by
Manjunath Golla , Technical Manager , Aptus Value Housing Finance ltd, Bangalore
In concrete mix, the ratio of the amount of water to the amount of cement used (both by weight) is called the water to cement ratio (w/c). A w/c ratio of 0.4 means that for every 100 lbs of cement used in the concrete, 40 lbs of water is added. The more the w/c ratio is increased (that is, the more water that is added for a fixed amount of cement), the more the strength of the resulting concrete is reduced. This is mostly because adding more water creates a diluted paste that is weaker and more susceptible to cracking and shrinkage. Shrinkage leads to micro-cracks, which are zones of weakness. Once the fresh concrete is placed, excess water is squeezed out of the paste by the weight of the aggregate and the cement paste itself. When there is a large excess of water, that water bleeds out onto the surface. The micro channels and passages that were created inside the concrete to allow that water to flow become weak zones and micro-cracks.
