The design philosophy adopted applies principally to the design of single/single sprayed seal using conventional bitumen as the binder. Designs of other seal types are based on the design procedure for single/single seals, with appropriate amendments in the procedure, and additional information as required.
Assumptions used in the design of single/single seals are:
- aggregate is single-sized and of appropriate quality
- average least dimension (ALD) of the aggregate is an important input into the design method and must be representative of the aggregate being used
- design traffic volume is expressed in vehicles/lane/day (v/l/d) and based on average annual daily traffic (AADT)
- aggregate is spread in a uniform layer of one stone thickness, with particles in continuous, partly interlocked contact and the least dimension near vertical
- there is no separate allowance to be made for whip-off in the design aggregate spread rate
- aggregate spread rate determines the inter-aggregate void space in the seal layer, and hence the amount of binder required; failure to achieve, within practical limits, the design aggregate spread rate will result in the design binder application rate being incorrect
- a single layer of aggregate particles settles with, typically, 40–60% voids after orientation and packing of the aggregate by rolling and trafficking
- binder rise should be a minimum of about 35–40% up the height of the aggregate particle after initial rolling and trafficking, increasing to between 50–65% (i.e., 1/2–2/3) about two years after construction
- aggregate particles may penetrate (embed) into the base
- reseals interlock with the existing surfacing
- binder may be absorbed into the base and, sometimes, by the aggregate
- the proportion of voids to be filled with binder may be varied to optimise requirements such as surface texture, maximum seal life, and for specific applications such as non-traffic areas; a minimum texture is generally required for skid resistance
- preliminary treatments such as primes and initial seals have been correctly designed and applied; if this has not been achieved, remedial work should be undertaken prior to, and well in advance of, the commencement of sealing
- all application rates determined by this method are expressed in L/m2 of residual binder at the standard reference temperature of 15 °C.
Sprayed seals are a system and sealing trials and subsequent work have shown that the design of the rates of application of binder and aggregate spread rates are both of major importance in achieving a satisfactory performance for the service conditions.