Bituminous sprayed seals - sealing and resealing

Discussion of primes and primer seals is not included in this chapter. Further the discussion in this chapter is general. For further and more detailed guidance refer to the Pavement Surfacings Manual (PSM) (TMR 2011b), the Austroads Guide to Pavement Technology Part 3: Pavement Surfacings (AGTPT Part 3) (Austroads 2009a) and the Update of the Austroads Sprayed Seal Design Method (Austroads 2006c).

A bituminous sprayed seal consists of a thin film of sprayed bitumen onto which a layer of single-sized cover aggregate is spread and compacted. The seal may involve more than one application of aggregate and binder. This system of surfacing is used extensively on both lightly and moderately trafficked roads. The performance of the seal as a waterproofing layer is vital to the overall performance of the pavement itself.

Resealing is the application of a seal to an existing bituminous surface to maintain the existing asset (e.g. to maintain integrity/’waterproofness’ of surface, restore surface texture and improve skid resistance).

Generally, a single/single seal is used, but under some circumstances multiple applications of binder and aggregate may be used (e.g. double/double). It is not essential, or necessarily more economical, to use the large stone size seals/reseals.

Where the seal or reseal is used as the final surfacing it is vital that it is also hardwearing with adequate surface texture and skid-resistance. Together with good construction practice and use in appropriate circumstances (refer to the PSM (TMR 2011b) and AGTPT Part 3 (Austroads 2009a)), a detailed design in accordance with the PSM (TMR 2011b) and AGTPT Part 3 (Austroads 2009a) is essential. The types of sprayed work alternatives that are usually within scope for a rehabilitation treatment are as follows:

• Seals and reseals using binders such as:

o (unmodified) Class 170 bitumen;

o modified (e.g. polymer) bitumen;

o emulsions with unmodified bitumen; and o emulsions with modified bitumen.

• Geotextile seals and reseals. Often these designed and constructed to become a Strain Alleviating Membrane Interlayer (SAMI) or Strain Alleviating Membrane (SAM). Refer to Sections 4.4.5 and 4.4.6.

• Seals and reseals using modified bitumen that are designed and constructed to become a SAMI or SAM.

4.4.1.1 Functions

The functions of a seal coat include the following:

• To protect the underlying pavement and subgrade from water and other damaging effects of the environment.

• Where used as a surfacing to also provide a wearing surface that is resistant to abrasion by vehicles, has adequate skid resistance and adequate surface texture.

• Where used as a SAM or SAMI to also mitigate the risk of cracks reflecting though the SAM or SAMI, but only where movements are not excessive and the pavement is moving horizontally.

The functions of a reseal are the same as a seal but also include the following:

• To maintain and protect the existing asset.

• To improve or restore skid resistance and surface texture such that they are adequate for the circumstances.

4.4.1.2 Appropriate Uses

Sealed pavements may exhibit block cracking, stripping or ravelling over time (caused by environmental effects) and loss of skid resistance or surface texture (e.g. caused by polishing of the aggregate or wear). In appropriate circumstances sealing or resealing treatments can be effective techniques to correct these deficiencies. Depending on the type, condition and extent of cracking other treatments may be more effective or also be required (e.g. see Sections 4.4.5, 4.4.6 and 4.4.7)

Resealing may be used as a preventative maintenance treatment to arrest surface deterioration and extend the service life of a pavement. In such cases an effective seal is restored to the existing pavement to prevent the entrance of air and water and thus protect underlying pavement layers and the subgrade, and to reduce oxidation of the existing bitumen surface.

If a pavement shows distress caused by structural inadequacy, a reseal treatment may be an appropriate temporary measure to arrest further deterioration until an overlay or other rehabilitation measures are taken (i.e. placement of a 14/7 double/double seal used as a ‘holding measure’).

In tropical and temperate climates, the presence of a permeable surfacing is a critical factor in accelerating pavement deterioration. In arid climates, ravelling and surface break-up caused by oxidation are important factors leading to pavement deterioration.

It is noted that the Pavement Design Manual (PDM) (QDMR 2009) requires a Polymer Modified Binder (PMB) seal at the bottom of all asphalt surface layers except in areas subject to high shear forces. If the omission of this seal is proposed specific advice is to be sought from Pavements and Materials and the decision is to be subject to a risk assessment. Refer to the PDM (QDMR 2009) for further details.

4.4.1.3 Inappropriate Uses

In heavily trafficked sections or sections with ‘tight’ horizontal curves, the treatment may be inappropriate (e.g. due to stripping). It may also be inappropriate to apply seals or reseals at intersections or the like with

A sealing or resealing treatment does not provide any additional structural strength to a pavement unless several layers build up to approach the thickness of an asphalt surface course. Therefore, if a pavement exhibits load-associated crocodile, transverse, or longitudinal cracking, a seal or reseal will not remove the cause of the distress. However, SAM and geotextile seals and reseals have proven cost-effective in reducing reflective cracking and extending the life of pavements (see Sections 4.4.5 and 4.4.6). When cracks are being sealed or resealed, maintenance patching should be thorough and carried out well in advance of the sealing or resealing process.

A seal coat cannot correct shape deficiencies such as rutting, shoving or (high) roughness. However, a seal coat may be used in conjunction with other rehabilitation techniques.

It is essential that sealing/resealing be done before cracking or ravelling becomes widespread, there is significant irreversible loss of pavement shape or degradation of pavement materials. For instance resealing an asphalt surface that is extensively fatigue cracked is not appropriate.

Seals and reseals generate the highest road tyre noise levels. Therefore this treatment is usually inappropriate wherever road related noise levels exceed the criteria given in the Road Traffic Noise Management - Code of Practice (QDMR 2008b). Reference must be made to the Road Traffic Noise Management - Code of Practice (QDMR 2008b).

Seals should not be placed over lively asphalt patches. If asphalt patches are lively options include either;

• removing, replacing them/patching with a suitable material other than asphalt, and sealed; or

• delaying enrichment until the youngest asphalt patch is at least six months old.

It is noted that, for pavements with more than one layer of asphalt at locations subject to heavy braking and/or tight cornering (e.g. intersections, roundabouts and approaches), excluding the waterproofing seal at the bottom of the asphalt surface layer can reduce the risk of shearing, but increase the risk of stripping of lower layers. Provision of a seal in these locations is not mandated. Refer to the PDM (QDMR 2009) for further details.

Reference should also be made to Section 4.11 which deals with the use of polymer modified and multigrade binders.

Sealing over a pavement can trap moisture contained within it. Therefore care should be exercised when sealing or resealing over a pavement or asphalt that contains excess water.

4.4.1.4 Materials

Relevant TMR specifications and technical standards include the following:

• MRS11 and MRTS11 Sprayed Bituminous Surfacing (Excluding Emulsions) (TMR 2010f).

• MRS12 and MRTS12 Sprayed Bitumen Emulsion Surfacing (TMR 2009a).

• MRTS17 Bitumen (TMR 2009c).

• MRTS18 Polymer Modified Binder (TMR 2009d).

• MRTS19 Cutter Oil and Flux Oil (TMR 2009e).

• MRTS20 Cutback Bitumen (TMR 2009f).

• MRTS21 Bituminous Emulsion (TMR 2009g).

• MRS22 and MRTS22 Supply of Cover Aggregate (TMR 2010g).

• MRS57 and MRTS57 Geotextiles for Paving Applications (TMR 2009j).

4.4.1.5 Design Considerations

Seals and reseals must be designed in accordance with the PSM (TMR 2011b), and the Update of the Austroads Sprayed Seal Design Method (Austroads 2006c). They must also comply with TMR policies, contracts, specifications and technical standards. AGTPT Part 3 (Austroads 2009a) provides additional guidance.

4.4.1.6 Construction Considerations

Seals and reseals must be constructed in accordance with the PSM (TMR 2011b) and TMR policies, contracts, specifications and technical standards. AGTPT Part 3 (Austroads 2009a) and the Update of the Austroads Sprayed Seal Design Method (Austroads 2006c) provide additional guidance.

It is critical that the aggregate adheres properly to the binder and that the binder adheres to the existing surface. Therefore, the preparation of the existing surface, close monitoring of the weather conditions, application of the bituminous material and aggregate, rolling and traffic management are all key considerations. Cleaning off the excess aggregate is critical in preventing whip-off and possible windshield damage. The appropriate amount of pre-coating and adhesion agents can be beneficial, in fact absolutely necessary in some circumstances, in preventing loss of aggregate.

4.4.1.7 Expected Performance and Comments

All sprayed bituminous treatments are susceptible to damage early in their life by weather and (construction or general) traffic, particularly during the first few hours. Traffic must be managed to avoid damage to them (e.g. reduce the speed limit until the binder has set and the aggregate is well embedded and adhering to the binder, use sidetracks). In addition the following discussion should be noted.

4.4.1.7.1 Excess Binder

The main forms of corrective treatment for removing excess binder are as follows:

• If the binder is still soft enough, spread and roll more aggregate into the binder. The aggregate size used for spreading is generally smaller than the original aggregate. The most common aggregate used for this type of operation is 7 mm aggregate.

• If the binder is not picking up and the situation is not serious or likely to cause safety issues, wait for warm weather to soften the binder, then spread and roll more aggregate into it (alternatively, crusher dust could be spread).

• By applying a high pressure water spray specially designed for the task and applied by a suitably qualified supplier/applicator.

• By using products made from gilsonite (a hard natural rock asphalt with a softening point of 140°C).

When applied, the gilsonite combines with bitumen to form a compound that has a much higher softening point than straight bitumen. After application of the gilsonite crusher dust or 7 mm aggregate must be spread to absorb the excess but “modified” binder.

If none of the above work then a reseal, with the design adjusted accordingly, or removal of the seal in conjunction with the construction of a new seal may be required.

4.4.1.7.2 Stripping

Stripping of aggregate can be caused either by loss of adhesion as a result of water being present on the aggregate, due to weather conditions (e.g. it is cold), by a deficiency in the quantity of binder applied or by incorrect seal design (e.g. variable spray rates not used where they are required). It can also be caused by the action of traffic.

If the aggregate is stripping because of moisture and it is possible to do so, trafficking should be delayed until all water has evaporated. Alternatively, while the seal is drying, the aggregate may be rolled and slow, controlled traffic allowed on it after some adhesion has occurred. Traffic management should remain in place until the seal has completely dried out and the binder has set up.

If there is a deficiency in binder quantity, then partial or total stripping may occur. For partial stripping, a surface enrichment or a light reseal may be applied. For total stripping, resealing with an appropriate design will be necessary.

4.4.1.7.3 Flushing

If sealing is carried out on cool days, followed by two or more days of continuous hot weather, the binder may flush up because of the amount of cutter required when the sealing work was undertaken. The corrective treatments for removing excess binder stated above could be used (see Section 4.4.1.7.1).

4.4.1.8 Further reading

Relevant publications include the following:

• The PSM (TMR 2011b).

• AGTPT Part 3: Pavement Surfacings (Austroads 2009a).

• AGTPT Part 4K: Seals (Austroads 2009d).

• AGTPT Part 4F: Bituminous Binders (Austroads 2008b).

• AGTPT Part 9: Pavement Work Practices (Austroads 2008c).

• Guide to the Selection and Use of Polymer Modified Binders and Multigrade Bitumens (Austroads 2006b).

• Update of the Austroads Sprayed Seal Design Method (Austroads 2006c).

• Pavement Work Tips (Austroads and AAPA various).

• Bituminous roads in Australia (Dickinson 1984).