The Civil Engineering Contractor September 2018 | Page 31

INSIGHT
“Shrinkage is the most common force on repairs, with most
failures being cracking and delamination in which the repair
material no longer adheres to the initial fault. The tensile
strength of the repair material is greater than the bond
strength to the host concrete.”
“The most common type of repair
is patching, which is involved in
up to 60% of all repairs. Steel
corrosion and water damage are the
most common causes of concrete
failure, especially in Johannesburg.
To thereafter perform a proper
repair requires understanding the
root cause of the failure, whether
structural or wear and tear. This
is crucial — you cannot simply
perform a symptomatic repair.
Often the cause of failure is that
solutions are rushed to try cover up
the underlying problem,” he says.
“Shrinkage is the most common
force on repairs, with most failures
being cracking and delamination
in which the repair material no
longer adheres to the initial fault.
The tensile strength of the repair
material is greater than the bond
strength to the host concrete. This
brings about shrinkage of the patch
repair, either by drying shrinkage
or actual shrinkage of the material.
When I hear people say the repair
was ‘left to dry’, that is wrong — it
‘cures’, not ‘dries’.
“In the case of insufficient bond to
host, either the material or bonding
agent is not correct. Shrinkage in
the material arises due to unstable
formulation that allows shrinkage,
or on incorrect water:cement ratio
used in the mixing. Primarily, the
repair material has to be such that
it will adhere to the concrete and
make it monolithic again,” says
Dillon. Success, he explains, is 90%
in the preparation.
Repair agent specifics
“A lot of repair agents are tailored
to a specific climate and are made
robust to conditions. Therefore,
great care needs to be taken in its
selection. Moisture needs to stay
in the repair — we suggest an
epoxy system that has sufficient
‘open time’. Otherwise the bonding
strength can be higher than that of
the host connection,” says Dillon.
Cracks can appear in the repair due
to the bond between host and repair
being stronger than the tensile
strength of the material. Such
cracks in the repair are most often
associated with loss of moisture
— the bond between the host and
the new material is greater than
the tensile strength of the green
uncured repair.
“The host and repair materials must
be of similar strength or it will
not act like a monolithic structure.
Some products give a lot of early
strength — but what of, say, 28 or
56 days later? If the repair material
is too strong it is simply shifting a
problem from one area to another.
You are walking a tightrope in
structural repairs,” says Dillon.
“In achieving a good repair job that
will not later result in cracking or
curling, teamwork is required —
no one aspect is more important
than another. The cause must be
clearly identified and understood,
and the repair material must be
aligned with this and the specs of
the manufacturer. It is not a one-
size-fits-all — each repair requires
a unique treatment. For this reason,
subsequent failure often comes from
lack of experience and knowledge.”
In conclusion
• T
 he bonding agent must have
sufficient strength to form a
structural bond, but allows
enough open time for application
while a suitable final set allows
for tensile stress relaxation to
avoid curling.
• Repair material has a suitable
strength curve: it must achieve a
high enough early strength; yet,
with a final strength not so high
as to make it brittle.
• 
Repair material is robust and
resists shrinkage in its many
forms. Polymer modified for
adhesion and to retain moisture
to resist drying shrinkage. Has
an advanced cementitious design
to compensate for various stages
of cement shrinkage.
• 
Repair material has sufficient
open time for good placement.
Has an initial set and strength
gain curve to allow material not
to create excess stress. nn
Brian Dillion, director of Stanton
Construction Chemicals offers practical
steps when repairing concrete.
CEC September 2018 - 29