the  strength  of  walls  constructed  with  cored  brick and  those  constructed  with  solid  brick.  Resistance to moisture penetration is about the same for both types of walls. The most easily available brick that will meet the requirements should be used whether the  brick  is  cored  or  solid. SAND-LIME  bricks  are  made  from  a  lean mixture  of  slaked  lime  and  fine  silicious  sand, molded under mechanical pressure and hardened under steam pressure. Types of Bonds When the word  bond  is  used  in  reference  to masonry, it may have three different meanings: STRUCTURAL  BOND  is  a  method  of  inter- locking  or  tying  individual  masonry  units  together so  that  the  entire  assembly  acts  as  a  single structural  unit.  Structural  bonding  of  brick  and tile walls may be accomplished in three ways: first, by overlapping (interlocking) the masonry units; second,  by  the  use  of  metal  ties  embedded  in connecting  joints;  and  third,  by  the  adhesion  of grout  to  adjacent  wythes  of  masonry. MORTAR BOND is the adhesion of the joint mortar   to   the   masonry   units   or   to   the   re- inforcing  steel. PATTERN  BOND  is  the  pattern  formed  by the  masonry  units  and  the  mortar  joints  on  the face  of  a  wall.  The  pattern  may  result  from  the type of structural bond used or may be purely a decorative one in no way related to the structural bond.  Five  basic  pattern  bonds  are  in  common use  today,  as  shown  in  figure  7-52.  These  are running   bond,   common   bond,   stack   bond, Flemish  bond,  and  English  bond. RUNNING BOND is the simplest of the basic pattern  bonds;  the  running  bond  consists  of  all stretchers. Since there are no headers used in this bond, metal ties are usually used. Running bond is  used  largely  in  cavity  wall  construction  and veneered  walls  of  brick  and  often  in  facing  tile walls where the bonding may be accomplished by extra width stretcher tile. COMMON   or   AMERICAN   BOND   is   a variation  of  running  bond  with  a  course  of  full- length headers at regular intervals. These headers provide  structural  bonding,  as  well  as  pattern. Header  courses  usually  appear  at  every  fifth, sixth,   or   seventh   course,   depending   on   the structural bonding requirements. In laying out any bond pattern, it is important that the corners be started  correctly.  For  common  bond,  a  three- quarter brick must start each header course at the corner.  Common  bond  may  be  varied  by  using a  Flemish  header  course. STACK  BOND  is  purely  a  pattern  bond. There is no overlapping of the units, all vertical joints   being   aligned.   Usually,   this   pattern   is bonded  to  the  backing  with  rigid  steel  ties,  but when 8-in.-thick stretcher units are available, they may  be  used.  In  large  wall  areas  and  in  load- bearing construction, it is advisable to reinforce the  wall  with  steel  pencil  rods  placed  in  the horizontal  mortar  joints.  The  vertical  alignment requires  dimensionally  accurate  units,  or  carefully prematched  units,  for  each  vertical  joint  align- ment.  Variety  in  pattern  may  be  achieved  by numerous combinations and modifications of the basic  patterns  shown. FLEMISH  BOND  is  made  up  of  alternate stretchers   and   headers,   with   the   headers   in alternate  courses  centered  over  the  stretchers in  the  intervening  courses.  Where  the  headers  are not  used  for  structural  bonding,  they  may  be obtained  by  using  half  brick,  called  blind-headers. Two  methods  are  used  in  starting  the  corners. Figure   7-52   shows   the   so-called   FLEMISH corner in which a three-quarter brick is used to start  each  course  and  the  ENGLISH  corner  in which 2-in. or quarter-brick closures must be used. ENGLISH  BOND  is  composed  of  alternate courses of headers and stretchers. The headers are centered  on  the  stretchers  and  joints  between stretchers.  The  vertical  (head)  joints  between stretchers in all courses line up vertically. Blind headers are used in courses that are not structural bonding  courses.  The  English  cross  bond  is  a variation of English bond and differs only in that vertical joints between the stretchers in alternate courses  do  NOT  line  up  vertically.  These  joints center  on  the  stretchers  themselves  in  the  courses above  and  below. Masonry  Terms Specific   terms   are   used various  positions  of  masonry to   describe   the units and mortar 7-33