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far from the adjacent joints and cracks as possible but within the void area, so the grout can flow
from the injection hole toward the joint or crack.
7.5. Drilling Holes. Grout holes may
be drilled with pneumatic, hydraulic,
or diamond core drills.
An important factor is hole size. Holes should not be larger than 2 inches (50 millimeters) in
diameter. The downward pressure, whether by hand or mechanical means, should be less than
200 pounds (91 kilograms), particularly at the bottom portion of the slab. Excessive down
pressure can cause breakout of the concrete adjacent to the injection hole. This breakout can
seriously weaken the slab and may result in premature cracking. Usually, the breakout material
drops in such a way that it seals the hole, thereby preventing the grout from reaching the void.
The grout holes should be drilled vertically and round and to a depth completely through the base
material. Grout holes should not be left ungrouted overnight and preferably should be grouted
within 4 hours.
7.6. Grout Mixtures. Grout mixtures for subsealing are typically cement grouts consisting of
approximately one part portland cement to three parts pozzolan, either natural or artificial, or
three parts limestone dust with enough water to achieve the required consistency. Other
additives may include super plasticizers, water reducers, fluidifiers, expanding agents, and
calcium chloride. Each must be tested and evaluated in the laboratory to ensure compatibility of
the materials. Consistency should be checked by a flow cone (CRD-C 611/ASTM-C-939) at least
twice each day.
Flow cone time varies between 9 and 20 seconds depending on the types of materials used in the
grout mix. Typically flow cone times for limestone grouts are 16 to 22 seconds. Flyash grouts
generally have flow times from 10 to 16 seconds. Strength requirements of the grout mixtures
should be specified; a common requirement is 600 pounds per square inch (4,134 kilopascals) at
7 days as determined by ASTM-C-109.