When Concrete Cracks: Repair the Pathway, Strengthen the Concrete
A crack in concrete is more than a visible defect.
It can become a direct pathway for water, chlorides and other contaminants to penetrate the concrete matrix. When those contaminants reach reinforcing steel, corrosion can begin or accelerate—leading to internal expansion, delamination, spalling and progressive loss of durability.
That is why durable crack repair must accomplish more than filling an opening. It must address the concrete surrounding the crack and reduce the conditions that caused—or could accelerate—the deterioration.
Why Concrete Cracks
Concrete can crack because of:
Plastic or drying shrinkage
Thermal contraction and expansion
Improper curing
Settlement or structural movement
Traffic, vibration and repeated loading
Freeze-thaw exposure
Reinforcing-steel corrosion
Chemical reactions within the concrete
Poor joint placement or construction practices
Before selecting a repair method, the crack should be evaluated to determine its width, depth, activity and likely cause. Active structural cracks, displacement and continuing water infiltration may require evaluation by a qualified engineer.
The Hidden Risk: What Enters Through the Crack
The crack itself may not be the greatest threat. The real danger is what the crack allows into the concrete.
Moisture can carry chlorides and other contaminants deeper into the structure. As permeability increases and reinforcing steel loses its protective environment, corrosion can develop beneath the surface.
This can create a destructive cycle:
Cracks admit moisture and contaminants.
Moisture and chlorides reach reinforcing steel.
Corrosion products expand around the steel.
Internal pressure creates additional cracking and spalling.
New openings allow even more contamination.
Covering the visible crack without addressing this cycle can leave the underlying deterioration mechanism active.
Why Filling the Crack May Not Be Enough
Routing, sealing, injection, patching and coatings all have legitimate applications. But a repair material placed into weak or porous concrete is only as dependable as the substrate supporting it.
A surface-only repair may not:
Strengthen the sides of the crack
Reduce permeability in the surrounding concrete
Address microcracking beyond the visible opening
Protect against future moisture and chloride entry
Improve the bond between the existing concrete and repair material
Reduce the risk of continued crack propagation
SURTREAT approaches crack repair as a system: stabilize the surrounding concrete, seal the opening and protect the repaired area against future exposure.
Ottawa LRT Stage 2: Crack Repair in a Demanding Tunnel Environment
The Confederation Line Extension tunnels in Ottawa, Ontario, presented a clear example of why crack repair must address both the opening and the surrounding concrete.
During construction, cracks developed in the tunnel’s walls, ceilings and portal wing walls. They were identified as shrinkage cracks likely associated with concrete placement and curing.
Although shrinkage cracks are not necessarily signs of structural failure, their location created a serious durability concern. The openings provided possible entry points for moisture, road overspray and chloride-bearing water. Left untreated, these pathways could contribute to additional cracking, reinforcing-steel corrosion and long-term degradation. Ottawa LRT tunnel case study
Two Crack Categories—Two Levels of Treatment
The cracks were divided into two categories:
Cracks less than 0.5 mm: These received surface protection intended to prevent water infiltration.
Cracks 0.5 mm or greater: These required a more intensive process to strengthen the concrete, stabilize the crack and seal the opening.
This distinction mattered. A fine crack and a larger crack do not necessarily require the same repair procedure. Matching the intervention to the crack condition allowed the project team to target treatment where it was needed without imposing an unnecessarily invasive repair throughout the structure.
A Challenging Concrete Surface
The tunnel concrete had been cast against steel forms and treated with curing compounds. This produced smooth, relatively nonporous surfaces that initially hindered penetration.
That condition highlights an often-overlooked part of crack repair: surface preparation and application technique directly affect performance. A penetrant cannot work inside the concrete unless it is given adequate access to the pore structure and crack faces.
The Ottawa application therefore incorporated repeated treatment, brushing and controlled curing to promote penetration.
Step 1: Stabilize and Strengthen With TPS II
For cracks measuring 0.5 mm or greater, TPS II was applied three successive times. The material was brushed into the cracks to encourage penetration and then allowed to cure for 24 hours.
TPS II was used to strengthen the sides of the cracks and the surrounding concrete. On the portal wing walls, it was also applied more broadly to improve the concrete’s tensile capacity and reduce the likelihood of additional cracking.
The case study reports an increase in tensile strength of up to 400 psi.
This step is important because it changes the repair sequence. Instead of immediately placing a filler into the existing crack, the surrounding substrate is treated first. The objective is to create stronger, more reliable crack faces capable of supporting the subsequent repair.
Step 2: Seal the Cracks With SurCoat
After the TPS II treatment cured, SurCoat was worked into the larger cracks with a spatula.
The material was extended approximately one inch beyond each side of the crack to:
Completely bridge and seal the opening
Create a continuous repair
Improve the transition between repaired and existing concrete
Produce a durable, smooth finish
SurCoat is a non-shrink, cement-based repair coating containing corrosion inhibitors. Its cementitious character allowed the repaired areas to blend with the surrounding tunnel concrete in color and texture.
Step 3: Protect Against Water With Repel WB
After the cracks were stabilized and sealed, Repel WB was applied as the final protective treatment.
Two layers were installed wet-on-wet over the repaired cracks, creating hydrophobic protection intended to reduce the penetration of water, road overspray and chloride-bearing moisture.
Cracks narrower than 0.5 mm received Repel WB directly as their designated moisture-protection treatment.
Because Repel WB works within the concrete rather than forming a heavy surface film, the objective was to reduce liquid-water entry while preserving the concrete’s ability to breathe.
The Ottawa Results
The completed treatment delivered three complementary levels of protection:
Crack stabilization: TPS II strengthened the concrete adjacent to the cracks and helped resist continued propagation.
Durable sealing: SurCoat closed the larger openings and provided a non-shrink, cementitious repair.
Moisture resistance: Repel WB established hydrophobic protection against water and chloride ingress.
The repaired areas were described as seamless and compatible with the surrounding concrete’s texture and color. More importantly, the work addressed the full durability pathway—from the crack faces to the completed surface.
What the Ottawa Project Teaches Us
The Ottawa tunnel application demonstrates a basic but powerful principle:
A crack should be treated as a pathway through a concrete system—not merely as a line on its surface.
A more complete repair strategy asks:
What caused the crack?
Is the crack active or dormant?
Is the adjacent concrete strong enough to support the repair?
What contaminants can enter through the opening?
Is embedded steel at risk?
How will the repaired area be protected from future moisture?
By treating the surrounding concrete first, sealing the opening second and protecting the surface third, the Ottawa repair addressed both the immediate defect and the conditions that could compromise its long-term performance.
Applications Beyond Tunnels
This systems approach can be adapted for cracks in:
Parking garages and elevated decks
Bridges and transit structures
Condominium balconies and walkways
Foundations and basement walls
Marine and waterfront structures
Industrial and utility facilities
Water and wastewater infrastructure
Precast concrete
Post-tensioned structures
Don’t Just Fill the Crack—Improve the Concrete Around It
Some cracks require structural injection. Others require flexible treatment because movement continues. Severely deteriorated concrete may need removal and replacement.
But when the existing concrete can be preserved, SURTREAT offers a less invasive strategy:
Diagnose the crack.
Strengthen the surrounding substrate.
Seal the opening.
Reduce moisture and contaminant intrusion.
Protect the concrete from within.
The Ottawa LRT Stage 2 tunnel project shows what becomes possible when crack repair is treated as a durability system rather than a cosmetic correction.
Repair the crack. Strengthen the concrete. Protect the structure.
JF Jad