So far we have discussed about different relationship between water content or water/cement ratio and strength of concrete or durability parameters; but do we know how does water is measured in calculating water/cement ratio? Is it total quantity of water?
Actually a more precise definition is required in this respect. We should consider that quantity of water which takes it position outside surface of aggregate particles; the time is also important i.e. the water should be at the time of stabilizing total volume of concrete.
This is assumed that measuring during setting is the right time. Thus there will have three terms
• Effective water/cement ratio
• Free water/cement ratio
• Net water/cement ratio.
Are you confused? Let’s explain component of water in concrete:
Water content consists of added water during mixing and water held within the aggregate while entering into the mixers.
The aggregates have wide range of pores; some are as large as can be seen with microscope or can even be seen with naked eyes. Some pores are entirely within solid portion of aggregate; but some are open to surface of aggregate particles.
Water has access to pores of aggregate and penetration rate and amount depends on
• Size of pores
• Their continuity
• Total volume of them
The held water on aggregate is partly absorbed through the surface to fill the void and if there have continuity the whole structure is filled. Some part of this water remains as free on surface of these aggregates.
So there have difference between these free water and added water in to mix. It is expected to treat aggregates before mixing so that if doesn’t absorb water from mix. Again like excess water on surface, as discussed above, there may have deficiency of water within aggregate i.e. aggregate is not entered in saturated surface dry condition.
In this case, unsaturated aggregates have some pores filled with air. When water is added to concrete mix, a portion is absorbed by aggregate and entrapped air will come out from aggregate pore and during compaction they will be reached to surface.
This absorption continues up to an hour starting from just after mixing. In this situation it is quite difficult to provide a demarcation between free and absorbed water.
So excess water remain in the mix, after absorption of aggregate to become saturated-surface-dry, is taken as effective water. Now can you remember during mix design a calculation for water adjustment is required? Sometimes water adjustment requires adding water (unsaturated aggregate) and sometimes requires subtracting water when there have excess water that is held by surface of aggregate.
Therefore water/cement ratio defined by mix design is based on added water excluding absorbed water i.e. free water in the mix. Sometimes laboratory test refer to gross water added to dry aggregate.
Interpretation of laboratory test results should be done carefully while making a mix proportion decision on the job site and all references of water/cement ratio should be clear whether it is based on total or free water.