_Concrete and Cement Investigations._--The investigations relating to
concrete include the examination of the deposits of sand, gravel, stone,
etc., in the field, the collection of representative samples, and the
shipment of these samples to the laboratory for analysis and test. These
tests are conducted in connection with the investigation of cement
mortars, made from a typical Portland cement prepared by thoroughly
mixing a number of brands, each of which must meet the following
requirements:
Specific gravity, not less than 3.10;
Fineness, residue not to exceed 8% on No. 100, nor 25% on No. 200
sieve;
Time of setting: Initial set, not less than 30 min.; hard set, not
less than 1 hour, nor more than 10 hours.
Tensile strength: Requirements applying to neat cement and to 1 part
cement with 3 parts standard sand:
-------------------------------+--------------+----------
| Neat cement. | 1:3 Mix.
Time specification. | Pounds. | Pounds.
-------------------------------+--------------+----------
24 hours in moist air | 175 | ...
7 days (1 day in moist air, | 500 | 175
6 days in water) | |
28 days (1 day in moist air, | 600 | 250
27 days in water) | |
-------------------------------+--------------+----------
Constancy of volume: Pats of neat cement, 3 in. in diameter, ½ in.
thick at center, tapering to a thin edge, shall be kept in moist air
for a period of 24 hours. A pat is kept in air at normal temperature
and observed at intervals for at least 28 days. Another pat is kept in
water maintained as near 70° Fahr. as practicable, and is observed at
intervals for at least 28 days. A third pat is exposed in an
atmosphere of steam above boiling water, in a loosely-closed vessel,
for 5 hours. These pats must remain firm and hard and show no signs of
distortion, checking, cracking, or disfiguration.
The cement shall not contain more than 1.75% of anhydrous sulphuric
acid, nor more than 4% of magnesium oxide.
A test of the neat cement must be made with each mortar series for
comparison of the quality of the typical Portland cement.
The constituent materials are subjected to the following examination and
determinations, and, in addition, are analyzed to determine the
composition and character of the stone, sand, etc.:
1.--Mineralogical examination,
2.--Specific gravity,
3.--Weight, per cubic foot,
4.--Sifting (granulometric composition),
5.--Percentage of silt and character of same,
6.--Percentage of voids,
7.--Character of stone as to percentage of absorption, porosity,
permeability, compressive strength, and behavior under treatment.
Physical tests are made to determine the tensile, compressive, and
transverse strengths of the cement and mortar test pieces, with various
preparations of cement and various percentages of material. Tests are
also made to determine porosity, permeability, volumetric changes in
setting, absorption, coefficient of expansion, effect of oil, etc.
Investigation of concretes made from mixtures of typical Portland
cement, sand, stone, and gravel, includes tests on cylinders, prisms,
cubes, and other standard test pieces, with various proportions of
materials and at ages ranging from 30 to 360 days. Full-sized plain
concrete beams, moulded building blocks, reinforced concrete beams,
columns, floor slabs, arches, etc., are tested to determine the effect,
character, and amount of reinforcement, the effect of changes in volume,
size, and composition, and the effect of different methods of loading
and of supporting these pieces, etc.
These investigations include detailed inquiry in the field and research
in the chemical and physical laboratories regarding the effects of
alkaline soils and waters on structures of concrete being built by the
Reclamation Service in the arid regions. It has been noted that on
certain of the Reclamation projects, notably on the Sun River Project,
near Great Falls, Mont., the Shoshone Project, near Cody, Wyo., and the
Carlsbad and Hondo Projects in the Pecos Valley, N. Mex., structures of
concrete, reinforced concrete, building stones, brick, and tile, show
evidence of disintegration. This is attributed to the effects of
alkaline waters or soils coming into contact with the structures, or to
the constituent materials used. In co-operation with the Reclamation
Service, samples of the waters, soils, and constituent materials, are
collected in the field, and are subjected to careful chemical
examination in the mineral laboratories at Pittsburg.
[Illustration: PLATE XV.
Fig. 1.--Characteristic Failures of Reinforced Concrete Beams.
Fig. 2.--Arrangement of Static Load Test for Reinforced
Concrete Beams.]
The cylinders used in the percolation tests are composed of typical
Portland cement mixed with sand, gravel, and broken stone of known
composition and behavior, and of cement mixed with sand, gravel, and
broken stone collected in the neighborhood of the Reclamation projects
under investigation.
[Illustration: Fig. 5.