Aggressive ground conditions can cause significant deterioration to concrete. Understanding ground conditions informs both the specification of concrete in new construction and the assessment of the likelihood of deterioration in existing concrete structures.
Sandberg specialises in evaluating the aggressivity of ground conditions, providing practical guidance on specifying concrete for installation in natural ground and brownfield locations, addressing the common occurrence of sulfates, sulfides, acids, and aggressive carbon dioxide.
Aggressive ground testing
Testing and assessing ground aggressivity is crucial to identify chemical agents in soil and groundwater that can deteriorate concrete. This helps in mitigating risks associated with sulfate, acid, and other chemical attacks, ensuring the longevity and durability of structures.
Deterioration Mechanisms:
- Pyrite oxidation: Pyrite in disturbed ground is liable to oxidise and in doing so generates acidic ground conditions and elevated sulfate levels, both of which are deleterious to concrete.
- Sulfate Attack: Sulfate ions from soil or groundwater react with components in the concrete, leading to the formation of expansive products like ettringite and gypsum. This expansion causes internal stress, cracking, and ultimately, structural failure.
- Sulfate-Reducing Bacteria (SRB): These bacteria thrive in anaerobic conditions and produce hydrogen sulfide, which can form sulfuric acid, causing significant concrete corrosion.
- Sulfur-Oxidising Bacteria (SOB): These bacteria convert hydrogen sulfide into sulfuric acid, further accelerating the deterioration of concrete.
Testing concrete for resistance to Sulfate attack
Where concrete properties are unknown, petrographic examination in accordance with BS 1881-211: 2016 can be used to measure w/c ratio and cement content and to diagnose sulfate attack in existing concrete. W/C ratio and cement content are two of the parameters that are required to classify the durability of concrete in aggressive ground conditions according to BS 8500-1: 2023. A common form of sulfate attack is the thaumasite form of sulfate attack, a typical example of which is illustrated above, where thaumasite-filled cracks are seen in a petrographic thin section.
Concrete Classification
Further to our specialised testing, we can provide classification based on the guidance in BRE Special Digest 1 (SD1): Concrete in Aggressive Ground and BS 8500-1:2023 Concrete – Complementary British Standard to BS EN 206 Part 1: Method of specifying and guidance for the specifier.
AC and ACEC Classes:
- AC (Aggressive Chemical) Classes: These classes categorise the level of chemical aggressivity in the ground, ranging from AC-1 (mild) to AC-5 (very severe). The classification helps in determining the necessary protective measures for concrete.
- ACEC (Aggressive Chemical Environment for Concrete) Classes: These classes provide a more detailed assessment of the chemical environment, considering factors like pH levels, sulfate concentrations, and other harmful chemicals. The ACEC classification ensures that the concrete mix is designed to withstand specific environmental conditions.
Selecting Appropriate Concrete Properties
Choosing the right concrete mix is essential for maintaining structural integrity and resilience. Properly specified concrete can withstand aggressive environmental conditions, prevent chemical damage, and reduce maintenance costs over time.
The appropriate concrete mix is designed to resist the specific aggressivity levels identified in the AC and ACEC classes, ensuring durability and performance under challenging conditions. This means that the concrete will have the necessary strength, permeability, and chemical resistance to endure the harsh environments it is exposed to, ultimately safeguarding the structure’s longevity.
