Crack Injection And Structural Concrete Repair

Crack injection is a reliable repair for the right crack and a waste of money for the wrong one.

The difference is whether the crack is still moving. A dormant crack can be bonded and the member restored. A live crack injected with a rigid resin will crack again, usually within millimetres of the original.

Establish the cause before choosing the repair

Every repair decision follows from why the crack is there.

Structural overload, meaning the member is carrying more than it can. Injection does nothing useful; the member needs strengthening or the load needs reducing.

Shrinkage and restraint, which produces cracks early in life that then stabilise. Good candidates for injection once movement has stopped, and the mechanism is in [shrinkage and creep in concrete structures](/blog/shrinkage-and-creep-in-concrete-structures).

Thermal and moisture movement, which is cyclic. A crack that opens and closes with the seasons is not a crack to glue shut; it is a joint the structure wanted and did not get, and the answer is in [expansion and control joints in concrete](/blog/expansion-and-control-joints-in-concrete).

Settlement, which may be ongoing or complete. Injecting before it has stopped is premature, and the diagnosis is in [differential settlement and what causes it](/blog/differential-settlement-and-what-causes-it).

Corrosion of the reinforcement, which is the most common cause of cracking in older concrete and the one where injection is actively wrong. The crack is a symptom of expanding steel, and sealing it traps the problem. This is the condition described in [concrete cancer in apartment buildings](/blog/concrete-cancer-in-apartment-buildings).

Serviceability cracking under load, which is normal in reinforced concrete and does not need repair at all unless the width exceeds the limit for the exposure. The thresholds are in [crack width in concrete and when it matters](/blog/crack-width-in-concrete-and-when-it-matters).

Measure the movement rather than guessing

A crack monitor, or a set of pins read over months across a seasonal range, settles the question.

Which is a delay nobody wants and it is cheaper than the repair that fails. A few months of readings tells you whether you are injecting a dormant crack or detailing a joint, and those are different projects.

The injection options

Epoxy resin, rigid, high strength, and capable of restoring structural continuity across the crack. The right choice for a dormant crack in a member whose capacity needs restoring. It requires a dry or near dry crack and it will not tolerate movement.

Polyurethane resin, flexible and water reactive, which expands on contact with water and seals a leaking crack. The right choice for water stopping in a wet crack, and it does not restore structural capacity.

Cementitious grout, for wide cracks and voids rather than fine ones.

Which means the two most common uses are different jobs with different materials: restoring strength with epoxy, or stopping water with polyurethane. Specifying one for the other is the usual error, and a leaking basement crack injected with epoxy is a crack that leaks again.

Doing it properly

Clean the crack and seal the surface so the resin travels through the member rather than out of the face.

Install ports at a spacing related to the member thickness, so the resin reaches the full depth.

Inject from the lowest point upward on a vertical crack, so air is pushed ahead of the resin instead of being trapped.

Watch for resin at the next port, which is the only evidence the crack is actually filling rather than the resin finding a void.

Core a sample afterwards on anything consequential, because a cut core through the repair is the only direct proof of penetration. A surface that looks neat tells you nothing about the middle of the member.

The repairs that are not injection

Patch repair, where cover concrete has spalled and the reinforcement is exposed. The steel is cleaned or replaced, treated, and a repair mortar applied. The critical part is cutting back to sound concrete well beyond the visible damage, because a patch over partially corroded steel fails at its edge within a couple of years.

Cathodic protection or corrosion inhibitors, where chloride contamination is widespread and patching individual areas would be endless. The context is in [coastal exposure and concrete durability](/blog/coastal-exposure-and-concrete-durability).

Externally bonded plates or fibre reinforcement, where capacity has to be added rather than restored. A strengthening design rather than a repair, as in [strengthening an existing slab for a new load](/blog/strengthening-an-existing-slab-for-a-new-load).

Doing nothing, which is the correct answer for a crack within the serviceability limits that is not leaking and not corroding. More repair money is spent on acceptable cracks than on unacceptable ones.

Who specifies it

The repair method, the material, the extent and the acceptance criteria are an engineering specification rather than a contractor selection.

Which matters because the cost difference between the right repair and the wrong one is usually small, and the difference in outcome is a repair that lasts decades against one that is back in three years. The assessment that precedes it is the work in [balcony remediation structural assessment](/blog/balcony-remediation-structural-assessment) and [structural assessment after a fire](/blog/structural-assessment-after-a-fire).

ACSES provides structural engineering and remediation assessment across Sydney. Talk to us about a project.