Foundation Cracks Around Seattle Author: Pacific Northwest Inspections Group, LLC Date: 05-Dec-2016. Category: Structural Add to Favorites
The cracks are normal concrete shrinkage cracks. Concrete shrinks as it dries and cures. It is not uncommon to see 1/16th inch of shrinkage for every
10 feet of concrete length. This shrinkage tends to pull or tear the concrete apart. Sharp 90 degree corners at windows, doors, and offsets in
slabs and foundations are excellent locations for the cracks to begin.
Some soils are very sensitive to moisture gain or loss. Certain clay soils can expand nearly 20 times their volume if they get saturated with water.
If this happens because an under-slab plumbing pipe fails, a large hump can develop in an interior slab. Other expansive clay soils can shrink
and take a structure with them as they dry out. If you build in areas that have these types of soils, you need to keep soil moisture as constant
as possible. One way to accomplish this is to install perforated drain pipe around the foundation. As part of the piping layout , install a
tee fitting and a riser pipe up to the surface. If a drought sets in, use a garden hose to inject water into the piping system. This will trick
the soil around the foundation into thinking it is raining at the surface.
Concrete footings and foundations for wood framed construction.
(1) Concrete used for footings and foundations must have a minimum compressive strength of two thousand pounds per square inch (psi). Concrete must be
mixed and delivered in accordance with the requirements of ASTM C94 (Ready-Mix Concrete), or may be field mixed. Field mixed concrete will be subject
to independent compressive strength testing and special inspection.
(2) Concrete footings must be placed on firm, undisturbed soil.
(3) Concrete footings must be continuous, be a minimum of twelve inches wide by six inches thick, be reinforced with a minimum of two No. 4 continuous
rebar, and be at least eighteen inches below finished grade measured from the bottom of the footing.
(4) Concrete foundations must be a minimum of six inches thick, be reinforced with a minimum of one continuous horizontal No. 4 rebar within twelve inches
from the top, and one continuous horizontal No. 4 rebar near mid-height, be reinforced vertically with No. 4 at forty-eight inches on center, extend
at least six inches above the finished grade, and have a total height of not greater than forty-eight inches.
(5) Concrete foundation stem walls that are formed by a thickened concrete slab edge as part of a slab on grade floor must be reinforced with one piece
of No. 4 rebar in the upper part and one piece of No. 4 rebar in the lower part of the foundation. The concrete floor will be reinforced according
to WAC 246-359-430
. The thickened concrete slab edge must extend at least eighteen
inches below finished grade, be at least twelve inches in width, and provide a slab height of at least six inches above finished grade.
(6) Where the walls are of wood construction, the treated foundation plates or sills must be bolted to the foundation or foundation wall with not less
than one-half inch nominal diameter steel bolts embedded at least seven inches into the concrete and spaced not more than seventy-two inches apart.
There must be a minimum of two bolts per piece with one bolt located within twelve inches of each end of each piece. A properly sized nut and washer
must be tightened on each bolt to secure the place.