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Chemical analysis of concrete can provide extremely useful information regarding the cause or causes of failure of concrete. The tests most frequently carried out are listed below:

1. Chloride content 4. Sulfate content

2. Cement content 5. Type of cement

3. Depth of carbonation 6. Alkali content

In the following, an explanation of the reason why each parameter is important is given, followed by an explanation of the test itself.

Chloride content test methods

The generally accepted method of test for chloride in hardened concrete is described in BS 1881 : Part 124: 1988 (BSI, 1988). The test involves crushing a sample of the concrete to a fine dust, extracting the chloride with hot dilute nitric acid and then adding silver nitrate solution to precipitate any chloride present. Ammonium thiocyanate solution is then titrated against the remaining silver and the amount of chloride determined from the difference between the added silver nitrate and that remaining after precipitating the chloride. Faster and more precise methods based on ion-selective electrodes are now available. One of the more modern automated analysers is illustrated in Figure 1.24. Automated testing is

10 100 3.7 3.9 4.1 4.3 4.5 4.7 4.9 Eq ui va le nt c ub e s tren gth N/mm2 UPV km/s

Equivalent cube strength N/mm2 Expon. (Equivalent cube strength N/mm2) Figure 1.23 Typical relationship between cube strength and UPV.

described in a paper from the Structural Faults and Repair Conference (Grantham, 1993)

Cement content

It is a fundamental requirement of good-quality concrete that it contains an adequate cement content, or more precisely, a sufficiently low water/cement ratio, to provide adequate durability for the intended exposure conditions. In the absence of chemical admixtures, a certain amount of water is required to provide an adequate workability; essentially to simply lubricate the aggregate particles and the cement. To achieve the desired water/cement ratio, the amount of cement required is therefore automatically defined. This can be altered only by changing the physical properties of the aggregate, or by the addition of a water-reducing admixture.

If the cement content is too low (i.e. the water/cement ratio too high), the concrete will be attacked by the weather and be liable to freeze–thaw damage and the effects of carbonation. If the cement content is too high, heat of hydration can cause thermal cracking in large pours, the risk of shrinkage increases (because of the higher water content) making curing doubly important, and, if a high-alkali cement is used, the risk of ASR increases with susceptible aggregates and DEF is more likely to occur. TESTMETHODS

The test to determine the cement content of concrete is given in BS 1881 : Part 124 : 1988 (BSI, 1988). It requires the crushed concrete to be extracted with dilute acid and dilute alkali solution to remove the cement. The extract

is then analysed for soluble silica and calcium oxide, being the two major components (expressed as oxides) of Portland cement. The cement content is determined by simple proportion from the two parameters. Where soluble components from the aggregate interfere by contributing to the calcium content (e.g. if a limestone aggregate is present) then the silica value would be used for the cement content determination. Conversely, if the silica value was inflated by some soluble component other than the cement, the lime value would be used, provided the analyst was confident that this was unaffected by soluble components from the aggregate. In practice, it is normal to analyse control samples of the aggregate, where these are available, to avoid these

problems. With control samples, an accuracy of better than ±25 kg/m3 _is

readily achievable.

Where cement replacement materials such as pfa (pulverised fuel ash) and ggbs (ground granulated blastfurnace slag) are present, the situation is more complex. Nevertheless, accurate results can often be obtained using total analyses by, for example, X-ray fluorescence methods and applying a simultaneous equations approach (Grantham, 1994) or by using petrographic methods combined with scanning electron microscopy (SEM). The topic is to be revisited in an update of Concrete Society Technical Report No. 32 (Concrete Society, 1989) by a Concrete Society working party.

Sulfate content

Exposure of concretes made with Portland cement to sulfate salts can cause damage due to an expansive reaction between the tricalcium aluminate phase of the cement and the sulfate salt to form crystals of ettringite. Given adequate space to form, the ettringite forms needle-like crystals, but in confined space causes an expansive reaction as the amorphous product develops. With good-quality concrete, significant sulfate attack is relatively rare, and research work suggests that concrete made with a reasonable

cement content (at least 330 kg/m3_{) and a reasonably low water/cement}

ratio, is attacked only very slowly. However, the most damaging salts are the more soluble sulfates based on magnesium or sodium sulfates. Calcium sulfate (gypsum) is only sparingly soluble and is less likely to cause damage. The rate of damage is also dependent on the rate of replenishment of the sulfate salts and hence on groundwater movement.

TESTMETHODS

Sulfate is usually determined by the method given in BS 1881 : Part 124 : 1988 (BSI, 1988). This involves an acid extraction and precipitation of the sulfate as barium sulfate with barium chloride solution. The resulting barium sulfate is filtered and weighed to determine sulfate gravimetrically.

Methods based upon ion selective electrodes and ion chromatography have also been employed. Petrography is usually the best initial indicator of a sulfate attack problem.

High-alumina cement (HAC)

HAC achieved some notoriety during the 1970s following the collapse of several buildings in which it had been used (Building Research Establishment, 1974). This was due to a conversion of the cement from one crystalline form into another, weaker, form. At normal temperatures, the hydration of HAC results in the formation of hydrated calcium monoaluminate (CAH10). Smaller amounts of C2AH8 and hydrous alumina are also formed. However, these hydrated calcium aluminates are metastable and can, at higher temperatures and in the presence of moisture, change to give the stable hydrated calcium aluminate C3AH6. This phenomenon is known as ‘conversion’, and the amount of the change occurring, ‘the degree of conversion’.

At normal temperatures, conversion may take many years but at temperatures in excess of 40°C a considerable amount of conversion can occur within a few months. Structures built with HAC are likely to have fully converted now and petrography and X-ray diffraction analysis are considered to be the best means of examination, with the degree of conversion test having fallen into disuse. Petrography should be accompanied by some means of strength evaluation and/or load testing.

Conversion results in a loss of strength, increased porosity and reduced resistance to chemical attack. There has been increasing concern regarding carbonation of high-alumina cement concrete. Following conversion, the increased porosity may permit rapid carbonation of the concrete, removing alkaline protection to the steel reinforcement, which may then suffer from corrosion.

TESTMETHODS

A test was devised by the Building Research Station to show whether HAC was likely to be present in a concrete (BRE, 1974). It essentially tests for a significant content of soluble aluminium in solution, following extraction with dilute sodium hydroxide solution.