The mixed concrete should be deposited in the forms within from 20 to 30
minutes from the time the water is added to the cement, as it begins to
set or harden after this time. To disturb the concrete after the set has
begun is risky, as it will lose some of its strength, the extent of the
injury depending upon the seriousness of the disturbance.
Concrete which has set before it can be placed in the forms should not be
tempered or softened with water, but should be discarded.
To prevent delay in placing, all forms should be examined before the
mixing is begun to see that they are properly braced, that all chips or
loose particles are removed, that the surface of concrete which has set
has been properly roughened and wetted to assure a bond, as described on
page 20, and that all reinforcement, bolts, inserts, etc., are properly
located and secured.
At the lunch' period, or at the end of a day's work, the mixing board
and equipment should be thoroughly washed, for if this is not done many
pounds of heavy concrete are needlessly carried around by the men and the
addition of a pound in the weight of tools will lower the efficiency of
the workers. Moreover, it will save time and wear and tear of equipment
incident to cutting the surplus concrete away with a cold chisel.
[Illustration: Fig. 14.--1592 Showing result of leaky forms and poor
placing. The soft cement mortar ran out, leaving areas of honeycombed
surface not necessarily harmful but unsightly.]
In depositing concrete in the forms care should be taken that the
materials do not separate.
If the mixing is done close to the place of depositing, the concrete
may be shoveled into the forms directly or through a chute. If it is
necessary to lift or transport the concrete, buckets and wheelbarrows are
convenient containers. The concrete should be deposited in horizontal
layers, preferably not over 6 inches thick, and a spade or paddle should
be worked up and down against the forms to push the coarse material
away from the surface, as illustrated in Figure 13. The object of the
spading is to eliminate impounded air that may form pockets in the mass
and to insure a smoother and more impervious surface. In addition to
being spaded, stiff concrete should be rammed until water flushes to
the surface. Tapping the forms with a hammer is a very effective way of
securing a smooth surface. Figure 14 shows the result of improper spading.
Fresh concrete will riot bond readily to concrete that has hardened and
a seam may be formed that will permit water to trickle through. When
bonding fresh concrete to that which has been in place for a short time
it is usually sufficient to roughen the hardened surface with a pick or
by other means so as to expose the gravel or stone, and to clean off
all loose particles. The hardened concrete should be soaked with water,
the excess water removed, and the surface then given a coat of grout (a
mixture of cement and water) of the consistency of cream just before the
new concrete is deposited.
When pouring of a wall is to be discontinued for some time, provision for
the bonding of future work should be made. This may be done by placing
short steel dowels in the concrete when it is poured, or a rebated joint
or groove may be made, as shown in Figure 15. In bonding a new wall to
old concrete, holes should be drilled for the dowels, which should be
grouted in, and the old surface should be roughened, cleaned, and wetted;
or a groove may be cut in the old wall to receive the new concrete.
[Illustration: Fig. 15.--Method of forming horizontal rebate.]