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RE: Repairing Older Concrete

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There are some other basic considerations which we think are important.


  1. Try to keep the repair strength compatible with the existing concrete strength.  There have been problems in the past with high strength repairs (7000 or 8000 psi and higher repair mortars are not uncommon) on much lower strength columns (say 4000 psi) failing because the repair mortar is relatively stiffer (higher modulus) which causes it to eventually delam.
  2. If the depth of repair doesn’t engage the rebar/tie cage, at times we have used small diameter (1/4”) threaded stainless rods embedded in the sound concrete with a 1” hook on the end.  This can help to tie things together more positively than just bond.  Usually we won’t require clearing around the rebar if the depth of repair is less than ½ depth of the rebar.
  3. Proper surface prep is the most important thing in any repair.  We typically will specify high pressure water blast (5000 psi or more) after chipping.  We also typically avoid bonding agents (particularly epoxy bonding agents).  You don’t need them if you have prep’d the surface properly, and if the bonding agent is not done properly it acts as a bond break and will delam the repair.
  4. For chipping, specify a light hammer, say 15 lbs up to maybe 30 lbs.  Anything higher will cause more harm than good.  After chipping, to avoid feathered edges, a sawcut or ground edge about 3/8” to ½” deep is usually sufficient.
  5. Inspection is as always important.  Consider inspector observing mixing of components, observing surface prep prior to placement, and possibly taking some cube samples for testing.  These also depend on how critical the repair is as well as your confidence in the contractor (ie specialty repair contractor or basic concrete contractor).


Eric Ober


-----Original Message-----
From: THunt(--nospam--at) [mailto:THunt(--nospam--at)]
Sent: Wednesday, September 17, 2003 7:50 PM
To: seaint(--nospam--at)
Subject: Re: Repairing Older Concrete



Not knowing the extent of the damage, here are some basic guidelines for concrete repair of the columns assuming the damage is local and overall the columns are structurally sound:

·         The damaged areas should be chip out to sound material.

·         The removed pocket should have edges that are perpendicular to the column face (i.e. do NOT feather out the edges).

·         Stop chipping if sound material is found before or at the face of the first rebar.

·         If chipping is required past the rebar continue so you have at least one inch clear behind the rebar.

·         Fill the pocket with a good prepackaged nonshrink grout from Master Builders, SIKA, 5-Star, etc.

·         If the pockets are deep you may need to extend the grout with pea gravel.  Consult the manufacturer for proper grout type and installation procedures.

·         Cure grout in accordance with the manufacturer's recommendations.

Thomas Hunt, S.E.
ABS Consulting

"Dave Adams" <davea(--nospam--at)>

09/17/2003 02:52 PM

Please respond to






Repairing Older Concrete




Hello all,
We did an investigation in the past where an existing concrete column had some fairly severe damage due to fork lift traffic, but our work was called off because the Owner made other arrangements for facilities and a new building.  We will be doing another investigation here shortly that may involve the same type of concrete column damage.  I've got my hands on all the references I need, but I'd like to solicit some practical advice from those on the list who have dealt with this, including issues such as shrinkage, prep work, curing, etc.
Dave K. Adams, S.E.
Lane Engineers, Inc.
Tulare, CA