What happens if more water is added to concrete?

If the w/c ratio is high, a large amount of water is available per unit weight of cement in the concrete mix. So if a fixed volume of concrete is poured into a cube of form-work, there are a large number of water-filled voids in the cube. So when the hydration reaction starts on the surface of the cement particle, the gel-like products of hydration get precipitated in the water – away from the surface of cement particles.

The gels formed in such a case are termed as Outer Products of Hydration. Now there are two reasons why the strength of concrete is low in this case, namely –

1. Since there is a large space for them to develop, the outer products of hydration are large in size. And we know, from the size effect, that larger sized particles have lower strength compared to particles of smaller size.

2. No matter how high the water consumption rate of the reaction is, due to the large amount of water present in the mix, some water will still be left when the concrete hardens and is ready for use. This trapped water will gradually evaporate, leaving some voids in the concrete block. The presence of voids results in greatly reduced strength.