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Repair Damages to Concrete Structures

Concrete can be easily damaged by carelessness, poor construction methods, failure of formwork, and even under design, or collapse of structure during construction stage or post construction stage. Generally once concrete is cracked, or crushed, nothing much can be salvages, and demolition, and reconstruction would be a simpler and better solution. However in some cases where damage is localised, eg a corner of a concrete column chipped off due to accidental impact, repairs can be made if the structural integrity of the concrete can be ascertained. In case of doubts, demolition and reconstruction should be employed.

Below is a picture taken from a Concrete Pier supporting Steel Truss crossing a river. It was ripped apart by their anchor bolts when the steel truss collapse due to poor construction method. Upon examination of the Pier Column, Foundation, and surrounding soils, none were damaged other than the localised concrete base supporting the steel trusses. It was thus decided by the Engineers to simply rectify the damaged concrete portions and in case all the bars and anchors.

Damaged Concrete Piers 

The Method Statement for the repairs are as follows :

  1. First check that there are no damages or cracks in any parts of the pier and that the foundation is still intact. If all in good condition proceed rectification works as described below: Similar Checks should also be made for the Upstream Piers and Foundation and any supporting soils and structures. 
  2. Using small hand held mechanical tools, strip all damaged concrete to at least 300mm below the lowest damaged portions, more if required to reach a good strata.
  3. The exposed concrete should be “horizontal” and “roughen” exposing the coarse aggregates. Wire brush all aggregates to remove any loose coatings and concrete pieces. Ensure the final finish is rough and coarse, clean.
  4. Strengthen all the bent bars and wire brush until raw iron are exposed. Any exposed rebars can be cutoff if desired, except that a minimum lap length of 45 dia should be maintained. Remove all links or loose bars.
  5. Install New Lap Bars and Links as before, assuming same detail with the lap length of 45mm dia minimum.
  6. Install the Base Plates as before. Note that to reuse any steelworks, they must be wire brush clean to raw metal, otherwise new base plate must be installed. This end is preferably a Sliding Joint.
  7. Before Re-Concreting, apply a primer if available from a Sika Repair Products suitable for this case, or equivalent. This will provide better adhesion and cleaner bonding surface with new concrete.
  8. Concrete as before, using leaner mix and ensure water or concrete do not bleed through the formworks.
  9. Allow to set for 30 days before fixing the any new loadings. You can use rapid hardening concrete if required, but it is better to let the concrete cure naturally. During the process of curing, care should be taken to ensure rapid drying out would not occur by wrapping damp cloth around, so a uniform temperature is provided.
  10. Caution - Just patching up the damaged portions with new concrete will not work as the Old and New Concrete will not mix and bond well and thus a shear surface is created, structure will split when loaded. If the pier is subjected to high horizontal shears, shear keys will need to be constructed and very lean concrete be cast into them. Also the Rebars must be wire cleaned to raw metal and any coatings of mortar must be removed. Where possible sand blasting is to be used for important structures.

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Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng)
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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng) MEC Engineers, Civil & Structural Engineer HP: 016-8672189, Email: Widget: email cloaker