2  Surface Preparation

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.  Surface preparation generally takes one of two forms:

1. Preparing the 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.
2. 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.

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

2.1  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.  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.  This subsection covers:

• Increasing subgrade support by compaction
• Increasing subgrade support by alternative means
• Subgrade elevation
• Primecoats for flexible pavements
• Other subgrade preparation practices

2.1.1  Increasing Subgrade Support - Compaction

In order to provide maximum structural support (as measured by M_R, CBR or R-value), a subgrade soil must be compacted to an adequate density (see Figure 7.1).  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 150 mm (6 inches) of subgrade of not less than 95 percent of maximum density determined in the laboratory.  In fill areas, subgrade below the top 150 mm (6 inches) is often considered adequate if it is compacted to 90 percent relative density.  In order to achieve these densities the subgrade must be at or near its optimum moisture content (the moisture content at which maximum density can be achieved).  Usually compaction of in situ or fill subgrade will result in adequate structural support. 

2.1.2  Increasing Subgrade Support - Alternative Means

If the structural support offered by the in situ compacted subgrade is or is estimated to be inadequate, there are three options (any one or combination of the three can be used): 

1. Stabilization.  The binding characteristics of these materials generally increase subgrade load-bearing capacity. Typically, lime is used with highly plastic soils (plasticity index greater than 10), cement is used with less plastic soils (plasticity index less than 10) and emulsified asphalt can be used with sandy soils.  For flexible pavements, a primecoat is not effective on silty clay or clay soils because the material cannot be absorbed into such a fine soil (TRB, 2000).
2. Over-excavation.  The general principle is to replace poor load-bearing in situ subgrade with better load-bearing fill.  Typically, 0.3 - 0.6 m (1 - 2 ft.) of poor soil may be excavated and replaced with better load-bearing fill such as gravel borrow.
3. Add a base course and perhaps a subbase course over the subgrade.  A base course offers additional load-bearing capacity.  New pavement structural designs often use some sort of granular base course unless subgrade structural support is extremely good and expected loads are extremely low.  Base courses are subjected to the same compaction and elevation requirements as subgrade soils.

2.1.3  Subgrade Elevation

After final grading (often called fine-grading), the subgrade elevation should generally conform closely to the construction plan subgrade elevation (see Figure 7.2).  Large elevation discrepancies should not be compensated for by varying pavement or base thickness because (1) HMA, PCC and aggregate are more expensive than subgrade and (2) in the case of flexible pavements, HMA compacts differentially – thicker areas compact more than thinner areas, which will result in the subgrade elevation discrepancies affecting final pavement smoothness.

2.1.4  Primecoat - Flexible Pavements

For flexible pavements, the graded subgrade or the top granular base layer may be prepared with a primecoat if necessary.  A primecoat is a sprayed application of a cutback or emulsion asphalt applied to the surface of untreated subgrade or base layers (Asphalt Institute, 2001).  Primecoats have three purposes (Asphalt Institute, 2001):

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

Generally, if a flexible pavement is to be less than 100 mm (4 inches) thick and placed over an unbound material, a primecoat is recommended (Asphalt Institute, 2001).

2.1.5  Other Subgrade Preparation Practices

Other good subgrade practices are (CAPA, 2000; APAW, 1995):

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.
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.  

2.2  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.  This subsection covers:

• Repair
• Tack coats
• Leveling (both by applying a leveling coarse and by milling)
• Flexible overlays on rigid pavement
• Rigid overlays on flexible pavement

2.2.1  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.  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 (see Figure 7.3).  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,

	Figure 7.3: Repairing Failed Pavement Sections Before Overlay

 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 (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 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.

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.

2.2.2  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.

2.2.3  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 (TRB, 2000).  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 (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.
2. 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.
3. 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.

2.2.3.1  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 (see Figure 7.4).  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.

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.

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.

2.2.3.2  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 (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 such as rutting, bumps, deteriorated surface material or stripping.  The primary advantages of milling are (Roberts et al., 1996): 

1. Eliminates the need for complicated leveling courses and problems with quantity estimates for irregular leveling course thicknesses used to fill existing pavement depressions.
2. Provides RAP for recycling operations.
3. Allows efficient removal of deteriorated flexible pavement material that is unsuitable for retention in the pavement structure.
4. Provides a highly skid resistant surface suitable for temporary use by traffic until the final surface can be placed.
5. Allows curb and gutter lines to be maintained or reestablished before flexible overlays.
6. Provides an efficient removal technique for material near overhead structures in order to maintain clearances for bridge structures, traffic signals and overhead utilities.

The basic components of a milling machine are a cutting drum to mill the existing pavement, a vacuum to collect the milled particles and a conveyance system to transport the milled particles to a dump truck for hauling (see Figure 7.5, 7.6 and 7.7).  Table 7.1 shows ranges for some key milling machine parameters, Figures 7.8 and 7.9 show two milling machine examples, Figures 7.10 and 7.11 show milled pavements and Video 7.1 shows the basic milling process. 

Table 7.1: Milling Machine Parameter Ranges (from ARRA, 2001)

7.5: Milling Machine Components

Video 7.1 Milling Machine

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 (see Figure 7.12 and 7.13).  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.

Figure 7.12: Sweeping the Existing Surface Prior to Overlay	Figure 7.13: Washing the Existing Surface Prior to Overlay

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).

2.2.3.3  Leveling Course vs. Milling

For many situations, milling may be a superior alternative to a leveling course.  Leveling course quantities are difficult to accurately estimate and leveling course thicknesses are usually small, precluding the use of nuclear gauge density testing.  Thus, adequate mix density is difficult to achieve and measure.  In some overlay projects a combination of milling and leveling course application may be best.

2.2.3.4  Diamond Grinding - Rigid Pavements

Although typically used for rigid pavement surface restoration, diamond grinding can be used to eliminate relatively small surface distortions in existing rigid pavement prior to flexible or rigid overlays.  Because it roughens the existing rigid pavement surface, diamond grinding also improves the bond between the existing pavement and the overlay.  Non-overlay applications of diamond grinding are covered in Module 10, Section 4, Rigid - Maintenance.

2.2.4  Flexible Overlays on Rigid Pavement

Placing a flexible overlay on a jointed rigid pavement involves some special considerations in addition to the usual repair and leveling.  jointed plain concrete pavement (JPCP) is placed in discrete slabs and both JPCP and continuously reinforced concrete pavement (CRCP) tend to crack into discrete sections.  These slabs/sections tend to move as individual units.  Although flexible overlays can accommodate small differential subgrade movement without cracking, the large differential movement at slab and crack interfaces is great enough to crack a flexible overlay (called 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 flexible 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 0.3 m² to 0.6 m² (1 ft² to 2 ft²) by repeatedly dropping a large weight (see Figure 7.14).  The pieces are then seated by 2 to 3 passes of a large rubber tired roller.  The result is a pavement made of small firmly-seated pieces (see Figure 7.15).  Video 7.2 briefly shows the process.
• 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 the flexible overlay.  Rubblizing is typically done with one of the following two pieces of equipment:
  • Resonant pavement breaker (see Figure 7.16 and Video 7.3).  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 (see Figure 7.17) (Roberts et al., 1996).  Usually it takes about 14 to 18 passes for a resonant pavement breaker to rubblize an entire 3.6 m (12 ft.) lane (NCAT, 2001).
  • Multi-head breaker (MHB) (see Figures 7.18, 7.19 and Video 7.4).  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 (0.3 - 1.5 m (1 - 5 ft.)) to create impact energies between 2,700 - 16,300 N-m (2,000 - 12,000 ft.-lbs.).  Hammers cycle at a rate of 30 - 35 impacts per minute.  MHBs can rubblize an entire lane (up to 4 m (13 ft.)) in a single pass (Antigo Construction, 2001). 

Figure 7.14: Drop Hammer Used for Cracking and Seating PCC	Figure 7.15: PCC Pavement After Cracking and Seating with Drop Hammer
	Video 7.2: Drop Hammer Used for Breaking and Seating PCC (video has no sound)
Figure 7.16: Resonant Pavement Breaker Used to Rubblize PCC Pavement	Figure 7.17: PCC Pavement After Rubblization With a Resonant Pavement Breaker
Figure 7.18: Multi-Head Breaker	Figure 7.19: Multi-Head Breaker with Following "Grid" Roller Used to Crush and Compact the Resulting Rubble

A 38-state survey published in 1999 (Ksaibati, Miley and Armaghani, 1999) revealed the following about rigid pavement rubblizing:

• Distresses in the subsequent flexible overlay such as fatigue cracking and rutting are most often traced to a weak subgrade.  This subgrade is also the most likely cause of the original rigid pavement distress.  Rubblization is risky when subgrade support conditions are not well known.
• A majority of rubblized particles are in the 25.4 - 76.2 mm (1 - 3 inch) range, although particles near pavement edged or under existing reinforcing steel can be as large as 380 mm (15 inches).
• Rubblizing is generally better than cracking and seating for reducing reflective cracking.

Given the expense of these techniques, some agencies just choose to live with joint reflection cracking rather than prevent it.  This is especially true on low volume, low speed roads where ride smoothness and structural integrity may not be given the high priority they are on high volume, high speed roads like interstates.

2.2.5  Rigid Overlays on Flexible Pavement (from ACPA, 2001b)

2.2.5.1  Unbonded Overlays

Unbonded rigid overlays do not require much surface preparation, which is one of the principal reasons they are used.

2.2.5.2  Bonded Overlays

Bonded rigid overlays of flexible pavement require several additional considerations.  First, the success of a bonded overlay is contingent on a good bond between the rigid overlay and the underlying flexible pavement.  In order to develop this bond, the underlying flexible pavement must have a clean, rough surface.  Preferably, the flexible pavement should be milled, however, as a minimum, water or abrasive blasting should be used to clean the HMA surface.  If water blasting is used, the surface must be allowed to air dry before the PCC is placed.

Once the flexible pavement surface is clean, it must be kept clean until the bonded overlay is placed.  Dust, dirt and debris that falls or blows onto the asphalt surface must be removed.  If the surface is cleaned on the day prior to paving, air cleaning may be required on the day of paving to remove dirt and dust.  If traffic is allowed on the milled surface, the surface must be cleaned again prior to paving.

2.3  Summary

Pavements should be placed only on properly prepared surfaces to ensure they perform properly.  Pavements constructed on inadequately prepared surfaces may be excessively rough, may not bond to the existing pavement (in the case of overlays) or may fail because of inadequate subgrade support.  For a new pavement, surface preparation involves compacting, grading and possibly stabilizing the underlying subgrade.  For an overlay, surface preparation involves repairing, leveling and cleaning the existing pavement.