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Flexible Pavement Types

An introduction to some key types of flexible pavements and their potential uses.

Figure 6. SMA Pavement Surface

Stone Matrix Asphalt

Stone matrix asphalt (SMA) is a gap-graded HMA (Figure 1) that is designed to maximize deformation (rutting) resistance and durability by using a structural basis of stone-on-stone contact (Figures 2-6). Because the aggregates are all in contact, rut resistance relies on aggregate properties rather than asphalt binder properties. Since aggregates do not deform as much … Read more »

Figure 2. Asphalt treated permeable base.

Open-Graded

An open-graded HMA mixture is designed to be water permeable (dense-graded and SMA mixes usually are not permeable). Open-graded mixes use only crushed stone (or gravel) and a small percentage of manufactured sands. There are two types of open-graded mixes typically used in the U.S.: Open-graded friction course (OGFC). Typically 15 percent air voids, no … Read more »

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Dense-Graded

A dense-graded mix is a well-graded HMA mixture intended for general use. When properly designed and constructed, a dense-graded mix is relatively impermeable. Dense-graded mixes are generally referred to by their nominal maximum aggregate size. They can further be classified as either fine-graded or coarse-graded. Fine-graded mixes have more fine and sand sized particles than … Read more »

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Full-Depth Reclamation

Although referred to as “full-depth reclamation”, this process is just an extension of the basic CIR principles to the entire HMA pavement depth plus a predetermined depth of the base material. FDR can be used to depths of 300 mm (12 inches) or more but the most typical applications involve depths of between 150 and … Read more »

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Cold Plant Mix Recycling

AAPTP Project 05-01 Cold plant mix recycling, the less common of the two cold recycling methods, involves mixing RAP with an asphalt emulsion or foamed asphalt at a central or mobile plant facility. A rejuvenating agent can be added to improve the recycled asphalt binder viscosity and new aggregate can also be added to improve … Read more »

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Mix Selection Guidance

The National Asphalt Pavement Association (NAPA) has published some general rules Based for HMA mix type use, which are summarized in Table 1.  Notice that dense-graded HMA is generally appropriate for all uses, SMA and OGFC are typically used as surface courses on high volume roads and ATPB is usually used for base courses on … Read more »

Figure 1. RAP Pile in Eastern Washington State

HMA Recycling

HMA is one of the most recycled products in the U.S. It is estimated that as much as 91 million tonnes (100 million tons) of HMA are milled off roads during resurfacing and widening projects each year (APA, 2001a[1]). Of this amount, 73 million tonnes (80 million tons) are recycled as “reclaimed asphalt pavement” (RAP … Read more »