Page 63 - Masonry and Concrete

Concrete Construction Techniques
62 CHAPTER THREE

is mixed. Delays result in evaporation of moisture from the mix and a
loss of both workability and strength. If the concrete is not placed
1
within 1 ⁄2 hours and shows signs of stiffening, it should be discarded.
Do not add water to a mix that has begun to stiffen. Even if you suc-
ceed in restoring some workability, the concrete will be of poor qual-
ity. To avoid delays, it’s important to make sure that all the necessary
preparations have been made before the ready-mix truck arrives or
before you begin mixing. Preparations should include wetting the
inside surfaces of plywood and kiln-dried lumber forms and the soil
subgrade to keep them from absorbing too much water from the con-
crete mix. Linseed oil or commercial form release oil can be used.
Oiled forms will also make form removal easier without damage to
the concrete surfaces. Oiling or wetting the forms and soil is espe-
cially important on a warm and windy day when moisture evapora-
tion is at its highest.
The second rule in pouring concrete is to place the mix as near to
its final location as possible. A ready-mix truck is equipped with metal
chutes which can be extended a moderate distance to deliver concrete
directly into the forms, and concrete pumps are often used on large
commercial projects. On residential projects, it is more common to use
wheelbarrows or buggies to move the concrete from the mixer to the
forms. You can build ramps and runways over the forms to keep from
bumping the boards or reinforcing out of place. Deep forms may
require pouring in more than one lift or layer. Work in lifts of 6-in. to
24-in. thickness, consolidating the concrete as you go to eliminate
voids and large air pockets. Continue with successive lifts until the
pour is completed. In wall or column forms, concrete should not be
dropped vertically more than three to five feet without appropriate
chutes or baffles to keep the concrete from segregating. Start placing
concrete at the ends or corners of the walls and work toward the mid-
dle. In slabs and footings, start placing the concrete in the farthest cor-
ner of the forms, depositing each load against the previously placed
concrete. Do not make separate piles of concrete and attempt to move
the mix horizontally because this will also cause segregation of ingre-
dients. Make sure you get enough concrete to fill the forms completely.
Fill low areas with a shovel if necessary, and tamp the concrete to fill
in corners. Settle concrete against the perimeter forms by tapping the
outside of the form boards with a hammer.

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