SOME BASIC CAUSES OF PAVEMENT FAILURE

Basics

INTRODUCTION

There are certain basics with respect to pavement failure that have existed since the first pavements were laid. Pavements crack, pavements slip, water damages them, and pavements rut. Irrespective of the tests used to evaluate pavements, failures have the same basic causes.

CRACKING

No matter where the cracking occurs, it is caused by the inability of the asphalt to relax the stresses, and must rupture.

Fatigue Cracking. Stress and strain are what are called tensors, which means that a pavement can be under compression and tension at the same time, but in different directions. While a tire compresses a pavement downward, it forms a deflection basin which causes the pavement to go into tension in both horizontal directions. Many years ago we used data from deflection testing and, assuming a parabola, did a line integral to calculate strain. If the pavement is not strong enough, the asphalt is stretched too far, separates and a crack forms in the wheel track. Also a crack may form between the wheel tracks.

Longitudinal Cracking on Joints. The joint between two passes are especially week. Inside any one pass of the paver, some aggregate willbe on both sides of any plane or slice inside of the pavement. In fact, when sample undergoes an indirect tensile test such as is done in stripping tests, rocks actually fracture. A joint, however, is held together only by the asphalt layer, which has a tensile strength of about 200-1000 psi, depending on the temperature and shear rate. If the asphalt in the mix can flow vertically in response to thermal stresses, the crack won’t form. However, if the stresses exceed that at the joint, a crack forms. As a result the pavement on either side of the crack can shrink or expand independently. Often what happens then is that the pavement sections shrink away from each other in the cold, but do not expand completely back together in the heat. For that reason it is crucial to follow proper technology of forming a joint.

Thermal Cracking.  The mechanism of formation of thermal or non-load associated cracks is again the lack of the asphalt to be able to relieve thermal stresses by flowing vertically up when the pavement is hot and vertically down when the pavement is cold.

PAVEMENT SLIPPAGE

From time to time the pavement will shift. In one project I has on at the LAX airport, a 2” lift was slipping on a 4” lift from landing of air traffic. A core was made of the section so it waw possible to observe a daily slippage. Two sources of the problem. First, it was supposed to be 4” over 2”. Secondly, if there was a tack coat, it had been ruined as a result of a dust storm. To prevent slippage a prime needs to be used between the base and pavement, and a tack coat between two lifts.

RUTTING

There are two causes of rutting, improper aggregate gradation and studded tires.

Gradation.  Asphalt itself is too weak to stopthe flow of the mix by itself. If the coarse aggregate in the mix cannot interlock themix has to rely on a mastic composed of the fines and asphalt, which cannot carry the load. The solution is a coarse gradation with no humps in the fine mastic area.

Studded Tires. Research is under way on how to solve this problem. Harder aggregate has helped, but no solution is available now.

WATER DAMAGE

If the pavement is not protected from water damage, all of the above is blowing in the wind. There are data that suggest that even pavement protected by amine or lime antistrips will lose much of its strength thus cannot complete its design life. Many aggregates are wetted by water better than asphalt so that if the surface cannot be permanently altered to prefer wetting by asphalt, eventually water will replace the asphalt.

Robert L. Dunning. www.petroleumsciences.com, blog asphaltwaterproofing.wordpress.com 

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UNRELIABILITY OF PG GRADING SYSTEM

Superiority of the AR Grading System

 

AR Grading. The Asphalt Residue (AR) grading system used in the Western part of the United States for decades grew out of the fact that the asphalts in this area differed greatly. While various grades were in use, the workhorse grade was AR 4000 which meant that the asphalt in the pavement, irrespective of crude source, would have the same consistency. AR 4000 meant that the viscosity at 60° C of the asphalt after the RTFO test would be 3000 (2500 in Washington) to 5000 poises. A viscosity of 4000 poise was selected as it was found that at 4000 poises tenderness in oversanded mixes was easier to handle.  60° C is used as in most cases that is about the highest temperature the pavement reaches although in the deserts it can reach considerably higher temperatures. On the other hand, the viscosity at 60° C from the RTFO of equivalent asphalts graded by the AC grading system (2000 ± 400 poises based on original viscosity at 60° F) or by penetration grading system (85/100 based on penetration at 25° C) can vary greatly. For the 85/100 penetration grade, the range of the 60° C viscosity after the RTFO of those asphalts evaluated during the development of the AR grading system varied from about 1600 to over 7000 poises. For an AC 2000 grade asphalt, the probable viscosity after the RTFO aging would range over about 4000-8000 poises, depending on the crude source. The equivalent PG grade is PG 64-XX.

PG Grading. There is an astounding number of PG grades, 7, and up to 6 subgrades within each grade, based upon low temperature properties. If there was consistency within the grades it might make sense, but we have regressed even back beyond the AC grading system. These grades were set up primarily to control tenderness and rutting even while leaving the gradation specification so open that gradations that would allow grievous rutting are included. The equivalent PG grade is based upon the Dynamic Shear test of G*/sinδ of 1.00 kPa at 64° C with no maximum. For a sinδ of 1.00 (close to that of unmodified asphalt) the viscosity is G*· sinδ or 1000 poises. The G*/sinδ value from the RTFO test would be 2.20 kPa min or 2200 poises with sinδ = 1.00 and again there is no maximum. Sinδ for modified asphalts is less than one thus that drops the specification minimum viscosity below that of non-modified asphalt.` In other words, for the asphalt as placed in the pavement, the AR 4000 specification is 3000-5000 poises at 60° C. For the PG 64-XX , the-in place viscosity at 64° C can vary from somewhat less than 2200 poises to as high as one wishes.

 

 

Philosophical Inconsistency of the PG Grading System. I am only addressing the grading system, not the value of the low temperature specification. I am not suggesting that there is anything wrong with the use of the DSR, as it is a handy tool. I am suggesting that the grading should have been based upon the consistency of the RTFO residue whether viscosity tubes are used or the DSR. The value of the DSR data is that we can get information about the effect of polymer modification from the phase angle, sigma (δ).

We have shown above that the range of the allowed viscosity from the RTFO test of any particular PG grade is greater than that of any previous grading system even though there is are 7 specific grades in order to control rutting. The implication is that controlling rutting requires fine tuning. Yet, at the same time there is a movement to use warm mixes, one of the benefits of which is that the asphalt will have a considerably lower viscosity than the intention of the grade.

Controlling Rutting. The prime control of tenderness and rutting should be with aggregate gradation.  As long as the gradation specification allows badly oversanded mixes, rutting will be a problem.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com

WATERPROOFING ASPHALT PAVEMENTS WITH SEALCOATS

Chip Seals

The application of a seal coat has a number of functions however one of the most important is to waterproof the pavements, protecting them from water damage and oxidation. If pavements were sealed early in their life, e.g. within a year, the pavements would last a lot longer. Chips seals are used especial on highways.

Chip Seal Emulsion. The emulsified asphalt used for chip seals are specially designed to break very fast on contact with aggregate. Emulsions can be either anionic (basic) or cationic (acidic) although the cationic are very popular. With asphalts from some crude oils the amount of emulsifier required for anionic chip seal emulsions is very small, approaching zero as a result naphthenic acids in the asphalt which serve as emulsifiers when neutralized with caustic soda.

Special Seal Emulsion. There is a product called PASS that has the ability to re-seal cracks and regenerate pavements.

Where to Use. A chip seal does an excellent job as a seal. While it can be used in cities, in my opinion a slurry seal would be better, unless it is a Capeseal in which a slurry is placed over the chip. The disadvantage of use in cities is that the chips can spread over lawns, in driveways, etc.

Mix Design. It is very important that a mix design is done, otherwise there can be failures.

Problems. One of the causes of failure is dirty aggregate. The chip seal emulsions are designed to break immediately on a surface thus when it hits the dust it breaks on the dust and not on the surface of the aggregate. An emulsion type called High Float is more tolerant of dust. Not enough emulsion can cause loss of chips while too much emulsion can called bleeding.  Also when used in cities, loss of chips can occur at the centerline as along the centerline there can be less asphalt as a result of less overlap of the spray. For rural roads this isn’t a big problem as there is not that much turning stress on the aggregate, however in the city, there can be turning traffic out of driveways. Also, there is another important problem; it is difficult to skate on chips.

It isn’t a good career move for a director of public works to place a chip seal on streets in expensive neighborhoods, especially if chips end up on the lawns, sidewalks and driveways.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com

REDUCING HOT MIXED ASPHALT COSTS

Controlling Voids in Mineral Aggregate (VMA)

Considerable effort is being made to reduce costs and amount of hydrocarbons that go into hot mixed asphalt (HMA) pavements. One such effort is to find ways to mix and compact at a lower temperature thus reducing the amount of fuel required. However, saving fuel can also be obtained by reducing the amount of asphalt used as asphalt can also be sold as a component of heavy fuel oil or cracked to make diesel, gasoline etc.

Mix Design.

Irrespective of the type of mix design or the amount of modification of the asphalt, the basic properties for an acceptable product remains the same. If we get down to basics, we want the gradation to be such that it inhibits rutting, want the gradation in the # 30 sieve size to be such that there isn’t a lack of material in that area and want the composition of the binder to be such that the film thickness is somewhere between 7 and 10 microns (based upon our experience. Idaho specifies 6 microns as a minimum) and, for example for a ½” nominal design, an effective asphalt content of 4-5%.

Trade off between % Asphalt and VMA. As the VMA increases, the % asphalt  required increases at a rate of about 0.25% per each percent of increased VMA, the exact amount depending on the actual specific gravities of the aggregate and asphalt. For a 400 ton an hour plant, the reduction of the VMA of 1% would reduce the asphalt by one ton per hour or a savings of $500/hour if asphalt is $500/ton.

Silliness of the “Forbidden Zone”. Some Superpave gradation specifications have a “forbidden zone” for the gradation through which the gradation must not go. It is supposed to be on the maximum density line (on the 0.45 power gradation curve) of the aggregate; however, in addition to being silly, it doesn’t even fall on the actual maximum density curve for the job mix formula.

Effect of RAP on VMA. With the introduction of SUPERPAVE the VMA, which used to be 13% if one was used, was increased to 14%. We were having problems in being able to make the 14% with granite aggregate, and found that we had to control this by blowing out -#200 material. On one project I used a factorial experimental design to aid in adjusting the gradation with considerable success. This allows evaluating the effect of numerous variables on mix properties. Of course saving money by reducing the VMA was not an option. With the introduction of RAP, however, the VMAs rose by as much as 2%, requiring as much as 0.5% more total asphalt (including that in the RAP).

Reducing VMA to Reduce Cost

A number of years ago I did a Gram-Schmidt orthogonalization on gradation data. I found that there were only three truly independent variables, one of which was the % -#200 material. By using three independent aggregate criteria and % asphalt as a fourth variable we should be able to determine what changes should be made in the mix to minimize the VMA within the specification criteria, thus minimizing cost. I would suggest the use of a 24 factorial design with triplicate centerpoint to find the most economical gradation. The following would be for a ½” nominal mix design. For variables I would use: 1) the % of the gradation between the ½” and the #4 screens; 2) the % of the gradation between the #4 and #30; 3) the  % -#200; and 4) the % asphalt. We have found that a Hveem compaction at the recommended compaction temperatures for a 75 gyration Superpave design give the same results as the gyratory compaction. We would suggest that this be done, therefore, with the Hveem compactor as it uses only 1/4th as much aggregate and asphalt as does the 6” gyratory design however gyratory compaction could be used. The advantage of the Hveem is we can also get as a bonus the stability. I would stipulate that one of the boundary limits would be that no gradation point should be above a line on the gradation curve (0.45 power graph) from the % passing through the first sieve that retains aggregate (1/2”) to the % passing of the #200 sieve. This would provide the information needed to minimize the VMA within the specification. The results could provide the starting gradation and asphalt needed for a gyratory design.

Decreasing the VMA from 16.5 to 14.5% for 100,000 tons of mix would save $ 250,000 of $500/ ton asphalt.

Petroleum Sciences, Inc. has the equipment and mathematical knowledge (as there is considerable mathematics involved) to provide a service should a contractor wish to reduce costs. We can set up the experiment to be done in the contractors own facility and then evaluate the results or do the complete project in our facilities.

Robert L. Dunning, www.petroleumsciences.com, chemistdunning@gmail.com

 

 

ASPHALT AND ASPHALTENE

Neither One is a Single Material

 

Asphalt and asphaltenes are names that show up in articles and papers discussing paving and roofing materials. Especial with people not very familiar with technical field, discussions often sound like each is a single well define material such as salt or water. However that is far from the fact. Some may even feel that asphaltenes are something in the way that needs to be isolated or corralled. Yet they are vital in controlling the properties of an asphalt. Also researchers may reach conclusions on an asphalt from a particular crude source and believe that those conclusions pertain to all asphalts.

Asphalt.

Asphalt is the part of crude oil that is left when all the other hydrocarbons have been removed. There are two main ways of separating the asphalt from the gasoline, kerosene and oils; distilling, and solvent extraction.

Source. The properties of a particular unmodified asphalt are controlled by the source of the crude oil. The differences can be profound. In California there are three crude sources that produce entirely different asphalts: California Valley, Coastal and LA Basin. Within those broad designations are subgroups such as the coastal crudes; Santa Maria and San Ardo. A specification can be developed such that it can be met by asphalts from all three sources however they will perform differently. There are some asphalts that have very poor cold temperature performance and others that perform very badly in hot weather.

Distillation. In the distillation of crude oil, one pipe goes into the distillation towers, and a number of pipes come out. Each tower system is designed for a particular crude or crude blend and there are pumps removing the products. What is left over is asphalt on the bottom of the tower also. Some crude oils have no asphalts while others may contain as much as 65% asphalt. If any one of the storage tanks gets full, the refinery has to shut down.

Propane Extraction. The other method is to extract the non-asphalt portion with propane.

Asphaltene

One of the components of asphalt is the asphaltenes. Here we have two problems: the misconception that asphaltenes are significantly different than other asphalt components, and the basic definition. While some methods define asphaltenes as n-pentane insoluble material, other methods use hexane or heptane or even iso-octane as the solvent. n-Pentane will produce the largest amount. Because certain asphaltenes are precipitated by a solvent doesn’t mean that there aren’t still other materials in the asphalt that are very similar to asphaltenes. Asphaltenes give body to the asphalt. If the asphaltenes are completely solvated, the asphalt won’t perform well. On the other hand, if they are in a second phase, again the asphalt may cause problems. In some cases, the asphaltenes will be at least solvated sufficiently at ambient temperatures for a single phase to be present, however they may form two phases in cold conditions, resulting in cracking in winter.

Modification

The addition of polymer modifiers further complicates the situation. Adding a polymer to any asphalt will result in two phases no matter how well the asphaltenes are solvated. When polymer modification was young problems with phase separation was a problem that had to be resolved. It can be seen that with a wide range of properties in asphalts, polymer modification can be more of an art than a science. One question I have is how well modified asphalts will perform in low temperatures even though they pass all of the low temperature test. For pavements to resist low temperature cracking the binder must be able to stress relax faster than thermal stresses build up. If the binder becomes more like a plastic with a yield force necessary, the pavement will crack.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com

WATER CAUSED PAVEMENT DISTRESS

Raveling

Raveling is the loss of the mastic matrix in the surface of a pavement. This would be expected to occur with time but is aggravated by the presence of water. If the aggregate surface is not protected from water, traffic will cause raveling. This can be seen near curbs where often water is flowing. The asphalt is not pulled off but is floated off.

Water, a blessing and a bane! To get compaction in a subgrade or base, the water content must be at an option. A blessing. Even with cold recycling systems, the total liquid content (including water) must be optimum to get proper compaction. A blessing again. Obviously we love water, especially on a hot day. If we had a choice of a cool glass of water or a glass of warm lard, we would obviously choose water. Most aggregates are no different. If they have a choice, they would choose water to imbibe into their pores, not asphalt.

Our production system forcibly removes water from aggregate and equally forcibly makes the aggregate accept asphalt. That doesn’t make the aggregate happy and if it has the chance it will invite water back in through any defect in the coating and gleefully kick the asphalt off. Many aggregates have hydroxyl groups sticking out on the surface, which attracts water. In addition there may be water loving sodium and potassium exchangeable ions on the surfaces. These ions are the result of defects in the silicate and aluminate structure in the aggregate. In the silicate structure there may be an aluminum atom instead of silicon resulting in a structural negative charge. Likewise a magnesium atom may replace aluminum in the aluminate structure.

To combat this, amines or lime can be added to change the nature of the aggregate surface. Unfortunately, the protection may not last, especially if salt or magnesium chloride is used for deicing. The chemical principle of mass action can reverse the action of these antistrips. One solution has been to graph onto the aggregate surface an organosilicon material that actually becomes an integral part of the aggregate and thus cannot be dislodged. The aggregate then changes allegiance so strongly that it actually forcibly rejects water and opens its pores to the asphalt.

So to control raveling, the adverse affect of water must be controlled. This is especially important with raveling as it occurs on the surface where the pavement will be often in contact with water. The best solution is to persuade the aggregates to distain the advances of their first love and turn to a new one that is not so transparent.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com

WORLD OF PAVEMENT CRACKING

Fundamentals of Non-Load Associated Cracking

There has been considerable research on the engineering basis of pavement cracking. Those interested in some of the basic studies on cracking might consult volume 41 (1972) of the Proc. Association of Asphalt Paving Technologists. Many of the concepts develop there were the basis of the PG grading system with regard to low temperature properties of asphalt. While those papers are 40 years old, they lay the basis of technical progress in understanding cracking. Later studies have been oriented toward understand how cracking can be predicted.

However, it is not the purpose of this blog to go into the engineering of pavement design but rather to speak of the basic physics involved.

Failure occurs either from tensile stress or crack propagation. The maximum tensile strength of asphalt and hot mix is about 1000 psi and that only happens if the asphalt is cold or stressed at a high rate. At higher temperatures or lower rates of strain the stress at failure would be less. When cracks appear, the stresses are concentrated at the apex of the crack accelerating the formation of a crack. Thus no matter what the crack might look like, it is caused by too much tensile stress.

Literature suggests that when the temperature drops down below about 100-110°F of the softening point of the asphalt in pavement one would expect damage to the pavement. That damage accumulates eventually resulting in transverse cracks showing up. The distance between cracks is related to hardness of the asphalt. If the temperature rapidly drops to, perhaps, 150° F below the softening point, the crack may occur that day. I actually observed that in the late ‘80s. There had been a very sharp drop in temperature in Spokane, Washington on one day. I was called in for several cases where even fairly new pavements showed block (traverse) cracking, including a new tennis court. The only answer was that the temperature drop had caused it.

If we recognize that the softening point of aged asphalt might approach 200° F it can be seen that the fast drop in temperature in deserts at night could even cause damage at surprisingly higher temperatures. Pavements can reach over 170° F in the deserts.

The effect of crack propagation can be seen in parking lots where asphalt pavement is adjacent to a portland cement area where there are 90° corners. A crack will be seen radiating out of the corner even if there is no other evidence of cracking in the pavement. If small cracks are formed inside a pavement and don’t heal themselves, they will grow and eventually show up.

When cracking occurs, the asphalt in a pavement is no longer performing as a liquid, but more as a solid. It responding to stress from cooling by pulling apart horizontally. When the pavement heats up again, the crack remains, although if they are small, traffic can knead them back together. If it can act as a liquid it flows vertically upward as the temperature increases and downward as the temperature decreases. The solution to cracking is to allow the asphalt to retain its liquid properties as long as possible.

As asphalts from different crude sources behave differently, there is no golden rule. Non-electrolytic solution chemistry can be involved but that is a discussion for another day.

One of the remedies for reducing the temperature related cracking in pavements is to seal them so that the rate of hardening of the asphalt is reduced. Also the HMA needs to be protected from water, both liquid and vapors. Even in the desert water accumulates under the pavement. If the bond between the asphalt and aggregate is susceptible to being compromised by the presence of water, the bond will be broken and failure will occur. Traffic accelerates the loss of strength as water propelled by changing pore pressure scours the asphalt off of the pavement. Even water vapor has been seen to do this. Weakening the bond between rock and asphalt will then be allowed to grow under less stress.

I also like to see primes used under the pavement to discourage water from entering the mix. Reducing the rate of hardening of the asphalt so it retains its liquid properties and protecting the pavement from water damage can reduce the rate of formation of non-load associated cracks.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com

GUARANTEEING UNPRODUCTIVE RESEARCH ON RUTTING

Expecting Binder Research to Solve the Problem

In a previous article “Fundamental Causes of Cracking, Potholes, Raveling, and Rutting in Asphalt Pavements” I touched on some of the causes of rutting. I wish to expand on this subject. However I wish to exclude rutting from studded tires as that problem has not been solved at this time.
Prior to the establishment of the Strategic Highway Research Program (SHRP) I attended a meeting in which it was stated that the goal was to develop an asphalt that could solve all of the problems that occur in pavements. The philosophy that problems reside primarily with the asphalt is still deeply encountered, however, in my opinion it is all “vanity and blowing into the wind”. That does not mean that there isn’t a place for asphalt research because great strides have been made in resolving pavement problems with modified asphalts. In fact, with rutting, I am sure many will state that with such and such binder, the rut tester shows an improvement in rut resistance. And I am sure that their data is correct. However I would suggest that those modifications only affect the rate at which rutting occurs not the basic cause. The misconception is that it is the properties of the asphalt that allows rutting. That is false. A properly performing asphalt is a liquid and is purposely designed to not resist rutting or any other stress that might prevent it from flowing. In fact one of the solutions to low temperature cracking is to modify the asphalt so that it can flow to relax thermal stresses before they reach the point where the asphalt fractures.

If one wishes a life time research project on rutting, concentrate only on the binder and work only with oversanded aggregate gradations. Do I mean that the aggregate gradation is part of the problem? Yes. In fact the aggregate gradation is the problem; and the present gradation specifications specifically allow oversanded mixes, thus, allowing the construction of tender and rut prone pavements to be built. I learned this from Vaughn Marker and Went Lovering of the Asphalt Institute back when I had more hair, and it was not so grey. (Went Lovering also had worked for CALTRANS and was a great source of knowledge and wisdom. He was instrumental in the development of the Hveem Design.)

How can we get oversanded mixes. First draw a straight line on the 0.45 power gradation chart from the % passing of first sieve that retains aggregate to that of the – #200. If you want an oversanded mix, make sure that the -# 4 gradation is above that line. If it is below the line, you can still meet that goal of an oversanded rut prone mix if the gradation in the -#30 range goes above that line. It is true that messing with the ability of the binder to flow will help reducing the rate of rutting, but, of course, non-load associated cracking is associated with the lack of ability for the binder to relax thermal stresses. In this manner research can be continually funded so that one can be an expert on how to not to stop rutting and tenderness.

You do want an oversanded mix for hydraulic structures, however.

Robert L. Dunning chemistdunning@gmail.com, www.petroleumsciences.com

CALCULATING CHIP SEAL DESIGN DATA

Spreadsheet for Comparing McLeod and Kearny Designs

Chip seals are widely used for maintenance of pavements. Two of the popular design methods are those developed by Dr. Norman McLeod and Mr. J. P. Kearny. I am finishing the development of a spread sheet that can be used to compare the two procedures, and that can be taken out in the field during construction so that changes can be made with regard to pavement conditions, traffic during construction, % oil in the binder or other changes that might occur.

The spreadsheet may also be used by agency engineers to establish budgets and for contractors in bidding projects.

At the present time the Kearny method on the spreadsheet does not include the modifications that Jon Epps et al. have made, but those will be added soon.

For more information contact Robert Dunning at chemistdunning@gmail.com, www.petroleumsciences.com