by Dennis Smith and Peter
During a recent remodel of a local building, the contractor
rented a gas-powered cutoff saw and attempted to cut a new
opening through an 8-inch-thick concrete-block interior wall.
After about an hour's hard labor, he'd managed to outline the
opening to a 1-inch depth and hack out a shallow recess where
the structural steel lintel would be installed. The dust
generated in the process reduced visibility to near-zero and
coated every surface. The job was made even more daunting by
the discovery that the blocks had been solid-filled during
construction. At this point, contractor and client agreed that
the situation called for stronger measures and called us.
Diamonds Are a Guy's Best
Diamond abrasives and concrete sawing have witnessed a
technological leap in their manufacture over the past five
years. The design and performance of industrial diamonds have
been dramatically improved, and diamond tool prices have come
down. Concrete sawing is now so economical that some tract
builders are actually pouring their foundations with blank
walls, allowing the homeowners to decide later where they want
their window and door openings.
Our company, Concrete Services of Cape Cod, has been
wet-sawing, coring, and altering concrete structures for six
years. The diamond abrasive saw blades we use yield neat,
accurate cuts with a smooth, terrazzo-like surface (see Figure
1). Correcting errors in the height of a pour or sawing back a
misplaced jog in a stepped foundation are all in a day's work.
Foundation contractors make their share of small errors, too,
like forgetting to allow for wood trimmers in a rough opening
or not dropping the footing across a garage entry.
1. Crisp edges and a smooth, terrazzo-like
surface are characteristic of wet-sawn concrete.
But our bread-and-butter is cutting access openings between
new crawlspace foundation additions and existing foundations,
and opening up walls to expand a foundation or add an exterior
We can schedule a couple of openings per day if the distance
between jobs isn't too great. The main on-site requirement is a
garden hose connection. Concrete sawing calls for plenty of
water to cool the blade and flush out the cut.
The tool of choice for many of these cuts is our Cushion Cut
diamond wall saw, which is available in hydraulic, electric,
and pneumatic models (Figure 2). Our pneumatically driven saw
operates at a low 90 to 100 psi, but requires the high-volume
output of a heavy-duty 185 hp, trailer-mounted compressor. We
can handle cuts up to 18 inches deep using this tool, but we
aren't usually called on to saw anything thicker than 8 to 10
inches. Although the saw itself is rated for blade diameters
from 24 to 42 inches, the largest diameter requires a
lower-rpm, higher-torque motor than the one we use. Our
workhorse — a 24-inch-diameter blade that costs about
$600 — is good for handling aged concrete, hard
aggregate, and moderate steel reinforcement.
Cut diamond wall saw can handle 24- to 42-inch-diameter
blades and cuts up to 18 inches deep. It takes about
three passes to cut through an 8-inch wall.
Embedded rebar doesn't
pose a problem, unless a horizontal rod coincides with the line
of cut. That tends to deflect the blade from its intended path
and cause excessive wear, so we'll occasionally have to
relocate a cut slightly. The manufacturer rates our 24-inch saw
blade for a life span of 5,000 inch/feet, or 625 linear feet of
8-inch-thick wall. Although the brazed sawing segments (Figure
3) can be replaced when worn, it makes more sense to buy new
blades — the blade blank is the cheapest part. On rare
occasions, a segment will get knocked out during use and go
zinging off the walls just like a ricocheting bullet, so the
work can be dangerous at times. Anyone working in the vicinity
should definitely wear safety glasses.
Figure 3. Various
combinations of metal hardness and diamond friability
produce industrial diamond saw teeth, called segments,
suitable for different types of aggregate and concrete
hardness. Blade design ensures that water is thrown
into the cut by centrifugal force.
The noise of sawing isn’t something you’re
likely to tolerate without good hearing protection. In fact,
OSHA requires you to provide hearing protection for noise
levels of 115 dB and higher, and we’re at least that