Asphalt/Bituminous Concrete

 

 

 

 

 

 

 

 

 

Asphalt, also called bituminous concrete, blacktop, hot top, etc. is a ubiquitous paving material which combines an oil-based pitch (bitumens) with graded aggregate to form a continuous surface. Its common applications include roadways, parking areas, and sometimes pedestrian walkways.

PROS & CONS

Advantages:

• Because asphalt forms a continuous surface, there will be no visible surface jointing
• Asphalt can be installed to fit just about any shape, can be very effective in irregular or curved layouts
• Owing to its darker color, may melt any accumulated surface ice or snow more readily in colder climates
• Cost-effective to install, with greater economy of scale the larger the area to be paved

Disadvantages:

• Depending on the layout, it can be difficult to create a defined “edge” without a curb or other pavement edge to constrain it. Without a broader base flaring out at the sides beneath it, the edges may break off or crumble when subjected to surface loading
• Economical material, can look “cheap” next to premium building materials such as stone or brick
• The smooth surface can also be a drawback; in large areas, it can be difficult or costly to add detail to articulate form, directionality, or scale to a space (this can be done, but it often involves re-heating the asphalt surface and impressing a pattern into the surface, sometimes with mixed results). This can leave large paved areas void of visual interest.

CONSTRUCTION

The sequence of construction is usually as follows:

EXCAVATION/GRADING

As a first step, the area to be paved will be excavated to allow for a proper building surface to be constructed beneath the pavement. Depending on the road or pathway’s intended use, the depth and bearing capacity of the base material will vary. Asphalt pavement designed for vehicles will usually require a __” base below the pavement, whereas pedestrian-only surfaces can be designed to __” below pavement. If you are unsure, a landscape architect or civil engineer can make a recommendation for your project taking into account specific attributes of your site.

Once the excavation is completed to attain the necessary depth, the exposed earth surface below is compacted, usually to a minimum of 95% Proctor Density, or an engineer’s recommendations.

AGGREGATE BASE

Once grading and subgrade preparation is complete, a base course will be laid to provide a stable platform for the pavement surface to rest on. In most roadway and parking areas, a dense-graded aggregate will be used, since the range of particle sizes can lock together to form a very supportive aggregate base. For applications of porous asphalt, an open-graded aggregate or crushed stone will be specified instead, to allow for free drainage below the pavement (this is less common, discussed in more depth below). The base is distributed over the compacted subgrade in the area to be paved, and is then mechanically compacted itself, either by a steamroller or manual vibrating plate compactor (for smaller areas).

Once the base course is in place, the asphalt itself is applied.

INSTALL PAVEMENT

For the average pedestrian walkway, the pavement will be a single application using a medium or fine aggregate; this will be applied either manually or with small equipment, and either hand-rolled or compacted in place.

For roadway applications, asphalt is commonly applied in 3 courses; a base course, a leveling course, and a wearing course. Each separate layer of pavement serves to distribute vehicle loading from the surface more evenly onto the course below, flexing under compression (this is why asphalt is known as a “flexible” pavement) to enhance the durability of the whole pavement profile.

The base course usually contains a coarse aggregate, its primary purpose being to support the upper layers of asphalt, and can be either unstabilized or stabilized with binder if a higher base stiffness is desired for engineering purposes.

The leveling course involves a finer mixture of aggregates, in many cases closely resembling a finished roadway. The primary purpose of the leveling course is load redistribution, and it constitutes the bulk of the asphalt pavement profile by volume.

The wearing course is the finished surface which is driven on, its purpose being to withstand vehicle wear on the pavement profile. It usually contains the finest aggregate mixture, to form a smooth surface and to be the “stiffest” layer of the profile, most resistant to deflection. The wearing course can also be engineered to provide friction, noise control, displacement resistance, and surface drainage. Most roadways will be designed with a crown, a subtle designed ridge running down the middle of the road to allow water to sheet off to the sides.

Because subsurface utilities must be installed before the asphalt surface above them is finished, you can often see utilities sticking out several inches from unfinished roadways – they are set to be flush with the road surface when it is finished with the wearing course.

POROUS ASPHALT TECHNOLOGY

Asphalt can also be engineered to drain water freely through the profile, typically done by eliminating much of the “fine” particle sizes from the asphalt aggregate. A pavement of this type will perform similarly to standard asphalt, but must be built very differently due to the free action of water movement within the pavement profile. It is common to require around 36” of open-graded aggregate or crushed stone as a base for porous asphalt, to serve as a storage reservoir for surface water draining into the subgrade.

Maintenance of this type of pavement should NOT include sanding for added traction in colder climates – the sand particles will clog the voids in the porous asphalt, negating the effect of the pavement and/or clogging the reservoir voids in the aggregate below.

A Demonstration of Porous Asphalt

Aside from research organizations and universities, where its properties continue to be studied and improved, porous asphalt’s most common applications include sensitive environmental sites, as well as areas where stormwater management practices are a key factor driving the design.

 

Further reading:

Asphalt being such a common construction material, there are entire books and trade institutes dedicated to the science of building with asphalt. This article should serve as a primer, but is only the tip of a much larger information iceberg. Further reading can be found at the links below (to be updated on an ongoing basis):

Homepage of the Asphalt Institute

www.asphaltinstitute.org

Commercial and Residential Paving – 3 Part Series

http://asphaltmagazine.com/commercial-and-residential-paving-part-1-an-overview/

Porous Asphalt Construction

http://www.asphaltpavement.org/index.php?option=com_content&view=article&id=558&Itemid=1148

 

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