CHIP SEAL

CHIP SEAL EMULSION APPLICATIONS

This paper gives a general overview of factors that need to be taken into account when considering chip seal application.  These notes are not intended to be used in the design of an actual chip seal application.

Should a specific design be required, samples of asphalt and aggregate chips should be sent to MeadWestvaco along with traffic design and an emulsion application can be calculated.

Traffic

The number, type and combination of vehicles has a marked influence on the performance of a seal.  Heavy vehicles have a much greater influence on the performance of a seal than light vehicles (a generally accepted conversion is that a heavy vehicle with an 800 kPa tire pressure is equivalent to 60 light vehicles).

It is also believed that a seal requires a minimum number of vehicles a day to keep the asphalt “alive” and flexible (in the order of 50 vehicles per day).

Aggregate

The aggregate in a seal has four main functions:

1. it provides resistance to the abrasion of moving wheel loads and transfers the wheel load to the underlying pavement structure;
2. it provides a skid resistant surface;
3. it provides a structure to accommodate the elastic and impervious bituminous binder and has sufficient voids to prevent the binder flushing to the surface under loading;
4. it protects the binder from UV rays.

Aggregate-related factors that affect performance are:

• Shape, Nominal Size and Grading;
• Spread Rate.

Shape, Nominal Size and Grading

The shape of the aggregate affects its interlocking in the compacted layer and, hence, the stability of the seal.  The more angular the aggregate, the better the interlock because there are more points of contact.  The shape and size also affect the void content in a seal and the ability of the seal to displace surface water (macro texture).

A single-sized stone develops good interlock and provides maximum contact between the tire and stone surface.  This increases the friction area, causes the stone to resist polishing and abrasion and gives good skid resistance, provided the correct quantity of binder is used.

More voids are available with large single-sized aggregates to accommodate a variation in binder application rate than in small single-sized aggregates. 
The smaller the aggregate size, the greater the possibility of the voids in the layer being filled with binder.  This will cause flushing unless very strict control is exercised during construction.

Larger aggregate allow more binder to be used, resulting in a more impermeable, longer lasting seal.

The use of aggregate that does not have a uniform size results in firm tire contract over a smaller area (which decreases the skid resistance, especially in wet weather), a higher noise level (because the larger particles are spaced further apart), loss of the larger stone by traffic action, and concentrated wear on the larger particles.                                                                                                       Changing specifications could lead to current design methods not being appropriate to ensure optimal performance.

Spread Rate of Aggregate

The aggregate protects the substrate against traffic abrasion and should be applied at such a rate that complete cover with a uniform mosaic is achieved.  The aggregate should lie shoulder –to-shoulder, in a single layer and in a tightly knit pattern.

Excessive ultra-violet damage to the binder raveling of the seal and subsequent damage of the lower pavement layers may occur if the spread rate is too low.  If the spread rate is too high, the excess aggregate will be forced into the mat, leading to whip-off of the bonded aggregate.  Degradation due to crushing and grinding of the aggregate, unnecessary waste of material, and claims for damage resulting from aggregate whip-off and broken windscreens may occur.

Asphalt Emulsion

Function

The service life and performance characteristics of a seal depend on good adhesion between the binder, stone and road surface and on the durability and flexibility of the binder under different climatic conditions.  Adhesion is one of the paramount functions of the binder.  Loss in retention of the aggregate, the degree of aggregate whip-off and durability are all related to the adhesive forces developed by the binder.  That depends on the type, grade and amount of binder applied.

A properly selected binder fulfills two functions – it develops both adhesive and cohesive strength.  Initially it is fluid to allow time for placing and wetting of the stone and then rapidly becomes harder.  The cohesive strength of the binder facilitates opening to the traffic and prevents the stone being pulled out of or whipped off the surface.

The binder should be able to withstand “softening-up” under the higher temperatures encountered in service and to retain the stone under the action of moving wheel loads.  On the other hand, at colder temperatures, the binder should stay flexible for as long as possible to prevent reflection cracking, to allow it to adapt to road deflections and to prevent, as far as possible, the ingress of moisture into the base.

Factors affecting seal performance

Binder-related factors that affect the performance of a seal are:

• Emulsion properties;
• Grade of base bitumen used;
• Spray rate.

The use of polymer modified emulsions has increased greatly in recent years because of improved performance regarding adhesion, elasticity, lower sensitivity to bleeding and durability – even at sub-zero temperatures.  Although these products are more expensive than conventional binders are, their improved properties can ensure extended pavement life, less maintenance and, consequently, lower life-cycle costs.

Emulsion Properties

Factors affecting the performance of a bitumen emulsion for chip seal application are as follows:

1. Viscosity – viscosity of the emulsion should be sufficiently high that no run-off of the emulsion occurs before aggregate chips are placed in the emulsion.
2. Stability – because of the low levels of emulsifier present in chip seal emulsions, these emulsions are inherently unstable.  The emulsion should be robust enough to stay together until it is sprayed on the road.
3. Rate of Break – this will determine how soon the road can be opened to traffic without excessive aggregate loss.  This performance characteristic is in direct opposition to emulsion stability and a balance needs to be struck between these two factors.
4. Cohesion/Adhesion – the emulsion should adhere to the stone surface.  Once the emulsion has cured, no stripping should occur under stress.

Grade of Base Bitumen Used

The selection of the appropriate base grade of bitumen to be used in the emulsion will be governed by climatic conditions at local ambient temperatures.

Generally, an increase in the softening point of the bitumen will allow a higher emulsion application rate to be used for similar conditions.

An increase in the softening point of the bitumen can be achieved by selecting a harder grade of bitumen or through the use of polymer introduction into the bitumen emulsion.

Spray Rate

For maximum performance the optimum amount of binder should be correctly determined and applied during the construction of seals.  A minimum amount is required to hold the stone firmly in place and bind it to the underlying surface.  There is also a maximum amount, which, if exceeded, will overfill the voids in the compacted layer, causing flushing and result in reduced skid resistance, particularly in wet weather.

The binder application rate which results in optimal performance is determined by the type of binder selected, volume of the voids in the compacted stone layer, the shape and size of the stone, the amount and type of traffic and the condition of the underlying surface.  Adjustments in spray rates are required for steep gradients.

Selection of Appropriate Surfacings

General

The selection of appropriate surfacings for both new construction and reseals, based on the experience of road authorities and practitioners, is discussed in this section.

For surfacings on new construction, the most important influencing factors, which identify those surfacings that will perform well under specific situations, are provided in tables.  A process is followed whereby inappropriate surfacing types are eliminated.

Surfacings on New Construction

A surfacing on new construction is defined as the first surfacing on a newly constructed road or street.  Guidelines for the selection of appropriate initial surfacings are discussed under the following headings:

-          Traffic Volume

-          Traffic Action

-          Gradient

-          Maintenance Capability

-          Surface Texture Required

-          Construction Techniques

-          Environmental Conditions

-          Quality of the Base

-          Special Conditions

-          Initial Cost Basis for Comparison

The recommended surfacing is based on history of good performance.  However, certain seals that are not recommended for specific situations may still perform reasonably well and their performance will depend largely on the skills and experience of local practitioners in the areas in which they are used.

Traffic Volume

Traffic volume is expressed as the number of equivalent light vehicles (elv) per lane per day, as follows:

            ELV = L + 60H

Where

            L = Number of Light Vehicles / Lane / Day

            H = Number of Heavy Vehicles / Lane / Day

It is usual practice to use surfacing seals for roads carrying from about 125 to 20 000 equivalent light vehicles (elv) per lane per day.  For roads carrying more traffic than this, a different type of surfacing is usually recommended.  However, there are surfacing seals that have performed well under much greater volumes of traffic (40 000 elv).  For roads carrying greater traffic volumes, all aspects relating to the materials, design and construction of the surfacing seal should be studied very carefully.

Lighter types of surfacing seals, such as sand seals, should only be used for roads carrying up to 2 000 elv per lane per day.  Where these seals are used for roads carrying greater volumes of traffic, it is possible that the polishing effect of the traffic may lower the skid resistance unduly.  Larger sizes of stone should be used for roads carrying traffic near the top end of the range, since heavier traffic causes greater embedment of the stone.

Table 1 gives guidelines on seal types appropriate for different traffic categories.

The following abbreviations are used in Tables 1 to 4:

            S3                    Sand Seal

            S1                    Single Seal

            S2(9)               Double Seal with 9,5 mm Aggregate and Sand

            S2(13)             Double Seal with 13,2 mm Aggregate and Sand

            S4(13)             Cape Seal with 13,2 mm Aggregate and One Layer of Slurry

            S2(13/6)                      Double Seal with 13,2 mm Aggregate and a Layer of 6,7 mm Aggregate

            S2(19/9)                      Double Seal with 19 mm Aggregate and a Layer of 9,5 mm Aggregate

            S4(19)             Cape Seal with 19,0 mm Aggregate and Two Layers of Slurry

TABLE 1                               Guidelines for Initial Surfacings for Different Traffic Volumes

Note:   a – Good performance has been noted in several cases.  The use of modified binders can reduce risks in these situations.

            x – Not suitable.

Traffic Actions

Table 2 gives appropriate seal types for different traffic and road type combinations.

TABLE 2                               Guidelines for surfacing types for different traffic and road type combinations

Note:   b – Preferably blinded with sand.

            x – Not suitable.

Gradients

On steep gradients, at traffic circles or in places where frequent stopping and starting occurs, traffic imposes such great stresses on the surfacing that a surfacing seal is liable to be damaged, particularly in its early life.  For these special conditions the choice of surfacing type and the design are very critical.  A normal single seal is likely to prove inadequate even for light traffic conditions.  Under these conditions, surfacing seals in which binders with higher viscosity than normal (modified binders) are used, together with the use of pre-coated stone, or seals with a relatively fine texture finished off with a fog spray, or seals in which the stone particles are firmly held in place by slurry, will perform better.

Table 3 gives appropriate seal types for different road gradient categories.

TABLE 3                               Recommended initial surfacing types for different road gradient categories

Note:   a – Not on stabilized base-courses constructed with fine material.

            b – Not if channeling of water flow is expected because of soil wash – common in developing areas.

            c – Not if urban drainage systems (kerbs) are present.

d – Not if communal water systems are present, since these result in detergents being washed onto the road with consequent erosion of the bitumen.

            e – Not on gradients above 10 per cent if channeling o flow is expected because of soil wash – common in developing and hilly areas.

            x – Not suitable.

Maintenance Capability

The majority of surfacing seals will normally lend themselves to simple maintenance techniques, such as the application of diluted emulsions or slurry seals.  The application of diluted emulsions has been found to be cost-effective where the seal is beginning to ravel.  If this is done before the seal starts to disintegrate, the life of the seal can be prolonged.

In table 4 the maintenance capabilities of road authorities are categorized and recommendations on appropriate seals for different situations are made.

TABLE 4                               Recommended initial surfacings for different maintenance capabilities

Notes:  b – Rural areas only

            c – The performance of surface seals is sensitive to design and construction problems.

# - It is not essential to have a maintenance management system (MMS), but its existence is indicative of a certain level of capability and sophistication.

            x – Not suitable

Surface Texture Required

Since the skid resistance of smooth-textured (fine) surfaces decreases much more rapidly with an increase in vehicle speed than that of rough-textured (coarse) surfaces, it is more important to provide a rough textured (coarse) surface for rural high-speed roads than for city streets.  Smooth-textured surfaces are also desirable for city streets since they are both easier to clean and generate less noise.

There is a limit to the coarseness of texture of the surface because of the nuisance if tire noise, its effect on riding comfort and the problem of windshield damage by large loose stones.

For single seals the largest size of stone generally recommended is 13 mm (19 mm in exceptional cases).

The higher the operating speed, the greater the texture depth required.  The greater the risk when braking, the higher the Sideways Force Coefficient (SFC) required.

Availability of Aggregate

For seals to perform effectively, aggregate of good quality should be used.  To increase the probability of a long maintenance-free life, consideration should be given to the importation of better quality materials if the stone available locally is of poor quality.

For lightly trafficked roads where suitable sand is available, it is usually worth considering one of the seals incorporating sand.

Construction Techniques

Although seal construction techniques are basically the same, irrespective of the type of seal, the experience of the construction team should be taken into consideration in the selection of the seal to be used.  Also, some types of seal are more “forgiving” than other, lending themselves more readily to the construction of an acceptable quality-wearing surface.

Conventional double seals (incorporating two layers of aggregate) or Cape Seals (incorporating a layer of aggregate followed by one or two applications of slurry) normally give good results.  As the binder is applied in up to three applications, there is room for correction.  Stone retention is good, since the lower layer of stone is wedged or grouted in by the succeeding layer.  In a conventional double seal, retention of the second layer of stone can easily be brought about by the use of pre-coated chips or the application of a fog spray.  In addition, its higher total binder content, by comparison with that of a single seal, makes the surfacing less permeable.

Single seals are more sensitive than double seals to the rate of application of binder, since there are less voids available and a small error in the application rate may have a significant effect on the percentage of voids filled.  This is particularly true for single seals when small sizes of aggregate are used.  Aggregate loss may also be greater, although this, to a certain extent, may also be overcome by pre-coating of the aggregate.  It is essential not to apply too much or too little aggregate, but to achieve a shoulder-to-shoulder pattern in the completed seal.  Judicious brooming and rolling are necessary to ensure that the above mentioned objective is met.  It should also be ensured that the aggregate particles are properly orientated and embedded in the binder film.

Sand seals also require experience in rolling and back-brooming techniques.

The experience of the construction team in the application and spreading of slurry, especially in the proper use of squeegees, could make a substantial difference to the life of slurry seals and Cape Seals.

Environmental Conditions

Although the temperatures prevailing in different  countries  affect the choice of grade of binder and its rate of application, they have very little bearing on the choice of type of surfacing seal.  In wetter areas it is desirable to use a surfacing that is as impermeable as possible, to prevent ingress of water into the underlying layers and to prevent the possibility of stripping of the binder in the surfacing itself.  Well constructed seals and Cape Seals will meet this requirement.

Where roads or streets are used to carry storm water, single seals or thin seals are not recommended because of the risk of erosion.  Table 1 may be used as a guideline for the selection of type of seal.

Quality of base

Since surfacing seals are relatively thin, any imperfections in the finish of the base will be reflected to the surface and will have an adverse effect on the riding quality of the road.  This is particularly true of single seals (since the layer is only a single stone thick), and also of sand seals (which are approximately 3 mm thick).  In the case of multiple seals, minor undulations and depressions in the base may, to a small extent, be evened out.  Some authorities have had good success in this regard, with the use of inverted double seals (smaller aggregate followed by the larger aggregate).

On relatively soft bases, such as those constructed with natural gravel where significant embedment of the surfacing stone is likely, good results will be obtained if large-sized stone is used, for example if 19 mm stone is used in a Cape Seal or a double seal.

Where a satisfactory finish to the base profile cannot be assured, or where appreciable embedment of the surfacing stone is likely, a surfacing seal should not be used.  Consideration should be given to the use of an asphalt surfacing or the strengthening of the base, e.g. by stabilization.

IAIN JACK