Crack injection is one of the few concrete repairs that actually deals with the cause instead of just patching the surface. Get the steps right and a slab or wall that was leaking or spreading stops moving and stops letting water through, for years rather than months.
In this article
Why concrete cracks in the first place
Concrete starts cracking almost as soon as it cures. Shrinkage as the mix loses water pulls the slab in on itself, and if there is nowhere for that movement to go, a crack opens up somewhere. Add the heat swings that are normal across Qatar and the wider Gulf, where surface temperatures can move by 30 degrees or more between a summer afternoon and a winter night, and the expansion and contraction cycle keeps working on any weak point long after the concrete has set.
Settlement is the other common driver. Poor compaction under a slab, a badly prepared subgrade, or a foundation reacting to nearby excavation all show up as cracking months or years later. Some of this is cosmetic. Some of it is not: a crack that keeps widening, that runs through a load-bearing element, or that lets water reach the rebar and start corrosion, needs proper concrete repair and restoration, not a cosmetic fill.
Reading the crack before you touch it
Not every crack gets the same treatment, and this is the step people skip when they are in a hurry. First question: is it active or dormant? An active crack is still moving, usually from ongoing thermal cycling or settlement that has not finished. A dormant one has stabilized. Injecting a rigid epoxy into a crack that is still moving is asking for a repeat failure, because the resin cannot flex with the concrete around it.
Second question: is water involved? A dry structural crack and a crack that is actively weeping water call for different resins entirely, which is covered in step three. Width matters too. Hairline cracks under about 0.3mm can sometimes be left alone or treated with a surface sealant, while anything wider, or anything on a wall retaining water or soil, is a candidate for full injection. A short site visit with a crack gauge and a straightedge answers most of this before any product gets opened.
Step one: cleaning and mapping
Injection only works if the resin can actually get into the crack, so the surface has to be clean along its whole length. That means grinding off paint, dust, laitance and any old sealant, then vacuuming the crack out rather than blowing it with compressed air, which just pushes debris further in. On outdoor work in the region, this stage often takes longer than the injection itself because sand and grit work their way into every surface crack over time.
While cleaning, the crack gets marked out end to end and its full length recorded. A crack that looks like it stops at a corner sometimes continues behind a finish or under a skirting, and missing that section is how a repair ends up only half done.
Step two: setting the injection ports
Small surface ports, usually plastic or metal fittings sometimes called packers, get set directly over the crack at intervals along its length, typically spaced at roughly the same distance as the concrete is thick. Between the ports, the crack surface is sealed over with a fast-curing epoxy paste, which does two jobs: it stops resin escaping out of the crack face during injection, and it holds each port firmly in place under pressure.
Spacing the ports correctly is what decides whether the resin actually reaches the back of the crack or just fills the first few centimetres near the surface. Too wide, and the middle of the run never gets properly filled. Too many ports wastes material and time for no real gain.
Step three: injecting the resin
This is where the two main resin families split. Epoxy is the choice for structural repair: it is rigid once cured, bonds extremely well to concrete, and can restore load-carrying capacity in a cracked beam, column or slab. It needs a dry crack to bond properly, so it is not the right pick where water is actively coming through.
Polyurethane is the flexible option and the one used on active leaks, movement joints, and below-grade or water-retaining structures where a small amount of ongoing movement is expected. PU resin reacts with moisture, expands to fill voids, and cures into a flexible seal that can accommodate minor shifting without cracking again. Anyone weighing up options for a basement wall, a lift pit or a water tank should be having this epoxy-versus-polyurethane conversation before a single port goes up, not after.
Injection itself runs from the lowest port upward, using a hand pump or a low-pressure injection gun, feeding resin steadily until it appears at the next port along, at which point that port is capped and work moves to the next one. Pressure stays low and controlled throughout. High pressure does not push resin further into a fine crack, it just risks blowing out the surface seal.
Step four: curing, removal and finishing
Once every port shows resin and the run is complete, the material needs to cure fully before the ports and surface seal come off, which depends on the product and the site temperature rather than a fixed number of hours. Cutting this short and grinding too early can pull uncured resin back out of the crack.
After curing, the ports are ground flush and the surface seal is removed, leaving a thin repaired line rather than a raised bead. On visible concrete, this can be finished further to blend with the surrounding surface. On structural elements, a follow-up check confirms the crack has been filled along its full depth, not just capped at the surface. Where the surrounding concrete itself has spalled or lost cover to the rebar, injection alone will not fix that, and it is worth combining it with broader concrete repair and restoration work rather than treating the crack in isolation. On floors that also need to perform under foot and vehicle traffic afterward, it is worth checking whether the finish should be upgraded to epoxy flooring at the same time, rather than coming back to redo the surface separately later.
Crack injection rewards patience more than speed. The diagnosis at the start decides which resin gets used, the port spacing decides whether the fill actually reaches the back of the crack, and the cure time decides whether any of it holds. Skip a step and the crack is very likely to reopen in the same place within a year.