The quantity of water used in mixing has a very great influence on the
strength of the concrete. An excess of water weakens the concrete, while
an insufficient amount prevents thorough mixing.
[Illustration: Fig. 2.--The result of using too dry a mixture, lack
of spading and careless placing; note irregularity of layers and poor
bonding.]
Therefore, only sufficient water should be used to produce a workable or
plastic mixture.
Recent tests have proved that to secure the greatest strength the
concrete should be mixed considerably drier than has heretofore
been customary. Of course, for thin walls containing closely placed
reinforcement, or for water-tightness, a fairly wet mix is necessary. A
little experience will show the proper amount of water to use.
A very rough estimate of the quantity of water required in mixing for
general work is 4 to 5 gallons to each sack of cement.
Three degrees of consistency (corresponding to different proportions
of water) are used in general practice, namely, wet, medium, and dry.
In the light of recent investigations it is thought the wet mixture of
present-day practice contains too much water. The following definitions
are therefore recommended:
=Wet mixture.= One that does not flow readily and yet can not be piled
up. It is recommended for thin sections when reinforcement is closely
placed.
=Medium mixture.= One that is between the wet and dry mixture. This
consistency is recommended for general work.
=Dry mixture.= One about like damp earth. If a handful is squeezed it
will retain its shape. This consistency requires thorough ramming to
eliminate voids and is used when forms are to be removed immediately,
but should not be used where a water-tight job is expected. The porous
structure of the concrete in Figure 2 is due to the fact that it was
placed as a dry mixture.