Aggressivity testing assesses the chemical properties of soil and groundwater to determine their potential to cause corrosion to buried concrete, steel, and other construction materials. It is a geotechnical investigation for infrastructure, pipelines, retaining walls, and building foundations.
What Is Aggressivity Testing?
Aggressivity testing evaluates the chemical environment of a site to identify corrosive conditions that could degrade buried construction materials over time. The test results inform material selection, protective coatings, concrete mix design, and the need for additional corrosion protection measures.
Why Is Aggressivity Testing Important?
Corrosion of buried structures is a major cause of premature failure in foundations, retaining walls, pipelines, and sheet piling. Aggressivity testing allows engineers to:
- Select appropriate concrete strength and exposure classification (AS 3600, AS 2159)
- Specify corrosion-resistant steel (e.g., galvanised, epoxy-coated, or stainless steel)
- Design protective coatings and cathodic protection systems
- Predict service life of buried infrastructure
- Avoid costly remedial work from undetected aggressive ground conditions
Tests Conducted
| Test | Standard | What It Measures |
|---|---|---|
| pH | AS 1289.4.3.1 | Acidity or alkalinity of soil/groundwater |
| Sulfate (SO₄²⁻) | AS 1289.4.4.1 | Water-soluble sulfate content |
| Chloride (Cl⁻) | AS 1289.4.4.1 | Chloride ion concentration |
| Electrical Resistivity | AS 1289.4.4.1 | Soil corrosivity (lower resistivity = more corrosive) |
| Organic Matter | AS 1289.4.1.1 | Organic content affecting concrete |
| Sulfide (S²⁻) | — | Presence of sulfides (acid sulfate soils) |
Classification of Aggressivity
Concrete Exposure Classification (AS 3600 / AS 2159)
| Parameter | Mild | Moderate | Severe | Very Severe |
|---|---|---|---|---|
| Sulfate (mg/L as SO₄) | < 500 | 500–1,500 | 1,500–10,000 | > 10,000 |
| pH | > 6.5 | 5.5–6.5 | 4.5–5.5 | < 4.5 |
| Chloride (mg/L as Cl⁻) | < 500 | 500–1,500 | 1,500–5,000 | > 5,000 |
| Resistivity (Ω·cm) | > 5,000 | 2,000–5,000 | 1,000–2,000 | < 1,000 |
AS 2159 Piling — Chemical Exposure Classification
| Class | Sulfate (mg/L) | pH | Description |
|---|---|---|---|
| A | < 500 | > 6.5 | Low aggressivity |
| B | 500–1,500 | 5.5–6.5 | Moderate aggressivity |
| C | 1,500–10,000 | 4.5–5.5 | High aggressivity |
| D | > 10,000 | < 4.5 | Very high aggressivity |
Sampling Requirements
Sample Collection
- Soil samples should be collected from multiple locations across the site
- Samples at foundation depth and at groundwater level (if encountered)
- Groundwater samples from boreholes or standpipes
- Sealed in airtight containers to prevent oxidation
Sample Quantity
| Material | Minimum Sample Size | Container |
|---|---|---|
| Soil | 500 g | Airtight plastic bag or jar |
| Groundwater | 1 L | Plastic bottle (fill to brim) |
| Fill material | 1 kg | Airtight container |
Interpreting Results
Sulfates
Sulfates react with concrete to form expansive compounds (ettringite) that cause cracking and spalling. Higher sulfate levels require:
- Sulfate-resisting cement (SRC)
- Increased concrete cover
- Lower water-cement ratio
Chlorides
Chlorides promote corrosion of reinforcing steel by breaking down the passive oxide layer. Effects:
- Localised pitting corrosion
- Accelerated corrosion rates
- Requires epoxy-coated or stainless steel reinforcement in severe cases
pH (Acidity)
Low pH (acidic conditions) attacks both concrete and steel:
- Dissolves cement paste in concrete
- Accelerates steel corrosion
- May require acid-resistant materials or protective linings
Electrical Resistivity
Low resistivity (< 2,000 Ω·cm) indicates highly conductive soil conditions that accelerate galvanic corrosion of buried steel.
Mitigation Measures
| Aggressivity Level | Concrete Protection | Steel Protection |
|---|---|---|
| Mild | Standard Portland cement, minimum 20 MPa | Unprotected steel generally acceptable |
| Moderate | Sulfate-resisting cement, increased cover, minimum 32 MPa | Galvanised or epoxy-coated steel |
| Severe | SRC cement, maximum w/c ratio 0.45, minimum 40 MPa | Epoxy-coated or stainless steel |
| Very Severe | SRC or specialised cement, protective membrane, minimum 50 MPa | Stainless steel with cathodic protection |
Australian Standards
| Standard | Title |
|---|---|
| AS 2159-2009 | Piling — design and installation (chemical exposure classification) |
| AS 3600-2018 | Concrete structures (exposure classification and durability) |
| AS 1289.4.3.1 | Determination of the pH value of a soil |
| AS 1289.4.4.1 | Determination of the electrical resistivity, chloride and sulfate content |
| AS 1289.4.1.1 | Determination of the organic matter content |
| AS 4312 | Corrosion of buried metal structures |