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referential to abstract: ... Comprehensive Truck Size and Weight (TS&W) Study; Phase 1—Synthesis; Working Paper 3—Pavements and TS&W Regulations; category 1 section 2 paragraph (a). [abridged]
1.2 Truck Characteristics Affecting Pavements
(a) Axle Weights
Load equivalence factors measure the relative effects of different types of loadings on pavements. Pavement engineers generally use the concept of an equivalent single-axle load (ESAL) to measure the effects of axle loads on pavement. By convention, an 18,000-pound single axle is 1.00 ESAL. The ESAL values for other axles express their effect on pavement wear relative to the 18,000-pound single axle. Stating, for example, that a given vehicle on a given type of pavement is 3.0 ESALs means that one pass by the vehicle has the same effect on the pavement as three passes by an 18,000-pound single axle.
AASHTO provides separate sets of ESAL values for flexible and rigid pavements. The principal difference between the flexible and rigid pavement ESAL values is that tandem axles were found to have a greater effect on rigid pavements (Exhibit 1 Axle load effects on pavements: top, flexible pavements). For example, a 34,000-pound tandem axle is about 1.1 ESALs on flexible pavement and about 2.0 ESALs on rigid pavements.
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