Surface Preparation

Before a pavement is placed 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.

Figure 1: Milling Machine Figure 2: Pavement Cleaning After Milling

Surface preparation generally takes one of two forms:

 Preparing the subgrade and granular base course for new pavement. This can involve
such activities as subgrade stabilization, over-excavation of poor subgrade, applying
a prime coat or compacting the subgrade.
 Preparing an existing pavement surface for overlay. This can involved such activities as
replacing localized areas of extreme damage, applying a leveling course, milling,
applying a tack coat, rubblizing or cracking and seating an underlying rigid pavement,
and replacing localized areas of extreme damage.

Specific actions for each method depend upon the pavement type and purpose, environmental
conditions, subgrade conditions, local experience and specifications.

Subgrade Preparation for New Pavements

Anything that can be done to increase the load-bearing capacity of the subgrade soil will most
likely improve pavement load-bearing capacity and thus, pavement strength and performance.
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

Generally, subgrade support can be increased by one or several of the following means:

 Compaction. Subgrade should be compacted to adequate density before pavement

placement. If it is not, the subgrade will continue to compress, deform or erode after
construction, causing pavement cracks and deformation. Generally, adequate density is
specified as a relative density for the top 6 inches of subgrade of not less than 95 percent
of a specified standard laboratory density. In fill areas, subgrade below the top 6 inches is
often considered adequate if it is compacted to 90 percent relative density.
 Stabilization. Lime, portland cement or emulsified asphalt can be mixed in with the
subgrade soil to increase its strength and stiffness. Construction geotextiles can be used to
help stabilize roadways with early signs of subgrade failure.
 Over-excavation. Poor in situ subgrade can be replaced with better load-bearing fill.
Typically, 1 – 2 ft. of poor soil may be excavated and replaced with high quality fill.
 Add a subbase course over the subgrade. A subbase course offers additional load-bearing
capacity. Subbase courses are subjected to the same compaction and elevation
requirements as subgrade soils.

Subgrade Elevation
After final grading (often called fine-grading), the subgrade elevation should generally conform
closely to construction plan subgrade elevation. Large elevation discrepancies should not be
compensated for by varying pavement or base thickness because (1) HMA, and aggregate are
more expensive than subgrade and (2) HMA compacts differentially – thicker areas compact
more than thinner areas, which will result in the subgrade elevation discrepancies affecting final
pavement smoothness.

Prime Coats
The graded subgrade or the top granular base layer can be prepared with a prime coat. A prime
coat is a sprayed application of a cutback or asphalt emulsion applied to the surface of untreated
subgrade or base layers in order to (Asphalt Institute, 2001[1]):

 Fill the surface voids and protect the subbase from weather.
 Stabilize the fines and preserve the subbase material.
 Promote bonding to the subsequent pavement layers.
Other Subgrade Preparation Practices
Other good subgrade practices are:

 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 and will
result in variable paving thicknesses as the HMA fills the wheel ruts and is displaced by
the rut ridges. The result can be a pavement with areas that are too thin (over the rut
ridges) and too thick (over the rut depressions) instead of a uniform depth.
 Remove all debris, large rocks, vegetation and topsoil from the area to be paved. 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.

In summary, subgrade preparation should result in a material (1) capable of supporting loads
without excessive deformation and (2) graded to the specified elevations and slopes.

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, cleaned and then coated with a binding agent.

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. 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. Small areas of localized structural failure in the existing pavement should be
repaired or replaced to provide this structural support (see Figure 3). If the existing pavement
contains areas of inadequate subgrade support, these areas should be removed and the subgrade
should be prepared as it would be for a new pavement.
 Figure 3: Replacing a Deteriorated Portion of
the Existing Pavement
Existing pavement crack repair methods depend upon the type and severity of cracks. Badly
cracked pavement sections, especially those with pattern cracking (e.g., fatigue cracking) must
be patched or replaced because these distresses are often symptoms of more extensive pavement
or subgrade structural failure (TRB, 2000[2]). 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 (TRB, 2000[2]). Cracks less than
about 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.

Tack Coats
A tack coat is a thin bituminous liquid asphalt, emulsion or cutback coating applied between
HMA pavement lifts to promote bonding (see Figures 4 and 5). Adequate bonding between
construction lifts and especially between the existing road surface and an overlay is critical in
order for the completed pavement structure to behave as a single unit and provide adequate
strength. If adjacent layers do not bond to one another they essentially behave as multiple
independent thin layers – none of which are designed to accommodate the anticipated traffic-
imposed bending stresses. Inadequate bonding between layers can result in delamination
(debonding) followed by longitudinal wheel path cracking, fatigue cracking, potholes, and other
distresses such as rutting that greatly reduce pavement life (TxDOT, 2001[3]).
 Figure 4: Tack Coat Application Figure 5: Tack Coat on Left Side

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. HMA tends to
differentially compact; a rule of thumb is that conventional mixes will compact approximately
0.25 inches per 1 inch of uncompacted thickness (TRB, 2000[1]). Thus, thicker pavements will
compact more. Therefore, before applying the final surface course the existing pavement is
typically leveled by one or both of the following methods:

 Applying a leveling course. 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. 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.
 Milling (also called grinding or cold planing). 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. Milling
machines are the primary method for removing old HMA pavement surface material prior
to overlay (see Figures 1 and 6). They can be fitted with automatic grade control to
restore both longitudinal and transverse grade and can remove most existing pavement
distortions. After milling, pavement surfaces should be cleaned off (see Figure 2) before
any overlay is placed otherwise dirt and dust may decrease bonding between the new
overlay and the existing pavement. 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 (TRB, 2000[1]). Milling is advantageous because it:
o Provides RAP for recycling operations.
o Efficiently removes deteriorated pavement that is unsuitable for retention in the
overlaid pavement.
o Provides a highly skid resistant surface suitable for temporary use by traffic until
the final surface can be placed.
o Allows curb and gutter lines to be maintained or reestablished before HMA
overlays.
o Provides an efficient removal technique for material near overhead structures in
order to maintain clearances for bridge structures, traffic signals and overhead
utilities.