Page 28 - Masonry and Concrete

Understanding Concrete
27
UNDERSTANDING CONCRETE

SEVERE
MODERATE
MILD
FIGURE 2-12
Continued

cracks, loose knots, and other defects that might reduce strength or mar
the finished surface.
Any type of lumber that is straight and smooth can be used for
temporary forms that will be removed when the concrete is cured.
No. 2 or No. 3 grade yellow pine, spruce, or fir make good, sturdy
form boards. Green lumber works better than kiln-dried lumber,
which will swell when it absorbs water from the concrete mix. Forms
that are too absorptive also reduce the quality of the concrete by
removing too much water from the mix and leaving insufficient
moisture for cement curing. Plywood for forms should be exterior
type with grade B face veneers. For forms or divider strips that
will stay in place, use redwood, cedar, cypress, or lumber that has
been pressure-treated with a chemical preservative. Coat redwood,
cypress, or cedar lumber with a clear sealer to protect it from the
alkalis in the fresh concrete. Pressure-treated lumber does not require
a sealer.

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