The rule for SMA surface preparation is the same that as for other binder courses. The surface of the underlying course should be even, clean, and free from dirt that could disturb the appropriate bonding of the laid-down mixture with the layer underneath. Any soiling of the surface (Figure 10.6) should be removed from the pavement. Oil and fuel stains should be cleaned with an absorbent material, and their residue should be washed off. The remains of an old mixture atop an underlying layer should be removed because they may cause partial unevenness of the spread SMA mixture (see Figure 11.33). When placing an SMA course on an old pavement, one should see that patches made of mastic asphalt have been removed and a new patch of asphalt concrete has been made. Leaving the mastic asphalt in place runs the risk of bleeding the binder from the patch onto the surface of the SMA course. Potholes and cracks should also be filled and sealed prior to placing the next course.
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FIGURE 10.6 Soiling of an intermediate course just before laying an SMA mixture. (Photo courtesy of Krzysztof Blazejowski.) |
10.2.4.1 Bonding Layer (Tack Coat)
Attention should be paid to the proper laydown of the tack coat beneath the SMA course. When the surface is uneven, with pockets collecting the binder of the tack coat, its excess should be completely removed, otherwise it is likely that fat spots may appear on the surface of the SMA course. The tack coat underneath the SMA is necessary, but the amount and type of required binder (bituminous emulsion or cutback binder) should be suitably estimated, depending on the state and type of surface. After all, we do not need a tack coat that is too thick or too soft because it can become a failure (sliding) plane under the SMA. A special binder (e. g., a special bituminous emulsion) should be used for interlayer bonding. Except for special cases, typical binder emulsions made of soft binder (e. g., 160/220 [Pen@25]) are not recommended. An average amount of binder (residual) for tack coating under an SMA varies within the range of 0.1-0.3 kg/m2. For example, in Germany according to ZTV Asphalt-StB 07, the recommended quantities of a polymer modified emulsion C60BP1-S[61] (former TL PmOB Art C U 60 K) for interlayer bonding for roads with heavy or very heavy traffic depend on the type of layer beneath the SMA as follows:
• Newly constructed asphalt intermediate layer: 150-250 g/m2
• Asphalt intermediate layer with cold milling surface: 250-350 g/m2
• Very porous asphalt intermediate layer or ravelled surface: 250-350 g/m2
Note: the above amounts of bituminous emulsion are NOT residual binder (from the emulsion).
Additionally there is a requirement for the strength of the connection between layers; the splitting force has to be at least 15 kN. A lack of interlayer bonding adversely affects the pavement’s service life; it can also cause some negative effects in the form of slippage under a roller (see Figure 11.32).
