Top Down Cracking

[BigH]

Does anyone have some detailed experience with top down cracking of asphalt pavement. What would the major causes of this phenomenae? Many thanks.

 

[SlideRuleEra]

BigH - Are you talking about "alligator cracking"?

http://www.SlideRuleEra.net

 

[BigH]

No . . . and yes. There is a phenomenon that seems to recently been "discovered" that many cracking of the highways are actually top down cracking rather than bottom-up variety due to subgrade fatigue. Recent studies in Florida (2001) indicate that they are discovering that the majority of cracking in their asphalt pavements are due to top down. Some other jurisdictions are seeing this too. Largely, the reasons given include bitumen aging; wide over-inflated tyres; softer modulus of asphaltic mix in very hot climates. Also, some jurisdictions are finding that segregation leads to this - although, I suspect, that the causes would differ from climate to climate. Normally, longitudinal or transverse cracking starts (turning into alligator with time). This starts between 3 to 10 years after laying. In our case, it has been <2 years but our average summer temp is over 35degC (95degF) - over the last few days we have been over 40degC. The hot weather ages the upper few millimeters of bitumen dramatically (reported to be 3x faster compared to a temperature 8degC lower). This then leads to cracking and propagating, etc. Interesting topic; not well known as ASTM doesn't even mention it in relation to cracking of pavements (see their standard on identification of distress for airfields).

Anyway - wanted to know if others have encountered it and what nuances they have "discovered" or "learned" from it.

 

[ACtrafficengr]

RE: high tempuratures: could it be due to asphalt hardening from oxidation?

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[BigH]

Well, that's the general idea. As indicated, we have had, say >35degC now for 2 months and it will continue for another 3 to 4 months - much more elevated than even Florida. After review of literature (to a search on Top Down Cracking of Asphalt Pavements) - it seems that the age hardening (oxidation) is a most primary cause.

Interesting how you do not find any mention of TDC in even recent manuals, etc. - always alligator from bottom up.

 

[emmgjld]

In Western Colorado, We are trying to break out TDC caused by temperature weathering of the AC, from other cases of slight heaving of low expansive subgrade silty clays and occ. lean clays. Swelling, of course, often results in cracking wider at the top. We too, have relatively high summer temperatures.

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.

 

[roncity]

Is the cracking over the entire surface or in spots ?
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.

 

[BigH]

Thanks for both inputs. I can understand emmgjld's points about expansive clays - our subbase and base were put on an embankment of flyash (85%) and sand (15%) mixture and the subgrade was same but 75:25. This material is non-expansive.
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.

 

[BigH]

emmgjld and roncity - in your experience how much does segregation play in the TDC?

 

[Ron]

BigH...most top-down cracking in asphalt is related to weathering phenomena. High temperatures, wet-dry cycles, type of coarse aggregate...all of these have influence on the top-down cracking occurrences. Here in Florida we see it often, though our mix designs generally accommodate it well so that it doesn't occur for quite a while in the life of the asphalt.

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.

 

[BigH]

Ron - thanks. You have certainly reinforced my thoughts although we had the alligators form quickly because, I believe of the high tyre pressures on the very very hot/wet monsoon seasons. We didn't get them where there was significant flushing (another problem). Do you have any links on Roth's works? TRRL of UK put out a nice paper on Top Down Cracking - Myth or Reality.

 

[Ron]

BigH...High tire (tyre) pressures generally cause more concentration of the load and thus more lateral strain at the bottom of the asphalt layer.

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?

 

[emmgjld]

This post has been very interesting and has resulted in some additional study on my part. I found I was distracted by more obvious problems and needed the have the blinders removed.

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://www4.trb.org/trb/crp.nsf/All+Projects/NCHRP+1-42

 

[BigH]

emmgild: Thanks for the site - the summary is good. I have done extensive searching and found a paper out of UK called Top Down Cracking - Myth or Reality was a great eye-opener on the subject. I must have some 20 references on disk now. I will find out exact URL and advise.
and thanks for the interest in this subject.