Highway maintenance workers have for some time recognized pavement slab pumping as one of the major factors contributing toward concrete slab failures. The value of preventive maintenance, correcting slab pumping by mud jack treatment before serious damage is done to the slab, cannot be underestimated. Experience has shown that pavement life is definitely prolonged. Slab pumping may be defined as follows: heavy axle loads deflect concrete pavement slabs at joints and cracks. "Free" water that has entered the subgrade through the joints and cracks is forced out, carrying soil particles in suspension. This, in turn, creates a void or reservoir in which more "free" water accumulates. Repeated deflections enlarge the void until the pavement becomes unsupported. The correction for this is mud-jacking. The jack-hammer operator operates the compressor an drills the holes. The mud-jacking crew pumps the slurry under the pavement until it begins to rise. When sufficient slurry mixture has been pumped, the nozzle operator signals the operator who stops the mud- jack pumping. He inserts a tapered wooden pug to retain the pressure and stop the flowing back of the mixture until the clean up man is ready to permanently plug the hole.