Tests on concrete are conducted in different stage after placing, of these, tests on hardened concrete are reliably (to some extent) established, but when analysis is concern, the determination of water/cement ratio and cement content are of primary interest and it should keep in mind that no method is considered satisfactory to determine these. We have already discussed about analysis of hardened concrete.
Now-a-days, modern concrete batching plants provides us a record for each batch which gives information about type and amount of materials incorporated in concrete. But detail information about moisture content and grading of aggregate is not available, we know moisture content and gradation of aggregate defines water demand for mix to ensure sufficient workability; cement requirement is also a function of aggregate grading and surface texture of the same.
In practical situation, mistakes and even intentional actions can produce wrong mix proportions. Sometimes it is required to determine composition of fresh concrete. Here we will learn about determinations of cement content of fresh concrete.
ASTM C 1078 provides a chemical method for determining cement content in fresh concrete. This method is based on calcium-ion concentration in mix. This method requires special operator and equipment which are very rare in usual laboratory.
The U.S. Army suggested a calcium-titration method for determining cement content which takes only 15 minutes in field. But there have a limitation of including fine particles of calcareous aggregate (passing No.100 sieve) in determining cement content.
For separating materials passing No. 100 sieve, pressure filter method was developed where filter material are pressed dry and cement content is determined; a correction is done for the aggregate fraction passing No.100 sieve (determined during gradation test before batching). This may produce error in pressure filter method. Sometimes cement is separated by floatation.
A unique method was developed based on application of heavy liquid to separated cement from mix with a centrifuge. Again this technique has draw-back and not successfully used especially when fine particles of aggregate have identical or very near specific gravity to specific brevity of cement (expected to be significantly lower).
‘Rapid Analysis Machine’ can be used in this purpose, here also a correction for aggregate particles passing No. 100 sieve is required which is also a source of error is determination. Thus there have no practicable and/or reliable method for determining cement content in fresh concrete that can suggest us acceptance of mix. We will discuss about rapid analysis machine in next post.