Structure of Bituminous Pavements
Flexible road structure is a combination of several layers, which serves the purpose of distributing the loads from the traffic (both dynamic and static) to the underlying layers in a level so that the layers can bear without failure. The stress is maximum at the top surface of the pavement. This value of stress decreases with the depth from top to bottom. So, a criterion about strength and the economy factor is decided based on the same concept. That is, the top material of the pavement must be of greater strength and the bottom layer used can be of lower strength bringing economy in construction.
Failure Modes in Bituminous Pavement Structure
There are mainly two ways in which the roads can undergo failure:- Functional Failure
- Structural Failure
Functional Failure of Bituminous Pavement Structures
The deterioration of the pavement surface with time creates a distress, which shows the functional failure. The main reasons behind functional failure are raveling, that involves stone loss or through fretting or the reduction of surface texture due to higher abrasion or polishing effect that would, in turn, decrease the skid resistance shown by the road.Structural Failure of Bituminous Pavement Structures
The wheel load application at a continuous rate over the pavement will result in the structural failure. During the early stages measurement of damage was very tough because the deterioration rate was very small. With time, the traffic rate and load increase making the rate of deterioration to accelerate. This made the structural change more clear and measurable. The breakdown can be through two modes. This is explained with the help of the figure-1 and figure-2. The Mode 1 is the permanent deformation represented in figure-1. These are deformation observed in wheel tracks. This cause rutting under the wheel application. The absence of bottom support or bottom layer instability causes this effect. This rutting is a structural failure that differs from other 'non-structural' type of permanent deformation. These non-structural permanent deformations are observed within bituminous materials, because of the accumulation of small areas due to the application of each wheel load, which cannot be recovered.
Fig.1: Mode 1- Permanent Deformation Observed in Flexible Pavement
The Mode 2 failure is cracking as explained in the figure-2. These modes of failure occur through the wheel tracks. When wheel load passes in the bound layers, tensile strain is created. Hence the cracking rate will be a function of the rate of tensile strain, the fatigue failure and repetitive nature of loading. The cracking is formed at the base of the bound layer as shown in figure-1. At this point, the tensile stress is higher, as shown in figure-3. This means that the damage existed in the pavement before the crack was visible.
Fig.2: Mode 2- Fatigue Cracking and critical strains
Fig.3: The vertical stress to radial stress variation below the centerline of a wheel application (with wheel load of 40kN) over a circular area of 160mm radius at a constant pressure rate of 0.5N/mm2 (As per Peattie, 1978)
Repetitive nature of loading and excessive strain developed in the structure are the main reasons behind both the above-mentioned failures. Hence, the life of the road is determined through the determination of the type of failure. This will also help in assessing the loads coming over the pavement and carry out performance evaluation. It is necessary that the road failure to be considered in terms of repairability or serviceability condition. This will ensure a road condition that is just acceptable for the drivers to safely ride. Based on three conditions sound, critical and failed the pavement state is determined. The table-1 shows the criteria based on which the pavement condition can be explained.Table.1: Pavement Condition explained based on Appearance of crack on the surface
Fatigue Characteristics of Bituminous Pavements
Due to the repeated application of tensile stress or tensile strain, the bituminous material is subjected to fracture. This cracking is termed as the fatigue cracking. The number of loads that is applied before cracking is less if the intensity or the value of each load is higher. The number of cycles that can occur before the formation of crack is dependent on:- The level of stress and strain
- The mix proportion of the whole material
- The nature of the bitumen
Fig.4: Methods for Testing Fatigue life of Bituminous Materials
The fatigue characteristics of bituminous materials can be carried out by a different range of tests. The figure-4 shows the flexure tests. Here it is simulated under repeated bending action in the stiff bound layer of a given pavement. The bending action caused here is due to the passage of each wheel (wheel load). Before the failure, the range of cycles it can endure depends upon:- Stress-strain Conditions
- The strain Criteria
- Effect of Mixture Variables
The Stress-Strain Conditions of Bituminous Structures
The fatigue test carried out in the bituminous structural sample can be of two types. They are:- Constant - Stress tests: Here each load that is applied will be at stress levels that are similar. This is irrespective of the amount of strain that is developed
- Constant-Strain Test: Here the load application is at the same strain level, which is regardless of the amount of stress that is required.
Fig.5: Graph showing fatigue lines as per constant stress test conditions; Nf is the number of cycles to failure, S is the stiffness for different test conditions
The results for constant-strain tests is given in the figure-5. Here unlike the results of constant stress tests, the results are reversed i.e. the mixture showed shorter life for higher stiffness. All other representations are similar to the above figure-5.
Fig.6: Graph showing fatigue lines as per constant strain test conditions; Nf is the number of cycles to failure, S is the stiffness for different test conditions
This contrast between the results of the two test can be explained with the help of failure mechanism. At the point of stress concentration, the crack is initiated. This will propagate through the material until the failure occur. Bringing the stress value to a constant rate will make the stress at the tip of crack to be higher, thus making the crack propagation rapid. But in the case of constant strain tests, as the propagation of crack contributes to more strain, the stress level is reduced. This will reduce the stress at the crack tip to reduce and make the propagation of crack to be slow. The choice of tests to understand the behavior of the given pavement is thus an important factor. The studies that were conducted showed that the constant strain test is applied for thin pavement layers (Surfacing layers). These thin layers are move with lower structural layers and they are effectively subjected to a strain control. While for thick structural layers, the constant stress test is the appropriate choice. This criterion is because the pavement layers are more subjected to stress-controlled load systems and the main structural layers of the pavement that are mainly thick will be stress controlled.