Flexible pavement failures; deflection overlay design

Common failures of flexible pavement

A flexible pavement fails when it can no longer give a safe, smooth ride or carry the traffic. Common failures:

Deflection approach for overlay design

The deflection approach estimates the overlay needed to raise the structural capacity of an existing pavement to that required for future traffic. It is based on the elastic rebound deflection measured under a standard wheel load (Benkelman beam as per IRC:81-1997, or FWD). Steps of the Benkelman beam method:

1. Select the section and test points: divide the road into homogeneous sections and choose at least 10 test points in each, at about 50 m spacing, staggered between the lanes. The points lie in the outer wheel path, 0.9 m from the pavement edge (0.6 m for lanes narrower than 3.5 m).
2. Standard test vehicle: a truck with a rear axle load of 8170 kg (dual wheels, 4085 kg on each dual-wheel set) and a tyre pressure of 5.6 kg/cm².
3. Measurement: place the beam probe between the dual wheels at the test point and note the initial dial reading $d_0$. Move the truck slowly forward 2.7 m (intermediate reading $d_i$) and then to 9 m (final reading $d_f$). Record the pavement temperature.
4. Rebound deflection: $D=2\,(d_0-d_f)\times$ least count, where the factor 2 is the beam lever ratio. If $d_i-d_f$ exceeds 0.025 mm, the front legs of the beam were inside the deflection bowl, and a leg correction is applied: $D=2(d_0-d_f)+2K(d_i-d_f)$ with $K=2.91$.
5. Corrections:
   • Temperature: the standard temperature is 35 °C. Correct by 0.01 mm for each °C difference from 35 °C (add when the pavement is colder, subtract when it is hotter). This applies when the bituminous layers are 40 mm or thicker.
   • Season: multiply by a seasonal correction factor (from the IRC:81 charts, based on subgrade soil type, plasticity index, field moisture content and annual rainfall) to convert the deflection to the weakest, post-monsoon condition.
6. Characteristic deflection: $D_c=\bar D+2\sigma$ for national and state highways ($D_c=\bar D+\sigma$ for other roads), where $\bar D$ is the mean and $\sigma$ the standard deviation of the corrected deflections.
7. Design traffic in msa over the overlay design life (usually 10 years), from the present commercial vehicles per day, growth rate and VDF.
8. Overlay thickness: read from the IRC:81 chart of characteristic deflection against design traffic (msa), which gives the thickness in mm of bituminous macadam (BM). It can also be estimated by the approximate formula $h=550\log_{10}(D_c/D_a)$ (h in mm), where $D_a$ is the allowable deflection for the design traffic.
9. Convert to the actual overlay material using the IRC:81 equivalence (1 cm of BM = 1.5 cm of WBM/WMM = 0.7 cm of DBM/BC). Repair badly cracked areas before overlaying, and provide the profile-correction (levelling) course and wearing course in addition.