Page 69 - Masonry and Concrete

Concrete Construction Techniques
68 CHAPTER THREE

On large slabs where it is not possible
to reach the entire surface from the
perimeter, floating and troweling requires
the use of knee boards. Knee boards can be
made from 24-in. long 1 10s with 1 4
handles on the ends (Figure 3-33), and are
used to distribute the weight of the fin-
isher without leaving deep depressions in
the surface. Start in the least accessible
areas and work your way backwards to the
edge. When working on knee boards, trow-
eling is typically done immediately after
floating. This requires waiting until the
concrete has hardened enough that water
FIGURE 3-26
and fine material are not brought to the
Cutting edge of concrete from form. (Photo courtesy
surface. Too long a delay will mean the
PCA).
surface is difficult to finish, but the ten-
dency is to begin too early. Premature floating and troweling can cause
scaling, crazing, or dusting of the concrete surface. For outdoor con-
crete subject to the extremes of weather, this is particularly harmful
because the concrete is less durable and less wear resistant.
Power floats can be used to reduce finishing time on large slabs,
and by changing blades, the same equip-
ment can be used for troweling. Power
floats can be rented at many tool rental
stores.

3.7.2 Special Finishes
An exposed aggregate finish will add color
and texture to a driveway, sidewalk or
patio, as well as a nonslip finish. The con-
crete should be poured in small, manage-
able areas so that the aggregate can be
seeded into the surface before the concrete
becomes too hard. The seeding method of
creating an exposed aggregate finish takes
FIGURE 3-27 about three times longer than normal finish-
Edging concrete. (Photo Courtesy PCA). ing so it is usually done in smaller sections.

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