Introduction
Asphalt is a familiar sight on roads, driveways, and parking lots across the world. The dark black color is iconic and immediately recognizable. But is asphalt always black? The quick answer is no, asphalt can also be brown, gray, or even red depending on its composition. However, the most common form of asphalt does appear black when laid down in roads or driveways. In this article, we’ll examine why asphalt is usually (but not always) black, what ingredients cause variation in color, and examples of non-black asphalt from around the world.
What is asphalt made of?
Asphalt is a viscous material made from the residues of petroleum distillation. The primary ingredients are bitumen and aggregates. Bitumen is the black or brown residue left over after crude oil is refined into gasoline, diesel and other fuels. Aggregates refer to granite, gravel, sand or other minerals added to the bitumen to make asphalt.
The typical recipe for asphalt is:
- 85-95% aggregates by weight
- 5-15% bitumen binder
- Small amounts of additional chemicals or polymers to modify properties
It’s the bitumen binder that gives asphalt its dark color. Bitumen is generally black or brown due to its high content of complex hydrocarbons. The black color helps asphalt absorb and retain heat from the sun, which helps it soften and flex to handle traffic loads. The aggregates added to make asphalt are usually crushed grey or white stone, sand and gravel. But the small proportion of aggregates compared to bitumen means the dark bitumen color dominates in the final asphalt mixture.
What makes asphalt different colors?
While conventional asphalt is black or dark brown, there are circumstances where asphalt can take on lighter or different shades of color, including:
1. Grey asphalt
Some asphalt mixes use a higher proportion of light-colored aggregates relative to bitumen. This dilutes the black color and can create a lighter grey asphalt after laying.
2. Polymer-modified asphalt
Polymers are sometimes mixed with asphalt to improve performance. Certain polymer modified asphalts take on a brownish grey color.
3. Reduced-bitumen asphalt
Bitumen is the most expensive part of asphalt. Some mix designs use less bitumen, which reduces the black coloration and can produce grey or brown asphalt.
4. Light-colored aggregates
While most aggregates in asphalt are dark gravel or crushed rock, sometimes light-colored limestone or dolomite aggregates are used, giving lighter shades of gray or brown.
5. Color additives
Pigments can be added to asphalt to deliberately make it a certain color for decorative or safety purposes. Red asphalt is sometimes seen on bike lanes, for example.
Examples of non-black asphalt around the world
While black asphalt dominates roads across North America, Europe and Australia, there are examples around the world of asphalt produced in lighter shades:
Indonesia
Indonesia uses a high-limestone aggregate mix for many urban roads, resulting in grey-colored asphalt:
Hong Kong
Hong Kong has hot and rainy weather that requires modified asphalt mixes. The polymers create a distinctive maroon-brown colored asphalt on many roads:
Decorative asphalt
Colored asphalt using pigments allows creation of decorative walkways, driveways and traffic calming measures. This driveway uses brick-red asphalt:
Porous asphalt
Porous asphalt contains more air voids allowing water drainage. The higher air void content creates a lighter brown or grey color:
Performance differences
Lighter shades of asphalt are not intrinsically better or worse performing than conventional black asphalt. The key factors affecting performance are using quality materials, proper mix design, and construction best practices.
However, some differences to consider:
- Darker asphalt may absorb more sun heat, staying more pliable in cold climates
- Lighter asphalt may provide better contrast and visibility at night
- Color additives may improve abrasion resistance in decorative asphalt
- Polymer-modified asphalt usually improves strength and durability
Performance depends much more on the overall mix composition, production quality, and implementation. Color alone does not determine the quality and lifespan of asphalt. Well-made lighter colored asphalt can perform just as well as black asphalt.
Cost comparison
There is generally little cost difference between standard black asphalt and lighter colored variants:
| Asphalt Type | Typical Cost per Ton |
|---|---|
| Standard black asphalt | $85-150 |
| Grey asphalt | $90-155 |
| Polymer-modified asphalt | $95-165 |
| Colored asphalt | $90-160 |
| Porous asphalt | $100-180 |
The small cost differences mainly come from:
– Higher cost light aggregates
– Polymer additives
– Pigment additives
– Extra production steps
But overall the base asphalt binder cost dominates, keeping costs similar regardless of color.
Environmental impact
Lighter colored asphalt is no better or worse for the environment compared to conventional black asphalt. The overall environmental impacts are similar:
– Asphalt production creates greenhouse gas emissions from heating aggregates and binder. Color does not change emissions.
– Runoff from asphalt can contaminate waterways. But proper stormwater management is needed regardless of asphalt color.
– Light colored asphalt does not significantly change heat island effects in cities or absorb less solar radiation.
– Asphalt binder and polymers will biodegrade slowly over time no matter the color.
– Life cycle impacts of mining aggregates and producing bitumen are comparable for all colors.
The only exception is porous asphalt designed to allow stormwater infiltration and reduce runoff. But porous asphalt can be any color.
Overall, the color of asphalt has minimal influence on sustainability or environmental impacts. Responsible production and construction methods are much more important.
Conclusion
While black asphalt is the norm on roads worldwide, light colored asphalt also has a place and performs equally well. The color primarily depends on the type and amount of aggregates and bitumen used. Any color of asphalt can deliver excellent performance if engineered and constructed properly. Specifying lighter asphalt on certain projects can provide decorative, visibility or safety benefits without sacrificing quality or durability. Asphalt’s color does not affect cost, lifespan or environmental impacts – good design and construction practices make the biggest difference. So black asphalt may dominate, but there are also applications where switching to lighter shades of asphalt can be beneficial.
