Frequently Asked Questions

What is the difference between injecting a crack with Nitofill SL polyurethane resin and injecting with a rigid epoxy resin and how does the choice affect the repair outcome?

The choice between flexible polyurethane crack injection (Nitofill SL) and rigid epoxy crack injection (such as Nitofill EP epoxy injection resin) is one of the most important decisions in concrete crack repair, and the correct choice depends on whether the crack is truly dormant (stable, not moving) or has any residual movement potential. A rigid epoxy injection resin cures to a hard, high-strength, high-modulus solid that is stiffer than the surrounding concrete — when injected into a dormant crack that has fully stabilised, an epoxy injection creates a repair that is stronger than the surrounding concrete in tension (pull-off strength typically 20 to 30 MPa), restoring full structural tensile and shear continuity across the repaired crack. For structural repairs where the cracked section must restore its full calculated load-carrying capacity, epoxy injection is the appropriate specification. However, if an epoxy-injected crack subsequently experiences any further movement (from thermal cycling, vibration, additional loading, or foundation settlement), the rigid epoxy does not accommodate the movement — instead, the crack propagates through the concrete immediately adjacent to the injected zone, opening a new crack parallel to the repair. The repair has failed, and the new crack is now in un-injected, sound concrete. Nitofill SL polyurethane injection cures to a flexible, elastic solid that can accommodate a small degree of further crack movement (typically 0.1 to 0.3 mm additional opening and closing from thermal cycling or vibration) without re-cracking. The flexible injection does not restore the full tensile structural capacity of the cracked section (the flexible resin has much lower tensile strength and modulus than epoxy), but it permanently seals the crack against water and chemical ingress even if the crack continues to move marginally. The practical rule for selection: if the crack is demonstrably dormant (no movement in the 3 to 6 months before repair) and the structural capacity must be restored, specify epoxy injection. If there is any residual movement risk, or if the repair objective is sealing rather than structural reinstatement, specify Nitofill SL polyurethane injection.

How are the injection ports installed and what injection pressure is used for Nitofill SL in a floor slab with fine hairline cracks?

Installing injection ports and applying the correct injection pressure are the two most critical aspects of crack injection technique that determine whether the Nitofill SL fills the complete crack depth and length or leaves sections of the crack un-injected and ineffective. Port installation: for floor slab cracks accessible from the top surface, there are two methods of port installation. The first method (surface-mounted ports) is used for cracks from 0.1 to 1.0 mm width that are accessible from one face: small surface-mounted injection ports (plastic fittings with a nozzle and a flange that is epoxy-bonded to the concrete surface over the crack) are bonded to the concrete at spacing of 150 to 250 mm along the crack length; the crack surface between ports is sealed with a thin layer of epoxy paste (surface sealing compound) to prevent Nitofill SL from running out of the crack rather than being retained in the crack. The second method (drilled packers) is used for wider cracks or for cracks that must be injected from the side in a wall: holes of 8 to 12 mm diameter are drilled at 45 degrees across the crack (drilling from the surface to intersect the crack plane at depth), and expanding rubber packer fittings are inserted and locked into the drilled holes. Injection pressure: Nitofill SL is a self-levelling, very low viscosity resin — in horizontal floor slabs, it can be allowed to flow along the crack under its own gravitational head (gravity injection) by simply pouring the resin into open ports and allowing it to travel the crack length. For cracks in walls or where the crack must be injected under pressure, low-pressure hand pumps or mechanical injection pumps at 0.1 to 0.3 MPa (1 to 3 bar) are used. Excessive injection pressure forces the resin out of the crack face before it has penetrated the full crack depth — low, patient injection at minimum pressure is consistently more effective than high-pressure rapid injection.

Can Fosroc Nitofill SL be used to inject cracks in a water-retaining concrete tank that is currently in service and retaining water?

Injecting cracks in a water-retaining tank that is currently in service (full of water and in use) with Nitofill SL is technically feasible but requires careful consideration of the specific crack condition and the potential consequences of an active leak during the injection process. The key issue is whether the crack to be injected is dry, damp, or actively weeping water at the time of injection. For dry or damp cracks (no active water flow through the crack at the time of injection), Nitofill SL can be injected from the accessible (typically inner) face of the tank wall without first emptying the tank — the very low viscosity of the resin and its adhesion to damp concrete allow it to penetrate the crack and cure against the water-wetted crack faces. The water pressure from the tank acts as a beneficial force that compresses the crack slightly, improving the effectiveness of the injection at the inner face. For actively weeping cracks (water is flowing through the crack at a measurable rate), Nitofill SL injection is less straightforward because the flowing water will flush the freshly injected resin out of the crack before it can cure — the water flow must be stopped or significantly reduced before resin injection. In this case, the crack should be pre-treated by injecting a hydrophilic polyurethane foam resin (a different product from Nitofill SL, such as a one-component water-reactive PU foam) that expands on contact with water and blocks the crack, stopping the active flow; after the foam treatment has arrested the water flow, the crack is then re-injected with Nitofill SL to provide the permanent structural and sealing repair. For a water supply tank in service with cracks that must be repaired without taking the tank out of service, Fosroc technical support or a specialist waterproofing contractor should be engaged to assess the specific crack condition and design the repair sequence.