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. CONCRETE  AND  CONCRETE  TESTING 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. COMPOSITION 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 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 13-19