Water cement ratio is the ratio of the amount of weight of water to the amount of cementitious materials (fly ash, ground granulated, blast-furnace slag, silica fume, rice husk ash and natural pozzolans) by weight is termed as the water-cement ratio. The strength and quality of concrete primarily depend upon water cement ratio.
The primary functions of water in concrete –
- The water enters into chemical action with cement and responsible for the hydration process, this action causes the setting hardening of concrete by strength gaining properties.
- The water lubricates the aggregates to increase workability and it facilitates the passage of cement through voids of aggregates and protect from forming honeycomb structure. It is responsible mainly for the porosity (porosity causes capillary rise, results strength decreases) of the hardened cement paste.
For proper workability the water cement ratio varies from 0.4 to 0.6 as per IS code 10262 (2009) for nominal mixes. For every kilogram of cement, about 0.38 kg or corresponding unit) of water is needed to fully complete hydration reactions. But if it is decreased to less than 0.4 that responsible for lack of consistency of cement and workability of concrete and honeycombed structure. So that A lower ratios are used to get higher strength and durability, and for getting better along with Workability/flowability along with higher strength use of plasticizers or super-plasticizers as admixtures.
Calculation of Water Cement Ratio
Generally, water cement ratio is not calculated and it is used as per required workability for different type of construction. But some researchers are given formulas to calculate water cement ratio –
- In 1897 Feret proposed a rule defining strength of concrete paste in terms of volume fractions of the constituents as
Where c = volume of cement
w = volume of water
a = volume of air
K = a constant
In this expression the volume of air is also included, which means the voids in concrete are taken into account in estimating the strength.
- Duff A. Abrams presented his classic law in 1918 as “For plastic mixtures using neat and clean aggregates the strength of concrete under specified conditions is governed by the net quantity of water mixed per bag of cement”. He gave the following equation to estimate the strength of concrete.
where S = strength at 28 days, and constants, A = 14000 lbs/sq.in and B = 7.
What if we add a large amount of water?
- Too much water will cause segregation i.e. the sand and aggregates will segregate from cement paste, result will not be able to make bonds tightly.
- Also, water that is not consumed by the hydration reaction cause to running of concrete through formwork as it hardens, leads to microscopic pores called bleeding of concrete that will reduce final strength of concrete.
- A concrete mix with excess water will experience more shrinkage as total volume consider at the time of construction. After completion excess water leaves from structure, results to forming internal fractures and visible cracks, which will reduce the durability of structure.