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model, including radial and tangential deflections of the arch and horizontal and vertical deflections on the abutments. The cracking was in accordance with expectations and a water seal was placed in the actual dam along with extensive grouting and drainage provisions.
After the model had cracked, small variations in temperature caused variations in the depth of the crack opening which made it very difficult to get constant strain conditions. To eliminate the stress concentration at the corners, the cracked portions were cut out and the sections rebuilt with fillets as shown in figure 13. A uniform compressed air load equivalent to the arch component was applied to the arch and to the middle of the fillets, and the full reservoir pressure was applied to the canyon walls and to the upper half of the fillets. The construction of the fillets successfully prevented cracking of the model and a complete set of strain and deflection measurements was made. Calculation of final results for these tests are still in progress at present.
Figure 14.—Uplift model before cracking; gages set to measure horizontal deflections.
Trial load analyses of thin arch dams with fairly long arch elements show that considerable tension often occurs at the upstream face of the cantilevers. Although the arches may be capable of carrying the load, the tension stresses often cause horizontal cracking in the cantilevers, accompanied by uplift. This uplift causes additional deflection of the cantilevers and throws increased load on the arches. A model of a cantilever clement of such an arch dam was built and tested to show what happens to the stress distribution in the remainder of the base after it has cracked and uplift pressure becomes effective over the cracked area.
Figure 14 shows the complete model arranged for measuring the horizontal deflection due to dead and live loads before cracking. Two systems of live loads were applied. One was the calculated cantilever component assuming the section capable of carrying tension and the other assuming all tension relieved by cracking.
Strain measurements were made at the base of the model to determine the depth of the tension area under the first system of loads. A horizontal crack was then cut along the top of the foundation at the upstream face and extended about two-thirds of the calculated depth of the tension area. A rubber bladder was fitted in the crack and connected to a mercury tank to provide the uplift pressure. The live load was the calculated cantilever component assuming the section to carry no tension.
Figure 15 shows the complete results of this first series of tests. The principal stresses show that the depth of the crack was not sufficient to relieve the tension. Several series of tests were run with greater depths of the uplift crack and with various intensities of loading. When the crack was extended to the full theoretical depth, the live load was adjusted to give zero stress at the end of the crack, this being the closest to actual conditions. The final series was run with the crack extended beyond the neutral axis and high compressive stresses were calculated with uplift present, so
the tests covered a greater range of cracking than would probably occur in the dam.
Experimental Work on Small-Scale Models of Arch Dams, by Ivan E. Houk, Reel. Era, October 1927, pp. 152-154.
Checking Arch Dam Designs With Models, by J. L. Savage and Ivan E. Houk, Civ. Eng., May 1931, pp. 695-699.
Tests of Models of Arch Dams, by J. L. Savage, Ivan E. Houk, H. J. Gilkey, and Fredrik Vogt, June 1, 1931; published by Engineering Foundation as Volume II of Arch Dam Investigation, May 1934, 542 pp.
Model Tests Confirm Design of Hoover Dam, by J. L. Savage and Ivan E. Houk, Eng. News-Rec, Apr. 7, 1932, pp. 494-499.
Dam Stresses and Strains Studied by Slice Models, by J. L. Savage, Eng. News-Rec, Dec. 6, 1934, pp. 720-723.
CONCRETE LABORATORY OPERATIONS
BY R. F. BLANKS, ENGINEER, BUREAU OF RECLAMATION
THE BUREAU has established, in Denver, complete laboratory facilities for investigating and testing concrete and all materials used in concrete construction. These facilities include a thoroughly trained personnel and modern equipment and apparatus for performing all kinds of tests of concrete and its constituent ingredients. The tests have proved invaluable in the solution of engineering problems, thereby making possible the economic completion of difficult engineering projects in record time.
It is impossible to evaluate the laboratory work in dollars and cents except for a few specific cases. The economic completion of the massive block of concrete comprising Boulder Dam, with practically no cracking, and with all contraction joints effectively grouted and the structure functioning as a monolith in accordance with its design, was largely possible because of the information supplied by the laboratory tests. The Denver laboratories have more than repaid their cost of installation and operation in benefits to specific projects which may be directly evaluated, not to mention the many intangible items. Some original accomplishments of the Denver cement and concrete laboratories are listed as follows:
1. Developed a satisfactory working specification for lowheat cement, which, in combination with artificial cooling, has been the greatest single factor contributing to the successful completion of Boulder Dam.
(a) Applied compound composition for the first time in the specification of portland cement. This has since proven to be the most satisfactory method of obtaining desired qualities in cement.
(b) Established specific surface as a measure and control of the fineness of portland cement, resulting in improved qualities both in cement and concrete. Such practice has now become national for nearly all important work.
2. Established the comparative physical and chemical properties of no less than five distinct types of portland cement, thus permitting more intelligent and efficient selection of the proper type of cement for specific jobs.
(a) Developed equipment and technique for measuring the heat-generating characteristics of cements in concrete under closely simulated field conditions for mass concrete.
(b) Determined various properties of mass concrete under closely approximated field conditions. These include strength, elastic properties, plastic flow under sustained loads, permeability and volume change.
3. Originated specifications for the so-called "modified" or "moderate heat" type of portland cement, which gives promise of becoming the all-purpose cement for general use, and which exhibits qualities superior to the present standard cement.
4. Established the thermal properties of mass concrete, including specific heat, thermal conductivity, diffusivity, and density, with various variable factors; also proved that such properties, as established in the laboratory, may be used to accurately predict temperature variations and movements under field conditions. Without such knowledge, the design and operation of effective methods for artificial cooling mass concrete would be greatly handicapped.
5. Developed equipment, methods, and technique for testing various properties of mass concrete in large specimens, including 36- by 72-inch cylinders.
(a) Established vibratory compaction as the most satisfactory method of obtaining uniform conditions in varying sizes of test specimens.
(b) Evolved the grinding of the ends of test cylinders as a satisfactory method of securing plane surfaces for test purposes.
(c) Developed equipment for controlling the rate of load application in testing concrete, which has subsequently been patented and included as a major feature on all modern testing machines.
6. Evaluated the effects on various properties of mass concrete of such variable factors as size of test specimen, size of aggregate, and wet screening. Such data are essential to the intelligent selection of design values and the correct interpretation of the results of adopted field-control measures.
7. Developed methods for testing the permeability of mass concrete under high heads, resulting in a better understanding and clearer conception of the structure of concrete and its suitability for hydraulic structures.
8. Applied the accelerated weathering tests of concrete to determine the suitability of various materials for use in concrete on various projects and to establish the necessary precautionary measures to be employed under a variety of conditions.
9. Developed petrographic studies as a valuable method of estimating the suitability of materials of construction for various purposes.
10. Developed methods for designing and adjusting concrete mixes which have proven efficient on the numerous recent projects.
11. Made worth while advances in the use of membrane curing compounds.
12. Developed methods and procedure for exploring aggregate deposits, which have resulted in more efficient use of available materials.
13. Developed advancements in specifications for concrete work which have been followed or adopted by various organizations throughout the country.
14. Prepared a manual for the control of concrete construction and a comprehensive bulletin on special cements for mass concrete together with numerous articles on cement and concrete.
15. Other items too numerous to mention in detail, or as yet incomplete, include the determination of extensibility properties or resistance to cracking, uplift characteristics, the evaluation of pozzolanic materials in combination with portland cement, and the behavior of concrete at early ages under restrained conditions.
In addition to the Denver laboratories, a field laboratory is established for each job in close proximity to the work, to handle routine control operations during construction. The size of the field laboratory and the amount of equipment installed is dependent upon the size and importance of the job. The field laboratory constitutes an indispensable part of the construction organization since the appearance, quality, economy and permanence of the finished concrete structures are all directly dependent upon the effectiveness of the field control operations.
One of the first steps in planning for the design and construction of a project is the investigation of all available sources of supply for construction materials. A thorough reconnaissance survey is made of all existing deposits, or quarry sites, and sufficient preliminary information collected to permit selection of the most suitable supply from a qualitative and economic standpoint. The selected source is then explored by test pits, borings, or other means, as illustrated in figure 1, to establish the depth, extent, and physical characteristics of the deposit and to obtain representative samples of the materials for test purposes. This work is usually accomplished by members of the field control organization.
The data from the field investigations, together with samples collected, which for the larger projects may comprise several carloads, are transmitted to the central laboratories in Denver for analyses and tests. The studies and tests made in Denver are for the purpose of obtaining data for design, as well as for guidance in processing, combining, and utilizing the materials in construction. Such procedure results in the maximum possible qualitative and economic benefit from the materials available for the job.
The procedure followed in the Denver laboratories in testing aggregate samples from a selected source for a
small mass concrete dam will illustrate the information obtained and facilities available. Aggregate sample shipments for such a job usually consist of a few sacks of materials for preliminary investigation and test, to make certain that the source of supply is satisfactory for the intended purpose. The preliminary shipment is then followed by a carload lot of about 50,000 pounds. The large shipment is made up of numerous small samples selected from representative locations and depths throughout the deposit. If the source of supply is a quarry site from which crushed aggregate is to be obtained, the test drilling and sampling operations are made commensurate with the local conditions.
Upon arrival of the shipment in Denver, each sack is cataloged according to size of material, test-pit location, and depth from which it was obtained. Physical properties of the individual samples are then determined; from which, in combination with the data obtained in the field, the weighted average characteristics of the deposit may be established.
The aggregate sizes actually used in construction of Bureau projects usually are divided as follows: sand, 0 to no. 4 sieve; fine gravel, no. 4 to three-fourthsinch; intermediate gravel, three-fourths to \% inches; coarse gravel. \% inches to 3 inches; and cobbles, 3 inches to 6 inches, all on the basis of square opening screens.