In some cases question about quality of concrete is raised whether concrete mix is designed as per specification; both chemical tests and physical test methods are available. Dear reader we are discussing here about hardened concrete which is relatively new topics in this blog.
In this regard primary investigation is focused on cement content and on water/cement ration. Here we will discuss about cement content determination. The latter will be discussed in next post.
So this method is based on chemical analysis and no specific method of analysis can be said applicable as we know concrete consists of wide range of materials. A fairly reliable test method can be adopted if original ingredients used in concrete mix are available which is even a matter of engineering judgment.
We know there have many types of cement other than Portland cement. Even if we know Portland cement was used in a sample, there have no direct method to determine cement content. The methods are based on determining soluble silica with calcium oxide which is an indicator of cement content.
Now question is about silica and calcium compound remaining in aggregate. It is considered that silicates in our conventional cement are more readily decomposable than silica compound found in aggregate i.e. silicates in cement are more soluble than that remain in aggregate.
The same consideration is applicable for lime compounds remain in cement and aggregate in respect of solubility of calcium oxide.
BS 1881: Part 124 and ASTMC1084 prescribed standard method to determine cement content (Portland cement) in hardened concrete. But it is found that the result is usually not so precise especially concrete of lean cement content.
Furthermore, to interpret the tests, details knowledge of chemical composition or of chemical content in aggregate is very important. When consisting aggregates liberate large amount of soluble silica and or calcium oxide, this result found is more unreliable.
Concrete society report #32, provides us a guide to determine different cementitious materials in hardened concrete. The report suggests us to calculate slag content in concrete depending on silicate content in concrete sample when composition of slag is known, but again precision is too low.
For determining Fly Ash content, no specific suggestion was provided. In case of admixture types and their dosages, it is not possible to determine as we use these ingredients at very low dosages and wide range of admixtures make it difficult to be estimated.