Following is a simple method used by engineers to calculate cement, sand and aggregate to produce different grade of concrete like M5, M7.5, M10, M15 and M20. if you don’t want to learn the math involved in the design, you can use the table given at the end of the article to produce different grade of concrete.
There are two ways to design the concrete mix. They are Design mix method and nominal mix method. Once you know the math behind the design you can easily design any grade of concrete for different purposes in different units.
DIFFERENCE BETWEEN NOMINAL MIX CONCRETE AND DESIGN MIX CONCRETE
In the design mix method, materials are proportioned based on the procedure and rules given in IS456(2000) and IS10262(2009) codebooks. In this method, cement, sand and aggregates are always batched in terms of weight, and concrete can be designed for different environmental conditions and different needs. Hence, if you prepare concrete using design mix method you can save cost and get good quality of concrete.
In nominal mix method, materials are proportioned based on prescribed approximation and experience to attain the different grade of concrete. Cement, sand and aggregates are usually batched in terms of volume using pans, batching boxes etc.., Nominal mix method is easier to follow in site and they are easier to design and measure hence they are often used in site to produce concrete up to M20 grade.
UNDERSTANDING CONCRETE GRADE AND CONCRETE MIX RATIOS
Concrete is classified into different grades like M5, M7.5, M10, M15 etc., where M stands for Mix and the following number stands for characteristic compressive strength(fck) of concrete in 28 days in the Direct Compression test. So, if you produce M20 grade concrete then you will get the compressive strength of 20 N/mm2 in 28 days after mixing and casting.
The strength of concrete is based on the ratio of cement, sand, aggregate used to prepare concrete. Hence it is important to determine the mix ratio through previous experience and Guidelines. Concrete mix ratios are usually in the form of cement : Sand: Aggregate. 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 taken to produce concrete.
Prescribed concrete Mix ratios for all grades of concrete are given below
|M 5||1 : 5 : 10|
|M 7.5||1 : 4 : 8|
|M 10||1 : 3 : 6|
|M 15||1 : 2 : 4|
|M 20||1 : 1.5 : 3|
Above concrete mix ratios are not strictly practised in the site. You will often notice that the contractors increase the quantity of sand and reduce the quantity of cement to reduce the cost. Increasing the sand content will increase the workability of concrete which is good for mixing and placing but increasing the sand content will decrease the percentage of cement in concrete hence the strength and durability of the concrete will be affected. It is advisable not to increase the sand content more than 30 per cent than the above-prescribed ratio.
DESIGN OF NOMINAL MIX CONCRETE FOR ONE CUBIC METER OF M20 CONCRETE
Though you want to prepare 1 cubic meter of concrete, the total volume of cement sand and aggregate required will be more than 1 cu.m. Because, while mixing, cement and sand has to fill the gaps between the aggregate(crushed stone) before taking their own place. Hence if you want to produce concrete with 20 mm aggregate then the total volume of cement sand and aggregate you require will be 1.55 cubic meters, and if you want to produce concrete with 40 mm aggregate then the total volume of cement sand and aggregate you require will be 1.52 cubic meters.
Let’s calculate material required for M20 grade to prepare one cubic meter concrete.
Cement sand and aggregate ratio for M20 grade concrete is 1:1.5:3
- Cement = 1 Part
- Sand = 1.5 Part
- Aggregate = 3 Part
- Total Parts = 1 + 1.5 + 3 = 5.5
- Total Material Required per cubic meter of concrete= 1.55
|Volume of 1 part||= (1 / 5.5) x 1.55|
|= 0.282 cu.m|
|Volume of cement||= Ratio x1 part|
|= 1 x 0.282|
|= 0.282 cu.m|
|Volume of sand||= Ratio x 1 part|
|= 1.5 x 0.282|
|= 0.423 cu.m|
|Volume of aggregate||= Ratio x 1 part|
|= 3 x 0.282|
|= 0.846 cu.m|
Conversion of Units of the Material for site conditions
Cement is often bought in 50kg bags hence we must convert our calculated quantity to bags.
|Weight of cement||= volume x density|
|= 0.282x 1440|
No of cement bags required to prepare 1 cubic meter = 400 / 50 = 8 bags
Hence we require nearly 8 bags to prepare one cubic meter of M20 grade concrete.
Aggregates are often bought in Units through lorries and tippers. One cubic meter is 35.32 Cubic feet. One unit is equal to 100 cubic feet.
|Sand in cft||= Volume x 35.32|
|= 0.423 x 35.32|
|= 15 cft|
Hence we require 15 cft of aggregate to prepare one cubic meter of M20 grade
|Aggregate in cft||= Volume of sand x 2|
|=15 cft x 2|
|= 302 cft|
|= 0.30 unit|
Hence we require 30 cft of aggregate to prepare one cubic meter of M20 grade concrete.
M20 GRADE – MATERIAL CALCULATION USING GAUGE BOX AND PAN
Gauge boxes, pans are used for Volume batching in site. Volume of a gauge box is 0.35 cu.m(1cft) which is a volume of 1 bag of cement. For M20 grade, use 1 bag of cement, 1.5 boxes of sand and 3 boxes of aggregate. These are our results….
Likewise, cement, sand and aggregate for different concrete grades with 20mm aggregate (Total dry material = 1.55)….
|220kg||16.5 cft||32 cft|
DESIGN OF NOMINAL MIX CONCRETE BY WEIGHT AS PER IS CODE
QUANTITY OF CEMENT SAND AND AGGREGATE IN WEIGHT PER BAG (50KG) OF CEMENT
IS456 Code book recommends the following quantities of total aggregate per 50kg of cement to produce nominal mix concrete.
|GRADE||TOTAL ( FINE + COARSE )|
|M 5||800 kg|
|M 7.5||625 kg|
|M 10||480 kg|
|M 15||330 kg|
|M 20||250 kg|
Let’s find out the quantity needed to produce 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.
|sand per bag of cement||= (1 / (1+2)) x 250 kg|
|=83 kg ~ 85 kg|
|aggregate per bag of cement||= (2 / (1+2)) x 250 kg|
|= 166 kg ~ 165 kg|
Likewise, the weight of cement, sand and aggregate per bag of cement for different concrete grades …
|M20||1bag||85 kg||165 kg|
|M15||1bag||110 kg||220 kg|
|M10||1bag||160 kg||320 kg|
|M7.5||1bag||210 kg||415 kg|
|M5||1bag||265 kg||535 kg|
WATER REQUIREMENT FOR NOMINAL MIX CONCRETE
Water is taken based on the weight of cement and the required workability. For design mix concrete codebook suggests to use suitable Water/Cement ratio based on different factors but for nominal mix concrete codebook suggests the following quantities per bag of cement.
|GRADE||WATER PER 50KG OF CEMENT IN LITERS|