“An Examination of Top-Down Cracking (or Surface Cracking) in Asphalt Concrete Flexible Pavements”
PROBLEM
Gerritsen et al (1987) reported that pavements in the Netherlands were experiencing premature cracking in the wearing courses. Further, the cracks did not extend into lower bituminous base layers. These surface cracks occurred both inside and outside the wheelpath areas, and, in some cases, soon after paving. This caused Gerritsen et al to conclude that there were likely more than one causative effect. The surface cracking outside of the wheelpaths had low mix strength characteristics at low temperatures. Further, they noted low binder penetration values could be related to higher thermal stresses. The surface cracks in the wheelpath areas were largely attributed to radial shear forces under truck tires near the tire edges. Their conclusion was that both thermal and load related effects caused the observed surface cracking. They recommended that the binder film thicknesses be increased to reduce early age hardening of the mixes.
Dauzats et al (1987) also published results that described surface initiated cracking on pavements in France. They noted that the cracks could be either longitudinal or transverse and occurred typically three to five years following construction. They estimated that these types of cracks were initially caused by thermal stresses and then further propagated by traffic loads. It was noted that a rapid hardening of the mix binder likely contributed to this type of pavement distress.
Work reported by Matsuno and Nishizawa (1992) noted that longitudinal surface initiated cracking of the AC wearing course was commonly observed in Japan about one to five years following construction. Their observations and analyses are of special interest. First, they observed that the longitudinal cracks did not extend under overpasses (shaded areas). Second, analysis of FEM results showed that very high tensile strains occur at the edge of truck tires at or near the surface of the AC wearing course. These high strains occur when the upper portion of the AC is at a low stiffness due to high surface temperatures. They also noted that if the AC is not hardened due to aging effects, the small cracks that form are eliminated by the kneading action of tires. This changes as the AC ages. They analyzed two thicknesses of AC: 200 mm (heavy traffic routes) and l00 mm (light traffic). For both thickness cases using a peak surface temperature of 60ºC (decreasing with depth) and associated stiffness of about 200 MPa at 60ºC, they reported similar tensile strains of over 1400x10-6 mm/mm near the pavement surface. Thus, they concluded that AC thickness is not a major factor with this type of cracking.
A study on large transport vehicles and their effects on pavements was reported by Craus et al (1994) in work done for the California Department of Transportation. Their analyses showed that large tensile strains occur at the top of the AC wearing course. Specifically, these strains are due to high tire edge stresses for conditions where the upper AC is at a low stiffness due to high surface temperatures (E1/E2 ratios of less than 0.5 produced the largest tensile strains). It is of special interest that the California and Japanese studies drew the same conclusions concerning the cause of surface initiated cracking.
More recently, Nunn (1998) reported that surface initiated cracking was common on UK motorways. Typically, these surface cracks were observed about 10 years after paving. Nunn noted that for pavements with asphalt thicknesses exceeding 180 mm, there was no evidence of fatigue cracking in the lower bituminous roadbase layers—only the wearing course. Additionally, he showed that there was a discontinuous relationship between the rate of rutting and the thickness of the asphalt layers. For combined asphalt thicknesses greater than 170 mm, the rutting rates on about 50 pavement sections were about 200 times less than for asphalt layers with thicknesses less than 170 mm (for sections with less than 170 mm of asphalt the rutting rates were about 100 mm per million equivalent axles and 0.4 mm per million equivalent axles for greater than 170 mm). Such dramatic measurements suggest that a very different failure mechanism occurs at the “breakpoint” thickness. Nunn also summarized recent work performed in the Netherlands that showed for asphalt thicknesses exceeding 160 mm, cracks initiated at the pavement surface and eventually penetrated to a depth of about 100 mm. He also noted that the Netherlands work indicated for full depth cracks in thinner pavements that the cracks propagated from the top of the pavement surface downward. Nunn showed that the surface initiated cracking in the UK could be either longitudinal or transverse. The transverse cracks were related to low binder penetration values (typically about 15). He also stated that the pavement sections with and without surface cracking had no significant difference in measured deflections. He concluded the cause of the surface initiated cracking was due to horizontal tensile stresses generated by truck tires at the asphalt surface. Wide based tires generated the highest tensile stresses.
Myers et al (1998) reported that surface initiated cracking in Florida was found to represent 90 percent of the observed cracking in pavements scheduled for rehabilitation. Thus, this type of cracking predominates in Florida. They noted that this type of cracking is generally observed on pavements five to ten years following construction. The asphalt concrete thicknesses in their study ranged from 50 to 200 mm. The cracks were most often longitudinal with surface crack widths of about 3 to 4 mm decreasing with depth. The total crack depths ranged from about 25 mm to the full depth of the asphalt concrete layer. Based on computer modeling, it was concluded that tensile stresses under the treads of the tire—not the tire edges—were the primarily cause of the cracks. Further, wide base tires caused the highest tensile stresses. They noted that the tensile stresses dissipate quickly with depth suggesting that this might be the reason the cracks essentially stop growing; however, they felt this needed further investigation. They concluded that surface initiated cracking is not a structural design issue but more related to mixture composition. Specifically, they concluded that more fracture resistant asphalt mixes are needed.
At the January 2000 TRB Annual Meeting, Uhlmeyer et al reported that top-down cracking is common to thicker Washington State DOT asphalt surfaced pavements (top-down cracking was typically observed when the average thickness of AC was about 160 mm or greater). Such cracks were generally contained in the wearing course and averaged 46 mm in depth. The top-down cracks generally initiated within three to eight years of paving. No hypothesis as to cause was made.
The bottom line is that AC top-down cracking is not thoroughly understood and is generally not considered as a causative factor for pavement cracking. Further, for two states that recently studied cracking origins (Florida and Washington State), both reported that top-down cracking is far more common than assumed. In fact, the Florida DOT reports that this type of AC cracking is dominant for their AC pavements due for rehabilitation. In general, we can summarize the problem by recapping the literature cited above:
· France: top-down cracks form within 3-5 years of paving.
· UK: top-down cracks form within 10 years of paving on AC thicknesses of 180 mm or more.
· Netherlands: top-down cracks common for AC thicknesses of 160 mm or greater.
· Japan: top-down cracks commonly observed and occur within 1-5 years of paving.
· California: analysis showed that top-down cracks could form due to truck tire edge stresses that produce high surface tensile strains.
· Washington State: top-down cracks form within 3-8 years of paving on AC thicknesses of 160 mm or greater.
· Florida: top-down cracks form within 5-10 years of paving on a wide range of AC thicknesses.
Additionally, there are three basic views on the cause of such cracking and their possible interrelationships. One is high surface horizontal tensile stresses due to truck tires (wide-based tires and high inflation pressures are cited as causing the highest tensile stresses). The second is hardening of the AC binder resulting in high thermal stresses in the AC (most likely a cause of the observed transverse cracks). The third is a low stiffness upper layer caused by high surface temperatures.
OBJECTIVES
The overall objective of the research is to define the causes of top-down cracking and predict or mitigate its effect. Specific objectives are:
· Quantify the extent of the phenomenon. Consideration should be given to AC thickness, depth of crack, and timing of crack initiation. This objective should be met largely by use of relevant literature and a survey of State DOTs.
· Analyze the cause of top-down cracking. This will require developing a hypothesis that explains the distress then testing the hypothesis. The causes cited in the existing literature can serve as a starting point.
· Develop a transfer function. By using appropriate mechanics (such as tensile strain or stress) and empirical data, a transfer function will be developed that will allow M-E procedures to predict the occurrence of top-down cracking. If a transfer function cannot or should not be developed, then suitable cracking mitigation measures should be developed and recommended (such as changes in AC mix composition, etc.).
EFFECTIVENESS
The results of this study would explain the causes of top-down cracking and quantify the distress so that it can be predicted or mitigated (or both). This will affect the nation’s pavement rehabilitation program in a significant manner.
KEY WORDS
Pavement performance, pavement distress, top-down cracking, surface cracking, transfer function.
RELATED WORK
There is little related, known work currently underway in the U.S. The recent study in Washington State reported at the 2000 TRB Annual Meeting showed that top-down cracking is common for their thick asphalt concrete pavements. The Florida DOT has reported similar findings except for a wider range of AC thicknesses. Several major international studies have examined the phenomenon and are of significant value.
An initial review of the literature shows that the following references are relevant to the topic:
· Gerritsen, A.H., van Gurp, C.A.P.M., van der Heide, J.P.J., Molenaar, A.A.A., and Pronk, A.C. (1987), “Prediction and Prevention of Surface Cracking in Asphaltic Pavements,” 6th International Conference Structural Design of Asphalt Pavements, The University of Michigan, Ann Arbor, Michigan, July 1987, p. 378-391.
· Dauzats, M. and Rampal, A. (1987), “Mechanism of Surface Cracking in Wearing Courses,” Proceedings, 6th International Conference Structural Design of Asphalt Pavements, The University of Michigan, Ann Arbor, Michigan, July 1987, p. 232-247.
· Nunn, M. (1998), “Design of Long-Life Roads for Heavy Traffic,” Proceedings, Industry Conference, Australian Asphalt Pavement Association, Surfers Paradise, Queensland, Australia.
· Myers, L.A., Roque, R., and Ruth B.E. (1998), “Mechanisms of Surface-Initiated Longitudinal Wheel Path Cracks in High-Type Bituminous Pavements,” Proceedings, Volume 67, Association of Asphalt Paving Technologists.
· Matsuno, S., and Nishizawa, T. (1992), “Mechanism of Longitudinal Surface Cracking in Asphalt Pavement,” Proceedings, Volume 2, 7th International Conference on Asphalt Pavements, The University of Nottingham, p. 277-291.
· De Beer, M., Fisher, C., and Jootse, F.J. (1997), “Determination of Pneumatic Tyre/Pavement Interface Contact Stresses Under Moving Loads and Some Effects on Pavements With Thin Asphalt Surfacing Layers,” Proceedings, 8th International Conference on Asphalt Pavements, University of Washington, Seattle, Washington, August 1997, p. 179-227.
· Himeno, K., Ikeda, T., Kamijima, T., and Abe, T. (1997), “Distribution of Tire Contact Pressure of Vehicles and Its Influence on Pavement Distress,” Proceedings, 8th International Conference on Asphalt Pavements, University of Washington, Seattle, Washington, August 1997, p. 129-139.
· Craus, J., Chen, A., Sousa, J., and Monismith, C. (1994), “Development of Failure Curves and Investigation of Asphalt Concrete Pavement Cracking From Super-Overloaded Vehicles,” Report to Division of New Technology, Materials, and Research, California Department of Transportation, August 1994.
URGENCY
This study must be given a high priority given that this type of distress widely affects flexible pavements and it is poorly understood at this time.
COST
The cost is estimated to range between $250,000 to $500,000.
SUBMITTED BY
TRB Committee A2B03, Flexible Pavement Design
Stephen B. Seeds, Chair, TRB Committee A2B03
605 Cliff View Drive
Reno NV 89523
Phone: 775-345-1999
e-mail: SBSeeds@aol.com
Joe P. Mahoney, Member, TRB Committee A2B03
(Author of Problem Statement)
University of Washington
Seattle, WA
Phone: 206-685-1760
e-mail: Jmahoney@u.washington.edu