MOISTURE INDUCED PRESSURES IN CONCRETE AIRFIELD PAVEMENTS

ABSTRACT The erosion of military concrete air- across the medium. The key premise is that the re-field pavements by jet exhausts is an expensive prob- sistance to flow through a high resistance porous lem expected to get worse. Scaling of pavements is medium can be modeled with friction coefficients being observed beneath the auxiliary power units analogous to the method used for simpler geom-(APUs) of F/A-18 and B-I aircraft. The AV-8B Har- etries. C rier and future vertical takeoff and landing aircraft Pore pressures predicted using this model show will generate a concrete environment much more that moisture in the pores of the cement is unlikely severe than that generated by the low power APUs. to be the primary cause of material failure in airfield In order to develop pavements impervious to pavements heated by the F/A-18 APU. Moisture is a this form of erosion, the failure mechanism must be probable cause of failure, however, when the ce-known. One plausible culprit, suggested by the ap- ment is being heated by the exhaust of a Harrier parent role of heating, is moisture. To examine this during takeoff and landing. For an initial pore satu-hypothesis, a mathematical model was developed to ration of 90 percent or greater, extremely high pore predict pore pressures caused by water vapor and pressures and pressure gradients result from air be-air migrating through the pores of a heated section ing compressed by the thermal expansion of liquid of concrete. water. Concrete temperatures beneath a VSTOL reach The foundation of this model is a constitutive 300'C in about 2 minutes. For a lesser pore satura-relationship developed to characterize the flow of tion, the water vapor pressure could eventually be high velocity, compressible, heated gases through sufficient to cause a tensile failure in Portland ce-concrete. This relationship equates a nondimensional ment concrete. flow parameter with pressure and temperature ratios Approved for public release; distribution is unlimited