19 June 2001
Chemical Admixtures for Concrete; Types A, D, E, F and G are
approved but not required.
Preparation and Construction. To obtain quality concrete repairs,
the following basics are mandatory for all jobs:
Properly prepare the surface of the old concrete to be adjoined.
Ensure a good bond between the old and new concrete.
Do not add more water than specified for the concrete mix.
Do not patch across active cracks or joints.
Cure the concrete properly.
7-18.104.22.168 Surface Preparation. All deteriorated concrete must be removed
down to sound concrete. For some old concretes, exposed surfaces will soften
after a few days of exposure; therefore, the surfaces should be checked closely
before patching. Clean the old surface thoroughly just before placing new
Normally, when removing concrete hand tools or light duty hand-held
power tools are used, particularly around the edges, to prevent damaging the
remaining concrete. Do not use an impact hammer greater than 67 Newtons (15
pound). Edges should be square, preferably cut by sawing to about a 25-mm (1
inch) depth. Feathered edges must be absolutely avoided. Reinforcing bars
should be exposed around their entire circumference by a 25-mm (1 inch)
clearance. Sandblasting surfaces removes loose concrete fragments and
scaling rust from steel. Once the steel is clean, protect the coating with a slurry of
Portland cement grout or latex modified Portland cement grout. This procedure
improves the life of the repair.
7-22.214.171.124 Bonding. Before patching, the existing base concrete should be kept
damp (except for epoxy concrete repair) for several hours, preferably overnight.
Remove free water or shiny wet areas by vacuuming or with oil-free compressed
air. Then scrub a bonding agent into the surface with a stiff brush. The bonding
agent can be Portland cement mortar or a latex modified Portland cement mortar.
Do not use epoxy bonding agents. The mortars should be one-part cement and 1
part sand passing the No. 30 sieve, and have a consistency of thick cream. In all
cases, it is important to place the repaired concrete immediately after placing the
7-126.96.36.199 Curing. Concrete used in repairs must be protected and cured more
carefully than usual. Curing should last for 7 days. The old concrete could absorb
moisture too fast from the new concrete, or the temperature of the old concrete
could be too low to permit early development of strength of a concrete patch.