by James Mackechnie

The New Zealand Standard NZS 3101:2006 Concrete Structures Standard provides guidance about chemical attack from soil and groundwater in section 3.4.3. This guidance rates the aggressivity of groundwater in terms of sulphate content and pH since acidic sulphate soils are found in some parts of New Zealand and can cause significant damage to concrete foundations. 

James Mackechnie - Education, Training & Research Manager

Generally, ground conditions in New Zealand are relatively benign with sulphate rich soil mostly confined to isolated parts of Northland, Taupo and Rotorua region and northern Westland.

SULPHATE ATTACK OF CONCRETE
Chemical attack of concrete occurs in aqueous solutions where aggressive agents react with normally stable phases in concrete. Attack of hardened concrete may occur by three main mechanisms (shown graphically in Figure 1):

• Exchange reactions involving acids reacting with alkaline hydration products and destroying the cement matrix from the surface inwards (most aggressive form).
• Leaching reactions where cement hydration products are dissolved by softwater that contain low calcium and magnesium salts.
• Spalling reactions where aggressive agents such as sulphates penetrate porous concrete and cause internal disruption by expansion of reaction products such as ettringite and softening from gypsum formation.

Synergies between these three mechanisms will accelerate the overall rate of attack since exchange reactions attack the surface while leaching increases the porosity of the near-surface, both of which allow more rapid ingress of sulphate into the concrete.

Figure 1: Chemical attack processes for concrete exposed to aggressive groundwater

Concrete is some parts of the country (e.g. Rotorua and Taupo volcanic zone) may be exposed to groundwater that is subject to all three modes of chemical attack. Volcanic zones may have significant sulphates in the ground, are low in carbonates and bacteria reduce sulphates to sulphides that can oxidise on exposure to form sulphuric acid. Many areas however have relatively moderate levels of aggression in terms of sulphate content and pH of the ground. Figure 2 show the typical signs of sulphate attack on the concrete surface of a bridge pier exposed to highly mineralised ground.

Figure 2: Sulphate attack of bridge pier exposed to magnesium sulphate (author, 1999)

Sulphate resistance of concrete improves with higher strength concrete since material is denser and less impermeable, but when sulphate contents are high, specific protection is usually taken by either:

• Using sulphate resisting cement that contains low calcium aluminate phases that react with sulphate ions (no locally manufactured SRPC in New Zealand but local GP cement usually complies with Australian cement standard limits for SRPC).
• Using supplementary cementitious materials (SCM) such as fly ash, slag or silica fume that densify the concrete microstructure and limits sulphate attack.
• Additional protective measures for extreme conditions that may include surface protection, extra sacrificial layers or providing drainage to reduce contact between groundwater and concrete.

NZS 3101 CLASSIFICATION
Chemical attack is classified based on chemical analysis of groundwater or soils samples and defines three categories (i.e. XA1, XA2 & XA3). Assumptions are made about mobility of water, temperature, and solubility of sulphate with a summary for static groundwater conditions shown in Table 1. Classification of likely exposure is intended to be read as being the combination of pH and sulphate content not conditions where either pH or sulphate content exist. For instance, where groundwaters are static, have low pH but contain low sulphate contents, chemical attack of concrete is likely to be low.

Table 1: Exposure classification for groundwater and recommendations (NZS 3101)

This approach is simple to apply provided accurate sampling of groundwater conditions can be undertaken on site, which are generally more reliable than extracting soil samples. The three categories are conservatively defined but this does mean that some relatively mild conditions need to be carefully considered. For instance, groundwater with pH of 6.0 and 1000 mg/L of sulphate may be considered to fall between XA1 and XA2 and further analysis is probably required using more detailed guidelines.

BRE SPECIAL DIGEST 1 CLASSIFICATION
The BRE special digest on chemical attack of concrete is a more comprehensive document that while providing similar recommendations to NZS 3101 has five main categories rather than three categories used in NZS 3101 Table 3.3. The BRE approach provides more guidance about special conditions such as mobile water, brownfield sites, precast concrete and other chemical interactions. The BRE classification system is more comprehensive and includes exposure conditions that are classified as U in NZS 3101.

Table 2 shows an extract from Table C1 where static groundwater conditions and mix design recommendations are given. It should be noted that the contents of the table are a basic summary since the BRE approach has numerous options and clauses that provide a more refined classification of environment, concrete type and structure.

Essentially, the BRE approach has two extra classifications; one that is for mild-moderate levels and an extreme exposure category similar to NZS 3101 exposure category U. The division between exposure category 2 and 3 for groundwater is related to the maximum solubility of calcium sulphate, which is around 1440 mg/L. Higher concentrations of sulphate in the groundwater indicate the presence of more soluble sulphates such as those of magnesium and sodium. Sulphate salts of magnesium and sodium are more aggressive to concrete as these are more mobile than calcium sulphate and diffuse more easily into concrete.

The BRE approach also allows analysis to be undertaken for specific types of aggressive water rather than using a coupled method of considering both pH and sulphate content. This is important in many environments where sulphates are not present but groundwater is aggressive to concrete due to its softness and/or the presence of acids (e.g. humic acid).

Table 2: Exposure classification for groundwater and recommendations (BRE Special Digest)

CONCLUSIONS
The NZS 3101 guidance for rating the exposure classification of chemical attack of concrete provides a broad and conservative method suitable for structural engineers. The approach is not comprehensive enough to cover all conditions such as when dealing with variable aggression levels of pH and sulphate content in groundwater and soils. It is satisfactory when projects have limited site data with which to assess the aggressivity of soil and groundwater. Larger infrastructure projects, particularly those using precast concrete such as concrete pipes, should probably consider more detailed analysis of ground conditions using more comprehensive guides such as BRE Special Digest 1. Better classification of exposure conditions will provide more confidence to designers and allow more cost-effective solutions.

REFERENCES

• New Zealand Standards. NZS 3101:2006 Concrete Structures Standard. Wellington, 2006.
• The Concrete Centre. BRE Special Digest 1 – Concrete in Aggressive Ground. UK, 2005.
• Australia Standard. AS 3972: 2010 General Purpose and Blended Cements. Australia, 2010.
• Whittaker, M. and Black L. Current Knowledge of External Sulphate Attack in Advances in Cement Research. ICE Publications, 2015.

PDF - Mackechnie's Lab 4