Pitching
The Driveway-
The driveway should also provide a pitch of from 1/4 to 1/2 inch
to the foot. This is considered a minimum in most areas. You can
give a sideways pitch to the entire drive, but the best plan is
to crown the driveway, providing pitch from the center to both sides.
To
create the proper pitch from the center to the edges, the slab is
placed in two stages. First, the form for the entire driveway is
built. Then a centered stop board is inserted lengthwise in the
form. It runs from the garage to the street, and must be high enough
to create the correct pitch.
If
your driveway is to be 20 feet wide, the center of the placement
must be 5 inches (10-1/2 inch) higher than the outside edges of
the slab. The form boards at the garage and street must correspond
to this pitch, and the top edge of the centered stop board must
be 5 inches higher than the side form boards. The center joint between
the two sections of the slab will be held together with a butted
construction joint.
Excavating/Base
Materials-
Lay out the dimensions of the pour and excavate as needed. The depth
and width of an excavation depends on the project, its specifications
and codes.
A correctly
prepared excavation has several characteristics. A uniform sub grade
ensures a consistent response to temperature and moisture changes.
Level the earth surface so there are no deep holes or protruding
objects. Remove all large rocks, roots, and other debris from the
area. Tamp the sub grade, either with a hand tamper or a mechanical
one. The sub grade should follow the angle or slope of the ground.
Setting
the Grade-
Set the grade for the driveway by staking it and leveling it, as
ring Stake B shown in illustration A. Excavating is easier to do
from this level string point as is building the concrete forms.
Construction joints occur (B) since a long driveway must be laid
in sections. Key the Slab 1 " below garage floor sections, if the
slab is more than 4 inches thick or butted if 4 inches or less.
A construction strip is used at the center of a driveway (C) pitched
from the center to each side for adequate drainage. If the driveway
runs downhill toward the garage, install a drain for water. Construction
or control joints are very important in large slabs of concrete.
They help prevent the concrete from cracking and heaving due to
frost and rain and normal settling of the slab.
Base
materials of sand, gravel and rock are placed at the bottom of the
excavation to prevent settling and heaving and to aid drainage.
They also provide a level surface on which to place the concrete.
The
depth of the base is often regulated by codes. Find out how much
base you need as you did for concrete, but order twice as much as
your figures indicate. This is because the base materials will compress
to half their original size when tamped.
Order
of Placement-
First, place the gravel in at least two layers. Tamp each layer
so the top of the final layer is 2 inches below the bottom edge
of the concrete pour. Then add sand. Keep adding and tamping until
you have a level layer that reaches to the top of the pour. Follow
this order. The depth of any given layer may vary according to code.
The level is checked with a strike board that you make.
plows would scrape the center and would have day light under both sides. I use 11/2" for every 10 feet but all you really need is 1" to every 10' to make the water runoff.