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Engineering Aid 3 - Beginning Structural engineering guide book
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CHAPTER 7 CONCRETE  AND  MASONRY This   chapter   provides   information   and guidance for the Engineering Aid engaged in or responsible for drawing structural and architec- tural  layouts  from  existing  plans,  engineering sketches,  or  specifications,  It  includes  informa- tion  on  basic  materials  commonly  used  in concrete  and  masonry  construction. Basic  principles  and  procedures  associated with the construction of reinforced, precast, and prestressed concrete and tilt-up construction are also discussed in this chapter. Terminology as it applies to masonry units is used to acquaint the Engineering Aid with the various terms used in this  type  of  construction. CONCRETE CONCRETE  is  a  synthetic  construction material  made  by  mixing  CEMENT,  FINE AGGREGATE   (usually   sand),   COARSE   AG- GREGATE (usually gravel or crushed stone), and WATER  together  in  proper  proportions;  the product  is  not  concrete  unless  all  four  of  these ingredients  are  present.  A  mixture  of  cement, sand, lime, and water, without coarse aggregate, is  NOT  concrete,  but  MORTAR  or  GROUT. Mortar  is  used  mainly  for  bonding  masonry units together. The term grout refers to a water- cement  mixture  (called  neat-cement  grout)  or water-sand-cement  mixture  (called  sand-cement grout)  used  to  plug  holes  or  cracks  in  concrete, to seal joints, and for similar plugging or sealing purposes. The fine and coarse aggregates in a concrete mix  are  called  the  INERT  ingredients;  the  cement and   water   are   the   ACTIVE   ingredients.   The inert ingredients and the cement are thoroughly mixed  together  first.  As  soon  as  the  water  is added,  a  chemical  reaction  between  the  water  and the cement begins, and it is this reaction (which is called HYDRATION) that causes the concrete to  harden. Always remember that the hardening process is caused by hydration of the cement by the water, not  by  a  DRYING  OUT  of  the  mix.  Instead  of being dried out, the concrete must be kept as moist as possible during the initial hydration process. Drying out would cause a drop in water content below   the   amount   required   for   satisfactory hydration  of  the  cement. The  fact  that  the  hardening  process  has nothing whatever to do with a drying out of the concrete  is  clearly  shown  by  the  fact  that  concrete will harden just as well under water as it will in the air. Concrete may be cast into bricks, blocks, and other  relatively  small  building  units  that  are  used in   concrete   MASONRY   construction. The proportion of concrete to other materials used in building construction has greatly increased in recent years to the point where large, multistory modern  building  are  constructed  entirely  of concrete,   with   concrete   footings,   foundations, columns, walls, girders, beams, joists, floors, and roofs. REQUIREMENTS  FOR GOOD  CONCRETE The  first  requirement  for  good  concrete  is  a supply of good cement of a type suitable for the work  at  hand.  Next  is  a  supply  of  satisfactory sand,  coarse  aggregate,  and  water;  all  of  which must  be  carefully  weighed  and  measured. Everything else being equal, the mix with the best graded,   strongest,   best   shaped,   and   cleanest aggregate   will   make   the   strongest   and   most durable  concrete. The best designed, best graded, and highest quality  mix  in  the  world  will  NOT  make  good concrete  if  it  is  not  WORKABLE  enough  to  fill the form spaces thoroughly. On the other hand, too  much  fluidity  will  result  in  certain  defects. Improper  handling  during  the  whole  concrete- making   process   (from   the   initial   aggregate handling to the final placement of the mix) will 7-1







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