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CAUSES OF PAVEMENT FAILURE 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 cal
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  CAUSES OF PAVEMENT FAILUREINTRODUCTION  There are certain basics with respect to pavement failure that have existedsince the rst pavements were laid. Pavements crack, pavements slip, waterdamages them, and pavements rut. Irrespective of the tests used to evaluatepavements, failures have the same basic causes. CRACKING No matter where the cracking occurs, it is caused by the inability of theasphalt to relax the stresses, and must rupture. Fatigue Cracking . tress and strain are what are called tensors, whichmeans that a pavement can be under compression and tension at the sametime, but in di!erent directions. hile a tire compresses a pavementdownward, it forms a de#ection basin which causes the pavement to go intotension in both hori$ontal directions. %any years ago we used data fromde#ection testing and, assuming a parabola, did a line integral to calculatestrain. If the pavement is not strong enough, the asphalt is stretched too far,separates and a crack forms in the wheel track. &lso a crack may formbetween the wheel tracks. Longitudina Cracking on !oint . The 'oint between two passes areespecially week. Inside any one pass of the paver, some aggregate will be onboth sides of any plane or slice inside of the pavement. In fact, when sampleundergoes an indirect tensile test such as is done in stripping tests, rocksactually fracture. & 'oint, however, is held together only by the asphalt layer,which has a tensile strength of about ())*+))) psi, depending on thetemperature and shear rate. If the asphalt in the mix can #ow vertically inresponse to thermal stresses, the crack wont form. -owever, if the stressesexceed that at the 'oint, a crack forms. &s a result the pavement on either  side of the crack can shrink or expand independently. ften what happensthen is that the pavement sections shrink away from each other in the cold,but do not expand completely back together in the heat. /or that reason it iscrucial to follow proper technology of forming a 'oint. T#er$a Cracking . The mechanism of formation of thermal or non*loadassociated cracks is again the lack of the asphalt to be able to relievethermal stresses by #owing vertically up when the pavement is hot andvertically down when the pavement is cold. PAVEMENT SLIPPAGE /rom time to time the pavement will shift. In one pro'ect I has on at the 0&1airport, a (2 lift was slipping on a 32 lift from landing of air tra4c. & core wasmade of the section so it was possible to observe a daily slippage. Twosources of the problem. /irst, it was supposed to be 32 over (2. econdly, if there was a tack coat, it had been ruined as a result of a dust storm. Toprevent 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 studdedtires. Gradation%  &sphalt itself is too weak to stop the #ow of the mix by itself. If the coarse aggregate in the mix cannot interlock the mix has to rely on amastic composed of the nes and asphalt, which cannot carry the load. Thesolution is a coarse gradation with no humps in the ne mastic area. Studded Tire . 5esearch is under way on how to solve this problem. -arderaggregate has helped, but no solution is available now.  &ATER DAMAGE If the pavement is not protected from water damage, all of the above isblowing in the wind. There are data that suggest that even pavementprotected by amine or lime antistrips will lose much of its strength thuscannot complete its design life. %any aggregates are wetted by water betterthan asphalt so that if the surface cannot be permanently altered to preferwetting by asphalt, eventually water will replace the asphalt. &#' Doe Pa(e$ent Crack) Installing asphalt pavement re6uires it to be placed in strips or paver passes. This means there is a construction 'oint 7 seam every +( to +8 feet across atypical parking lot. & crack will form along these seams within + year afterinstallation, depending on the weather conditions. 9racks also occur due tofailures in the surface layer of asphalt, lack of bonding between the asphaltlayers, and structural failures due to inade6uate design. -ere are a fewexamples of the various type of cracks. Sea$ Crack ã &s the pavement ages it becomes dry and brittle causing cracking. ã 0ow temperatures : winter free$e and thaw cycles causing the asphaltto expand and contract. ã 9racks form where each paver pass starts and stops. Sea$ Crack    These type of cracks can becleaned of debris and then sealedwith hot rubber. hich prevents water fromgetting into the base creatingpotholes. *ond Faiure+ Si,,age Crack ã lippage cracks develop when there is not a good bond between thesurface layer and the binder layer of asphalt. ã 0ack of tack coat used between asphalt layers. ã ;ehicles turning the wheels when the vehicle is not moving. *ond Faiure+ Si,,age Crack  & full depth asphalt patch isre6uired to repair this type of defect. Structura Faiure+ Fatigue Cracking
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