Concrete repair: patching, sprayed concrete and crack injection

How bridges get built · Finishing, articulation and maintenance · Concrete repair: patching, sprayed concrete and crack injection

Also called: patch repair, spall repair, hydrodemolition repair, sprayed concrete repair, shotcrete repair, crack injection, epoxy injection

How it is done

Diagnose first: map spalls and delamination (hammer sounding, GPR), cover, carbonation depth, chloride profile and half-cell potentials; find the cause (leaking joint, low cover, alkali-silica reaction) and choose the repair principle (EN 1504-9).
Mark out repair areas beyond the delamination, saw-cut the perimeter to avoid feather edges, and prop the member where breakout will reduce its capacity.
Break out by hydrodemolition or light breakers to sound concrete and about 20 mm behind corroded bars so the repair locks around the steel.
Blast the bars clean, add or replace bars where section loss exceeds the engineer's limit, and apply a bar primer only where the repair system calls for one.
Prepare the substrate (clean, rough, saturated surface dry) and rebuild with a compatible material: hand-applied polymer-modified mortar for small areas, poured or pumped flowable concrete behind formwork for deep repairs, wet-mix sprayed concrete for large vertical and soffit areas.
Cure by wet methods or curing compound for several days, protect from early load and vibration where possible, then apply a protective coating or silane where specified.
Inject cracks: seal the surface, set ports, and inject low-viscosity epoxy (structural bond across dry, dormant cracks) or polyurethane (wet or leaking cracks) from the lowest port upward until resin appears at the next.
Test: sound repairs for hollows, pull-off bond tests, cores through injected cracks to check penetration, and record repair locations for future monitoring.

Key numbers

Breakoutabout 20 mm clear behind corroded bars; perimeter saw-cut about 10–20 mm deep
Structural repair mortars (EN 1504-3)class R4 at least 45 MPa compressive and 2.0 MPa bond; class R3 at least 25 MPa and 1.5 MPa
Epoxy injectionseals cracks as narrow as about 0.05 mm
Chloride thresholdcorrosion risk rises above about 0.2–0.4 % chloride by mass of cement at the bar
Durabilitysurveys of European repairs (CONREPNET) found a large share deteriorating within about 10 years, mostly from poor diagnosis or workmanship

Where it fits

Spalling and delamination from reinforcement corrosion.
Vehicle and vessel impact damage.
Dormant cracks from shrinkage, restraint or past overload.
Preparation for overlays, FRP strengthening or cathodic protection.

Where it does not

Patching without removing the cause (leaking joints, blocked drains).
Repair materials much stiffer or less permeable than the substrate over large areas.
Rigid epoxy injection of cracks that are still moving.
Breaking out around prestressing steel without a structural check.

Choosing it

Choose patch repair alone when damage is local and caused by low cover or carbonation; add cathodic protection or galvanic anodes when chlorides are high outside the patches, or the repairs will trigger new corrosion at their edges.
Choose sprayed concrete over hand-applied mortar for large areas, soffits and vertical faces; choose formed and poured concrete for deep repairs and congested reinforcement.
Choose epoxy injection to restore strength across dry, dormant cracks and polyurethane for wet or leaking cracks; seal live cracks with a flexible system instead of injecting them.
Prefer element or deck replacement when the repair area is a large share of the surface or the cause cannot be removed.

Plant, pace and money

PlantHydrodemolition units, breakers, saws, blast equipment, mortar mixers, wet-mix sprayed concrete pumps, injection pumps, access platforms.

ProductivityIndicative: a few to about 20 m² of patch per crew per day depending on depth and access; sprayed concrete considerably more on large open areas.

CostIndicative: USD 300–1,500 per m² of patch area depending on depth and access; crack injection USD 50–200 per metre of crack.

Risks and controls

What goes wrong

Incipient anode corrosion around the patches.
Debonding and shrinkage cracking of the repair.
Loss of capacity during breakout.
Hidden corrosion of prestressing steel.
Dust, noise and falling debris over traffic.

Quality assurance

Inspection of breakout depth and bar cleaning before placing, hollow sounding after cure, pull-off and compressive tests, injection cores, as-built repair maps.

Origins

Patching with cement mortar is old practice; it became an engineered discipline in the 1980s and 1990s as chloride damage on post-war bridges grew, with ICRI guidance in the US and the EN 1504 series in Europe defining repair principles and material classes.

Examples

The bulk of most bridge owners' concrete maintenance budgets: pier crossheads under leaking joints, deck soffits, edge beams and marine piles.

Case studies

Champlain Bridge, MontrealCanada · 1990s–2019Years of concrete repair and strengthening of the salt-damaged precast prestressed approach spans, including a temporary steel 'super beam' installed in 2013 under a cracked girder, kept the 1962 bridge open until its replacement opened in 2019.

Midland Links motorway viaducts, BirminghamUK · 1980s onwardLong-running repair programmes on the elevated M5/M6 of the late 1960s and early 1970s, whose crossheads and columns were damaged by de-icing salt leaking through deck joints; patch repair was combined with cathodic protection and strengthening.