Gradation refers to the distribution of particle sizes in an aggregate sample. Well-graded aggregates with a continuous range of particle sizes achieve maximum density and stability. Gap-graded aggregates missing intermediate sizes are used in specific applications like stone matrix asphalt. Open-graded aggregates with limited fine particles provide permeability for drainage layers.
Asphalt Binder Materials
Asphalt binder, also called asphalt cement, is a petroleum product that binds aggregate particles together in asphalt concrete. The Performance Grade system classifies binders based on the temperature range over which they maintain adequate performance. PG 64-22 indicates a binder suitable for average pavement temperatures between 64 degrees Celsius high and minus 22 degrees Celsius low. pavement thickness design. roundabout intersection design.
Polymer modification adds synthetic polymers to asphalt binder to improve performance at extreme temperatures and under heavy traffic. Styrene-butadiene-styrene is the most common polymer modifier, enhancing elasticity and rutting resistance. Crumb rubber from recycled tires is used as a modifier in some states, improving binder performance and providing an end use for scrap tires.
Portland Cement Concrete
Concrete for highway pavements uses the same basic ingredients as structural concrete but with specific requirements for durability and ride quality. The water-cement ratio is typically limited to 0.45 for concrete exposed to freeze-thaw cycles. Air entrainment of 5 to 8 percent provides resistance to freeze-thaw damage by creating microscopic air voids that accommodate ice formation.
Concrete pavement mixtures use larger maximum aggregate sizes up to 1.5 inches for improved economy and reduced shrinkage. Flexural strength measured by modulus of rupture is the primary structural design parameter for concrete pavements. Typical 28-day flexural strength requirements range from 600 to 750 pounds per square inch depending on traffic levels.