# 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