A waterproofing membrane composition is provided which is strong, easy to use, trowelable, flexible, passes negative hydrostatic test ASTM D4068 and which can be flood tested after 24 hours @ 70.degree. F. of application. The waterproofing membrane composition contains a flexible polymer, preferably an epoxy containing polyether chains formed from a glycol, a flexible curing agent, sand, and an aramid fiber. The epoxy component also preferably includes an epoxy flexibilizer and a reactive diluent for epoxy chain termination. A p-aramid fiber is preferred and it is highly preferred to use the polymer fiber in a pulp form which has a fibrillated structure.