to sulfate attack or damage by the heat of hydration. Type  I  portland  cement  is  used  in  pavement  and sidewalk construction, reinforced concrete buildings and   bridges,   railways,   tanks,   reservoirs,   sewers, culverts,  water  pipes,  masonry  units,  and  soil-cement mixtures. Generally,  it  is  more  available  than  the other types. Type I cement reaches its design strength in about 28 days. TYPE II.— Type II cement is modified to resist moderate sulfate attack. It also usually generates less heat of hydration and at a slower rate than type I. A typical application is for drainage structures where the  sulfate  concentrations  in  either  the  soil  or groundwater are higher than normal but not severe. type II cement is also used in large structures where its moderate heat of hydration produces only a slight temperature  rise  in  the  concrete.  However,  the temperature rise in type II cement can be a problem when concrete is placed during warm weather. Type II cement reaches its design strength in about 45 days. TYPE  III.—  Type  III  cement  is  a  high-early- strength  cement  that  produces  design  strengths  at  an early age, usually 7 days or less. It has a higher heat of hydration and is more finely ground than type I. Type  III  permits  fast  form  removal  and,  in  cold weather  construction,  reduces  the  period  of  protection against low temperatures. Richer mixtures of type I can obtain high early strength, but type III produces it more satisfactorily and economically. However, use it cautiously in concrete structures having a minimum dimension  of  2  1/2  feet  or  more.  The  high  heat  of hydration can cause shrinkage and cracking. TYPE IV.— Type IV cement is a special cement. It  has  a  low  heat  of  hydration  and  is  intended  for applications requiring a minimal rate and amount of heat of hydration. Its  strength  also  develops  at  a slower  rate  than  the  other  types.  Type  IV  is  used primarily in very large concrete structures, such as gravity dams, where the temperature rise from the heat of hydration might damage the structure. Type IV  cement  reaches  its  design  strength  in  about  90 days. Types TYPE V.— Type V cement is sulfate-resistant and should be used where concrete is subjected to severe sulfate action, such as when the soil or groundwater contacting  the  concrete  has  a  high  sulfate  content. Type V cement reaches its design strength in 60 about days. Air-Entrained  Cement Air-entrained  portland  cement  is  a  special  cement that  can  be  used  with  good  results  for  a  variety  of conditions.  It  has  been  developed  to  produce  concrete that is resistant to freeze-thaw action, and to scaling caused by chemicals applied for severe frost and ice removal.  In  this  cement,  very  small  quantities  of air-entraining materials are added as the clinker is being ground during manufacturing. Concrete made with this cement contains tiny, well-distributed and completely separated air bubbles. The bubbles are so small that there may be millions of them in a cubic foot  of  concrete.  The  air  bubbles  provide  space  for freezing   water   to   expand   without   damaging   the concrete.  Air-entrained  concrete  has  been  used  in pavements in the northern states for about 25 years with  excellent  results.  Air-entrained  concrete  also reduces  both  the  amount  of  water  loss  and  the capillary/water-channel  structure. An  air-entrained  admixture  may  also  be  added  to types I, II, and III portland cement. The manufacturer specifies the percentage of air entrainment that can be expected  in  the  concrete.  An  advantage  of  using air-entrained cement is that it can be used and batched like  normal  cement.  The  air-entrained  admixture comes  in  a  liquid  form  or  mixed  in  the  cement.  To obtain  the  proper  mix,  you  should  add  the  admixture at the batch plant. AGGREGATES The  material  combined  with  cement  and  water  to make concrete is called aggregate. Aggregate makes up 60 to 80 percent of concrete volume. It increases the   strength   of   concrete,   reduces   the   shrinking tendencies  of  the  cement,  and  is  used  as  an economical filler. Aggregates  are  divided  into  fine  (usually consisting of sand) and coarse categories. For most building  concrete,  the  coarse  aggregate  consists  of gravel  or  crushed  stone  up  to  1  1/2  inches  in  size. However, in massive structures, such as dams, the coarse  aggregate  may  include  natural  stones  or  rocks ranging up to 6 inches or more in size. 6-4