While treating any crack may ultimately provide some benefit to the underlying pavement structure through the reduction of moisture intrusion, the most advantageous applications for crack sealing are block, longitudinal, reflection, and transverse cracks. Unless the crack treatment is done in early-stage distress development, crack treatments for fatigue cracking do not substantially improve pavement performance; however, the treatment may reduce further deterioration of the pavement.
Transverse Cracks - these cracks run perpendicular to the pavement lane and are typically caused by thermal conditions or reflection of underlying pavement joints. These cracks can experience large amounts of horizontal movement with temperature change. Transverse cracking will generally occur at intervals greater than one hundred feet early in the life of the pavement. As the pavement ages, cracking will develop at smaller intervals. For pavements in colder climates, transverse crack spacing is typically between forty and eighty feet when pavements are three to six years old. For old pavements in very cold climates, thermal transverse and/or longitudinal cracks can be spaced as close as five feet.
Longitudinal Cracks - these cracks run parallel to the pavement lane. Longitudinal cracks that form near the center line are typically caused by construction paving joints. Longitudinal cracks may also be formed due to thermal factors such as shrinkage of the asphalt layer and temperature cycles. Longitudinal cracks are also formed by reflection of cracks in the underlying layer.
Block Cracks - these cracks generally form in relatively old asphalt pavements due to shrinkage and thermal effects. They form into block shapes as a result of hardening of the asphalt coupled with shrinkage during cold weather. Block cracking will occur in both traffic and non-traffic areas.
Edge Cracks - these cracks can form within the outer two feet of asphalt concrete pavements. These cracks form due to lack of lateral support, settlement of underlying material, a weak base, or heavy traffic along the edge. These cracks should always be addressed with a crack treatment. Roads are designed and built to drain to the edge. The unsealed edge cracks act like a rain gutter catching the water as it runs off the pavement allowing it to erode the base quickly. Therefore, routing the crack as necessary and then sealing it is recommended.
Fatigue Cracks - these cracks, also known as alligator cracks, are caused by excessive traffic loadings, weak or thin surface, and poor drainage. Fatigue cracking indicates that the pavement system is experiencing structural failure of the base and asphalt layers. For properly designed and constructed pavements, fatigue cracking generally occurs late in the pavement life. However, for overloaded, high traffic pavements, or pavements that have design or construction deficiencies, fatigue cracking can occur earlier in the pavement life. Whereas most cracks are good candidates for crack sealing, fatigue cracks are an indication of structural failure and therefore may not be a candidate for crack sealing.
Crack density is a factor to help identify a pavement’s condition and determine whether routing is appropriate when designing a crack treatment. Crack density is calculated by assessing the linear footage of cracks that are present per square feet of pavement area. The percentage of crack density helps indicate potential movement of the cracks that are present, which in turn impacts the sealant selection. Typically, the higher the crack density, the more cracks that are present in the pavement and therefore less movement is anticipated. The lower the crack density, the less cracks that are present in the pavement and therefore higher movement is anticipated.
It should be noted that there are two fundamental reasons to rout the pavement prior to crack sealing. The first reason is to prepare and clean the edges of the pavement so that there is better adhesion and therefore a better seal that will last longer. When routing, the operator should remove one-eighth inch from each side of the crack to remove deteriorated asphalt and enable good bonding of the sealant to the asphalt that is still in good working condition. The second reason is to create a defined reservoir that holds an amount of sealant to accommodate the anticipated annual thermal movement. The defined reservoir has specific dimensions on width and depth in the pavement depending on climate and other factors which will be detailed later.
When pavements have a lower crack density, <20%, the pavement condition is typically appropriate to rout the cracks and then seal to accommodate the anticipated movement. In this case, routing prepares the edges for sealing and developing a defined reservoir.
When pavements have a higher crack density, >20%, the pavement condition is typically not able to sustain a defined reservoir. In this case, if the pavement can accept the routing without additional spalling, the crack is routed to prepare the edges for sealing and long life. Crack density should be used as a general guideline to help guide appropriate routing. The unique circumstances of each project may determine that routing is or is not appropriate outside of these guidelines.
Crack sealing is generally conducted in the winter months when cracks are at their widest. Depending on what climate region you are in, you will need to select the correct crack sealing material.