Concrete surfaces to which additional concrete is to be bonded shall be
prepared for receiving the next horizontal lift by cleaning the
construction joint surface with either air-water cutting, sandblasting,
high-pressure water jet, or other approved method. Concrete at the side of
vertical construction joints shall be prepared as approved by the
Contracting Officer. Air-water cutting shall not be used on formed
surfaces or surfaces congested with reinforcing steel. Regardless of the
method used, the resulting surfaces shall be free from all laitance and
inferior concrete so that clean surfaces of well bonded coarse aggregate
are exposed and make up at least 10-percent of the surface area,
distributed uniformly throughout the surface. The edges of the coarse
aggregate shall not be undercut. The surface of horizontal construction
joints shall be kept continuously wet for the first 12 hours during the
24-hour period prior to placing fresh concrete. The surface shall be
washed completely clean as the last operation prior to placing the next
lift. For heavy duty floors and two-course floors a thin coat of neat
cement grout of about the consistency of thick cream shall be thoroughly
scrubbed into the existing surface immediately ahead of the topping
placing. The grout shall be a 1:1 mixture of portland cement and sand
passing the 2.36 mm No. 8 sieve. The topping concrete shall be deposited
before the grout coat has had time to stiffen.
3.1.2.1
Air-Water Cutting
Air-water cutting of a fresh concrete surface shall be performed at the
proper time and only on horizontal construction joints. The air pressure
used in the jet shall be 700 kPa 100 psi plus or minus, 70 kPa 10 psi, and
the water pressure shall be just sufficient to bring the water into
effective influence of the air pressure. When approved by the Contracting
Officer, a surface retarder complying with the requirements of COE CRD-C 94
may be applied to the surface of the lift in order to prolong the period of
time during which air-water cutting is effective. After cutting, the
surface shall be washed and rinsed as long as there is any trace of
cloudiness of the wash water. Where necessary to remove accumulated
laitance, coatings, stains, debris, and other foreign material,
high-pressure waterjet or sandblasting shall be used as the last operation
before placing the next lift.
3.1.2.2
High-Pressure Water Jet
A stream of water under a pressure of not less than 20 MPa 3,000 psi shall
be used for cutting and cleaning. Its use shall be delayed until the
concrete is sufficiently hard so that only the surface skin or mortar is
removed and there is no undercutting of coarse-aggregate particles. If the
waterjet is incapable of a satisfactory cleaning, the surface shall be
cleaned by sandblasting.
3.1.2.3
Wet Sandblasting
Wet sandblasting shall be used after the concrete has reached sufficient
strength to prevent undercutting of the coarse aggregate particles. After
wet sandblasting, the surface of the concrete shall then be washed
thoroughly to remove all loose materials.
3.1.2.4
Waste Disposal
The method used in disposing of waste water employed in cutting, washing,
and rinsing of concrete surfaces shall be such that the waste water does
not stain, discolor, or affect exposed surfaces of the structures, or
SECTION 03300A
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