Asphaltic concrete cores have been used in dam construction around the world for over 50 years. They have been slow to catch on in North America, but their many advantages to traditional earthen core alternatives makes them worthy of a second look. You might wonder when should you use an asphaltic concrete core?
What is an Asphalt Concrete Core?
The oldest known use of an asphalt concrete core was a reservoir in the Indus Valley built 5,000 years ago. The reservoir builders used natural asphalt and the reservoir is still operational today. The first use of the asphalt concrete core in modern dam design was in 1962 in Germany. China first used asphaltic concrete core in 1970. The first asphalt core dam in North America was in 2008 (Nemiscu-1 dam).
Asphalt is a mixture of bitumen, a by-product of petroleum distillation, and various aggregates. It is well known as the material used to pave roads, however the composition of the road asphalt is a bit different than the asphalt used for dam construction. Since the asphaltic concrete core is protected from the environment, it does not degrade like the asphalt used on roadways.
Since the initial use, various mix designs and placement methods have been explored, initially with high bitumen content (30-40%) and subsequently high costs. The most economical mix developed consists of a bitumen content between 6.5 and 7.5%. The exact composition of the mix depends on local conditions, however typically engineers use fine grade component smaller than 0.063 mm and a maximum aggregate size of 18 mm to eliminate segregation and improve workability.
The asphalt core is placed in layers with a thickness of around 20 cm. To provide lateral support to the core, transition zones are simultaneously placed on either side of the asphalt. The core paving machine is followed by 3 rollers to compact all the zones at once. A heater in front of machine can dry and heat the previous layer.
Use in Areas with Seismic Activity
Multiple studies have shown that the asphaltic concrete cores have very small induced deformations and no signs of the core deteriorating or cracking during seismic loading. Asphaltic core dams have a unique ability to self-heal. When designed right, the viscoelastic-plastic and ductile properties of the asphalt seal cracks that might form during seismic activity. With over 130 asphalt concrete core dams worldwide, no serious failures due to seismic load have been reported. There have been reports of crest settlements and slope deformations, but this has no impact on the impermeability.
Use in Areas with No or Limited Clay Material
The main alternative to the asphalt core is a clay core. The cost of importing clay is often so high that finding local sources is vital. In many areas clay is not abundant. Asphalt is man-made and contains a by-product of petroleum distillation. It is readily available and easy to transport. A lack of local clay was the original reason for the development of the asphalt concrete core in Germany.
Use in Areas with Wet Weather
The main issue with an earthen core during highly rainy weather is poor compaction due to compaction energy dissipation. Rainy weather rarely causes issue during asphaltic concrete core placement like it does with earthen core placement. The design for the Yele dam in China only considered an asphalt concrete core, as the weather in the region was so poor.
There are several other advantages to an asphaltic concrete core dam, such as only small amounts of good quality aggregates are needed, fast construction, and low maintenance cost. All of these advantages make asphaltic concrete cores one of the best options for many dam designs.