Since the January 17, 1994, Northridge Earthquake, PACE Engineering, Inc., has been involved with the inspection and repair recommendations of commercial and residential properties for the mitigation of seismic damage. In some instances recommendations calling for the use of epoxy resin or mortar to repair structural damage to concrete members has been challenged. While epoxy injection is only one method of repair and should not be considered a cure-all fix in every instance, proper application of today's advanced epoxy resins by experienced concrete restoration and repair specialists has proven to be a valid and cost effective method to restore (and in some instances) increase the strength of damaged structural members.

The remainder of this article addresses some of the most common objections to the use of epoxy in concrete repair.

BACKGROUND

Structural epoxies in the form of resin or mortar have been in use for over 35 years for the repair and restoration of concrete and wood members. Modern epoxy adhesives are not sensitive to moisture and may be applied to damp or dry surfaces, and cracks need only be clean of debris and free of standing water. Manufacturers of approved epoxy resin based bonding systems must meet American Society for Testing Materials (ASTM) standards that regulate the modules of elasticity (stiffness of a material to imposed loading) tensile, compressive, and flexural strengths.

In the majority of cases, if the structure has not been wholly deformed and the original source of the cracking has been removed, the cracked concrete member may be effectively repaired in-place by epoxy injection. Recent development has led to CFRP (carbon fiber reinforced plate) technology, which involves the application of epoxy resin with impregnated layers of carbon fiber to the exposed outside faces of structural concrete members. This method has proved to be very effective in obtaining substantial increases in the strength of columns, beams, and shear walls already in place. This ultimately increases the structural integrity of the overall superstructure. This budding technology is currently being used for on-site repair and retrofit work.

FIVE COMMON OBJECTIONS TO EPOXY REPAIR

1. Objection: "There are no International Conference of Building Officials (I.C.B.O.) evaluation reports 'of record' which authorize the use of epoxy for concrete foundation repair."

The I.C.B.O. is an agency, which publishes the family of Uniform Codes, offers code interpretations, and provides evaluation reports for various applications of building products. Tests conducted by applicants are reviewed by I.C.B.O. for conformance to the code sections on which they are based. The lack of an evaluation report on the use of structural epoxy repair for concrete foundations or other members does not in any way preclude the issuance of a building permit for this very purpose. This is done everyday in many jurisdictions, including the City of Los Angeles. It is the discretion of the local Building Official to approve or disapprove alternate materials, designs, or methods of construction. Not the I.C.B.O.

2. Objection: "Epoxy is not approved as a structural repair by the City of Los Angeles."

Since the Northridge earthquake, individual manufacturers of epoxy products have been allowed to submit necessary product information to the City of Los Angeles, Research Division, for approval. The division has completed a list of "Products for Administrative Approval of Alternate Materials." All brands on that list are recommended for approval for a one-time usage. According to the general guidelines for administrative approval, structural damage to non-fire rated elements (i.e., foundations and stem walls of residential construction occupancy R-3, type V-N) with cracks up to 1/8-inch in width may be repaired by epoxy injection without limitation as long as the engineering report and special inspection provisions are met.

3. Objection: "Epoxy repair is unacceptable since there is no way to tell if the crack was properly prepared, the epoxy has penetrated all cracks, was adequately mixed and injected in the field, etc. Therefore, epoxy injection does not restore a slab or foundation to its previous monolithic condition."

A special Deputy Inspection is required throughout the injection process to insure the quality of preparation, correctness of the mix, and proper method of injection. In addition, core samples or (non-destructive) pulse velocity testing may be employed to verify the degree of penetration and compressive strength of the repaired concrete.

4. Objection: "The primary function of epoxy repair is to seal and fill cracks, prevent moisture migration, and retard rebar corrosion."

While all the above properties accompany repair by epoxy injection, the primary function is to provide a bond between the opposing crack faces having a greater compressive and flexural strength than that which existed in the original concrete mixture. Lab results on test cylinders have repeatedly exhibited that a properly injected high strength epoxy bond will remain intact while the sections of concrete to either side of the bond shatter catastrophically.

5. Objection: "The fire resistant properties of epoxy are limited, epoxy breaks down and loses strength when subjected to high temperatures."

The conditions of approval for the use of epoxy injection repair in the City of Los Angeles specifically state "use of epoxy injection for cracks exceeding 1/8 inch in fire rated elements requires that the epoxy be protected by fireproofing materials that provide the required fire rating of the element being repaired." In the context of residential footing and slab repair the fire resistance of epoxy is of no consequence since these members are not fire rated.

CONCLUSION

The outright objection to the use of epoxy in residential concrete repair should be vigorously contested. The responses provided above to many of the common objections should assist in countering the flagrant and costly "full replacement" demands which are so often made. In most cases, adequate on-site repair of distressed slabs and footings will restore and strengthen these members to a condition equal to or better than, that which existed prior to the damage.

For further information about PACE Investigative Services, please contact Sherrie Henderson at 818-251-1200 or e-mail her at shenderson@hfinc.com.