ASPHALT, A REALY INERESTING LIQUID

Resisting Failure if Treated with Care

 

A pavement is about 93-96% rock, by weight, however it seems that there is a strong belief that by properly modifying the asphalt all problems can be solved. Asphalt or more properly, asphalts have served us well, even before modification. The properties of asphalts are primarily determined by their crude sources, however blending crudes or asphalts can at times produce an asphalt that performs better than either of the components. Modifying asphalts can also enhance their properties. However, it is important that we keep in mind that its performance depends to a great extent to its ability to flow, and its ability to suppress hardening as time goes.

Rutting is Not an Asphalt Failure. Asphalt is a liquid whose job is to flow in response to stress. If a pavement ruts, it is either ground by studded tires, or the aggregate size or the gradation is improper. If the stress is greater than the aggregate can handle, rutting occurs with the asphalt doing what it is designed to do, flow. Modifying the asphalt can affect how fast the flow occurs, however it is the aggregate properties that affect the rutting.

Many Aggregates Prefer Water to Asphalt. Asphalt doesn’t work well if it can’t stick to aggregate. Water can interfere with adhesion. One cause can be in the asphalt itself. If it is produced from crude oil that had been treated with caustic soda, it will contain soaps that will make the asphalt itself water sensitive. That has been solved by lime treating the crude. Antistrips are used to aid adhesion; however it has been shown that with some antistrips the effect wears off which allows water to lift the asphalt off of the rocks. There is one antistrip that combines chemically to aggregate and provides long term durability.

Non-load Associated Cracking Occurs when the Asphalt Cannot Relax Stresses. The fluidity of the asphalt is essential to prevent cracking. Trying to make the asphalt stronger only makes the matter worse as its maximum tensile strength is about 1000 psi. Portland cement cannot defeat thermal stress so don’t expect asphalt to do so. The solution is to have a binder that can relax stresses faster than they build up.

Pavement Slippage. Slippage occurs when of tack coats and primes are not used properly.

Fatigue Failure. There are suggestions that asphalt could be modified to increase its stiffness so that the pavement thickness could be reduced. Again it must be remembered that it is the aggregate that carries the load, in compression, not the asphalt. However fatigue failure occurs in tension, and again the tensile strength of asphalt is much less than that of aggregate. The pavement is stretched underneath the wheel path, and between the wheel paths. However, tensile failure is often really crack propagation, thus additives that stop crack propagation such as tire buffings may be of value.

chemistdunning@gmail.com, http://www.petroleumsciences.com

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 

PAVEMENT DISTRESS. WHO’S TO BLAME

Tort or Penalties Called for, or the Result of Natural Aging

Once a pavement is laid it is expected to last a long time. However over time distress will occur at which time there can be a blame game, especially if tort lawyers get involved. Unfortunately the attorneys may team up with “experts” who have only limited understanding of pavement technology and absolutely no understanding of multivariate statistics upon which are based possible penalties where data collected during construction would suggest there would be future distress.

In a construction contract there are various contractors involved. One contractor may prepare the subgrade and base while another would lay the pavement. The design engineer may not have built in sufficient strength into the pavement or paid attention to the properties of the paving material with respect to the expected traffic. Also, whether the location is in a city or on a highway will affect judgments. Following are a few types of distress:

Residential Streets.

  1. There is a sunken “bird bath” in the street. There were separate contractors for the subgrade, base and pavement. There were alligator cracking in the sunken area. The thickness of the pavement is the design thickness. Who’s at fault and what can be done? Usually the paving contractor will be blamed; however the actual problem is an area in the subgrade that was not properly compacted. The pavement has to follow the consolidation that occurs in the subgrade thus the cracking comes from stretching the pavement as it sinks and is not the fault of the paving contractor. To repair it the section may be removed and reconstructed. If the pavement is to be slurry sealed, leveling can be done by the slurry seal contractor.
  2.  There is loss of matrix, called raveling, in the pavement in areas of continuing flowing water. Wet asphalt pavements are weaker than dry ones. This usually occurs on corners where there is shear stress from the tires. The source of the water needs to be identified and stopped. There are asphalt paving mixes designed for hydraulic structures, however a pavement in a street is not one of them. Another cause is the lack of use of additives to the asphalt that address the loss of wet strength. A discussion of such additives is outside of my goal at this time however it will be addressed later.

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Main Streets and Highways

  1. Longitudinal Crack in the Wheel Path. This usually starts in the right wheel path and later occurs in the left wheel path. This is caused by lack of structural strength. It can be accelerated by lack of proper compaction or lack of anti stripping additives in the asphalt. It isn’t unusual for an agency to mill off two inches and repave. That is like blowing in the wind, as the lack of structural strength continues, and the cracks will soon reappear. The lack of structural strength and stripping are design problems, not construction.
  2. Damaged from Studded Tires. In areas in which studded tires are used there will be ruts, whether the pavement in asphalt concrete or portland cement concrete. The width between ruts will be consistent to that of passenger tires. That problem still lacks a solution. Some say that such rutting is caused by trucks, but that is not true for ruts described above. Rutting caused by trucks is a mix design problem that is solvable. In that case the width between ruts is that of truck tires, and the effect of the duals is obvious. This is not caused by poor construction techniques.
  3. Traffic Cause Ruts. There is a considerable effort to solve rutting by changing the binder. Adjusting the binder can affect the rate of rutting; however the true solution to rutting is to make sure that the coarse aggregate particles can interlock. Yet the specifications generally allow over-sanded mixes in which a sand asphalt matrix is supposed to stop rutting. I would suggest that both the design engineer and the supplier of the (hot mixed asphalt) HMA are equally at fault. The design engineer specified gradation limits that allow oversanded mixes, and the HMA supplier crushed and blended to a gradation that would cause ruts when in the pavement.
  4. Block (Thermal) Cracking. Often called transverse cracking, however it really occurs in blocks if the pavement is wide enough. This cracking occurs when the asphalt in the pavement is too hard to relax thermal stress fast enough. Public agencies or other owners are at fault for not sealing the pavements, which reduces the hardening rate of the asphalt.
  5. Water Damage. If the aggregate would prefer being wetted by water rather than asphalt, the asphalt will at least get weak, and probably strip off. The result is raveling. While there are additives to asphalt that helps in this area, there are data that shows that some of the more popular antistrips may lose their ability to prevent stripping over time. Once the pavement loses it strength, fatigue cracking may also be prevalent. The fault here is the design specification that does not specify proper antistrips.

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