MG Associates can provide a comprehensive service in the chemical analysis of building materials, paints and sealants.  Listed below are some of the more commonly requested tests.  If the test you require is not shown, then please contact us.

Tests Available

Capabilities

The laboratories are equipped with state-of-the-art apparatus to carry out analysis of samples.  This includes autotitrators, atomic absorption spectroscopy, infra red and UV spectroscopy, gas-liquid chromatography, wet chemical analysis and HPLC.

Testing for High Alumina cement

Notes on the Techniques

Chemical Tests

Chemical analysis of concrete can provide extremely useful information regarding the cause or causes of failure of concrete.

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

Chloride when present in reinforced concrete can cause very severe corrosion of the steel reinforcement. Chlorides can originate from two main sources:

1. "Internal" Chloride, i.e. chloride added to the concrete at the time of mixing. In this category, calcium chloride accelerating admixtures, contamination of aggregates and the use of sea water or other saline contaminated water are included.
2. "External" chloride, i.e. chloride ingressing into the concrete post-hardening. In this category, both de-icing salt as applied to many highway structures and marine salt, either directly from sea water in structures such as piers, or in the form of air-borne salt spray in structures adjacent to the coast.

The effect of chloride salts depends to some extent on the method of addition. If the chloride is present at the time of mixing, the calcium aluminate (C3A) phase of the cement will react with the chloride to some extent, chemically binding it as calcium chloroaluminate. In this form, the chloride is insoluble in the pore fluid and is not available to take part in damaging corrosion reactions. The ability of the cement to complex the chloride is limited, however, and depends on the type of cement. sulfate resisting cement, for example, has a low C3A content and is therefore less able to complex the chlorides. In any case, experience suggests that if the chloride exceeds about 0.4% by mass of cement, the risk of corrosion increases. This does not automatically mean that concretes with chloride levels higher than this are likely to suffer severe reinforcement corrosion: this depends on the permeability of the concrete and on the depth of carbonation in relation to the cover provided to the steel reinforcement.

When the concrete carbonates, by reaction with atmospheric carbon dioxide, the bound chlorides are released. In effect this provides a higher concentration of soluble chloride immediately in front of the carbonation zone. Normal diffusion processes then cause the chloride to migrate into the concrete. This process, and normal transport of chlorides caused by water soaking into the concrete surface, is responsible for the effect sometimes observed where the chloride level is low at the surface, but increases to a peak a short distance into the concrete (usually just in front of the carbonation zone). The increase in unbound chloride means that more is available to take part in corrosion reactions, so the combined effects of carbonation and chloride are worse than either effect alone.

Passivation of the steel reinforcement in concrete normally occurs due to a two component system comprising a portlandite layer and a thin pH stabilised iron oxide/hydroxide film on the metal surface. When chloride ions are present, the passivity of the system is lost by dissolution of the portlandite layer, followed by debonding of the passive film. Physical processes operating inside the passive film may also contribute to its disruption.

The critical chloride content required to initiate corrosion depends on whether the chloride was present at the time of mixing, or has ingressed post hardening, as discussed above. Clearly this also depends on the microclimate of the concrete (temperature and humidity) and also whether the concrete has carbonated. A figure of about 0.2% by mass of cement is generally accepted for chloride ingressing post-hardening and 0.4% or 0.5% by mass of cement, where chlorides are added at the time of mixing. Good quality concrete can often show a remarkable tolerance for chloride without significant damage, however, at chloride contents up to about 1% by mass of cement (usually for chloride added at the time of mixing: reinforced concrete is much less tolerant of ingressed chloride).

When chlorides have ingressed from an external source particularly, in conditions of saturation and low oxygen availability, insidious pitting corrosion can occur, causing massive localised loss of cross section. This can occur in the early stages without disruption of the concrete underneath.

Test Methods

The generally accepted method of test for chloride in hardened concrete is described in BS 1881 pt 124. 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.

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 frost attack 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.

Test Methods

The test to determine the cement content of concrete is given in BS 1881 : Part 124 : 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 plus or minus 25 kg/m³ is readily achievable.

Where cement replacement materials such as pfa (pulverised fuel ash) and ggbfs (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.

Depth of Carbonation

In a normal, good quality reinforced concrete, the steel reinforcement is chemically protected from corrosion by the alkaline nature of the concrete. This alkalinity causes the formation of a passive oxide layer around the steel reinforcement. Concrete, however, reacts with atmospheric carbon dioxide (and sulphur dioxide) to cause gradual neutralisation of the alkalinity from the surface inwards: a process known as carbonation. The rate at which this occurs is a function of concrete quality, mainly the water/cement ratio and the compaction. It is generally accepted that the rate of the carbonation reaction is inversely proportional to the square root of the age of the structure. If the depth of carbonation is taken in mm. and the age of the structure in years, the constant of proportionality is approximately unity.

So for K (Rate Constant) = 1

i.e. Rate of carbonation (mm/yr) = 1 /(Age in years)^0.5

(NB the rate applies only at the particular age chosen. The rate cannot be used for other ages)

or Depth of carbonation (mm) = (Age in years)^0.5

Recent research suggests that the square root relationship holds only at about 50% RH. At higher humidities the power function drops off, so that above 90% RH the depth of carbonation is likely to equate to the (Age in years)0.3 and continues to fall at higher humidity. the effect of this is to mean that the carbonation depth will be lower for concrete continuously exposed to higher humidities.

On this basis, even with a cover of only 10 mm, steel reinforcement should be safe for more than 100 years. In practice, however, carbonation often occurs rather faster, either because the concrete is excessively permeable or due to microcracking in the concrete providing secondary paths to the steel other than by normal diffusion processes. Excessive permeability can result from a high water/cement ratio, but can also result from poor curing of the cover concrete. Most modern specifications fail to recognise the importance of curing on concrete quality.

For the reasons given above, the advice given in table 3.4 of BS 8110, for example, is rather more stringent, in recognition that concrete in practice is often less than perfect.

Test Methods

The depth of carbonation can be measured on a freshly exposed section of the concrete, such as a core, by spraying with an indicator spray such as phenolphthalein. This turns to a pink colour when the concrete is alkaline (above pH 9.2) but remains colourless where the concrete is carbonated, usually as a more or less even zone extending to some depth from the surface. It should be noted that the pH at which the colour of phenolphthalein changes is lower than that at which passivity is lost (which occurs progressively below about pH 11). The test is described in a BRE Information Sheet

It should be noted that carbonation along microcracks and along diffusion paths in poorly compacted concrete, or so called reconstituted stone, may not be readily revealed by the phenolphthalein spray method. Petrographic methods can reveal carbonation of this kind and are recommended.

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.

True 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.

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.

Test Methods

Sulfate is usually determined by the method given in BS 1881 pt 124:1988. 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.

HAC achieved some notoriety during the 1970's following the collapse of several buildings in which it had been used. 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. High alumina cement also is more prone to strength loss in a damp environment.  Not surprisingly, one of the first collapses in the UK occurred in a swimming pool roof, where the atmosphere was both warm and damp.

Conversion results in a loss of strength, increased porosity and reduced resistance to chemical attack. Recently, 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.

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

The presence of the carbonate minerals renders any determination of the degree of conversion of the concrete potentially inaccurate. The best procedures for examination of HAC are petrographic and X-Ray diffraction analyses.