Alligator Cracking Continued...

Over the last few days I received the following email in response to my entry Alligator Cracking:

Could the consistency/mixture of the pavement be different? Or maybe the thickness of the pavement is less toward the shoulder than the center. Or flooding or excessive rain at the time it was laid or before the pavement had time to dry, could that cause this? Maybe an underground spring or just ground moisture. Or maybe the ground wasn’t properly prepared before the pavement was laid. Could tree/plant roots cause something like this? How about an earthquake or tremor?

Just a thought.
-Alex

Yes!  Almost all of those are potentials for this type of cracking!  Typically most of those problems will cause the alligator cracking to appear in various other parts of the pavement as well, not just on the joint. 

Mixture variation in the pavement can cause a variety of different issues in the pavement, depending on how the mixture varied.  However, alligator cracking is typically a structural problem instead of a material problem.  A material issue typically means that the mixture of asphalt binder (oil) and/or rock has problems.  When I refer to structural issues it is referring to the "structure" below the mixture that is being laid down.  The structure is typically old pavement, base rock, or soil.  So when I say that there is a structural problem, that means that there is something wrong with the underlying pavement, base rock, or soil below the new mixture that you're putting down. 

The thickness of the pavement is an interesting idea, if this part of the pavement had a thinner layer of pavement than was designed to hold the load, it could easily crack under the pressure. 

Rain and moisture at the time of paving could cause this type of cracking but more likely it will cause raveling or shoving instead.  The one time that it could definitely be an issue is if the paving crew is paving over aggregate base or soil instead of existing pavement.  If the base course is too wet at the time of paving it will tend to move and adjust under the weight of vehicles, sort of like playdough would.  This would also happen if there were an underground spring or ground moisture.  I would consider this a way of poorly preparing the ground before laying the pavement because it is up to the contractor to assure that his base course is prepped and ready for the asphalt layer.  This includes obtaining the correct moisture and compaction of the underlying structure.  Another way that the alligator cracking could occur would be if they didn't compact the base rock enough before paving on it. 

Roots and earthquakes could absolutely cause these types of cracks as well, however, if it were caused by roots you would tend to see a raised portion in the alligator cracking, where the root is pushing up on the pavement.  I hope that answers your questions!  Keep them coming!
 

CT 216 Changes-June 2011

Caltrans has released a new version of CT 216: Relative Compaction of Untreated and Treated Soils and Aggregates.  The basic test method has remained the same but there were a few additional notes added and a few minor changes that will help to clarify the test method.  The changes worth mentioning can be found below:

·         It no longer specifies miscellaneous equipment in the apparatus section

·         In the test method of Laboratory Compacted samples, it now specifies that if there is less than 10% retained on the ¾”, you should divide the passing the ¾” into 5 test specimens of equal weight (within ± 1 g) in the prep stage.  Previously it did not specify the amount of specimens at this point in the process and did not specify the tolerance for variance in weight between the samples. 

·         It now notes that after compaction, when reading the graduated tamper shaft, if the graduation is not level with the top of the mold, you should read the next highest graduation.

·         It now notes that some materials may require different moisture content variations besides 2% but that if another increment is used it should be used throughout the test. 

·         It now notes that excessive moisture on the base plate denotes a failed test point and what to do if this happens. 

·         Relative compaction is now recorded as the nearest whole number, not to the nearest 0.1% as was previously specified.

·         It now specifies that the corrected density on the plot is denoted as a dot with a circle labeled “Adjusted Density”

The June 2011 version of CT 216 can be found on the Caltrans website and at: CT 216 June 2011

Happy reading and good luck!

CT 229 Changes-June 2011

Caltrans has issued a new version of CT 229: Method of Test for Durability Index.  There were very few changes but below are the ones I thought were worth mentioning:

• The test method has been changed over from metric to US units.
• The specifications for the the lateral reciprocating motion of the coarse aggregate agitator have been changed from 4.75 ± 0.17 Hz to 285 ± 10 cycles per minute.  This is due to the change from metric to US units.
• The detailed specifications on the mechanical sand equivalent shaker have been removed and the test method now refers you to the apparatus used in CT 217.
• It no longer specifies the sand equivalent test apparatus.
• You can no longer use tap water in this test, even if it does not affect the test results.  The only exception is that you may use tap water during the rinse process.

The June 2011 version of CT 229 can be found on the Caltrans website and at: CT 229-June 2011

Good luck and happy reading!
   

CT 217 Changes-June 2011

Caltrans has released a new version of CT-217 Method of Test for Sand Equivalent.  There were not many changes but the following are the things I thought were worth mentioning.  The new June 2011 version can be found on the Caltrans website and at the following link:  CT 217-June 2011

·         There is now more detailed information for manually-operated sand equivalent shakers.  It must be able to oscillate at the rate of 100 complete cycles in 45 ± 5 s with a hand-assisted half stroke length of 5.0 ± 0.2 in.

·        You can no longer use tap water in lieu of distilled or deionized water. 

·        It now specifies the weight of the total starting sample weight to be 1000-1500g of passing #4 materials.  Previously there was no weight specified.

·    There is now a note on dispute resolution for when manual shakers are used.  In dispute, a mechanical shaker will be used.

·         It is no longer specified to avoid exposing the plastic cylinders to direct sunlight any more than is necessary

·         The Health and Safety section has been rewritten to be less specific and generally puts the responsibility to determine the safety issues and best practices on the user. 

Happy reading and good luck!

Alligator Cracking

Welcome to post one of disaster roads!  The pictures I'll be using for this series were taken by our readers over the fourth of July weekend.  If you have a road you are curious about email it to me at toni@qualityincalifornia.com and I'll talk about it in the blog. 

This post will focus on alligator cracking based on the photo below:

For those of you who might not know, alligator cracking is the scaly looking pattern you see on the right side of the photo.  Here the cracks have been filled in with a crack sealer which can help to slow the cracking from spreading by keeping out additional moisture.  Alligator cracking is one type of fatigue cracking that can happen to a road for a variety of reasons but is most often related to overloading the pavement. 

Roads are designed for a variety of variables, two of the biggest are the amount of traffic and the type of traffic.  A road in a subdivision may be designed for a lighter load than a Wal-mart loading dock because it is assumed that only small vehicles will be driving there instead of heavy trucks.  If a road is designed for a lighter load than is applied to it, the pavement can form a pattern of these types of cracks.  Often roads are designed with a lighter load in mind but some vehicles are overlooked.  For instance, it seems like a fair assumption that a subdivision will only see small vehicles but the subdivision is still visited by heavy garbage trucks weekly and if there is a bus stop in the area their roads will be overloaded from the bus's weight as well. 

However, the road shown above does not seem to be a result of poor pavement design but more poor pavement construction.  Take a look at the photo above and notice that the black crack sealant fills in a patch of alligator cracks but then continues up the road in a straight line with a couple more patches of alligator cracks as it goes.  The long straight crack is the joint in which one length of pavement was laid down next to another length of pavement.  Joints are common places for distresses on roads because if they are not properly installed, the joint can allow for moisture and other debris to wriggle in between the two sections of pavement making the joint a weaker part of the road way.  For this very reason, Caltrans and several other government entities specify that joints be lain on lane dividers like the yellow dashed line you see on the left and the white line you see on the white. 

The next thing you might have noticed is that the joint is a foot or two to the left of the edge of the lane.  The wheels of vehicles travel in this exact area of the lane, placing half of the weight of a vehicle on the joint in this road.  The joint is already a weaker section of the road and the additional weight of the vehicle tires overload this section of the road and create a variety of small cracks in the joint that radiate out from the joint line.  Over time moisture will accumulate in these cracks.  Cold weather will cause the moisture to expand and the cracks will grow until they form a pattern like you see above.          

Can you think of any other reasons this road might look like this?

4th of July Road Pictures

Thank you to everyone who submitted photos to us over the past weekend!  There were quite a few "damaged" road pictures that I'm planning on analyzing for all of you over the next few weeks but for your viewing pleasure here are a couple of my favorite submissions of good roads.  California's roads are designed to last 10-20 years before needing replacement and from the looks of it, these roads will be around for quite some time.  Thanks again to all of the construction workers out there!  You are the ones that make this happen and we're so glad that you're good at what you do!

And my personal favorite.....

Call For Photos!

It's that time of year again!  The time for loud booms from the sky and the smell of bratwursts wafting through the air.  The fourth of July is finally upon us and what is more American than the sweat and hard work that our construction workers all over the country pour into the biggest asset that our country possesses.  Our Roads! 

In honor of the red, white, and blue I'd like to challenge you to take a picture of a road this weekend and submit it to us with a description of where it is and if there was a story behind it.  Come on, you're going to spend hours in the car this weekend on the way to the grand parents' house so why not take some really amazing photos for all of us here at Quality In California!  Take a picture of a good looking pave job, or a rutting road that looks like you could swim in the puddles, or even just of that dirt road you travel down to get to the family ranch.  Take the time to think about the amazing engineering that has taken place under our tires over the last century and how it has influenced our lives for the better.  Our country's infrastructure is our greatest asset and if you submit a photo you'll be helping to recognize the hard work of our melting pot's citizens!