## 3 Selection and Construction of Thin Chip and Cape Seal Surfacings
### Binder Selection
#### 3.2.2 Binder Selection: Tack Coat for the First Application of the 20 mm Chips (Stone)  
With the construction of the test sections on the anionic NME-stabilised base, binder options were considered with the specific intention to limit risks associated with construction and temperature-related aspects that could be challenging to meet under difficult construction and environmental conditions. The APT programme was inter alia, also designed to test at temperatures below 20 \({}^{\circ}\)C to evaluate stone loss during early trafficking while orientation of the stones took place. These aspects are not covered in this paper and are fully addressed elsewhere [29].  
Conventional modified binders commonly used in the construction of chip seals do not allow for lowering of the temperature criteria, as stated above. In addition, an elastomer-modified binder has a disadvantage that it \"can\" or \"does restrict the evaporation of moisture\" [11] from a newly constructed base layer. This could result in moisture build-up underneath the surfacing and failures, as discussed. Hence, any unconventional modified binder must, over and above all the other considerations, also allow moisture evaporation to continue until a state of moisture equilibrium has been reached within the pavement layers (which usually takes at least two seasons under trafficking to occur).  
SASOBIT(r) (Fischer-Tropsch (FT)) wax modifications [30] to bituminous binders were developed more than 25 years ago with well-known advantages, as confirmed all over the world (e.g., [31; 32; 33]). These advantages included the compaction of asphalt at temperatures as low as 80-90 \({}^{\circ}\)C (compared to unmodified binders, where a minimum of 120 \({}^{\circ}\)C is the norm). The original formulation of SASOBIT(r) has recently been improved, with a crude-oil additive to allow for compaction at temperatures as low as 60 \({}^{\circ}\)C. This new formulation is known as SASOBIT-M(r) [34], which also prevents an increase in viscosity at lower temperatures (associated with SASOBIT(r)). The comparison between the viscosities and compaction end temperatures of standard bitumen, 3% SASOBIT(r)-modified bitumen and 1.5% SASOBIT-M(r)-modified bitumen is demonstrated in Figure 3.  
Logically, if the use of SASOBIT-M(r) allows for the lowering of compaction temperatures in asphalt, the same principle should also be applicable when used in a tack coat for the construction of chip seals, allowing for the placement of the chips at lower temperatures. If proved in practice, this modification will allow for chip seal surfacings to proceed at temperatures considerably lower than currently used specifications, without any negative effects (such as chip loss). Hence, contractors would also have more time to apply chips during the construction of seals without an increase in the risk of failures. Consequently, it was decided to use a tack coat consisting of a 60/70 pen bitumen modified with 1.5% SASOBIT-M(r) for the 20 mm chip seal. This tack coat was placed at an application rate of 1.6 L/m2 at a surface temperature of 18 \({}^{\circ}\)C and rising (compared to the minimum specification of 25 \({}^{\circ}\)C and rising). These experimental sections are the first where Sasobit-M(r) modification has been used in the construction of chip seals.