Basic design principles for single and double seals

Most of the seal design methods used in South Africa evolved from Hanson’s concept of partially filling the voids in the covering aggregate and that the volume of these voids was controlled by the Average Least Dimension (ALD) of the sealing chips. Through formal and informal experiments and experience, different institutions have refined their methods over a period of 30 years to provide guidance to designers in the design of seals within their areas of jurisdiction.

For various reasons, the recommended application rates for apparently similar situations, calculated according to the different methods, are different. During the compilation of this document, several logical reasons were found for these differences. The main reasons are:

• _{Most design methods recommend only one application rate for a given situation, whereas theory and}

practice indicate that a range of application rates could suffice.

• Perceptions of the ideal aggregate matrix differ. The more open the aggregate matrix, the more binder can be accommodated (in case of low flakiness aggregate).

• _{Perceptions of the ideal texture depth differ, which may be related to the maintenance strategy of the}

authority, the risk of bleeding and poor skid resistance.

• _{Climatic differences.}

• _{Different sources of binders.}

To a large extent the design method described in this section satisfies the requirements of the major road authorities.

The principles applied are as follows (see also Figure 7-1):

a) The minimum amount of voids to be filled with binder to prevent stone loss when there is no embedment, is 42 per cent for single seals and 55 per cent for double seals. (Wetting 30 per cent of the aggregate height, as shown in Figure 3-1, requires approximately 42 per cent of the voids to be filled.)

b) The amount of void loss due to traffic wear is dependent on the hardness of the stone and traffic as given in APPENDIX K, Table K-5. For purposes of simplification it is assumed that the hardness is not less than 210 kN (10% FACT).

c) The required texture depth to provide adequate skid resistance is taken as 0,7 mm. However, design charts are available for seals with low ALDs and for texture depths of 0,3 mm and 0,5 mm.

d) The degree of embedment during construction may vary but, for simple design purposes, it is taken as 50 per cent of the embedment with time.

e) The total embedment potential is determined from corrected ball-penetration tests (test method ST4 in TMH6)12.

f) The effective layer thickness (ELT) of a single seal is a function of the average least dimension (ALD).

g) ELT = 0,85679 x ALD + 0,46715 mm.

h) The effective layer thickness of a double seal is a function of the sum of the ALDs of the two aggregates.

i) (ELTd) = 0,86028 x (ELT1 + ELT2) + 0,19188 mm.

(The ELT and percentage voids for any aggregate/binder combination may be determined by the modified tray test). (See APPENDIX L.)

j) The percentage void content in the aggregate layer is a function of the ELT. k) Estimated void content for a single seal (%) = 45,3333 - 0,333 x ELT.

Estimated void content for a double seal (%) = 63,01263 + 0,04743 x ELTd 2

- 2,41172 x ELTd.

void loss due to aggregate wear texture for

skid resistance maximum voids to be filled

void loss due to embedment total voids minimum voids to be filled 3 0 % 1 0 0 % a ld

void loss due to aggregate wear texture for

skid resistance maximum voids to be filled

void loss due to embedment total voids minimum voids to be filled 3 0 % 1 0 0 % a ld

Figure 7-1 Principles applied for design of the binder application rate 7.6.2 Design process for single seals

The following procedure is given as a guideline:

a) Equivalent light vehicles (ELV)

Obtain or determine the most probable traffic scenario for each unique section per lane. (Recent traffic counts should be used.)

Total ELV/lane/day = Number of light vehicles + (Number of heavy vehicles x 40).

b) Potential embedment

per road section. (Refer to Figure 3-1 for suggested road temperatures, T0 as described in Method ST4) 12

.

Note:

Additional guidelines on interpretation are provided in paragraph 3.3.

c) Minimum and maximum binder application rates

Use the average least dimension (ALD) and the required texture depth, together with the average corrected ball-penetration value and read off the theoretical minimum and maximum residual binder application rates (net cold binder) from the relevant charts in APPENDIX E.

Note:

Aggregate samples should be taken on site and not from stockpiles at the crusher.

One of the major reasons for differences in application rates by various road authorities lies in the required texture depth after the service life. The design charts (APPENDIX E) make provision for texture depths of

0,3 mm, 0,5 mm, 0,7 mm and 1,0 mm where appropriate.

Use straight line interpolation if the ALD falls between those supplied on the charts.

d) Adjustments (refer to paragraph 7.6.3.3)

Existing texture

- Measure the average texture depth (TMH6 - Method ST1)12 and use Figure 7-2 to determine the quantity of extra binder required to prevent whip-off on coarse textures.

Climate (See Figure 1-3).

- The design curves are appropriate for moderate climates.

- Subtract up to 10 per cent of the net cold binder for wet or humid areas (Weinert N-value < 2). - Add up to 10 per cent to the net cold binder for dry areas (Weinert N-value > 5).

Slow-moving and channelised traffic

- Obtain information regarding the typical speed and actions of heavy vehicles for specific uphills, downhills, stopping places, turning places and sections where heavy vehicle speeds are low and reduce binder content with up to 10 percent to prevent bleeding and fattiness.

Note:

Sections where binder application should be reduced relate to the speed of heavy vehicles and not the gradient. Reduction of the binder application rate is normally required when the speed of heavy vehicles reduce to below 40 km/h

Aggregate spread rate

- Decide on the appropriate matrix of aggregate by referring to APPENDIX F and add up to 10 per cent binder for the medium-dense or up to 20 per cent binder for the open “shoulder-to- shoulder” matrices.

Note:

This adjustment is only valid for aggregates with low flakiness indices (guideline < 10%) and only if there is a specific need for these type of aggregate spread.

e) Sensitivity analysis

It is important to note that variations of input parameter values will occur and that different assumptions used in the design process may be valid. It is, therefore recommended that the sensitivity of these parameters to the minimum and maximum application rates be analysed. Extreme scenarios would result from the following:

Minimum: Highest expected traffic

Highest ball penetration (Average corrected ball penetration + 1 Standard deviation) Smoothest texture (Average texture depth - 1 Standard deviation)

Lowest design ALD (Average texture depth - 1 Standard deviation)

Maximum: Lowest expected traffic

Lowest ball penetration (Average adjusted ball penetration - 1 Standard deviation) Coarsest texture (Average texture depth + 1 Standard deviation)

Highest design ALD (Average texture depth + 1 Standard deviation).

f) Practical minimum and maximum binder application rates

- Select possible binders from Table 5-1 (Chapter 5).

- Convert to hot spray rates of the appropriate binders using Table 7-3. - Check for practical minimum spray rates (accuracy) = 0,7 ℓ/m2 (hot).

- Check for practical maximum spray rates to prevent run-off by evaluating the maximum gradient/cross fall combination, texture and binder viscosity (approximately 1,75 ℓ/m2 for hot conventional binders and 1.5 ℓ/m2 for emulsions).

Note:

Each type of binder has its own minimum practical spray rate. Cognisance should be taken of this. In the case of polymer-modified binders, it should be borne in mind that the use of minimum spray rates will tend to defeat the object for which the polymer-modified binder was selected in the first place.

g) Final decision and specification of a target spray rate.

It is essential that the contractor be given a specified application rate for each unique road section.

In selecting the target application rate cognisance should be taken of the 5 per cent permissible variation in binder application rates (See COLTO specifications4).

The final decision should be supported by documentation stating the input parameters and the rationale for adjustments.

h) Policy and maintenance strategy

Evaluate the policies and strategies of the road authority concerned with regard to maintenance (risk of aggregate loss) and required skid resistance (risk of fattiness) as well as the level of uncertainty with regard to traffic. If there is a high level of uncertainty with regard to traffic and the ability exists to add binder (application of diluted emulsion) when necessary, a strategy can be followed of applying the minimum amount of binder.

Figure 7-2 Binder adjustment for existing texture 7.6.3 Design process for double seals

7.6.3.1 Application rate

The steps are similar to those taken for the determination of application rates for single seals except for the following:

a) The design ALD of the double seal is calculated as follows:

Design ALD (double seal) = ALD of first layer + ALD of the second layer.

b) Use the relevant charts in APPENDIX E to determine the total binder application rate required for the double seal.

c) Binder distribution

Current practice differs from one road authority to the next with regard to the split in binder between the tack coat and penetration coat. However, it is generally agreed that this split is governed by the minimum application rates required for each layer to prevent whip-off or by the minimum rate that can be sprayed accurately (in the case of a fog spray).

The following guidelines may be used:

- Determine the total net cold binder required for the double seal.

- Subtract half of the binder required for the fog spray e.g. 0,33/2 = 0,17 ℓ/m2 (if a fogspray will be applied). It is assumed that only half the binder will flow down - the remainder will stick to the top and sides of the aggregate, i.e. will be non-effective in terms of filling the voids.

The minimum quantity of net cold binder required for the penetration coat depends on the aggregate size of the second layer. (Table 7-4 may be used as a guideline.)