Chemical analysis of hardened concrete can provide a wealth of information about the mix constituents and possible causes of deterioration. Deterioration in concrete can be caused by environmental factors, inferior materials, poor workmanship, inherent structural design defects and inadequate maintenance. This analysis is usually required for one of two reasons: the most common being that something has gone wrong and the cause and/or blame for the problem is thought to be related to cement content. The other is that an older structure is being repaired or expanded and it is desired to match the existing materials. Some methods describing how to do such determinations:1)chemical composition of crumbled sample of concrete, especially regarding two components: soluble silica in HCl and /or CaO,2) the content of insoluble parts in HCl,3)sieve analysis of concrete,4)differential thermal analysis,5)microscopy analysis,6)density of mortar. The laboratory method is based on determination: apparent density of concrete, ascertainment of presence of slag and/or ash, the content of insoluble parts in HCl and the content of component affiliated to binder during hardening.
The largest difficulty in chemical analysis of concrete is heterogeneity of this material. This is why the way and the place of taking specimen is very important for precision results. The mass of sample depends on grain’s size of aggregate. The minimum mass of concrete’s sample (the sum of individual pieces) should be 3 kg if the aggregate’s size is up to 40mm. If aggregate size is bigger the sample’s mass – up to 5 kg. Choosing place of taking samples one should consider: external appearance of concrete, local changes of colour, presence of efflorescence and others in whole building.
It is necessary to determine apparent density of concrete, amount of insoluble residue in HCl and amount of affiliated components during hydration, hydrolysis and carbonization and make proper calculation using these data.
Determination of apparent density (Apparent density - the weight (dried to constant mass in temp. 105 - 110ºC) per unit volume of a material including voids inherent in the material.) Determination of aggregate contentThe amount of insoluble residue in HCl is assumed to be aggregate. Sample of concrete (1 kg) one should crumble and sieved through sieve 1 mm. According to proper standard reduce mass of sample to 50 – 100 g and dry to constant mass in temp. 105ºC. After drying and precise mixing one should weigh 5 – 10 g of sample and powder in mortar until the grain’s size will be less than 0,2 mm. Sample (1 – 2 g) is taken for further investigation: determination of insoluble parts in HCl. The weighed sample should be placed into 250 cm3 beaker, add 100 cm3 water solution of HCl 1:3 (1,19). Mix all with glass rod, leave for 15 min. and decant the liquid over the precipitate. Then add to the precipitate 50 cm3 HCl and keep on water bath (temp. 90 - 100ºC). Content of the beaker wash twice hot water and decant. Add to the beaker 50cm3 5% Na 2CO3 and keep it on water bath for 15 min. and wash twice hot water and decant. Place 50 cm3 of water into the beaker with precipitate, then acidify with HCl at the presence of methyl orange and filter. The precipitate wash 6 times with hot water until chlorides are completely washed. In the end make the test for chloride ions: place some drops of the solution on watch glass, add a drop of Ag NO3 - if you don’t get precipitate it means that there are no chloride ions. Put the paper filter with content into the porcelain crucible (first weight the crucible) and burn the paper filter, then ignition of the precipitate in the temp.1000ºC. After the cooling in the exsiccator weight the crucible with the precipitate (m1). Calculate the content of insoluble components according to the formula: [%]
m1 – the mass of the crucible with the precipitate [g], m2 – the mass of the crucible [g], m3 – the mass of the sample [g].
ρ – apparent density [kg/m3]
There is assumed that the content of insoluble parts equals the content of aggregates.
If aggregates consists components soluble in HCl it will be necessary to change formula:
[%] Rk – content in aggregates of component soluble in HCl [%].