Cracking in the bituminous pavement is the most common distress. These cracks may be load associated (Bottom-Up cracks ) or Nonload associated (Top Down cracks ). In many cases, we can identify the cause of cracks from the pattern of the cracks. Ingress of water through wide cracks is harmful to the pavement structure.
Type of failure of flexible pavement
- Hairline Cracks
- Alligator cracking
- Longitudinal Cracking
- Shrinkage and Transverse Cracking
- Edge Cracking
Hairline Cracks: These are generally Top-Down cracks. Due to oxidation of bitumen (aging), Aromatics converted into resins, and that resins converted into Asphaltenes . which makes the bitumen harder and causes Top-down cracks. These cracks may be also due to insufficient bitumen, or sometimes due to excessive moisture in the subgrade. These cracks are not interconnected. The width and depth of the hairline cracks are less than 1 mm. If we don’t provide any treatment to these cracks. Then these cracks change into alligator cracks.
Treatment of these hairline cracks
These cracks may be treated by fog seal, application of liquid rejuvenation agents, slurry sealing, latex modified emulsion, and micro surfacing.
As the name suggests “ALLIGATOR” as a series of interconnected cracks, creating small irregularly shaped pieces of pavements surface resembling the skin of Alligator. These cracks are load associated fatigue cracking due to high tensile strains below bituminous layers. The size of irregular shape blocks of cracks varies in size. Normally these cracks appear along the wheel path.
Causes of Alligator Cracking
Due to excessive deflection of pavement surface normally at wheel path caused by overloading and excessive loads repetitions. Hence the failure of flexible pavement takes place. There may be other reasons too, like the use of stiff (hard ) binder which contains more percentage of Asphaltenes like VG-10 in wearing a coat, unsound pavement, poor drainage, Low CBR of subgrade, poor PI, etc. Such cracking starts at the bottom of the surface, where tensile stresses are maximum. Pavement flexes under the repeated wheel loading, resulting upward movement of cracks .initiation of Bottom-up fatigue cracking starts below of bituminous layers due to high tensile strain.
Severity of Cracking
Alligator cracking may be further classified as low severity, medium severity and high severity.
Low Severity: Low severity cracking is identified by an area of cracking with narrow cracks with almost no deterioration of the surface. The cracking is often isolated and many times, the cracking may not be interconnected to other areas. The depth & width is in the range of 1 to 3mm.
Medium Severity: Medium severity cracking is identified by interconnected cracks forming a small area similar to the skin of an alligator. The cracks may have signs of slight spilling, with no pumping of water and fine material from the bottom , invisible from the top. The depth and width of cracks lie between 3 to 6 mm.
High Severity: High Severity alligator cracking is identified by an area of moderate to severe spalled interconnected cracks creating a full pattern of cracking similar to the skin of Alligator. Pieces of the bituminous surfaces may be loose or missing. The pumping of water and fine material from lower pavement layers may be seen on the surface through high severity cracks
Treatment of Alligator Cracking
Use of SAM(stress absorbing membrane ) of rubberized bitumen in the pavement , Use of Geotextiles in the strengthening of crust and subgrade, Drainage improvement of the road for proper drainage of water, and slurry sealing, etc.
Longitudinal Cracking type failure of flexible pavement
Cracks parallel to the centerline of the pavement are called longitudinal cracks These cracks may appear at joints between two paving lanes and paved shoulders. These are non-load associated cracks . Joint cracks and edge cracks are also non load associated longitudinal cracks. After repeated loading, the longitudinal cracks develop into the same pattern similar to the back of an alligator
Severity of Longitudinal Cracking
Low severity : Cracks width upto 5 mm and infrequent cracks.
Medium severity: cracks width greater than 5 mm and less than 10 mm and infrequent.
High Severity: It is assigned to cracks width greater than 10mm and frequent (numerous) cracking.
Possible causes of longitudinal cracking
Improperly constructed joints at the centre of the pavement, As roller, starts compacting from both ends towards centre hence there is a density gradient at the centre of the pavement. In thin pavements, cracking starts at the bottom of the bituminous layers, where the tensile stress is the highest and then it spreads to the surface as one or more longitudinal cracks. And In thick pavements, the cracks usually commence from the top because of high localised tensile stresses due to pavement interaction.
Treatment for Longitudinal Cracking
Low and Medium Severity Cracks: Sealing of cracks using rubberized bitumen and latex modified bitumen emulsion.
High Severity Cracks: Remove and replace the cracked pavement layer and affected area with fresh bituminous paving material.
Shrinkage & Transverse Cracks in asphalt pavement
These interconnected cracks appear in the transverse directions ( perpendicular to the road length ) forming series of large blocks perpendicular to the direction of the road. These are non-load associated cracks.
Causes of Transverse cracks in asphalt pavement
Stiff (Hard) bitumen used for laying the pavement at low temperature or a temperature susceptible bitumen, rapid and repeated cooling of pavement, wax in bitumen used for surfacing. Due to low temperature, there is a brittleness in bitumen, or maybe the Aging (oxidation ) of the Bitumen, and also the structural failure of a concrete base course are also causes of such cracking.
Treatment of Shrinkage and Transverse Cracks
Transverse cracks may be treated using rubberized bitumen
Edge cracking is defined as cracks that develop parallel to the outer edge of the pavement. Hence leads to breaking of edge subsequently. The location of such failure of flexible pavement is normally at 0.3 to 1.5 m inside of the pavement edge.
Causes of Edge Cracking
Lack of lateral support to the pavement from the shoulder, Shrinkage in subgrade and settlement the Sub-base and base below the bituminous layers, accumulation of water due to inadequate surface drainage, use of inferior quality of material in shoulders, the Thinner bituminous surface at the edge of the pavement, inadequate compaction of shoulders, Sub Grade, Sub Base, and base layer.
Treatment for Edge Cracking
- Improve drainage along the edge of the road and it is very important that surface water should run to the nearby drain and should not be ponded along the pavement edge.
- Remove the poor draining shoulder material and replace it with a more permeable material in shoulders
- If there is higher truck traffic, place a considerably thick structural overlay of the order of 75-100 mm of hot mix DBM+BC to provide adequate support for heavier vehicle loads.
- Shoulders should be paved
- Rubberized bitumen can be used for crack sealing.