The Master Builders Association meeting held in Maitland last week had a very entertaining and informative guest speaker from the NSW Department of Primary Industries. The topic was urban salinity.
Salt from the soil can have a very destructive and catastrophic effect on masonry in buildings. Salt is drawn up into masonry with rising damp, and over many years with wet and dry cycles, more and more salt is deposited into the point of evaporation, typically in brickwork or stonework below damp proof course [DPC] level. The pressure exerted by crystallising salt can exceed 200MPa, whilst the tensile resistant strength of even high strength concrete is only 10MPa, so the salt wins every time.
According to the Department, salt attack is a potential risk anywhere in Australia, and needs to be a design consideration for builders. In some instances, salt has proved destructive within the first 5 years!
For builders, the main objective in order to prevent salt attack is to prevent water absorption. There are 3 simple measures that takes care of most typical construction:
- Sand binding as foundation material. The open pores in sand are considered to reduce the capillary effect of rising damp.
- High impact concrete underlay (for slab on ground construction). This is not simply referring to the minimum thickness of 200μm, but the strength testing of the plastic. It has been mandatory since 2007 to use high impact underlay, yet low impact is still available for purchase in many trade stores.
- Exposure grade bricks. This is very important! These are tested for less susceptibility to salt attack, as opposed to general purpose bricks.
- Efflock - (this is a 4th point we've added to the list!) Efflock included into the mortar mix prevents water absorption and salt attack. It provides a DPC at every joint, and has the bonus performance of keeping mortar clean. For more information please contact Ben at Efflock on 0414 730 736.
For further reading check out this link published by the DPI.
Images of salt attack on brickwork in the suburbs of Sydney.