Chapter 3: Crack Sealing, Crack Filling & Joint Sealing of Flexible & Rigid Pavements

2.0 Materials

2.1 Materials

2.1.1 Materials for Crack Sealing

Crack sealing materials are designed for three main functions:  to adhere to the walls of the crack, stretch with the movement of the crack over the range of conditions and loads associated with the crack location, and resist abrasion and damage caused by traffic.  For sealing working cracks, the preferred sealant is usually elastomeric.  This means the sealant has a low modulus of elasticity and will stretch easily to high elongations (usually around 10 times its non-strained dimensions) without fracture. Such sealants also recover over time to close to their original dimensions.  Sealants are usually applied at elevated temperatures due to their high viscosity at ambient temperatures and they have a thermoplastic property that allows them to set or cure by cooling and reforming into complex structures.  These materials are sometimes incorrectly referred to as thermosets, a term that describes materials that undergo chemical cross-linking when heated.  Such materials have structures that are retained with cooling and are not reversible by reheating. Thermoplastics form physical structures on cooling, but this process is reversible with reheating. Hot application ensures good adhesive bond to the crack walls.

Cold poured materials for crack sealing are usually silicone-based and often used prior to paving.  These materials cure either by exposure to moisture in the air, or by mixing a hardening agent with the base silicone.  They often have poor abrasion resistance and should not be used in trafficked areas. Other materials such as epoxies and polyurethanes are almost always cured by addition of a second chemical.

2.1.2 Materials for Crack Filling

For crack filling applications, the cracks are basically inactive (non-working).  Crack filling materials are designed to adhere to the walls of the crack, and resist abrasion and damage caused by traffic.

Crack filling materials may be hot applied rubber or polymer asphalts, or cold applied emulsion-based products. The emulsion products assist with forming a good adhesive bond with the crack wall and additives such as Styrene Butadiene Rubber (SBR) latex ensure that the material can endure some degree of movement.  In some cases, hot applied fiber modified asphalt binders may be used.

2.2 Storage and Handling of Materials

This section identifies procedures for material storage and handling.  In all cases, the manufacturer’s recommendations for storage and handling should be closely followed.

 Hot pour materials require very high temperatures, typically between 188 to 200°C (370 to 390°F) (3).  These materials may degrade or cross-link when exposed to excessive temperatures for long periods of time.  For this reason, the manufacturer’s recommendations must be followed closely. 

2.3 Material Placement Methods

Once a suitable seal or fill material has been selected, the appropriate placement method must be determined.  Placement methods vary according to the nature of the distress.  When selecting the placement method, one should consider the method’s applicability to: 1) the type of distress, 2) the dimensions of the crack channel, 3) the type of crack channel (cut or uncut), and 4) the finish requirements.  Each method carries its own set of job equipment and preparation requirements.  Placement methods include:

• Flush Fill
• Overband
• Reservoir
• Combination: Reservoir w/Band-Aid
• Combination: Sand Fill w/ Recessed Finish
• Backer Rod

2.3.1 Flush Fill Method

In the flush fill method, fill material is forced into an existing uncut crack.  Once filled, the crack is struck off flush with the pavement.  Figure 11 illustrates the flush fill method.

2.3.2 Overband Method

In the overband method, fill material is forced into and placed over an uncut crack.  If the fill material is squeegeed flat, it is referred to as a ‘Band-Aid’; if not; it is referred to as capped.  Overbanding and capping should not be done if silicone has been chosen as the fill material.  This is due to silicone’s poor abrasion resistance.  Figure 12 illustrates the overband method with both finishing options.

The capped overband method is not recommended.  All crack sealing and filling should be squeegeed if material is left above the surface.  Overbanding can create a rough ride and/or excess road noise and causes problems when placing subsequent overlays.

2.3.3 Reservoir Method

In the reservoir method, the crack is cut or routed to form a reservoir that is filled with a sealant.  The sealant may be left flush or slightly below the surface of the reservoir.  The depth and width of the reservoir varies according to job requirements.  Saw depths will be greatest when working with very active cracks and cracks in PCC pavements.  Crack cutting will often depend on the number of cracks and whether the cutter can follow the shape of the crack.  Typical reservoir widths range from 12 to 25 mm (0.5 to 1.0 in), and even up to 38 mm (1.5 in) in very cold climates.  Reservoir depth ranges from 12 to 25 mm (0.5 to 1.0 in).  Reservoir use is appropriate for pavements in good condition without extensive cracking.  Crack cutting units, when operated by trained, experienced personnel, can follow meandering random cracks.  Figure 13 illustrates the reservoir method.

2.3.4 Combination Method:  Reservoir with Band-Aid

This combination method involves the formation of a Band-Aid over the top of a cut reservoir.  Figure 14 illustrates the combination method.  Like the overband method, the combination method should not be used with materials that are prone to pickup due to traffic or materials with poor wearing characteristics (3).  The combination method can be used on heavily trafficked roads, but care must be taken to squeegee excess material off the surface.

2.3.5 Combination Method: Sand Fill with Recessed Finish

Thermal cracking can develop over time and penetrate the full depth of asphalt pavement in a roadway.  As thermal cracks progress down through the asphalt layers, they typically continue to widen.  It is not unusual for such cracks to be 12 to 25 mm (0.5 to 1 in) or wider and exceed 102 mm (4 in) in depth.  If these types of cracks are sealed or filled full depth, the large volumes of filler or sealer tend to soften and migrate under loads in hot weather and begin to pull out under traffic. If an overlay is applied, the heat of the new mat will draw the filler and sealer materials up through the overlay.  In areas with heavy sealer or filler applications, fat spots, flushing, and shoving in the overlay can occur.  These symptoms can only be remedied by changes in construction procedures or the removal and replacement of the affected materials.

Sealant application should not exceed 25 mm (1 in) in depth.  For full depth wide cracks, backer rod can be used to limit sealant depth.  Another method is to partially fill the crack with sand.  Blow out any debris with air, fill the crack with clean sand to a point approximately 19 to 25 mm (0.75 to 1 in) below the adjacent pavement surface, and tamp lightly as needed with a steel rod or piece of rebar to reduce any large voids in the sand.  Then apply the crack sealer over the top of the sand and along the crack faces.  The surface of the sealant should be cupped slightly below the adjacent pavement surface.  This recessed finish allows some movement of the crack and sealer material without creating an undesirable hump on the surface.  It fills and seals the deep wide crack while limiting the impact on subsequent paving operations.  Figure 15 illustrates this combination method.

2.3.6 Backer Rod (PCC Pavements)

Joint sealing applications for PCC pavements may require the incorporation of a backer rod or bond breaker.  The backer rod, typically polyethylene foam, is placed within a crack or joint to prevent the sealant from sticking to the reservoir bottom and to restrict the sealant depth to the upper portion of the joint.  A backer rod is also incorporated in very large cracks or joints,  particularly when a self-leveling silicone is being used.  A backer rod is only used if it is cost effective and the cracks are relatively straight like those occurring in PCC joints.  Figure 16 shows three typical backer rod configurations.

2.4 Selecting the Appropriate Placement Method

The appropriate placement method should be based on a project’s governing considerations such as:

• Type and extent of the sealing or filling operation
• Traffic conditions
• Crack characteristics
• Material requirements
• Desired performance or expectations
• Aesthetics
• Cost

Table 3 outlines method placement issues in relation to governing project considerations.