Saline Soils In Western Sydney And What They Do To Concrete And Brick

Saline soil attacks a building slowly, from the ground up, and on many Western Sydney sites the salt is already in the clay before anyone arrives. The answer is to test for it during the geotechnical investigation and design the footings, slabs, brickwork and drainage for it from the start, because none of those are cheap to change once they are built.

Salt damage rarely looks dramatic in the first year. It turns up later as white powder on the brickwork, mortar that crumbles under a fingernail, and concrete that flakes away along the bottom of a wall.

Why Western Sydney has salt in the ground

The clay soils of the Cumberland Plain hold salt that has built up over a very long time. While it stays deep and dry, it does no harm.

The trouble starts when water moves it. Clearing deep-rooted trees, watering gardens, leaking pipes and stormwater soaking into the ground all lift the groundwater, and rising groundwater carries dissolved salt with it. In low ground, along creek lines and at the foot of slopes, that salty water ends up within reach of footings, slabs and the bottom courses of brickwork.

The NSW Government has mapped salinity potential across Western Sydney, and councils in the region, Blacktown, Penrith and Liverpool among them, carry salinity controls in their development controls. A site does not have to be on a map to be saline, though. The test is the soil itself.

How salt damages concrete and masonry

Water carrying salt is drawn into concrete and brick through their pores. When it reaches a surface that can dry out, the water evaporates and the salt stays behind as crystals.

Those crystals grow inside the pores and push the material apart. On brickwork the face of the brick falls away and the mortar joints wash out. On concrete the surface scales and the edges break off.

Chlorides do a second kind of damage. Once they reach the reinforcement, they break down the protection the concrete normally gives the steel, the steel rusts, and rust takes up more room than the steel it came from. That expansion cracks the cover off from the inside. Sulfates in the soil add a third problem by softening the cement paste itself.

How salinity is tested

We test for salinity in the same boreholes and test pits used to classify the site.

Soil samples go to the laboratory for electrical conductivity, which measures how much salt is dissolved in the soil, along with pH, sulfate and chloride. The conductivity is converted to a saturated extract value, known as ECe, and that number places the soil in a salinity class from non-saline through to highly saline. How the wider investigation is planned is set out in [site classification and soil investigation, the basics](/blog/site-classification-soil-investigation-basics).

The result has to reach the designers before the footings are drawn. A salinity report that arrives after the slab is detailed is a report nobody acts on.

What the design changes

The concrete exposure class. AS 2870 links the measured soil salinity to an exposure classification for residential slabs and footings, and AS 3600 does the same for larger buildings. A higher class means stronger concrete, longer curing and more cover to the steel. The reasoning behind cover is covered in [concrete cover and why it decides durability](/blog/concrete-cover-and-why-it-decides-durability).

The membrane under the slab. On saline ground the plastic under a slab on ground is upgraded to a heavier damp-proofing membrane, lapped and taped at every joint and kept intact while the steel is placed. A membrane full of holes from careless bar chairs stops nothing.

Masonry below the damp-proof course. Brickwork in contact with saline ground is specified with exposure grade units and a stronger mortar under AS 3700. The damp-proof course has to stay continuous and clear. Render taken down over it, paving laid above it or garden soil heaped against the wall all give the salt a bridge around it.

Footings on reactive clay. Saline sites in Western Sydney are often reactive clay sites as well, so the footing system is designed for both movement and durability at once, as explained in [reactive clay and how it decides your footing](/blog/reactive-clay-sites-footing-design-nsw).

The civil side of salinity

Salt needs water to move, so the civil design matters as much as the concrete.

On a saline site we do not soak stormwater into the ground beside the building, because every litre added lifts the groundwater a little further. Roof and paved runoff is collected and piped to the street system. Where groundwater sits high, subsoil drainage keeps it away from the slab edge, and finished ground levels fall away from the walls so water never ponds against them.

Basements need their own thinking. Walls and slabs below ground sit in that salty groundwater permanently, so the exposure class and the waterproofing are chosen for constant contact, as discussed in [basement tanking and hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift).

How we approach it

We treat salinity as one question answered by three disciplines. The geotechnical investigation measures it, the structural drawings carry the concrete grade, cover, membrane and masonry that follow from it, and the civil drawings stop water from feeding it.

When those three come from one office, the salt is designed out once, before the first footing is poured. Salt works slowly and it does not stop, so the drawing is the cheapest place to deal with it.

ACSES provides geotechnical engineering, including salinity assessment, across Western Sydney. Talk to us about a project.

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