Fairbanks 12-1492 - 12-1496 A.A.R. Combination Railroad Track/MTS - Manual (Page 57)

04/12 57 50538 Rev. 7
APPENDIX VII: Concrete & Slump Testing:
Concrete is produced from the mixing of sand (fine aggregate), stone (coarse aggregate),
cement and water. The water combines with the cement to form a fluid paste often referred to
as "plastic". The paste is combined with the sand and stone to make a workable "plastic" con-
crete that can be poured, shaped, smoothed, and molded. The plastic concrete then hardens
around the sand and stone forming a solid mass. Concrete in it's plastic state can be formed
into structures such as pavements, walls, or footings. When the plastic concrete hardens, the
structures formed can then support the anticipated loads.
The reaction of water with cement is called hydration. The water to cement ratio is an impor-
tant factor in the ultimate strength of the concrete and its subsequent load carrying ability. The
lower this ratio is, the tighter the microscopic crystals of concrete, and the stronger the con-
crete will be. Fewer shrinkage cracks from excess water will also result from this low ratio.
For a given amount of cement, a smaller quantity of water will produce a higher strength con-
crete. Too small a quantity of water, however, will not allow for adequate mixing of the sand
and stone, and will also make the concrete difficult to "work" and to form, which in turn will
produce a lower strength concrete.
The amount of water in a particular concrete mixture is thus carefully selected to balance the
desired strength of the concrete. Whether a "soupy" mix, or "firm" mix, the ability of the con-
crete to be shaped, worked, and placed is called fluidity. The slump test is a quality control
measure of the fluidity of the concrete mixture.
The procedure for the slump test is covered in ASTM (American Society for Testing &
Materials) Document C-143 from a sample of concrete obtained per ASTM C-172. The slump
is measured in inches. The measured slump is then compared to the desired specifications.
The equipment for the slump test is usually a metal cone, a metal base plate and a metal rod.
The "cone" is 12 inches in height, 4 inches in diameter at the top, and 8 inches in diameter at
the bottom. The slump test must be performed within 2 1/2 minutes after obtaining the sam-
ple.
1: The cone, base plate, and rod are moistened with water.
2: The cone is placed on the base plate with the 8" opening at the bottom.
3: The cone is filled in three (3) equal layers.
4: Each layer is rodded 25 times to settle the concrete, before the next layer is added.
5: The cone is then pulled straight up and off of the sample. The cone must come off within
3-7 seconds for an accurate test, per ASTM standards.
6: When the cone is removed, the concrete mixture "slumps" down. It is then measured to
determine how far down it has slumped, and compared to specification.