Concrete mix design is a crucial process that involves determining the appropriate proportions of cement, sand, and aggregates required to achieve the desired strength in concrete structures. This process is typically employed for large construction projects where the consumption of concrete is significant and higher grades of concrete, such as M25 and above, are required. The objective of concrete mix design is to obtain the right mix proportions that will ensure the concrete’s strength and durability, while also making the construction process economical.
To achieve this goal, concrete mix design involves several steps, including calculations and laboratory testing. These steps help to determine the right proportions of each material needed to achieve the desired strength of the structural members. One of the significant benefits of concrete mix design is that it ensures the correct proportions of materials are used, which can reduce the cost of construction. As the quantity of concrete required for large construction projects is substantial, reducing the quantity of materials such as cement can make the project more economical.
Concrete mix design is particularly useful for calculating the proportions of higher grade concrete, such as M20, M25, M30, and beyond. By following the steps and calculations involved in the mix design process, construction teams can ensure that they are using the correct proportions of materials and achieving the desired strength in their structures. Overall, concrete mix design is an essential process that can help to make construction projects more economical while ensuring the durability and strength of concrete structures.
Concrete Mix Design
Data Required for Concrete Mix Design
(i) Concrete Mix Design Stipulation
The field requires a characteristic compressive strength of M25 at 28 days. The nominal maximum size of aggregate is 20 mm, and the shape of the coarse aggregate is angular. The degree of workability required at the site is 50-75 mm slump. Quality control measures will be in accordance with IS:456. The structure will be subjected to mild exposure, as defined in IS:456. The type of cement used is PSC conforming to IS:455. The method of concrete placing is pumpable concrete.
(ii) Test data of material (to be determined in the laboratory)
The specific gravity of cement is 3.15, while the specific gravity of fine aggregate (FA) is 2.64. The specific gravity of coarse aggregate (CA) is 2.84. The aggregates are assumed to be in a saturated surface dry condition, and the fine aggregates conform to Zone II of IS – 383.
Procedure for Concrete Mix Design of M25 Concrete
Step 1 — Determination Of Target Strength
The Himsworth constant for a 5% risk factor is 1.65. In this specific case, the standard deviation is taken from the Indian Standard code IS:456 against M 20 and is determined to be 4.0. To calculate the ftarget value, which represents the target mean strength, we can use the formula ftarget = fck + 1.65 x S. Here, fck represents the characteristic compressive strength of concrete and has a value of 25 N/mm2. Using the given Himsworth constant and standard deviation value, we can calculate ftarget to be 31.6 N/mm2. This value is obtained by multiplying 1.65 and 4.0 and adding the product to the value of fck, which is 25 N/mm2. The standard deviation value of 4.0 is obtained from Table-1 of the Indian Standard code IS 10262-2009.
Step 2 — Selection of water / cement ratio:-
Table 5 of IS 456, which can be found on page 20, specifies the maximum water-cement ratio for Mild exposure condition as 0.55. However, based on practical knowledge and experience, it is suggested to use a water-cement ratio of 0.5. As 0.5 is lower than the maximum value allowed by the table, which is 0.55, it is considered acceptable.
Step 3 — Selection of Water Content
Table 2 of IS 10262-2009 provides guidelines for determining the water-cement ratio for concrete mixes based on the desired compressive strength and the nominal maximum size of the aggregate. In particular, the table specifies the maximum water content that can be used for a given nominal maximum size of aggregate.
For instance, according to the table, if the nominal maximum size of aggregate is 20 mm, the maximum water content that should be used in the mix is 186 kg. This means that the amount of water added to the mix should not exceed 186 kg per cubic meter of concrete.
It is important to follow these guidelines to ensure that the resulting concrete mix has the desired strength and durability properties. Using too much water in the mix can result in a weaker concrete that is more susceptible to cracking and other forms of damage.
Table for Correction in water content
| Parameters | Values as per Standard reference condition | Values as per Present Problem | Departure | Correction in Water Content |
| Slump | 25-50 mm | 50-75 | 25 | (+3/25) x 25 = +3 |
| Shape of Aggregate | Angular | Angular | Nil | – |
| Total | +3 |
Estimated water content = 186+ (3/100) x 186 = 191.6 kg /m3
Step 4 — Selection of Cement Content
In the given example, the water-cement ratio is 0.5, and the corrected water content is 191.6 kg/m3. The minimum cement content for mild exposure condition, as per Table 5 of IS 456, is 300 kg/m3. As the calculated cement content of 383.2 kg/m3 is greater than the minimum requirement, it is considered acceptable. However, it needs to be checked for durability requirements as per IS 456. For mild exposure conditions and reinforced concrete, the minimum cement content is 300 kg/m3, which is lower than the calculated value of 383.2 kg/m3. Therefore, the adopted cement content is 383.2 kg/m3. As per clause 8.2.4.2 of IS 456, the maximum cement content allowed is 450 kg/m3.
Step 5: Estimation of Coarse Aggregate proportion:-
The context provided is a reference to Table 3 of the Indian Standard Code IS 10262-2009, which specifies the proportioning of concrete mixes. The context specifically pertains to the selection of the nominal maximum size of aggregate, the zone of fine aggregate, and the volume of coarse aggregate per unit volume of total aggregate for a given water-cement (w/c) ratio of 0.5.
According to Table 3 of IS 10262-2009, for a nominal maximum size of aggregate of 20 mm and a zone of fine aggregate of Zone II, the recommended volume of coarse aggregate per unit volume of total aggregate is 0.62. This means that out of the total volume of aggregates used in the mix, 62% should be coarse aggregate and 38% should be fine aggregate.
The proportioning of concrete mixes is an important aspect of concrete design, as it directly affects the strength, workability, and durability of the resulting concrete. The use of the appropriate proportions of aggregates, water, and cement is essential in achieving the desired properties of the concrete.
The selection of the nominal maximum size of aggregate, the zone of fine aggregate, and the volume of coarse aggregate per unit volume of total aggregate is based on a number of factors, including the properties of the materials, the desired properties of the concrete, and the construction requirements. The guidelines provided in Table 3 of IS 10262-2009 are based on extensive research and testing, and are widely used in the construction industry in India.
Table for correction in estimation of coarse aggregate proportion
| Parameter | Values as per Standard reference condition | Values as per present problem | Departure | Correction in Coarse Aggregate proportion | Remarks |
| W/c | 0.5 | 0.5 | Nil | – | See Note 1 |
| Workability | – | pump able concrete | – | -10% | See Note 2 |
| Total | -10% |
Note 1:
The proportion of coarse aggregate in concrete needs to be adjusted based on changes in water-cement (w/c) ratio. For every 0.05 change in w/c, the coarse aggregate proportion should be altered by 0.01. When the w/c ratio is less than the standard value of 0.5, the volume of coarse aggregate should be increased to reduce the amount of fine aggregate used in the mixture. Conversely, if the w/c ratio is greater than 0.5, the volume of coarse aggregate should be reduced to increase the proportion of fine aggregate.
If the coarse aggregate used is not angular, it may be necessary to increase its volume appropriately based on past experience. In situations where pump-able concrete or congested reinforcement is involved, the proportion of coarse aggregate may be decreased by up to 10%. As a result, the volume of coarse aggregate per unit volume of total aggregate would be 0.62 multiplied by 90%, which gives a value of 0.558. The remaining proportion of the total aggregate would be occupied by fine aggregate, which would amount to 1 minus 0.558, or 0.442.
Step 6: Estimation of the mix ingredients
The given context is about calculating the volume and mass of various materials involved in making concrete.
Firstly, it is mentioned that the volume of concrete to be made is 1 cubic meter.
Then, the volume of cement and water needed for making the concrete is calculated. The volume of cement is determined using its mass and specific gravity, and is found to be 0.122 cubic meters. Similarly, the volume of water is calculated using its mass and specific gravity, and is found to be 0.1916 cubic meters.
Next, the volume of total aggregates is calculated by subtracting the sum of the volume of cement and water from the total volume of concrete. This value is found to be 0.6864 cubic meters.
Further, the mass of coarse aggregates and fine aggregates are calculated using the volume of total aggregates and their respective specific gravities. The mass of coarse aggregates is found to be 1087.75 kilograms per cubic meter, while the mass of fine aggregates is found to be 800.94 kilograms per cubic meter.
Concrete Mix proportions for Trial Mix 1
To conduct a trial -1 casting of concrete in a lab, the mass of ingredients required must be calculated. The concrete mix must satisfy both durability and economy. For this trial, the concrete will be cast in the form of 4 cubes, assuming 25% wastage. The volume of concrete required for the 4 cubes is calculated to be 0.016878 m3.
To determine the quantities of each ingredient needed, the proportions given must be used. The water-cement ratio (W/C) for this mix is 0.5. Based on this, the following mass calculations are made:
- Cement: The proportion of cement required is 383.2 kg/m3. Multiplying this by the volume of concrete needed (0.016878 m3) gives a total mass of 6.47 kg.
- Water: The proportion of water required is 191.6 kg/m3. Multiplying this by the volume of concrete needed (0.016878 m3) gives a total mass of 3.23 kg.
- Coarse aggregate: The proportion of coarse aggregate required is 1087.75 kg/m3. Multiplying this by the volume of concrete needed (0.016878 m3) gives a total mass of 18.36 kg.
- Fine aggregates: The proportion of fine aggregates required is 800.94 kg/m3. Multiplying this by the volume of concrete needed (0.016878 m3) gives a total mass of 13.52 kg.
Step 7: Correction due to absorbing / moist aggregate:-
The given statement suggests that because the aggregate is in a saturated surface dry condition, there is no need for any correction to be made. In other words, since the aggregate is already at its maximum capacity to hold water on its surface, there is no further need to make any adjustments.
It is important to note that the term “saturated surface dry” is used to describe a specific condition where the pores on the surface of the aggregate are filled with water, but there is no excess water on the surface. This condition is often used as a reference point for determining the water content of aggregates in various applications, such as concrete production or soil engineering.
In summary, the statement suggests that because the aggregate is already in a saturated surface dry condition, no further correction is needed. This condition indicates that the aggregate has reached its maximum water holding capacity on its surface, making it a useful reference point for various applications.
Step 8: Concrete Trial Mixes:-
Concrete Trial Mix 1:
In Step 6 of the concrete manufacturing process, the mix proportion is calculated to form trial mix 1. Once the mix proportion is determined, the concrete is produced and tested for its fresh concrete properties, such as workability, bleeding, and finishing qualities. Upon testing, it was found that the trial mix 1 had a Slump value of 25 mm and a Compaction Factor of 0.844. The slump test indicated that the mix was cohesive, workable, and had a true slump of approximately 25 mm, while also being free from segregation and bleeding. However, the desired slump value for the mix was between 50-75 mm, which meant that modifications needed to be made to the trial mix 1 to achieve the desired workability.
Concrete Trial Mix 2:
To increase the workability of concrete from 25 mm to 50-75 mm, an increase in water content by +3% is required. The corrected water content is calculated to be 191.6 x 1.03 = 197.4 kg. However, the water-cement ratio will not be changed to adjust the fresh concrete properties, which means the cement content needs to be recalculated to maintain the desired ratio. The calculated cement content is 394.8 kg/m3, which also meets the durability requirement.
To determine the volume of all in aggregate, we use the formula: 1 – [{394.8/(3.15×1000)} + {197.4/(1 x 1000)}]. The calculated value is 0.6773 m3. Using this value, we can calculate the mass of coarse and fine aggregate. The mass of coarse aggregate is calculated to be 0.6773 x 0.558 x 2.84 x 1000 = 1073.33 kg/m3, while the mass of fine aggregate is calculated to be 0.6773 x 0.442 x 2.64 x 1000 = 790.3 kg/m3.
Concrete Mix Proportions for Trial Mix 2
The given information provides details regarding the ingredients required for casting trial-2, with a 25% wastage assumption, for the production of four cubes. The required volume of concrete for the four cubes is calculated to be 0.016878 m3 using a formula based on the dimensions of the cubes. The mass of cement, water, coarse aggregates, and fine aggregates required for the mix is then calculated using their respective densities and the calculated volume of concrete.
The slump value and compaction factor of the mix are also provided. The slump test indicates that the mix is highly cohesive and workable, with a true slump value of about 60 mm. Despite flowing during vibration, the mix does not exhibit any segregation or bleeding. The desired slump value for the mix is within the range of 50-75 mm, and the current mix satisfies this requirement.
Based on these details, the next step is to move forward with trial mix-3.
Concrete Trial Mix 3:
The context discusses a trial mix where the water cement ratio is varied by +10% while keeping the water content constant. Specifically, in the example being presented, the water cement ratio is increased from 0.5 to 0.55. This increase in w/c ratio results in a reduction in the coarse aggregate fraction by 0.01, which means that the coarse aggregate as a percentage of the total aggregate is now 0.548. The water content is kept constant, and the cement content is calculated to be 358.9 kg/m3 using the given formula.
Using the volume of all in aggregate formula, the volume of all in aggregate is determined to be 0.688 m3. The mass of coarse aggregate is then calculated to be 1070.75 kg/m3, and the mass of fine aggregate is determined to be 821 kg/m3 using the given formulas.
Concrete Mix Proportions of Trial Mix 3
The given context provides information about the ingredients required for casting trial-3 for 4 cubes assuming 25% wastage. The volume of concrete required for 4 cubes is calculated and found to be 0.016878 m3. The mass of each ingredient, including cement, water, coarse aggregate, and fine aggregates, is calculated based on their respective densities and the volume of concrete required for 4 cubes.
The slump test is conducted to determine the workability of the mix. The obtained slump value is 75 mm, indicating that the mix is stable, cohesive, and workable. The desired slump range is 50-75 mm, and the mix meets this requirement, achieving the desired workability.
With the mix meeting the desired workability, the next step is to proceed with trial mix-4.
Concrete Trial Mix 4:
The context given describes a scenario where the water/cement ratio (w/c) is reduced by 10% while keeping the water content constant. The initial w/c value is 0.45, and the reduction of 0.05 in w/c results in an increase of the coarse aggregate fraction by 0.01. As a result, the new coarse aggregate fraction is 0.568, which is calculated by adding 0.01 to the initial value of 0.558.
The water content is 197.4 kg/m3, and the cement content is calculated by dividing the water content by the w/c ratio, which yields a value of 438.7 kg/m3. The volume of all in aggregate is then calculated by subtracting the sum of the cement and water contents from 1. The result is 0.664 m3.
The mass of the coarse aggregate is obtained by multiplying the volume of all in aggregate by the coarse aggregate fraction and the density of the coarse aggregate, which is 2.84 x 1000 kg/m3. The result is 1071.11 kg/m3. The mass of the fine aggregate is then calculated by multiplying the volume of all in aggregate by the fraction of fine aggregate (which is 1 minus the fraction of coarse aggregate) and the density of the fine aggregate, which is 2.64 x 1000 kg/m3. The result is 757.28 kg/m3.
Concrete Mix Proportions of Trial Mix 4
In order to conduct a casting trial, the required mass of ingredients for four concrete cubes with a 25% wastage is being calculated. The volume of concrete needed for the four cubes is determined to be 0.016878 m3. The proportions of cement, water, fine aggregates, and coarse aggregates are then calculated based on this volume. Specifically, the cement required is 7.4 kg, the water required is 3.33 kg, the coarse aggregate required is 18.07 kg, and the fine aggregate required is 12.78 kg per m3. However, a local correction is applied to these proportions due to the moisture condition of the aggregate. Once corrected, three concrete cubes are cast and left to set for 28 days. The compressive strength of the cubes is then tested and the results are summarized in a table.
Recommended mix proportion of ingredients for grade of concrete M25:
The given information presents a compressive strength versus water-cement ratio (c/w) graph for a target strength of 31.6 MPa. Based on the graph, the water-cement ratio is found to be 0.44, and the corresponding water content is 197.4 kg/m3.
Using the water content and water-cement ratio values, the cement content is calculated to be 448.6 kg/m3. Additionally, the volume of all in aggregate is determined to be 0.660 m3 by subtracting the sum of the volume of cement and water from 1.
The reduction of 0.05 in the water-cement ratio will lead to an increase of 0.01 in the coarse aggregate fraction. This, in turn, increases the coarse aggregate fraction from 0.558 to 0.568. Therefore, the volume of fine aggregate can be calculated by subtracting the coarse aggregate fraction from 1, which results in a value of 0.432.
Furthermore, the mass of coarse aggregate is determined to be 1064.65 kg/m3, and the mass of fine aggregate is calculated to be 752.71 kg/m3. These calculations are based on the volume of aggregate, its density, and the respective aggregate fractions.