About 2.5 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.
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 placement of concrete with proper consolidation 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, inch, 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 tapping the cone, occur; the cone is not lifted up smoothly in one movement; the cone tips over because of filling from one side or pulling the cone to one side; or if the measurement of slump is not made to the center vertical axis of the cone.
|Schematic diagram of slump test|
Various penetration tests are quicker and more suitable for untrained personnel than the standard slump test. In each case, the penetration of an object into a flat surface of fresh concrete is measured and related to slump. These tests include use of the patented ‘‘Kelley ball’’ (ASTM C360, ‘‘Ball Penetration in Freshly Mixed Hydraulic Cement Concrete’’) and a simple, standard tamping rod with a bullet nose marked with equivalent inches of slump.
A field test frequently requires measuring the air entrapped and entrained in fresh concrete. Various devices (air meters) that are available give quick, convenient results. In the basic methods, the volume of a sample is measured, then the air content is removed or reduced under pressure, and finally the remaining volume is measured. The difference between initial and final volume is the air content. (See ASTM C138, C173, and C231.)
Tests on fresh concrete sometimes are employed to determine the amount of cement present in a batch. Although performed more easily than tests on hardened concrete, tests on fresh concrete nevertheless are too difficult for routine use and usually require mobile laboratory equipment.