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Surface Preparation

Before a pavement is placed the surface to be paved must be prepared. 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.

Milling machine Sweeper
Figure 1: Milling machine. Figure 2: Pavement cleaning after milling.

Surface preparation generally takes one of two forms:

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:

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):

Other Subgrade Preparation Practices

Other good subgrade practices are:

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.


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.

Repairing the surface before paving
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). 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). 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).

Applying tack coat
Tack cost
Figure 4: Tack coat application. Figure 5: Tack coat on left side.


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). 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:

Table 1: Milling Machine Parameter Ranges (from ARRA, 2001)

Typical Range
Cut Width
1.5 inches to 8 feet
(although narrower and wider drums are available)
Drums come in specific widths. Varying widths can be made with multiple passes.
Cut Depth
up to 10 inches per pass
It may be easier to make several shallow passes than one deep pass.
Production Rate
100 to 200 tons/hr
for large machines
Depends on machine and pavement conditions.
Material Size After Milling
95% passing the
2-inch sieve
Typical size.

Milling machine

Figure 6: Milling Operation

Figure 7: Milling Machine Components

Video 1: Milling Machine in Manoa Valley

HMA Overlays on Rigid Pavement

Placing a flexible overlay on a jointed rigid pavement involves some special considerations. Rigid pavement in Hawai'i is placed in discrete slabs, which tend to crack into discrete sections that move as individual units. Although HMA overlays can accommodate small differential subgrade movement without cracking, the large differential movement at slab and crack interfaces is great enough to crack an HMA overlay (called reflection cracking, one type of which is joint reflection cracking). There are several techniques to prevent (or at least delay the onset of) reflection cracking:

Multi-head breaker
Figure 8: Resonant pavement breaker. Figure 9: Multi-head breaker (MHB).


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