Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/128531
RoadScience by Tom Kuennen, Contributing Editor To Prevent Failure, Begin with Better Bases Fatigue cracking usually links to inadequate road foundations F atigue cracking in bituminous pavements is experienced by the public so often that it is considered just part of driving. Motorists endure it and assume it's just part of an aging pavement. But the perceptive road manager knows that fatigue cracking in less-deep asphalt pavements is a symptom of distressed base layers, and is indicative of a pavement on its way to failure. This fatigue cracking usually, but not always, is manifested by so-called alligator cracking. Named for its similarity to the pattern on an alligator's hide, alligator cracking appears as many sided, sharp-angled pieces, usually less than 12 inches on the longest side. This characteristic alligator or chicken wire pattern appears in later stages of deterioration. There are three types of fatigue cracking, according to the Federal Highway Administration's Distress Identification Manual for the Long-Term Pavement Performance Program (to view, search for FHWA-RD-03-031). A concise pocket guide for field use – Distress Identification Guide – also is available from FHWA (download by searching FHWA-RD-05-001). Neither of these guides explain the why of pavement distresses, instead offering precise identification of distresses in order to provide a common, standard definition for use by pavement managers. In these documents FHWA categorizes types of cracking for asphalt pavements, jointed concrete pavement, and continuously reinforced concrete pavements. • Low-severity fatigue cracking is an area of cracks with no or only a few connecting cracks; the cracks are not spalled or sealed; pumping of base materials out the cracks is not evident. • Moderate fatigue cracking is manifested by interconnected cracks forming a complete pattern. The cracks may be slightly spalled and may be sealed, and pumping is not evident. • High-severity fatigue cracking is an area of moderately or severely spalled interconnected cracks forming a complete pattern; pieces may move when subjected to traffic, cracks may be sealed, and pumping may be evident. It's easy to look at fatigue cracking in thinner pavements and assume it's a surface problem, but subsurface investigation will find fatigue cracking is bottom-up cracking, in which stresses propagated to asphalt pavement foundations cause cracks in inadequate base layers. As the asphalt pavement structure flexes under loads, these foundation cracks work their way upward through the pavement. Typically they are found in pavements subjected to repeated traffic loadings, like wheel paths, and can be a series of interconnected cracks. "[Fatigue cracking] generally occurs when the pavement has been stressed to the limit of its fatigue life by repetitive axle load applications," according to Hot Mix Asphalt Materials, Mixture Design and Construction, published by the National Asphalt Pavement Association's Research and Education Foundation. "Fatigue cracking is often associated with loads that are too heavy for the pavement structure or more repetitions of a given load than provided for in design." Poorly drained bases exacerbate the problem. As the cracks in the base layers work their way upward, capillary action will draw water from undrained bases up into the pavement, where it damages the pavement structure through reflection cracking, cracks along longitudinal joints, cracks in wheel paths, alligatoring, raveling and potholes. "The problem is often made worse by inadequate pavement drainage, which contributes to this distress by allowing the pavement layers to become saturated and lose strength," NAPA says. "The HMA layers experience high strains when the underlying layers are weakened by excess moisture and fail prematurely in fatigue. Fatigue cracking also is often caused by repetitive passes with overweight trucks and/or inadequate pavement thickness due to poor quality control during construction." New Look at Top-Down Cracking Not all fatigue cracking is bottom-up; in thick pavements, cracks may start from the top of the pavement in areas of high localized tensile stresses. 20 May 2013 Better Roads RoadScience_BR0513.indd 20 4/29/13 3:54 PM