Asphalt overlay of cracked pavement 2

[dirtdoctor]

To all -

Doing a runway rehabilitation project in SE U.S. where the existing asphalt pavement is experiencing longitudinal cracking along joints (where lanes of paving match up). Some isolated transverse cracking beginning to appear where bitumen has begun to oxidize with age and tear at surface. Nearly all of longitudinal and some of transverse cracks are full depth. In-place pavement section consists of:

2" asphalt that was placed approx 30 years ago
2" asphalt placed at time of initial paving (50 yrs old)
8" to 10" of crushed aggregate base material
Clayey to sandy SILT subgrade (CBR values of 6 to 10)

Overall drainage characteristics are very good to excellent. No signs of rutting, shoving, or lateral distortion of pavement. Some of the cracks approach 1.5" in width - a poor crack sealing job was done 5 years ago and is starting to become ineffective.

Now the hard part - I am considering three possible solutions and would greatly appreciate some input. Solutions are:

1) Remove all crack sealant and fill cracks with a non-shrink cement/epoxy grout. Then, install non-woven paving fabric, using AC-20 to hold the fabric in-place. Overlay entire runway with 2" of surface mix.

2) Cold Process Recycle - CPR - (using an asphalt emulsion) the existing 4" asphalt and up to 2" of underlying aggregate base - will provide a 6" treated base. Install a 2" surface course overlay - would 2" be enough, or do I need to consider adding an intermediate course?

3) Mill out the longitudinal cracks approximately 4' in width. Install full-depth patches in the milled sections, then place paving fabric and 2" overlay.

Is a 2" overlay sufficient over paving fabric? Any idea on estimated life of an overlay if fabric is not used. The existing pavement is in good to very good condition, except for the cracking.

Any help would be most appreciated.

 

[VAD]

Hello Dirtdoctor:

I would apply your solution 3. I presume that your full depth patches will take you to the top of the aggregate base.You may encounter wet conditions in the aggregate base as the open longitudinal joints would have allowed water to enter the pavement section.

The paving fabric, if acting as an impermeable inclusion in addition to being a "suppressant" of reflective cracking would be good insurance as it would tend to reduce the depth at which crack sealing will have to be done re future maintenance.

My personal preference is at least 3 inches of overlay, but it does not look like it is warranted in your situation.

The emulsion treated section in 2 appears at face value okay but you may be providing a weaker section within the pavement and may not give uniform performance. One of the things you should look at is trying to provide close to the same stiffness across the structure or else rutting is likely to occur, if area is subjected to wheel loads.

Solution 1 has the disadvantage that it does not allow you the opportunity to determine the condition below re moisture etc.

I think your overall concepts are good and there may be things that you know from the site that we wouldn't. Would be interested in hearing your final choice. Remember to stagger the paving joints which you no doubt are fully aware of.

Regards

 

[dirtdoctor]

VAD -

Thanks for your input. I also have regards about cold process recycling, but a local contractor uses one quite regularly around here. I don't think it's wise to disturb a relatively stable, but fractured 4" thick mass of asphalt.

I have obtained 18 full depth asphalt cores from both uncracked and cracked areas. Cracks are full depth, but the 8" to 10" of macadam appears to be stable and not too wet at depth (obviously, the upper 2" got wet from coring operations, but no excess moisture at depth). Static cone penetrometer testing on subgrade materials indicated uncorrected in-situ strengths of 1.4 to 2.8 tsf - roughly equivalent to a CBR of 6 to 12. That's not too shabby for a clayey to sandy SILT w/ mica flakes. So, I don't think the subgrade has been damaged greatly due to good lateral drainage and a thick stone base course above it. Knowing this, do you think option 1 would be sufficient. I believe it would be the least expensive solution - and we all know that's what an owner wants.

I have also been toying with the idea of using a rubberized asphalt (wet process - mixed with bitumen) surface course mix for the overlay. If not an asphalt rubber mix, I've thought of using a polymer asphalt (at least a PG76-22 bitumen)to help reduce potential for reflective cracking. But, I am not sure how either of these mixes would hold up with the impact loads from airplane traffic. The runway serves low volume, light traffic and no current distortion, slippage, or rutting problems exist. However, given that runway is 6000' long, it COULD see heavier traffic in the future. Primary purpose of overlay is to get rid of cracking in the 30 year old pavement.

Again, thanks for your input.

 

[Ron]

Consider direct sealing the cracks with an asphaltic sealer, then add a double chip seal as a crack relief layer, then overlay.

 

[dirtdoctor]

Ron -

You like a chip seal better than paving fabric? Maybe I've just seen too many bad surface treatments in the past, but it always seems like the chip seals cause a tender mix (like the P-401 used in this situation on airports) to spread under the breakdown roller, thereby creating minor transverse in the mat surface.

What size aggregate do you typically see specified for a double chip seal?

Thanks for the info.

 

[GabionGuy]

You may want to look into asphalt reinforcement, provided you feel you aren't experiencing any problems due to your granular base layers.

Asphalt reinforcement (Road Mesh) has been used in asphalt overlayers throughout the world.

 

[VAD]

Hello dirtdoctor:

Seems that you have done a comprehensive invsestigation. Solution 1 seems promising but I am afraid that you may never stop the cracking from reflecting but you can delay its time of reappearance and certainly reduce maintenance costs in the future. I would go along with the fabric over the cracked area after you have done the joint sealing. Joint sealing should be done when cracks are open the widest not when they are closing, as such spring may be the time, but this is best determined from local experience.

Yes cost is a factor that both Client and Egineer has to address. It would be an opportubity to undertake testsections of your alternatives if you can swing it, but these must be monitored. Too often these are donw and forgotten.

Spray patching has been used to fill the cracks as well - finecrushed aggregate with emulsion or cutback, however cracks reappear. Thick overlays have some chance of retarding propogation but no solution exists as far as I am aware that would completely eliminate cracking. I think we need to have solutions that would prevent moisture from getting below and to reduce the amount of maintenance. As such solution 1 looks attractive.

In the end you will have to pick one and go with it. Sonme people would argue that solution 3 would result in two cracks rather than one. Here is wher local experience comes in. Others would argue that removal by milling would remove the problem. As well do not forget that many cracks penetrate the subgrade. In my experience I have seen less problems where large thickneses of granular material is used above the subgrade, but this is costly to construct.

You are facing the agonizing period which we all face in making a decision. Very often many feel that it is easy to design asphalt pavements. However, we are not good at fixing many of the problems associated with performance.

In the end I would discuss the pros and cons with the Client and give my best opinion. I think you are on the right track and I am confident based on how you have investigated the problem come up with a sound solution. i have done enough of these and know the feeling of coming up with a solution that is perfect, well I would say it cannot be done but we are still along way off.

The PG grade asphalts are perhaps a step in the righr direction as they can be tailored to suit the both superimposed and environmental loads. We are (the community) still not good at the latter. You are therefore not alone. It is a pity that some of the esoteric research in the pavement engineering field are more concentrated in finding solutions to the problems facing the Engineer in his/her daily practice.

Nice to see someone taking then pains to examine cores etc and not just winging solutions and walking away.

Good Luck.

 

[Ron]

dirtdoctor...you raise an excellent point but it can be covered in the mix design of the overlay asphalt. We have had good experience in my area with chip seal crack relief (should be a double chip seal, not single). A No. 57 stone is generally used for this purpose. It is course enough to provide good shear bond with the overlay and has enough surface area to "bite" the asphalt used to hold it down.

I practice in a hot climate so we are fairly sensitive about tender mixes. Use a high penetration or high viscosity AC in the mix and keep the asphalt content on the low side of the range. Then specify high levels of compaction to keep the secondary bleed-thru potential down. Since you are dealing with a low traffic, high weatherability application, your bleeding or flushing potential of the asphalt will be lower than you would have for a roadway.

 

[Laser28]

Cold-in-place will cure all your cracking problems. Foamed asphalt is even better, as it cures more rapidly and allows the immediate placement of the asphalt overlay. the pulverizing of the old asphalt and a portion of the aggregate base will remove all of the cracking, and reduce the propoensity of reflective cracking. this is the simplest, easiest to inspect and most cost-effective way to accomplish what your client wants.