Cracking of the mixture under rollers may be observed when there is no bond between an SMA course and the layer under it or when the temperature of the SMA mixture is too high. The temperature of the mixture during compaction is easy to control, therefore it will not be a topic of this section.
Figure 11.32 shows an old cement concrete pavement without a tack coat covered with a new SMA overlay. This resulted in a “dry slide” and the tearing of the SMA.
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FIGuRE 11.32 An example of the tearing of an SMA mat due to the lack of a tack coat on the cement concrete bottom layer. (Photo courtesy of Krzysztof BlaZejowski.) |
A similar accident might happen while paving SMA on an old asphalt course with a very polished surface. By contrast, a course with too much tack coat may lead to a “wet slide” on a layer, also resulting in the tearing of the mat.
The conclusion to be drawn from these examples are self-evident—SMA paving should be preceded with proper tack coat. The amount of tack coat should be carefully selected, taking into consideration the state of the underlying layer. Examples are shown in Chapter 10.
