Concrete Spalling Repair: Treat the Steel, Then Rebuild the Concrete

Concrete spalling and exposed rusted rods: check the damage first

Ceiling plaster falling in chunks with the iron rods visible, rust stains running down a column, a chajja with its edge broken off, cracks along the line of the bars in a beam: all of these are the same problem at different stages. The steel inside the concrete is rusting, and rust takes up more room than the steel it replaces. The pressure cracks the cover concrete and finally pushes it off. This is concrete spalling. Replastering over it hides the rods for a season or two, and then the patch falls again, because the steel underneath is still rusting. Concrete spalling repair works only in one order: treat the steel, bond, then rebuild the concrete.

Find the right concrete repair method for spalled or cracked RCC

Match the damage to one route. Each route names the Armstrong products used for it and takes you to its section. Where more than one mortar suits the job, all of them are listed.

Treat the steel, then rebuild the concrete. All loose, hollow and flaking concrete and old plaster comes off first, back to sound material. Repair is always the first choice. Breaking out and recasting a member is the last resort, for concrete with no sound material left.

Concrete repair mortar finder: polymer, epoxy or micro concrete

Armstrong makes rebar coatings, polymer and epoxy bond coats, polymer-modified and epoxy repair mortars, micro concretes and an underwater epoxy mortar. Every one is a sound choice for its job. Answer three questions for a starting recommendation, then confirm it with an Armstrong expert and, for load-bearing members, a structural engineer.

Rebar corrosion treatment: open up, clean and protect the steel

Every structural concrete repair starts the same way, whatever mortar goes on afterwards. The concrete is opened until the steel is sound, and the steel is cleaned and protected before any cover goes back. Patching over cleaned but unprotected steel is why many repaired spalls reappear within a few monsoons.

1. 1
   Prop if needed

   Where a beam, column or slab is badly damaged, the engineer decides on propping before any concrete is removed.Why: A badly damaged member may not carry its own load once concrete is cut away, so the engineer decides on propping first.
2. 2
   Break out to sound concrete

   Chip back all hollow, cracked and loose concrete until the substrate is sound and the bar is no longer rusted. Follow the rust along the bar. Expose the bar all round so its back can be cleaned.Why: New mortar bonds only to sound concrete, and anything hollow or cracked left behind will fail again behind the repair.
3. 3
   Cut a square edge

   Cut the edges of the repair 10 to 15 mm deep so the mortar ends against a square shoulder and not a feather edge. This is stated on the ROOFCRETE AR 576 data sheet.Why: A feathered edge cracks and breaks off. A square shoulder gives the mortar something solid to end against.
4. 4
   Clean the steel

   Wire-brush or grit-blast the bars to remove rust and scale. Where brushing cannot reach, ALSTRONG RR-170 is brushed on, left for 15 to 30 minutes, scrubbed and rinsed, and the bar is primed as soon as it is dry.Why: Rust under a primer carries on and lifts the coating, and the corrosion goes on spalling the repair.
5. 5
   Check the bar

   If a bar has lost section, is pitted deeply or is broken, stop. The engineer decides on extra steel. New bars are lapped or anchored into the old concrete with RAPIDSTRONG AG, a polyester anchor grout of which 1 kg fills 0.5 litre.Why: A bar that has lost section is weaker and may be pitted too far to rely on, so the engineer decides on added steel.
6. 6
   Protect the steel

   Coat every exposed bar. The options are below.Why: Exposed steel rusts again fast. A coat of protection on every bar stops the corrosion that caused the damage.
7. 7
   Clean the concrete

   Remove dust, oil, soot and loose grit from the broken face, so the bond coat meets clean concrete.Why: The bond coat grips clean concrete, and dust, oil or soot form a layer it cannot bond through.

How to protect exposed reinforcement before concrete repair

Armstrong makes three coatings for rusted reinforcement. Each suits this job, so pick by the state of the steel and the system that follows, or ask an Armstrong expert.

Ceiling, soffit and stair repair: the standard sequence

When concrete has spalled and the rods are showing, the repair has to deal with the corroded steel first and the concrete second. The order below protects the steel, bonds the new mortar to the old concrete and cures it so it reaches full strength. Doses, thicknesses and coats are the standard figures; the site and the exposure decide where they move.

1. 1
   Chip back

   Chip off loose and deteriorated concrete until sound concrete is reached, then clean off dust and dirt.Why: New mortar bonds only to sound concrete; loose material underneath makes the repair hollow.
2. 2
   Protect the steel

   Clean the exposed bars of rust with emery paper and wire brushes. Fix extra bars by welding wherever the bar has lost section. Apply one coat of ALSTRONG ZP 777, the zinc-rich epoxy primer, to the bars.Why: Rust expands, and expanding rust is what burst the cover in the first place. Removing it and priming the bar stops the cycle, and welded bars restore the lost strength.
3. 3
   Bond coat

   Apply ROOFBOND SBR 401 mixed with cement over the concrete. For beams and heavy sections an epoxy bonding agent, ROOFBOND EP, is applied before concreting instead.Why: The bond coat joins the old concrete to the new mortar, which would otherwise shrink away from it.
4. 4
   Polymer mortar on the wet bond coat

   While the bond coat is wet, lay a 1:4 polymer mortar with ROOFCRETE AR 576 at 2 L per 50 kg of cement, 25 to 30 mm thick, finished neatly.Why: Laying the mortar on a wet bond coat gives a monolithic bond. The polymer makes the mortar dense and keeps water and chlorides away from the steel.
5. 5
   Cure

   Apply one coat of ROOFEXCURE WB over the mortar straight away.Why: Curing keeps the mortar from drying out in the first days, which is when it gains its strength.
6. 6
   Cracks in beams and slabs

   Inject cracks with the low-viscosity epoxy grout RAPIDSTRONG GROUT EP 403, mixed 1 : 0.5 base to hardener by weight. On columns, fill cracks with the three-component epoxy putty ROOFCRETE EP.Why: Epoxy restores the bond across the crack so the member acts as one piece again.
7. 7
   Add steel connectors

   Where a beam is jacketed, drill, clean the holes with compressed air and water, and anchor L-shaped 8 mm shear connectors with RAPIDSTRONG AG, the polyester resin anchor grout.Why: The connectors tie the new jacket to the old beam so they carry load together.

Ceiling concrete falling with rods visible: the Armstrong epoxy repair sequence

A spalled ceiling is the hardest place to make a repair hold. The mortar is applied overhead, the slab above may still be damp from a bathroom or terrace, and the cover is thin. For spalled concrete with exposed steel, the Armstrong specification is an epoxy system in three steps: ROOFBOND EP as the primer, ALSTRONG ZP 777 on the steel, and ROOFCRETE EP as the repair mortar.

1. 1
   Break out and clean

   Remove all loose, hollow and flaking concrete and plaster back to sound material, expose the rusted length of each bar and clean the steel, as in the steel section. The ROOFCRETE EP data sheet asks for a sound, clean surface: laitance removed by scabbling or grit blasting, and oil, grease and fungus by detergent scrubbing.Why: The epoxy bonds to sound concrete, so everything loose, hollow or flaking has to go, and the rust is exposed to be dealt with.
2. 2
   Prime with ROOFBOND EP

   Add the whole hardener tin to the base tin and mix thoroughly. The stated mix is Part 1 to Part 2 at 2:1, and the pot life is 90 to 120 minutes. Brush a thin, continuous film onto the prepared concrete with a stiff brush. On very porous or absorbent concrete a second coat follows straight after the first. ROOFBOND EP bonds to damp or dry concrete, but water must not be standing on the surface.Why: The primer soaks into the concrete and holds the epoxy mortar to it. The tin ratio has to be exact to cure properly.
3. 3
   Coat the steel with ALSTRONG ZP 777

   Apply the zinc-rich primer to all the exposed reinforcement. It is surface dry in 1 hour. ROOFCRETE EP is placed on the ROOFBOND EP primer while it is still tacky, within 30 to 50 minutes. The sequence used on site is: zinc-rich primer on the bars, then the bond coat, then the mortar on the wet bond coat, then curing.Why: The zinc-rich primer protects the bars where the cover has failed, so the rust does not restart.
4. 4
   Mix ROOFCRETE EP

   Pour the hardener into the base and mix for 3 minutes with a slow-speed drill. Add the silica filler slowly while mixing until the mortar is uniform. Where graded aggregate is added for a deeper fill, reduce the filler to suit. Pot life is 90 to 120 minutes at 27°C.Why: Mixing the base and hardener fully, and adding the filler as stated, gives a mortar that cures evenly and reaches its stated strength.
5. 5
   Place the mortar on the tacky primer

   Apply ROOFCRETE EP while the primer is still sticky. Pack it tightly around and behind the bars to a uniform thickness, level it and finish with a steel trowel. Do not overwork the surface.Why: Placed on tacky primer, the mortar bonds as one piece. Packed tightly round the bars, it leaves no voids.
6. 6
   Clean up and let it cure

   Clean tools with acetone straight after use. The epoxy mortar cures without water and is unaffected by humidity. It reaches 93 N/mm² in compression and 21 N/mm² in flexure at 7 days, with a bond to concrete of 4 N/mm².Why: Epoxy tools must be cleaned before it sets, and the mortar cures without water, so the repair is protected from damage while it hardens.
7. 7
   Finish the ceiling

   Once cured, the ceiling is finished and painted, and the leak above is fixed.Why: The leak above is what started the corrosion, so it is fixed too, or the repair fails again.

A ceiling almost never spalls on its own. Look for the bathroom, balcony or terrace above it. See bathroom waterproofing and terrace waterproofing for the leak that fed the rust.

Structural repair mortar for columns, beams, chajjas and honeycombed concrete

Most spalled columns, beams, chajjas, balcony edges and dry ceiling patches are rebuilt with a polymer-modified structural repair mortar. The polymer makes the mortar bond, resist shrinkage cracking and keep water, chlorides and carbon dioxide away from the steel, which a plain cement and sand patch does not do. This is Armstrong's reinstatement sequence: steel primer, bond coat, repair mortar placed while the bond coat is tacky, and curing straight away.

How to repair a cracked or spalled RCC column, beam or chajja

1. 1
   Open up and treat the steel

   Break out, cut square edges, clean the bars and coat them with ALSTRONG ZP 777, ALSTRONG AC or ALSTRONG RC, as in the steel section.Why: The steel is treated before the concrete goes on, since a repair over rusting steel fails.
2. 2
   Apply the bond coat

   Brush on a slurry of ROOFBOND SBR 401 and cement at 1:1 by volume. Where the repair is heavily loaded or the concrete is damp, use ROOFBOND EP as the bond coat. The ROOFCRETE AR 576 data sheet allows either, as site conditions require.Why: The bond coat holds the new mortar to the old concrete, which would otherwise shrink away from it.
3. 3
   Mix the mortar

   For ROOFCRETE AR, mix the polymer liquid with the water, at the stated 2 litres of liquid and 4.5 to 5.5 litres of water per 25 kg bag of powder, then add the powder and mix for 3 to 4 minutes with a slow-speed drill paddle. For a site mix, add ROOFCRETE AR 576 to the gauging water first, mix for 2 minutes, then add the cement and sand. The stated dosage is 2 to 5 litres per 50 kg bag of cement.Why: The polymer and the water have to be in the stated proportion for the mortar to have its stated strength and bond.
4. 4
   Place it wet on wet

   Apply the mortar while the bond coat is still wet and tacky. Press it hard against the substrate and all around the bars. The data sheet gives an average layer of 20 mm. Build deeper repairs in layers.Why: On a tacky bond coat the mortar and concrete bond together, but on a dried coat the bond is lost.
5. 5
   Finish

   Strike off with a straight edge and finish with a wooden or plastic float or a damp sponge.Why: A struck-off, floated surface is dense and even, and it sheds water instead of holding it.
6. 6
   Cure at once

   Spray ROOFEXCURE WB straight after finishing, or moist-cure for at least 7 days. A repair mortar that dries too fast cracks before it gains strength.Why: Repair mortar that dries early cracks and loses strength, so water or a curing compound keeps the moisture in.

Chajja broken at the edge and balcony concrete repair

A chajja or balcony slab is thin, exposed to rain on top and rusts from its edge inward. The repair is the same sequence with two extra points. Rebuild the edge against a board fixed as formwork, so the mortar is supported and the profile is true. Then form a drip groove under the edge and waterproof the top, because the water that rusted the edge came from above. For more than about 50 mm of build-up, the ROOFCRETE AR 576 data sheet recommends a polymer-modified reinstatement concrete, made by adding graded aggregate in equal proportion.

Honeycomb repair in concrete columns and beams

Honeycombing is concrete with voids left by poor compaction. It is a defect from casting, not from rust, but it gives water and air a direct path to the steel. Shallow honeycombs are cut back to sound concrete and rebuilt with ROOFCRETE AR over a bond coat. Deep honeycombs and voids that run into the member are pressure grouted with a non-shrink grout made with RAPIDSTRONG GROUT ADMIX at 250 g per 50 kg of cement. See injection grouting for the method. A dormant structural crack with no rust is bonded by injecting RAPIDSTRONG GROUT EP 403.

Micro concrete for column jacketing, deep repairs and difficult conditions

When the damage is too large or too deep to trowel, the repair is cast. Formwork is fixed around the member and a micro concrete is poured or pumped in. Micro concrete is a pre-packed, free-flowing, non-shrink concrete with small aggregate. It fills tight spaces around congested steel without a vibrator. This is the route for whole column faces, beam soffits and sides, jacketing and strengthening, and it is always designed by a structural engineer. For beam jacketing with shear connectors, slab strengthening with added steel and retrofitting a whole building, see structural strengthening and retrofitting.

How to strengthen an old column with micro concrete jacketing

1. 1
   Engineer's design

   The engineer fixes the propping, the extra bars and links, the jacket thickness and the grade of micro concrete.Why: A column carries the building, so the engineer designs the extra steel, the jacket and the propping.
2. 2
   Break out and treat the steel

   Remove all loose and delaminated concrete to a sound base and treat the existing steel, as in the steel section.Why: The jacket bonds to sound concrete, and rusting steel inside it would burst it from within.
3. 3
   Fix the new steel

   New bars are tied and anchored into the existing concrete. Starter bars and dowels are fixed in drilled holes with RAPIDSTRONG AG. See grouting and anchoring.Why: New bars have to be tied and anchored into the old concrete to share the load with it.
4. 4
   Bond coat

   Apply the bonding agent to the prepared concrete and let it become tacky. ROOFBOND EP has an overlay time of 2 to 3 hours at 30°C, which gives time to close the formwork.Why: The bond coat holds the micro concrete to the old column, and it has an overlay time that must be kept.
5. 5
   Fix watertight formwork

   The formwork must be tight, with a pouring point and a way for air to escape.Why: The micro concrete is fluid, so leaking formwork loses grout and leaves voids, and air needs a way out.
6. 6
   Mix and pour without a break

   Mix the micro concrete with the stated amount of clean water until it is smooth and free of lumps. Pour or pump it from one side, straight after mixing.Why: A smooth, lump-free mix poured without a break flows into every space in the jacket and has no cold joint.
7. 7
   Strike and cure

   After the formwork is removed, cure with water for 7 days.Why: Cured with water for a week, the micro concrete reaches its strength and does not crack.

Concrete repair on damp surfaces, under chemicals and under water

When repair is not enough: the extreme case

After the repair: curing, protection and stopping concrete spalling in new work

A repair that is not cured cracks. A repair that is cured but left open to the same water spalls again a few years later, often right beside the patch. Three things finish the job.

How to prevent rebar corrosion and spalling in new construction

Spalling is easier to prevent than to repair. Four things at the time of casting help keep water and air away from the steel.

1. 1
   Give the steel its cover

   Use cover blocks so every bar has the cover the drawing asks for. Thin cover is the most common cause of early spalling on ceilings and chajjas.Why: Thin cover is the commonest cause of rebar corrosion, because the steel is too close to the weather.
2. 2
   Make dense, watertight concrete

   Add an integral waterproofing compound such as CONPROOF LW 200 at 200 ml per 50 kg bag of cement and compact well, so there are no honeycombs. See waterproof concrete.Why: Dense concrete keeps water and chlorides away from the steel. An integral compound reduces the water paths.
3. 3
   Cure fully

   Cure for the full period, or spray ROOFEXCURE WB where water curing is difficult. It is sprayed 1 to 2 hours after the concrete is placed, or straight after the shuttering is struck.Why: Concrete that dries out early stays porous, and porous concrete lets corrosion in.
4. 4
   Waterproof the wet areas

   Terraces, bathrooms, balconies and chajja tops are waterproofed before finishes. See new construction waterproofing.Why: Terraces, bathrooms and balconies leak water into the slab, and that water is what corrodes the steel later.

Polymer or epoxy repair mortar: compare Armstrong concrete repair products

Every product below is a sound choice for the job it is made for. They differ in chemistry, in how deep they can be placed and in the conditions they tolerate. Figures are from Armstrong data sheets and brochures.

Which repair mortar for which concrete damage

Concrete repair chemicals and products by stage

How much repair mortar for a spalled ceiling patch

Approximate quantities for a ceiling patch of 1 sq.m, about 10.8 sq.ft, broken out to an average depth of 25 mm, with 6 m of 10 mm rods exposed. The volume to fill is 25 litres. Figures are worked out from each product's stated yield, density or coverage. An Armstrong expert confirms the final quantity after seeing the site.

Standards and tests for structural concrete repair

• IS 456:2000 is the Indian code for reinforced concrete. Its cover and crack width limits are what the original concrete should have met.
• ROOFCRETE AR 576 is tested to BS 4551 Part 14:1980 for setting time and BS 1881:1970 for water absorption.
• ROOFBOND EP strengths are tested to BS 6319. It is also listed against AS 4020:2018.
• ROOFCRETE MC 240 is listed against BS 1881 Part 116:1983 and ASTM C1107.
• ROOFEXCURE WB complies with ASTM C-309 Type 1D Class B and IS 15865:2018.
• Site checks: no hollow sound when the cured repair is tapped, no cracks at the edges of the patch, and no fresh rust stain after the next monsoon.

Concrete repair material calculator

Enter the area and average depth of the repair and the exposed steel. The calculator gives the volume to fill and the steel coating, bond coat, repair mortar and curing compound. Quantities only.

Seven mistakes that make a concrete repair fall off again

1. Replastering over rusted rods. The rust keeps growing under the plaster and pushes it off again.
2. Repairing only what has fallen. The hollow area around it is next. Tap, mark and open all of it.
3. Cleaning the front of the bar only. Rust left on the back restarts the damage. Expose the bar all round.
4. Skipping the steel primer. Cleaned steel with no protection rusts again under the new cover.
5. No bond coat, or a bond coat that has dried. The mortar then sits on the old concrete and comes away. It goes on while the bond coat is tacky.
6. Feather edges. A thin edge cracks and lifts. Cut a square edge 10 to 15 mm deep.
7. No curing and no waterproofing above. The mortar cracks, the water returns, and the steel rusts beside the patch.

Concrete spalling repair questions and answers