depth of repair is less than 1-1/2 inches and the exposure conditions are such that relatively constant temperatures can be expected. Epoxy mortars have thermal coefficients of expansion that may be significantly different from
conventional concrete. If such mortars are used under conditions of wide and frequent
temperature fluctuations, they will cause failure just below the bond surface in the base
concrete. For this reason, current Reclamation practice precludes the use of epoxy mortars under conditions of frequent or large temperature fluctuations.
The application of epoxy mortar to repair areas of concrete deterioration caused by corroding reinforcing steel is also not recommended. The epoxy bond coat and epoxy mortar create zones of electrical potential that are different from the electrical potential in the surrounding concrete.
This difference in potential can result in the formation of a galvanic corrosion cell with accelerated corrosion at the repair perimeters.
Epoxy mortar is properly used to make thin repairs (1/2-inch to 1-1/2-inch thickness) to concrete under relatively constant temperature exposure conditions. Such applications could include tunnel linings, indoor or interior concrete, the underside of concrete structures such as bridge decks, continuously inundated concrete such as stilling basin floors, canal linings below water line, or concrete pipe.
Applications to concrete exposed to the daily temperature fluctuations caused by exposure to direct sunlight are not appropriate for epoxy mortar repair.
Properly applied epoxy mortar repairs have a long history of successful performance on Reclamation concrete when used under appropriate conditions. A 1991 inspection of the epoxy mortar repairs made at Yellowtail Dam in 1968 showed that less than 2 percent of the repairs had suffered failure in over
20 years of service. This is considered out-standing performance for a repair material.
(a) Preparation.—Concrete to be repaired with epoxy mortar should be prepared in accordance with section 8. Prior to application of the epoxy mortar, the concrete should be heated in
sufficient depth, when necessary, so that the surface temperature (as measured by a surface temperature gage) does not drop below 40 EF during the first 4 hours after placement of an epoxy bond coat. This may require several hours of preheating with radiant heaters or other approved means (figures 49 and 50). If existing conditions prohibit meeting these temperature requirements, suitable modifications should be adopted upon the approval of the inspector or other responsible official. The concrete temperature during preheating should never exceed 200 EF, and the final surface temperature at the time of placing epoxy materials should never be greater than 100 EF.
(b) Materials.—Epoxy resins used to prepare epoxy mortar for use in concrete repair should be two-component, 100-percent solids type meeting the requirements of specification ASTM C-881 for type III, grade 2, class B or C.
Class B epoxy is used between 40 and 60 EF.
Class C epoxy is used above 60 EF up to the highest temperature defined by the epoxy manufacturer. The sand used in epoxy mortar must be clean, dry, well graded, and composed of sound particles. For most applications, sand passing a No. 16 screen and conforming to the following limits should be used:
Screen number
Range shown is applicable when 60 to 100 percent of pan is retained on No. 200 screen.
When 41 to 100 percent of pan passes the No.
200 screen, the percent pan should be within the range of 10 to 20 percent, and the individual percentages retained on the Nos. 30, 50, and 100 screens should be adjusted accordingly.
Sand processed for use in concrete rarely contains the required quantity of pan size sand.
As a result, problems often arise in obtaining additional pan size material to supplement sand available on the jobsite. A source of silica pan size material may be obtained by contacting the Materials Engineering and Research
Laboratory, Code D-8180, Bureau of
Reclamation, Denver Federal Center, Denver, Colorado 80225. A sand graded as shown above and properly mixed with an epoxy meeting ASTM C-881 specifications will provide a dense, high strength, workable epoxy mortar.
Guide to Concrete Repair
Figure 50.—An enclosure has been constructed over an area to be repaired with epoxy mortar to keep the concrete warm.
Figure 49.—A gas-fired forced air heater is being used to heat concrete prior to application of epoxy mortar.
The sand should be maintained in a dry area at not less than 70 EF for 24 hours immediately prior to the time of use. Filler materials other than sand, such as portland cement, can be used. However, for general applications, a natural sand is recommended.
It is also acceptable to obtain and use brand name prepackaged epoxy mortar repair systems that contain resin and sand, provided that the resin systems meet the ASTM C-881 specifications previously listed. Such mortar systems are manufactured specifically for concrete repair and must be used in exact accordance with the manufacturer's recommendations.
On critical repair jobs such as areas of high velocity flow or on repairs requiring a considerable quantity of materials, the contractor should be required to submit samples of epoxy resin and graded sand to the Materials Engineering and Research
Laboratory, Denver, for use in mix design determinations. The samples should consist of 1 gallon total quantity of epoxy components and a minimum of 50 pounds of graded sand. Samples should be submitted at least 30 days prior to use in the work and be labeled or otherwise identified with the specifications number under which the material is to be used.
(c) Mixing.—Preparation of epoxy mortar
involves premixing proper quantities of epoxy resin and hardener and then mixing the resin system with sand to make the epoxy mortar.
The epoxy resin used for mortar preparation is a two-component (part A and part B) material which requires accurate combination of components and mixing prior to use. Once mixed, the material has a limited pot life and must be used immediately. (Pot life refers to the period of time elapsing between mixing of ingredients and their stiffening to the point where satisfactory use cannot be achieved.) The repair resin should be prepared by adding the required quantity of hardener (normally, part B) to the resin (normally part A) in proportions
recommended by the manufacturer, followed by thorough mixing. Since the pot life of the mixture depends on the temperature (longer at low
temperature, much shorter at high temperature), the quantity to be mixed at one time should be that quantity that can be applied within approximately 30 minutes. The addition of nonreactive thinners or
diluents to the resin mixture is not permitted since it weakens the epoxy.
The epoxy mortar is composed of sand and epoxy resin suitably blended to provide a stiff, workable mix. Mix proportions should be established, batched, and reported on a weight basis, although the dry sand and mixed epoxy may be batched by volume using suitable measuring containers that have been calibrated on a weight basis. Epoxy meeting ASTM specification C-881 will require approximately 5-1/2 to 6 parts of graded sand to 1 part epoxy, by weight. This is equivalent to a ratio of approximately 4 to 4-1/2 parts sand to 1 part epoxy, by volume. If equivalent volume
proportions are being used, care must be taken to prevent confusing them with weight proportions. It will be necessary to adjust the mix proportions for the particular epoxy and sand being used. The epoxy mortar should be thoroughly mixed with a slow-speed mechanical device. The mortar should be mixed in small size batches so that each batch can be completely mixed and placed within approximately 30 minutes. Figure 51 shows a simple bucket mixer that is adequate to mix epoxy mortar for small repairs.
(d) Application.—Application of epoxy mortar repairs first requires application of a resin bond coat followed by application and finishing of the epoxy mortar. Surfaces of existing concrete to which epoxy mortar is to be bonded should be prepared as discussed in section 8. Steel to be embedded in epoxy mortar should be prepared, cleaned, and dried in the same manner as the concrete being repaired. The exposed steel should be completely coated with epoxy bonding agent when the agent is applied to the surfaces of the repair area.
A resin bond coat consisting of the same type epoxy resin used to mix the epoxy mortar is applied to the prepared concrete surface immediately before placing the epoxy mortar. After the bond coat resin is mixed, it must be uniformly applied to the prepared, dry, existing concrete at a coverage of not more than 80 square feet per gallon, depending on surface conditions. The area of coverage per gallon of resin depends on the roughness of the surface to be covered and may be considerably less than the maximum specified. The epoxy bonding agent may be applied by any convenient, safe method such as
Guide to Concrete Repair
Figure 51.—A bucket mixer can be used to mix epoxy mortar for small repair areas.
squeegee, brushes, or rollers which will yield an effective coverage. Spraying of the material is permitted if an efficient airless spray is used and if the concrete surfaces to receive the agent are at a temperature of 70 EF or some-what warmer.
Before approving spraying, it should be demonstrated that spraying will provide an adequate job with minimum overspray. If spray application is used, the operator must wear a compressed air-fed hood, and no other personnel should be closer than 100 feet if downwind of the operator.
During application of the epoxy bond coat, care must be exercised to confine the material to the area being bonded and to avoid contamination of adjacent surfaces. However, the bond coat should extend slightly beyond the edges of the repair area.
The applied epoxy bonding resin must be in a fluid condition when the epoxy mortar is placed.
If the resin cures beyond this fluid state but is still tacky, a second bond coat should be applied over the first coat. If any bond coat has cured beyond the tacky state, it must be completely removed by sandblasting, the concrete properly cleaned, and a new bond coat applied.
Special care must be taken to prevent the bond coat from being spread over concrete surfaces not properly cleaned and prepared.
Appropriate solvents may be used to clean tools and spray guns, but in no case should the solvents be incorporated in any bonding agent.
All tools must be completely dried before reuse.
The prepared epoxy mortar should be tamped, flattened, and smoothed into place (figure 52) in all areas while the bonding resin is still in a fluid condition, except that on steep slopes, the bond coat can be allowed to stiffen to a very tacky condition to assist in holding the mortar in place. Special care must be taken to thoroughly compact the epoxy mortar against the bond coat.
The mortar should be worked to grade and given a steel trowel finish (figure 53). Special care must be taken at the edges of the area being repaired to assure complete filling and leveling
and to prevent the mortar from being spread over surfaces not having the epoxy bond coat application. Steel troweling should best suit prevailing conditions; in general, it should be performed by applying slow, even strokes.
Trowels may be heated to facilitate the
finishing, but the use of thinner, diluents, water, or other lubricants on placing or finishing tools is not permitted. After leveling the epoxy mortar to the finished grade where precision surfaces are required on sloping, vertical, or overhead surfaces, the mortar should be covered with plywood panels smoothly lined with polyethylene sheeting and weighted with sandbags or otherwise braced by suitable means until the possibility of slumping has passed.
When polyethylene sheeting is used, no attempt should be made to remove it from the epoxy mortar repair before final hardening.
Surfaces of all epoxy mortar repairs should be finished to the plane of surfaces adjoining the repair areas. The final finished surfaces should have the same smoothness and texture of surfaces adjoining the repair areas.
(e) Curing.—Epoxy mortar repairs should be cured immediately after completion at not less than the temperature range prescribed by the class of the epoxy until the mortar is hard.
Postcuring, if required by the specifications, can then be initiated at elevated temperatures by heating in depth the epoxy mortar and the concrete beneath the repair. Postcuring should continue for a minimum of 4 hours at a surface temperature generally not less than
90 EF nor more than 110 EF. The heat could be supplied by use of portable propane-fired heaters, infrared lamp heaters, or other approved sources positioned to attain the required surface temperatures (figure 54).
In no case should epoxy-bonded epoxy mortar be subjected to moisture until after the specified postcuring has been completed.
Epoxy mortars generally produce patches that are darker than the surrounding concrete.
Guide to Concrete Repair
Figure 52.—Epoxy mortar is consolidated and compacted by hand tamping.
Figure 53.—Applying the steel trowel finish required by epoxy mortar repairs.
Figure 54.—Postcuring heating enclosure installed over an epoxy mortar repair area.
Some available epoxies produce a gray- colored mortar resembling concrete. However, these materials will rarely produce an exact color match. Grinding hardened epoxy mortar may lighten its color to about that of the surfaces adjoining the repair areas. Epoxy mortars can be colored by the addition of such materials as iron oxide red, chromium oxide green,
lampblack and titanium dioxide white for gray, and ocher yellow; although Reclamation rarely uses any materials to color the epoxy other than the sand for the mortar. Use of white silica sand in the mortar will produce a white-looking patch; most natural riverborne sands will produce darker colored mortars. Whenever epoxy mortar repair materials must be colored to match adjacent concrete, laboratory mixes should be made to ascertain the proper quantities of coloring constituents.
(f) Safety.—All personnel must be carefully instructed to take every precaution in pre-venting epoxy resins and their components from contacting the skin and in preventing the breathing of epoxy fumes or vapors. Protec-tive clothing must be worn, including gloves and goggles, and protective creams for other exposed skin areas should be provided when
handling epoxies, as severe allergic reactions and possible permanent health damage can result when these materials are allowed to con-tact and remain upon the skin. Any deposits acquired through accidental contact of these materials with unprotected skin must be
removed immediately by washing with soap and water, never with solvents. Solvents, such as toluene and xylene, may be used only for cleaning epoxy from tools and equipment. Care must also be exercised to avoid contact of cleaning solvents with the skin and to provide adequate ventilation for mixing, placing, and cleanup operations. All safety equipment used must conform to the requirements of the Occupational Safety and Health Standards of the Occupational Safety and Health
Administration.