Following are the Methods of Repair for Dormant Cracks in Concrete:

1. Sealing of Cracks:

Sealing  of  cracks  as  stand  alone  repair  should  be  used  in  conditions where  structural  repair  is  not  necessary.  Isolated  cracks  whether extending through the concrete section or partially into it, should be sealed at  the  concrete  surfaces.    For  this  a  slot  of  approx.  25mm  wide  should  be saw  cut  upto  10mm  deep  along  the  crack  keeping  crack  at  the  center  of the  slot.    The  concrete  should  be  chiseled  out  from  between  the  two  saw cut  edges  and  concrete  should  be  further  undercut  beyond  the  10mm depth up to say 20mm depth so that the base width is slightly greater than the  surface  width.  After  the  slot  is  thoroughly  cleaned,  soaked  with  water for  10  hrs.  and surface  dried,  a bond coat/  primer  coat,  of  an  approximate latex  bonding  compound  should  be  applied.  Once  the  primer  becomes tacky, high strength  polymer  modified  cementitious  mortar  should  be  filled  in  the  slot,  properly tamped  and  surface  finished.  Curing  compound  should  be  applied  as soon as surface becomes touch dry. 7 days wet curing should be done by covering with wet Hessian and polythene sheet.

2.  Routing and Sealing of Cracks:

Alternatively a V-groove should be prepared along the crack at the surface ranging  in  depth  from  6  to  25mm  and  minimum  opening  at  surface  of 6mm (Fig. 1)

 Repair of crack by routing and sealing

Fig: Repair of crack by routing and sealing

A  concrete  saw,  hand  tools  or  pneumatic  tools  may  be  used.  The  groove is  then  cleaned  by  air  blasting,  sand  blasting  or  water  blasting  and  dried. A sealant is placed into the dry groove and allowed to cure. The  sealant  may  be  any  of  several  materials,  including  epoxies, urethanes,  silicones,  polysulphides,  asphaltic  materials  or  polymer mortars.  A  bond  breaker  may  be  provided  at  the  bottom  of  the groove  to allow  the  sealant  to  change  shape,  without  a  concentration  of  stress  on the bottom.  The bond breaker maybe polyethylene strip or tape which will not bond to the sealant.

3.  Bond Breaking method:

In  some  cases  over  bonding  (strip  coating)  is  used  independently  of  or  in conjunction  with  sealing.  For  this  an  area  approx.  25  to  75mm  on  each side of the crack is sand blasted or cleaned by other means, and a coating (such  as  urethane)  1  to  2mm  thick  in  a  band  is  applied  over  the  crack.  A bond  breaker  may  be  used  over  the  crack  or  over  a  crack  previously sealed  (Fig. 2).  Cracks  subject  to  minimal movement  may  be  over banded,  but  if significant  movement  can  take  place,  sealing  must  be  used in conjunction with over banding to ensure a waterproof repair.

Effect of bond breaker

Fig: Effect of bond breaker

4.  Epoxy Injection method:

Cracks  as  narrow  as  0.3mm  can  be  bonded  by  the  injection  of  epoxy successfully in buildings, bridges and other concrete structures.  However, unless  the cause  of  the cracking  has been corrected,  it  will  probably  recur near the original crack.  If the cause of the crack cannot be removed and it is  not  causing  reduction  in  strength  of  the  structure,  then  either  the  crack could  be  sealed  with  flexible  sealant  thus  treating  it  as  a  joint  or  establish a  joint  that  will  accommodate  the  movement  and  then  the crack should  be grouted  with  epoxy.    With  the  exception  of  certain  moisture  tolerant epoxies,  this  technique  is  not  applicable  if  the  cracks  are  actively  leaking and cannot be dried out.  Epoxy injection requires a high degree of skill for satisfactory  execution,  and  the  ambient  temperature  may  limit  application of the technique.