Concrete Masonry Building Hints

Good concrete masonry walls depend just as much on good construction as on a good unit, and many cracked and leaky walls are due to the method of construction rather than to the units themselves.

Please take note of the following tips:

GENERAL

Use only CMA quality concrete masonry

•Use units of the correct compressive strength for the correct application

•Masonry units of dissimilar materials should not be built into the same wall unless separated by a horizontal damp proof course or a vertical control joint

MORTAR

A good general purpose (class II) mortar mix is 50kg cement, 0-40lt lime, 200lt sand (or 1 cement, 1 lime, 6 sand by volume)

•Mortar not used within 2 hours (less in hot weather) should be discarded

PLASTER

A good general purpose plaster mix is 1 cement, 2 lime, 8 sand by volume

•Make sure that the wall surface is dry before applying plaster

SETTING OUT

The wall should be set out by first laying units without mortar to check the spacing. Joints can then be adjusted to fit

LAYING

Concrete masonry should NOT be wetted prior to laying in a wall

•Hollow units should be laid with the thicker shell uppermost (upside down)

•Work should stop during heavy rain unless adequately protected

•Care should be taken to ensure that mortar is not smeared onto the exposed face of the wall as this will show up after painting

JOINTING

The joint thickness should be 10mm

•Joints should be tooled when the mortar has become thumbprint hard

•Concave joints are preferred for exterior work. Avoid raked joints as they reduce strength and cause water penetration and efflorescence

REINFORCMENT

Care should be taken to use sufficient mortar in the horizontal joints in which brickforce or wall ties are bedded to ensure that the whole surface of the steel is in contact with mortar so that there will be an adequate bond and protection against corrosion

CHASING

Chasing of walls should not be made by impact methods as this encourages localised cracking

•Horizontal chasing should be avoided where possible

CONTROL JOINTS

Control joints will not normally be required in interior walls of dwellings, and as a general rule for other walls, vertical joints should be provided at intervals of 6m to 7m

WEATHER PROTECTION

Partially completed or unfinished walls should be covered for protection from excessive rain

For further information please refer to the relevant manual.