Curing Concrete

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rubbing if plywood or lined forms are used. The first rubbing  should  be  done  with  coarse  carborundum stones as soon as the concrete has hardened so that the aggregate  is  not  pulled  out.  The  concrete  should  then be  cured  until  final  rubbing.  Finer  carborundum stones are used for the final rubbing. The concrete should  be  kept  damp  while  being  rubbed.  Any  mortar used in this process and left on the surface should be kept damp for 1 to 2 days after it sets to cure properly. The  mortar  layer  should  be  kept  to  a  minimum thickness  as  it  is  likely  to  scale  off  and  mar  the appearance of the surface. EXPOSED AGGREGATE FINISH An exposed aggregate finish provides a nonskid surface. To obtain this, you must allow the concrete to  harden  sufficiently  to  support  the  finisher.  The aggregate is exposed by applying a retarder over the surface and then brushing and flushing the concrete surface  with  water.  Since  timing  is  important,  test panels should be used to determine the correct time to expose  the  aggregate. CURING  CONCRETE Adding  water  to  Portland  cement  to  form  the water-cement paste that holds concrete together starts a  chemical  reaction  that  makes  the  paste  into  a bonding   agent. This   reaction,   called   hydration, produces   a   stone-like   substance—the   hardened cement paste. Both the rate and degree of hydration, and  the  resulting  strength  of  the  final  concrete, depend  on  the  curing  process  that  follows  placing  and consolidating   the   plastic   concrete.   Hydration continues indefinitely at a decreasing rate as long as the  mixture  contains  water  and  the  temperature conditions are favorable. Once the water is removed, hydration ceases and cannot be restarted. Curing  is  the  period  of  time  from  consolidation  to the  point  where  the  concrete  reaches  its  design strength. During this period, you must take certain steps to keep the concrete moist and as near 73°F as practical. The properties of concrete, such as freeze and thaw resistance, strength, watertightness, wear resistance,  and  volume  stability,  cure  or  improve  with age  as  long  as  you  maintain  the  moisture  and temperature   conditions   favorable   to   continued hydration. The length of time that you must protect concrete against moisture loss depends on the type of cement used,  mix  proportions,  required  strength,  size  and shape  of  the  concrete  mass,  weather,  and  future exposure conditions. The period can vary from a few days to a month or longer. For most structural use, the curing period for cast-in-place concrete is usually 3 days  to  2  weeks. This  period  depends  on  such conditions   as   temperature,   cement   type,   mix proportions,  and  so  forth.  Bridge  decks  and  other slabs  exposed  to  weather  and  chemical  attack  usually require  longer  curing  periods.  Figure  7-51  shows how moist curing affects the compressive strength of concrete. Curing  Methods Several curing methods will keep concrete moist and,  in  some  cases,  at  a  favorable  hydration temperature.  They  fall  into  two  categories:  those  that Figure 7-51.-Moist curing effect on compressive strength of concrete. 7-29

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