Codes referenced
Indian code gives short repair guidance; most technical detail comes from ACI 546R and ACI 562.
Comprehensive American guide: assessment, surface preparation, material selection, application methods, quality control.
Code requirements for assessment, design, and quality assurance of structural repairs.
International federation model code for repair of concrete structures, European perspective.
Rebound hammer + UPV for assessment before repair; core test per IS 516; patch compressive strength per IS 4031 Pt 11.
Assessment before any repair
The first step determines everything else. Without proper assessment, even the best repair fails. The standard sequence:
- Visual mapping — crack pattern, spall extent, efflorescence, rust staining. Photo-document every defect with scale.
- NDT survey per IS 13311 — rebound hammer (IS 13311 Pt 2) and UPV (IS 13311 Pt 1) to map strength variation and detect delamination.
- Cover survey — pachometer or profometer to locate rebar and measure cover. A common defect cause is cover < specified.
- Chloride profile — drill powder samples at 5 mm increments, titrate for chloride content. Threshold 0.6 kg/m³ of concrete for carbon-steel rebar in reinforced concrete, per IS 456 Cl 8.2.5.2 and Table 7.
- Carbonation depth test — phenolphthalein spray on fresh core. Depth > cover means depassivation has reached the rebar.
- Core sampling per IS 516 — for petrographic examination and compressive strength. 75 mm dia minimum, three cores per defect zone.
- As-built verification — compare actual rebar size, spacing, cover with drawings. Many "design defects" are construction defects.
Method 1 — Crack injection
Crack injection forces a resin into the crack under pressure, bonding the two faces back together (epoxy) or sealing against water ingress (polyurethane).
- Epoxy injection — structural repair for dry, dormant cracks ≥ 0.2 mm wide. Epoxy viscosity 200-500 cps, bond strength 5-15 MPa. Used in beams, columns, slabs. Not for active (moving) cracks.
- Polyurethane injection — flexible, water-reactive. Expands 5-10× on contact with water to form a watertight gasket. Used in basements, water tanks, expansion joints, where future movement is expected.
- Procedure: clean the crack surface, mount injection ports at 200-300 mm spacing along the crack, seal the crack between ports with epoxy paste (V-cut if needed to seat the seal), inject from the bottom port upward until resin appears at the next port, move to next port, repeat.
- Acceptance: cores through the injected crack (after cure) should show resin penetration to full crack depth. Pressure-test for water-tightness if required.
Method 2 — Jacketing (section enlargement)
Jacketing restores or adds structural capacity to a column or beam by enlarging its cross-section. Used when load capacity needs to increase, or when significant concrete has spalled off.
- Concrete removal — chip back to sound concrete, ≥ 25 mm beyond the last visible defect. Avoid damaging existing rebar.
- Rebar preparation — clean exposed rebar to bright metal (wire brush or sandblast), replace heavily corroded bars with new bars lap-welded or lapped to existing. Add new corner bars and stirrups at the larger section.
- Shear connectors — welded studs or bent bars anchored into the existing concrete, typically 12 mm dia at 300 mm c/c, to transfer shear between old and new concrete.
- Bonding coat — epoxy or SBR bonding agent on the old concrete surface (still tacky when new concrete is placed).
- Micro-concrete or shotcrete — pour or spray the new layer to the enlarged dimensions. Formwork on three sides, open at top for placement, or shotcrete applied directly.
- Curing — minimum 7 days wet curing before load is applied to the enlarged section.
Method 3 — FRP wrap
Fibre-reinforced polymer (FRP) wraps — carbon (CFRP), glass (GFRP), or aramid (AFRP) — bonded to concrete with epoxy to add flexural, shear, or confinement strength. CFRP is the strongest and most common; GFRP is cheaper but lower modulus.
- Surface prep — grind concrete to expose aggregate (CSP 3-5 profile per ICRI), fill bug-holes, round corners to 10 mm minimum radius.
- Primer — apply epoxy primer to consolidate the surface, let cure to tack-free.
- Putty — trowel epoxy putty to level any surface irregularities.
- Resin + fibre — wet lay-up: apply saturating resin, lay fibre fabric, roll to impregnate, repeat for additional plies. 2-4 plies typical for column confinement.
- Cure — 24-48 hours at ambient; post-cure at elevated temperature for full properties.
- Protective coating — UV-resistant topcoat for exposed FRP, fire-protection coating where fire rating is required.
Method 4 — Patch mortars
Patch mortars fill small spalls and surface defects. Two chemistries cover most cases:
- Polymer-modified cementitious — SBR or acrylic-modified mortar, applied 6-50 mm thick, used for general repair. Bonds well, breathes, can be trowelled to profile.
- Epoxy mortar — 100% solids epoxy + aggregate, applied 3-25 mm thick, used for chemical resistance, rapid return to service, thin patches where cementitious would shrink.
- Application: square-cut the perimeter of the patch (avoid feather-edges), clean substrate to sound concrete, dampen for cementitious or prime for epoxy, build up in lifts if thickness exceeds 25-50 mm, trowel to profile, cure 3-7 days.
Method 5 — Shotcrete
Shotcrete is pneumatically projected concrete or mortar at high velocity onto a prepared surface. Two processes: dry-mix (water added at nozzle, less equipment, dusty) and wet-mix (water pre-mixed, less rebound, better for thick layers). Used for large repair areas (typically > 50 m²), tunnel linings, retaining walls, and where formwork is impractical.
- Trial panels mandatory — shoot a 1 m × 1 m panel before the actual repair to confirm crew skill, equipment settings, and bond.
- Application thickness — 50-100 mm in one pass for vertical, 40-60 mm overhead. Build up in multiple passes for thicker sections.
- Finishing — screed to profile, then trowel. Curing compound or wet burlap for 7 days.
- Quality — cores through the shotcrete per ASTM C1144 / IS 13846 to confirm bond and compressive strength.
Repair material selection
ACI 546R §6 groups repair materials by application thickness, exposure, and required set time. Generic selection rules: thin patches (< 25 mm) use polymer-modified or epoxy mortars; thick patches (> 50 mm) use concrete or shotcrete; water-active cracks need polyurethane; structural cracks need epoxy; large areas use shotcrete. Match the repair material stiffness to the substrate — too stiff and the repair debonds from creep and thermal movement; too flexible and it creeps under load.
Source: ACI 546R-14 §6 Material selection; FIB Bulletin 33 §5.2.
Worked example — spalled column repair
| Step | Action |
|---|---|
| 1. Assessment | Visual: cover spalled 200 × 300 mm, rebar exposed and corroded. Rebound hammer: 35 MPa (acceptable). Cover scan: 25 mm. Carbonation depth: 18 mm — below the 25 mm cover, so the carbonation front has not reached the reinforcement. |
| 2. Preparation | Chip back to sound concrete, 25 mm beyond visible spall. Sandblast corroded bars to bright metal. Replace two heavily-corroded bars with 12 mm dia Fe500, lap-welded 50d. |
| 3. Jacket reinforcement | Add 4 × 12 mm corner bars + 8 mm ties at 150 mm c/c. Weld 12 mm shear studs at 300 mm c/c into existing concrete. |
| 4. Bonding + pour | Apply epoxy bonding coat. Form three sides, pour M35 micro-concrete with 10 mm aggregate, 100 mm jacket thickness. |
| 5. Cure | Wet burlap + 7 days water curing. Strip formwork at 3 days. |
| 6. Test | Cube test 7 and 28 days: 28 and 38 MPa. Rebound hammer at 28 days on jacket: 36 MPa (acceptable). Load test on jacketed column. |
| Result | Column capacity restored + 20% additional capacity from jacket. Repair accepted. |
Common deviations seen on Indian sites
Skipping NDT, cover survey, and chloride profile means the underlying cause is not addressed. A patch over corroding rebar fails in 1-2 years. Assessment must drive the repair strategy, not the other way around.
If carbonation depth exceeds cover, rebar corrosion will continue even after patch repair. Apply anti-carbonation coating on the entire repaired surface, not just the patch. Or use corrosion-inhibiting admixture in the patch.
New concrete / mortar will not bond to old without a bonding coat (epoxy or SBR). Feather-edging and direct application gives a debond in months. Always square-cut the perimeter and use a bonding agent.
A jacket adds concrete + rebar but only works if load transfers from the old to the new section. Shear connectors, surface preparation, and bonding are not optional — they are the load-transfer mechanism.
Field-checklist blocks
A short questionnaire, an inspector's checklist, and three worked examples that show how assessment and material selection resolve into an accepted repair.
Questionnaire — asked at the assessment stage, before the method is written into the method statement
- Has the defect been assessed before a method was chosen? Visual mapping with scale photographs, NDT per IS 13311, cover survey, chloride profile, carbonation depth on a fresh core, and 3 cores of 75 mm dia minimum per defect zone per IS 516. The assessment drives the repair strategy, not the other way round. A patch over corroding rebar fails in 1 to 2 years.
- What does the chloride profile say, sampled at 5 mm increments? The threshold is 0.6 kg/m³ of concrete for carbon-steel rebar in reinforced concrete, per IS 456 Cl 8.2.5.2 and Table 7.
- Is the crack dry and dormant, and is it at least 0.2 mm wide? Then epoxy, for structural cracks in beams, columns and slabs. If it is water-active or still moving, it is polyurethane, and epoxy must never go into a moving crack.
- Is the spall shallow enough for a patch mortar, and does the method statement say which chemistry? Polymer-modified cementitious goes on 6 to 50 mm, epoxy mortar on 3 to 25 mm, and the perimeter is square-cut, never feather-edged.
- Has enough concrete come off that a jacket is the right answer rather than a patch? Jacketing needs the section chipped back at least 25 mm beyond the last visible defect, new corner bars and stirrups, shear studs at 12 mm dia and 300 mm c/c, a bonding coat, and 7 days of wet curing before load is applied.
- Is the area large enough for shotcrete, and has the trial panel been shot and accepted? Above 50 m² shotcrete beats forming and mortaring, and a 1 m × 1 m panel comes first, every time.
- For FRP, is the substrate ready? Ground to a CSP 3 to 5 profile per ICRI, bug holes filled, corners rounded to 10 mm minimum radius, primer cured to tack-free, putty levelled, and the plies rolled to impregnate.
- Has the underlying cause been addressed, not just the symptom? Carbonation past the cover, chloride above the threshold, or cover below the specified value all keep working after a patch unless they are dealt with across the whole repaired surface.
Inspector's checklist — what to verify on site
- Assessment pack: defect map with scale photographs, rebound hammer and UPV per IS 13311 Pt 2 and Pt 1, pachometer or profometer cover survey, chloride profile at 5 mm increments, phenolphthalein on a fresh core, and 3 cores of 75 mm dia minimum per defect zone for petrographic examination and strength.
- As-built verification: actual rebar size, spacing and cover against the drawings. Many defects that look like design defects are construction defects, and the repair changes when you know which.
- Injection record: ports at 200 to 300 mm spacing, crack sealed between ports with epoxy paste, injection from the bottom port upward until resin appears at the next port, cure before coring, and acceptance by a core through the injected crack showing resin penetration to full crack depth.
- Jacketing: chipping ≥ 25 mm beyond the visible defect, exposed bars cleaned to bright metal by wire brush or sandblast, heavily corroded bars replaced and lap-welded, new corner bars and stirrups at the larger section, 12 mm dia shear studs at 300 mm c/c, epoxy or SBR bonding coat applied while still tacky, formwork on three sides, and 7 days of wet curing before load.
- FRP: CSP 3 to 5 surface with corners at 10 mm radius, epoxy primer to tack-free, putty levelled, saturating resin and fibre rolled to impregnate (2 to 4 plies typical for column confinement), 24 to 48 hours cure at ambient, UV-resistant topcoat, and fire protection where a fire rating is required.
- Patch mortar: square-cut perimeter, substrate cleaned to sound concrete, dampened for cementitious or primed for epoxy, built up in lifts where thickness exceeds 25 to 50 mm, trowelled to profile, cured 3 to 7 days.
- Shotcrete: trial panel 1 m × 1 m accepted before the real work, application at 50 to 100 mm per pass for vertical work and 40 to 60 mm overhead, screeded to profile then trowelled, cured with compound or wet burlap for 7 days, and cored per ASTM C1144 / IS 13846 for bond and compressive strength.
- Close-out: 7 and 28 day cube results, an in-place check on the repair, and confirmation that the cause (cover, chloride, carbonation) was treated across the repaired surface rather than at the patch only.
What happens if… three worked examples
What happens. The rebound hammer reads 35 MPa and the cover scan reads 25 mm, so the parent concrete is sound and the problem is loss of section. That makes jacketing the right answer, not a patch. Chip back 25 mm beyond the visible spall, sandblast the corroded bars to bright metal, replace the two heavily corroded bars with 12 mm dia Fe500 lap-welded at 50d (which on a 12 mm bar is 600 mm), add 4 × 12 mm corner bars and 8 mm ties at 150 mm c/c, weld 12 mm shear studs at 300 mm c/c, apply an epoxy bonding coat, form three sides and pour M35 micro-concrete with 10 mm aggregate to a 100 mm jacket.
Likely outcome. Wet burlap for 7 days, formwork stripped at 3 days. The 28-day cube reads 38 MPa against M35 (38 ≥ 35) and the rebound hammer on the jacket reads 36 MPa. The column's capacity is restored and the jacket adds 20 percent more. Repair accepted.
What happens. Two opposite problems in the same repair package. The 60 mm spall exceeds the 50 mm maximum single lift for a polymer-modified cementitious patch, so it is built up in two lifts rather than one (30 + 30 will do, and each lift stays inside the 6 to 50 mm application range). The 4 mm spall is below the 6 mm minimum for cementitious patch mortar, which cannot accommodate aggregate packing and tends to shrink and crack, so it either gets built up to 6 mm or it is filled with epoxy mortar, where the 3 mm minimum applies.
Likely outcome. One visit, the right material in each place. In both cases the perimeter is square-cut, the substrate is cleaned to sound concrete and dampened for cementitious or primed for epoxy, the patch is trowelled to profile and cured 3 to 7 days. Feather-edging is what produces the debond in months, and it is the one shortcut this method does not allow.
What happens. At 60 m² the area is past the 50 m² threshold where shotcrete becomes the sensible choice and formwork becomes impractical. Thickness then sets the number of passes: 120 mm cannot go on in one pass, because the vertical limit is 50 to 100 mm per pass. Two passes of 60 mm each work comfortably. Overhead, the limit is 40 to 60 mm per pass, so the same 120 mm needs three passes of 40 mm.
Likely outcome. A 1 m × 1 m trial panel is shot first to confirm crew skill, equipment settings and bond, and only then is the real area shot. The surface is screened to profile and trowelled, cured with compound or wet burlap for 7 days, and cores taken per ASTM C1144 / IS 13846 to confirm bond and compressive strength. The trial panel is not paperwork. It is the only thing standing between you and a 60 m² surface that has to be cut off.
Frequently Asked Questions
When should epoxy injection be used versus polyurethane injection?
What is the minimum patch mortar thickness for a concrete repair?
Is FRP wrapping a permanent repair solution?
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Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years experience in construction QA/QC, concrete technology, RMC plant operations, consulting and technical training across India. For repair specification review, FRP design check or on-site repair supervision, contact amit@concreteinfo.in.
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