Top Down Cracking
Top Down Cracking
(OP)
Does anyone have some detailed experience with top down cracking of asphalt pavement. What would the major causes of this phenomenae? Many thanks.
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RE: Top Down Cracking
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RE: Top Down Cracking
Anyway - wanted to know if others have encountered it and what nuances they have "discovered" or "learned" from it.
RE: Top Down Cracking
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RE: Top Down Cracking
Interesting how you do not find any mention of TDC in even recent manuals, etc. - always alligator from bottom up.
RE: Top Down Cracking
I am coming to the conclusion that most of the AC pavement cracking we are seeing is subgrade heave of dessicated, Low expansive, silty clay and lean clay, alluvial/debris fan deposits.
We are experimenting by overexcavation, water conditioning and replacing the subgrade clays, to break up the dessicated soil structure. The amount of overexcavation and water conditioning ranges from 18" to 36", based upon the swell portion of the R-Value (Hveem-Carmany) test. The CBR test is not real common here and doesn't seem to be sensitive to low to medium swelling conditions.
RE: Top Down Cracking
Spot Cracking:
Some top down cracks may start during the construction process if the surface is over rolled or it’s rolled after it’s cooled down. The roller leaves hairline cracks in the surface that expand as the pavement ages. This may show up in isolated areas where the roller changes directions or at the end of a truckload where the mix has cooled. You may want to compare construction conditions (weather, rolling patterns , contractor experience) where cracking exists and where it doesn’t exist.
Generalized Cracking:
If cracking exists over the entire area, it may be related to lower asphalt content in the mix design and/or greater asphalt absorption by the aggregate . Both would produce a dryer mix that would have less resistance to weathering and surface cracking. Weathering includes wet/dry cycles as well as hot/cold cycles.
RE: Top Down Cracking
For roncity, your points are well taken. The paving was done in March to August 2003 - daily temperatures would be in the order of 35degC (95degF) or so consistently. Do admit that there was over-rolling to a degree as the spec is for min 98% (and if under 98%, there are reductions - serious ones) for each percentage. Doubt that cold rolling was a problem - with the high temps, low haul distance from plant to site, etc, temps should have been >100degC for more than 1/2 hour. Some of placement in monsoon season - but we were careful not to place in rain or shortly after raining.
The cracks are generally in the wheel paths - or adjacent - starting as longitudinal to transverse then going alligator. Bitumen content was in the order of 4.1% for a coarse DBM (Dense bitumen macadam - British style). Our specific gravity of aggregates is 3.0 and minimum bitumen content was 4%. (in my view, the bitumen content should be 3.8% or so - to keep air voids up after secondary compaction) We have a 'ellish history of flushing due to bitumen rising to surface during over-secondary compaction - especially in the BC (i.e., wearing course). Air voids after secondary compaction likely less than 2%. We found if badly flushed, cracking is either not occuring due to "protective" course of excess bitumen at surface - or the excess bitumen is masking the cracking.
Anyway, we have, on recommendations of top university here to remove the cracked areas except where still widely spaced - then using stone chipping as stress relief or fog sealing.
Again - thanks for inputs.
RE: Top Down Cracking
RE: Top Down Cracking
Another factor that is often overlooked is the LACK of traffic on the asphalt. Sections with little or no traffic tend to exhibit this problem faster than those sections with normal or heavy traffic. This is due to the continous "remolding" of the asphalt by traffic loads.
Some of the better rheology studies have been done by Byron Ruth at the University of Florida. Byron has retired, but his publications are certainly still around.
RE: Top Down Cracking
RE: Top Down Cracking
If you are experiencing early alligator cracking, it is likely from overstress, not weathering. What are the shrinkage characteristics of your asphalt? Have you done any tensile tests on the asphalt?
RE: Top Down Cracking
My focus on Top Down Cracking has been on subgrade soil reaction because the vast majority of our local pavement failures actually point to subgrade failures. I have observed a local tendency, featuring local governments, to blame the asphalt layer for most failures because then the contractor can be pressured to redo projects which do not work out. I am finding that our municipal project geotechnical exploration, design and final specifications often result in a minimal amount of subgrade preparation. Of course, more subgrade preparation implies more initial project cost, which is undesirable.
I am also distressed by the idea of 'Standard Specifications' which prove to be very resistant to change, or even serious discussion of change. As our area grows, the new construction gets into areas which have differing soils, climatic issues and other changes from 'the old part of town'. You know the drill, "this has worked for x number of years and how dare you suggest a problem". But, as construction continues in previously undeveloped (read arid, non-irrigated) areas, failures continue to occur with more frequency.
I have gone this far in order to say that local experience is STILL struggling through the basics. My issue of true Top Down Cracking is probably hidden within more elementary problems of design, specification and construction. In the course of my recent research, I 'googled' this. Hope it helps.
National Cooperative Highway Research Program - Completed Project Project 1-42 Top-Down Fatigue Cracking of Hot-Mix Asphalt Layers - Phase I
http:
RE: Top Down Cracking