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The determination of the percentages of materials falling within each of the above size ranges is one of the first steps in analyzing an aggregate source. The complete grading of the materials in the deposit is also determined. The grading and distribution figures supply most of the information needed for establishing what processing operations, such as crushing, rolling, screening, and wasting, will be required to prepare the materials for use in concrete. The silt content, or percentage of dirt, clay, etc., in the aggregate samples, provides an indication of the thoroughness with which the aggregates must be washed in the field. Specific gravity, absorption, and organic material tests supply information for judging the soundness or general suitability of the materials for concrete aggregate purposes. Figure 2 illustrates apparatus developed in the Denver laboratories for determining specific gravity, moisture, content, and absorption, either in the laboratory or for field control.
A petrographic analysis or mineral classification of the aggregates with the aid of a microscope provides very valuable information concerning a material proposed for use in concrete. Soft particles, clay lumps, rock types, unsound, or deleterious substances may be found by this process. The degree of alteration, chemical activity, and general stability of the aggregate are all indicated by the information obtained from the petrographic studies. Thin sections for observation under the microscope and photomicrographs are some of the methods employed in the mineral classification of aggregates.
The soundness of sand and gravel or crushed rock is estimated or compared with other materials with the aid of a number of different tests. The structural soundness of sands is tested by the compressive strength of small mortar cylinders made under standardized conditions. The weath
ering resistance and stability of sand, gravel, and rock are indicated by their behavior when subjected to alternate soaking in sodium sulfate solution and drying in warm air, and to alternate cycles of freezing and thawing when immersed in water. The aggregates are then used in concrete to fabricate test cylinders, which are also subjected to the sodium sulfate and freezing and thawing treatment. Soundness tests afford some measure of the ability of concrete made with a given aggregate to withstand the attack of disintegrating agencies such as temperature and moisture variations and alkali conditions.
The concrete making properties of the aggregates, after they have been processed in a manner that will simulate the probable treatment in the field during production, is determined by using the materials in making concrete mixes. Four series of mixes, having maximum size aggregate of three-fourths, \%, 3, and 6 inches, are usually included in the concrete mix tests. The variable factors included in each of the four series are water-cement ratio, consistency as measured by the slump test, gravel-sand ratio, and cement content. The various properties of the individual mixes and concretes directly determined by tests include slump, workability, finishing qualities, 28-day compressive strength, and elastic properties. These properties are analyzed in a number of ways and the recommended mixes to be used in various parts of the work are derived. The information obtained from the mix tests also furnish data for design purposes.
The tentative concrete mixes selected for use in the field are finally tested under conditions closely approximating those that will obtain on the job. Specimens to be tested for strength, volume change, permeability, elasticity, plastic flow, temperature rise, and heat of hydration are cast in thin metal containers and stored in specially constructed rooms wherein the temperature may be caused to follow any predetermined or automatic cycle. Figure 3 shows a view of the control panel for these rooms which are termed "adiabatic calorimeter" rooms. Coincident with the above tests which are made under conditions called "mass curing" the thermal properties of the selected mixes are determined with the aid of specially designed and constructed apparatus as illustrated in figure 4. The determinations for thermal properties include specific heat, or the amount of heat required to raise the temperature of the concrete 1 °, thermoconductivity and diffusivity, having to do with the rate at which heat is transferred through the concrete, and density or unit weight. The tests just outlined supply essential data for accurately predicting the behavior of the concrete in the completed structure, thereby permitting the use of
optimum design conditions and values, and the intelligent design and planning of artificial cooling and grouting systems and operations.
Sufficient data and knowledge concerning the materials available for the construction of a given project are obtained to insure that they will be processed, combined, and utilized in a manner to obtain the greatest practicable, economic, and qualitative benefit. In addition to the investigational work described, the Denver laboratories are constantly being called upon experimentally to solve a myriad of problems encountered in design and construction and in the utilization of numerous materials of construction.
The data and information collected in the Denver laboratories are transmitted to the field for the use and guidance of the field laboratory and control organization during construction. The field laboratory performs routine tests on the concrete materials as they are processed and made ready for use, of the concrete as it is mixed and placed in the structures, and of the hardened concrete, to insure that the desired quality, workmanship, and economy are obtained. Figure 5 shows the plans for a typical field laboratory.
While the investigational and control operations, as above outlined, may sound elaborate, it should be realized that a large percentage of the cost of most of the Bureau's projects lies in the concrete work. The cost of the investigation and control work usually constitutes a very small proportion of the total cost of the project. It is insignificant in comparison with the insurance afforded byway of increased integrity, quality, performance, and economy obtained in the concrete construction.
EARTH MATERIALS LABORATORY OPERATION
BY L. W. HAMILTON, ASSOCIATE ENGINEER, BUREAU OF RECLAMATION
IN VIEW of the larger number of earth dams proposed for construction, the Bureau of Reclamation established the Earth Materials Laboratory in the fall of 1933. Primarily the duties of the laboratory were to determine the characteristics of proposed embankment and foundation soils, to work with the design section in planning field control tests on the foundation and compacted embankment, and to train construction inspectors in the test procedure.
Earth testing along these lines is a comparatively new field. Consequently, the laboratory staff had to design the tests and test apparatus, a matter which required much time and thought. Shortly after the establishment of the laboratory it was decided to follow, in general, the technique of laboratory testing and control developed by the Los Angeles Bureau of Water Works and Supply. This technique was described by R. R. Proctor in a series of four articles published by Engineering News Record beginning August 31, 1933.
The routine tests now made in the laboratory are: (1) Mechanical analysis, (2) compaction and penetration resistance, (3) percolation and settlement, (4) consolidation, (5) shear, (6) specific gravity, and (7) soluble solids determinations. The laboratory has made numerous tests and studies using different amounts of compaction with various impact or hammer compactions. The laboratory has also conducted studies and experimentation on hydrostatic pressure travel through soils, how the internal pressure lags with decrease in external pressure, how hydrostatic pressure is created by consolidation, and how rapidly this pressure is dissipated.
BRIEF DESCRIPTION OF TESTS AND
Mechanical analysis is the determination of the distribution of particle sizes. This analysis includes: (1) the determination of the percent by weight of particles retained on 6-, 3-, \ )'2-, %-, and %-inch screens; (2) the sieve analysis on the standard sand screens plus the 200-mesh screen; and (3) the hydrometer analysis of the fines passing the 200mesh screen. These analyses are combined into a total analysis. The material passing the 200-mesh screen is analyzed by the rate of sedimentation in water, using a Bouyoucos hydrometer as the means of measurement, and using Stokes' law as a basis for the computation of particle sizes. The mechanical analysis gives data for the classifica
tion of the soil, and may indicate many of the soil characteristics such as permeability and shearing strength.
The compaction test is made to determine the relationship between moisture content and dry density. From the information obtained by this test the maximum density, the optimum moisture content, and the range of moisture contents which will cause only small variations in resultant dry density, may be observed. This test also indicates the density that may be expected in the compacted embankment. The test is made by compacting a series of samples at different moisture contents, with these variations in moisture covering a much wider range than would logically be expected in construction practice. Using the information obtained from this test the moisture content of the embankment material is controlled to secure the greatest practical embankment density.
In conjunction with the compaction test the penetrationresistance test is made. This test is a measurement of the resistance of the soil to penetration. It is determined by forcing a cylindrical-ended needle into the compacted soil to a depth of approximately three inches, and reading the force required to cause an additional penetration of onehalf inch per second. The factors affecting this test are particle lubrication (moisture), degree of compaction, and density. By standardizing the amount of compaction it has been found that for any one soil this resistance to penetration becomes a function of the moisture content and may be successfully used for construction moisture control.
The percolation test is a measure of the seepage capacity of a soil. It is made by measuring the flow of water through a loaded soil specimen. In conjunction with the percolation test the settlement of the soil specimen is observed before and after loading and with complete saturation. The amount of consolidating load used depends upon the probable load conditions in the structure; but for routine testing the load is usually made equivalent to 20 feet of fill.
The consolidation test is the observation of the volumetric change of a soil specimen under load with respect to time. From the test data information is obtained which will indicate the settlement of the actual structure.
The shear test is made in a two-piece container so constructed that the upper half may be held stationary while the lower half is pulled out, thus causing the soil to slide on soil. The shearing strength resulting from any one normal load is obtained by placing the material in the abovementioned container, applying a constant load to the soil