etermine Drainage Coefficient. The drainage coefficient is used to modify the
design thickness for drainage conditions. Moisture affects the pavement performance
by decreasing the strength of the subgrade and subbase material and affects the warping
and curling behavior of the concrete slabs. The intent of the drainage coefficient is to
allow performance prediction for pavements without a proper drainage system.
Increasing the pavement thickness should not be used in lieu of a properly designed
drainage system. Recommended values for the drainage coefficient are given in Table 3.24.
The Federal Highway Administration’s “Highway Subdrainage Design Manual,”
Report No. FHWA-TS-80-224, provides a procedure that may be used to determine
drainage times for base material. (See Arts. 5.4.5 and 5.4.6.)