Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.7
proportions adjusted accordingly. If the specified slump for the basic mix was to
be reduced, weight, lb/yd 3 , should be increased, because less water and cement
would be used and the cement paste (water plus cement) weighs 864/7.68 � 113
lb/ft 3 � 149.4 lb/ft 3 . If the same batch weights are used for all deliveries, and the
slump varies erratically, the yield also will vary. For the same batch weights, a
lower slump is associated with underyield, a higher slump with overyield. With a
higher slump, overyield batches are likely to be understrength, because some of the
aggregate has been replaced by water.
The basic mix proportions in terms of weights may be based on surface-dry
aggregates or on oven-dry aggregates. The surface-dry proportions are somewhat
more convenient, since absorption then need not be considered in calculation of
free water. Damp sand and gravel carry about 5 and 1% free water, respectively.
The total weight of this free water should be deducted from the basic mix weight
of water (300 lb/yd 3 in the example) to obtain the weight of water to be added to
the cement and aggregates. The weight of water in the damp aggregates also should
be added to the weights of the sand and gravel to obtain actual batch weights, as
reported on truck-mixer delivery tickets.
9.6 PROPERTIES AND TESTS OF FRESH
(PLASTIC) CONCRETE
About 2 1 ⁄2 gal of water can be chemically combined with each 94-lb sack of cement
for full hydration and maximum strength. Water in excess of this amount will be
required, however, to provide necessary workability.
Workability. Although concrete technologists define and measure workability and
consistency separately and in various ways, the practical user specifies only one—
slump (technically a measure of consistency). The practical user regards workability
requirements simply as provision of sufficient water to permit concrete to be placed
and consolidated without honeycomb or excessive water rise; to make concrete
‘‘pumpable’’ if it is to be placed by pumps; and for slabs, to provide a surface that
can be finished properly. These workability requirements vary with the project and
the placing, vibration, and finishing equipment used.
Slump is tested in the field very quickly. An open-ended, 12-in-high, truncated
metal cone is filled in three equal-volume increments and each increment is consolidated
separately, all according to a strict standard procedure (ASTM C143,
‘‘Slump of Hydraulic-Cement Concrete’’). Slump is the sag of the concrete, in,
after the cone is removed. The slump should be measured to the nearest 1 ⁄4 in which
is about the limit of accuracy reproducible by expert inspectors.
Unless the test is performed exactly in accordance with the standard procedure,
the results are not comparable and therefore are useless.
The slump test is invalidated if: the operator fails to anchor the cone down by
standing on the base wings; the test is performed on a wobbly base, such as formwork
carrying traffic or a piece of metal on loose pebbles; the cone is not filled by
inserting material in small amounts all around the perimeter, or filled and tamped
in three equal increments; the top two layers are tamped deeper than their depth
plus about 1 in; the top is pressed down to level it; the sample has been transported
and permitted to segregate without remixing; unspecified operations, such as tap-