An ESAL is a standard unit that is used to measure the amount of damage vehicles inflict on a surface. 1 ESAL = the impact from a single 18,000-lb axle load. The damage to a pavement caused by one pass of a large tractor-trailer in exponentially greater than that caused by a two door sedan.
It takes roughly 20,000 passes of the sedan to cause the same amount of same amount of damage as one pass of a tractor-trailer. In these scenarios, the design ESALs will be predetermined. This is usually based on the expected traffic mix. Reference ASCE and ICPI Tech Spec 4 to calculate the appropriate ESAL rating to a design.
It takes roughly 20,000 passes of the sedan to cause the same amount of same amount of damage as one pass of a tractor-trailer. In these scenarios, the design ESALs will be predetermined. This is usually based on the expected traffic mix. Reference ASCE and ICPI Tech Spec 4 to calculate the appropriate ESAL rating to a design.
Pedestrian or Light Vehicular Loads
For light loads on normal soils the cross section of the layers are proportioned as 1 : 0.5 : 1 : 1. That is; the unit thickness at the paver level, 1/2 the unit thickness in sand, the unit thickness in aggregate, and the unit thickness of compacted aggregate in contact with the earth. For intermediate loads on strong soils the thickness of the unit can be increased and intern the layers below can be proportionally increased. The overall cross section composition results in a depth of approximately 8” to 10” (200mm-250 mm)
Intermediate Vehicular Loads
For Intermediate vehicular loads and normal soils, the cross section is increased in the dimension of the base material and typically the thicker paver of 3.125" (800mm) is used. The added thickness will enhance the load distribution of the sub-grade earth improving overall performance and stability. Note that the base and sub-base materials can be replaced by a single base with an overall proportion of 2 1/2 times the unit thickness. The final composition of this application is approximately 12” in total depth (305mm).
Heavy Vehicular Loads or Weak Soil
For very heavy loads on normal or good soil, the cross section of the layers will vary basically only in the thickness of the paving units and the depth of the base and sub-base. By increasing the depth of the unit and base material, more shear force will be developed between units and in the aggregate of the base. This additional shear will mobilize units and material further away from the load therefore reducing the overall stress on the earth below. The proportions on the layers are 1 : 0.5 : 2.5 : 1(minimum). In some cases the base and sub-base compositions are combined into one layer that may be treated with cement resulting in a total thickness of stabilized base and sub-base of about seven times the thickness of the unit. The overall final composition of this application ranges from 15” to 28” in depth (380mm to 710mm)
Reliability Ratio
Based on AASHTO’s design procedure, a reliability ratio will increase the design traffic load logarithmically. For example, If the actual ESALs (Equivalent Single Axle Load) were a value of 1,000,000 a reliability ratio of 75% would increase the ESAL value to 2,000,000. A reliability ratio of 90% would increase the ESAL value to 3,750,000.
Choosing a reliability ratio is based on the amount of uncertainty on the expected traffic loads over the life the pavement, the soil type or ability of the soil to support the pavement especially if saturated and weakened from wet weather. A reliability ratio of 75% is typical for low speed municipal roadways.
Choosing a reliability ratio is based on the amount of uncertainty on the expected traffic loads over the life the pavement, the soil type or ability of the soil to support the pavement especially if saturated and weakened from wet weather. A reliability ratio of 75% is typical for low speed municipal roadways.
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