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Short answer: cracks in concrete are injected with one of two resins, depending on the goal. For dry, non-moving cracks 0.1 – 1 mm wide, a low-viscosity epoxy injection resin is used to bond the structure back together. For leaking, wet or moving cracks, a polyurethane injection resin that foams on contact with water is used to stop the water. In both methods holes are drilled on both sides of the crack, packers are installed and the resin is pumped in under pressure. The cause of the crack should be established before injection; for structural cracks the decision is made with a structural engineer.
The values below are taken from the technical data sheets of DEMCO injection resins and repair mortars.
Which injection for which crack?
| Crack | Goal | DEMCO product |
|---|---|---|
| Dry, non-moving, 0.1 – 1 mm wide | Fill the crack and bond the concrete back together | DEMCO EPOX 120 INJECTION RESIN |
| Water-bearing, wet crack | Stop the water flow | DEMCO PUR 500 INJECTION |
| Moving cracks and cold joints | Waterproofing | DEMCO PUR 500 INJECTION |
| Voids between steel jacketing and concrete or stone in strengthening work | Fill the void | DEMCO EPOX 120 INJECTION RESIN |
| Cracks wider than 1 mm and cavities | Filling and repair | DEMCO EPOX 281 MORTAR · DEMCO EPOX 283 DBA |
The two resins compared
| Property | DEMCO EPOX 120 INJECTION RESIN | DEMCO PUR 500 INJECTION |
|---|---|---|
| Type | Solvent-free, two-component epoxy | Solvent-free, water-reactive flexible polyurethane |
| Mixing ratio (by weight) | A/B (70/30) | 100/10 (resin/catalyst) |
| Mixture viscosity (25 °C) | 250 – 450 mPas | 400 – 800 mPas |
| Pot life (23 °C) | 25 – 45 minutes | 80 – 150 minutes |
| Application thickness / expansion | 0.1 – 1.00 mm | Expands up to 30 times its volume |
| Strength | At 7 days: compressive > 67 N/mm², flexural > 30 N/mm², adhesion to concrete > 2 N/mm² | Elastic, closed, fine-cell foam |
| Application temperature | 15 – 30 °C, at least 10 °C | +5 – +35 °C, not applied when there is a risk of frost |
| Substrate | Moisture below 4% | Wet; in a dry crack water is injected first |
| Packaging | 5 kg (A: 3.50 kg / B: 1.50 kg) | 22 kg (20 kg resin + 2 kg catalyst) |
DEMCO EPOX 120 INJECTION RESIN has been tested to EN 1504-2 under CE marking and reaches full cure in about 7 days at 20 – 25 °C.
Epoxy injection step by step
- Surface preparation: the plaster over the crack is removed; the concrete is cleaned of dust, oil and dirt, and damaged and loose parts are removed. If water is running on the surface, it is drained or sealed with a suitable plug.
- Holes: depending on the width and depth of the crack and site conditions, holes are drilled on both sides of the crack at intervals of 20 – 25 cm. The holes are drilled at about 45° to the crack plane and deep enough to pass through it to the other side.
- Packers: dust and loose particles are blown out of the holes with air, and the packers are driven in and tightened.
- Surface sealing: the areas around the packers and the surface of the crack are covered with DEMCO EPOX 283 DBA to make them tight and rigid.
- Mixing: all of component B is poured into A and mixed at low speed for at least 3 minutes; no thinner is added. As the pot life is 25 – 45 minutes, only prepare as much as can be used in that time.
- Injection: the mixture is poured into the hopper of a single-component injection pump and injected at a suitable pressure between 0 and 40 bar, starting from the lowest packer. When resin comes out of the next packer up, the air in the crack has been displaced; the hose is moved to that packer and the process continues up to the top packer. When resin comes out of the top packer, the crack is considered full.
- Finishing: after about 12 – 24 hours the packer ends can be cut off and further work can be carried out on the surface. Do not touch the surface or let water reach it for 24 hours after application.
The injection pressure depends on the width and depth of the crack; according to the data sheet it must be set separately for each project.
Stopping water with polyurethane injection
DEMCO PUR 500 INJECTION expands up to 30 times its volume on contact with water and forms an elastic foam that stops the water flow. It is used in water-bearing and moving cracks, cold joints, water tanks and swimming pools, tunnels and underground storage, and for waterproofing basements from the inside.
- The resin and catalyst are mixed with a mixer running below 400 rpm until uniform. Take care not to draw humid air into the mixture; otherwise the reaction starts before it reaches water.
- The mixture is injected with a single-component injection pump. Moisture or water in the crack starts the foaming reaction; if the crack is dry, water is injected into the hole first.
- Do not leave tools standing for more than 15 minutes; when the work is finished, clean the pump and equipment with DEMCO PUR 1000 THINNER.
- Do not apply if the surface or ambient temperature is below +5 °C or above +35 °C, or if there is a risk of frost. Do not deviate from the stated mixing ratio.
Before injecting: the cause of the crack
Injection fills a crack but does not remove what caused it. If there is settlement, overloading, reinforcement corrosion or ongoing movement, the injected crack can open again or new cracks can form next to it. For cracks in load-bearing members, the cause and the repair method should be determined by a structural engineer; where necessary, injection should be planned together with strengthening such as carbon fiber strengthening or jacketing.
Frequently asked questions
What crack widths are suitable for epoxy injection?
DEMCO EPOX 120 INJECTION RESIN is designed for cracks up to 1 mm; the application thickness in the data sheet is 0.1 – 1.00 mm. Wider cracks and cavities are repaired with filled repair mortars such as DEMCO EPOX 281 MORTAR.
How is a leaking crack sealed?
Water-bearing cracks are injected with the water-reactive polyurethane injection resin DEMCO PUR 500 INJECTION. On contact with water the resin expands and foams and stops the water flow. Epoxy injection, by contrast, needs a dry crack.
What injection pressure should be used?
DEMCO EPOX 120 INJECTION RESIN is injected at a suitable pressure between 0 and 40 bar. The right pressure depends on the width and depth of the crack and is set separately for each project.
When can the packer ends be cut off?
About 12 – 24 hours after epoxy injection, the packer ends can be cut off and further work can be done on the surface. Full cure takes about 7 days at 20 – 25 °C.
Is it enough to seal the top of the crack with filler?
Surface sealing only closes the mouth of the crack; it does not fill it. To restore the integrity of the concrete, the crack is filled by injection; the surface sealing mortar is used to keep the resin from escaping during injection.
Share the location and width of the crack, whether it leaks and, if possible, photos in the quote form, and our technical team will recommend a suitable resin and method. For anchoring and starter bars in concrete repair, see the Epoxy rebar anchoring guide.
