Page 45 - Masonry and Concrete

Concrete Construction Techniques
44 CHAPTER THREE

The corner reinforcing bar stakes mark
BATTER BOARD the outline of the slab or footing, and exca-
NAIL
vations should extend one or two feet
beyond these limits to allow room for
building the forms. Untie the string lines
and pull them aside so that the excavation
work can be done.
3.2.2 Excavating or Filling
NAIL
Concrete footings, slabs, driveways, patios,
and sidewalks will usually require some
excavating or filling to establish the finished
STAKES AND BRACES
work at the proper elevation above or below
the finish grade. Structural fill to support
foundations or slabs-on-grade must be spec-
ified by the engineer as to the required type,
FIGURE 3-4 compaction, and moisture content. This is
Batter boards. work which should be performed by a qual-
ified subcontractor with the proper equip-
ment to achieve the strength and stability
necessary for structural fill.
Excavation for a sidewalk or patio will usually consist of removing
a few inches of topsoil. For grade beams or footings, excavation may be
much deeper. The actual depth of foundations and footings will be
indicated on the drawings by the project engineer or dictated by build-
ing code. In cold climates, footings must be placed below the winter
frost line so that they are not destabilized by frost heave as the moisture
in the soil freezes and expands. The map in
>>>TIP Figure 3-8 gives winter frost depths in
To make sure that cor-
inches for the continental United States.
ners are square, use the 3-4-5 triangle
This will give you a general idea how deep
method. From the outside corner point,
measure 4 ft. along one string and 3 ft. the bottoms of footings or grade beams
along the other. The string lines are square must be set. The local building official can
when the diagonal between the two points tell you exactly what the requirements are
measures exactly 5 ft. (Figure 3-7). To in a given area. In the northern tier of states
square a string line against an existing and in the Rockies, foundations must be
wall, use a steel carpenter’s square. dug so deep that it is usually economical to
excavate for a full or half basement.

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