Corrugation and shoving are two defects that usually occur in flexible pavements. Weak sub-grade conditions, improper rolling, poor mixing, temperature effect of bitumen, and weak bottom layers are different causes of corrugation and shoving in flexible pavements.
What is Corrugation?
The formation of ripples or waves on the flexible pavement generally perpendicular to the traffic flow is called corrugation or washboarding. It occurs at the points where traffic starts and stops.
What is Shoving?
The bulging of the pavement surface due to plastic movement of pavement is called shoving. It occurs at points where a vehicle pulls its brakes or at points where pavement meets rigid objects.
Causes for Corrugation and Shoving
1. Weak Sub-grade Conditions
If the soil subgrade is poor then the problems like corrugation and shoving arise in flexible pavements. Subgrade soil is said to be weak when it consists of highly plastic clays, organic soil etc. Presence of water table nearer to subgrade soil also makes it weak to take loads.
In general, weak soil is excavated and is replaced with boulders, crushed stones for the stability of subgrade course. If the depth of excavation is not up to the hard stratum, then sinking of stones, boulders may occur which leads to the formation of ripples on the pavement surface.
2. Improper Rolling
Improper rolling or inadequate compaction during construction of flexible pavement may lead to the formation of corrugations as well as shoving of pavement. Due to improper rolling, the layers of pavement are not held together at some places and whenever traffic enters the layers at those places, it disrupts the layer into the formation of waves on the pavement.
3. Poor Mixing
The mix or gradation of surface course should be done carefully. Inappropriate mixing of surface course in flexible pavement is also a reason behind corrugation and shoving. Corrugations formed due to defective mixing may spread to larger areas due to the continuous flow of traffic or vehicular movement.
4. Temperature effect
Bitumen is the surface course material used in flexible pavements. The viscous state of bitumen varies according to the temperature. The temperature of the bitumen mix during mixing and rolling should be maintained in a way so as to attain the maximum stability after drying.
If the temperature is very high during any of the above es, the stability may reduce and also waves or corrugations are formed during compacting.
5. Weak bottom layers
If the bottom layers such as binder course, base course, sub-base course etc. are poorly compacted or defectively graded then the chances of occurrence of corrugation and shoving increase. When those layers are continuously subjected to vehicular movements, the surface course gets deformed and depressed, ultimately resulting in the formation of corrugations.
Remedial Measures for Corrugation and Shoving
Remedial measures to be taken against corrugation and shoving in flexible pavements are as follows :
- If the subgrade soil is poor, then stabilize the whole subgrade using suitable stabilizing agents such as lime, cement, chemical admixtures etc.
- Remove inadequately compacted layers and reconstruct them with proper compaction.
- Lower the water table by providing a suitable sub-surface drainage system.
- If failure is due to poor mixing of surface course, then lay another layer of surface course with proper mixing.