Choose the Right Epoxy Repair Before You Mix a Batch

Epoxy resin can repair selected dormant cracks, holes, spalls, voids, damaged edges, and bonded interfaces, but it is not a universal concrete-repair material. Diagnose the defect and its cause first. Then choose pressure injection, gravity-fed resin, non-sag paste, aggregate-filled epoxy mortar, or a different repair system based on movement, geometry, moisture, orientation, temperature, loading, access, and the required return-to-service window.

Start Here: Decide Whether Epoxy Belongs in the Repair

Do not start at the product shelf. Start by classifying what the crew is looking at:

• A narrow crack
• A moving crack or movement joint
• A spall or delamination
• A hole or localized void
• A broken or failed joint edge
• A damaged corner or step
• Broad surface deterioration

The cause matters as much as the visible shape. Drying shrinkage, thermal movement, settlement, inadequate control joints, overload, and restrained movement are among the typical causes identified in ACI RAP-1, Structural Crack Repair by Epoxy Injection (2003). Because that field guide dates to 2003, current project documents, governing requirements, and current ACI standards control where they differ.

The repair objective matters too. Restoring continuity across a dormant structural crack is different from controlling water, rebuilding a traffic edge, or preserving movement at a joint.

Rigid epoxy may be appropriate when a crack is dormant and the objective is to bond the concrete across it. If movement is expected to continue, ACI advises that epoxy repair may not be applicable.

Stop work and escalate the repair when any of these conditions is present:

• Structural significance is uncertain.
• The crack is moving or changing.
• Settlement, displacement, or overload may be continuing.
• Rust staining, delamination, exposed steel, or crack geometry suggests reinforcement corrosion.
• Water is entering under active pressure.
• Unsound concrete extends well beyond the visible defect.
• The repair involves safety-critical anchorage, sustained loading, or a load-bearing detail.
• The cause cannot be distinguished from the symptom.

Do not simply inject a crack caused by corroding reinforcement. ACI RAP-1 advises against that approach because continuing corrosion can produce new cracking. The reinforcement and affected concrete require evaluation rather than concealment behind a cap seal.

Generic field guidance cannot establish structural significance, design a repair, remediate corrosion, or set acceptance criteria. Those decisions belong in the condition assessment, current project requirements, and qualified professional judgment.

Defect-to-Method Decision Table

Use this table to narrow the method, not to authorize the work. Project requirements and current product documentation still control.

Defect	Likely method	Orientation and key conditions	Stop or reconsider when
Narrow, dormant crack	Low-viscosity pressure injection	Horizontal, vertical, or overhead; crack accessible and compatible with resin moisture limits	Movement, corrosion, uncertain cause, active water pressure, or uncontrolled branching
Dormant horizontal crack	Pressure injection or gravity-fed resin	Geometry must allow penetration and containment	Crack is moving, contaminated, too wet for the product, highly porous, or open through an unsealed underside
Hole, spall, localized void, or broken edge	Aggregate-filled epoxy mortar or another purpose-made patch	Commonly horizontal; vertical or overhead only with an approved non-sag system	Unsound concrete continues beyond the repair or corrosion remains unresolved
Vertical or overhead surface loss	Approved non-sag patching formulation	Product must permit the orientation, lift, exposure, and substrate condition	Material slumps, required lift exceeds its limits, or exterior use is not approved
Moving joint or active crack	Designed movement-capable system	Must accommodate expected movement	Do not routinely bridge movement with rigid epoxy
Active pressurized leak	Project-specific water-control evaluation	Selection depends on water source, pressure, crack condition, and system approval	Do not assume moisture tolerance permits hydrostatic-pressure exposure
Corrosion-driven crack or spall	Corrosion assessment and designed concrete repair	Affected reinforcement may need exposure and treatment as specified	Do not inject over an unresolved corrosion mechanism
Extensive deterioration or delamination	Removal and replacement, cementitious repair mortar, or another designed repair	Depends on area, depth, exposure, reinforcement, and loading	A small patch would merely cover unsound concrete

ACI RAP-1 describes injection as potentially applicable to suitable cracks approximately 0.002 inch and wider, depending on project requirements. That is a guidance point, not a promise that every resin, pump, crack, or concrete section can achieve that threshold.

Epoxy mortar gains that body from specified aggregate. Cementitious repair mortar is a separate material category that may suit some larger or thicker repairs.

As a formulation-specific comparison, SikaRep SA, a polymer-modified cementitious mortar, lists a placement range of 5–30 mm per layer. That example illustrates a different placement model from thin resin injection or product-specific epoxy lifts; it does not establish that cementitious mortar is universally better, cheaper, or more durable.

Select the Formulation From the Crack and the Data Sheet

“Epoxy” can describe several materials that are not interchangeable:

1. Low-viscosity injection resin flows under controlled pressure into suitable cracks.
2. Gravity-fed resin penetrates suitable horizontal cracks without pressure.
3. Non-sag sealing or bonding paste holds ports, creates a cap seal, bonds components, or acts as a mortar binder where approved.
4. Aggregate-filled epoxy mortar rebuilds holes, spalls, voids, and edges.

Selection should account for crack width and depth, section thickness, access, orientation, moisture, repair objective, viscosity, modulus, working life, application temperature, and the required reopening window.

For cracks 0.010 inch or narrower, ACI RAP-1 recommends considering epoxy with a viscosity of 500 cP or less. Treat that as a screening guide: actual penetration also depends on blockage, branching, depth, pressure, section thickness, and the selected system.

ASTM C881 addresses epoxy-resin bonding systems for concrete and identifies basic grade and class criteria, as summarized in ACI RAP-1. A claim that a product “meets ASTM C881” is meaningful only when its applicable type, grade, and class align with the work and project documents; it is not a complete repair specification.

Two products illustrate why the category name is insufficient.

SikaFlow-647 is described as a flowable, two-component epoxy grout for pressure injection or gravity feed. It may also be extended with the manufacturer’s specified oven-dried aggregate.

Sikadur-31 Hi-Mod Gel, by contrast, is a non-sag paste used for bonding, cap sealing, and selected interior vertical or overhead mortar repairs. Ratios, temperature limits, placement restrictions, and cure requirements cannot be transferred from one formulation to the other.

Before procurement, pull the current local product data sheet and record:

• Approved use and repair objective
• Horizontal, vertical, or overhead approval
• Interior or exterior approval
• Dry, damp, or wet-substrate limits
• Standing-water, underwater, and hydrostatic-pressure exclusions
• Component ratio and whether partial units are permitted
• Required component conditioning
• Mixed viscosity
• Air, substrate, and material temperature limits
• Dew-point limitation, if applicable
• Pot life or open time at expected temperature
• Bond-line, neat-resin, or lift-thickness limits
• Primer or conditioner requirements
• Approved aggregate type and loading
• Cure and return-to-service requirements
• Cleanup method
• Professional- or approved-installer restrictions

A moisture-tolerant formulation is not automatically approved for standing water, underwater installation, or active hydrostatic pressure. Each condition must be expressly permitted by the selected system’s current local documentation.

Prepare Sound Concrete Before Opening the Epoxy

A dependable repair must bond to sound concrete, not laitance, dust, oil, coating residue, weak paste, or the loose edge of a delamination. Remove deteriorated material and bond-inhibiting contamination until the repair perimeter and base are competent.

For injection, ACI RAP-1 recommends cleaning approximately 1/2 inch on each side of the crack so the cap seal can bond. It favors wire brushing at narrow cracks because mechanical grinding may force dust into the opening and impede resin penetration.

Depending on the defect and selected system, preparation may include:

• Wire brushing
• Vacuuming
• Oil-free compressed air
• High-pressure water cleaning
• Blast cleaning
• Other appropriate mechanical preparation

Match the preparation method to the defect and product instructions. Water cleaning can create a moisture condition the resin does not permit. Compressed air must be clean and oil-free. Abrasive preparation must not leave dust behind or damage the required repair geometry.

Where concrete beside a crack is deteriorated, open or remove that material until sound concrete is reached. A surface seal over weak concrete creates a tidy cover, not a sound substrate.

Describe moisture accurately:

• Dry: No moisture condition prohibited by the product.
• Damp: Moisture is present, but no standing film exists unless the product defines otherwise.
• Wet or water-containing: Liquid water is present in the crack or substrate.
• Pressurized: Water is being driven through the defect.

Some systems accept damp concrete or water-containing cracks; others require dry surfaces and prohibit wet cracks, standing water, or pressure. For example, SikaFlow-647 permits certain water-containing cracks but excludes underwater repair, while the Sikadur Crack Repair Kit prohibits wet cracks and osmotic or hydrostatic pressure. The named formulation decides—not the word “epoxy.”

Before application, document:

• Concrete soundness and removed material
• Coatings, laitance, oil, grease, dust, and other contamination
• Crack width, condition, branching, and visible movement
• Moisture condition and water pressure
• Air temperature
• Substrate temperature
• Material temperature
• Dew point where required
• Access to one or both faces
• Horizontal, vertical, or overhead orientation

Injection Workflow for Horizontal, Vertical, and Overhead Cracks

The following sequence is planning guidance for a crack already judged suitable for injection. It is not a universal structural-repair specification.

1. Prepare the crack and adjacent concrete. Remove contamination and weak material without packing the crack with dust.
2. Lay out and place ports. Surface-mounted ports suit many cracks. Socket-mounted ports may help where the crack is obstructed.
3. Install the cap seal. Seal between ports and let the material set as directed. When a crack passes through the section, sealing both accessible faces helps contain resin during pressure injection.
4. Prepare the delivery equipment. Confirm that cartridges, pumps, hoses, couplers, and spare ports are compatible and ready before activating the resin.
5. Mix or activate the resin exactly as directed. Follow ratio, conditioning, mixing, nozzle, and batch-size requirements.
6. Inject in a planned sequence. Continue at each port to the specified endpoint—commonly refusal or resin appearing at the adjacent port—subject to system instructions.
7. Cap completed ports. Control leakage while moving to the next location.
8. Allow the repair to cure. Maintain the required temperature and protection.
9. Remove ports and cap-seal material where specified. Finish without damaging the repair.
10. Verify and document the work.

ACI RAP-1 describes the usual sequence for a vertical crack as bottom-up: inject the lowest port, continue until resin reaches the next port, cap the completed port, and move upward. For a horizontal crack, injection may start at an accessible or wider section. Gravity filling may be an option when the geometry and selected product permit it.

Higher pressure may improve penetration into a narrow crack, but ACI warns that it can also blow out a port or cap seal.

Field injection checklist

• Pump or cartridge functional and compatible
• Ports numbered in working sequence
• Cap seal cured and checked
• Batch or cartridge start time recorded
• Working-time deadline posted
• Leakage watched continuously
• Resin appearance at adjacent ports recorded
• Completed ports capped
• Material use, anomalies, and unfilled sections logged

Epoxy Mortar Workflow for Spalls, Holes, Voids, and Edges

Epoxy mortar combines resin and hardener with a specified aggregate. It is a patching material, not neat injection resin mixed with whatever sand happens to be available.

A bounded field workflow is:

1. Remove unsound concrete and establish the required repair geometry.
2. Clean the area to sound, contaminant-free concrete.
3. Confirm substrate moisture and air, substrate, and material temperatures.
4. Apply a primer or conditioner only when the selected system requires it.
5. Mix complete resin and hardener components in the stated ratio.
6. Add only the specified dry aggregate, following the required sequence and loading.
7. Place within the documented working time.
8. Compact the mortar into the repair and finish it to the required profile.
9. Protect the patch through the specified cure.
10. Reopen only after the applicable cure and loading requirements are met.

Do not alter component ratios, add water or solvent, substitute unapproved aggregate, or thin the binder. These changes can interfere with cure or alter placement and performance.

Excessive high-speed mixing can entrain air. SikaFlow-647, for example, specifies slow-speed mixing and warns against forming a vortex.

Orientation controls formulation. A flowable material may suit a horizontal void but run out of a wall or soffit. Vertical and overhead work requires a non-sag system expressly approved for that use.

Maximum lift, minimum depth, aggregate loading, feather-edge approval, primer requirements, and exposure limits vary. As one bounded example, the Sikadur-31 data sheet limits its epoxy-mortar lifts to 1 inch and its mortar application to interior use. Those limits belong to that formulation, not epoxy mortar as a category.

Plan Temperature, Batch Time, Cure, and Reopening

These schedule terms are not interchangeable:

An April 2020 Ireland-market Sikadur Crack Repair Kit data sheet provides an illustrative, product-specific example. Its listed injection-resin open time changes from 20 minutes at 30°C to 30 minutes at 20°C and two hours at 5°C. Listed cure time changes from 12 hours to 24 hours and 72 hours at those temperatures. These figures are not a local scheduling specification; verify the current data sheet for the market and formulation being used.

SikaFlow-647 likewise lists shorter working times as temperature rises: 30–40 minutes at 55°F, 20–30 minutes at 75°F, and 10–20 minutes at 90°F, assuming the stated component conditioning.

Rapid-return claims apply only to the named product, stated temperature, repair geometry, and specified load. A seller’s foot-traffic or forklift figure is not a general benchmark for epoxy concrete repair.

Use a batch log for every repair period:

Batch control	Field entry
Product, formulation, and lot
Material temperature
Air temperature
Substrate temperature
Dew-point check, if required
Batch or cartridge start
Working-time deadline
Placement completed
Cure conditions maintained
Authorized reopening time and load

Base reopening on the current local data sheet, actual jobsite conditions, project requirements, and the load the repaired area will receive—not appearance or a generic “overnight cure.”

Safety, Verification, and Applicable References

The current safety data sheet controls exposure measures. ACI RAP-1 identifies general controls including protective clothing, eyewear, gloves, eyewash, ventilation, respirators when required, secure hazardous-material storage, and occupant notification. Exact PPE, ventilation, respiratory protection, spill response, cleanup, disposal, and occupant controls must come from the selected product’s current SDS and applicable workplace requirements.

Cleanup varies by formulation. Some uncured products permit specified soap, water, citrus degreaser, or proprietary cleaner; hardened epoxy generally requires mechanical removal. Confirm the method before work begins so cleanup does not become an improvised solvent exposure.

A neat cap seal proves only that the surface was sealed. It does not prove the crack was filled internally. Verification can include observed resin travel between ports, material-use records, and documented leakage or refusal.

Where project requirements warrant, ACI RAP-1 identifies test cores and nondestructive methods including impact echo, ultrasonic pulse velocity, and spectral analysis of surface waves. Establish acceptance criteria before structural or safety-critical work begins.

Use each reference according to its scope:

• ACI RAP-1, Structural Crack Repair by Epoxy Injection: field guidance for epoxy injection
• ACI RAP-2, Crack Repair by Gravity Feed with Resin: gravity-fed crack repair
• ACI SPEC-548.14-25, Repairing Concrete with Epoxy Mortar—Specification: epoxy-mortar repair
• ACI SPEC-548.15-20, Specification for Crack Repair by Epoxy Injection: epoxy-injection work
• ACI PRC-546.3-23, Guide to Materials Selection for Concrete Repair: repair-material selection
• ACI PRC-515.3-20: assessment and surface preparation for concrete protection systems
• ACI CODE-562-25: assessment, repair, and rehabilitation of existing concrete structures
• ACI SPEC-563-25: repair of concrete in buildings
• ASTM C881: epoxy-resin bonding systems for concrete

The ACI concrete-repair catalog confirms that ACI publishes separate resources for injection, gravity feed, epoxy mortar, material selection, assessment, and existing-concrete repair. The catalog establishes document names and broad scopes; it does not provide their detailed procedures or acceptance criteria. It also lists multiple editions, so identify the edition required by the project documents or governing authority.

Use this control-document hierarchy:

1. Project specifications and engineering requirements
2. Applicable codes and standards
3. Current local product data sheet and label
4. Current safety data sheet
5. Qualified installer procedures consistent with all of the above

The field rule is straightforward: diagnose the defect; stop when movement, corrosion, water pressure, or structural uncertainty is present; and choose the repair form rather than merely choosing “epoxy.” A sound substrate, correct formulation, controlled batch, workable temperature window, and planned verification matter more than broad strength or rapid-cure marketing claims.

How narrow a concrete crack can epoxy injection fill?

ACI RAP-1 says suitable cracks approximately 0.002 inch wide and greater may be injectable, depending on project requirements. It also recommends considering epoxy at 500 cP or less for cracks 0.010 inch or narrower. Neither figure guarantees successful filling: depth, blockage, branching, concrete thickness, access, moisture, pressure, and the selected product all affect penetration.

Can epoxy resin be applied to damp concrete?

Some formulations can; others cannot. One product may accept damp concrete or a water-containing crack, while another requires a dry surface and prohibits wet cracks. Even a moisture-tolerant epoxy is not automatically suitable for standing water, underwater repairs, or active hydrostatic pressure. Use the current local data sheet for the exact formulation and condition.

Does sealing the visible crack prove that epoxy filled it completely?

No. The cap seal contains injection resin and closes the visible surface, but it does not demonstrate complete internal filling. Verify resin travel during injection and use the project’s pre-established acceptance method, which may include records, test cores, or appropriate nondestructive evaluation.