Spalled Parking Garage Beams With Post-Tensioning
A 25-year-old parking garage has spalling and delamination on the soffits of several 400 mm by 600 mm beams. Cover over the bottom reinforcement is about 20 mm. Half-cell potential readings are more negative than -350 mV over large areas, and chloride content near the bar depth is above the threshold in the local repair standard. The beams contain unbonded post-tensioning tendons in ducts. The engineer requires removal of unsound concrete to a minimum 25 mm depth around the perimeter of each patch, with sawcut edges. A subcontractor proposes using a 15 kg pneumatic chipping hammer to speed up the work and wants to chip within 20 mm of the tendon ducts. The garage must remain open to traffic below.
1. What are the main risks in the proposed demolition method, and what sequence should be followed before chipping begins?
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Main risks:
- Tendon damage: unbonded tendons carry high force. A chipping hammer near a duct can nick or crush the sheathing, expose the strand, or damage the anchorage. That can lead to loss of prestress or a sudden release of stored energy.
- Structural damage: impact tools can crack the beam beyond the repair area and loosen concrete around the tendons.
- Falling concrete: the garage is open below, so debris and spalled concrete are a public safety hazard.
- Corrosion acceleration: leaving chloride-contaminated concrete at the patch edges can cause the repair to fail.
Sequence before chipping:
- Locate the tendons and reinforcement with ground-penetrating radar or radiography, and mark them on the soffit.
- Erect containment and debris netting, and close or protect the area below.
- Sawcut the patch perimeter to the required depth to create a clean, square edge.
- Remove concrete with light hand tools, working away from the tendon path. Do not use a 15 kg hammer within the tendon zone; use a smaller chipping hammer or hand tools and maintain the specified clearance.
- Sound the area and extend the patch until sound concrete is reached.
If a tendon is exposed or damaged, stop and call the engineer and the post-tensioning specialist.
2. How should the repair material and application be selected for these patches?
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The repair material should match the substrate and the exposure:
- Low permeability and low shrinkage: a polymer-modified or silica-fume repair mortar with shrinkage compensation reduces cracking and debonding at the patch edges.
- Chloride resistance: the material should be suitable for a chloride-contaminated parking structure, and a corrosion inhibitor may be specified.
- Strength compatibility: the repair should not be so stiff that it concentrates stress at the interface. It should be compatible with the parent concrete and the expected movement.
- Bond: the substrate must be sound, clean, and roughened. A bonding agent or a slurry coat should be used only if the system requires it; some repair mortars are applied to a saturated-surface-dry substrate without a separate bond coat.
Application:
- Remove all unsound and chloride-contaminated concrete to the specified depth and profile the edges.
- Clean the reinforcement; if section loss is significant, the engineer must approve bar replacement.
- Pre-wet the substrate as required and apply the repair material according to the manufacturer's thickness limits. Deep patches may need multiple lifts.
- Cure the repair properly; curing is critical because repair mortars are thin and exposed.
- Protect the repair from traffic and drying until it has gained sufficient strength.
A repair that only fills the hole without addressing the chloride source and the tendon zone will likely fail again.
3. What quality-control checks should be made before the repair is accepted?
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Acceptance should be based on both condition and performance:
- Patch extent: verify that all unsound concrete was removed and that the sawcut perimeter and depth meet the specification. Hammer-sound the edges; a hollow sound indicates a bonded-area defect.
- Tendon and reinforcement condition: confirm that no tendon or bar was damaged. If reinforcement was exposed, check cover after repair with a cover meter.
- Surface preparation: check that the substrate is clean, roughened, and properly saturated or primed as required.
- Material quality: verify batch records, mixing water, pot life, and application thickness against the product data.
- Curing: confirm the curing method and duration.
- Bond and strength: where specified, use pull-off testing or another approved bond test. Visual inspection alone does not prove adhesion.
- Appearance and profile: the patch should be flush or slightly proud as specified, with no cracking or shrinkage separation at the edges.
- Documentation: record locations, quantities, test results, and any deviations.
If a patch fails a bond test or shows cracking, it should be removed and redone. The acceptance decision should be made against the project specification, not simply whether the patch looks better than the spall.