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Salinity and structures

In the last article we looked at the causes of corrosion and scaling and how to test for them. In this article we go into depth about measurement and the interpretation of results, and measures than can protect structures from salinity.

Recap: scaling, aggressiveness and corrosion

Scaling is the deposition of insoluble salts, oxides and hydroxides from water on surfaces. Inside pipes, this can block flow. Common scaling substances include calcium carbonate, calcium sulphate, magnesium hydroxide and iron oxides. Scaling is most common in the presence of hard water: that is, water with a lot of dissolved calcium and magnesium salts.

Aggressiveness is the propensity of a soil or water to dissolve concrete structures. It depends on the types of ions present. Different ions will cause either physical expansion or a loss of cementing properties, reducing concrete strength. The permeability of concrete to air and moisture is the main factor influencing resistance to aggression.

Corrosion of metals is essentially an electrical phenomenon. Salts in water themselves have no direct role in corroding metal; rather, they promote electrical conductance. Thus, a wet environment high in oxygen and high in chemical ions is the most corrosive.

Structures at risk

• Structures in acidic groundwater
• Structures in soil with sulphates in solution
• Industrial tips or tunnels through them
• Piled foundations
• Rafts, strip foundations and column bases
• Structures holding mildly aggressive liquids
• Water reservoirs
• Contact tanks and sewage tanks holding trade effluent
• Sludge digestion tanks
• Precipitators for flue gases
• Sewer pipelines and ancillary structures
• Seawater distillation plants
• Blow-down pits from boilers
• Industrial chimneys and floors
• Lining of chemical tanks and structures holding industrial cooling water

Many industrial processes rely on concrete and steel structures, which are prone to corrosion and scaling. Processes include heating and steam generation, cooling water systems, hydroelectric power generation, and the manufacture of chemicals, foods and drinks, iron and steel, textiles, leather, pulp and paper, and petroleum.

When to test

It will be necessary to test for scaling, aggressiveness or corrosion if concrete structures (including pilings and pipes) or steel or other metal structures are subjected to:

• acids: organic acids from silage, processing industries or landfill (domestic, industrial and mining waste)
• sulphates: naturally occurring sodium, potassium, calcium or magnesium sulphates in soil or water or industrial wastes; soils containing naturally occurring or deliberately added gypsum; water containing added gypsum
• sulphuric acid: proximity to sulphurous gases, which combine with moisture to form sulphuric acid; materials that contain pyrite (FeS₂), such as acid sulphate soils
• sea water
• soft water, which will dissolve Ca(OH)₂ out of hardened concrete
• soluble ammonium and Mg salts, which dissolve Ca(OH)₂ out of hardened concrete
• high levels of salts: saline soils with an EC(1:5) of >150 mS/m.

Measurement of scaling, aggressiveness and corrosion

Salinity is measured as electrical conductivity (EC) in water or in a dilute extract of soil in water. EC can be converted to mg of salt per litre of solution by multiplying by 640 (i.e. 1 dS/m [EC] = 640 mg salt/L). However, salinity is only part of the story. In the context of corrosion and aggression assessment, the propensity of the total soil mass to promote the flow of electrical current and hence corrosion is the critical factor. Measurement of this must additionally take into account the inherent electrical conductivity of the soil minerals (e.g. sand vs clay). Poorly permeable soils tend to insulate the structure from rapid flow of salts and electric current. Thus, at the same salinity, a sandy soil may prove more rapidly corrosive or aggressive than a heavy clay.

Test methods for the assessment of scaling, aggressiveness and corrosion risk measure permeability, pH, total salinity, electrical resistivity, and sulphate, chloride, ammonium, calcium and magnesium ions. Some can be measured on site; the rest must be measured in the lab. The results allow a level of risk to be assigned.

Interpretation of testing results

The assessment of the potential impact of salinity on built structures is not a precise science. Risk depends on a complex interaction of exacerbating factors.

Currently the interpretation of results is judgmental and qualitative. We have to consider not only the worst aspect of the soil or water (be it acidity, salinity or whatever), but also how the various factors may interact. For example, a given salinity level is less problematic in an impermeable soil than in a permeable soil. Or the ability of the soil to buffer pH changes might be more important than the actual pH.

For this reason, the interpretation of results requires a thorough assessment of all potential factors involved and a broad understanding of the many contributing factors.

Protecting structures

The type of intervention required to protect structures in a saline environment increases in cost and complexity with increasing risk. In most situations, avoidance (burial of saline soil) and good building practices (vapour barriers, coverage of reinforcing) are adequate to protect the structure. Only in highly saline environments are specific measures such as stainless steel reinforcing, sacrificial anodes and high-density waterproof concrete justified.

The first and most necessary step in protecting structures is to keep saline material and the structure separate. Soil stripping and correct reinstatement are paramount. For example, a non-saline B horizon can be set aside to cap cut-and-fill on a saline C horizon. In landscaping on a saline C horizon, 20 cm of a non-saline B horizon topped with 10 cm of topsoil is preferable.

Aside from avoidance, good damp-course integrity is vital. Salt damage from wick effects is usually confined to older buildings, where footings are porous brick and damp courses are either non-existent or inadequate. Where more modern structures suffer damage, this is most often the result of poor building practices, such as allowing contact with soil above the vapour barrier. Good site assessment is essential to preventing deep saline material from ending up on the surface, and rigorous building inspection and public education are required to ensure the integrity of vapour barriers.

More information

The staff at SESL will be happy to advise you on all aspects of testing for scaling, aggressiveness and corrosion and protecting structures.

Further reading

AS 1289.4.4.1 (1997). Methods of testing soils for engineering purposes – Soil chemical tests – Determination of the electrical resistivity of a soil – Method for sands and granular materials. Standards Australia.

AS 2159 (1995). Piling – design and installation. Standards Australia.

Basson JJ (1989). Deterioration of concrete in aggressive waters – measuring aggressiveness and taking countermeasures. Portland Cement Institute, Midrand, South Africa.

Cement Concrete & Aggregates Australia.

DIN 4030-1 & -2 (2008). Assessment of water, soil and gases for their aggressiveness to concrete. Deutsches Institut für Normung.

Perkins PH (1997). Repair, Protection and Waterproofing of Concrete Structures. Taylor & Francis.

Tomlinson MJ (1980). Foundation Design and Construction. Pitman.