ADDITIVES AND ADMIXTURES FOR PORTLAND CEMENT CONCRETE Additives are substances or agents added in small amounts to a basic ingredient of a mixture prior to mixing. Admixtures are substances or agents added in small amounts to the basic ingredients of a mixture during the mixing. The same substances or agents may be either, or both, depending on when they are added to the mixture. Investigation regarding the use of admixtures in portland cement concrete has been carried on in this country since about 1885. Prior to 1950, however, there was a reluctance on the part of highway departments to specify and use them, atrributed by some research groups to a lack of understanding the chemical nature of these materials and ability to predict how the physical characteristics of the concrete could be improved by their use. Constant research by industry and by many Government agencies and State highway departments has produced a vast amount of technical information in this area. For example, the American Concrete Institute Committee No. 212 on Admixtures in Concrete published a report in 1954 suggesting the classification of additives and admixtures into the following groups: 1. Accelerators 2. Retarders 3. Air entraining agents 4. Gas forming agents 5. Cementitious materials 6. Grouting agents Accelerators 7. Pozzolans 8. Alkali-aggregate inhibitors 10. Workability agents These materials speed up the strength development of concrete. This permits earlier stripping of the forms and use of the structure and shortens the time for protecting concrete placed in winter. Calcium chloride is one of the most widely used accelerators used in concrete. Retarders These materials sometimes called dispersing agents delay the setting or stiffening of concrete which may be desirable because of hot weather or to aid in the placement of complicated geometric shapes or concrete containing closely spaced reinforcement. Among the agents which produce this effect are sugars, starches, calcium lignosulphate, and certain organic compounds. Air entraining agents These materials, now the most widely used admixtures for concrete, introduce very small separate air bubbles into the concrete. This entrained air materially aids the concrete to resist damage from freezing and thawing, improves the workability and plasticity which permits lower water-cement ratios, helps to reduce segregation and the tendency to bleed. This in turn permits earlier finishing of flat surfaces. In February 1954, the Physical Research Branch of the Bureau of Public Roads, published a report entitled "Evaluation of Air Entraining Admixtures for Concrete," which gave the results of tests performed on 27 different materials produced by 19 manufacturers. These were classified with respect to their major constituents into the following seven groups: A. Salts of wood resins E. Salts of proteinaceous mate- F. Fatty resinous acids and their salts G. Organic salts of sulfonated hydrocarbons This report includes a list of "Admixtures Approved by the Bureau of Public Roads for Use on Direct Government Contracts in the National Parks and National Forests." A companion report "Chemical Analysis and Sources of Air Entraining Admixtures for Concrete" also by the Physical Research Branch, was published by the Bureau in February 1954. This report presented the results of an investigation of the practicability of applying chemical analyses as a means of identifying and controlling the uniformity of the various admixtures used in concrete. Grouting agents These materials are used as lubricants to aid in the penetration of grout into seams, cracks, and gravel pockets at reasonable pressure. Bentonite, pumicite, fly ash, and various proprietary materials have been used for this purpose. Research has also been conducted by the Bureau on other admixtures including retarders and accelerators. Workability agents As the name implies, these agents assist in the handling and placing of concrete. Some materials used for this purpose are powdered or finely ground inert materials such as hydrated lime, bentonite, pumicite, fly ash, diatomaceous earth, and some sulfonated organic compounds. Pozzolans These materials have the ability or reacting with the lime freed by the hydration of portland cement to form additional cementitious compounds which add to the strength of the hardened concrete. Some of these are natural materials such as volcanic ash. Some are artificial such as fly ash. A number of proprietary products with these pozzolanic properties are available from industry. Dampproofing agents These agents as the name implies are intended to reduce the penetration of moisture through hardened concrete. In concrete of moderate to high cement content, air entraining agents have been effective for this purpose. For concretes of low cement content, other admixtures such as certain soaps, stearates, and asphalt emulsions have been used to reduce concrete permeability. Trends in acceptance That the reluctance on the part of State highway departments to use additives and admixtures in portland cement concrete pavements and structures has disappeared is evidenced by the AASHO designation M-154 (ASTM designation C-260), which was developed in 1948 and revised in 1960, "Standard Specifications for Air Entraining Admixtures for Concrete" and AASHO designation T-157 "Standard Methods of Testing Air Entraining Admixtures for Concrete" and the inclusion in many State highway standard specifications of requirements for air entrained concretes, agents, and air entraining portland cement. Scope of use BITUMINOUS MATERIALS Bituminous materials used in highway work are products resulting from processing asphalts from native deposits, the refining of petroleum, and the destructive distillation of bituminous coal. Coal tar products were used as binder materials in our early paving program, especially in the Northeast, where coal tar was a byproduct of the coal coking industry. Tar is still used in these areas to a limited extent. Chemical demands for coal tar have limited the use of this product in highway construction and maintenance. The market for bituminous materials has increased greatly in recent years. The current annual requirements for highway construction and maintenance are running about 14 million tons. Based on projected highway activities and expenditures, some 172 million tons of bitumen, including asphalt and coal tar products, will be required in the 1962-72 period for highway construction and maintenance purposes. Trends in prices of petroleum asphalt The prices of asphalt cement from six sources of delivery within the United States were nearly 3 percent lower in the 1960-61 period than the price 10 years earlier. Cutback asphalt (asphalt cement liquified with volatile solvents) in this same 10-year period has shown an increase of only 10 percent. Quality control For many years the need for methods of evaluating the quality of asphaltic materials has been recognized. The petroleum industry, individually and through its associations, including the Asphalt Institute and the National Bituminous Concrete Association, is cooperating with State highway departments and the Bureau of Public Roads in seeking these improvements. Bureau studies of asphalt consistency. In 1954 and 1955, the Bureau of Public Roads began a study of asphalt cements being manufactured for use in highway construction. The samples were taken by our field offices and were representative of the materials then in use in the States. A total of 323 samples from more than 100 refineries were obtained. In February 1959, the first report of this study, which included the results for the samples in the 85-100 penetration grade, was presented. Of the 119 materials included in the first report, 12 were out of grade by a few penetration points, but only 3 would fail to meet any of the other standard requirements of the American Association of State Highway Officials. However, the results of our tests show a very wide range in the engineering properties of these materials. If the mixing temperature was to be set at 275° F., the viscosities of the asphalts during mixing would range from a minimum of 85 seconds to a maximum over 300 seconds. Other tests indicate that the hardening that occurs when these materials go through the normal mixing process may vary from only a few penetration points to as much as 50 or 60 percent of the original penetration. Industry, the highway departments, and the Bureau are endeavoring to develop improvements among which are the following: (1) Laboratory tests that directly measure the quality of the material; (2) Greater knowledge of the fundamental characteristics of asphalts and the relation between these characteristics and the performance of the pavement; (3) More uniformity of materials meeting the same specifica tions. New laboratory tests.-Progress has been made toward improved laboratory tests. The Bureau's efforts along this line have resulted in the development of the thin-film oven test. This test has now been accepted for many State specifications, by the Asphalt Institute, and by the American Association of State Highway Officials. This is a simple test. A one-eighth-inch film of asphalt is heated in an oven for 5 hours at 325° F., the penetration is determined before and after heating, and the amount of hardening is expressed as the percentage of retained penetaration. This hardening correlates relatively with the hardening that is likely to occur in the mixing plant. That is, those asphalts that harden the most in this test are likely to harden most during the mixing process under the same conditions. Efforts are being made to develop tests which will provide better methods to define engineering stability of asphalts. The Asphalt Institute, asphalt producers, and the Physical Research Division of the Bureau are experimenting with tests for obtaining fundamental consistency relationships which eventually may supplement or even eliminate the empirical test methods now used. The development of new tests and instrumentation is opening the way for determining fundamental viscosities of asphalts. Perfection of these tests will enable highway departments to define construction temperatures more precisely in their specifications. In the case of hot asphaltic concrete, it will probably be necessary to control consistency at more than one temperature. Coal tar products In 1959, an industrial firm developed a bituminous binder, produced by the digestion of powdered coal in coal tar and tar oils, for highway construction and maintenance. The producer claimed the product to be comparable to, and could be used in the same manner as, asphaltic cements in hot-mix bituminous concrete. Several coal-producing States investigated the feasibility of utilizing the new binder material in their highway construction programs. Kentucky, however, was the first and only State to set up a production plant and incorporate the coal modified coal-tar binder into pavement surfaces for experimental purposes. At the same time, control sections using standard asphaltic binder were constructed for comparative evaluation. The experimental sections of coal tar totaled 10.2 miles of two-lane road sections, and the standard asphalt control sections totaled 13 miles. During the first year, continuous evaluations of these sections were carried on by laboratory tests and visual observation of performance. |