Bituminous Surfacing and Construction

Introduction

Bituminous surfacing is any surfacing that involves the use of bituminous materials to waterproof the pavement surface and/or provide structural strength.

Such a construction may consist of an application of a thin film of bituminous binder onto a base course or pavement surface followed by the spread of a single layer of chippings as in a chip seal (surface dressing), or it may involve the use of high-strength asphalt concrete to waterproof and provide structural strength to a pavement.

Other types of bituminous surfacing include slurry seal, fog seals, etc. In all such constructions, the principal materials are bituminous binders (penetration or viscosity grade, cutbacks, emulsified asphalts) and aggregates of specified gradation.

Priming

Priming by itself is not a bituminous surfacing but it is the first step in the construction of bituminous surfacing.

It involves the application of bitumen or asphalt, which has been made more fluid, to a road base so that the bituminous material (called primer), will soak and penetrate the surface.

The primer should be fluid enough to penetrate the pavement surface to a depth of between 3 to 10 mm. 

Priming of a road base is for any or all of the following reasons;

  • coating and bonding of fine dust particles to the surface of the pavement
  • plugging capillary pores in the pavement to waterproof the surface
  • penetrating and strengthening the surface of the base layer by increasing cohesion
  • coating the surface of the base to provide better adhesion between the surface and subsequent bituminous surfacing
  • providing a strengthened short-term running surface prior to subsequent paving.

The main types of primers are cutbacks of the MC-30, MC-70, SC-70, SC-250 grades. Asphalt emulsions are not recommended for priming because of their tendency to form a skin on the surface of the pavement.

The choice of the grade of primer and its rate of application is usually dictated by the nature of the pavement surface to be primed, and the period of time that will elapse before the main bituminous surfacing is applied. 

The following table provides a guide to the type of primer and rate of application appropriate for the nature of the surface to be primed.

primer

Extremely porous surfaces may need two applications of the primer. If the rate of application of the primer to get adequate coverage and penetration exceeds 1.4 l/m2 then a more viscous primer needs to be selected.

In the case where the primed surface will be blinded with sand or quarry dust to allow short-term use by traffic, it is recommended to increase the rate of primer application by 0.2 l/m2.

Priming should take place when the weather is hot and dry and never when rain is impending or the weather looks threatening. In addition, surfaces to be primed should be damp but not too wet.

A primed surface should be left for 2 to 3 days to allow the volatile cutter in the cutback to evaporate otherwise the residual volatiles can affect subsequent bituminous treatments.

A primed surface is blinded with sand or quarry dust if the road is to be opened to traffic for some short period before the application of the main bituminous layer. 

This will serve to prevent vehicle tires from picking the sticky residual bitumen remaining on the pavement surface and destroying the primed surface. 

Blinding is not necessary if the main bituminous layer will follow within one or two days during which period traffic will not be allowed on the primed section.

Primer-seal

A primer seal results if a primed road base is covered with aggregates.

In this case, the binder used for priming must be more viscous than would be used for normal priming (see surface dressing binder) since it is intended to serve both as a primer and as a binding material to hold the chippings onto the road surface. 

A primer seal provides a riding surface for traffic for a period longer than would be provided by a normal primed base that is blinded with sand or quarry dust.

In general, a primer-seal   construction provides several advantages including the following: 

  1. Traffic is able to use the primed surface the same day and for a much longer period without significant surface deterioration even  if the main seal coat is considerably delayed.
  2. The  construction results in less disruption to traffic.
  1. The seal can last up to 24 months and is suitable for situations where the placement of the  main seal is expected to be considerably delayed.
  2. When the final seal coat is placed, a surfacing intermediate between a single seal and a double seal is obtained. This leads to enhanced surface integrity which improves the durability of the chip seal as a whole.
  3. It results in minimal loss of chippings as about 95% chip embedment is achieved. This in turn almost eliminates aggregate whip-off and associated windshield breakage and results in an environmentally friendly surfacing.

Surface dressing (chip sealing)

This is a type of bituminous surfacing consisting of an application of a very thin layer of liquid bituminous binder such as a cutback or emulsified asphalt on a primed pavement surface and the subsequent spread of single-sized stone chippings.

This type of bituminous construction does not provide any structural strength to a pavement but it enables the strength of the pavement and the sub-grade to be preserved.

The thin layer of bitumen or asphalt acts as a waterproofing seal to prevent the entry of surface water into the pavement structure.

The chippings provide a durable skid-resistant and dust-free wearing surface as well as a protective cover against damage of the asphalt film by vehicle tires.

Surface dressings may be used to restore the skid resistance of an existing pavement whose wearing surface has been polished by traffic.

The construction is generally adequate for lightly trafficked roads with traffic levels not exceeding 500 veh/lane/day. Where roads carry up to 1,000 veh/lane/day or even more, a double surface dressing may be appropriate.

A double surface dressing is simply a surface dressing applied on a surface dressing.

It is recommended that the application of the second dressing takes place at least 2-3 weeks after the construction of the first surfacing to allow traffic to help the chippings of the first dressing assume a stable interlocking structure and become a firm foundation for the second dressing.

But successful performances have been observed for constructions that ignored this recommendation.

The nominal size of chippings to use in the second dressing must be half that of the first.

Design and construction-GHA’s Method

i. Selection of aggregate size

The current design approach selects aggregate size on the basis of traffic intensity and whether the chippings are to be used in primer seal construction or the main seal.

For primer seal construction, the recommended chipping sizes are 7, 10, and 14mm whereas only the 10 and 14mm sizes are specified for the main seal.

The table below provides designers with a guide to the selection of aggregate size for the main seal using information about the average daily traffic expected on the road.

Aggregate size selection for main steel
Aggregate size selection for main steel

Where there are two  sizes to choose from, it is recommended to use the smaller aggregates for roads in  urban areas where traffic loads are low and the larger ones on rural highways where commercial vehicle traffic and axle loads are high.

ii. Pre-coating of chippings (aggregates)

Poor binder–aggregate bond is one of the major causes of loss of cover aggregates in surface dressings.

Loss of cover aggregates leads to damage of the thin bituminous cover which subsequently results in pitting and pothole formation.

To overcome this, the current design approach specifies the pre-coating of the aggregates for the seal with a bitumen-based (cutback) material.

This includes the aggregates to be used for the primer seal construction and the main seal coat as well. The cutback for the pre-coating is formulated as 90% gas oil and 10% AC-10  bitumen. 

The recommended application rate for uniform coverage is 12 litres/m3.

Pre-coating must take place at least three days before the chippings are to be used and if rain is imminent, the adequate cover must be provided to prevent the pre-coating material from being washed off.

iii. Primer seal construction

Instead of a primed surface  a primerseal is constructed. The primer binder to use is an  AC-10  (or 80-100 pen grade) with a 16-20% kerosene cutter.

The size of chippings for the primer seal may be selected from any of the following; 7, 10, and 14mm.

However, for low-medium traffic, 7mm  chippings are recommended for use as this will provide a more uniform and less hungry surface for the final seal.

The rate of primer binder application is a function of the size of aggregate to be used for the primerseal;  the volume of traffic anticipated on the road; the spray temperature, and the porosity of the pavement surface.

The spray temperatures considered are for either cold or hot applications and are respectively 15oC and 135oC.

The porosity of the pavement surface is placed into four types, namely,

  • tightly bonded,
  • fine bonded,
  • coarse and
  • crushed rock.

For both cold and hot binder applications the aggregate spread rate is dependent only on the size of chippings selected.

The Table below from GHA Spraying Seal Manual provides an example guide to materials application for a tightly bonded pavement surface with the primer seal applied cold.

A guide to material selection and application for primerseals

A guide to material selection and application for primerseals

Similar tables are available for other surface types and for cold and hot binder applications.

iv. Seal coat construction

A more viscous bituminous binder is specified for use as tack coat for the main seal.

The tack coat may be a cutback or an emulsion but it is more preferable to use a cationic rapid setting emulsion grade CRS-70 containing 70% AC-10 base bitumen.

The rate at which the aggregate selected is to be applied to the pavement surface following the spraying of the bituminous tack coat is dictated by the Average Least Dimension (ALD) of the aggregate.

The Table below provides a guide to the chippings application rate for a broad range of ALD values.

A guide to aggregate application rates.
A guide to aggregate application rates.

v. Rolling (Compaction)

Only pneumatic-tyre rollers of mass between 12 and 15 tonnes and a wheel load greater than 1 tonne with  a tyre pressure of 550kNm2 are specified for use. 

Steel-wheeled rollers are not recommended for use as they  tend to compact only the high spots and crush aggregates.

Pneumatic-tire rollers result in kneading compaction and are able to re-orient  the aggregates to lie on their flat side with their least dimension vertical so that there is also embedment rather than just mere compaction.

This results in a more interlocking aggregate layer for the seal. Where necessary two rollers may be employed to keep pace with the tack coat application and aggregate spread.

Problems associated with surface dressing

Surface dressings that are not properly constructed may experience one or more of the following problems:

  1. Streaking

This is an alternating series of light and dark longitudinal parallel lines on the road surface due to uneven application of binder on the finished surface. Such unevenness may result from clogged spray bar nozzles or incorrect height of spray bar above pavement surface.

2. Bleeding

This is the flushing to the surface of bituminous binder in the surfacing so that the passage of a vehicle leaves an imprint of the tyres on the surface. Bleeding causes pavement surfaces to be shiny and extremely smooth. The problem is caused by the application of too much binder or by the loss of cover aggregates.

3. Loss of cover aggregates

This occurs when there is insufficient embedment of cover aggregates in the thin asphalt layer. Insufficient embedment may arise if;

  • inadequate binder was applied
  • dusty and dirty aggregates were used for the surfacing
  • the asphalt binder was too hard to develop good bond with aggregates
  • the binder or tack coat was applied on a primed or an existing surface that had a layer of dust on it thus preventing a good bond from developing between the surface and the new seal coat.  

Fog seal

A fog seal consists of a very light application of binder onto the surface dressings followed by an application of quarry dust. The binder used is usually emulsified asphalt sprayed at a rate of 0.6-0.9 l/m2.

The fog seal is used to improve retention of aggregates that under normal sealing operations may not become sufficiently embedded in the bitumen layer and hence are whipped-off to the sides of the road.

Because of the enhanced chipping retention, surface defects such as potholes are delayed on surface dressings that have been  fog-sealed.

In general the benefits to be derived from a fog seal application on a surface dressing include:

  • complete retention of surface dressing chippings
  • longer pavement life from improved retention of cover aggregates
  • improvement in the surface riding quality
  • windshield breakage and paint chip from whip-off aggregates are completely eliminated
  • noise from vehicle tyres is substantially reduced
  • enhanced roadway aesthetics from a well-defined carriageway

Slurry seal

A slurry seal is a bituminous surfacing placed in the form of thick workable slurry of a mixture of asphalt emulsion, water, aggregate and filler.

The slurry, which is premixed in a specially equipped truck, is spread on the road surface by means of a spreader box to a thickness of about 5-10mm. When placed, slurry of optimum design should set in about 30-60mins.

Depending on the aggregate gradation used, three slurry types, namely, type I, type II, and type III may be produced.

The following are the gradation requirements for the three types of slurry.

Slurry seal aggregate gradation
Slurry seal aggregate gradation

Type I slurry is suitable for sealing cracks, filling voids and correcting surface defects on airfields where surface sealing is the primary needs. The slurry is applied at the rate of 3.3-5.4kg/m2.

Type II slurry is suitable for filling surface voids,  correcting severe surface erosion conditions, providing a wearing surface on bituminous base course or soil-cement bases, or as a sealer on stabilised base courses. The rate of application is 5.4-8.2kg/m2.

Type III slurry is suitable for providing a new wearing surface to improve aesthetics or the skid resistance of a polished surface or building a crown. The rate of application is 8.2kg/m2 or more.

Asphalt overlay construction

An asphalt overlay construction is a bituminous surfacing in which hot-mix asphalt concrete (HMA) is placed as the wearing course and a structural component of a pavement.

Overlay construction may be as low as 50mm thick to as much as full pavement depth.

Preparatory works

Surfaces to be overlaid must be prepared prior to placement of the hot-mix asphalt.

Preparation includes regulating the pavement surface if there are irregularities in the surface profile or shape, sealing cracks, and planing or grooving surfaces that have been polished by traffic and are likely to develop poor adhesion with the new construction.

The new flexible  construction must have the base course already primed. The final stage of the preparatory works involves the application of a tack coat to the surface to be overlaid.

The tack coat (a solution of a slow setting asphalt emulsion and an equal amount of water) is applied to the pavement surface at a rate of about 0.25-0.7 litres/m2.

Placing of mix

An approximate way of knowing the thickness of loose material to place is to multiply the desired compacted thickness by 1.25. 

After placing about 6-9m of the mat, the operation should be stopped and the unrolled mat checked for uniformity in texture so that deficiencies can be corrected by making the necessary adjustment in the screed.

During placement of the HMA, the amount of material carried ahead of the screed should be kept uniform in height as variations in height have the potential of causing surface roughness.

Compaction

Compaction of asphalt paving mixtures is aimed at reducing the volume of air in a HMA by the application of external energy.

The process is intended to achieve a uniform high density of the asphalt layer. The expulsion of air enables the mix to occupy a smaller volume thereby leading to an increase in density.

Though the amount of compaction energy put into a field material cannot be determined, its effectiveness can be determined by measuring the density achieved by the compaction energy. In view of this, field compaction operations of asphalt paving mixtures are guided by a roller pattern established using a test section or strip at the beginning of the operations.

The roller pattern enables the number of passes required to achieve a desired density in a compaction operation to be established.

The compaction process is affected by confining conditions, which in the field are provided by the surrounding HMA material, the underlying layer and the compactor contact area in its zone of influence.

Because temperature has a profound effect on the workability of asphalt paving mixtures, compaction should be carried out at the proper compaction temperature determined on the basis of the viscosity-temperature characteristics of the asphalt binder used.

Whatever the case, compaction should be carried out before the asphalt mat cools below 80oC; below this temperature, the mix will become stiff and no amount of compaction can compress it to occupy a smaller volume.

At no time during the compaction operations should rollers be parked on the hot mat since parking will cause depressions that may be difficult to remove even with additional rolling.

When compacting unsupported mats with constant cross slope, the first pass of the roller should be on the low edge of the mat with each successive forward pass moving toward the high edge of the laid mat.

Special care must be taken in compacting longitudinal joints as an improperly compacted joint may be a future location for water damage, ravelling or even a potential driving hazard.

Where possible, especially when paving two-lane carriageways, the longitudinal joint can be deleted by paving in echelon.

This of course will require the use of two pavers. In other cases, about 50-75mm of the cold mat adjacent to the new mat to be placed is first cut back to expose a clean dense vertical face.

The face is then tack- coated and the adjoining run placed to overlap the cut by about 25-50mm. Using lutes, the overlap is pushed back immediately after placing to form a small ridge along the joint.

Longitudinal joints are the areas to be rolled first and a steel-wheeled roller must be used. Rolling should take place close behind the paver with the roller compressing the joint and overlapping onto the fresh asphalt by about 150mm. This is continued until a smooth joint is obtained.

Rolling is then continued on the opposite side and then worked back toward the joint. During all rolling operations, the next forward pass should overlap the previous by at least 150mm.

Also, each roller pass should end beyond the preceding pass by at least one meter to prevent the development of a bump.

When the general rolling operations have been completed, visual inspection has to be conducted to ascertain the condition of the freshly compacted surface. If rolling was properly carried out, there should be no roller marks, scuff marks, and undulations.

i. Compaction equipment

Compaction equipment for asphalt works includes steel wheeled rollers, vibratory steel-wheeled rollers, pneumatic-tyred rollers and plate compactors.

To be suitable for asphalt works, the compacting surfaces of the equipment must be smooth and free from marks.

To prevent the sticking of the asphalt concrete mix to the compactor tyres, the equipment must be provided with a watering system and a spray bar for the controlled application of water on the compacting wheel or steel drum and scrappers and a series of mats mounted on a transverse bar which are kept wet and in contact with the drum to remove any asphalt mixture that sticks to the wheel.

The water system is used during the breakdown rolling but not for finish rolling.

ii. Compaction  pattern

The compaction of the hot mix asphalt involves breakdown rolling (initial rolling) intermediate rolling and finish rolling.

Breakdown rolling is carried out as soon as possible after the mix is laid.

This is usually carried out using steel-wheeled rollers though pneumatic-tired rollers can also be used.

However, the problem that sometimes arises with using pneumatic-tired rollers in the initial rolling is the possibility of having rollers marks that are difficult to remove during the finish rolling.

There may also be sticking of mix to the rubber tires resulting in surface deficiencies.

The breakdown roller should be operated as close to the paver as practicable in order to take maximum advantage of the temperature of the mix and thus obtain the maximum increase in density.

Where the speed and productivity of the paver is high more than one compactor may be deployed in the breakdown rolling. Initial rolling should normally begin at the lower side of the run with the roller moving longitudinally and reversing along the same track.

The roller should then move progressively across the run to the opposite side, completing a forward and backwards passes on each track and with each track overlapping the previous by about 150mm.

Normally two passes of the breakdown roller may be adequate but up to four passes may be necessary if a thick layer is placed.

Intermediate or secondary rolling is carried out right after the initial compaction and should only be delayed if the asphalt temperature is higher than the optimum required for proper compaction so that excessive deformation or shoving by the compacting drum does not occur.

Rolling commences along the longitudinal side of the run and returns on the same track before moving across the run in full roller width to the opposite side. Rolling then continues until the required density is achieved.

This may require between 6 to 12 passes depending on the thickness of mat and temperature conditions.

Traditionally pneumatic-tired rollers are used for intermediate rolling as they are very effective in removing roller checking (fine hairline transverse cracks) produced by steel-wheeled rollers during breakdown rolling.

Finish or final rolling is carried out while the mat is still warm enough to allow removal of all roller marks so as to obtain a smooth uniform finish without causing shoving and fine or hairline transverse cracks in the surface of the mat.

Rolling may be accomplished by using steel-wheeled rollers or pneumatic tyred-rollers with relatively high tyre pressure.

iii. Density specifications

Density values are needed to control compaction and to verify a satisfactory void content or to establish whether the mix is being compacted to too low or too high an air voids content that can have serious implications for rutting.

Densities achieved in the field may be expressed as some percentage of a target density, which may be defined to be any of the following:

  • theoretical maximum density
  • laboratory bulk density
  • density of a control strip constructed at the beginning of the project

Thus, for example, the field density to be achieved may be specified as minimum 95% of laboratory density. There are no special criteria for selecting which of the above to specify as target density.

In fact, all the above types of densities have been specified for density control on many projects with varying degrees of success and failure. It appears however that minimum density values are commonly specified with respect to the laboratory bulk density.

Relative compaction levels that are consistently above 100% may indicate over-compaction that can have serious consequences on rutting.

Over-compacting the asphalt mat may result in air voids content below 3% which will cause rutting to develop early in the life of the pavement.

But generally, relative compaction levels consistently approaching or exceeding 100% may likely be more the problem of a low reference density rather than of over-compaction.

Therefore the interpretation of the degree of compaction must be approached with caution.

Quality control and assurance(QC/QA)

Quality control refers to those tests necessary to control a product to determine the quality of the product being produced.

The contractor usually carries this out. During asphalt overlay operations, QC may be necessary to ensure that material being supplied to the lay-down crew is consistent and within specifications and/or to detect at the onset any deficiencies in mix productions if and when they occur so that any problems attributed to the mix can be fed back to the design laboratory or production plant for corrections or adjustments to be made.

On the other hand quality assurance are those tests necessary for the owner to make a decision on acceptance of a project and to ensure that the product being delivered is indeed what the owner specified.

The Engineer’s representative carries out the tests. 

A successful QC/QA requires that the Materials Engineer, the paving team, and the personnel at the hot-mix asphalt plant work together.

Tests to undertake

In line with quality control and assurance, the following are considered important tests to perform during the manufacture and placement of asphalt concrete mixtures:

  • Aggregate gradation
  • Asphalt content
  • Temperature
  • Compaction and voids analysis
  • Density tests (theoretical maximum density and in-situ density)
  • Visual inspection

a. Aggregate gradation

Even though several aggregate properties are important, routine testing during construction is usually limited to gradation only. Aggregates for grading tests must be sampled from the stockpile, cold feeder belt, hot bins and extracted asphalt mixture.

Gradation of the asphalt mixture is the most important since it is the final product.

Evaluation at other points will however allow the engineer to troubleshoot the gradation problem and quickly identify the location where it is occurring.

b. Temperature

This parameter is very important during mixture production and during field compaction.

The asphalt concrete must be produced at just the right temperature to ensure good coating on the aggregate and a satisfactory compaction.

Excess temperature causes oxidation and loss of volatiles to occur to the asphalt cement reducing mixture durability. 

An indication of asphalt concrete being produced at an unacceptably high temperature is the blue smoke syndrome which literally means a bluish smoke coming out of the mixture as it is discharged into  a delivery truck.

Monitoring the temperature of the laid mixture is very necessary to ensure that compaction is carried out at the proper compaction temperature as determined per the viscosity-temperature characteristics of the asphalt cement.

Where the temperature of the laid mix is high, compaction has to be delayed to allow the mix to come to the compaction temperature.

It is also necessary to ensure that the temperature of the mix does not fall below a value that makes compaction ineffective. The recommended minimum temperature for compaction is 80oC.

c. Asphalt content

This is very important in ensuring that the asphalt concrete is being produced per the Job Mix Formula and also in guaranteeing a satisfactory performance.

Asphalt content has direct effect on mixture properties such as film thickness, voids, stability and flow.

A mixture with low asphalt content may have durability problems whereas one with high asphalt content is likely to have stability and bleeding problems.

Asphalt concrete from the spreader must be sampled and the solvent extraction test carried out in order to determine the asphalt content. Improper asphalt content can be caused by inaccurate aggregate proportioning in the batch or drum mixing facility, improperly calibrated asphalt cement meter, segregation, and even sampling and testing errors.

d. Laboratory compaction

Samples of the asphalt concrete must be taken regularly and for every batch production during construction for laboratory compaction and other tests in order to verify that mixture characteristics or properties meet specification requirements.

If the required density levels are not achieved, the mix may deform under load, harden prematurely, lose fatigue resistance and be more susceptible to moisture-induced damage (stripping).

The laboratory density values are the standard against which the level of compaction being achieved in the field is evaluated. The compacted samples also enable voids analysis to be carried out.

e. Filed density

To check the level of in-place density achieved, nuclear gauges may be used or cores may be taken for laboratory analysis. The results of the density tests are used to establish the relative compaction achieved by comparing the field density to the laboratory density.

f. Visual inspection

As test results can never identify all the problems that may occur, visual inspections must be carried out during QC/QA evaluations. This way, potential trouble areas can be identified and corrected or areas that appear deficient can be sampled to determine if deficiencies really exist.

Testing frequency

Primarily the scale and importance of the project and material variability determine the frequency of testing. The following is only a guide to the minimum number of tests to be conducted on an asphalt overlay project.

Test and frequency

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