are therefore not so important.
intervals of not more than about 12 PRECAUTIONS ft. With a maximum shrinkage of The kind of cracking with which 0.04 we are concerned is that caused by movement of 12 ft. is 0.06 or 1 16 in. taken by the user to keep down the
per cent.. the total possible
The following precautions can be the shrinkage of the units. It is If this total movement were to be shrinkage. and to distribute shrink- likely to occur when units having a manifested as one crack it would be age stresses in such a way that the relatively high drying shrinkage are built in a moist condition into a various factors tend to operate na- that cracking is confined to mortar conspicuous. Generally. however, risk of major cracks is reduced, or structure that restrains their move- turally, to reduce the total shrinkage ment during the drying-out process. occurring, or to distribute the stresses Unless precautions are taken to keep set up and, under normal conditions the total amount of cracking as in practice. the total crack width small as possible or to distribute it in does not often exceed about half the the form of finer and less noticeable maximum possible (0.03 or 1 32 in. in cracks, the appearance of the build- the above example). ing may be spoiled.
TYPES OF CRACK
Shrinkage cracks in buildings usually run more or less vertically and they occur mostly at places where the cross section of the brick- work is reduced, as for instance above or below the corners of door or window openings. They may be of either of two types-running straight through the unit and joint in alter- nate courses (crack A in the figure), or following a zigzag path along the joints (crack B). Which of these two types of crack will occur depends mainly on whether the mortar is stronger or weaker in tension than
the unit.
Since the type of crack shown at B is more easily made good if this should become necessary, it is ob- viously better to make the joint weaker in tension than the unit. With a weak mortar joint, moreover, there is a tendency for the units to shrink individually without stressing the wall as a whole, and thus for negligible hair cracks to form around each unit rather than for wider cracks to form. On both these counts, therefore, it is better to use a mortar that is weaker than the units. The units must also, of course, have a reasonable strength in tension if they are themselves to resist crack- ing.
AMOUNT OF CRACKING
The total amount of cracking to be expected is of interest. In house construction. openings such as doors and windows forming weak points in the wall will usually occur at
As mentioned above, all concrete products shrink to a certain extent on drying from a wet condition; the magnitude of the shrinkage varies according to the type of product and the nature of the curing treatment before delivery. The British Stan- dard Specifications for products of this type limit the permissible shrinkage to 0.08% in partition blocks and 0.06% in external wall blocks.
Compliance of the units with the requirements of the relevant B.S. does not imply that there will be no risk of shrinkage cracking, but that the manufacturer has reduced the shrinkage as far as is practicable with present knowledge; other pre- cautions must be taken by the user.
(above) Shrinkage cracking in block construction. (Below) Concrete block construction at Shek Kip
Mei, Hong Kong.
joints.
1. STORAGE. Units should be stored on the site in such a manner used. If they are to be held for any that they are reasonably dry when
length of time the stacking should be sufficiently open to allow air cir- assist drying culation, which will after rain.
2. WETTING. The more absor- bent types of unit sometimes have to be wetted in order to reduce the suction and facilitate proper laying and bedding. Such wetting should only be done where necessary, and even then it should not be overdone
3. MORTAR. Mortar should be
of a relatively weak type. A mix of one part of Portland cement. with one part of hydrated lime and six to nine parts of sand, all by volume is generally suitable.
4. REINFORCEMENT. The use of reinforcement in the bed joints of brickwork or block construction will reduce the risk of cracking.
5. DESIGN. Where the design of the building permits, dividing the wall into suitable lengths or panels with freedom from restraint at the ends will eliminate the risk of crack- ing. Various methods of treatment may be used at the edge of such panels to exclude moisture or to secure a suitable finish.
Where, with external work. a ren- dered finish is to be applied over units there will be less risk of the rendering being cracked by move- ments of the background if the ren- dering is applied when the wall has dried out. It is thus preferable, particularly during a wet period. to defer application of the rendering as long as possible.
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