CHAPTER 6 CONCRETE Concrete  is  one  of  the  most  important  con- struction materials. It is comparatively economical, easy to make, offers continuity and solidity, and will bond with other materials. The keys to good-quality concrete  are  the  raw  materials  required  to  make concrete and the mix design as specified in the project specifications.   In   this   chapter,   we’ll   discuss   the characteristics  of  concrete,  the  ingredients  of  con- crete,  concrete  mix  designs,  and  mixing  concrete. We’ll   conclude   the   chapter   with   a   discussion   of precast  and  tilt-up  concrete.  At  the  end  of  the discussion,  we  provide  helpful  references.  You  are encouraged  to  study  these  references,  as  required,  for additional  information  on  the  topics  discussed. CONCRETE   CHARACTERISTICS LEARNING  OBJECTIVE:  Upon  completing this section, you should be able to define the characteristics  of  concrete. Concrete  is  a  synthetic  construction  material made  by  mixing  cement,  fine  aggregate  (usually sand),  coarse  aggregate  (usually  gravel  or  crushed stone),  and  water  in  the  proper  proportions.  The product   is   not   concrete   unless   all   four   of   these ingredients  are  present. CONSTITUENTS OF CONCRETE The fine and coarse aggregates in a concrete mix are  the  inert,  or  inactive,  ingredients.  Cement  and water are the active ingredients. The inert ingredients and the cement are first thoroughly mixed together. As  soon  as  the  water  is  added,  a  chemical  reaction begins   between   the   water   and   the   cement.   The reaction,  called  hydration,  causes  the  concrete  to harden.  This  is  an  important  point.  The  hardening process occurs through hydration of the cement by the water, not by drying out of the mix. Instead of being dried out, concrete must be kept as moist as possible during   the   initial   hydration   process.   Drying   out causes a drop in water content below that required for satisfactory hydration of the cement. The fact that the hardening process does not result from drying out is clearly shown by the fact that concrete hardens just as well underwater as it does in air. CONCRETE AS BUILDING MATERIAL Concrete  may  be  cast  into  bricks,  blocks,  and other relatively small building units, which are used in  concrete  construction.  Concrete  has  a  great  variety of applications because it meets structural demands and  lends  itself  to  architectural  treatment.  All important building elements, foundations, columns, walls, slabs, and roofs are made from concrete. Other concrete applications are in roads, runways, bridges, and  dams. STRENGTH OF CONCRETE The  compressive  strength  of  concrete  (meaning its ability to resist compression) is very high, but its tensile strength (ability to resist stretching, bending, or twisting) is relatively low. Consequently, concrete which 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  only  compression may  not  require  reinforcement.  As  you  will  learn later,   the   most   important   factor   controlling   the strength of concrete is the water-cement ratio, or the proportion  of  water  to  cement  in  the  mix. DURABILITY OF CONCRETE The durability of concrete refers to the extent to which the material is capable of resisting deteriora- tion   caused   by   exposure   to   service   conditions. Concrete  is  also  strong  and  fireproof.  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. The  major  factor  that  controls  the  durability  of concrete is its strength. The stronger the concrete, the more  durable  it  is.  As  we  just  mentioned,  the  chief factor  controlling  the  strength  of  concrete  is  the water-cement  ratio.  However,  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.  However, 6-1