concrete will not be as watertight as a cement-only concrete,  nor  will  it  have  as  much  initial  strength. Additional tests may have to be made to determine when to remove the forms. Its final strength, however, will be as great as a cement-only concrete. Densifiers Dense  concrete  is  required  in  some  types  of construction, such as in prestressed structures. This density is achieved when cement particles are separated evenly  throughout  the  mix  or  at  least  prevented  from attaching  to  each  other  (flocculating).  A  detergent admixture  will  disperse  the  particles  individually  and will create a more uniform paste. These admixtures also reduce the formation of a cement gel that expands at the early stages of hydration and pushes the particles apart, thus increasing the volume. Prevention of this expansion results in a denser paste. Waterproofing  Agents Watertightness can be controlled to a great extent by lowering the water-cement ratio. This may not always be  practical,  and  sometimes  even  with  a  low water-cement ratio, capillaries still form through the concrete.  Densifying  or  using  an  accelerator  like calcium chloride improves the watertightness. Air-Entraining  Agents The  greatest  improvement  in  watertightness  and resistance to deterioration under freezing and thawing is obtained by incorporating 4 to 6 percent, by volume, of  entrained  air  into  the  mix.  Workability  of  fresh concrete  is  enhanced  by  such  entrained  air.  Air- entrained cement contains the necessary admixture. Soaps, butylstearate, some of the fine pozzolans, and several proprietary compounds are available for use as air-entraining admixtures with ordinary cements. These agents  minimize  the  formation  of  capillaries  and  plug the tiny holes with a water-repellant or sealing material. They  provide  small,  uniformly  spaced,  discrete  air  voids that prevent the buildup of damaging pressures from the expansion of freezing water into ice. CURING Concrete does not develop its full strength until the chemical  process  of  curing  (or  hydration)  is  complete. Curing takes place over an extended period—the most critical portion of which is from the day of placement through the 10th day. The extent and rate of curing depends upon the presence of moisture and the correct temperature within the mix. Temperature The  ideal  temperatures  for  concrete  work  are between 55°F and 70°F. Above this, rapid evaporation of moisture creates a problem. At lower temperatures, the curing or setting is delayed. Temperatures below 32°F stop the hydration process. Since the chemical action gives off some heat, some method must be used to keep the heat within the structure during times of low temperatures. Cold weather construction may even require heating the ingredients, or mix, and covering the emplaced  concrete  or  providing  a  heated  enclosure.  In hot weather, extra care is required to prevent a high temperature  rise  and  too  rapid  drying  of  the  fresh concrete.  Moistening  the  aggregate  with  cool  water  will lower the generated temperature. The water is kept cool as possible by the application of reflective white or aluminum paint to water supply lines and storage tanks. On  massive  construction,  such  as  dams  and  heavy retaining  walls,  the  mixing  water  is  often  cooled artificially or ice is substituted for part of the water. This ice must be melted by the time the concrete is fully mixed  and  ready  to  leave  the  mixer.  Cement  replace- ment materials (such as pozzolans of diatomaceous earth, pumicites, or fly ash) may be used to depress concrete  temperature  by  reduction  of  the  heat  of hydration  in  a  structure;  however,  pozzolans  vary widely and may have adverse effects on strength, air content,  and  durability,  if  used  in  excessive  amounts. Moisture Concrete  curing  depends  upon  chemical  action  in the presence of water. Any loss of moisture during the process  by  seepage  or  evaporation  prevents  complete hydration  and  development  of  optimum  strength  and watertightness. Saturating the subgrade on which the concrete  will  be  placed  will  delay,  if  not  prevent, seepage. Wood forms should be thoroughly wetted if they have not been treated otherwise. Covering the concrete without marring the surface as soon as possible after finishing is one method used to reduce evaporation. This covering may be some material, such as burlap, straw, or plastic film, or it may be a chemical curing compound that is sprayed over the finished surface. After the initial set is attained, water can be applied directly to the surface to keep the hydration process in action.  This  water  application  can  be  part  of  the temperature   control.   The   increase   on   concrete compressive strength with age is shown by curves in 13-28