geotextile use in paving 1

[twalkengr]

Does anyone have recommendations regarding the use of geotextiles in preventing reflective cracking when constructing asphalt overlays? We are considering whether or not they are economical in extending the life of the pavement.

 

[Focht3]

We've had some local experience with geotextiles - but not for overlays. In short, they work quite well.

The problem had to do with reconstruction of residential streets in older sections of San Antonio. The streets saw very little traffic, but were being built with 8 inches of cement treated base (CTB), 4 inches of "black base", and 2.5 inches of HMAC - on top of expansive soils. (The top 6 inches of the clay was "lime stabilized.&quot Longitudinal cracks were appearing within 6 months; it was a political problem for some of the city councilmen.

That pavement section is clearly overkill. We had an informal meeting with the City engineer about eight years ago, and I recommended a test section with a geotextile between the HMAC and the black base, while eliminating the CTB and replacing it with an equal thickness of crushed limestone base.

The City installed a test section, and that pavement is holding up better than the approach section built using the City's standard design. I'm a believer -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[layfieldman]

There is quite a debate on whether geotextiles use in overlays provide enough benifit to justify the cost. For instance, it is standard practice in California to do so but not in very many other locations. Up here in Canada due to the freeze/thaw cycle there is very little benefit. The main contribution of geotextiles seems to be in preventing the movement of water and therefore reduce the deterioration due to moisture. Geotextiles do not offer very much in terms of structural support as there is usually unacceptable deformation in the road before the full strength of the geotextile is mobilized. Other products such as a steel mesh or glassgrid are better at this as they mobilize their strength at much lower strains. As Foch3t has suggested a test section is probably the best way to determine whether the cost is worth the benefit.

 

[KRSServices]

I know that EBA has done these for airports with reasonable success. I tried it once on an overlay project and it was a horrible failure due to an inexperienced contractor. There were folds in the fabric and the long and short is that the asphalt overlay was rejected.

I have however seen some sections of road with the fabric under the overlay and it seems to be holding reasonably well, mind you, the cross section was well draining and therefore the effects of frost heave were never that much of a consideration in this location.

If you are intending on the use of fabric, I suggest some very tight specifications be drafted as to the application and finished product. I would tender this item as provisional, because I would think the costs would be a little too rich for the project.

KRS Services

http://www.krs-services.com

 

[VAD]

Interesting topic that has engaged many thoughts and numerous experiments. Many have shown some reduction in deterioration of a pavement by incorporating paving fabrics between overlays others have found the benefit to be subtle.

Most of the time no one spends the time to keep monitoring. After a few years many stop doing so and make their own conclusions. Glasgrid was ruled out in parts of Western Canada a long time ago as a material that would minimize reflective cracking. Others have reported success with this material. Dedicated monitoring plays a significant role. Some engage test sections because of political pressure brought to bear by suppliers and manufacturers so many times test sections are put in and poor scientific approach is used to monitor etc etc.

In cold climates, it has been the norm for pavements to crack and in Canada this is somewhat acceptable. In my travels and observations, I have noted that a factor that plays an important role is the type of granular base. Where gravel is abundant and used as base course, there seems to be more evidence of pavement cracking than where crushed stone is used. I believe this has to do for the same subgrade type with the cleanliness of the material.

Some jurisdictions look at the cost of procuring the material and base their acceptance on percent passing the N0 200 sieve and play lip service to the plasticity characteristics. Some like a bit of binder as the base compacts well and stays together longer. There is a downside, however, in that these bases crack quite easily.

Crushed rock, on the other hand, if it comes form a first rate source has fines that are generally non plastic and hence displays lesser tendency for cracking. Subgrades tend to crack in cold climates and very often these cracks reflect through the overlays after one or two seasons despite that thicker overlays are used to reduce the time for crack propogation.

Of course many pavement engineers regard gravel and crushed rock behaviour as the same because they are both classified as granular materials. This is generally the case because of the tests to understand the material behaviour is traditionally looked at by the geotechnical community.

A large thickness of granular base in excess of 300 mm could help to mitigate this problem but this becomes expensive and only used where there are copious amounts of aggregate within short haul distances. Apart from cracks promoted by the subgrade and base there is the cracking of the asphalt mix due to the cold weather. Bottom up or top down cracks have their reason for occuring. The problem with the asphalt cracks is that they tend to propogate into the base and subgrade as well due to the adhesion of the various materials and stresses imposed during contraction of the asphalt mat during freezing temperatures.

Once these cracks have propogated then moisture travels down to various layers and if these are not sealed early then deterioration occurs and rough riding etce.

Most jurisdictions have a maintenace program to seal cracks but to arrest the problem of water travelling via the cracks the crack sealing does not seem to be totally effective. The reason being is that the cracks tend to open and close and the sealant used cannot respond to ensure that the sealant always provides a closure. We see that in homes as well when sealant is used. However, there has been tremendous work in the area of sealants but the other problem has been workmanship. After all while it looks easy when one sees the crack sealing crew in action there is a lot more than blowing out and passing the wand to effect a proper seal. You no doubt have noticed sometimes that the crack sealant comes out as ribbon from the cracks.

The role of the paving fabric in my mind is to reduce the maintenance costs associated with crack repair and to reduce the costs of repairs to cracks that some jurisdictions use - spray pathcing before paving. Some claim that this reduces the emergemce of reflective cracking. Pehaps it does but much money is wasted in this process. I would put the money in a paving fabric for the following reasons and in the following way.

Having built a new pavement, I would first apply a fabric above the base and place my lift of asphalt. The purpose of the fabric would be to prevent the physical communication of the crack from the asphalt mat to the base, and below. In other words the fabric acts to have the crack discontinuous. Assuming that I crack seal later then my crack sealant would also be stopped by the fabric and hence a smaller amount of sealant would be used. In a subsequent overlay say after 12 years then I would use another fabric between the existing pavement and the new overlay. By this means I would be saving on my crack sealant and preventing water from going into the base which causes the major damage to the pavement structure and results in rough joints etc through pumping of fines etc.

Now this approach may look costly in terms of firt time costs but we have to look at the savings etc re overlays, milling because pavement is too rough etc etc. Unfortunately these aspects are not always easy to evaluate and people tend to dismiss these as they can hide behind practices that were used by a Department during its history and people especially in some Government departments are afraid to innovate or push the leading edge.

Let us for argument sake place a piece of steel plate over a crack would a reflective crack propogate from below the steel. Well we do not build pavements with steel inserts of this type and it is not meant to say we should , all it brings to light is that with some thought we can move toward processes and materials that would allow us to combat the problem more effectively.

Another observation I have made is the cracks that are associated with curb placed at roadsides in streets etc. Invariably one notices transverse cracks in line with every curb sometime every second curb. In some highways one can count transverse cracks at ever 30 to 40 feet like clockwork etc etc. There are many observations which are interesting as well as puzzling and when you think you have come on a pattern and possible solution some thing turns up to destroy your hypothesis.

Coming back to the curb situation. It is my belief that in the cold climate areas the transverse cracks are promoted more by curbs that are cast in place versus curbs that are precast and placed on site. This is my hunch as with the cast in place the process of contarction results in more traction to the base course below and causes higher stresses. With the precast curbs there is less frictional forces as the concrete is not bonded with the base as with the cast in place curb. Has anyone taken a look at this while driving one's streets. What do you think.

Well back to the question raised. It is my belief that there are benefits to be gained in the retardation of overall pavement deterioration. To eradicate the cracks in a cold climate may not at this time be feasible but the driving public is concerned with smoothness. So if we can have a cracked pavement and it is smooth then we would have won a good portion of the battle.

However, most asphalt paving contractors do not like this idea as they depend on placing asphalt for a living. Unfortunately, for innovative engineers these guys often have the greatest lobbying power as they contribute heavily to the party in power and spend a lot on dinner plates. Of course, one has to spend money to generate money would be the counter argument for economic progress. I am not sure.

I think I have said enough. Sorry to ramble so much. I have spent a number of years on this topic from a geotechnical as well as a pavement engineering perspective as I am a firm believer that no real solution will occur unless we can integrate these disciplines in pavement engineering. However, just like some other discplines there is the same gap betweeen geotechnical and pavement engineers. I, however,wear both hats as well as others.

Focht3's experiment of removing the CTB on an expansive soil and replacement with granular is one such example where the nature of expansive soil behaviour on soil cement or cement treated material would tell anyone that a semi rigid material would be more prone to cracking whereas the granular material would accommodate some heave pressures and because of its nature would not crack as much - Brittle/work softening behaviour versus work hardening behaviour etc. The Lousiana State University, I think, did some work realated to crushed stone as a sandwich layer in mitigating crack propogation from CTB bases. I think it was published by TRB in 2000.

[Cheers]

 

[dirtdoctor]

There can be several obstacles to using fabric - the greatest is perhaps mobilization costs - most true fabric installers will not give you a good price unless you have at least 15,000 SY to do. And believe me, you want to use an experienced fabric installer. A normal paving contractor usually does not have the experience to do it.

As far as the performance of fabric goes, it is hit or miss. For cracks larger than 3/8 inch, I have not seen a fabric work well, unless some crack sealing is done first. And true crack sealing involves CLEANING out the cracks first - most don't do this effectively. If grade changes are not an issue, I prefer to use a 'popcorn mix' as a bond breaker to stop the propagation of reflective cracking. The popcorn mix is an open-graded mix (789 stone, 5% fines, and 6% AC) that is usually installed at 140 to 200 pounds per square yard. We have had tremendous success in stopping reflective cracks and have been monitoring performance for nearly 7 years. It is a bit more expensive, but when you figure in mobilization costs for fabric contractor, it's not that much more.