DBST

DBST SURFACINGDBST SURFACING

1. INTRODUCTION• Surface Dressing (SD) is a simple, highly

effective, inexpensive road surface treatment, provided

• Adequate care is taken in the planning and execution of work.

• Used throughout the world for surfacing both medium and highly trafficked roads and

• As a maintenance treatment for all kinds of roads.

PART I

A thin film of binder – pen.gr.of Bit, Cutback or Emulsion sprayed on the road surface

Covered with a layer of stone chippings.

Compaction:- rolling with suitable rollers –

i. Steel rollers (> 8 tonnes)ii.Pneumatic Tired rollers (PTR)

2. 2. MethodologyMethodology

3. Functions• Thin film of binder acts, as a water proofing seal, preventing

the entry of surface water into the road structure.

• The stone chippings protect the binder from damage by vehicle tires and form a durable ,skid resistant and dust free, wearing surface.

• In some circumstances, the process may be repeated to provide Double or Triple layers of chippings.

4. Other functions (a)

• S.D. is a very effective maintenance technique - capable of greatly extending the life of a structurally sound road pavement if the process is under taken at the optimum time.

• S.D. may also retard the rate of failure of a structurally inadequate road pavement by preventing ingress of water, thus preserving the inherent strength of the pavement layers and the sub grade.

5. 5. Other functions - contd (b)Other functions - contd (b)

S.D. can provide an effective and economical running surface for newly constructed road pavements, in addition to its maintenance role.

Existing roads with bituminous surfacing, carrying in excess of 1000 vehicles per lane per day have been successfully surfaced with multiple surface dressings.

6. 6. Life span of a S.DLife span of a S.D

• A correctly designed and constructed S.D. A correctly designed and constructed S.D. should should last at least 5 years,last at least 5 years, before resealing before resealing withwith another S.D becomes necessary.another S.D becomes necessary.

• If traffic growth over a period of several years If traffic growth over a period of several years necessitates a more substantial surfacing or necessitates a more substantial surfacing or increased pavement thickness, increased pavement thickness, A bituminousA bituminous OVER LAYOVER LAY can be laid over the original S.D can be laid over the original S.D when the need arises.when the need arises.

7. 7. Life span of a S.D (contd)Life span of a S.D (contd) Success of a S.D depends primarily on the

Adhesion of the chippings to the road surface

Hence the road surface must be clean and free from dust during the S.D process.

Inappropriate specifications, poor materials and bad workmanship can also drastically reduce the service life of a S.D

8. 8. 2. TYPES OF SURFACE 2. TYPES OF SURFACE DRESSINGDRESSING

S.D can be constructed in S.D can be constructed in a number of ways to suit a number of ways to suit the conditions. The the conditions. The common types of S.D are common types of S.D are illustrated in Fig. illustrated in Fig. 1(a),Fig.1(b).1(a),Fig.1(b).

8 (a) 8 (a) Types of Surface Dressing – contd Types of Surface Dressing – contd (a)(a)

8 (b) 8 (b) Types of Surface Dressing – Types of Surface Dressing – contd (a)contd (a)

9. 9. Single S.DSingle S.D When applied as a maintenance operation

to an existing Bituminous road surface, a single S.D can.

Fulfill the functions, required of a maintenance

re-seal - viz. water proofing the road surface arresting deterioration and restoring skid resistance.

A single S.D would not be normally used on a new road base because of the risk that the film of bitumen will not give complete coverage.

A double S.D should be more durable than a single S.D .

10. 10. Double S.DDouble S.D

Double S.D are robust and should be used when

A new road base is surface dressed. Extra cover is required on an existing

bit. road surface, because of its condition (e.g. when the surface is slightly cracked or patched.

There is a requirement to maximize durability and minimize the frequency of maintenance and re-sealing operations.

11. 11. Double S.D contd(a)Double S.D contd(a)

The quality of a Double S.D will be greatly enhanced if the Traffic is allowed to run on the first dressing.

For a minimum of 2-3 weeks (preferably longer) before the second dressing is applied.

This delay allows the chippings of the first dressing to adopt a stable inter locking mosaic which provides a firm foundation for the

second dressing. Picture of a Mosaic

12. 12. Double S.D - contd(b)Double S.D - contd(b)

Such cleaning of dust and debris before the second dressing may be difficult.

Sand may sometimes be used as an alternative to chippings for the second dressing but it cannot contribute to the overall thickness of the surfacing.

13.13.Triple S.DTriple S.D

A triple S.D may be used to advantage when a new road is expected to carry high traffic volumes from the out set.

The application of small chippings in the third seal will reduce noise generated by traffic and the additional binder will ensure a longer maintenance – free, service life.

14. Racked – in S.D .14. Racked – in S.D .

This system is recommended for use when traffic is particularly heavy or fast.

A heavy single application is made and a layer of large chippings is spread to give approximately 90%coverage.

This is followed immediately by the application of smaller chippings which should “lock – in” the larger aggregate and from a stable mosaic.

The amount of bitumen used is more than that would be used with a single seal but less than for a double seal.

15. Racked-in S.D - contd.15. Racked-in S.D - contd.

The main advantage of the Racked - in S.D are

Less risk of dislodged chippings. Early stability through good mechanical interlock.

Good surface texture.

16.Other types of S.D16.Other types of S.D

“Sandwich” S.DPrincipally used on existing binder rich surface.

Sometimes on gradients to reduce the tendency for the binders to flow down the slope.

17. ‘Pad coats’17. ‘Pad coats’Used when the hardness of the existing

road surface allows very little embedment of the first layer of chippings, such as on a newly constructed cement stabilized road base or dense crushed rock base.

A first layer of 6mm chippings will adhere well to the hard surface and well provide a ‘key’ for larger 10 or 14mm chippings in the second layer of the dressing

18. 18. CHIPPINGS for S.D . CHIPPINGS for S.D .

The selection of chippings sizes is based on Vol. Com. Vehicles having unladen weights of more than 1.5 tonnes and hardness of the existing pavement.

Ideally, chippings for S.D should be single sized. cubical in shape, clean and dust free, durable and not susceptible to polishing under the action of traffic.

In practice, the chippings available fall short of this ideal.

19. 19. Props. Of Chippings - contd Props. Of Chippings - contd (a)(a)

It is recommended that Chippings used for S.D should comply with the requirements of B.S 63 Part 2 (1987) Refer Appendix A – Road Note 3 (page 28).

Normal sizes are specified in Table A1, Grading limits, Flakiness Index in Table 2.

Specified limits, F.I for S.D aggregate for lightly trafficked roads carrying up to 250 vehicles per day given in Table A3

20. 20. Props. Of Chippings contd (b)Props. Of Chippings contd (b)Sample should be tested for Grading,

F.Index, Agg. Crushing value (ACV) and when appropriate, for polished stone value (PCV) and Agg. Abrasion value (LAAV).

Typically ACV lies in the range 20-35, for more heavily trafficked road a max. value of 20 is recommended.

Some values for PSV are given in appendix B (Road Note 3) page 29 based on traffic flows No. of Veh per Lane per Day and road geometry.

21. 21. Props. Of Chippings contd (c)Props. Of Chippings contd (c)Some recommended value for F.I- 25% for heavy

traffic,35% for light traffic.

Usually recommended value for LAAV should be less than 40%.

The nominal sizes for S.D are 6,10,14 and 20mm The proportion of flaky chippings clearly affects

the Average thickness of a single layer of chippings.

Hence the concept of average least dimension is introduced (ALD) .

Effectively, the ALD is the average thickness of a bedded single layer of chippings when they have bedded down into their final interlocked positions.

22. 22. Props. Of Chippings - Props. Of Chippings - contd (d)contd (d)

The amount of binder required to retain a layer of chippings is thus related to ALD, rather than their nominal size.

The most critical period for a S.D occurs immediately after the chippings have been spread on the binder film.

At this stage, chippings have yet to become an interlocking mosaic and are held in place solely by the adhesion of the binder film.

DUSTY, chippings can seriously impede adhesion and can cause immediate failure of the dressing.

23. 23. Props. Of Chippings - Props. Of Chippings - contd (e)contd (e)

The effect of dust can be reduced by dampening the agg. prior to spreading.

With cutback bitumen or emulsion good adhesion develops.

Most road aggregate have a preferential attraction for di-polar water molecules rather than for non- polar bitumen.

24. 24. Props. Of Chippings - Props. Of Chippings - contd (f)contd (f)

With heavy rain within the first few hours,

Loss of chippings due to poor adhesion.An adhesion agent may be added to the

binder or in dilute solution to pre-coated chippings.

Pre-coated chippings are beneficial with very dusty, and poorly shaped chips if traffic is severe.

25. 25. Pre- coating of chips- Pre- coating of chips- contd (g)contd (g)

Basically two methods are available. I. Spray the chips with a light application

of creosote, diesel oil or kerosene at ambient temp. in a simple concrete mixer.

Creosote is a black oil prepared by burning coal, used to preserve wood.

26. 26. Pre- coating of chips- Pre- coating of chips- contd (h)contd (h)

II. A harder grade of bitumen can be used for coating at about 140°C, the dosage being 0.5 to 1.0 percent by mass of agg. based on the nominal size of agg.

Chips should not stick together and flow freely.

27.27.Pre- coating of Pre- coating of chippings-contd (i)chippings-contd (i)

Early strong adhesion and this helps to obtain high quality dressings.

Pre- coating is usually under taken in hot mix plants or in a concrete mixer.

The hardness of the coating and adhesion to each other can be controlled by the mixing temp. and or the duration of mixing.

28.28.Pre- coating of Pre- coating of chippings-contd (j)chippings-contd (j)

The amount of binder for coating is given by the following Table 1.

Table 1 – Binder contents for lightly coated chippings.

Size of chips (mm) Bitumen by mass of agg. (TRL 1996) 6 1.0 10 0.8 14 0.6 20 0.5

29.29.Pre-coating of chippings-Pre-coating of chippings-contd (k)contd (k)

Pre- coated chippings should NOT be used with Emulsions because the breaking of the Emulsion will be adversely affected.

If the loss of chippings has occurred, it is advisable to check the viscosity of the binder and the packing arrangement of different agg. layers.

30. 30. BITUMENSBITUMENS. . Part IIPart II

It is essential that good bonding is achieved between the S.D and the existing road surface. This means that non – bituminous materials must be primed, before S.D is carried out.

Prime coats When a S.D is to be applied to a previously

untreated road surface, it is essential that the surface should be dry, clean and as dust free as possible. On granular, cement or lime stabilised surfaces, a prime coat of bitumen ensures that these conditions are met.

31. 31. The Functions-The Functions-

The functions of a prime coat of bitumen can be summarised as follows:

It assists in promoting and maintaining adhesion between the road base and the S.D by pre – coating the road base and penetrating the surface voids.

It helps to seal the surface pores in the road base thus reducing the absorption of the first spray of the binder of the S.D .

32. 32. The Functions- contd.The Functions- contd.

It helps to strengthen the road base near its surface by binding the finer particles of aggregates together.

If the application of the S.D is delayed for some reason, it provides the road base with a

temporary protection against rain fall and light traffic until the surface can be laid.

The depth of penetrations of the prime should be between 3-10mm and the quantity sprayed should be such that the surface is dry within a few hours.

33. Some other props of the prime 33. Some other props of the prime coatscoats

The correct viscosity and application rate are dependent primarily on the texture and density of the surface being primed.

Application rate 0.3-1.1 kg/m² or l/m².Low viscosity cutbacks are necessary for

dense cement or lime stabilized surface and higher viscosity cutbacks for untreated coarse – textured surfaces.

34. Some other props. – contd (a)34. Some other props. – contd (a)

Beneficial to spray, the surface lightly with water before applying the prime coat. This helps to suppress dust and allows the primer to spread more easily over the surface to penetrate.

Bit. Emulsions are not suitable for priming as they tend to form a skin on the surface.

Low viscosity, medium curing cutback bitumen such as MC -30,MC-70 or in rare circumstances MC-250 can be used for prime coats.

35. Some other props. – contd 35. Some other props. – contd (b)(b)

The relationship between grade and viscosity are given in Table 2.

Table 2- Grade of cutback binder permitted

viscosity range centistokes at

60°c MC 30 30-60 MC 70 70-140 MC 250 250-500

36.Bitumens for S.D .36.Bitumens for S.D .

The correct choice of bitumen for S.D work is critical. It must fulfill a number of impt. requirements. They must

be capable of being sprayed, “wet” the surface of the road in a continuous film,

not run off a cambered road or form pools of binder in local depressions,

“wet” and adhere to the chippings at road temp.

be strong enough to resist traffic forces and hold the chippings at the highest prevailing temp.

37.Bitumens for S.D - contd (a).37.Bitumens for S.D - contd (a).

remain flexible at the lowest ambient temp, neither cracking nor becoming brittle enough to allow traffic to pick-off the chippings and

resist premature weathering and hardening.Some of these requirements conflict hence the

optimum choice of binder involves a careful compromise. For example the binder must be sufficiently fluid at road temp. to ”wet” the chippings whilst being sufficiently viscous to retain the chippings against the dislodging effect of vehicle tires when the traffic is first allowed to run on the new dressings.

38. Fig. 2 (Overseas Road Note 3) – 38. Fig. 2 (Overseas Road Note 3) – page 8page 8

Shows the permissible range of binder viscosity for successful S.D at various road surface temps.

In the tropics, the day time road temps. lie between about 25°C and 50°C, normally in the upper half unless heavy rain is falling.

For these temps. The viscosity of the binder should lie between approximately 104and 7x105centistokes.

At the lower road temps, cutback grades of bitumen are most appropriate whilst at higher road temps, penetration grade Bitumen can be used.

39. Fig. 2 contd (a)39. Fig. 2 contd (a)The temp, viscosity relationship shown in

Fig.2 does not apply to Bitumen Emulsions. These have a relatively low viscosity and “wet” the chippings readily, after which the emulsion “breaks”, the water evaporates and particles of high viscosity bitumen adhere to the chipping and the road surface.

40. Types of bitumen.40. Types of bitumen.

Depending upon the availability and local conditions at the time of construction, the following types of bitumen are either commonly used in the tropics or are becoming so:

Penetration gradeCutbackEmulsionModified bitumen.

41. Penetration grade of bitumen.41. Penetration grade of bitumen.

These vary between 80/100 to approximately 700 pen.

The softer grades are usually produced in the refinery, but can be made in the field, by blending appropriate amounts of kerosene or diesel or a blend of kerosene and diesel.

With higher solvent contents, they are referred to as cut back bitumen which for S.D work is usually MC or RC 3000 grades.

In very rare circumstances. a less viscous grade such as MC or RC 800 may be used if the pavement temp, is below 15°C for long periods of the year.

42. Bitumen Emulsion.42. Bitumen Emulsion.

Cationic Bit. Emulsion with a Bit. content 70-75% is recommended for most S.D work.

This type of binder can be applied through whirling spray jets. at a temp. of 70-85°C. Once applied, it will break rapidly on contact with chippings of most mineral types.

The cationic emulsifier is normally an anti- stripping agent and this ensures good initial bonding between chippings and the bitumen

CRS emulsions have anti – stripping props with normal road aggregates.

43.Props.of Bit. Emulsion- contd (a)43.Props.of Bit. Emulsion- contd (a)

With a gradient or considerable road camber, the emulsion is likely to drain off, when high rates of spray are required.

In this situation, the emulsion “breaks” early.“split” bitumen application is possible –

reduced initial rate of spray and a heavier application after the chippings have been applied.

For a single seal coat, the second application of binder will have to be covered with SAND or quarry fines to prevent adhering to roller and vehicle wheels.

44.Props.of Bit. Emulsion- contd (b)44.Props.of Bit. Emulsion- contd (b)

Double surface dressing on a gradient – apply sufficient binder in the second spray to give the required rate for the finished dressing.

Use minimum excess of chippings to reduce whip off.

Apply second coat of binder before traffic is allowed onto the dressings.

45.Cutback Bitumens.45.Cutback Bitumens.

MC or RC 3000 grade cutback, normally the most fluid binder for S.D in the tropics.

Basically an 80/100 pen. Bitumen blended with 12-17% cutter – solvent 3:1 mixture of kerosene oil and diesel.

The above cutbacks may be prepared with 80/100 Bit blended with 2-10% diesel to give viscosities 1x104 - 7x105 centistokes for 25-45°C temp. range. (Refer Fig.2- page 8)

Fig 4 (page 9) shows the temp/ viscosity relationship for 5 blends.

46. Polymer modified bitumen. 46. Polymer modified bitumen. (e.g. Rubberised Bitumen.)(e.g. Rubberised Bitumen.)

Polymers can be used in S.D to modify pen. Grades, cutbacks and emulsions.

Great advantage at road surfaces, experiencing high stresses and strains.

47. Polymer modified bitumen – 47. Polymer modified bitumen – contd(a)contd(a)

These reduce temp. susceptibility (small variations with temp.), improves cohesive strength of the binder. (more able to retain chippings under stress from the action of traffic)

Improves early adhesive qualities (early re - opening to the traffic)

Increased elasticity in bridging hair line cracks.

Improved Durability.

48. Props of rubber modified 48. Props of rubber modified bitumen – contd(b)bitumen – contd(b)

Rubber modified bitumen may consist typically a blend of 80/100 bitumen with 2-3% un - vulcanised natural rubber powder prepared in static tanks. The blending temp. 170-200°C, very much cheaper to use natural rubber latex than powder.

49. Props of rubber modified 49. Props of rubber modified bitumen – contd(c)bitumen – contd(c)

Cationic Emulsion can be modified in purpose made plants by the addition of 3% latex Rubber. This is a very useful binder with good adhesion characteristics with normal acidic road aggregate.

50. Props. of rubber modified 50. Props. of rubber modified bitumen – contd(d)bitumen – contd(d)

Bitumen modified with SBS (styrene- Butadiene- Styrene) exhibits thermoplastic qualities at high temp. while having a rubbery nature at lower ambient temps.

51. Props. of SBS modified 51. Props. of SBS modified bitumenbitumen

With 3% SBS, noticeable changes in binder viscosity, and temp. susceptibility occur and

Good early adhesion is achieved. SBS can be obtained in a carrier bitumen

blocks of approximately 20kg.

52. Natural Rubber Latex modified 52. Natural Rubber Latex modified Bitumen.Bitumen.

Extensive local research carried out in Sri Lanka confirms that Nat. Rub. Latex modified bitumen is an ideal binder in road construction for Tropical and Temperate Climates. Easily prepared using about 2% by weight of Bitumen.

53. ADHESION AGENTS.53. ADHESION AGENTS.

Proprietary additives, known as adhesion agents are available for adding to binders to help to minimize the damage to S.D that may occur in wet weather with some type of stones.

When correctly used in right proportions, these agents can enhance adhesion between the binder film and the chippings even though they may be wet.

Immersion Tray Test (Appendix C- Road Note3) is used to determine the amount and the effectiveness of Adhesion agents.

54. DESIGN54. DESIGN. . PART III PART III

The key stages in the S.D procedures are outlined in Fig. 5 (page11).

The main factors to consider are:I. Existing site conditions Total traffic (all classes) – vehicle/lane/day. Hardness of the existing surface. Climatic conditions.

II. Type of chippings. Grading limits, nominal sizes. Maximum flakiness Index (F.I) Pre – coated chips.

55. Site conditions.55. Site conditions.

Larger chippings are required on soft surface or when traffic is heavy.

Whilst small chippings are best for hard surface.

For example – 20 mm chippings are appropriate on very soft surfaces carrying 1000 vehicles/lane/day and 6mm chippings on a very hard surface such as concrete. (Ref. Table 3, page 12)

56. Road Surface Hardness.56. Road Surface Hardness.

This is assessed by a simple penetration probe test (TRL 1996)

described in Appendix D (page 31).Alternately, assessment may be based on

judgment with the help of definitions given in Table 4 (page 12).

If larger chippings are used, it may result in “whip off” in early life of the S.D and significantly affect the durability on low volume traffic roads.

57. Selection of nominal size 57. Selection of nominal size of chippings.of chippings.

For double S.D, size of the 1st layer should be selected on the basis of the hardness of the existing surface and the traffic category indicated in Table 3 (page 12).

The nominal size of the chippings for the second layer should preferably have an average least dimension (ALD) not more than half that of the 1st layer.

This will promote good interlock between layers (If ALD of 1st layer is 1, than ALD of 2nd layer >1/2).

58. Hard existing surface.58. Hard existing surface.

For example, on a newly constructed cement stabilized road base or a dense crushed rock base, “a Pad Coat” of 6mm chips followed by 10 or 14mm chips may be applied.

The 1st layer adheres well to the hard surface and provides a “key” for the larger stone of the second dressing.

59. Selecting the binder.59. Selecting the binder.

Temp. of the road surface during construction.

Type of binder – pen. grade of Bit, cutback or emulsion.

Viscosity at the road surface temp, With reference to Fig.2 (page 8) it is

observed that for the surface temp. range 25-45°C, the viscosity of the binder should be 1x104-7x105 centistokes.

60.Choice of binder.60.Choice of binder.

a) MC 3000 or 400/500 pen. Bitumen. 400/500 pen, bitumen can be prepared from

80/100 pen bitumen, blend with 2-10% diesel or with a cutter – 3:1 by volume of kerosene oil and diesel.

b) Fig. 4- viscosity / temp. for blends of 80/100 is useful to determine the % diesel for the required viscosity.

c) Anionic emulsion may not adhere well to certain acidic agg. such as Granite and Quartzite.

d) Cationic emulsions are better with the usual road agg.

61.Choice of binder -contd 61.Choice of binder -contd (1).(1).

Fluid binders (e.g. emulsions) are not suited at gradients or steep cross falls due to drain off before “breaking”. “split application” of the binder may be used.

The choice of cut backs or pen. gr. bitumen is controlled by the road temp, at or shortly after the time of construction.

MC 3000 cut back with 12-17% cutter even under warm condition is very tolerant of short delays.

62.Choice of binder -contd 62.Choice of binder -contd (ii).(ii).

If the road temp. increases soon after construction, MC 3000 is likely to be tender and the seal can easily be damaged.

The use of pen gr. bitumen in the range 80/100 to 400 is preferred even at high road surface temps.

63.Choice of binder -contd 63.Choice of binder -contd (iii).(iii).

Factors which may influence the final selection of a binder include.

cost ease of use flexibility with regard to adjusting binder viscosity on site.

quality of the finished dressing.

64.Choice of binder -contd 64.Choice of binder -contd (iv).(iv).The use of pen. gr. bitumen in the range 80/100 to 400 is preferred to MC 3000 where circumstances allow this.

In either situation, early trafficking is very likely to dislodge chippings and seriously damage the seal.

65. Choice of binders . - contd 65. Choice of binders . - contd (v).(v).

For high volume fast traffic, when very early adhesion of the chippings is essential, consideration should be given to the use of pre - coated chippings.

A polymer modified or rubberised binder can provide immediate strong adhesion in addition to improvements in other props, Emulsions provide good “wetting”.

In surface dressing, it is better to use a more viscous binder with adequate “wetting”.

66. Choice of binders . - contd 66. Choice of binders . - contd (vi).(vi).

Consideration of these factors, will usually narrow the choice of binder to one or two options.

The final selection will be determined by other factors such as the past experience of the S.D Team.

67. DESIGNING THE SURFACE DRESSING 67. DESIGNING THE SURFACE DRESSING (S.D)(S.D)

Basis for the design method. Having selected the nominal size of chippings and

the type of binder to be used, the next step in the design of a S.D is to determine the rate of spread of the binder.

In this respect, recommendations are given in Road Note 39 (TRL 1996).

Conditions in U.K are not appropriate for most tropical or subtropical countries.

Differences in climate, uniformity of road surface, the quality of agg, traffic characteristics and construction practice, necessitate a more general approach to the determination of the rate of spread of the binder for application in tropical countries.

68. Basis for the design method-68. Basis for the design method-contd (a)contd (a)

To develop national or regional standards, the To develop national or regional standards, the method of S.D design proposed bymethod of S.D design proposed by Jackson(1963), Jackson(1963), modified by Les Hitch(1981) ismodified by Les Hitch(1981) is suitablesuitable..

This method relates voidsThis method relates voids in a layer of chippings in a layer of chippings to the amount of binder necessary to hold the to the amount of binder necessary to hold the chippings in place. chippings in place.

Hanson calculated that in a loose single layer of Hanson calculated that in a loose single layer of chippings, such as spread for a S.D, VOIDS are chippings, such as spread for a S.D, VOIDS are initially 50%,initially 50%, decreasing to about decreasing to about 30% after 30% after rollingrolling and subsequently and subsequently to 20%to 20% by the by the action of action of traffictraffic. .

69. Basis for the design 69. Basis for the design method-contd (b)method-contd (b)

For best results, between For best results, between 50 and 70% of the 50 and 70% of the voids in the compacted agg. should be filled voids in the compacted agg. should be filled with binder.with binder.

Hence it is possible to calculate the amount Hence it is possible to calculate the amount of binder required to retain a layer of of binder required to retain a layer of regular, cubical chippings of any size. regular, cubical chippings of any size.

However, in practice, However, in practice, chippings are rarely chippings are rarely thethe ideal cubical shapeideal cubical shape (especially when (especially when unsuitable crushing plants have been used) unsuitable crushing plants have been used) and this is and this is whywhy the concept of average least the concept of average least dimensiondimension (ALD) was originally introduced. (ALD) was originally introduced.

70. Determination of ALD. 70. Determination of ALD.

The ALD of chippings is a function of The ALD of chippings is a function of both the averageboth the average size of the chippingssize of the chippings, as determined by normal square – , as determined by normal square – mesh sieves and the degree of flakiness. Two methods are mesh sieves and the degree of flakiness. Two methods are available to determine ALD. available to determine ALD.

Method AMethod A – A grading analysis is performed on a – A grading analysis is performed on a representative sample of the chippings in accordance with representative sample of the chippings in accordance with BS812:1985. The sieve size through which 50% of the BS812:1985. The sieve size through which 50% of the chippings pass is also determined. chippings pass is also determined. The ALD of theThe ALD of the chippings is then determined from the Nomographchippings is then determined from the Nomograph (Fig. 6 – page 14) (Fig. 6 – page 14)

Method BMethod B – A representative sample of chippings is – A representative sample of chippings is carefully subdivided (BS 812: 1985) to give approx. 200 carefully subdivided (BS 812: 1985) to give approx. 200 chippings. chippings. The least dimension of each chippings isThe least dimension of each chippings is measured and the mean value ormeasured and the mean value or ALD is calculated. ALD is calculated.

71. Determining the overall 71. Determining the overall weighting factor (F).weighting factor (F).

The ALD of the chippings is used with an overall The ALD of the chippings is used with an overall weighting factor (F) to determine the basic rates weighting factor (F) to determine the basic rates of spray of bitumen. F is determined by adding of spray of bitumen. F is determined by adding together the four factors that represent:together the four factors that represent:

the level of traffic.the level of traffic. the condition of the existing road surface.the condition of the existing road surface. the climatic conditions.the climatic conditions. the type of chippings.the type of chippings.The factors appropriate to the site, to be surface The factors appropriate to the site, to be surface

dressed are selected from Table 5 (page 15).dressed are selected from Table 5 (page 15).

72. The rating for the amount 72. The rating for the amount of binder.of binder.

The rating for the existing surface allows for the The rating for the existing surface allows for the amount of binder which is required amount of binder which is required to fill the to fill the surface voids surface voids and which is therefore and which is therefore not not available to contribute to the film that retains the available to contribute to the film that retains the chippings.chippings.

If the existing surface of the If the existing surface of the road is roughroad is rough, it , it should be rated as “should be rated as “very lean bituminous.” evenvery lean bituminous.” even if its overall color is darkif its overall color is dark with bitumen.(+4) with bitumen.(+4)

Similarly, when determining the rate of spread of Similarly, when determining the rate of spread of binder for the binder for the secondsecond layer of a double S.D, layer of a double S.D, thethe first layer should also be rated very leanfirst layer should also be rated very lean bituminousbituminous.(+4).(+4)

73.Estimation of total traffic.73.Estimation of total traffic.

The Jackson method of determining the The Jackson method of determining the rate of spread of binder requires the rate of spread of binder requires the estimation of traffic in terms of estimation of traffic in terms of thethe number of vehicles onlynumber of vehicles only..

However, if the proportion of commercial However, if the proportion of commercial vehicles in the traffic stream is high (say vehicles in the traffic stream is high (say more than 20%), more than 20%), the traffic factor selectedthe traffic factor selected should be for the next higher category of should be for the next higher category of traffictraffic than is indicated by the simple than is indicated by the simple volume count.volume count.

74.Determining the basic 74.Determining the basic bitumen spray rate (R) .bitumen spray rate (R) .

Using the ALD and F (weighting factor) Using the ALD and F (weighting factor) values in Equation (1) – page 15, will give values in Equation (1) – page 15, will give the required basic rate of spread of binder the required basic rate of spread of binder R asR as

RR = 0.625+[FX0.023]+[0.0375+(F X0.0011)]ALD – = 0.625+[FX0.023]+[0.0375+(F X0.0011)]ALD – (1)(1)

Where F = overall weighting factor.Where F = overall weighting factor. ALD = the average least dimension of theALD = the average least dimension of the chippings (mm).chippings (mm). R = Basic rate of spread of R = Basic rate of spread of

binder(kg/m²).binder(kg/m²).

75. Example – DBST parameters 75. Example – DBST parameters (C3)(C3)

11STST seal seal 11stst seal seal 22ndnd seal seal Nom. Max chippings size (mm) 19.0 10.0Nom. Max chippings size (mm) 19.0 10.0 Median size(50%)(mm) 16.7 7.8Median size(50%)(mm) 16.7 7.8 Flakiness Index (F.I) % 0.0 18.0 Flakiness Index (F.I) % 0.0 18.0 ALD (mm) fromALD (mm) from (Nomograph Fig. 6) 12.9 5.8(Nomograph Fig. 6) 12.9 5.8Weighting factor for S.D (F) – Table 5 (page 15) Weighting factor for S.D (F) – Table 5 (page 15) 11stst seal seal 22ndnd seal seal Total traffic (Veh/ lane/ day) Total traffic (Veh/ lane/ day) medium heavy (500-1500) -1 -1medium heavy (500-1500) -1 -1Existing surfaceExisting surface – ABC, primed base +6 +4 – ABC, primed base +6 +4Climatic conditionClimatic condition – semi. Arid (hot and dry) -1 -1 – semi. Arid (hot and dry) -1 -1Type of chippingsType of chippings – (pre coated) – (pre coated) -2 -2 -2 -2 Total (F) Total (F) +2 +2 0 0

76. Rate of application of 76. Rate of application of binder (R).binder (R).

Basic bitumen spray rate R- kg/m² from equation (1) Basic bitumen spray rate R- kg/m² from equation (1) section 5.26 (page 15) can be obtained by the section 5.26 (page 15) can be obtained by the appropriate values for F and ALD.appropriate values for F and ALD.

11STST seal seal F = +2, ALD = 12.9F = +2, ALD = 12.9

Hence Hence R R2020(mm) =0.625+(2x0.023)+[0.0375+2x0.0011]12.9(mm) =0.625+(2x0.023)+[0.0375+2x0.0011]12.9 = = 1.183(kg/m²)1.183(kg/m²) 22ndnd seal seal F = 0, ALD = 5.8F = 0, ALD = 5.8 RR10(mm) 10(mm) = = 0.8420.842 Alternatively the two values can be used in Alternatively the two values can be used in the design chart Fig.7 the design chart Fig.7

(page 16) to determine R.(page 16) to determine R. The corresponding valuesThe corresponding values – – RR20(mm) = 20(mm) = 1.184 1.184 and and RR10(mm) = 10(mm) = 0.850.85

77. Correction for Durability 77. Correction for Durability w.r.t. MC 3000.w.r.t. MC 3000.

The basic spray rates are multiplied by The basic spray rates are multiplied by (1.08) for increased Durability as shown in (1.08) for increased Durability as shown in table E1 (Appendix E-page 32) where Rtable E1 (Appendix E-page 32) where RDD= = Design rateDesign rate

Then the rates will be ,Then the rates will be , RRDD20 20 == 1.184x1.081.184x1.08== 1.281.28(kg/ m²)(kg/ m²) RRDD10 10 == 0.85x1.080.85x1.08== 0.920.92(kg/ m²)(kg/ m²)

78. Correction for high road 78. Correction for high road surface temp,surface temp,

High road surface temp. approx 50°C prevails in High road surface temp. approx 50°C prevails in C3. Hence penetration grade binder C3. Hence penetration grade binder such as 400 such as 400 pen. Bitumen should be used.pen. Bitumen should be used. Therefor the basic Therefor the basic spray rates have to be corrected.spray rates have to be corrected.

As given as Table E1 (page 32), the basic spray As given as Table E1 (page 32), the basic spray rates should be multiplied by 0.9x1.067 to rates should be multiplied by 0.9x1.067 to correct for 400 pen. gr. bitumen. The final correct for 400 pen. gr. bitumen. The final adjustments are as follows:adjustments are as follows:

11stst seal R seal R2020 = 1.184x1.08x0.9x1.067 = 1.228kg/m²(l/m²). = 1.184x1.08x0.9x1.067 = 1.228kg/m²(l/m²). 22ndnd seal R seal R2020 = 0.850x1.08x0.9x1.067 = 0.882kg/m²(l/m²) = 0.850x1.08x0.9x1.067 = 0.882kg/m²(l/m²)

79. Rate of application of 79. Rate of application of chippings(1chippings(1stst seal). seal).

Estimate.Estimate. Loose bulk density of nom. 20mm = 1.44Mg/m³ or Loose bulk density of nom. 20mm = 1.44Mg/m³ or

1440kg/m³1440kg/m³ With reference to para 5.41,(page 17), for a l.b.d. of 1.35 With reference to para 5.41,(page 17), for a l.b.d. of 1.35

Mg/m²,Mg/m², The spread rate of 20mm chippings.=The spread rate of 20mm chippings.=1.364 1.364 x1.44xALD x1.44xALD 1.351.35 = 1.455X12.9= 1.455X12.9 = 18.77kg/m²= 18.77kg/m² or = 1877kg/100m²or = 1877kg/100m² Rate of Application = Rate of Application = 18771877 (cum/100m²) 1440(cum/100m²) 1440 =1.303=1.303 with an allowance of 10%with an allowance of 10% for whip off =for whip off =1.4331.433cum/100m²cum/100m²

80. Rate of application of 80. Rate of application of chippings(2chippings(2ndnd seal). seal).

Loose bulk density of nom 10mm = 1.406Mg/m³ or Loose bulk density of nom 10mm = 1.406Mg/m³ or 1406kg/m³1406kg/m³

Again with ref. of Para 5.41,(page 17), Again with ref. of Para 5.41,(page 17), The spread rate of 10mm chippings. =The spread rate of 10mm chippings. =1.364 1.364

x1.406xALD x1.406xALD 1.351.35 spread rate = 1.421x5.8spread rate = 1.421x5.8 = 8.24kg/m²= 8.24kg/m² or = 824kg/100m²or = 824kg/100m² Rate of Application = Rate of Application = 824824 (cum/100m²) 1406(cum/100m²) 1406 =0.586=0.586 with 10% for whip off =with 10% for whip off =0.6450.645cum/100m²cum/100m²

81.Spray rates adjustment 81.Spray rates adjustment factors.factors.

Traffic speed and road gradients.Traffic speed and road gradients.For slow traffic and gradients, steeper For slow traffic and gradients, steeper

than 3%, R should be reduced approx by than 3%, R should be reduced approx by 10%(Rx0.9)10%(Rx0.9)

For fast traffic and down grades, steeper For fast traffic and down grades, steeper than 3% R should be increased approx. by than 3% R should be increased approx. by 10%(RX1.1)10%(RX1.1)

Note – Definition of traffic speed is not Note – Definition of traffic speed is not precise to differentiate between road precise to differentiate between road carrying high proportion of heavy vehicles carrying high proportion of heavy vehicles and those carrying mainly cars at 80 and those carrying mainly cars at 80 km/hr or more.km/hr or more.

82. Modification of rate (R).82. Modification of rate (R).

Type of binderType of binder – (Ref. Table 6 – page 17). – (Ref. Table 6 – page 17). For cutback binders – e.g. MC/RC 3000 - no modification.For cutback binders – e.g. MC/RC 3000 - no modification. For pen. gr. Binders – e.g. 80/100Bit -Decrease R by 10%For pen. gr. Binders – e.g. 80/100Bit -Decrease R by 10%

(RX0.9) (RX0.9) 300pen. Bit -300pen. Bit -Decrease R by 05%(RX0.95)Decrease R by 05%(RX0.95) 400pen. Bit - Multiply R by (0.9x1.067) 400pen. Bit - Multiply R by (0.9x1.067) For Emulsion binderFor Emulsion binder - - Multiply R by (90/ %binder)Multiply R by (90/ %binder)

e.g. cutback Bit 70-75%-(90/75)=1.2e.g. cutback Bit 70-75%-(90/75)=1.2 NoteNote - % Binder is the percentage of - % Binder is the percentage of

bitumen inbitumen in the emulsion.the emulsion.

83. Amount of cutter required.83. Amount of cutter required.

For on “site” blending, amount of For on “site” blending, amount of cutter should be determined in the cutter should be determined in the LAB based on viscosity test.LAB based on viscosity test.

MC 3000 can be prepared in the MC 3000 can be prepared in the field by blending 90 pen.Bitumen field by blending 90 pen.Bitumen with 12-14 cutter by volume of a with 12-14 cutter by volume of a mixture of 3:1 of kerosene oil and mixture of 3:1 of kerosene oil and diesel.diesel.

84. Adjustment for 84. Adjustment for different grades of binder.different grades of binder.

If a different grade of binder in required, If a different grade of binder in required, then the adjustment factor should reflect then the adjustment factor should reflect the different amounts of cutter.the different amounts of cutter.

For instance, a 200 pen. Binder may have For instance, a 200 pen. Binder may have 3% cutter and therefore the spray rate is 3% cutter and therefore the spray rate is 103% of the rate for a 80/100 bitumen - 103% of the rate for a 80/100 bitumen - (R(RDD X0.9X1.03) for 400pen. gr. Bitumen X0.9X1.03) for 400pen. gr. Bitumen (made by cutting 80/100 pen. Bit. with (made by cutting 80/100 pen. Bit. with 6.7% cutter, the spray rate is (R6.7% cutter, the spray rate is (RDD X0.9X1.067) Refer –Table E1 page 32X0.9X1.067) Refer –Table E1 page 32

85. Adjustment of spray rates85. Adjustment of spray rates

The spray rates, arrived after applying the The spray rates, arrived after applying the adjustment factors in Table 6(page 17), adjustment factors in Table 6(page 17), will provide a very good surface texture will provide a very good surface texture and useful economic quantity of binder.and useful economic quantity of binder.

Because of the difficulties experienced in Because of the difficulties experienced in many countries in carrying out effective many countries in carrying out effective maintenance, there is considerable merit maintenance, there is considerable merit in sacrificing some surface texture for in sacrificing some surface texture for increased durability of the sealincreased durability of the seal..

86. Maximum Durability.86. Maximum Durability.

On flat terrain and carrying moderate to high On flat terrain and carrying moderate to high speed traffic, after increasing the spray rates speed traffic, after increasing the spray rates for factors given in Table 6(page 17),multiply for factors given in Table 6(page 17),multiply the net rate by the net rate by 8%(Rcx1.08).8%(Rcx1.08).

Heavier spray rates may result in a bitumen Heavier spray rates may result in a bitumen rich appearance in the wheel paths carrying rich appearance in the wheel paths carrying appreciable traffic volumes.appreciable traffic volumes.

Additional binder should not result in Additional binder should not result in BLEEDING. It can still be expected to retain BLEEDING. It can still be expected to retain more surface texture than is used in more surface texture than is used in Asp/Conc. – W/C.Asp/Conc. – W/C.

87. S.D design for low 87. S.D design for low volume roads.volume roads.

If a low volume road, carryingIf a low volume road, carrying less than 100 less than 100 vehicles/day isvehicles/day is surface dressed, it is very surface dressed, it is very important that important that the seal is designed to be as the seal is designed to be as durable as possible to minimize the need durable as possible to minimize the need for subsequent maintenance. for subsequent maintenance.

A double S.D should be used on new road A double S.D should be used on new road bases and the max. durability of the seal bases and the max. durability of the seal can be obtained by using can be obtained by using the heaviest the heaviest application which does not result in application which does not result in bleedingbleeding..

88. ALD of the two agg. sizes88. ALD of the two agg. sizes

Ideally, the ALD of the two Agg. sizes used Ideally, the ALD of the two Agg. sizes used in the double S.D. should differ by at least in the double S.D. should differ by at least a factor of two. If the ALD of the chippings a factor of two. If the ALD of the chippings in the second seal is more than half the in the second seal is more than half the ALD in the first seal, then the texture ALD in the first seal, then the texture depth will be further increased.depth will be further increased.

If ALD (2If ALD (2ndnd layer) > ½ ALD (1 layer) > ½ ALD (1stst layer) – texture depth further layer) – texture depth further increasedincreased

If ALD (2If ALD (2ndnd layer) = ½ ALD (1 layer) = ½ ALD (1stst layer) – ideal. layer) – ideal. If ALD (2If ALD (2ndnd layer) < ½ ALD (1 layer) < ½ ALD (1stst layer) – good interlock. layer) – good interlock.

89. Spread rate of 89. Spread rate of chippings.chippings.

An estimate of the rate of application of the An estimate of the rate of application of the chippings assuming a loose balk density of chippings assuming a loose balk density of 1.35Mg/m³ can be obtained from the 1.35Mg/m³ can be obtained from the following equation. following equation.

chippings application rate (kg/m²) =1.364x ALD (mm) chippings application rate (kg/m²) =1.364x ALD (mm) For any other loose bulk density (x) expressed in Mg/m³ or For any other loose bulk density (x) expressed in Mg/m³ or

10³kg/m³ 10³kg/m³ Rate of application = Rate of application = 1.3641.364 x X x ALD x X x ALD (kg/m²) 1.35(kg/m²) 1.35

The above rate should be a rough guide The above rate should be a rough guide onlyonly..

90. Allowance for losses 90. Allowance for losses and precise chippings and precise chippings application.application.

An addition of 10% is allowed for whip off. An addition of 10% is allowed for whip off. Storage and handling Storage and handling losseslosses must also be must also be allowed for when stockpiling chippings.allowed for when stockpiling chippings.

The precise chipping application must be The precise chipping application must be determined by observing on site whether determined by observing on site whether any exposed binder remains after any exposed binder remains after spreading the chips or whether spreading the chips or whether chips are chips are resting on top of each other. resting on top of each other.

Best results are obtained when the Best results are obtained when the chippings are tightly packed togetherchippings are tightly packed together, one , one layer thick.layer thick.

91.Precise application – contd 91.Precise application – contd (a)(a)

To achieve this, a slight excess of To achieve this, a slight excess of chippings must be applied.chippings must be applied.

Too great an excess of chippings will Too great an excess of chippings will increase the risk of whip – off and wind increase the risk of whip – off and wind screen damage.screen damage.

It is important that these increased spray It is important that these increased spray rates are adjusted on the rates are adjusted on the basis of TRIALbasis of TRIAL SECTIONSSECTIONS and local experience. and local experience.

92. PLANT AND EQIPMENT. 92. PLANT AND EQIPMENT. Part Part (iv).(iv).

Method of distributing binder. The success of a S.D is very dependent on

the binder being applied uniformly at the correct rate of speed. The method adopted for distributing the binder must therefore:

be capable of spreading the binder uniformly at pre – determined rate of spread: and

be able to spray a large enough area in a working day to match the required S.D programme.

93. Small scale methods.93. Small scale methods.

The use of hand – held containers such as watering cans, perforated buckets etc. has a place for minor works. Any type of binder from pen. grades to emulsions can be applied in this way but uniform spreading at pre- determined amounts cannot be achieved by this method and hence it is not recommended for anything other than small scale work.

94. Methods - contd (a).94. Methods - contd (a).

The use of either of these hand held methods of binder application for large scale work, invariably results in waste of valuable binder and a poor quality of S.D which will have a short life.

The spreading of binder on a large scale requires the use of a bulk binder distributor which may be either a self – propelled or a towed unit (BS 1707:1989, and BS 3136 :part2: 1972)

95. Large scale methods.95. Large scale methods.

There are two basic types of bulk binder distributors - pressurized tank, constant rate of spread, constant volume and constant pressure machines.

Most distributors manufactured in U.S.A are constant volume type, most distributors made in U.K are constant pressure type, constant volume distributors are quite common in tropical and developing countries.

96. Rollers and other 96. Rollers and other equipment.equipment.

The rolling of a S.D plays an impt. part in ensuring the retention of the chippings by assisting in the initial and bedding down the chippings in the binder.

Traditionally, steel – wheeled rollers have been used but these tend to crush weaker aggregates and to crack poorly shaped chippings.

Accordingly if steel – wheeled rollers are used, they should not exceed 8 tonnes in weight and should only be used on chippings which are strong enough. Some steel wheeled rollers are fitted with rubber sleeves which make them more suitable for S.D work, but as for any other roller of this type, they will “bridge” depressions in the existing road surface.

97. Other type of rollers.97. Other type of rollers.

In general pneumatic tired rollers (PTR) are preferred because tires have a kneading action which tends to maneuver the chippings into a tight mosaic without splitting them and do not bridge depressions.

In favourable conditions, adhesion should be well established within 30 mins. of rolling after which considerable benefit can be obtained by allowing slow moving traffic, particularly heavy lorries to traverse the dressing provided that traffic speed is below 20-30km/hr. This is very important and the use of a lead vehicle to “convoy” traffic at slow speed is recommended.

98. Other equipments.98. Other equipments.

Other important items of equipment required for S.D are mechanical brooms, binder heaters, decanters, and transporters, and front end loaders. Mechanical brooms either towed or powered, are invaluable for obtaining a clean road surface prior to spraying the binder.

Whilst hand booming is an alternative, it is difficult to obtain as good results by this method, particularly when sweeping the surface of a newly constructed road base, from which all loose particles should be removed.

99. THE SURFACE 99. THE SURFACE DRESSING PROCESS.DRESSING PROCESS.

Planning. A typical sequence of events in the

planning of a S.D. operation section 7.1 (pages 22,23 – Road Note 3).

The surface dressing operation. A sequence of events is given under

section 7.2 (page 23 – Road note 3).

100. After - Care :100. After - Care :

After care is an essential part of the surfacing and consists of removing excess chippings within 24 to 48 hours of the construction of a dressing.

Some loose chippings remaining on the surface are a hazard to wind screens of vehicles moving along on the new surface. It is usually a public complaint.

The loose particles can be removed by booming or by purpose made suction cleaners. Care must be taken with brooming to avoid damage to the new surface and it is usually best to do this work in the early morning when the S.D is still relatively stiff.

101. After - Care (contd –101. After - Care (contd –a).a).

It is important to stress that over – chippings can reduce the quality of a dressing, make after - care, a more time consuming process and also un necessarily increase the costs.

102. Other surface treatments102. Other surface treatments Part v.Part v.

Other types of treatments. Apart from S.D there are several other

kinds of surface treatments that complement S.D, five of which are described briefly below:

Slurry seals. A slurry seal is a mixture of Aggregates, Portland cement filler, bitumen emulsion and additional

water (ASTM, D. 3910, 1996 BS 434 part 1+2 1984)When freshly mixed, they have a thick creamy consistency and can be spread to a thickness of 5 to 10mm.

103. Slurry seals -contd 103. Slurry seals -contd (a).(a).

This method of surfacing is not normally used for new construction. Because it is more expensive than S.D ,does not provide as good a surface texture, and is not as durable as properly designed and constructed S.D. Slurry mixes are best made and spread by purpose made machines as shown in Fig.8 (page 24- Road Note 3).

Slurry seals are often used in combination with a S.D to make a “Cape seal”. In this technique the slurry seal is applied on top of a single S.D to produce a surface texture which is less harsh than a S.D alone and a surface which is flexible and durable.

104. 104. Slurry seals - Slurry seals - contd contd (b).(b).

However, the combination is more expensive than a double S.D and requires careful control construction.

Both Anionic and Cationic emulsions may be used in slurries containing acidic aggregates and its early breaking characteristics are also advantageous even when rainfall is likely to occur..

Suitable specifications for slurry seals and for a “Cape seal” are given in tables 10 and 11 (page 24 – Road Note 3).

105. Otta seal.105. Otta seal.

An Otta seal is different to S.D. in that a graded gravel or crushed agg. containing all sizes, including filler, is used instead of single sized chippings. There is no formal design procedure but recommendations based on case studies have been published. (NPRA, 1999).

An Otta seal may be applied in a single or double layer. Evidence on the performance of these types of seals has shown them to be satisfactory for 12 years on roads carrying up to 300 vehicles per day. (Overby 1998).

106. Otta seal - 106. Otta seal - contd(a).contd(a).

The grading of the materials is based on the level of traffic expected. Recommended grading envelops are given in Table 12(Page 25 – Road Note3).

Generally for roads carrying light traffic.(<100 veh./days.), a coarse grading should be chosen while a “dense” grading should be to one carrying (>100 veh./day).

The viscosities of binders used in construction should reflect the quality of agg. employed but cut back bitumen MC 800,MC 3000 or 150/200 pen. gr. Bitumen is used depending upon traffic volumes and type of Agg. cover.

107. Otta seal - 107. Otta seal - contd(b).contd(b).

Spray rates cannot be calculated by design and must be chosen empirically. Typical spray rates (hot) for single seals are between 1.6-2.0 l/m²(NPRA 1999).

It is because of the broad range of materials that may be used and the empirical nature that pre – construction trials must be carried out.

108. Otta seal - 108. Otta seal - contd(c).contd(c).

An important aspect of Otta seal construction is the need for extensive rolling by PTR for 2 or 3 days, after construction . The action of rolling ensures the binder is forced upwards, coating the Agg. and thereby initiating the process, continued by subsequent trafficking , of forming a premix like appearance to the surface.

After - care, can take as long as 12 days and involves sweeping dislodged agg. back into wheel paths for further compaction by traffic.

109. Sand seal.109. Sand seal.

Where chippings for a S.D are unobtainable or are very costly to provide, sand can be used as a “cover material” for a seal. Sand seals are less durable than S.D, the surface tends to abrade away under traffic.

Nevertheless, a sand seal can provide a satisfactory surfacing for lightly trafficked roads carrying less than 100/vehicles/lane/day.

It is not possible to design a sand seal in the same sense that a S.D can be designed. The particles of sand become submerged in the binder film, and the net result is a thin layer of sand – binder mixture adhering to the road surface.

110. Sand seal (a).110. Sand seal (a).

The sand should be clean coarse sand with a max size of 6 mm, containing upto 15% of material finer than 0.3mm and upto 2% of material finer than 0.15mm.

The sand should be applied at a rate of (6 to 7x10-3)m³/m² (CSRA 1986).

The binder which may be a cutback or an emulsion should be spread at a rate of approx. 1.0 to 1.2kg/m² depending on the type of surface being sealed.

111. Synthetic Agg. and Resin 111. Synthetic Agg. and Resin

Tretments.Tretments.

These treatments are costly and are used on relatively small areas, usually in urban situations, where high skidding resistance is required.

The agg. is normally a small single sized calcined Bauxite(Al2O3 soil with Al(OH)3) having a high resistance to polishing under traffic. The agg. is held by a film of epoxy resin binder (Denning 1978).

The process requires special mixing and laying equipment and is normally undertaken by special contractors.

112. Application of light 112. Application of light Bitumen sprays.Bitumen sprays.

There are two main uses for light sprays of Bitumen.A light film of binder which can be applied as the

final spray on a new S.D. The advantage of this procedure is that the risk of whip-off of chippings under fast traffic is reduced. This is particularly useful where management of traffic speed is difficult.

A light spray of binder can be used to extend the life of a bituminous surfacing. This is particularly useful when a surfacing is showing signs of bitumen ageing by fretting or cracking.

These applications may be referred to by different authorities as Fog sprays or Enrichment sprays.

113. Fog sprays.113. Fog sprays.

A light spray of bitumen emulsion is ideal for improving early retention of chippings in a new dressing. The road is usually dampened before spraying or if a low bitumen content emulsion (45%). is available, dampening can be omitted.

Complete breaking of the emulsion must occur before traffic is allowed onto the dressing and it may be necessary to dust the surface with sand or crushed fines to prevent pick up by traffic.

The spray rate is likely to be between 0.4 and 0.8 l/m². It is important to avoid over application of bitumen which could result in poor skid resistance.

114. Enrichment Sprays.114. Enrichment Sprays.

Surfaces which are showing obvious signs of disintegration through bitumen ageing can be enriched by apply

ing a stable grade of anionic emulsion which has been diluted at a rate of 1:1 with water (CSRA, 1972).

The rate of application will depend upon the texture of the surfacing and this must be determined by trial sprays. However it is likely to be between 0.2 and 0.5 l/m² of residual bitumen.

Great care must be taken to avoid leaving a slippery surface and a light application of sand sized fines may be required in some cases.

Take home messageTake home message

“Knowledge, Hard work and Dedication are pillars of Success”