Readings taken with the hydrometer require correction due to the following factors: (1) the difference between the test temperature at each reading and the standard temperature of 68F, (2) the affect of the dispersing agent on the liquid density of the soil-water suspension, and (3) the difficulty of reading the hydrometer at the meniscus of the murky soil-water suspension. The manner in which the correction factors are determined and applied to obtain corrected readings can be found in ASTM D 422 and NAVFAC MO-330. The corrected readings are used to determine the percent passing and the particle sizes corresponding to each reading.

As you know, the larger particles in a soil-water suspension settle more rapidly than the smaller particles; therefore, for each hydrometer reading, the percent passing is the percentage of soil remaining in suspension at the level at which the hydrometer measures the density of the soil-water suspension. That percentage, for each hydrometer reading, can be calculated using the formulas given in NAVFAC MO-330 or ASTM D 422. The particle sizes corresponding to each of those percentages is calculated on the basis of Stokes law, that relates the terminal velocity of a free-falling sphere in a liquid to its diameter. To calculate the sizes, use the formulas given in ASTM D 422 or use the nomograph procedure discussed in NAVFAC MO-330.

As you learned in the EA3 TRAMAN, concrete is one of the most economical, versatile, and universally used construction materials. It is one of the few building materials that can be produced directly on the jobsite to meet specific requirements. In this section, you will study the materials used to produce concrete and you will further your knowledge of concrete testing.

Concrete is a synthetic construction material made by mixing cement, fine aggregate, coarse aggregate, and water together in proper proportions. The following paragraphs discuss each of these materials:

Cement is a substance that hardens with time and holds or entraps objects or particles in a definite relationship to each other. For concrete, portland cement is usually used. Portland cement is manufactured by a standardized process consisting of grinding limestone and clay, mixing them in proportions, heating the mixture to a high temperature to form clinkers, and then pulverizing the clinkers so that 95 percent of the material will pass through a No. 200 sieve. The following paragraphs describe the various types of Portland cement:

. TYPE I. Normal portland cement is an all-purpose type used to make ordinary concrete pavements, buildings, bridges, masonry units, and the like.

. TYPE II. Modified portland cement is a type that generates less heat during the curing process than Type I. The hydration process generates heat that, in a large mass of concrete, can become high enough to affect the concrete adversely. Type II is also more sulphur-resistant than Type I. Sulphur exists in water or soil having a high alkali content and has an adverse effect on the concrete.

. TYPE III. High-early-strength portland cement, as the name suggests, is used where a high strength is needed quickly. That maybe due to a demand for early use, or in cold-weather construction to reduce the period of protection against low or freezing temperatures.

. TYPE IV. Low-heat portland cement has the heat-resistant quality of Type II, but to a higher degree. It develops strength at a slower rate than Type I but helps prevent the development of high temperatures in the structure with the attendant danger of thermal cracking upon later cooling.

. TYPE V. Sulphate-resistant portland cement has a higher degree of sulphate resistance than Type II and is for use where high sulphate resistance is desired. Other types of cements maybe variations of the five types above or may be special types. Some of these types are as follows:

. TYPE IS. Portland blast-furnace slag cement uses granulated slag, which is rapidly chilled or quenched from its molten state in water, steam, or air. The slag (from 25 to 65 percent of the total weight of the cement) is interground with cement clinker. This cement is for general use in concrete construction.

. TYPE IP. Pozzolan cement uses a mixture of from 15 to 40 percent of pozzolan with the cement clinker. Pozzolan is a siliceous or siliceous and aluminous material, such as fly ash, volcanic ash, diatomaceous earth, or calcined shale. The strength of concrete made with pozzolanic cements is not as great as concrete made with the same amount of Portland cement, but its workability may be better for some uses.

. AIR-ENTRAINED CEMENT. Concrete made with air-entrained cement is resistant to severe frost action and to salts used for ice and snow removal. It is produced by adding air-releasing materials to the clinker, as it is ground. In general, air-entrainment may be controlled to a much greater extent by the use of admixtures with normal cements during mixing. This combination results in a concrete with tiny, distributed, and separated air bubbles (up to millions per cubic foot). The entrained air bubbles improve the workability of the fresh concrete. These bubbles reduce the capillary and water channel structure within hardened concrete and restrict the passage of water. That prevents the buildup

of damaging water pressure in the pores when concrete is frozen; therefore, air-entrained concrete has greatly increased durability in outdoor locations exposed to freezing weather. types I, II, III, IS, and IP cements are available as air entrained. The letter A is added after the type to signify that it is air entrained; for example, air-entrained pozzolan cement is known as Type IP-A. In addition to the types described above, there are white cement, waterproofed cement, and oil well cement. White cement is made from selected materials to prevent coloring, staining, or darkening of finished concrete. Waterproofed cement has water-repellent materials added. The finished and set concrete has a water-repellent action. Oil well cement is specially made to harden properly when used under high temperature in deep oil wells.