Local governments began using asphalt rubber materials for applications like sealing joints and membranes during the late 1930s. More research on rubber and asphalt cement mixtures was conducted throughout the 1940s and 1950s. Concurrently, technological advances were being made in the use of vulcanized and unvulcanized rubber materials in asphalt pavements. However, the first successful formulation or "wet process" was discovered and pioneered by City of Arizona engineer, Charles H. MacDonald, in the 1960s. In addition to Arizona, states including California, Florida and Texas have used asphalt rubber extensively in road construction.
Chances are, you have traveled or driven on a road that was built using rubberized asphalt cement. Rubberized asphalt cement is used to make rubber asphalt concrete applied to roads for repair and structural purposes. From common neighborhood streets to mountainous and flat desert highways, rubberized asphalt cement is a key ingredient in producing safe and durable roads.
Rubberized asphalt is made from crumb rubber -- scrap tires and high natural rubber materials -- and paving grade asphalt. The rubber component comprises at least 15 percent of the cement, and swells when the cement is heated. It is also combined with other additives during the heating and pressurization process. Additives may include extender oils, polymers and anti-strip agents.
The high natural rubber used in asphalt rubber cement improves adhesion and flexibility, while additives aid interaction between the asphalt and rubber. Interaction between rubber and asphalt cement depends on factors such as temperature, blending methods and rubber type or source. The asphalt cement grade or source also affects the level of interaction between the asphalt and rubber components.
Rubberized asphalt cement mixed with mineral aggregates such as sand and gravel produce rubber asphalt concrete. According to the Integrated Waste Management Board publication “Asphalt-Rubber & RAC Materials and Mix Design,” construction companies use gap-graded, open-graded or dense-graded rubberized asphalt concrete. Gap graded rubber asphalt concrete is the most common because it provides strong resistance to rutting, cracking and other structural stress. It is primarily used for road pavement construction and maintenance. Open-graded mixes are used as thin surface overlays, rather than for structural elements in roads. Dense-graded rubberized asphalt concrete is the least used, as it provides minimal road improvement versus its application and maintenance costs.
The benefits of asphalt rubber cement combined with aggregated minerals, such as concrete, include improved durability, increased friction and reduction of surface defects on roads. As roads age, cracked and cratered pavement pose road bottlenecks and safety hazards for motorists. Rubberized asphalt concrete as a surface overlay also reduces hydroplaning and skidding on web pavement, facilitates drainage and significantly reduces tire road noise.
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