Failures in flexible pavements.pptx

Flexible pavement failures types

1. HIGHWAY MAINTENANCE

2. Causes of Pavement failure Defects in quality of materials used. Defects in construction methods and quality control during construction. Inadequate surface or sub surface drainage resulting in stagnation of water.

3. Increase in magnitude of wheel loads and the number of load repetitions due to increase in traffic load. Settlement of foundation of embankments of the fill material itself. Environmental factors including heavy rainfall, soil erosion, high water table, snow fall, frost action etc ., Poor workmanship Improper design

4. Flexible pavement failures A flexible pavement failure is defined by formation o f pot holes, ruts, cracks, depressions and settlements. Failure of any one or more pavement layers develops waves and corrugations on pavement surface or longitudinal ruts and shoving. Pavement unevenness itself is considered as failure. Various factors contribute to pavement distress or failure like, aging and oxidation of bituminous film causes failure in bituminous pavements.

5. Failure in Flexible Pavements

6. Failures in Subgrade One main cause of flexible pavement failure is excessive deformation in subgrade . Failure can be noticed in the form of undulations or waves and corrugations in pavement surface and also depressions followed by heaving of pavement surface. Lateral shoving of pavement near edge along the wheel path of vehicles is due to insufficient bearing capacity or a shear failure in subgrade

7. Failure of subgrade is due to two basic reasons: Inadequate stability Excessive stree application. Inadequate stability may be due to inherent weakness of soil or excessive moisture or improper compaction. Stability is the resistance to deformation under the stress. Excessive stress application is due to inadequate pavement thickness or loads in excess of design value. Deformation of soil subgrade and other pavement materials is increased with increase in number of load repetitions.

8. If the applied stress on the subgrade or pavement is very low when compare to its bearing capacity, the deformation due to load would be elastic or fully recovered when load is released. If compaction of layers is not adequate and after loading, part of the deformation may be permanent due to compaction of soil, this may be called as consolidation deformation . If applied stress is excessive with respect to stability and if plastic flow takes place as in the case of wet clayey soil, deformation is called as plastic deformation and is not partly recoverable.

9. Soil deformation under loads

10. Damage in flexible pavement caused by traffic to subgrade failure or inadequate compaction of subgrade and other pavement layers can be seen in below fig. Failure due to improper compaction of subgrade and pavement layers

11. Failures in Sub-base or Base Courses Chief Failures: Inadequate stability or strength. Loss of binding action. Loss of base course materials. Inadequate wearing course. Use of inferior materials and crushing of base course materials. Lack of lateral confinement for granular base course.

12. TYPES OF FLEXIBLE PAVEMENT FAILURES Alligator (map) cracking . Consolidation of pavement layers. Shear failure . Longitudinal cracking. Frost heaving. Lack of binding (keying) to the lower course. Reflection cracking. Formation of waves and Corrugations.

13. Alligator Cracking: This is the most common type of flexible pavement failure and occurs due to relative movement of pavement layer materials. This may be caused by repeated application of heavy wheel loads resulting in fatigue failure or due to moisture vibrations resulting in swelling and shrinkage of subgrade and other pavment materials. Weakness in underlying course also causes a cracking of surface course.

14. Alligator or Map cracking

15. Consolidation of Pavement layers: Ruts are mainly formed due to consolidation of one or more layers of pavement. Repeated application of loads along the same wheel path cause deformation resulting in consolidation deformation or longitudinal ruts. Shallow ruts on surface course can also be caused due to wearing along wheel path. Depending on depth and width of ruts, it can be estimated whether the consolidation deformation has been caused in subgrade or in subsequent layers.

16. Formation of ruts

17. Shear failure and cracking: Shear failures are caused due to inherent weakness of pavement mixtures and shearing resistance being low due to inadequate stability or excessive heavy loading. Shear failure causes upheaval of pavement materials by forming a fracture or cracking. Shear failure cracking

18. Longitudinal Cracking: Due to frost action and differential volume changes in subgrade longitudinal cracking is caused in pavement traversing the full pavement thickness. Settlement of fill and sliding of side slopes also would cause longitudinal cracking.

19. Frost heaving: Frost heaving is often misunderstood with shear or other types of failures. In shear failure, the upheavel of portion of pavement is followed with a depression. In this case, there is mostly a localized heaving-up pavement portion depending upon ground water and climatic conditions. Failure due to frost nature

20. Lack of binding with lower layers: Slipping occurs when surface course is not keyed/bonded with the underlying base. This results in opening up and loss of pavement materials forming patches or pot holes. This will occur when bituminous surfacing is provided over existing concrete base course or soil-cement base course. This can be caused when there is lack of prime/tack coat in between two layers.

21. Reflection Cracking: This type of failure is observed when bituminous overlays provided over existing concrete pavements. The crack pattern as existing cement concrete pavements are mostly reflected on bituminous surfacing in the same pattern. These allow surface water to seep through and cause damage to the soil subgrade or result in mud pumping.