Foundations are the backbone of any structure, providing the essential support that ensures the building’s stability and safety. However, foundations often face various forms of attack from the surrounding soil and groundwater, which can severely compromise their integrity over time. Understanding these threats and applying proper protective measures is crucial for extending the life of foundations and maintaining structural safety.
Causes of Attacks on Foundation Structures
Different types of foundation materials face unique challenges from their environment:
- Concrete Foundations often suffer from chemical attacks such as sulfates and other harmful wastes found in the soil, alongside erosion and mechanical abrasion.
- Steel Piles are vulnerable to corrosion caused by specific environmental factors in soil and groundwater.
- Timber Piles are susceptible to decay due to biological organisms in the soil and water. Additionally, physical damage may occur from floating objects like ships or ice, and wave action can exacerbate wear through abrasion.
The severity of these attacks depends not only on the concentration of harmful substances but also on climatic conditions and fluctuations in the groundwater table.
Soil and Groundwater Exploration
Before designing protective measures, it is critical to analyze the soil and groundwater conditions at the construction site. This process involves:
- Collecting disturbed and undisturbed soil samples for chemical analysis.
- Installing standpipes in boreholes to monitor groundwater levels and fluctuations over time.
- Determining the concentration of aggressive agents like sulfates at different soil depths.
Accurate data prevents both under- and over-designing protective strategies, ensuring cost-effectiveness and reliability.
Protection of Concrete Foundation Structures
Concrete foundations are primarily threatened by sulfate attack, which deteriorates the concrete matrix. Other damaging agents include organic acids, chemical wastes, deleterious aggregates, reinforcement corrosion, and seawater exposure.
Protection Against Sulfate Attack
ASTM classifies cements by sulfate resistance:
- Type II Portland Cement offers fair resistance.
- Type V Portland Cement provides high resistance.
In severe sulfate environments, super-sulfate or high alumina cement may be used. However, high alumina cement can undergo “conversion,” a process that reduces concrete strength, but this can be managed by controlling cement content, avoiding heat, and protecting curing concrete.
Physical barriers like plastic sheets or bituminous membranes are often wrapped around foundations to block sulfate ingress. For more durability, galvanized steel or rigid PVC tubing may be used, though these are costlier options.
Protection Against Organic Acid Attacks
Organic acids, common in peat soils, and free sulfuric acid (from pyrite oxidation) can harm concrete. Protection depends on soil pH:
- If pH ≥ 6, no special measures are needed.
- For lower pH values, sulfate-resistant cements combined with rapid hardening cement and pozzolanic materials like fly ash or slag provide effective protection.
Protection Against Chemical and Industrial Wastes
When a site contains unknown or aggressive chemicals, precast concrete piles with a hollow interior filled with concrete and an outer sacrificial shell (such as PVC) are recommended. This design protects the core from chemical damage.
Protection of Steel Piles Against Corrosion
Corrosion occurs when steel piles are exposed to air and water, causing some areas to act as anodes (corroding) and others as cathodes. To combat this:
Steel Pile Paint Treatment
The steel surface is cleaned by sand or grit blasting, then coated with a zinc silicate primer followed by an epoxy or vinyl topcoat. This is suitable above splash zones but is not ideal for long-term protection in marine environments. Alternatives include adding steel plates or increasing pile thickness.
Cathodic Protection
This electrochemical method protects steel by making it the cathode of an electrical circuit, stopping metal loss.
- Powered Systems use DC generators to supply current via anodes (carbon or scrap iron).
- Sacrificial Anodes are metal masses that corrode instead of the steel pile.
Sacrificial anodes are more feasible in marine settings but require periodic underwater replacement.
Protection of Timber Piles
Timber piles decay mainly due to biological organisms, especially when subjected to alternating wet and dry conditions.
Creosote Preservation
Impregnating timber with creosote effectively prevents biological decay. It penetrates better in softwood than hardwood. Hardwood requires longer treatment and special attention to bolt holes to ensure protection.
Concrete Protection
Where creosote is insufficient (e.g., varying groundwater levels), composite piles are used: timber submerged underwater with a concrete upper part. Alternatively, piles can be cut at the water table level and capped with concrete to protect exposed wood.
Protection Against Marine Borers
Choosing naturally borer-resistant timbers, such as African padauk, Belian, and Afrormosia, minimizes damage. The sapwood, which is susceptible to borers, should be removed or treated with creosote. Maintaining an intact protective layer is crucial; any damage (from bolts, lifting hooks, or cuts) can allow borers to penetrate and deteriorate the timber.
In conclusion, protecting foundation structures against soil and groundwater attacks requires careful assessment of environmental conditions and selecting appropriate materials and treatments. Properly implemented protection strategies can significantly extend the lifespan of concrete, steel, and timber foundations, ensuring the long-term safety and durability of structures.