Sulphate analysis

The sulphate and sulphoaluminate content of concrete can be analysed by the methods given in BS 1881-124 (2015) and AS 1012.20 (2016) for example. In this case the acid solution prepared as described previously is analysed for sulphate ions. This is done by adding barium chloride and gravimetrically determining the amount of barium sulphate precipitated.

Knowledge of the sulphate content can indicate whether sulphate attack has occurred or is likely to occur in the future if the concrete remains exposed to its environment. Normal Portland cements contain a concentration of sulphate arising from the original gypsum added to prevent flash set of about 5%. Sulphate in excess of this may be evidence of sulphate attack and so sulphate analyses are typically carried out in conjunction with original cement content determinations.

As indicated at Section 2.4.4, the common sources of sulphates which may attack concrete are groundwater, soils, industrial chemicals, seawater, and products of sulphur oxidising bacteria. Groundwater and soils may contain sulphates of calcium, magnesium, sodium, potassium and ammonium in various quantities depending on the location with magnesium sulphate being the most aggressive to concrete followed by ammonium sulphate, refer Section 2.4.4.

Petrographic examination, refer Section 7.16, of thin sections from core samples is most commonly employed to identify the form of sulphate attack. The smell of ammonia gas may also be present in the case of ammonium sulphate induced concrete deterioration (Collins & Green, 1990).

Laboratory methods such as x-ray fluorescence (XRF) and x-ray diffraction (XRD) on crushed and pulverised concrete core sections to identify inorganic elements and compounds may also be necessary to confirm the type of sulphate attack.

Alkali aggregate reaction

The presence and extent of alkali silica reaction (ASR) (or alkali carbonate reaction, ACR) within a concrete structure or building can only be unequivocally established by taking core samples and performing a laboratory study, including petrographic examination (Thomas et al., 2011), refer Figure 7.6, and Section 7.16.

Laboratory studies can also be utilised to measure the residual expansion in ASR-affected concrete by accelerated exposure testing of cores. Structure and building elements may also be instrumented and monitored for future behaviour before deciding on a course of action.

Uranyl acetate staining of concrete core samples followed by examination under fluorescent light has also been undertaken to detect the presence or otherwise of ASR (Natesaiyer and Hover, 1989).

ASR gel deposition through both coarse aggregate and cement paste

Figure 7.6 ASR gel deposition through both coarse aggregate and cement paste.

Alkali content

In some circumstances, the risk of AAR is managed by specifying a maximum concrete alkali content. For example, in New Zealand a maximum alkali limit of 2.5 kg/m3 (equivalent Na,0) is used for many concretes when potentially reactive aggregate is used. Determination of the alkali content of hardened concrete may therefore be required to assess compliance with such specifications, or to evaluate the cause of AAR in an existing structure to help identify appropriate precautions for future construction (Concrete Institute of Australia, 2015).

BS 1881-124 (2015) provides a method for determining acid soluble sodium and potassium contents. Barnes and Ingham (2013) reported that inter-laboratory test results from this method were consistently higher than the target, which may reflect extraction of alkalis that would normally not be available for AAR, such as alkalis bound within minerals in the aggregate. Of potentially greater concern is that the range of results they reported exceeded 1.0 kg/m3 (equivalent Na,0) for all concrete tested. Thus, this method may not be accurate enough for practical use (Concrete Institute of Australia, 2015).

Other in-house methods may be more appropriate, such as methods that measure water soluble alkalis, and/or that separate the binder fraction from the aggregate and analyse only the binder fraction (Concrete Institute of Australia, 2015).

Delayed ettringite formation

Delayed ettringite formation (DEF) is identified in hardened concrete by petrographic examination, refer Section 7.16.

Acid attack

Inorganic acid attack of concrete can be indirectly determined by spraying concrete breakouts, core samples, etc with phenolphthalein pIT indicator solution and noting the depth of neutralisation from the inorganic acid attack rather than the depth of carbonation.

As indicated at Section 2.4.7, the effects of organic acids on concrete cannot be determined by depth of neutralisation (pH) indication or concentrations of solutions, which may reasonably be applied to inorganic acids as a whole, as virtually each organic acid must be considered individually with regard properties such as its solubility in water and, most importantly, the solubility of its calcium salt (Lea, 1998).

Laboratory methods such as XRF and XRD can be utilised on crushed and pulverised concrete core sections to identify inorganic elements and compounds of relevance in terms of the products of reaction from inorganic acid attack.

Organic based compounds can be determined using laboratory-based methods such as fourier transform infra-red (FTIR) spectroscopy, for example, and so the products of reaction of organic acids-based attack may be detectable. Consideration may need to be given however to any organic- based admixtures already present in the concrete.

 
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