Flexible Pavement Failure: Alligator Cracking Explanation

The failures that occur in the flexible pavement are as follow:- Alligator cracking or Map cracking (Fatigue)- Consolidation of pavement layers (Rutting)- Shear failure cracking- Longitudinal cracking- Frost heaving- Lack of binding to the lower course- Reflection cracking- Formation of waves and corrugation- Bleeding and - Pumping. (Suryakanta, 2015)Alligator cracking or Map cracking (Fatigue) Fatigue failure is a common type of flexible pavement failure and the main reasons are relative movement of the coating layer material, repeated application of heavy loads on wheels and swelling or shrinkage of subgrade or other layers due to moisture variation.

Consolidation of pavement layers (Rutting) Ruts formation falls into this type of failure. A rut is a depression or groove worn used on a path through the wheel's displacement. This type of failure is caused by repeating application of the load, which is along the same path of the wheel, resulting in longitudinal grooves and by wearing of the surface course along the wheel path, resulting in shallow ruts.

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Shear failure cracking The cutting failure causes the pavement material to reverse, forming a fracture or cracking. The following are the main causes of cracking due to cutting failure. They are too much wheel load and low resistance to shear of the pavement mixture.Longitudinal cracking This type of cracks extends to the full thickness of the pavement.

The main causes of longitudinal cracking are differential volume changes in subgrade soils, dismantling of filling material and sliding occurs due to side slopes. Frost heaving The accumulation of frost heaving causes interference with the located part of the pavement. The degree of frost formation depends on the water table and the weather conditions.Lack of binding to the lower course When there is no compartment between the surface and the substrate, some of the surface of the material releases material that creates patches and potholes. Slip cracking is a form of lack of binding with lower layer failure. The lack of primer layers or tack layers between the two layers is the main failure cause.Reflection cracking This type of failure occurs due to the bituminous surface layer is placed on the concrete cement pavement with some cracks and this crack is reflected in the same pattern on the bituminous surface.Formation of waves and corrugation Because of the unstable surface that is happened by stop-and-go traffic make the transverse waves at the regular intervals.Bleeding The excess bituminous binder present on the surface of the pavement causes bleeding. Bleeding causes a shiny surface like glass, which may be tacky to the touch. There are usually on the paths of the wheels.Pumping Seeping or ejection of water and fines from the undercoat through cracks is called pumping. 3.4.2Failure of Rigid Pavement Failures in the rigid pavement (or concrete cement pavement) can be identified by cracking on the pavement surface. The two main factors responsible for rigid pavement failures are use of low-quality materials and insufficient stability of the pavement structure.The poor quality of the material is composed of the following elements are use soft aggregate, subgrade soil quality is in poor condition and joint filler R sealer materials is poor.Insufficient stability in the pavement structure may be due to the following reasons are insufficient pavement thickness, lack of subgrade support, incorrect subgrade compression and incorrect distance between joints. Failures of rigid pavement are as follow:- Scaling of cement concrete- Shrinkage cracks- Joint spalling- Warping cracks and- Pumping (Suryakanta, 2015)Scaling of cement concrete Scaling of rigid pavement simply means peeling or flaking off to the top layer or surface of the concretes. This may be due to the following reasons are incorrect mix design, excessive vibration at concrete compaction time, concrete laitance and performing the finishing while wet water is on the surface.Shrinkage cracks The indication of shrinkage cracks is the formation of shallow cracks in the concrete slab. Shrinkage cracks develop on the surface of the concrete during setting and curing. These cracks can be formed in the longitudinal direction as well as in the transverse direction.Joint spalling The common division is the breakdown of the slab near the edge of the joint. Normally, this occurs within 0.5 m of the joints. The common causes of this defect are incorrect alignment of incompressible material under the concrete slab, insufficient resistance of the concrete slab near the joints, freeze-thaw bicycle and excessive joint load due to wheel load.Warping cracks When the weather becomes hot, the concrete slab tends to expand. Therefore, the joints must be designed to accommodate this expansion. When the joints are not properly designed, it prevents the expansion of concrete slabs and, therefore, results in the development of excessive stress. This stress causes the formation of warping cracks in the concrete slab near the edge of the joint. This type of crack can be prevented by providing a correct reinforcement in the longitudinal and transverse joints. Hinge joints are generally used to relieve stress due to warping.PumpingWhen the material located below the road surface is discharged through joints or cracks, it is called pumping. When the soil hits, it will be called mud pumping.The common causes of this defect are the infiltration of water through the joint, the cracks or the edge of the pavement forms a mud of the soil, The movement of heavy vehicles on the pavement forces this mixture of earth out which causes the pumping of clay, when there is a gap between the slab and the underlying base of the subgrade, poor joint sealant that allows the infiltration of water and repeated loading of the wheel causing erosion of the underlying material.Pumping can also cause cracks. This is because the expulsion of sub-grade material below the slab causes the loss of subgrade support. When traffic movement occurs in these locations, it cannot support the loading of the wheels due to reduced subgrade and develops cracks. This type of defect can be identified when there is presence of base material or subclass on the surface of the pavement near joints or cracks.