BigH
Geotechnical
- Dec 1, 2002
- 6,012
I figured I could double post this one! A Tip and a Question - can't seem to get them both to light up!!
SlideRuleEra - thanks for your interest. Basically the situation is this:
Location: India; West Bengal; Howrah District
We laid a DBM (dense bitumen macadam) - a base course asphalt. For various reasons especially that the embankment is on very soft clays and large settlements were expected - it was decided to run the traffic on the DBM before putting on the wearing course. That would given an option to use a regulating layer and then the wearing course.
Traffic ran for 1.5 to 2 years - through 1 full and 1 partial monsoon (lots of rain). Temperatures run in the mid 30s to 40s for 6 months a year. After the first monsoon season and the hot summer months preceeding the second one, we started to notice cracking in the pavement - alligator cracking in the wheel paths. At first we thought it was fatigue (inadequate pavement) but we have 500mm subgrade CBR>6; 330mm crushed stone subbase (150 open graded for drainage and 180 dense graded); 250mm crushed stone base (in two layers) and 175mm DBM (in two layers). This is pretty thick - but the truck here are way overloaded and high over-inflation of tyre pressures.
We cored the apshalt and noted that the cracking went down only partially through the top layer and did not extend down into the lower DBM layer. This was at so many core locations. (I also noted this on one of our other contracts but much more minor (only 6 months exposure before laying BC). This surprised us. Cracking more pronounced in "non-flushed" areas rather than flushed areas (asphalt in India traditionally flushes badly - various reasons).
I started doing searching of internet for top-down cracking. Ran into some very good publications - I will send the articles to you; you might wish to reference them on your web page until I can get mine started (have the name, now need to do the page and publish it).
As well, the contractor hired Indian Institute of Technology of Kharagpur (top IIT in India) to study the problem. They did coring, found the same. They did Benkelman Beam and found no structural deflection difference in cracked vs uncracked section. They basically attributed the top down cracking to:
1. Aging of the bitumen due to the very high temperatures
2. High tensile stresses at the surface of the pavement due to the highly over-inflated wide tyres
3. Softening of modulus due to elevated temperatures.
4. Segregation.
This pretty well corresponds to the literature.
As indicated in the other thread, pavement engineers seemed to have "ignored" top-down cracking for decades - first started to be "noticed" in the mid-1980s. What I find is that most studies are in temperate climates. How many weeks does Colorado or Wisconsin have temps greater than 100 deg C?? British study found that for each 8degC of lower temperature, rate of aging is 1/3 that of the higher temperature. So, if Colorado sees such cracks at 8 years, it seems reasonable that we, in this climate, might see them in 1 to 2 years. If Florida sees them in 3 to 8 years, again, 3/3 or 8/3 - 1 to 2 years seems reasonable for us. To back up - we found that the softening point had risen from 46degC to 53 to 55deg C in just the 2 years - much higher than the Shell estimation of 1degC per year.
Anyway - this is the gist and was/am hoping that others can collaborate similar experiences or that they might have other information.
One final note - here, it might not have been noted too much in that they use 60/70 pen with relatively high bitumen content - meaning that we get bad flushing within a year of the hot weather - and the high surface bitumen content protects the asphalt from cracking although it leads to other "bad" things.
![[cheers]](/data/assets/smilies/cheers.gif "[cheers] [cheers]")
SlideRuleEra - thanks for your interest. Basically the situation is this:
Location: India; West Bengal; Howrah District
We laid a DBM (dense bitumen macadam) - a base course asphalt. For various reasons especially that the embankment is on very soft clays and large settlements were expected - it was decided to run the traffic on the DBM before putting on the wearing course. That would given an option to use a regulating layer and then the wearing course.
Traffic ran for 1.5 to 2 years - through 1 full and 1 partial monsoon (lots of rain). Temperatures run in the mid 30s to 40s for 6 months a year. After the first monsoon season and the hot summer months preceeding the second one, we started to notice cracking in the pavement - alligator cracking in the wheel paths. At first we thought it was fatigue (inadequate pavement) but we have 500mm subgrade CBR>6; 330mm crushed stone subbase (150 open graded for drainage and 180 dense graded); 250mm crushed stone base (in two layers) and 175mm DBM (in two layers). This is pretty thick - but the truck here are way overloaded and high over-inflation of tyre pressures.
We cored the apshalt and noted that the cracking went down only partially through the top layer and did not extend down into the lower DBM layer. This was at so many core locations. (I also noted this on one of our other contracts but much more minor (only 6 months exposure before laying BC). This surprised us. Cracking more pronounced in "non-flushed" areas rather than flushed areas (asphalt in India traditionally flushes badly - various reasons).
I started doing searching of internet for top-down cracking. Ran into some very good publications - I will send the articles to you; you might wish to reference them on your web page until I can get mine started (have the name, now need to do the page and publish it).
As well, the contractor hired Indian Institute of Technology of Kharagpur (top IIT in India) to study the problem. They did coring, found the same. They did Benkelman Beam and found no structural deflection difference in cracked vs uncracked section. They basically attributed the top down cracking to:
1. Aging of the bitumen due to the very high temperatures
2. High tensile stresses at the surface of the pavement due to the highly over-inflated wide tyres
3. Softening of modulus due to elevated temperatures.
4. Segregation.
This pretty well corresponds to the literature.
As indicated in the other thread, pavement engineers seemed to have "ignored" top-down cracking for decades - first started to be "noticed" in the mid-1980s. What I find is that most studies are in temperate climates. How many weeks does Colorado or Wisconsin have temps greater than 100 deg C?? British study found that for each 8degC of lower temperature, rate of aging is 1/3 that of the higher temperature. So, if Colorado sees such cracks at 8 years, it seems reasonable that we, in this climate, might see them in 1 to 2 years. If Florida sees them in 3 to 8 years, again, 3/3 or 8/3 - 1 to 2 years seems reasonable for us. To back up - we found that the softening point had risen from 46degC to 53 to 55deg C in just the 2 years - much higher than the Shell estimation of 1degC per year.
Anyway - this is the gist and was/am hoping that others can collaborate similar experiences or that they might have other information.
One final note - here, it might not have been noted too much in that they use 60/70 pen with relatively high bitumen content - meaning that we get bad flushing within a year of the hot weather - and the high surface bitumen content protects the asphalt from cracking although it leads to other "bad" things.