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Concrete

ENGINEERING METHODS AND MATERIALS

Gang vibrators.-Gang vibrators have been used in concrete placement in Europe for about 7 years. Engineers in the United States look favorably upon the development and use of such vibrators in this country, and some contracts for large mass-concrete dams have been written to permit their use. The possibility of mounting & bulldozer blade on the same vehicle with gang vibrators should be considered to permit spreading the fresh concrete to the proper depth. The Bureau of Reclamation would cooperate with industry in development of domestic models.

Cast-in-place concrete slabs, and immersion-type air meter to measure air content.-Labor and materials in the construction of massconcrete dams can be saved by casting in-place concrete slabs as forms for interior concrete. Mass production methods for slabs require study, and economical physical dimensions of plain, reinforced, and prestressed slabs should be established. Industry could contribute to the development of method for anchorage and alinement, and on properties of the concrete mixture to enable it to withstand weathering effects. To be resistant to freezing-thawing action, concrete must contain myriads of small, well-dispersed bubbles of entrained air. The Bureau has developed an immersion-type air meter to measure air content. Further work is to be done to reduce the size, and weight of the equipment in improving portability.

Removal of lightweight aggregate constituents.-Unless concrete ag gregate is processed to remove lightweight constituents, the finished surface becomes disfigured with popouts caused by freezing-thawing action. Heavy media separation and elastic fractionation are two methods used to improve aggregate quality. Large-scale industrial equipment is available for this processing, and is frequently required to meet Bureau specifications.

Water-reducing set-retarding agents.—Results of laboratory and field tests have shown that many commercial water-reducing set-retarding agents will increase the compressive, tensile, and shearing strengths of mass and structural concrete, as well as improve resistance to freezing-thawing deterioration. These agents also extend the time in which concrete can be consolidated. Reduction in workability also occurs with some agents, however, and the cause of this characteristic should be identified and eliminated.

Sulfate resistance and absorption of concrete pipe.-Studies of the relationship between sulfate resistance and absorption of concrete pipe have been conducted toward establishing safe limits for absorp tion. The data are as yet insufficient, but will be supplemented to enable formulating an acceptable specification.

Cement and admixtures

Lignin and hydroxylated carboxylic acid.-Tests of several commer cial agents of the lignin and hydroxylated carboxylic acid classes of admixtures show that optimum dosages in concrete will increase com pressive, tensile, and shearing strengths, and extend the time in which concrete can be consolidated by vibration. Industry can help to discover and eliminate the characteristics of these materials which hinder workability.

Bituminous

Roofing material.-Past experience with roof construction on Bureau structures has shown a need for improvement in construction techiques and materials for built-up roofing. Common defects are blisering and buckling. Roofing materials of a breathing type to prevent blistering effects and use of sprayed-on-type roof construction are desirable. The Bureau is working with industry on evaluating materials being developed toward this use.

Asbestos fibers, rubberized asphalts and epoxy resin additives.—The Bureau is also working with industry on use of additives for improving asphaltic concrete in canal linings and dam facings. Such additives as asbestos fibers, rubberized asphalts, and epoxy resins have been investigated in the laboratory. All show some degree of promise. Industry could further explore the possibilities of other additives for this work.

Corrosion

Evaluation.-Accurate evaluation of potential corrosivity of soils and water for laboratory samples and during field studies is a serious problem. The corrosion of buried and underwater metalwork is like

wise of major concern. Present-day methods for laboratory and field evaluation of potential corrosivity of soil and water, or the determination of structural corrosion, involve a mixture of traditional and new techniques and equipment. Some of these are resistivity, Corfield corrosion tests, electrical resistance corrosion meter measurements, determination of pH, chemical analysis of waters and soils, and metallurgical considerations.

Electrical resistance corrosion meter.—The electrical resistance corrosion meter is a new instrument introduced recently for determination of actual corrosion rates experienced by a standard specimen. The measure is based on the electrical resistance of a specially prepared specimen or probe. The method can provide answers in a matter of hours or days where previous determinations would require several months to obtain significant data. The electrical resistance corrosion meter now used provides one of the best known means of evaluating the potential corrosivity of soils and waters, but must be used with considerable judgment. The meter has been used to determine the potential corrosivity of project waters and soils, and to determine the best metal to be used for cooling tubes or other mechanical features exposed to water.

Corrosivity of soils and waters instruments.-Industrial research can assist in the search for improved instruments for both laboratory and field evaluation of the potential corrosivity of soils and waters, and the progress of corrosion of buried and submerged structures.

Cathodic protection.-An important means of corrosion mitigation is the use of cathodic protection, usually in conjunction with protective coatings. The Bureau uses this method to correct high corrosion developed after construction or where excessive corrosion is expected on new projects. The process is based upon the electrochemical nature of corrosion. A current may be supplied as rectified power or through sacrificial anodes, such as magnesium or zinc, which create their own voltage necessary to supply the protective current.

nological advances are extending the possible application of cathodic

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FIGURE 12.-Several vibrators mounted on a tractor can be used for consolidation of concrete in large masses, such as in gravity and arch dams.

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FIGURE 13.—An immersion-type air meter for determining the air content of fresh concrete in place has been recently developed.

protection in reclamation work. These advancements include: High-current sacrificial anodes; high-strength, low-consumption anodes for impressed current systems; new rectifier designs; automatic control of impressed currents; deep well anode design; and anodic protection. The Bureau has applied cathodic protection to buried penstocks, well casings, water tanks, and underwater portions of control gates. Research is continuing to develop advanced techniques and equipment for the more efficient use of this tool of corrosion prevention.

Industry can contribute to the solution of corrosion problems by developing better anodes with longer life, high current capacity anodes, techniques for improvement of cathodic protection, and new processes for corrosion prevention.

Epoxy coal tar, catalyzed phenolic and epoxy phenolic paints.— Recently developed epoxy coal tar, catalyzed phenolic and epoxyphenolic paints are being tested on submerged metalwork under field service conditions. A direct comparison of these materials and plastic vinyl resin paint under similar service conditions will be available. Synthetic rubber and epoxy coatings are being field tested for use in water-carrying conduits.

Vinyl resin and cuprous oxide paints. Two vinyl resin paint systems developed by the Bureau are being used extensively for corrosion protection. Because of long life expectancy and general applicability, these systems considerably reduce painting maintenance costs. Antifouling paints of cuprous oxide, vinyl resin are now being specified for use in controlling algae on irrigation structures.

Earth

Nuclear instruments in placement density and moisture content.Methods for rapid measurement of placement density and moisture content in compacted embankments have been investigated, using both commercial equipment and Bureau devices. Nuclear instruments have been purchased for trial experiments and comparison with laboratory equipment. A corresponding need to control compaction criteria of gravelly soils remains unsatisfied.

Automatic, mechanical, compaction machines.-Commercial automatic, mechanical, compaction machines for the laboratory Proctor compaction test have been found satisfactory after calibration with previous standards. Bureau field laboratories are now equipped with these machines.

Conventional types of compaction equipment.-Conventional types of compaction equipment include sheepsfoot and pneumatic tire rolling, vibration, and impact. Comparative tests of these methods are needed with particular attention devoted to effects on soil properties. One example would be to compare soil permeabilities after treatment with sheepsfoot and pneumatic rollers. More rapid compaction methods are desirable, either with present equipment and new techniques; or with novel equipment.

Instruments to study earth- and rock-fill dams.-Earth- and rock-fill dams are being proposed for more difficult sites and increased heights. Experience data on settlement during construction, distribution of stress, the nature of failure surfaces, and cracking must be accu

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