Type N— One part portland cement, one part hydrated lime or lime putty, and six parts sand; or, one part type II masonry cement and three parts  sand.  Type  N  mortar  is  suitable  for general use in above-grade exposed masonry where high compressive or lateral strength is not  required. Type O— One part portland cement, two parts hydrated  lime  or  lime  putty,  and  nine  parts sand;  or,  one  part  type  I  or  type  II  masonry cement and three parts sand. Type O mortar is recommended   for   load-bearing,   solid-unit walls  when  the  compressive  stresses  do  not exceed 100 pounds per square inch (psi) and the  masonry  is  not  subject  to  freezing  and thawing in the presence of a lot of moisture. MIXING  MORTAR The manner in which mortar is mixed has a lot to do with the quality of the final product. In addition to machine  and  hand  mixing,  you  need  to  know  the requirements  for  introducing  various  additives, including  water,  to  the  mix  in  order  to  achieve optimum results. Machine  Mixing Machine  mixing  refers  to  mixing  large  quantities of  mortar  in  a  drum-type  mixer. Place   all   dry ingredients  in  the  mixer  first  and  mix  them  for 1  minute  before  adding  the  water.  When  adding water,  you  should  always  add  it  slowly.  Minimum mixing  time  is  3  minutes.  The  mortar  should  be mixed until a completely uniform mixture is obtained. Hand  Mixing Hand  mixing  involves  mixing  small  amounts  of mortar by hand in a mortar box or wheelbarrow. Take care  to  mix  all  ingredients  thoroughly  to  obtain  a uniform mixture. As in machine mixing, mix all dry materials together first before adding water. Keep a steel drum of water close at hand to use as the water supply. You should also keep all your masonry tools free of hardened mortar mix and dirt by immersing them in water when not in use. Requirements You  occasionally  need  to  mix  lime  putty  with mortar. When machine mixing, use a pail to measure the  lime  putty.    Place the putty on top of the sand. When hand mixing, add the sand to the lime putty. Wet pails before filling them with mortar and clean them  immediately  after  emptying. Mixing  water  for  mortar  must  meet  the  same quality  requirements  as  mixing  water  for  concrete. Do  not  use  water  containing  large  amounts  of dissolved  salts.  Salts  weaken  the  mortars. You can restore the workability of any mortar that stiffens on the mortar board due to evaporation by remixing it thoroughly. Add water as necessary, but discard  any  mortar  stiffened  by  initial  setting. Because  it  is  difficult  to  determine  the  cause  of stiffening, a practical guide is to use mortar within 2  1/2  hours  after  the  original  mixing.  Discard  any mortar you do not use within this time. Do not use an antifreeze admixture to lower the freezing pint of mortars during winter construction. The quantity necessary to lower the freezing point to any  appreciable  degree  is  so  large  it  will  seriously impair the strength and other desirable properties of the mortar. Do  not  add  more  than  2-percent  calcium  chloride (an  accelerator)  by  weight  of  cement  to  mortar  to accelerate its hardening rate and increase its early strength.  Do  not  add  more  than  1-percent  calcium chloride to masonry cements. Make a trial mix to find the  percentage  of  calcium  chloride  that  gives  the desired hardening rate. Calcium chloride should not be  used  for  steel-reinforced  masonry.  You  can  also obtain  high  early  strength  in  mortars  with high-early-strength  portland  cement. MODULAR PLANNING Concrete masonry walls should be laid out to make maximum  use  of  full-  and  half-length  units.  This minimizes cutting and fitting of units on the job. Length and height of walls, width and height of openings, and wall  areas  between  doors,  windows,  and  corners  should be planned to use full-size and half-size units, which are usually available (figure 8-6). This procedure assumes that window and door frames are of modular dimensions which fit modular full- and half-size units. Then, all horizontal dimensions should be in multiples of nominal full-length masonry units. Both horizontal and vertical dimensions should be designed to be in multiples of 8 inches. Table 8-2 lists nominal   length   of   concrete   masonry   walls   by stretchers. Table 8-3 lists nominal height of concrete masonry walls by courses. When 8-by-4-by-16 units are used, the horizontal dimensions should be planned in multiples of 8 inches (half-length units) and the vertical dimensions in multiples of 4 inches. If the thickness of the wall is greater or less than the length of a half unit, a special-length unit is required at each 8-8