Guidelines

BRIEF GUIDELINES FOR MANUFACTURING, MASONING CLC BLOCKS AND MEASURES SUGGESTED TO AVOID & REPAIR CRACKS IN MASONRY
(Kindly refer provisions of relevant clauses of IS: 6042 &IS:2572 for details)

1.0 Manufacture, Curing & Maturing of CLC Blocks

1.1 CLC slurry must be made using quality input raw materials {Portland Cement Grade 53, Clean river sand, Grade 1 Fly Ash, Clean water and Neopor Foam density 80 gms/litre} in defined proportions, mixed in a gravity mixer and poured into leak-proof battery moulds.

1.2 Blocks de-moulded on hardening are stacked, water spray cured for about two weeks like any other concrete product and then allowed to mature and get thoroughly dried till completion of 4-weeks after casting to enable their initial shrinkage to get completed, before being allowed to be used in masonry work.

2.0. Masonry Work & Plaster

(a) Masonry mortar should be leaner than the strength of block. As per provisions of Table-1 of IS: 2572; preferably 1-Cement, 1-Lime & 6-Sand. If Cement Sand mortar 1:6 is used then partial substitution of cement be done with Fly ash.

(b) The blocks should be laid dry, but the surface on which mortar is being laid should be moistened to obviate it absorbing water from the mortar.

(c) Both horizontal and vertical joints should be ~ 10 mm thick.

(d) The mortar shall be well spread on both horizontal and vertical surfaces of the joints to produce fully filled-up and well compacted joints. It may be necessary to add mortar especially to vertical joints to ensure that they are well filled.

(e) The mortar shall be racked out of joints with a trowel to a depth of about 10mm as each course is laid to ensure good bond for plaster.

(f) When the mortar has stiffened somewhat, it shall be firmly compacted with a jointing tool. The mortar shall be pressed against the units to affect intimate contact between the mortar and the masonry to obtain a water tight joint. Please refer two attachments reproduced from one of the referred IS codes.

(g) Weldmesh ladder of 75mm width of 3 or 4mm Ø mesh of 25x75 mm rods be placed in every third course of 100mm thick partition walls.

(h) Masonry shall be cured for at least 7-days and then allowed to dry before applying rendering over it.

(i) Rendering of walls may be done with 1:1:6 Cement, lime, sand mortar or 1:4 cement mortar. When smooth finish is desired internally 2-3 mm thick lime neeru finish may be applied.

(j) The masonry walls, abutting against structural elements like columns, beams may deflect, move because of load, settlement, shrinkage or thermal effects. In order to avoid cracking as a result of such movements, a slip joint shall be provided, preferably packed with a resilient material. (This could be material like “Sika Flex Construction”, “Dr. Fixit Crack-X etc.)

If, however, no joint line is desired to be seen at the junction of columns and beams with masonry from aesthetic considerations, then the junction, as treated above may be over laid with a 200mm wide chicken mesh, equally spaced on either side of the joint line, before rendering the wall with plaster.

3.0 Observation of cracks and Remedial Measures

It is felt that deficiencies in observance of broad stipulations stated under 1.2, 2(a) to 2(g) etc. above have resulted in appearance of some cracks at some locations in the plastered walls. The types of cracks seen are: -

3.1 Hair cracks in Putty finish: -

Some hair cracks have been observed at isolated locations in the putty finish. We concur with the assessment of your Chief Project Manager, that these are only in the putty layer and do not extent into the plaster.
(It is likely that if use of cement based putty or Neeru finish is adopted, such a situation may not occur).

However in the current situation, it is suggested that the joint be cleaned of dust, removing any flaky material, moisten the surface slightly paint with a brushable consistency paste of 1 part Sika Latex & 1 part Cement.

Alternatively apply with a putty knife a mix of 2.5 parts Dr. Fixit Crack-X powder mixed with 1 part water by volume, remove excess material and let it stay for 24 hours before paining.

3.2 Crack at some masonry junction with Beam/ Column or other locations.

3.2.1 The joint between the structural elements and masonry be treated as stated under 2(j) above. Make a 6x6-8 mm groove along the line of joint with a grinder, clean and wash with water and fill the groove with “Sika Flex Construction” or equivalent polyurethane single compound sealant.

3.2.2 Isolated cracks in the body of the masonry be also opened-up into grooves of 6x6-8 mm with the help of grinder, cleaned and washed and then filled up with a putty made of : -

• 2-Sika Latex + 5-Cement+ 15 Sand and water over a primer coat, if recommended by Sika.
  or
  
• Dr. Fixit Crack Filler and Water in volumetric proportion 2.5 to 1.
  or
  
• 1 Bond Aid (of Asian Laboratories, Okhla) :2 parts water mixed with equal amount of Cement and twice the quantity of Sand.

(Note: A typical sample repair work, noticed at one location, done by some agency by making 200-250 mm wide repair bands over the crack locations, is un-called for and would be wastage of costly input materials).

General:

• It was learnt that all plaster work is being done using Recron fibre as additive. However it does not seem to be effective. Use of Polypropylene fibre could be tried as a better alternative.
  
• Use of any kind of bond aid (to the extent of 2% -3% of cement content) as an additive in the masonry mortar could improve bonding efficiency and reduce water requirement for hydration.