2  Flexible - Maintenance

Pavement maintenance describes all the methods and techniques used to preserve pavement condition, safety, and ride quality, and therefore aid a pavement in achieving its design life (Hall et al., 2001).  The performance of a pavement is directly tied to the timing, type and quality of the maintenance it receives.  This section, taken largely from Roberts et al. (1996), describes the more common U.S. preventative and corrective maintenance options for HMA pavement.  The timing of these maintenance items is discussed in Module 11, Pavement Management.

2.1  Crack Seals

Crack seal products are used to fill individual pavement cracks to prevent entry of water or other non-compressible substances such as sand, dirt, rocks or weeds.  Crack sealant is typically used on early stage longitudinal cracks, transverse cracks, reflection cracks and block cracks.  Alligator cracks are most often too extensive to warrant filling with crack sealer; they usually require an area treatment such as a patch or reconstruction.  Crack filler material is typically some form of rubberized asphalt or sand slurry.

2.2  Fog Seals

A fog seal is a light application of a diluted slow-setting asphalt emulsion to the surface of an aged (oxidized) pavement surface.  Fog seals are low-cost and are used to restore flexibility to an existing HMA pavement surface.  They may be able to temporarily postpone the need for a surface treatment or non-structural overlay. 

2.3  Rejuvenators

Rejuvenators are products designed to restore original properties to aged (oxidized) asphalt binders by restoring the original ratio of asphaltenes to maltenes.  Many rejuvenators are proprietary, making it difficult to offer a good generic description.  However, many rejuvenators contain maltenes because their quantity is reduced by oxidation.  Rejuvenators will retard the loss of surface fines and reduce the formation of additional cracks, however they will also reduce pavement skid resistance for up to 1 year (Army and Air Force, 1988).   Because of this, rejuvenators are generally appropriate for low-volume, low-speed roads or parking lots.    

2.4  Slurry Seals

A slurry seal is a homogenous mixture of emulsified asphalt, water, well-graded fine aggregate and mineral filler that has a creamy fluid-like appearance when applied.  Slurry seals are used to fill existing pavement surface defects as either a preparatory treatment for other maintenance treatments or as a wearing course.  There are three basic aggregate gradations used in slurry seals:

1. Type I (fine).  This type has the finest aggregate gradation (most are smaller than the 2.36 mm (No. 8) sieve) and is used to fill small surface cracks and provide a thin covering on the existing pavement.  Type I aggregate slurries are sometimes used as a preparatory treatment for HMA overlays or surface treatments.  Type I aggregate slurries are generally limited to low traffic areas (ISSA, 2001).
2. Type II (general).  This type is coarser than a Type I aggregate slurry (it has a maximum aggregate size of 6.4 mm (0.25 inches)) and is used to (1) treat existing pavement that exhibits moderate to severe raveling due to aging or (2) to improve skid resistance.  Type II aggregate slurry is the most common type.
3. Type III (coarse).  This type has the most coarse gradation and is used to treat severe surface defects.  Because of its aggregate size, it can be used to fill slight depressions to prevent water ponding and reduce the probability of vehicle hydroplaning.       

Microsurfacing
Microsurfacing is an advanced form of slurry seal that uses the same basic ingredients (emulsified asphalt, water, fine aggregate and mineral filler) and combines them with advanced polymer additives.  Figures 10.1 through 10.4 show a microsurfacing slurry seal project. 

Figure 10.1: Microsurfacing Truck	Figure 10.2 Microsurfacing Placement
Figure 10.3: Microsurface Close-Up	Figure 10.4: Finished Microsurface

2.5  Bituminous Surface Treatments (BST)

A bituminous surface treatment, also known as a seal coat or chip seal, is a thin protective wearing surface that is applied to a pavement or base course.  BSTs can provide all of the following:

• A waterproof layer to protect the underlying pavement.
• Increased skid resistance.
• A fill for existing cracks or raveled surfaces.
• An anti-glare surface during wet weather and an increased reflective surface for night driving.

A single layer BST is constructed in the following steps:

1. Surface preparation.  Surface defects, such as potholes, are repaired and the existing surface is cleaned (e.g., by a street sweeper).
2. Asphalt material application.  Typically, an asphalt emulsion is applied from a spray truck to the surface of the existing pavement (see Figure 10.5).  
3. Aggregate application.  A thin aggregate cover (only one stone thick) is spread over the asphalt material before it has set (see Figure 10.6).  The aggregate usually has a uniform gradation.
4. Aggregate embedding.  A roller (usually a pneumatic tire roller) is used to push the aggregate into the asphalt material and seat it firmly against the underlying pavement (see Figure 10.7).  Generally, about 50 percent of each aggregate particle should be embedded in the asphalt material (see Figure 10.8) after final rolling.  About 70 percent of each aggregate particle will be embedded after several weeks of traffic.  It is common to place an aggregate "chokestone" on top of the uniformly graded larger aggregates after embedment.  Chokestone is essentially a finer aggregate gradation (e.g., less than 12.5 mm (0.25 inches)) used to make a more dense aggregate matrix at the level of embedment (see Figure 10.9).  This more dense matrix helps prevent excessive aggregate loss due to traffic. 

Multiple layer surface treatments are done by repeating the above process for each layer.  Figure 10.10 shows a BST in Washington State.

Figure 10.5: Placing the Asphalt Emulsion	Figure 10.6: Placing the Aggregate
Figure 10.7: Embedding the Aggregate	Figure 10.8: BST Before Chokestone Application (note asphalt emulsion is visible between aggregates)
Figure 10.9: BST After Chokestone Application (note small chokestone between the larger aggregates)	Figure 10.10: BST on SR 2 near Coulee City, WA

2.6  Non-Structural Overlays

Figure 10.11: Non-Structural Overlay

Non-structural overlays (see Figure 10.11) do not involve extensive structural design and generally contribute little, if anything, to a pavement's structural capacity.  Non-structural overlays are generally thin surface overlays on the order of 12.5 mm (0.5 in.) to 37.5 mm (1.5 in.) that are used to (NAPA, 1995):

• Improve ride quality.
• Correct minor surface defects.
• Improve safety characteristics such as skid resistance and drainage.
• Enhance appearance.
• Reduce road-tire noise.

2.6.1  Categories

Non-structural overlays can vary widely in composition depending upon local practice, traffic and general purpose.  A loose classification of non-structural overlays follows (NAPA, 1995):

1. Light volume/residential traffic.  The primary objective in light traffic areas is to retard asphalt binder aging of the underlying pavement.  Since heavy traffic loads are not of great concern, overlays are generally less stiff (resulting in a more workable mix, increased durability and flexibility and a potential for the overlay to reheal under traffic) and use smaller-sized aggregates.
2. Heavy, high-speed traffic.  The primary objective in heavy, high-speed traffic areas is to prevent rutting and provide good friction.  Because of this, overlays typically use larger angular aggregate and more durable mixes such as SMA or OGFC. 

2.6.2  Construction Notes

Non-structural overlays are generally quite thin.  This results in several construction concerns (NAPA, 1995):

• Thin lifts require less HMA per foot of road length than thick lifts.  This can result in high paver speeds (in excess of 21 m (70 ft.) per minute).  Compaction may not be able to keep pace with these high speeds. 
• Thin lifts will cool quicker than thick lifts.  This can result in little time available for compaction before the thin overlay reaches cessation temperature (sometimes as little as 3 to 5 minutes).  Therefore, roller variables should be set to account for this (e.g., enough rollers and an adequate roller pattern to compact the material before it reaches cessation temperature).
• Thin lift construction produces greater screed wear.  If the lift depth is less than about twice the maximum aggregate size, the HMA may tear under the paver screed.  Very thin lifts (less than 25 mm (1 inch)) can be damaged by the screed dragging large particles.
• Thin lifts are more sensitive to vibratory rolling.  Incorrectly chosen amplitude, frequency or roller speed can result in aggregate degradation (i.e. breaking) and damage of the bond between the overlay and the existing pavement.
• Density control is difficult.  Thin lifts provide fewer options for aggregate particles to rearrange under compaction.  Thus, mat densities will tend to be less uniform than those associated with a thicker lift.  This should be recognized if pay is in any way tied to mat density.

In general, compaction is more difficult and more variable on thin lifts.

2.7  Patches

Patches are a common method of treating an area of localized distress.  Patches can be either full-depth where they extend from the pavement surface to the subgrade (see Figure 10.12) or partial where they do not extend through the full depth of existing pavement (see Figure 10.13). 

Full-depth patches are necessary where the entire depth of pavement is distressed.  Often times, the underlying base, subbase or subgrade material is the distresses root cause and will also need repair.  Partial depth patches are used for pavement distresses like raveling, rutting, delamination and cracking where the depth of crack does not extend through the entire pavement depth. 

Patching material can be just about any HMA or cold mix asphalt material as well as certain types of slurries.  Typically some form of HMA is used for permanent patches, while cold mix is often used for temporary emergency repairs.

Figure 10.12: Full-Depth Patch	Figure 10.13: Partial-Depth Patch

One form of patching, pothole patching, probably receives the greatest amount of public attention.  Pothole patching procedures cover a wide range of methods and intentions from permanent full-depth patches to temporary partial depth patches.  Two general patching procedures are described next.

Semi-Permanent Pothole Patch (see Figures 10.14 and 10.15) (from FHWA, 1998)

1. Remove all water and debris from the pothole.
2. Square up the pothole sides so they are vertical and have in-tact pavement on all sides.
3. Place the patching material into the clean squared-up hole.  The material should mound in the center and taper down to the edges so that it meets flush with the surrounding pavement edges.
4. Compact the patching material starting in the center and working out toward the edges.  Compaction can be accomplished using a vibratory plate compactor or a single-drum vibratory roller.  Check the compacted patching material for a slight crown.  This is done so that subsequent traffic loading will compact it down to the surrounding pavement height.

Figure 10.14: Pothole Patching Truck with a Hotbox	Figure 10.15: Semi-permanent Pothole Repair

Throw-and-roll (from FHWA, 1998)

1. Place the patching material into the pothole without any preparation or water/debris removal.
2. Compact the patching material using the patching truck tires (usually 4 to 8 passes).
3. Check the compacted patch for a slight crown.  If a depression is present add more patching material and compact.

Although it may seem that the semi-permanent technique would produce a higher quality patch than the throw-and-roll technique, the FHWA's Long Term Pavement Performance (LTPP) Study found that the "throw-and-roll technique proved just as effective as the semi-permanent procedure for those materials for which the two procedures were compared directly" (FHWA, 1998).  Since the semi-permanent technique is more labor and material intensive, the throw-and-roll technique will generally prove more cost effective if quality materials are used.

2.8  Summary

Pavement maintenance prolongs pavement life by slowing its deterioration rate.  This section has described some of the more common maintenance options in the U.S.  Each option's effectiveness is dependent upon a multitude of local conditions.  For most smaller agencies, the best advice when considering pavement maintenance options is to talk to local contractors and nearby agencies about what types of maintenance options have worked best in your local area.