Table of Contents

6.2.2 Allowances Applied to the Basic Binder Application Rate

The following allowances need to be considered to complete the design. Allowances are determined to the nearest 0.1 L/m2 and are cumulative. They must be added to or subtracted from the basic binder application rate, Bb (L/m²), to determine the design binder application rate, Bd (L/m2).

Allowances in L/m2 are made for the following:

  • surface texture of existing surface
  • potential aggregate embedment into the existing surface
  • potential binder absorption into the existing pavement
  • potential binder absorption into the sealing aggregate.

Surface texture allowance (As)

Measurement of surface texture

Surface texture allowance is based on measurement of the existing surface texture using the sand patch method provided in Austroads (2008) or in Transit New Zealand (1981).

Texture measurements should be taken at least every 400 to 500 m or where there is a visual change in texture, such as a change to a seal of different aggregate size. For shorter sites, measure at a minimum of two locations.

It is recommended that the texture depth be measured in the wheelpaths, between wheelpaths, and outside of the wheelpaths. This will assist in deciding if separate design rates of binder need to be considered across the lane. If the difference in texture allowance is 0.3 L/m² or greater, one of the following alternatives may assist in achieving optimal performance across the full width of the seal:

  • Regulate the surface with a 5 or 7 mm seal.
  • Pre-spray the coarse-textured areas using the techniques described in Austroads (2003).
  • Use a bitumen sprayer with a variable rate spray bar.
Surface texture allowance for existing seals

Table 6.3 provides a guide to binder application rate allowances for different sizes of aggregate for a seal over various existing seal sizes and textures. The allowances are based on an assumption of satisfactory interlock between aggregates. Aggregates that have unusual (atypical) shape or size may require minor variations from the tabulated values.

Some aggregate sizes will not be readily compatible with existing seal sizes and texture depths. For example, small‑sized reseals will generally not give good results over flushed large-sized seals, and 10 mm reseals may sometimes not interlock well with existing hungry coarse textured 16, 14 and 10 mm seals.

Allowances to be applied for existing surface texture may be substantial and require a degree of judgement by the designer.

Consideration should be given to changing the size of aggregate and/or the treatment if surface texture allowances required become excessive, say exceeding 0.5 L/m².

On surfaces with low texture depths (e.g. < 0.5 mm), it may not be possible to measure the texture adequately, and a visual assessment may be necessary. This may particularly apply to asphalt, slurry, concrete, timber and primed surfaces (see notes below).

Table 6.3: Surface texture allowance for existing surfacing, As (L/m2)

Existing surfaceAggregate size of proposed seal (Note 4)Measured texture depth (mm)Surface texture allowance (L/m2)
14, 16 or 20 mm seal5 or 7 mm0 to 0.3Note 1
0.4 to 0.6Note 2
0.7 to 0.9+0.1
1.0 to 1.3+0.2
1.4 to 1.9+0.3
2.0 to 2.9+0.4
> 2.9+0.5
10 mm0 to 0.3–0.1
0.4 to 0.50
0.6 to 0.7+0.1
0.8 to 0.9+0.2
1.0 to 1.3+0.3
1.4 to 1.8+0.4
> 1.8Note 3
14 mm0 to 0.3–0.1
0.4 to 0.50
0.5 to 0.6+0.1
0.6 to 0.7+0.2
0.8 to 0.9+0.3
1.0 to 1.3+0.4
1.4 to 1.8+0.5
> 1.8Note 3
10 mm seal5 or 7 mm0 to 0.3Note 1
0.4 to 0.9+0.1
1.0 to 1.4+0.2
1.5 to 2.0+0.3
2.1 to 2.7+0.4
> 2.7+0.5
10 mm0 to 0.3Note 1
0.4 to 0.7+0.1
0.8 to 1.1+0.2
1.2 to 1.7+0.3
> 1.7Note 3
14 mm0 to 0.2Note 1
0.3 to 0.6+0.1
0.7 to 0.9+0.2
1.0 to 1.2+0.3
1.3 to 1.7+0.4
> 1.7Note 3
5 or 7 mm seal5 or 7 mm0 to 0.3Note 1
0.4 to 0.9+0.1
1.0 to 1.5+0.2
1.6 to 2.2+0.3
2.3 to 3.2+0.4
> 3.2+0.5
10 mm0 to 0.3Note 1
0.4 to 0.7+0.1
0.8 to 1.1+0.2
1.2 to 1.8+0.3
> 1.8Note 3
14 mm0 to 0.2Note 1
0.3 to 0.6+0.1
0.7 to 0.9+0.2
1.0 to 1.4+0.3
1.5 to 2.0+0.4
> 2.0+0.5
Asphalt/microsurfacingAll0 to 0.10
0.2 to 0.4+0.1
0.5 to 0.8+0.2
0.9 to 1.4+0.3
> 1.4+0.4


  1. Embedment considerations are dominant.
  2. Specialised treatments are necessary.
  3. This treatment might not be advisable depending on the shape and interlock of aggregates so alternative treatments (surface enrichment, small size seal or others) should be considered.
  4. For application of aggregate sizes greater than 14 mm, adopt the allowances applicable to 14 mm aggregate.
Texture allowance for asphalt

For an asphalt surface, the sand patch test may not be appropriate. Based on experience and depending on the type of mix, the texture allowance for hardened and aged asphalt surfaces is typically between +0.0 to +0.3 L/m2. Embedment into freshly placed asphalt or asphalt that is slick with fatty patches, may also need to be considered (see Section 6.2.3).

Texture allowance for microsurfacing

For microsurfacing, the sand patch test is, again, usually not appropriate. Similar to asphalt, a typical allowance is between +0.0 to +0.3 L/m2. Embedment into freshly placed microsurfacing may also need to be considered (see Section 6.2.3).

Texture allowance for concrete surfaces

The concrete must be primed and, in order to get a satisfactory seal over a well-primed concrete surface, the allowance should be +0.2 to +0.4 L/m2, even on smooth surfaces, to compensate for the lack of aggregate embedment and interlock into the texture of the concrete surface. For broom-dragged or tyned surfaces, the allowance can be as high as +0.4 to +0.5 L/m2.

Texture allowance for timber surfaces

Timber may be untreated, primed, coated or impregnated. Similar to concrete, and as a guide, an allowance of between +0.2 to +0.4 L/m2 may be appropriate.

Texture allowance for pavement and primes

Some pavement materials and primes present a coarse-textured surface and it may be possible to measure surface texture. If not, and based on experience, the texture allowance is generally in the order of +0.0 to +0.3 L/m2.

Texture allowance for initial seals

A texture allowance is determined as per existing sealed surfaces.

Texture allowance for regulation or patched areas

Shape correction of pavements and maintenance patching prior to sealing is often carried out using asphalt, microsurfacing or premix. Allowance may need to be made for patches that are much smoother in texture than the surrounding seal, particularly if the affected area is substantial.

In addition, if the regulation/patching has not had time to cure, there is an increased risk of aggregate embedment, with resulting flushing of the seal over those areas. The regulation/patching should be allowed to cure to minimise the risk of flushing/bleeding. Recommended minimum curing times are three months in hot weather and six months in cooler weather (see also Austroads (2010b)).