The weight ratio of water to cement in a concrete mix is known as the water-cement ratio. It is typically expressed as a decimal or a percentage. The water-cement ratio can be calculated using the following formula:

Water-cement ratio = weight of water/weight of cement

The water-cement ratio can also be expressed as a percentage by multiplying the ratio by 100.

There is no 'law' that dictates the specific water-cement ratio that must be used in concrete, as the optimal ratio will depend on the specific properties of the cement being used and the intended use of the concrete.

The water-cement ratio is an important factor in determining the strength and durability of concrete. 

A lower water-cement ratio will produce stronger, more durable concrete, but the concrete will be more difficult to work with and may require longer curing times. 

A higher water-cement ratio will produce concrete that is easier to work with and requires shorter curing times, but the concrete may be weaker and less durable. The optimal water-cement ratio will depend on the specific properties of the cement being used and the intended use of the concrete.

Significance of Water cement ratio

The water-cement ratio is important because it affects the chemical reactions that take place within the concrete as it cures. When water is added to cement, it starts a chemical reaction called hydration, which causes the cement to harden and bind the other ingredients together. The amount of water used in the mix determines the amount of hydration that occurs and therefore affects the strength and durability of the concrete.

If too much water is used in the mix, the excess water will fill the spaces between the cement particles, resulting in weaker, less durable concrete. On the other hand, if not enough water is used, the cement will not fully hydrate, which can also result in weaker, less durable concrete. It is important to use the right amount of water in the mix to ensure that the concrete has the desired strength and durability.