We all are usually concerned about water/cement ratio of fresh concrete; here we will learn about water/cement ratio of hardened concrete. It should be kept in mind that we will determine water/cement ratio from hardened concrete sample but at the time of placing concrete mix i.e. fresh concrete.
This ratio is expressed as a function of cement content. We have already discussed about determination of cement content of hardened concrete. So if we can predict cement content with the determination of actual water content, the parameter can be determined.
Now question is how actual water can content in fresh concrete be determined from hardened concrete as majority portion of water is used for hydration (depending on maturity) and evaporation during placing and curing.
The water content during placing is expressed as a function of mass of water combined in the cement grains and volume of estimated capillary pores.
Capillary pores contain remaining water (say hydrated) of actual water content. Capillary water is also called non-evaporable water. This water is considered as 23 percent of mass of cement (obviously anhydrous, L.E. COPELAND and J.C. HAYES, 1983). For Type II cement this water content may even lower than 19 percent.
We can determine original water content in laboratory by igniting concrete sample under 10000C and determining drive out water (BS 1881: Part 124). But this method is not applicable for concrete sample containing blended cements (concrete society Report No.32).
Now consider the accuracy of this test method. Regarding this we can include the water/cement ratio determined may be of 10 % of actual water/cement ratio. There have also other methods like Reflected Light Fluorescence Microscopy.