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

1.0 Introduction

Cracking in pavements occurs when a stress is built up in a surface layer that exceeds the tensile or shear strength of the pavement causing a fissure or crack to open.  Crack sealing and crack filling are methods that can be used to repair these cracks in pavement surfaces.  The cause of the crack and its activity play a dominant role in determining the success of crack sealing or filling operations.

This chapter addresses crack sealing and filling techniques associated with flexible hot mix asphalt (HMA) pavements and joint and crack sealing of rigid Portland cement concrete (PCC) pavement systems.  The reader is advised to pay close attention to the type of pavement system being addressed, as treatment techniques can vary.

1.1 Types of Cracks

Cracking may be associated with various distress mechanisms.  Crack types include: fatigue cracks, longitudinal cracks, transverse cracks, block cracks, reflective cracks, edge cracks, slippage cracks, and joints in PCC pavements.  Each crack type is discussed below:

1.1.1 Flexible (AC) Pavements

Fatigue Cracking:  These cracks form a pattern similar to an alligator’s skin as illustrated in Figure 1.  They are the result of repetitive traffic loads or high deflections often due to wet bases or sub grades.  This type of cracking can also lead to potholes and pavement disintegration.  Neither crack sealing or filling can treat this type of failure.  Alligator cracking can be preceded by longitudinal cracking in the wheel paths. 

Longitudinal Cracks:  These cracks run longitudinally along the pavement, as shown in Figure 2, and are caused by thermal stress and/or traffic loadings.  They occur frequently at joints between adjacent travel lanes or between a travel lane and the shoulder, where hot mix density is lower and voids are higher.  Longitudinal cracking may be associated with raveling and poor adhesion or stripping.  These cracks can be treated effectively with crack sealants. 

Figure 1: Fatigue Cracking

Figure 2: Longitudinal Cracking

Transverse Cracks:  These cracks occur perpendicular to the centerline of the pavement, or laydown direction, as shown in Figure 3.  Transverse cracks are generally caused by thermally induced shrinkage at low temperatures.  When the tensile stress due to shrinkage exceeds the tensile strength of the HMA pavement surface, cracks occur.  These cracks can be effectively treated with crack sealants. 

Block Cracking:  These cracks form regular blocks (Figure 4) and are the result of age hardening of the asphalt coupled with shrinkage during cold weather.  They can be effectively treated with crack sealants. 

(Direction of Travel >>) Figure 3: Transverse Cracking

Figure 4: Block Cracking

Reflection Cracking: Reflection cracks are caused by cracks or other discontinuities in an underlying pavement surface that propagate up through an overlay due to movement at the crack.  They exhibit any of the crack patterns mentioned and must be treated according to the original distress mechanism.  Figure 5 illustrates reflection cracking in asphalt concrete over Portland cement concrete

Edge Cracking: These are crescent-shaped or fairly continuous cracks intersecting the pavement edge.  They are located within 0.6 m (2 ft) of the pavement edge, adjacent to an unpaved shoulder.  They include longitudinal cracks outside of the wheel path and within 0.6 m (2 ft) of the pavement edge (1).  Figure 6 illustrates edge cracking.  Edge cracks are caused by overloading at the edge of the pavement, shear failure or erosion in the shoulder.  This type of cracking cannot always be treated effectively with crack sealants.

Slippage Cracks: These cracks produce a characteristic crescent shape, as shown in Figure 7.  They occur when the top layer of the asphalt shears due to high deflections and a poor bond between the layers. This type of cracking cannot be treated effectively with crack sealants.

Figure 5: Reflection Cracking

Figure 6: Edge Cracking

Figure 7: Slippage Cracking

1.1.2 Rigid (PCC) Pavements

Joints:  Joints in rigid pavements are designed and constructed to permit expansion and contraction of rigid pavements so as to prevent cracking of the slabs between the joints.  Typically they are constructed by sawing the concrete shortly after placement of the concrete.  Joints may be transverse or longitudinal.  Normally they are sealed during construction and resealed as needed throughout the life of the pavement.  Joints are generally straight with vertical cut faces.

Cracks:  Cracks in rigid pavements are generally associated with load or excessive thermal movement that is not adequately controlled by the joint system.  Cracks may be transverse, longitudinal, or angled, especially at slab corners.

1.2 Project Selection

Crack sealing and or crack filling may be an option for either surface preparation or surface sealing of a cracked PCC or HMA pavement. Projects are selected on the following criteria:

• The base should be sound.
• Cracks are only sealed or filled when greater than 3mm (0.1 inches) or up to 25mm (1 inch).

1.3 Project Planning

Ideally, crack-sealing treatments should be applied during moderately cold weather conditions when the crack width is at its midpoint to widest, usually in the fall, winter, or early spring.  Weather conditions during installation need to be appropriate, i.e., not too cold or wet.  Because non-working cracks do not widen significantly with temperature, crack filling treatments can be applied at any time of year when weather conditions are appropriate.  Traffic passing over a hot applied sealed or filled crack is usually not an issue; however, traffic control during the application of the treatment should be in force long enough to allow for adequate curing of the product to prevent tracking.  Sand is typically needed for cold applied systems to prevent tracking.  Planning considerations will vary according to the treatment method chosen.  For example, cold pour materials require different handling than hot pour. Hot pour must be preheated before work may commence, but cold pour may be used immediately.

1.4 Seal or Fill

The first question is whether to seal or fill a crack.  Cracks may open and close horizontally with temperature and moisture changes and may undergo vertical movements as the result of load applications.  Figures 8 and 9 illustrate these mechanisms of crack movement.

To determine whether to seal or fill a crack, it must be established whether the crack is working or non-working and whether the crack undergoes horizontal or vertical movement.  The total horizontal movement of a crack over the period of one year is the primary determining factor of whether a crack is a working or non-working crack.  The FHWA criterion for a working crack is an annual horizontal movement of at least 3 mm (1/8 inch) (3).  Vertical movement is not usually considered (3).  Additionally, the width of the crack plays a role in deciding whether it is a working or non-working crack.  Crack sealing is usually triggered when the crack width exceeds 6 mm (1/4 in).  Also, the type of the crack can provide an indication of whether it is a working crack or not.  Working cracks can be transverse or longitudinal to the pavement, but are most often transverse.  Working cracks with limited edge deterioration should be sealed, rather than filled.

When the criteria for working cracks are not met, or when cracks are closely spaced and have little movement, crack filling is less expensive (3).  FHWA criteria for deciding whether to seal or fill a crack are listed in Table 1.

1.4.1 Crack Sealing

Crack sealing and filling prevent the intrusion of water and incompressible materials into cracks.  The methods vary in the amount of crack preparation required and the types of sealant materials that are used. 

Crack sealing is the placement of materials into working cracks.  Crack sealing requires thorough crack preparation and often requires specialized high quality materials placed either into or above working cracks to prevent the intrusion of water and incompressible materials.  Crack sealing is generally considered to be a longer-term treatment than crack filling.

Due to the moving nature of working cracks, a suitable crack sealant must be capable of:

• Remaining adhered to the walls of the crack,
• Elongating to the maximum opening of the crack and recovering to the original dimensions without rupture,
• Expanding and contracting over a range of service temperatures without rupture or delamination from the crack walls, and
• Resisting abrasion and damage caused by traffic.

Section 2.1 discusses material requirements in further detail.

1.4.2 Crack Filling

Crack filling is the placement of materials into nonworking or low movement cracks to reduce infiltration of water and incompressible materials into the crack.  Filling typically involves less crack preparation than sealing and performance requirements may be lower for the filler materials.  Filling is often considered a short-term treatment to help hold the pavement together between major maintenance operations or until a scheduled rehabilitation activity.

Crack filling is for active or non-active cracks created by aging of the binder.  Such cracks are not completely inactive and require some flexible characteristics.  A suitable filler material must be capable of:

• Remaining attached to the walls of the crack,
• Possessing some elasticity, and
• Resisting abrasion and damage caused by traffic.

Section 2.1 discusses material requirements in further detail.

1.5 Treatment Performance

The performance life of a treatment depends on the amount of crack preparation and the type of material used (3).  It has been found that, depending on the amount of preparation and material selection, crack sealants can provide up to 9 years of service and fillers up to 8 years of service (3).  Overband treatments may contribute to poor ride, ride noise, and poor surface appearance unless the material is finished (squeegeed) flush with the road surface.  It should not be placed more than 12.5mm (1/2 inch) wider than the width of the crack (on both sides of the crack).

Emulsions or asphalt materials placed in a flush configuration in unrouted cracks (see Section 2.4) can provide 2 to 4 years of service while hot applied rubber and fiber-modified asphalt fillers placed in flush or overbanded configurations (Section 2.4) can provide 6 to 8 years of service (3).

Several methods exist for evaluating a treatment’s performance.  One method is based on determining a treatment’s effectiveness.  Treatment effectiveness is the success of the treatment measured as a percentage of the total treatment that has not failed (3).  In order to determine the condition of a treatment, visual inspections of the treated areas are required.  Inspections for treatment failure should be carried out once per year (3).

1.5.1 Treatment Failures

Treatment failures can be attributed to improper treatment selection, improper material selection, poor workmanship, and improper application or lack of post-treatments.  Common treatment failures include:

• Adhesion loss:  The sealant does not adhere to the sides or bottom of the crack.
• Cohesion loss: The sealant fails in tension by tearing.
• Potholes:  The crack is not completely sealed, allowing water into the pavement. Continued deterioration leads to pumping and pothole formation.
• Spalls: The edges of the crack break away as a result of poor routing or sawing.
• Pull-on: The sealant is pulled out of the crack by tire action.

1.5.2 Treatment Effectiveness

The first step in determining a treatment’s effectiveness is to establish how much of the treatment has failed in relation to the total length of treatment applied (% failure).  Once the amount of treatment failure is determined, the treatment’s effectiveness can be calculated using the following expression (3).

By routinely monitoring treated areas, a graphical representation of a treatment’s effectiveness can be generated like the one shown in Figure 10.  From this figure, the projected life of the treatment used on this cracked area can be projected as the time at which the effectiveness has dropped to 50% (as defined above).  Graphs like these can be used to determine when additional treatments may become necessary (3).

Figure 10: Treatment Effectiveness (3)

1.5.3 Cost Effectiveness

The cost-effectiveness of a treatment can be determined readily once the treatment effectiveness has been determined.  Cost effectiveness is the total cost of a treatment divided by its effectiveness.  Cost effectiveness may be converted into an annual cost by dividing the cost effectiveness by the number of years required to reach 50% effectiveness.