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maximum  strength  and  durability  will  still  not  be attained unless the sand and coarse aggregate you use consist   of   well-graded,   clean,   hard,   and   durable particles  free  of  undesirable  substances  (figure  6-1). WATERTIGHTNESS OF CONCRETE The  ideal  concrete  mix  is  one  with  just  enough water  required  for  complete  hydration  of  the  cement. However,  this  results  in  a  mix  too  stiff  to  pour  in forms.  A  mix  fluid  enough  to  be  poured  in  forms always contains a certain amount of water over and above that which will combine with the cement. This water eventually evaporates, leaving voids, or pores, in the concrete. Penetration of the concrete by water is  still  impossible  if  these  voids  are  not  inter- connected. They may be interconnected, however, as a result of slight sinking of solid particles in the mix during the hardening period. As these particles sink, they  leave  water-tilled  channels  that  become  voids when  the  water  evaporates.  The  larger  and  more numerous these voids are, the more the watertightness of the concrete is impaired. The size and number of the voids vary directly with the amount of water used in  excess  of  the  amount  required  to  hydrate  the cement. To  keep  the  concrete  as  watertight  as possible,  you  must  not  use  more  water  than  the minimum  amount  required  to  attain  the  necessary degree  of  workability. GENERAL REQUIREMENTS FOR GOOD CONCRETE The first requirement for good concrete is to use a cement type suitable for the work at hand and have a satisfactory  supply  of  sand,  coarse  aggregate,  and water. Everything else being equal, the mix with the best  graded,  strongest,  best  shaped,  and  cleanest aggregate  makes  the  strongest  and  most  durable concrete. Second,   the   amount   of   cement,   sand,   coarse aggregate, and water required for each batch must be carefully  weighed  or  measured  according  to  project specifications. Third, even the best designed, best graded, and highest quality mix does not make good concrete if it is  not  workable  enough  to  fill  the  form  spaces thoroughly.  On  the  other  hand,  too  much  fluidity  also results  in  defects.  Also,  improper  handling  during  the overall  concrete  making  process,  from  the  initial aggregate  handling  to  the  final  placement  of  the  mix, causes  segregation  of  aggregate  particles  by  sizes, resulting  in  nonuniform,  poor-quality  concrete. Finally, the best designed, best graded, highest quality, and best placed mix does not produce good concrete if it is not proper] y cured, that is, properly protected against loss of moisture during the earlier stages of setting. CONCRETE INGREDIENTS LEARNING  OBJECTIVE:  Upon  completing this section, you should be able to identify the ingredients essential for good concrete. The  essential  ingredients  of  concrete  are  cement, aggregate, and water. A mixture of only cement and water  is  called  cement  paste.  In  large  quantities, however, cement paste is prohibitively expensive for most  construction  purposes. PORTLAND CEMENT Most  cement  used  today  is  portland  cement.  This is  a  carefully  proportioned  and  specially  processed combination of lime, silica, iron oxide, and alumina. It is usually manufactured from limestone mixed with shale,  clay,  or  marl. Properly   proportioned   raw materials are pulverized and fed into kilns where they are  heated  to  a  temperature  of  2,700°F  and  maintained at  that  temperature  for  a  specific  time.  The  heat produces  chemical  changes  in  the  mixture  and transforms it into clinker—a hard mass of fused clay and  limestone.  The  clinker  is  then  ground  to  a fineness  that  will  pass  through  a  sieve  containing 40,000  openings  per  square  inch. Types of Cement There are five types of Portland cement covered under  “Standard  Specifications  for  Portland  Cement.” These  specifications  are  governed  by  the  American Society   for   Testing   and   Material   (ASTM)   types. Separate  specifications,  such  as  those  required  for air-entraining portland cements, are found under a separate ASTM. The type of construction, chemical composition of the soil, economy, and requirements for  use  of  the  finished  concrete  are  factors  that influence  the  selection  of  the  kind  of  cement  to  be used. TYPE I.— Type I cement is a general-purpose cement for concrete that does not require any of the special  properties  of  the  other  types.  In  general,  type I cement is intended for concrete that is not subjected 6-2

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