There are some other basic considerations
which we think are important.
- 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.
- 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.
- 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
- 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.
- 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
Sent: Wednesday, September 17,
2003 7:50 PM
Subject: Re: Repairing Older
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
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
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.
09/17/2003 02:52 PM
Repairing Older Concrete
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.
K. Adams, S.E.