Chapter 3: Repair Materials and Procedures

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Section 1: Minor Spall Repair

Description

Most minor concrete spalls are repaired using neat Type VIII epoxy (no sand) to help protect against deterioration caused by exposure to the water, chlorides, and other contaminants. Minor spalls are defined in Section 2.1 of this Manual.

Minor spalls can be built up using Type VIII epoxy mortar. Generally building up minor spalls with mortar provides no additional corrosion protection or capacity vs. neat epoxy only. Filling the voids removes the appearance of spalling and is typically done based on aesthetic concerns.

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Material

TxDOT Type VIII neat epoxy compound (ASTM C 881 Type I or IV) is produced by mixing two proprietary liquid components in the ratio required by the manufacturer.

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Refer to DMS 6100, DMS 6110, and the MPL for Epoxies and Adhesives for additional information.

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Use materials from TxDOT’s preapproved list for Type VIII epoxy. Contractor may use other materials only if specifically authorized by the Engineer in writing.

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Follow manufacturer’s published recommendations for storage including temperature and humidity controls. Retain manufacturer lot tags with packaged date and shelf life for inspection prior to product use.

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Follow manufacturer's material requirements.

TxDOT Type VIII epoxy mortar is produced by combining the neat epoxy compound and an aggregate (usually silica sand) approved by the epoxy producer and TxDOT.

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Ratio by Volume:
- Adhere to the requirements from the manufacturers’ technical data sheets when proportioning the sand to add to the neat epoxy.

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Silica Sand:
- Most manufacturers recommend that 20/40 mesh sand be used to produce epoxy mortar. It is acceptable to use sand that passes a No. 16 sieve but is retained by a No. 50 sieve (16/50 mesh).
- Use sand other than 16/50 mesh or 20/40 mesh only if authorized by the epoxy manufacturer and the Engineer.
- If the repair will remain visible after a member has been erected, the Engineer may require use of a combination of gray and white sand to make the epoxy mortar material closely resemble the surrounding concrete.

Do not exceed 130°F if heating any of the epoxy liquid components to ease mixing or application.

NOTE: Keep a separate container that is protected from rain and other water for storing sand used to mix Type VIII epoxy mortar. Sand from a stockpile is typically far too wet to be used for mixing epoxy mortar.

Do not exceed 230°F when heating sand to dry it. Allow sand to cool to a maximum temperature of 130°F before mixing with the neat epoxy.

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Repair Procedure

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Surface preparation.
- Remove any damaged or loose concrete.
- Avoid damage to sound concrete that is to remain in place by saw cutting the perimeter of the repair area or taking other appropriate measures acceptable to the Engineer.
- Unless otherwise approved by the Engineer, use only hand tools or power-driven chipping hammers (15-lb. class maximum) to remove concrete.
- If the damage occurs at the end of a member and prestressing strand is exposed, recess the strands a minimum 3/8-inch using a torch or other approved method. Do not overheat or damage the surrounding concrete.
  NOTE: In the past some Contractors and Fabricators opted not to recess prestressing strands in spalled areas so the protruding sections could serve as dowels for the repair material. While the strands would serve well as dowels in those circumstances, they could be exposed to moisture and chlorides if the repair fails over the life of the structure. For that reason it is more important that the strand be completely recessed.
- Ensure substrates are clean and sound. Remove any contaminants, including laitance, oil, dust, debris, or other foreign particles.
- Just prior to coating or repairing, blast the repair area using a high-pressure air compressor equipped with filters to remove all oil from the compressed air.
- For minor spalls in which a short longitudinal section (less than 4 inches) of mild reinforcement or prestressing strand is exposed, the steel should NOT be removed. Nor should sound material behind the steel be excavated unless more than half the bar circumference is exposed, in which case the spall should be classified as Intermediate.
- It is not necessary to install dowels or provide other mechanical anchorage in applications less than 1 ½ inch thick. The Engineer may require dowels, typically stainless steel expansion anchors, in thicker applications to help tie the repair material to the parent material in case of a delamination.

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Mixing.
Measure the proper quantity of each component per the manufacturer’s requirements, then dispense into a clean container. Do not estimate the proper amounts while adding the different components.

Mix the liquid components thoroughly using a low-speed electric drill (400 – 600 rpm) and a clean “Jiffy” type mixing paddle. Do not mix Type VIII neat epoxy or epoxy mortar by hand.
- If utilizing whole batches, mix the liquid components for a minimum of 3 minutes or in accordance with the manufacturer’s instructions.
- If using partial batches, mix for at least 1 minute or until the material is well-blended and uniform, whichever is longer.
Slowly add the sand or other approved aggregate to the epoxy compound while mixing with an electric drill and paddle. Mix the material until the epoxy mortar is well-blended and all sand particles are coated (1 minute minimum after the sand is added).

Mix an adequate amount of additional neat epoxy compound for use as a waterproofing and bonding layer.

Set times vary significantly depending on type of epoxy and ambient conditions (temperature, wind, humidity). In hot weather (greater than 90°F) place a partial batch in a cup to determine set-up time, and adjust production volumes accordingly. Adhere to manufacture's storage and shelf life recommendation.

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Neat Epoxy Application.
- Surface must be dry and clean prior to application of the repair material.
- Brush, roll, or scrub the material into the prepared substrate to ensure that all small voids are filled.
- Cover the entire damaged area, including exposed steel reinforcement and dowels when applicable, with at least 10 mils of the neat epoxy compound.
  NOTE: Members of the repair crew should have a wet-film thickness gauge to periodically check that neat epoxy is being installed in adequate application depth. Inspectors should also carry wet-film thickness gauges so they can verify adequate minimum thickness.

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Epoxy Mortar Application.
- Apply a layer of neat epoxy compound to the substrate as outlined above to serve as a bond coat layer for the repair mortar.
- Trowel-apply the epoxy mortar into the repair area while the bonding layer is still tacky. If the bonding layer loses its tackiness prior to repairing, clean the epoxy surface and apply additional neat epoxy before proceeding.
- Limit repair depth to 1 inch when using epoxy mortar unless otherwise approved by the Engineer. In multiple lift applications wait until previous lift has cured sufficiently to prevent sagging prior to applying the next lift. Apply a bonding layer of neat epoxy between each lift.

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Finishing.
The Contractor should consult the Engineer before repairing minor spalls in which an aesthetic treatment will later be applied.

As noted above, the Engineer may require that white or gray sand be used to produce epoxy mortar if a repair will remain visible in its final configuration. Such repair should not be easily discernible if viewed from more than 25 feet away. The Engineer will review other methods proposed for blending repairs.

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Curing.
The required time for the material to cure properly can increase significantly when the ambient temperature is below approximately 50°F. If artificial means are used to heat the in-situ neat epoxy or epoxy mortar, ensure that the air around the repair material does not exceed 130°F.

Moist curing is not required.

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Commentary

The high bond strength of properly mixed and applied Type VIII epoxy mortar makes it a good option for use in thin applications where inclusion of dowels is not practical and excavation behind partially exposed steel would unnecessarily necessitate removal of sound material.

Do not use Type VIII epoxy mortar for structural repairs or in other areas where significant movement from loading or temperature variations are anticipated. Stress caused by differential movement at the bond line can develop because the coefficient of thermal expansion of Type VIII epoxy mortar varies significantly from the concrete substrate. This problem can occur even in thin applications if the damage covers a large area.