cause segregation of aggregate particles by sizes, resulting  in  nonuniform,  poor  concrete. Finally, the best designed, best graded, highest quality, and best placed mix in the world will not produce   good   concrete   if   it   is   not   properly CURED—meaning,   properly   protected   against loss   of   moisture   during   the   earlier   stages   of setting. As  you  can  see,  the  important  properties  of concrete are its strength, durability, and water- tightness.  These  factors  are  controlled  by  the WATER-CEMENT  RATIO  or  the  proportion  of water to cement in the mix. Strength The  COMPRESSIVE  strength  of  concrete  is very  high,  but  its  TENSILE  strength  (meaning  its ability to resist stretching, bending, or twisting) is relatively low. Consequently, concrete that must resist   a   good   deal   of   stretching,   bending,   or twisting, such as concrete in beams, girders, walls, columns,  and  the  like,  must  be  REINFORCED with steel. Concrete that must resist compression only  may  not  require  reinforcement. Durability The  DURABILITY  of  concrete  means  the extent to which the material is capable of resisting the  deterioration  caused  by  exposure  to  service conditions.  Ordinary  structural  concrete  that  is to be exposed to the elements must be watertight and weather resistant. Concrete that is subject to wear, such as floor slabs and pavements, must be capable of resisting abrasion. It has been found that  the  major  factor  controlling  durability  is strength—in   other   words,   the   stronger   the concrete  is,  the  more  durable  it  will  be.  As mentioned previously, the chief factor controlling strength  is  the  water-cement  ratio,  but  the character,  size,  and  grading  (distribution  of particle  sizes  between  the  largest  permissible coarse  and  the  smallest  permissible  fine)  of  the aggregate  also  have  important  effects  on  both strength  and  durability.  Given  a  water-cement ratio   that   will   produce   maximum   strength consistent  with  workability  requirements,  max- imum  strength  and  durability  will  still  not  be attained  unless  the  sand  and  coarse  aggregate consist  of  well-graded,  clean,  hard,  and  durable particles, free   from   undesirable   substances (fig.  7-1). Watertightness The  ideal  concrete  mix  would  be  one  made with  just  the  amount  of  water  required  for complete hydration of the cement. This would be a  DRY  mix,  however,  too  stiff  to  pour  in  the forms.  A  mix  that  is  fluid  enough  to  be  poured into  forms  always  contains  a  certain  amount  of water  over  and  above  the  amount  that  will combine  wit  h  the  cement,  and  this  water  will eventually  evaporate,  leaving  voids  or  pores  in  the concrete. Even so, penetration of the concrete by water would still be impossible if these voids were not interconnected.   They   are   interconnected, however,  as  a  result  of  a  slight  sinking  of  solid particles in the mix during the hardening period. As  these  particles  sink,  they  leave  water-filled channels,  which  become  voids  when  the  water evaporates. The larger and more numerous these voids are, the more the watertightness of the concrete will be  impaired.  Since  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,  it  follows  that  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. PLAIN  CONCRETE Plain concrete is defined as concrete with no reinforcement, This type of concrete is most often used where strength is not essential and stresses are minimal, such as sidewalks or driveways and floors  where  heavy  loads  are  not  anticipated. REINFORCED  CONCRETE Reinforced  concrete  refers  to  concrete  con- taining  steel  (bars,  rods,  strands,  wire,  and  mesh) as reinforcement and designed to absorb tensile and  shearing  stresses,  Concrete  structural members,  such  as  footings,  columns  and  piers, beams,   floor   slabs,   and   walls,   must   be   re- inforced  to  attain  the  necessary  strength  in tension. Reinforced Concrete Structural Members A reinforced concrete structure is made up of many  types  of  reinforced  structural  members, including footings, columns, beams, slabs, walls, and  so  forth.  Their  basic  functions  are  briefly described  below. 7-2