Page 46 - Masonry and Concrete

Concrete Construction Techniques
45
CONCRETE CONSTRUCTION TECHNIQUES

Concrete slabs and footings must be
supported on soil that is hard, uniformly STRING
graded, and well drained. A poorly or LINES
improperly prepared subgrade can cause
uneven settlement and cracking. Remove NAIL
all vegetation, roots, and large rocks from
an area at least two to three feet wider
than the slab or footing perimeter. Remove
the soil to the necessary depth, allowing
plenty of room around the outside to build
the formwork. If you are digging very
deep, leave a generous slope on the outer FIGURE 3-5
soil walls to prevent dangerous cave-ins. String lines.
Dig out any soft or spongy areas in the
subgrade and fill them with compacted
soil, or with gravel, crushed stone, or sand. Loosen and tamp hard
spots to provide uniform support for the concrete, but wherever pos-
sible, leave the subgrade undisturbed. Smooth loose surface soil and
fill in holes left by stones or roots with sand or gravel. Level the sub-
grade surface and then compact it by hand, with mechanical rollers or
vibratory compactors. In areas with poor drainage, excavate deeply
enough to place a 4- to 6-inch layer of crushed rock or gravel under
the concrete. Crushed rock is better than smooth gravel because it
compacts firmly and provides more stable support for the concrete.
This aggregate drainage layer will stop the
capillary rise of soil moisture into the bot-
tom of the concrete. PLUMB
Only small areas should be excavated BOB
by hand because the work is labor inten-
TRUE CORNER
sive and backbreaking. Larger areas are
LOCATION
more economically excavated with heavy
equipment or subcontracted to an excava-
tion company.

3.3 Building Formwork

Small, shallow concrete footings can FIGURE 3-6
sometimes be formed by earth trenches if Locating corners.

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