Resources

Purpose

This notice provides the guidance to assist maintenance contract managers to more easily respond to proposed works aimed at remedial works for areas of binder rise.  The guidance has been released, will come into effect from 1 April 2019 and should be applied to all resealing works in the Maintenance Renewals Programme.

The standard interventions included within the guidance are proven road surfacing counter-measures that deliver beneficial outcomes by improving long-term surfacing performance. Further work is continuing assessing other interventions and once they have been approved, they will be included in future updates of this guidance. 

The guidance is now available online at https://www.nzta.govt.nz/roads-and-rail/highways-information-portal/

General information

Definition of flushing

Flushing is a pavement surface defect with bituminous binder near the top of or above the surfacing aggregate, which results in minimal surface texture.  (Note: Bituminous binder is made up of bitumen, bitumen additives and fine aggregate particles generally smaller than 2.36 mm. This bituminous binder, or mastic, will be referred to as binder throughout this document.) In severe cases, the binder covers the aggregate completely.  In hot weather, the binder in a flush surface can become sticky and soft, leading to pick-up and tracking of the binder.  In wet conditions, a flush surface can be a safety hazard due to very poor skid resistance.

What causes flushing?

The main cause of the flushing must be identified before the most appropriate flushing treatment can be selected. 

Flushing can result from:

1. Chip embedment into soft substrate
2. Layer instability
3. Moisture-induced binder rise
4. Poor seal selection, design and construction
5. Chip breakdown
6. Insufficient time for pre-seal repairs to cure prior to placing the reseal
7. End-of-life reduction in macrotexture. 

1. Chip embedment
Chip embedment occurs when sealing chip is punched into a weak underlying pavement while the binder level remains unchanged. Chip embedment occurs during vehicle loading, resulting in progressive reduction in chipseal macrotexture and wheel path flushing.
Chip embedment can occur in first coat seals due to weakness of the basecourse surface caused by a build-up of fine particles on the surface, too much moisture in the surface, or a combination of the two just prior to construction of the first coat seal.

2. Layer instability
Layer instability occurs where successive chipseal layers combine together over time to produce an unstable mix of binder and aggregate.  Layers becomes unstable as binder-to-stone ratio increases, resulting in chipseals flushing prematurely.  In extreme cases, structural instability within seal layers can lead to shoving and shear failures.

Key indicators of layer instability include:

• Decreasing seal lives – achieving less than half the expected design life.
• Maintenance surfacing costs – increasing at an abnormal rate.
• Multiple seal layers (generally greater than four).
• A binder-to-stone ratio of greater than 20%.  The binder mass used in this calculation is the mass of bitumen plus aggregate fines passing the 2.36 mm sieve.
• A pavement age of greater than 40 years.

See references for further information on layer instability.

3. Moisture induced binder rise
Research has shown that water can strip binder from chip near the bottom of multiple chipseal layers, and this water within the pavement generates a pressure (from pumping action of heavy vehicles and/or temperature) capable of pushing the binder upwards and depositing on the road surface which will flush after time. If a core is taken, this can be seen as a binder-rich top layer and a lean layer at the bottom of the seal.  On the surface of a road, water blisters, or volcanoes, are a common sign of water-induced flushing.  They are caused by water vapour venting through the binder.

4. Seal – selection, design, construction and repair
Successful chipseals rely on appropriate seal selection, use of proven design methods, and good construction practices.  Issues that can contribute to chipseal flushing include:

• Inappropriate seal selection that requires excessive bitumen to be applied to construct the chipseal, e.g. applying a Grade 2 chipseal over an existing coarse Grade 2 seal.
• Using aggregate that is not suitable for the SCRIM site category, resulting in short seal lives and contributing to layer instability.
• Resealing too soon over fresh chipseals that have not had sufficient time for cutters in the bitumen to evaporate and for the chip to re-orientate into a compacted layer.
• Application of too much bitumen due to incorrect seal design calculations, errors in bitumen application, or out of season sealing where application rates are increased to ensure chip retention.
• Repair of failed chipseals – the two most common repair methods (described below) can lead to early flushing of repaired chipseals.  Two repair methods are generally used to repair chipseals where chip loss has occurred: either replace chip that has come off with chip that is one grade smaller, or replace it with the same size chip along with additional bitumen.

5. Chip breakdown
Research carried out by WSP Opus International has found that flushing is often associated with high quantities of aggregate fines found in the seal matrix.  These fines are generated by the sealing chips breaking down under traffic.  The fine material falls or is driven into the interstices between the chips and adds to the binder to form a mastic. The binder mastic has a greater volume than the bitumen binder and consequently the level of the binder mastic rises and can eventually flush.

See references for further information on flushing caused by chip breakdown.

6. End of life reduction in macrotexture
The designed mode of failure for chipseals in New Zealand is loss of texture therefore it is expected that a successfully constructed chipseal will quickly lose texture initially in the first year which is regarded as “bedding in” where a certain amount of embedment and chip reorientation takes place. Subsequently, there will be a slow reduction in texture until the seal has reached the end of its life and approaches a flushed condition. The reason this is mentioned here is to remind us that not all flushing is caused by a fault and in these cases the treatment is to simply apply another seal.

Flushing prevention

As stated previously, flushing of road surfaces is a natural process that occurs as pavements reach the end of their design lives. The onset of flushing can be delayed and potential pavement life can be extended when flushing and other pavement failures are well-managed by following good maintenance practices.

To minimise the risk of chip embedment, site supervisors should ensure that:

• Basecourse surfaces meet the NZTA B/2 dry back requirements.
• All running course and fine material is removed from the surface.
• A tight stone mosaic surface is achieved when the first coat seal is applied.

Water can enter the chipseal layers and pavement in various ways; however, it is important to ensure that basecourse layers never become saturated. Maintaining adequate pavement drainage minimises the risk of flushing in road surfaces.

Chip can embed and cause flushing when a new chipseal is constructed over uncured or fresh bituminous surfaces (e.g. chipseals, asphalt mixes, slurries and micro surfacing).  It is advisable to leave these types of surfaces for a minimum of six months, and preferably for 12 months, before attempting to seal over them.

Constructing chipseals outside the recognised sealing season should be avoided as these surfaces can be prone to premature flushing.  This typically occurs because too much bitumen is applied to ensure chip retention, or because additional bitumen is needed to repair chipseals that have lost chip due to unfavourable weather conditions.

Flushing treatment options

The treatments below have been ordered from lowest to highest overall cost; however, costs are likely to vary from region to region, depending on local availability of equipment and materials.

Treatment of flush surfaces

Notes:

Low risk

Medium risk

 High risk         

¹ Geotextile seals are most suited to straight sections of road and can be difficult to construct. Refer to manufacturer’s instructions.
² Some suggest that water cutting of hot surfaces is less effective than cooler surfaces.  A single application is often insufficient, which is indicated in the life of the treatment.
³ The asphalt requires internal voids to allow absorption of free binder without saturating the asphalt. For this reason, Open Graded Porous Asphalt is generally used.
⁴ The pavement deflections and the curvature need to be appropriate.  See New Zealand Guide to Pavement Evaluation and Treatment Design.
⁵ In regions where pavement aggregates are readily available, granular overlays can be cheaper than recycling.

1. Application of sealing chip
A relatively low-cost treatment for flushing involves applying sealing chip to flush road surfaces to increase chip content. This treatment uses a range of different processes for the chips:

• Dry chip
• Hot chip
• Pre-coated chip
• Hot pre-coated chip.

Each of these chip variations will generally require the use of cutter to soften the binder surface and increase adhesion of the chip.  The cutter must be applied is a safe manner. Care should be taken to ensure the cutter doesn’t over-wet the pavement surface.

Sealing chip size should be chosen based on the depth of free binder at the site to be treated. The intention is that the sealing chip is retained, and that the top of the chip remains above the surface of the flushing once embedded.  A blend of sealing chip sizes can be used to ensure chip retention out to the edges of an area of flushing where the depth of binder decreases.

Rolling can be completed by using either a steel drum or pneumatic-tyred (PTR) or combination rollers. Steel drum rollers can provide good initial chip embedment into an area of flushing, but the number of passes should be limited to minimise chip crushing.  Pneumatic-tyred rollers should be used on sites where wheel paths are rutted, to ensure chip embedment occurs.

1.1 Dry chip
Dry chip is applied to flush areas after an application of cutter, and this is followed by rolling. This treatment is limited to very warm weather conditions to facilitate chip adhesion.

Dry chip

Summer

Spring/Autumn

Winter

1.2 Hot chip

Hot chip is produced by running sealing chip through an asphalt plant. The chip should be heated so that chip temperature is above 100°C when applied to the road surface. A cutter may be required, particularly on aged binder, to improve adhesion prior to application of the heated chip. This treatment is limited to warm weather conditions to assist with chip adhesion.

Hot chip

Summer

Spring/Autumn

Winter

1.3 Pre-coated chip
Pre-coating involves applying a light application of bitumen or diesel to the sealing chip.  Pre-coating can use diesel, bitumen or adhesion agent, or any combination of these. Emulsion can also be used for pre-coating, but the method of application needs to allow for sufficient breaking of the emulsion. The amount of pre-coat to be applied to the chip will be determined by the pre-coat material or blend, the method of applying the pre-coat, and desired chip coverage. A cutter may be required, particularly on aged binder, to improve adhesion prior to the application of the pre-coated chip. This treatment is limited to warm weather conditions to assist with chip adhesion.

Pre-coated chip

Summer

Spring/Autumn

Winter

1.4 Hot pre-coated chip
Hot pre-coated chip is the ultimate treatment for applying sealing chip to flush road surfaces. In addition to using a chip’s heat to soften the adjacent binder, the pre-coating on a chip creates a stronger bond between chip and binder than is possible with a standard, uncoated chip. A cutter may be required, particularly on aged binder, to improve adhesion prior to application of the hot pre-coated chip. This treatment is best completed during warm weather conditions to achieve maximum chip adhesion.

Hot pre-coated chip

Summer

Spring/Autumn

Winter

2. Sandwich and combination seals
Sandwich seals can be used to absorb surplus binder on a road surface. They are particularly effective for treating flushing which is the result of seal layers approaching an unstable condition.  The aim is to improve the binder/stone ratio by absorbing surplus surface binder into the voids created during seal construction.

Figure 1: Sandwich seal (Chipsealing in New Zealand 2005)

A sandwich seal is constructed as follows:

• A layer of large chip is applied and spread directly onto the existing surface (with chips more tightly packed than for a two-coat seal).
• A light application of binder is sprayed on.
• A layer of smaller chip is applied over top of the binder.

Sandwich seals are most commonly constructed using a grade 2/4 chip.  Grade 3/5 sandwich seals are not recommended as the smaller voids absorb less surplus binder, resulting in lower rates of treatment success.

Combination seals are a variation on sandwich seals.  For this type of treatment, large chip is applied only to areas where flushing has occurred, while bitumen and small-sized chip is applied to the full pavement width (including over the repaired area of flushing) as a void fill seal. This treatment option is particularly useful to correct minor rutting where this occurs in tandem with wheel path flushing.

Combination and sandwich seals

Summer

Spring/Autumn

Winter

3. Geotextile seals
A geotextile is generally a non-woven, needle-punched polyester or polypropylene fabric.  Polyester is preferred when using hot bitumen due to its higher melting point.

A geotextile seal is constructed by firstly spraying a uniform tack coat of binder at a light application rate of approximately 0.5- 0.6 L/m2(residual).  This can be adjusted to cater for the level of free binder on the surface. 

The geotextile is laid over the tack coat using a frame to tension the fabric to minimise creasing. Once the fabric has been rolled by a pneumatic-tyred roller (PTR), a chip seal is applied using standard sealing practice.

Figure 2: Geotextile seal (Chipsealing in New Zealand 2005)

The manufacturer guidance should be considered when determining the binder application rates. Care should be taken to allow for surplus binder on the pavement surface, as this needs to be balanced with applying sufficient binder to retain the sealing chip over winter.  It is recommended that only two-coat seals be used over geotextile.  In New Zealand, experience with using geotextile seals to treat flush pavement surfaces is still limited, and results to-date have not always been successful.

The main benefit of using a geotextile seal to treat flushing is the prevention of chip embedment into the underlying flush surface, so reductions in bitumen application rates that allow for chip embedment should not be applied.

Figure 3: Geotextile being laid over tack coat

Geotextile sealing should be performed in warm weather to allow surplus binder to rise into the fabric to retain the sealing chip.  Geotextile sealing should be limited to relatively straight, stress free sections of road due to the risk of the fabric slipping on curves and high stress areas.

Geotextile seals

Summer

Spring/Autumn

Winter

4. Water cutting
Water cutting involves using high pressure water to remove excess binder from the road surface to reduce the overall amount of binder.  This is a fast and efficient means of removing surplus binder and restoring surface texture, and can be undertaken at any time of year.   Water cutting may not improve skid resistance when underlying sealing chip has been polished.

Care should be taken when using water cutting to reduce flushing on the following sites:

• Water cutting is not recommended on first coat seals as there is a risk of damaging the chipseal and basecourse pavement.
• Water cutting is not recommended on pavements with visible fatigue cracking because water will be pushed into the pavement, resulting in more extensive pavement failure.
• Water cutting at the same location on multiple occasions can have a detrimental effect on pavement structure.
• On roads where layer instability is identified, water cutting should only be used as a temporary treatment until a more permanent solution such as sandwich sealing or recycling can be programmed.
• Variations to the normal water cutting practice such as reduced cutting speed and changes to cutting pressures may be required where the binder is very soft and mobile.

Water cutting

Summer

Spring/Autumn

Winter

5. Asphalt overlay
Overlaying a flush pavement with asphalt can be considered where:

• Other treatments are not suitable for technical reasons, such as local weather conditions or plant availability
• Flushing corresponds with pavement roughness issues, but the pavement is otherwise sound
• A rapid means of making the surface safe is required.

High pavement deflections and curvatures will have a detrimental effect on the performance of an asphalt overlay, please refer to the NZ Guide to Pavement Evaluation and Treatment Design.  A pavement’s structural condition needs to be assessed to reliably estimate the life of an asphalt overlay.

In areas of low stress, an open graded porous asphalt (OGPA) may be suitable as the high void content of the mix has the capacity to absorb surplus binder from the underlying surface.  In areas of moderate to high stress, a high strength (HS) OGPA may be used. 

OGPA may not be appropriate on pavements with moderate to severe wheel path rutting, as the OGPA may not be able to drain effectively. It is also not recommended to use stone mastic asphalt (SMA) mixes to treat flushing as these can be sensitive to small changes in binder content, which can in turn lead to flushing of the mix.

Asphalt overlay

Summer

Spring/Autumn

Winter

6. Recycling
A recycling option is generally appropriate where layer instability is suspected, and where a surface treatment option such as a sandwich seal is expected to have a short life. Key indicators for layer instability would include shallow shear, very early onset of flushing, binder to stone ratio greater than 20%.  It is essential to undertake a thorough pavement investigation and design as required under the New Zealand Guide to Pavement Evaluation and Treatment Design.

Recycling is an effective way to distribute unstable seal layers into the upper pavement, where testing confirms there are no underlying pavement weaknesses or deep-seated subgrade issues, and where there is sufficient depth in the existing pavement.  This treatment involves milling unstable surface layers into the underlying basecourse to a depth of 200-300mm with the addition of cement (generally 2% by weight), followed by resealing.  Milled surface layers should be limited to 30% or less of the total composite pavement. Additional granular top-up material can be applied to the pavement surface prior to milling to enhance the pavement’s load-carrying capacity or to ensure sufficient granular pavement for the surfacing layer depth.

Recycling

Summer

Spring/Autumn

Winter

7. Granular overlay
Depending on the availability of aggregate, a granular overlay can be more cost-effective than recycling in some areas, e.g. this can be the case for many South Island roads.  An overlay has the advantage of treating deficiencies in pavement depth that lead to subgrade rutting; however, restrictions on increasing pavement height need to be taken into consideration.

Granular overlay

Summer

Spring/Autumn

Winter

Definition of bleeding and tracking

Bleeding of binder occurs when pavement and air temperatures are sufficiently high that the binder in chipseals becomes soft and starts to flow. When the binder is in this soft state, it adheres to vehicle tyres, which then redeposit it on adjacent road surfaces. This is referred to as tracking binder. In New Zealand, bleeding usually occurs over the warmer months from November through to the end of February. Bleeding and tracked surfaces can be differentiated from flushed surfaces because although they will look black they will have reasonable texture.

Causes of bleeding and tracking

Bleeding and tracking can result from:

• Venting surfaces, where moisture escaping from a pavement causes bubbles of binder to rise above a road surface and come into contact with a vehicle’s tyres.
• Flush road surfaces that have not been treated sufficiently before the onset of high summer temperatures. Treatments for flush road surfaces are discussed above, and should be implemented at an appropriate time to remove the risk of bleeding and tracking.
• Fresh maintenance repairs, where areas of excess soft binder occur due to deficiencies in repair design and/or construction.
• Two coat, sandwich and combination chipseals constructed during periods of high summer temperatures.
• Chip rollover on newly constructed chipseals.

It has been found that vehicle tyres, under normal use, can reach down between road surface chips for depths of more than 1mm.  As a result, all seals where the binder has settled to within 1mm of the road surface are susceptible to tracking whenever the binder becomes soft.

1. Tracking from moisture vents
Moisture is sometimes trapped in pavements during construction or maintenance treatment. Water can also enter a pavement either from above, when a surface is not adequately waterproof, or from below, when drainage is ineffective. During hot weather, typically over the summer months, any moisture trapped within a pavement will heat up, and bubbles of vapour will be released.  Warm weather also releases a thin film of binder along with the vapour, and these then combine to create bubbles of binder containing moisture. These bubbles rise and eventually rupture, depositing binder onto the road surface.  This then adheres to vehicles’ tyres and is tracked along the road.

Moisture venting is typically observed from November or December each year, coinciding with the earliest period of consecutive hot summer days, and continues until all pavement moisture has evaporated sometime in the new year.

2. Maintenance repairs
It is quite common in early summer to see binder tracking from chipsealed maintenance repairs that were completed during the previous winter or spring. It can be difficult to successfully construct chipsealed maintenance repairs during periods of cooler wet weather, and any weakness in pavement strength or chipseal construction will often cause bleeding to occur.

Careful consideration should be given to the type, design and construction of chipsealed maintenance repairs during periods of cooler wet weather to ensure that instances of binder tracking are minimised or avoided wherever possible.

3. Construction of reseals during hot weather
The design and construction of some types of chipseal can result in resealing binder being exposed to vehicle tyres following chipseal construction, once a road reopens to traffic. On days when temperatures are high, fresh binder will remain very soft and lively, meaning that chips can roll over and expose binder to the surface, and that binder is then picked up by vehicle tyres and tracked.

Construction of chipseals that are likely to cause binder tracking on hot days should be avoided wherever possible, particularly when consecutive hot days are forecast.

On New Zealand roads, binder tracking often results from flush road surfaces that have not been treated appropriately before the onset of high summer temperatures. To ensure this does not happen, isolated patches of flushing must be treated as a pre-reseal repair before resealing can be carried out. Treatments for flush road surfaces are discussed above and should be implemented at an appropriate time so that bleeding and tracking risks can be avoided.

Public information and protection

Whenever sustained periods of hot temperature occur that result in network-wide bleeding and tracking, and when there are also insufficient resources available to treat the problem, the following courses of action are available to help alleviate public risk and concern: provision of accurate information, traffic management, and resealing.

1. Information on tracking
During network-wide or other significant binder bleeding events, the risk to motorist safety and the potential for damage to either vehicles or the road surface itself is often very low, particularly during dry weather. The extent of bleeding and tracking can make it impossible to treat the problem on a network scale; and in any case this is sometimes not required. Nevertheless, accurate information should be provided to motorists via the news media and traffic management devices such as variable message signs (VMS).

2. Traffic management
During significant bleeding events, localised areas of the network may require increased management and information provision. Traffic management devices should be used for protection where safety concerns or vehicle damage may occur. This would generally include a temporary reduction in the posted speed limit, and additional information may also need to be disseminated using variable message signs (VMS).

The use of temporary reductions in posted speed limits and traffic management devices will require careful planning and implementation, particularly where long lengths of road network are affected.

3. Resealing in hot summer temperatures
During hot summer weather, traffic management – including temporary speed restrictions – is essential on newly constructed chipseals where chip rollover and binder tracking is likely to occur. This helps to minimise the risk of damage to the chipseal and to vehicles from sticky surplus chip being picked up on vehicle tyres and thrown around.

One effective solution is to avoid constructing the types of sealed surfaces that by design result in vehicle tyres coming into contact with fresh binder when the weather is hot. These seals include two coat seals, sandwich seals, combination seals and wet lock seals.

Treatments for bleeding and tracking

There are three main reasons to treat tracking and splashing of binder:

• Safety risks – specifically skid resistance.
• Protection of vehicles – from bitumen and thrown sealing chip.
• Protection of the road asset – typically mitigation of chipseal patches pulled from the road surface.

Treatments should be selected and applied following careful consideration of safety risks and required levels of protection for both the road pavement and motorists.

Where bleeding has resulted from flushing that has not been sufficiently treated before the onset of hot summer temperatures, an appropriate treatment should be selected and applied from the section in this document that covers treatment of flush surfaces. Other treatment alternatives are listed below.

1. Cooling the road surface using water
Spraying water onto a road surface using a water cart can help to cool the surface and reduce the onset of bleeding and tracking.

This treatment is only effective for relatively small, localised areas, and is typically applied to prevent the onset of bleeding and tracking, particularly in urban areas. Applying water to prevent bleeding and tracking has significant limitations for network-wide treatment.
It is recommended that water not be applied once bleeding and tracking has occurred.  Research has shown that skid resistance decreases significantly when water is applied to a road surface over freshly tracked binder.

2. Application of sealing chip
Sealing chip can be applied to flushed and bleeding road surfaces to minimise tyre contact with the binder.  Chip application reduces the risk of binder splashing, binder tracking, and surface delamination where a flushed road surface adheres to hot vehicle tyres, resulting in patches of chipseal lifting up from the road surface and being tracked along the road.  

The following process should be followed to ensure optimum success of this treatment and to minimise potential risks and damage to road users:

1. The sealing chip applied should be of an appropriate size for the depth of free binder and the size and shape of the voids in the underlying surface. This would generally be a combination of chip sizes from grade 3 to grade 5, i.e. a grade 3 and 4 mix, or a grade 4 and 5 mix. Use of these chip mixes will allow for retention of the smaller grades of chip as the depth of binder decreases toward the outer edges of the flushing. It is important that, after rolling, the top of the chip remains above the top of the free binder.
2. Chip may be heated and/or precoated before application to increase adhesion and embedment of sealing chip into the surplus binder.
3. Chip application rates should be sufficiently light that chip can easily lie on its greatest dimension and re-orientate to sit into the underlying surface texture. This will allow for maximum embedment and binder displacement up the chip, and in turn will achieve optimum chip retention. It is preferred that two or more light applications of chip be applied rather than one heavy application, which may result in chip not embedding well into the free binder and loose chip appearing on a road surface.  It is vital that excessive loose chip is not left on the road surface as this can cause a safety hazard, particularly for motorcycles.
4. Typically, rolling of a treated surface should be fully completed before a road is reopened to normal traffic.
5. A drag broom may be used to redistribute chip during the final rolling stage and at intervals during managed traffic rolling.
6. Application of sealing chip should be completed on the same day that bleeding occurs or immediately prior to the hottest part of the first hot day following the onset of bleeding and tracking.
7. Traffic management will need to be established as required for the completion of chip sealing, and should remain in place for the duration of the work and until any loose chip has been removed. A 30km/h temporary speed limit is required.

NOTE: Sand, crusher dust and very fine aggregate should never be used to treat bleeding during hot temperatures because the fine material will become encased in binder and will increase the volume of binder mastic, resulting in an increase in the amount of surplus binder available for bleeding on the next hot day.

3. Water cutting
Water cutting can be used to remove flush, bleeding and tracking binder. Guidance on water cutting is provided in the relevant section above – see Flushing Treatment Options, section 4. Water cutting. Water cutting during warm summer conditions can be challenging as the binder can be very soft and mobile. This may require variations to the normal practice such as reduced cutting speed and changes to cutting pressures.

References

Alderson, A (2006) Update of the Austroads Sprayed Seal Design Method, Austroads Report AP-T68/06

Austroads (2014) Pavement Work Tip No. 25; Geotextile Reinforced Seals

Gribble, M (2017) New Zealand Guide to Pavement Evaluation and Treatment Design, NZ Transport Agency

Herrington PR, Kodippily S, Henning TFP (2015) Flushing in Chipseal, New Zealand Transport Agency research report 576

Jones, A (2016) Improving Layer Instability Prediction and Management, Report for NZ Transport Agency by Opus International Consultants

NZ Transport Agency (2011) Chipsealing in New Zealand, Chapter 3: Practice Note 1; Combination Seals. www.nzta.govt.nz/resources/chipsealing-new-zealand-manual/practice-notes.html

Transit New Zealand, Road Controlling Authorities and Roading New Zealand (2005) Chipsealing in New Zealand. Accessed 14 June 2017, www.nzta.govt.nz/resources/chipsealing-new-zealand-manual/

Publication details

Author: New Zealand Transport Agency
Published: February 2019
Version: 1
Found at: https://www.nzta.govt.nz/roads-and-rail/highways-information-portal/

Further information

To submit documents or if you have any questions or feedback, please contact the System Design and Delivery Pavements Team at pavements@nzta.govt.nz

David Darwin                 Vanessa Browne