Surface Preparation

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 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 in order to (Asphalt Institute, 2001):

1. Fill the surface voids and protect the subbase from weather.
2. Stabilize the fines and preserve the subbase material.
3. Promote bonding to the subsequent pavement layers.

Other Subgrade Preparation Practices

Other good subgrade practices are:

1. 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.
2. 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.
3. 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 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 Figures 3 and 4).  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	Figure 4: Replacing a Rigid Pavement Slab Before an HMA Overlay

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 layer applied between HMA pavement lifts to promote bonding (see Figures 5 and 6).  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).

Figure 5: Tack Coat Application	Figure 6: Tack Coat Close-up on a Test Pad

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).  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:

1. Applying a leveling course (Figures 7 and 8).  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.

Figure 7: Left Side = Leveling Course Right Side = Leveling + Surface Course	Figure 8: Paving a Leveling Course

2. 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 (Roberts et al., 1996).  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 by sweeping or washing (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).  Milling is advantageous because it:
   • Provides RAP for recycling operations.
   • Efficiently removes deteriorated pavement that is unsuitable for retention in the overlaid pavement.
   • Provides a highly skid resistant surface suitable for temporary use by traffic until the final surface can be placed.
   • Allows curb and gutter lines to be maintained or reestablished before HMA overlays.
   • Provides an efficient removal technique for material near overhead structures in order to maintain clearances for bridge structures, traffic signals and overhead utilities.

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

Figure 7: Milling Machine Components

HMA Overlays on Rigid Pavement

Placing a flexible overlay on a jointed rigid pavement involves some special considerations.  Rigid pavement in Washington State 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:

• Prevent the slabs or sections from moving by stabilizing the material beneath them.  This involves drilling holes in an unstable PCC slab or section and injecting an asphaltic or cementitious material to fill any underlying voids.  Typically, this method is only an option for isolated instances of instability.  It does not work well as a general roadway treatment.
• Make the overlay structure strong enough to resist cracking.  This usually involves extra granular base layers between the flexible overlay and the existing rigid pavement or extremely thick flexible layers, both of which are often not cost effective.  Even if these types of preventative measures are used, they still cannot be guaranteed to prevent reflective cracking.
• Crack/break and seat the underlying rigid pavement.  This involves breaking the underlying rigid pavement into relatively small pieces (on the order of about 1 ft² to 2 ft²) by repeatedly dropping a large weight.  The pieces are then seated by 2 to 3 passes of a large rubber tired roller. 
• Rubblize the underlying rigid pavement.  This involves reducing the underlying rigid pavement to rubble.  This rubble is then used as a high quality base course to support a flexible overlay.  Rubblizing is typically done with one of the following two pieces of equipment:
  • Resonant pavement breaker (see Figure 8).  This equipment strikes the rigid pavement at low amplitude with a small plate at the resonant frequency of the slab (usually about 44 Hz) causing the slab to break apart  (Roberts et al., 1996).  Usually it takes about 14 to 18 passes for a resonant pavement breaker to rubblize an entire 12 ft. lane (NCAT, 2001).
  • Multi-head breaker (MHB) (see Figure 9).  This equipment uses a series of independently controlled high amplitude drop hammers to smash the slab.  Typically, there are between 12 and 16 hammers, each weighing between 450 - 680 kg (1000 - 1500 lbs.).  Hammers can be dropped from variable heights (1 - 5 ft.) and cycle at a rate of 30 - 35 impacts per minute.  MHBs can rubblize an entire lane (up to 13 ft.) in a single pass (Antigo Construction, 2001). 

Figure 8: Resonant Pavement Breaker	Figure 9: Multi-head Breaker (MHB)