From previous discussion here we mentioned on water cement ratio ranging from 0.40 to 0.60 per bag of cement. Knowing that commercially available and widely used in the country is a 40kg bag of cement (portland).

To put into computation say 0.40 x 40kg = 16kg of water, which normally equated in construction sites as 16 liters as conversion reference is 1kg=1L.

16 liters pail from Boysen
16 liters image from boysen

Therefore:

  • For a 0.50 ratio to 40kg = 20kg or 20L of water for every bag of cement.
  • same process if using 0.60 ratio to 40kg bag of cement = 24kg or 24 liters

Since portland cement is used, it has its own density of 1506kg/cu.m compared to the general cement of 1440kg/cu.m we normally used as basis for computation of quantity of cement, sand and gravel per cubic meter.

1 cubic meter

With this given data divide 1506kg/40kg = 0.02656cu.m for one bag of cement multiply this with 1000 and you get 26.56 liters equivalent.

note: in 1cu.m of water = 1000 kg

So a 1cu.ft of measuring box filled with 1 bag of 40kg of portland cement is 26.56 liters which is not that far from the usual computation of 40kg from  a density of 1440kg/cu.m = 0.02777cu.m x 1000 = 27.77 liters or roughly 28 liters.

density of water:  1000kg/cu.m 

therefore: 1000kg/36 = 27.7777 or 28 liters

In Summary:

  1. general cement  40kg = can fit in 1cu.ft = 28 liters
  2. portland cement 40kg = can fit in 1cu.ft = 26.56 liters
  3. using 1kg = 1 liter 40kg = can fit in 1cu.ft = 40 liters

In general practice, the 40 liters multiplied by factor of 0.40-0.60 is commonly used but will incur problems later on if delivered aggregates are not totally dry but rather have some amount of water content, compromising your mixture since it will add up to your water cement ratio.

To be safe start your water ratio with the first two liters 26.56 to 28 liters to be multiplied with your 0.40-0.60 factors that would start from 10-14 liters vs  16-20 liters from the usual practice, be sure to check on the quality of materials and actual site conditions as well.

Always remember even if you have the right dry mix ratio if you fail to balance the amount of water then most likely the targeted compressive strength will not be achieved.

"More water, the lesser the strength of concrete becomes."

Disclaimer: Provided data and computation is for general info and educational purposes only, always follow what is specified as per your structural plans and advise from your Structural Engineer in charged.