The document discusses methods for preparing surfaces for new and overlay highway construction. For new pavements, adequate subgrade preparation is important and may involve compacting, stabilizing, or adding a prime coat to the subgrade. For overlays, the existing pavement must be repaired, leveled through milling or adding a leveling course, cleaned, and tack coated to bond to the new overlay. Proper preparation of both the subgrade and existing pavement surface is critical to the long-term performance of the highway.

1. HIGHWAY CONSTRUCTION METHODS
SURFACE PREPARATION
ENG. S.A.S.T SALAWAVIDANA
CEng FIESL, MCIHT, MEng (Highway & Traffic), BSc.Eng (Hons)

2. INTRODUCTION
• Before a pavement is actually placed at the
construction site the surface to be paved must be
prepared.
• Adequate surface preparation is essential to long-
term pavement performance.
• Pavements constructed without adequate surface
preparation may not meet smoothness
specifications, may not bond to the existing
pavement (in the case of overlays) or may fail
because of inadequate subgrade support.
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3. INTRODUCTION
• Surface preparation generally takes one of two forms:
– Preparing subgrade and granular base course for new
pavement. This can involve such activities as subgrade
stabilization (e.g., with lime, cement or emulsified
asphalt), over-excavation of poor subgrade, applying a
prime coat or compacting the subgrade.
– Preparing an existing pavement surface for overlay. This
can involve such activities as removing a top layer
through milling, applying a leveling course, applying a
tack coat, rubblizing or cracking and seating an
underlying rigid pavement, and replacing localized areas
of extreme damage.
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4. INTRODUCTION
• Specific actions for each method
depend upon the pavement type and
purpose, environmental conditions,
subgrade conditions, local experience
and specifications.
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5. Subgrade Preparation for New Pavements
• The overall strength and performance of a
pavement is dependent not only upon its design
(including both mix design and structural design)
but also on the load-bearing capacity of the
subgrade soil.
• Thus, anything that can be done to increase the
load-bearing capacity (or structural support) of
the subgrade soil will most likely improve the
pavement load-bearing capacity and thus,
pavement strength and performance.
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6. Subgrade Preparation for New Pavements
• Additionally, greater subgrade structural capacity can
result in thinner (but not excessively thin) and more
economical pavement structures.
• Finally, the finished subgrade should meet elevations,
grades and slopes specified in the contract plans.
– Increasing subgrade support by compaction
– Increasing subgrade support by alternative means
– Subgrade elevation
– Prime coats for flexible pavements
– Other subgrade preparation practices
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7. PRIMECOAT - FLEXIBLE PAVEMENTS
• For flexible pavements, the graded subgrade or the
top granular base layer may be prepared with a
primecoat if necessary.
• A prime coat is a sprayed application of a cutback or
emulsion asphalt applied to the surface of untreated
subgrade or base layers.
• Prime coats have three
– Fill the surface voids and protect the subbase from weather.
– Stabilize the fines and preserve the subbase material.
– Promotes bonding to the subsequent pavement layers.
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8. PRIMECOAT - FLEXIBLE PAVEMENTS
• Generally, if a flexible pavement is to be less than 100
mm (4 inches) thick and placed over an unbound
material, a prime coat is recommended (Asphalt
Institute, 2001).
• This work consists of an application of a prime coat on a
base or subbase newly constructed using gravelly soil,
stabilized soil or aggregate, prior to laying of a surface
dressing or a surface course, so as to provide a proper
bond between the layers and also to serve as a
protective measure for the base or subbase.
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9. Materials
• Materials used must meet the following
requirements.
• Binder must be medium curing cutback bitumen
(20-45% or MC 30 or MC 70 or MC 250 or MC 800
or bituminous emulsion of grade CSS-1 or grade
CSS-1h.
• Blotting (blinding) material must be coarse sand.
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10. Materials
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11. Weather Limitations
• The prime coat must generally be applied in
dry weather and when the ambient
temperature is not below 150C.
• It must not be applied when the rain is
imminent.
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12. Preparation of Surface
• Before the application of the prime coats, the
surfaces must be well brushed to remove all dust,
loose particles and other objectionable material
with a power broom or any other suitable method.
• In the case of aggregate bases, such cleaning must
continue until the entire surface shows a pattern of
exposed large particles free from dust as far as
practicable.
• The surface must be wetted with water before the
application of the binder.
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13. Application of Binder
• The prime coat must be applied by means of a
mechanical sprayer or a hand sprayer at the
temperatures given in the following table.
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14. Aftercare
• Traffic must not be allowed on the
primed surface for a period of minimum
3 hours until the binder has penetrated
and dried up and will not be picked up
by traffic.
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15. Other Subgrade Preparation Practices
• Other good subgrade practices
• Ensure the compacted subgrade is able to support
construction traffic.
• If the subgrade ruts excessively under construction
traffic it should be repaired before being paved over.
• Left unrepaired, subgrade ruts may reflectively cause
premature pavement rutting.
• Remove all debris, large rocks, vegetation and topsoil
from the area to be paved.
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16. Other Subgrade Preparation Practices
• These items either do not compact well or cause non-
uniform compaction and mat thickness.
• Treat the subgrade under the area to be paved with
an approved herbicide.
• This will prevent or at least retard future vegetation
growth, which could affect subgrade support or lead
directly to pavement failure.
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17. Other Subgrade Preparation Practices
• In summary, subgrade preparation should
result in a material capable of supporting
loads without excessive deformation and
graded to specified elevations and slopes.
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18. Existing Surface Preparation for Overlays
• Overlays make up a large portion of the roadway paving
done today.
• The degree of surface preparation for an overlay is
dependent on the condition and type of the existing
pavement.
• Generally, the existing pavement should be structurally
sound, level, clean and capable of bonding to the overlay.
• To meet these prerequisites, the existing pavement is
usually repaired, leveled (by milling, preleveling or both),
cleaned and then coated with a binding agent.
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19. Existing Surface Preparation for Overlays
• Repair
• Tack coats
• Leveling (both by applying a leveling
coarse and by milling)
• Flexible overlays on rigid pavement
• Rigid overlays on flexible pavement
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20. Repair
• Generally, pavement overlays are used to restore
surface course (both flexible and rigid) characteristics
(such as smoothness, friction and aesthetics) or add
structural support to an existing pavement.
• However, even a structural overlay needs to be placed
on a structurally sound base.
• If an existing pavement is cracked or provides
inadequate structural support these defects will often
reflect through even the best-constructed overlay and
cause premature pavement failure in the form of cracks
and deformations.
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21. Repair
• To maximize an overlay’s useful life, failed sections of
the existing pavements should be patched or replaced
and existing pavement cracks should be filled.
• At most, overlays are designed to add only some
structural support; the remaining structural support
must reside in the existing pavement.
• Therefore, small areas of localized structural failure in
the existing pavement should be repaired or replaced
to provide this structural support.
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22. Repair
• Often, existing pavement failure may be caused
by inadequate subgrade support or poor
subgrade drainage.
• In these cases, the existing pavement over the
failed area should be removed and the subgrade
should be prepared as it would be for a new
pavement.
• Existing pavement crack repair methods depend
upon the type and severity of cracks.
• Badly cracked pavement sections,
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23. Repair
• Especially those with pattern cracking (e.g., fatigue
cracking) or severe slab cracks, must be patched or
replaced because these distresses are often symptoms
of more extensive pavement or subgrade structural
failure.
• Existing cracks other than those symptomatic of
structural failure should be cleaned out (blown out with
pressurized air and/or swept) and filled with a crack-
sealing material when the cracks are clean and dry.
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24. Repair
• Cracks less than about 10 mm (0.375 inches) in width
may be too narrow for crack-sealing material to
enter.
• These narrow cracks can be widened with a
mechanical router before sealing.
• If the existing pavement has an excessive amount of
fine cracks but is still structurally adequate, it may be
more economical to apply a general bituminous
surface treatment (BST) or slurry seal instead of filling
each individual crack.
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25. Repair
• In all, pavement repair should be extensive
enough to provide an existing pavement
with adequate structural support.
• Pavement management techniques should
provide for overlays before an existing
pavement has lost most or all of its
structural support capability.
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26. TACK COATS
• Before overlaying, a tack coat should be placed on an
existing pavement to ensure adequate bonding of the
overlay to the existing pavement surface.
• Proper tack coat application can be critical to long-
term pavement performance.
• This work consists of an application of a tack coat to an
existing pavement before the construction of a
surfacing so as to provide a bond between the two
layers.
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27. Materials
• Binder used must be medium curing cutback
bitumen (10-20%) or MC 800 or MC 3000 or
bituminous emulsions of any specified grade.
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28. Weather Limitations
• The tack coat must generally be applied in
dry weather and when the ambient
temperature is not below 150C.
• It must not be applied when the rain is
imminent.
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29. Preparation of Surface
• Before the application of the tack coat, the
surface must be dried, thoroughly swept and
rendered clean of clay, dust and any other
extraneous material.
• The tack coat must be applied only when the
surface is dry.
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30. Application of Binder
• Where cutback bitumen is used the temperature of
application must be 105 to 1300C.
• Where emulsions are used, they must normally be
applied at ambient temperatures.
• However, the emulsions must be warmed to facilitate
uniform application, where necessary.
• They must be applied using a mechanical sprayer, hand
sprayer or by any other appropriate means.
• The rate of application of tack coats generally range
between 0.25 and 0.55 l/m2 depending on the surface
condition. Eng. Suneth Thushara Salawavidana 30

31. Application of Binder
• In areas being treated, precautions must be taken to prevent
the surfaces structures and trees being spattered or marred,
and the binder getting into channels, catchpits and drains.
• The emulsions may be diluted with clean water, if necessary,
in order to control the rate of spread.
• However, dilution of CRS types is normally not
recommended.
• Tacked surfaces must be allowed to dry until the proper
condition of tackiness is achieved to receive the surface
course.
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32. Application of Binder
• In order to obtain the proper condition of tackiness,
tack coat must be applied only so far in advance of
the surface course placement as is necessary.
• Normally the tack coats must not be left applied for
more than about 6 hours without the placement of
the surface course.
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33. Leveling
• The existing pavement should be made as smooth as possible
before being overlaid.
• It is difficult to make up elevation differences or smooth out ruts by
varying overlay thickness.
• For flexible overlays, HMA tends to differentially compact; a rule of
thumb is that conventional mixes will compact approximately 6 mm
per 25 mm (0.25 inches per 1 inch) of uncompacted thickness.
• Therefore, before applying the final surface course the existing
pavement is typically leveled by one or both of the following
methods:
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34. Leveling
• Applying a leveling course (flexible pavements).
– The first lift applied to the existing pavement is used to fill in ruts
and make up elevation differences.
– The top of this lift, which is relatively smooth, is used as the base
for the wearing course.
• Milling(flexible pavements).
– A top layer is milled off the existing pavement to provide a
relatively smooth surface on which to pave.
– Milling is also commonly used to remove a distressed surface layer
from an existing pavement.
• Diamond grinding(rigid pavements).
– A thin top layer can be milled off of an existing pavement to
smooth out relatively small surface distortions prior to flexible or
rigid overlay.
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35. Leveling Course - Flexible Pavements
• Leveling courses (or prelevel) are initial lifts placed
directly on to the existing pavement to fill low spots
in the pavement.
• Typically, pavers use an automatic screed control,
which keeps the screed tow point constant
regardless of the tractor unit’s vertical position.
• This allows the paver to drive over a rough, uneven
pavement yet place a relatively smooth lift with
extra HMA making up for low spots in the existing
pavement.
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36. Leveling Course - Flexible Pavements
• Leveling course lifts need to be as thick as the deepest low
spot but not so thick that they are difficult to compact.
• Because it is not the final wearing course, leveling course
elevation and grade are sometimes not tightly specified or
controlled.
• However, contractors and inspectors alike should pay
close attention to leveling course thickness because an
excessively thick leveling course can lead to large overruns
in HMA and thus large overruns in project budget.
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37. Leveling Course - Flexible Pavements
• Although leveling courses can help produce a
smoother pavement, they suffer from the previously
discussed differential compaction and therefore may
not entirely solve the smoothness problem.
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38. Milling - Flexible Pavements
• Milling (also called grinding or cold planing) can be used to
smooth an existing flexible pavement prior to flexible or rigid
overlays.
• Rather than filling in low spots, as a leveling course does,
milling removes the high points in an existing pavement to
produce a relatively smooth surface.
• For flexible pavements, milling can help eliminate differential
compaction problems.
• Milling machines are the primary method for removing old
flexible pavement surface material prior to overlay.
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39. Milling - Flexible Pavements
• They can be fitted with automatic grade control to
restore both longitudinal and transverse grade
and can remove most existing pavement
distortions such as rutting, bumps, deteriorated
surface material or stripping.
• The primary advantages of milling are as follows:
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40. Milling - Flexible Pavements
• Eliminates the need for complicated leveling courses and
problems with quantity estimates for irregular leveling course
thicknesses used to fill existing pavement depressions.
• Provides RAP for recycling operations.
• Allows efficient removal of deteriorated flexible pavement
material that is unsuitable for retention in the pavement
structure.
• Provides a highly skid resistant surface suitable for temporary
use by traffic until the final surface can be placed.
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41. Milling - Flexible Pavements
• Allows curb and gutter lines to be maintained
or reestablished before flexible overlays.
• Provides an efficient removal technique for
material near overhead structures in order to
maintain clearances for bridge structures,
traffic signals and overhead utilities.
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42. Milling - Flexible Pavements
• After a pavement has been milled the resulting
surface is quite dirty and dusty.
• The surface should be cleaned off by sweeping or
washing before any overlay is placed otherwise the
dirt and dust will decrease the bond between the
new overlay and the existing pavement.
• When sweeping, more than one pass is typically
needed to remove all the dirt and dust.
• If the milled surface is washed, the pavement must
be allowed to dry prior to paving.
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43. Milling - Flexible Pavements
• Milling also produces a rough, grooved surface, which will
increase the existing pavement’s surface area when
compared to an ungrooved surface.
• The surface area increase is dependent on the type, number,
condition and spacing of cutting drum teeth but is typically in
the range of 20 to 30 percent, which requires a
corresponding increase in tack coat (20 to 30 percent more)
when compared to an unmilled surface.
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44. Eng. S.A.S.T Salawavidana