A series of acceptance tests are carried out after finishing the placement of a mixture. They usually comprise measuring the content of air voids and the compaction factor, conducted on specimens of cores taken from the finished pavement. In many countries, nuclear density gauges are used for testing the homogeneity of compaction. When this is the case, cores are only required for calibration and comparisons with the nuclear gauge. Other properties checked on finishing the layer are skid resistance and the macrotexture depth. The methods used for these tests depend on national specifications.
10.6.1 Air Voids in a Compacted SMA Layer
The content of voids in the compacted SMA layer is the most commonly found parameter checked at the acceptance of a finished layer and is always mentioned as a fundamental. According to most analysis documents worldwide, the content of air voids should be lower than 6.0% (v/v). Lately in German guidelines ZTV Asphalt-StB 07 (September 2008 issue), this value has been lowered to 5.0% (v/v). This is the requirement most closely related to the durability of the compacted layer, including its susceptibility to water permeability, frost heave, and deicers. In countries with no significant drops of temperature below 0°C, higher contents of voids in the SMA layer (e. g., up to as much as 5-8% [v/v]) have usually been permitted.
An insufficient content of voids in a layer is also disadvantageous; recent experience shows that less than 3% (v/v) brings about the risk of premature rutting. It has been underlined in the literature (Voskuilen, 2000, Voskuilen et al., 2004) that when designing SMA with a determined content of voids in laboratory specimens (usually 3-4% [v/v]), one should remember that the final amount of voids in a compacted layer depends, among other things, on the arrangement of skeleton particles and voids among them. The air voids achieved in a layer on a work site are different from those achieved in the laboratory, just as particles compacted using a Marshall compactor are arranged differently than those that are rolled. Some authors (Voskuilen, 2000) also claim that during the life of the pavement, the content of voids in an SMA layer decreases due to the gradual decrease of voids in the chipping skeleton (e. g., postcompaction, crushing particles). That is why, for instance, in the Netherlands, designing SMA with an initial (laboratory) content of voids at the level of about 5% (v/v) has been practiced. Then, after some time of service, the air void content in the field was lowered to 2-3% (v/v) (Figure 10.13).
