In a variety of situations concrete members may undergo cracking that seriously jeopardizes their structural performance or their ability to prevent seepage of water. When the cracks are narrow it is difficult to repair them in any other way than by injecting a bonding agent. The most satisfactory agents for this purpose at the present time are epoxy adhesives which can be used to repair cracks as small as .002 inch in width. To say epoxy adhesives is not enough. The repair of concrete places certain physical and chemical requirements on the numerous categories of epoxies that are available. Physically the epoxy should have a low viscosity, not more than about 20 poise for very fine cracks, so that it will penetrate the full depth of the crack and penetrate pores at the working pressures to be applied. It should be capable of bonding to moist concrete if necessary because it is usually impossible to remove moisture present within the member. The chemical formulation should provide a sufficiently long pot life to permit the epoxy to be handled conveniently: a minimum of 30 minutes is needed before gelling. No solvent should be included in the formulation and not more than one percent of reactive diluent. To prepare a crack for injection the repairman must first seal the crack at the concrete surface to keep the grout from leaking out before it has gelled. If extremely high injection pressures are needed the crack is cut out to a depth of one-half inch and a width of about three-fourths of an inch and a width of about three-fourths of an inch in a V-shape, filled with an epoxy of a grease-like, nonsagging consistency and struck off flush with the surface. When the job is completed the dry film can be stripped away to expose the gloss-free surface. When all adhesive used to seal the crack and to bond the entry ports has hardened, the epoxy grout is mixed and injected. It is often best to do this operation mainly between midnight and morning when the concrete is coolest and cracks are widest. If the crack is vertical, the operator injects epoxy into the lowest port until it oozes out of the port above. With horizontal cracks he proceeds from one end of the crack to the other in essentially the same manner. The crack is full when the operator is able to maintain pressure.