by Rita B Leahy, APACA Technical Director
Cracking – Types and Treatments
Cracking in pavements occurs when stress in the layer exceeds the strength of the material. Cracks provide paths for surface water to infiltrate and weaken the pavement structure. The cause of the crack and its activity play a dominant role in determining the success of crack sealing or filling operations.
Cracking may be associated with various distress mechanisms, ie, cracking may be caused by traffic or the environment. Crack types include the following: fatigue, longitudinal, transverse, block, reflective, edge, and slippage. A brief discussion of each crack type follows.
Fatigue Cracking: These cracks are generally referred to as alligator cracks as the pattern is similar to that of an alligator’s skin, as shown in Figure 1. In South America, it’s called crocodile cracking. Only kidding! Fatigue cracking is load-related, ie, it is the result of repetitive traffic loads or high deflections often due to weak and/or wet base or subgrade. This type of cracking is indicative of a structural failure and, if not repaired, develops into potholes and pavement disintegration. Neither crack sealing nor crack filling can remedy this type of structural failure. Longitudinal cracking in the wheel paths is often the first visible sign that alligator cracking is starting to develop.
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 density is lower (voids are higher). Longitudinal cracking may be associated with raveling and poor adhesion or stripping. These cracks can be effectively treated with crack sealants.
Transverse Cracks: These cracks occur perpendicular to the centerline of the pavement, or laydown direction, as shown in Figure 3. Transverse cracks are environmentally induced, ie, by shrinkage at low temperatures. Thermal cracks often penetrate the entire layer and typically widen over time. These cracks can be effectively treated with crack sealants, but deep cracks should be filled first to avoid excessive application of crack sealant.
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 too can be effectively treated with crack sealant.
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 or differential stresses across the crack. They “mirror” the underlying crack pattern and must be treated according to the original distress mechanism. Figure 5 illustrates reflection cracking in asphalt concrete over a jointed portland cement concrete pavement.
Edge Cracking: These are crescent-shaped or fairly continuous cracks that intersect the unbound pavement edge and are typically located within 2 ft of the pavement edge, adjacent to an unpaved shoulder, as shown in Figure 6. Edge cracking may be caused by overloading at the unbound edge of the pavement, shear failure, or erosion (loss of support) in the shoulder. Generally, this type of cracking cannot be effectively treated with crack sealant.
Slippage Cracks: These cracks produce a characteristic crescent shape, as shown in Figure 7, and are caused when the top layer of the asphalt shears and separates from the underlying material. Slippage cracking is often caused by high deflections and a poor bond between the layers. This type of cracking cannot be effectively treated with crack sealants.
Next week’s cracking topic, to fill or seal?
FHWA, 2005. Memorandum on Pavement Preservation Definitions, FHWA, September 12, 2005.
TRIP, At the Crossroads –Preserving Our Highway Investment, 2007.
Caltrans, Maintenance Technical Advisory Guide, Volume I, Flexible Pavement Preservation Second Edition, 2008.