The second approach includes the use of “truck factors” (the average ESALs per truck number) along with the number and type of trucks that are expected to use the facility.Įxample A two-lane highway has the following characteristics and resulting overlay requirement: One approach provides broad traffic classifications and the associated ESAL amounts, as illustrated in Table 3. To estimate the ESALs for the overlay design period, at least two approaches can be used, depending on availability of site specific traffic information. The Asphalt Institute treatment of traffic includes consideration of volume composition, and axle weights, with the goal being to develop the equivalent number of ESALs. *Equivalent thickness of new asphalt concrete PCC pavement stable, undersealed and generally uncracked pavement Granular subbase or base CBR not less than 20Ĭement modified subbases and bases constructed from low PI soilsĬement or lime-fly ash bases with pattern crackingĮmulsified or cutback asphalt surfaces and bases with extensive cracking, rutting, etc.Īsphalt concrete surface and base that exhibit extensive cracking Improved subgrade - predominantly granular material Table 2: Example of Asphalt Institute Conversion Factors for Estimating Thickness of Existing Pavement Components to Effective Thickness (after Asphalt Institute, 1983) Typical layer thickness Conversion Factors are shown in Table 2. The Effective Thickness of the complete pavement structure is the sum of the individual Effective Thicknesses. Third, the Effective Thickness for each layer is determined by multiplying the actual layer thickness by the appropriate Conversion Factor. Second, “Conversion Factors” are selected for each layer (judgment by the designer is very important at this point). First, the significant pavement layers are identified and their condition determined. One common Asphalt Institute approach is illustrated here. AASHTO refers to effective thickness as D eff. The goal of this portion of the design method is to determine the “Effective Thickness (T e)” of the existing pavement structure. The design subgrade strength/stiffness is then selected from the plot according to Table 1 or similar criteria. Basically, this is a cumulative frequency plot. To do this, a plot is prepared of the percent equal to or greater than (y-axis) versus subgrade strength/stiffness test results (x-axis). If subgrade strength or stiffness test results are available, a conservative value is chosen as a function of the design traffic ( ESALs). These soils can be expected to retain a substantial amount of their strength when wet and include clean sands and sand-gravels. Typical properties are: M R = 80 MPa (12,000 psi), CBR = 8, R-value = 20.
These soils can be expected to lose only a moderate amount of strength when wet. Include soils such as loams, silty sands, and sand-gravels which contain moderate amounts of clay and silt. Typical properties are: M R = 30 MPa (4,500 psi), CBR = 3, R-value = 6. Soft and plastic when wet, generally composed of silts or clays. If test data in terms of M R, CBR, or R-value are not available, subgrades can be placed into one of three classes for design purposes as follows: Therefore CBR and R-values must be correlated to resilient modulus values. For actual design, the design strength of the subgrade must be characterized in terms of resilient modulus. The resilient modulus (M R), CBR or R-value tests appear to be the easiest to use with this procedure. Subgrade material testing is encouraged even if original design records are available.
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