Types of Bituminous Mixtures used in Pavement Construction
A bituminous mixture is a combination of bituminous materials (as binders), properly graded aggregates and additives. Since tar is rarely used in bituminous mixtures in recent years and asphalt is the predominant binder material used, the term “asphalt mixture” is now more commonly used to denote a combination of asphalt materials, aggregates and additives. Asphalt mixtures used in pavement applications are usually classified by
(1) Their methods of production, or
(2) Their composition and characteristics.
Classification by Method of Production
Hot-mix asphalt (HMA)
Hot-mix asphalt (HMA) is produced in a hot asphalt mixing plant (or hot-mix plant) by mixing a properly controlled amount of aggregate with a properly controlled amount of asphalt at an elevated temperature. The mixing temperature has to be sufficiently high such that the asphalt is fluidic enough for proper mixing with and coating the aggregate, but not too high as to avoid excessive aging of the asphalt. A HMA mixture must be laid and compacted when the mixture is still sufficiently hot so as to have proper workability. HMA mixtures are the most commonly used paving material in surface and binder courses in asphalt pavements.
Cold-laid plant mix
Cold-laid plant mix is produced in an asphalt mixing plant by mixing a controlled amount of aggregate with a controlled amount of liquid asphalt without the application of heat. It is laid and compacted at ambient temperature.
Mixed-in-place or road mix
Mixed-in-place or road mix is produced by mixing the aggregates with the asphalt binders in proper proportions on the road surface by means of special road mixing equipment. A medium setting (MS) asphalt emulsion is usually used for open-graded mixtures while a slow setting (SS) asphalt emulsion is usually used for dense-graded mixtures.
Penetration macadam is produced by a construction procedure in which layers of coarse and uniform size aggregate are spread on the road and rolled, and sprayed with appropriate amounts of asphalt to penetrate the aggregate. The asphalt material used may be hot asphalt cement or a rapid setting (RS) asphalt emulsion.
Classification by Composition and Characteristics
Dense-graded HMA mixtures, which use a dense-graded aggregate and have a relatively low air voids after placement and compaction, are commonly used as surface and binder courses in asphalt pavements. The term Asphalt Concrete is commonly used to refer to a high-quality, dense-graded HMA mixture.
A dense graded HMA mixture with maximum aggregate size of greater than 25 mm (1 in.) is called a large stone dense-grade HMA mix. A dense-grade HMA mix with 100% of the aggregate particles passing the 9.5 mm (3/8 in.) sieve is called a sand mix.
Open-graded asphalt mixtures, which use an open-graded aggregate and have a relatively high air void after placement and compaction, are used where high water permeability is desirable. Two primary types of open-graded mixes are
(1) open-graded base mix and
(2) open-graded friction course (OGFC).
Open-graded base mixes
Open-graded base mixes are used to provide a strong base for an asphalt pavement as well as rapid drainage for subsurface water. Open-graded base mixes usually use a relatively larger size aggregate that contains very little or no fines. Due to the lower aggregate surface area, these mixes have relatively lower asphalt content than that of a dense-graded HMA mix. Open-graded base mixes can be produced either hot or cold in an asphalt plant.
Open-graded friction courses (OGFC)
Open-graded friction courses (OGFC) are placed on top of surface courses to improve skid resistance and to reduce hydroplaning of the pavement surface. OGFC mixtures use aggregates with a small proportion of fines to produce high air voids and good drainage characteristics. Even though the voids content is higher, the asphalt film thickness is usually greater than that for a dense-graded HMA, and thus a typical OGFC mixture has about the same or higher asphalt content than that of a dense-graded HMA. A typical OGFC uses an aggregate of ½ in. (12.5mm) maximum size, and is placed at a thickness of ¾ in. (19 mm). An OGFC mixture is produced in a hot-mix plant in the same way as a dense-graded HMA mixture. Crumb rubber modified asphalt has been used in OGFC mixtures in recent years to improve their performance and durability. Due to the higher viscosity of the crumb rubber modified binder, thicker film thickness can be used. This results in a higher binder content and thus better durability for the crumb rubber modified OGFC mixtures.
Stone Matrix Asphalt (SMA), which was originally developed in
Europe, was a special asphalt mixture of improved rutting resistance and increased durability. SMA mixtures are designed to have a high coarse aggregate content (typically 70–80%), a high binder content (typically over 6%) and high filler content (typically about 10%). Asphalts modified with polymers and/or fibers are typically used. The improved rutting resistance of the SMA mixture is attributed to the fact that it carries the load through the coarse aggregate matrix (or the stone matrix), as compared with a dense-graded HMA, which carries the load through the fine aggregate. The use of polymer and/or fiber modified asphalts, which have increased viscosity, and the use of high filler content, which increases the stiffness of the binder, allow the SMA mixtures to have a higher binder film thickness and higher binder content without the problem of draindown of asphalt during construction. The increased durability of the SMA mixtures can be attributed to the higher binder film thickness and the higher binder content. SMA mixtures require the use of strong and durable aggregates with a relatively lower L.A. Abrasion Loss. SMA mixtures can be produced in a hot-mix plant in a similar way as a dense-grade HMA mixture. The main disadvantage of using a SMA as compared with a dense-grade HMA is its relatively higher cost due to the requirement for the use of higher quality aggregates, polymer, fibers and fillers.