asphalt pavement distress

[cphi]

I have been asked to determine what caused an asphalt parking lot to crack. The cracks are spaced more than about 30 feet apart and are random but most are tracking between islands and light standards. Some cracks are 0.1± feet wide. The pavement has no depressions and no elevation difference across the cracks. I have an opinion what the cause is, but I am curious of what the forum's opinions are. Thanks for your time.

 

[Ron]

Additional information would be helpful. There are several reasons for asphalt to crack.

Please answer the following and maybe we can help....

1. How old is the asphalt surfacing?
2. What type of mix is it? (fine, coarse, dense, open graded)
3. What type of base material is it? (graded aggregate, soil-cement, asphalt, other)
4. What type of subgrade is it? (sand, clay, etc.)
5. What area are you in?

The two most common causes of cracks like you have describe are asphalt surface weathering (shrinkage) or base shrinkage (typical of soil-cement, lime treated clays, cement treated base, some calcareous aggregates).

Based on the spacing you have noted, I would surmise that you have base shrinkage due to a soil-cement or similar base material.

If that's the case, the cracks are common and should be sealed with a hot asphaltic pavement sealer.

 

[cphi]

Hello Ron,
Thank you for your reply.
What experience do you have with forensic evaluation of pavements?
Here are some answers to your questions:
1. about 9 years, but time of crack appearance unknown - my guess is the cracks are more than 2 to 8 years old
2. visually about 3/4 inch max. and dense; extraction testing not completed yet
3. graded sand and gravel, not stabilized
4. silty sand to sandy silt
5. the pavement is in Western Nevada, I am in Wisconsin

 

[BigH]

If the pavement has been in place that long - and the spacings are so far apart as you have indicated - Ron probably has the best solution - just fill the crack and (as bad as it sounds) not worry too much about the reasons for it. Seems like the cracks are not multiplying, becoming more concentrated in small areas and the like. Seems like the pavement is behaving extremely well and I wouldn't touch it for even any small remedial work other than the filling of the cracks with sealer - just ensure that the sealer has enough viscosity that it doesn't 'melt' in the hot weather.

 

[Ron]

cphi...I have done lots of pavement investigations and evaluations over the last 25 years or so, ranging from laboratory and field testing to visual condition assessments to forensic investigations of roadway and airfield pavements.

OK..based on your answers, it probably is not a base shrinkage issue. If your subgrade is silty clay or clayey silt, then dessication can be occurring, causing shrinkage. You'll need to see if the cracks continue through the base or below, and if generally the moisture content of that material is decreasing with time (runoff patterns changed, less water available to that layer because of reduction in groundwater levels, etc.) I'm not that familiar with the area, but my guess is groundwater is probably not an issue.

That predominately leads to the surface paving itself. With spacing that far apart, the surface would likely be relatively thick (over two inches of asphalt) and/or the surface was placed with no prime or tack coat and is not bonded to the base. Without restraint to inherent material shrinkage, the crack spacing increases quite a bit. The fact that your cracks generally occur between an island and a light standard indicates they are occurring at reduced lateral cross sections..thus further evidence of shrinkage.

If the cracks are wider at the top than at the bottom of the asphalt layer, that's a good indication of material drying/weathering shrinkage. If not, the shrinkage could be below the asphalt surface.

All of this means that there could be several reasons, alone or in concert, for your cracks. Only further investigation will tell for sure.

Given that, if you have no faulting and the pavement is holding up well except for the cracks, then BigH has it....just fill the cracks and don't bother the rest of it.
With cracks that wide, you need to be sure to select a sealer that will span the distance and remain flexible and bonded. Using an airfield type hot asphalt sealer (a rubberized asphalt) is likely your better choice. If the pavement surfacing is rather thick, you might also consider adding sand to the lower layer of sealant to help fill the space.

Let us know what you do and how it works out.

 

[cphi]

Thanks guys for your replies. One more question for now. What is the thermal volume change coefficient of asphalt?

 

[Ron]

cphi...it's higher than concrete, but because of the rheology of asphalt, it doesn't work the same way. For example, Poisson's ratio of concrete is about 0.15; for asphalt, about 0.35. If you had the same volumetric expansion in the materials, the strain attenuation is asphalt would be tremendously greater, thus you'd "show" less movement.

I'm curious as to why you ask...it is a parameter that's rarely used outside cold climates for asphalt.

 

[BigH]

Ron - you are correct in that as the aspahlt binder approaches its softening point (which, by the way increases with age due to hardening of the binder), the asphalt will/may act more in a viscous manner than as a "solid". This can cause flushing, lowering of voids in the asphalt and the like - but cphi's problem is addressed only by infilling the cracks - see what it is like in 2 or 5 years - surely wouldn't mess with a pavement that has a few cracks some 30 ft apart.