Proper calculation and relative proportioning of materials are very important to prepare the cost-effective good quality of concrete. This post explains two simple methods used by engineers(like me) to calculate cement, sand, aggregate (coarse aggregate) and water to prepare different Nominal mix concretes like M5, M7.5, M10, M15 and M20 for construction.
Different trusted sources like IS456, IS10262 and the government’s Rate of Analysis have been verified to provide you with the following calculations and results.
If you don’t want to learn the calculation, you can simply use the tables given in the middle of the article to get the quantities of materials required to prepare different grade of concrete.
- Method 1: Calculating cement sand and aggregate required in volume and weight using Concrete Mix Ratio given in the IS Codebook
- Method 2: Calculating the weight of sand and aggregate needed per bag of cement ( 50kg bag) using the Total weight of aggregate given in the IS Codebook.
CONCRETE GRADES AND THEIR CONCRETE MIX RATIOS
Concrete is classified into different grades like M5, M7.5, M10, M15, M20 based on the strength of concrete. In concrete grades, the term M stands for “Mix” and the following number stands for characteristic compressive strength(fck) of concrete in 28 days in the Direct Compression test.
For example, in M20 grade concrete M stands for “Mix” and 20 is the strength of concrete in terms of Newton per millimeter square at the end of 28 days if the concrete is properly cured.
Concrete mix ratios are prescribed ratio of cement, sand, and aggregate to get the desired strength of concrete. For example, if the concrete mix ratio of M20 concrete is 1:1.5:3, then 1 part of cement, 1.5 part of sand and 3 part of aggregate in volume should be batched to prepare M20 grade concrete.
However, above concrete mix ratios may not be accurately followed in the construction site. This may be due to a few reasons such as to increase workability (by increasing fine aggregate – River sand and M sand) or to reduce cost (by reducing cement content) etc.,
Excess reduction of cement content or increase of sand content will adversely affect the strength of concrete. Hence it is advisable not to increase the fine aggregate (river sand and M sand) content more than 30% above the prescribed ratio in any case.
Before diving into calculation it is very important to understand the Difference between Nominal Mix Concrete and Design Mix Concrete and their pros and cons.
METHOD:1 CALCULATION OF CEMENT SAND AND AGGREGATE NEEDED FOR CONCRETE FROM CONCRETE MIX RATIO
While mixing of ingredients, cement and sand(fine aggregate) have to fill the gaps between the coarse aggregate before taking their own space. Hence in order to prepare 1 cubic meter of M20, M15 and M10 concrete you need 1.57 cubic meters of “total dry volume:” of cement, sand, and aggregate and incase of M7.5 and M5 concrete you need 1.52 cubic meters total dry volume of cement sand and aggregate.
Many websites are giving different value for “total dry volume”, but the values given above have been personally verified, many times, and they are in par with different government’s Rates of Analysis.
CEMENT SAND AND COARSE AGGREGATE NEEDED FOR M20 GRADE CONCRETE
Prescribed Concrete Mix Ratio of M20 grade concrete is 1:1.5:3 as per codebook.
- Cement = 1 Part
- Sand = 1.5 Part
- Aggregate = 3 Part
- Total dry volume of Material Required = 1.57 cu.m
WEIGHT OF CEMENT REQUIRED TO PREPARE 1 CUM OF M20 GRADE CONCRETE
Weight of cement required can be calculated from multiplying volume of cement we found above with density of cement. The density of frequently used cement varieties (both PPC and OPC) is 1440 kg/cu.m.
Hence 8 bags of cement required to prepare one cubic meter of M20 grade concrete.
Alternative method:- 1 cu.m of cement is approximately 28.8 bags.
VOLUME OF SAND AND AGGREGATE REQUIRED IN CUBIC FEET TO PREPARE 1 CUM OF M20 GRADE CONCRETE
Suppliers sell sand aggregate in the measurement of Cubic Feet (CFT), UNITS and in lorry or tipper LOADS. One UNIT measurement is equal to 100 cubic feet. One cubic meter is 35.32 Cubic feet.
Hence 15.08 cft of sand and 30.16 cft of aggregate is required to prepare one cubic meter of M20 grade concrete.
QUANTITIES OF CEMENT SAND AND AGGREGATE REQUIRED TO PREPARE ONE CUBIC METER OF DIFFERENT CONCRETE GRADES
*As I have explained above while calculating M7.5 and M5 grade concrete, total dry material required for concrete is taken as 1.52 instead of 1.55 and the quantity of sand and aggregate is adjusted for M10 and M5 based on experience.
NUMBER OF CEMENT BAGS AND CUBIC FEET OF SAND AND AGGREGATE REQUIRED TO PREPARE ONE CUBIC METE OF DIFFERENT CONCRETE GRADES
METHOD:2 CALCULATION OF WEIGHT OF SAND AND COARSE AGGREGATE NEEDED PER BAG OF CEMENT AS PER IS456
IS456 Codebook recommends the following quantities of total aggregate per 50kg bag of cement to produce nominal mix concrete.
WEIGHT OF SAND AND COARSE AGGREGATE NEEDED PER 50KG BAG OF CEMENT FOR M20 GRADE CONCRETE
Let’s find out the quantity needed to prepare M20 grade by the codal procedure. Sand is confirmed to Zone II – average grading. The ratio of fine to coarse aggregate is chosen as usual as 1:2. From the above table we know we need 250kg of total dry aggregate for our concrete.
The general ratio of fine aggregate to coarse aggregate is 1:2, but it can be adjusted between 1:1.5 to 1:2.5 based on the grading of fine aggregate and size of coarse aggregate.
- Sand = 1 Part
- Aggregate = 2 Part
- Total aggregate required = 250 kg
WEIGHT OF SAND AND AGGREGATE NEEDED IN KG PER 50KG BAG OF CEMENT FOR DIFFERENT GRADE OF CONCRETE
VOLUME OF SAND AND AGGREGATE NEEDED IN CUBIC FEET (CFT) PER 50KG BAG OF CEMENT FOR DIFFERENT GRADE OF CONCRETE
Convert the weight of sand and aggregate by dividing it with a bulk density to find the volume of sand and aggregate needed per bag of cement. Bulk density of the
WATER REQUIREMENT FOR DIFFERENT CONCRETE GRADES OF NOMINAL MIX CONCRETE AS PER IS456 CODE
For design, mix concrete water content is calculated from the Wate/Cement ratio which depends on various factors like the weight of cement, workability, etc. But for nominal mix concrete IS456 codebook suggests the following quantities per bag of cement.
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Er.SP.ASWINPALANIAPPAN., M.E.,(Strut/.,)
Structural Engineer
Madras Terrace Architectural Works
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