Structural Strengthening and Retrofitting: Restore, Then Add Strength

Repair, strengthening or retrofitting: know which job you have

Repair puts a damaged member back to what it was. Strengthening makes it carry more than it was built for. Retrofitting is strengthening an existing building to meet a newer need, such as an extra floor, a change of use, heavier loads or earthquake safety. Most old buildings need both: the damage is repaired first, then strength is added where the engineer's check shows it is short.

Signs that a building needs more than a patch repair

• Cracks across beams or slabs, or cracks that keep growing after they are filled.
• Ceiling or beam concrete falling off with rods visible and thinned by rust.
• Sagging slabs or beams, or floors that bounce.
• An extra floor, a water tank, machinery or a change of use that adds load.
• An older building in a higher seismic zone, or one damaged by fire or a flood.

Assess the structure before anyone designs a fix

Strengthening is designed from facts about the existing concrete and steel, not from the look of the cracks. The engineer combines a survey, a few tests and the original drawings where they exist.

For reinforced concrete buildings, the Indian guideline for evaluating and strengthening existing buildings is IS 15988:2013. It was written for earthquake safety and lists methods such as RC jacketing, steel jacketing, fibre wrapping and new shear walls or bracing. The engineer chooses the method and the details.

Restore cracked beams, slabs, columns and walls with epoxy injection

Cracks that have stopped moving are filled with a low-viscosity epoxy so the member acts as one piece again. This is done before any jacket or new concrete is added, because a jacket adds strength but does not rejoin a cracked core. This is the sequence Armstrong applicators use on beams, ceilings, pillars and external walls.

1. 1
   Open and clean the crack

   Widen the crack along its length with a cutting machine to a groove, and clean out dust and loose material.
2. 2
   Drill and fix the packers

   Drill 12 to 14 mm holes into the crack at about 300 mm centres, to about 75 mm deep. Clean each hole with water and blow it dry with compressed air. Fix aluminium or steel non-return packers in the holes with the instant-setting plugging compound RAPIDSTRONG PLUG 31.Why: The surface must be sealed and the packers fixed firm, or the epoxy escapes at the face instead of travelling into the crack.
3. 3
   Seal the surface of the crack

   Seal the crack face between the packers so the grout stays inside.
4. 4
   Inject the epoxy

   Inject RAPIDSTRONG GROUT EP 403, mixed 1 : 0.5 base to hardener by weight, packer by packer from the lowest point up, until each packer refuses. Site specifications allow about 0.5 kg of mixed grout per packer.Why: The specification on these jobs calls for an injection epoxy reaching 70 N/mm² compressive, 30 N/mm² flexural and 15 N/mm² tensile strength at 7 days, tested to BS 6319.
5. 5
   Remove the packers and seal

   Cut the packers off and seal the holes with plugging compound, about 0.2 kg per packer.
6. 6
   Fill surface cracks on columns

   Where a column has surface cracks, cut a U-groove about 6 mm wide and 12 mm deep. Prime it with the epoxy bonding agent ROOFBOND EP and fill it with the three-component epoxy mortar ROOFCRETE EP.

More on crack injection, packers and pumps: injection grouting.

Beam jacketing with shear connectors and added steel

A beam jacket adds new steel and new concrete around the old beam, tied to it with shear connectors. This is the sequence used to jacket beams on a large heritage building, to the engineer's drawing.

1. 1
   Scaffold and prop the beams

   Erect scaffolding and fix shuttering supports under each beam before any concrete is removed.Why: Chipping takes load-bearing concrete away. The beam must be propped until the jacket has gained strength.
2. 2
   Chip to sound concrete

   Chip the beam with pneumatic power chisels until the hard parent concrete is reached. Remove the debris and clean the surface with a water jet.
3. 3
   Treat the exposed steel

   Remove loose rust scale with emery paper and wire brushes, and give the exposed bars one coat of the zinc-rich primer ALSTRONG ZP 777.
4. 4
   Anchor the shear connectors

   Drill holes to the depth on the drawing and clean them with compressed air and a water jet. Fix L-shaped 8 mm shear connectors in them with the polyester resin anchor grout RAPIDSTRONG AG. The specification on this job called for an anchor grout of 75 N/mm² compressive strength that resists fresh and salt water.
5. 5
   Fix the new reinforcement

   Fabricate the new bars to the drawing, place them and tack-weld them to the shear connectors.
6. 6
   Apply the bond coat

   Apply one coat of the epoxy bonding agent ROOFBOND EP to the old concrete just before the new concrete goes in.Why: New concrete does not stick well to old concrete on its own. The epoxy bond coat makes the jacket and the beam work together.
7. 7
   Cast the jacket

   Fix formwork and pour the jacket in a non-shrink micro concrete such as ROOFCRETE MC 240 or the fibre-reinforced ROOFCRETE PFMC 122, to the thickness on the drawing. Micro concrete flows around closely spaced bars without a vibrator.
8. 8
   Inject the cracks and cure

   Inject any remaining cracks in the beam with epoxy, as in the previous section. Keep the jacket moist for at least 7 days, then remove the props when the engineer allows.

Slab and ceiling strengthening with added steel and polymer concrete

Where a slab soffit has lost cover and steel, new bars are tied to it with shear connectors and covered with a polymer-modified concrete. This is the sequence used on a large ceiling strengthening job, together with epoxy injection of the ceiling cracks.

1. 1
   Prop, chip and clean

   Prop the slab, chip the damaged concrete back to sound concrete and clean the surface and the bars.
2. 2
   Treat the steel

   Remove rust from the existing bars and prime them with ALSTRONG ZP 777.
3. 3
   Fix shear connectors and new bars

   Fix shear connectors in drilled holes with RAPIDSTRONG AG. Fabricate the new reinforcement to the engineer's sketch, place it and tack-weld it to the connectors, with extra bar supports where needed.
4. 4
   Bond coat

   Apply the bond coat over the prepared concrete.
5. 5
   Lay the polymer concrete

   While the bond coat is still wet, lay polymer screed concrete made with ROOFCRETE AR to an average of about 50 mm, with a neat finish. Where a site mix is used, the specification is 1:2:4 with 6 to 12 mm graded aggregate and ROOFCRETE AR 576 at about 2 litres per 50 kg bag of cement.
6. 6
   Inject the ceiling cracks

   Inject the cracks in the ceiling with RAPIDSTRONG GROUT EP 403 through packers, as on the injection section.
7. 7
   Cure

   Cure the new concrete with water or a curing compound such as ROOFEXCURE WB.

External walls, columns and new anchors

The same order holds on every member: sound concrete, clean steel, a bond coat, then the new material. These are the other strengthening sequences Armstrong applicators carry out.

Structural repair of external RCC walls

1. 1
   Chip and clean

   Chip off loose concrete until a sound substrate is reached, then clean off dirt and dust. Work from a cup-lock scaffold.
2. 2
   Clean and add steel

   Clean the bars free of rust with sandpaper and wire brushes. Weld in new bars where the engineer asks for them.
3. 3
   Prime the steel

   Apply one coat of the two-component zinc-rich epoxy primer ALSTRONG ZP PRIMER 210.
4. 4
   Bond coat

   Apply one coat of ROOFBOND SBR 401 mixed with cement, at about 300 g per sq.m.
5. 5
   Rebuild with fibre-reinforced mortar

   While the bond coat is wet, apply the fibre-reinforced polymer repair mortar ROOFCRETE AR to an average of about 30 mm, with a neat finish.
6. 6
   Cure at once

   Spray ROOFEXCURE WB as soon as the mortar is finished. Inject structural cracks in the wall with epoxy, as on the injection section.

Column jacketing

Columns are jacketed with new ties and bars and a cast micro concrete sleeve. The full sequence is on the structural repair page: how to strengthen an old column with micro concrete jacketing. Retrofit the beam-column joints with the columns. Jacketing only the column can move the weak point to the joint.

Starter bars, dowels and anchors

New bars for jackets, extensions and added members are fixed into the old concrete in drilled holes with the chemical anchor grout RAPIDSTRONG AG. Hole size, depth and spacing come from the engineer. See machine grouting and anchoring.

Dams, weirs and other heavy structures

Weir and baffle blocks have been repaired with consolidation grouting, polymer mortar on the honeycombs and 16 mm dowel bars anchored about 600 mm deep. See infrastructure waterproofing and repair.

Products used in strengthening, and other methods

These are the Armstrong products used at each stage of the sequences on this page.

Other strengthening methods

Steel jacketing with angles and plates, fibre-reinforced polymer wrapping, and new shear walls or steel bracing are also used to strengthen buildings. They are designed member by member by the structural engineer. Each still needs the cracks injected and the damaged concrete repaired before it goes on.

Structural strengthening and retrofitting questions

Short answers to the questions owners and engineers ask most.

What is the difference between structural repair and retrofitting?

Repair restores a damaged member to its original strength. Retrofitting adds strength to an existing building so it can take more load or meet earthquake requirements. Most jobs on old buildings do both, repair first.

Can I strengthen a beam or column without a structural engineer?

No. The size of the jacket, the added steel, the connectors and the propping all come from the engineer's check. The applicator carries out that design.

Why is epoxy injected before jacketing?

A jacket adds strength around the member but does not rejoin a cracked core. Injection makes the old concrete act as one piece first.

Why are shear connectors needed?

They tie the new steel and concrete to the old member so the two carry load together. Without them the jacket can separate from the beam.

Can the building be used during the work?

Often yes, area by area, if the members being worked on are propped and the engineer agrees. Loads above the working area are kept off until the new concrete has cured.

Which Armstrong products are used most?

Epoxy injection grout for cracks, an epoxy bonding agent for old-to-new concrete, a chemical anchor grout for connectors and dowels, zinc-rich primer on the steel, and micro concrete or polymer mortar for the new section.