Concrete mix design involves determining the appropriate proportions of cement, sand, and coarse aggregates for a specific concrete mix. These proportions are expressed as ratios, such as 1:2:4, which indicate the relative amounts of cement, fine aggregate, and coarse aggregate in the mix. For instance, a mix with a ratio of 1:2:4 would mean that there is one part cement, two parts fine aggregate, and four parts coarse aggregate.
The ratios used in concrete mix design can be expressed either by volume or by mass. Regardless of the method used, the goal is to find the correct proportions of the ingredients that will result in a concrete mix with the desired properties.
One crucial factor to consider when designing a concrete mix is the water-cement ratio. This ratio is expressed as a mass and indicates the amount of water needed per unit of cement in the mix. Finding the appropriate water-cement ratio is essential since it can significantly affect the strength, durability, and workability of the resulting concrete. Therefore, careful consideration of the water-cement ratio is crucial in the concrete mix design process.
Requirements for concrete mix design
Concrete strength is determined by its grade designation, which specifies the required characteristic strength. The strength of concrete also depends on the type of cement used, as it affects the rate at which compressive strength develops.
The maximum size of aggregates to be used in concrete can be as large as possible within the limits specified in IS 456:2000. However, it is important to limit the cement content to prevent shrinkage, cracking, and creep.
Workability is a crucial factor in ensuring that concrete is satisfactorily placed and compacted. It is affected by various factors such as the size and shape of the section, quantity and spacing of reinforcement, and the technique used for transportation, placing, and compaction.
Procedure for Concrete Mix Design as per IS 456 : 2000
The process for designing a concrete mix involves several steps. The first step is to determine the mean target strength of the concrete using the characteristic compressive strength at 28-day fck and the level of quality control. This is done by adding 1.65 times the standard deviation, obtained from the table of approximate contents, to the fck value.
Next, the water-cement ratio is obtained using an empirical relationship between compressive strength and water-cement ratio. The chosen ratio is then checked against the limiting water-cement ratio for the requirements of durability given in a table, and the lower of the two values is adopted.
The amount of entrapped air for the maximum nominal size of the aggregate is estimated from the table. The water content for the desired workability and maximum size of aggregates (for aggregates in saturated surface dry condition) is selected from the table.
The percentage of fine aggregate in the total aggregate is determined by absolute volume from the table for the concrete using crushed coarse aggregate. The values of water content and percentage of sand are adjusted based on any differences in workability, water-cement ratio, grading of fine aggregate, and for rounded aggregate, the values are given in the table.
The cement content is calculated from the water-cement ratio and the final water content after adjustment. The cement content is checked against the minimum cement content for the requirements of durability, and the greater of the two values is adopted.
Finally, the content of coarse and fine aggregates per unit volume of concrete is calculated from the quantities of water and cement per unit volume of concrete and the percentage of sand already determined in the previous steps.
The given context involves the process of determining the mix proportions of concrete and preparing trial mixes until the final mix proportions are achieved. The mix proportions are determined using various factors such as the absolute volume of concrete, the specific gravity of cement, the mass of water and cement per cubic meter of concrete, the ratio of fine aggregate to total aggregate by absolute volume, and the specific gravity of saturated surface dry fine and coarse aggregates.
To begin with, the absolute volume of concrete is calculated by subtracting the volume of entrapped air from the gross volume, which is taken as 1 cubic meter. The specific gravity of cement is denoted by Sc, and the mass of water and cement per cubic meter of concrete is denoted by W and C, respectively. The ratio of fine aggregate to total aggregate by absolute volume is represented by p, while the masses of fine and coarse aggregates per cubic meter of concrete are represented by fa and Ca, respectively. The specific gravity of saturated surface dry fine and coarse aggregates is represented by Sfa and Sca.
After determining the mix proportions, the concrete is prepared using the calculated proportions and cast into three cubes of 150 mm size. These cubes are then tested wet after 28 days of moist curing to check for their strength. If the desired strength is not achieved, trial mixes are prepared with suitable adjustments until the final mix proportions are arrived at. This iterative process continues until the desired strength of the concrete is achieved.
Concrete Mix Design Example – M50 Grade Concrete
The given information pertains to the specifications of various construction materials for a particular project. The grade designation of the concrete to be used is M-50, which indicates its compressive strength in mega pascals. The type of cement specified is O.P.C-43 grade, which stands for ordinary Portland cement with a strength of 43 mega pascals. The brand of cement to be used is Vikram (Grasim), a commonly used cement brand in the construction industry.
An admixture called Sika [Sikament 170 (H)] is also specified for the project. Admixtures are chemical substances added to concrete to modify its properties, such as setting time, workability, and strength. Sika is a well-known brand of admixtures that is widely used in the construction industry.
The fine aggregate specified for the project is from Zone-II with a specific gravity of 2.61. Fine aggregate is typically sand or crushed stone and is used in the production of concrete to improve its workability and strength.
The coarse aggregate specified for the project is of two sizes – 20mm and 10mm. The specific gravity of the 20mm size aggregate is 2.65, while that of the 10mm size aggregate is 2.66. Coarse aggregates are larger stones or crushed rock used in the production of concrete to provide bulk and strength.
The minimum amount of cement required for the project, as specified in the contract, is 400 kg per cubic meter of concrete. The maximum allowable water-cement ratio specified in the contract is 0.45. The water-cement ratio is an important factor in the production of concrete as it determines the strength and durability of the final product.
Concrete Mix Design Calculation
In the context provided, a target mean strength is being calculated. This value is determined by adding 50 to the product of 5 and 1.65, resulting in a target mean strength of 58.25 megapascals (MPa). The exact application of this target mean strength is unclear, but it is likely related to some kind of engineering or construction project where strength requirements are crucial.
2. Selection of water cement ratio:
The context given is related to the water cement ratio, which is an important parameter in the field of civil engineering. This ratio determines the amount of water required to create a workable mixture of cement and water. In the given scenario, the water cement ratio is set at 0.35, which means that for every unit of cement, 0.35 units of water are used.
The water cement ratio is crucial in determining the strength and durability of the concrete. It directly affects the compressive strength, permeability, and workability of the concrete. Therefore, it is important to choose an appropriate water cement ratio to achieve the desired properties in the concrete.
A water cement ratio of 0.35 is considered to be a relatively low ratio, which can result in a more cohesive and durable concrete mixture. This ratio is often used in construction projects that require high-strength concrete, such as bridges, dams, and high-rise buildings. However, it is important to note that the appropriate water cement ratio may vary depending on the specific requirements of each project.
In conclusion, the given context refers to the water cement ratio of 0.35, which is an important parameter in determining the strength and durability of concrete. This ratio can have a significant impact on the properties of the concrete mixture and is often chosen based on the specific needs of the project.
3. Calculation of water content:
According to Table No. 5 in IS : 10262, for a maximum size of aggregate of 20mm, the recommended water content is 180 kg/m3. However, it has been proposed to use a plasticizer in the mixture, which can help reduce the amount of water required by 20%.
As a result, the new water content calculation is done by multiplying the original value of 180 kg/m3 by 0.8, which results in a new value of 144 kg/m3. This adjusted water content takes into account the use of a plasticizer and ensures that the concrete mixture remains workable while also reducing the amount of water required.
4. Calculation of cement content:
The given information pertains to the water cement ratio and water and cement content of concrete. The water cement ratio is stated to be 0.35, while the water content per cubic meter of concrete is 144 kg. By using the formula for calculating cement content, the cement content is determined to be 411.4 kg per cubic meter of concrete. To comply with the contract’s minimum cement content requirement of 400 kg per cubic meter of concrete, the cement content is increased to 412 kg per cubic meter of concrete. Therefore, the cement content satisfies the contract’s minimum requirement, and the concrete mix is deemed acceptable.
5. Calculation of Sand & Coarse Aggregate Quantities:
The given context provides information regarding the calculation of the mix details for concrete per cubic meter. The total volume of concrete is 1 cubic meter, which is composed of various materials such as cement, water, admixture, fine aggregate, and coarse aggregate.
To determine the volume of each material, calculations are performed based on the given values. The volume of cement is calculated by dividing the weight of cement (412 kg) by the density of cement (3.15 x 1000), which equals 0.1308 cubic meters. Similarly, the volume of water is calculated by dividing the weight of water (144 kg) by the density of water (1 x 1000), which equals 0.1440 cubic meters. The volume of admixture is calculated by dividing the weight of admixture (4.994 kg) by the density of admixture (1.145 x 1000), which equals 0.0043 cubic meters.
The total volume of other materials except for coarse aggregate is calculated by adding the volumes of cement, water, and admixture, which equals 0.2791 cubic meters. The volume of coarse and fine aggregate is determined by subtracting the total volume of other materials from the total volume of concrete, which equals 0.7209 cubic meters.
Assuming that the volume of fine aggregate is 33% of the total aggregate volume, the volume of fine aggregate is calculated by multiplying the total aggregate volume by 0.33, which equals 0.2379 cubic meters. The volume of coarse aggregate is determined by subtracting the volume of fine aggregate from the total aggregate volume, which equals 0.4830 cubic meters.
The weight of fine aggregate is calculated by multiplying the volume of fine aggregate by the density of fine aggregate (2.61 x 1000), which equals 620.919 kg/cubic meter. Similarly, the weight of coarse aggregate is calculated by multiplying the volume of coarse aggregate by the density of coarse aggregate (2.655 x 1000), which equals 1282.365 kg/cubic meter.
Assuming a ratio of 20mm to 10mm coarse aggregate of 0.55:0.45, the weight of 20mm coarse aggregate is calculated by multiplying the total weight of coarse aggregate by 0.55, which equals 706 kg. The weight of 10mm coarse aggregate is calculated by multiplying the total weight of coarse aggregate by 0.45, which equals 578 kg.
To perform a trial, the values of cement, water, and admixture are increased by 2.5%. Hence, the weight of cement is calculated by multiplying the original weight of cement by 1.025, which equals 422 kg. Similarly, the weight of water is calculated by multiplying the original weight of water by 1.025, which equals 147.6 kg. The weight of admixture is calculated by multiplying the weight of cement by 1.2%, which equals 5.064 kg.
Finally, the ratio of water, cement, fine aggregate, and coarse aggregate is given as 0.35:1:1.472:3.043 for one cubic meter of concrete.
Observations from Concrete Mix Design
The mix used for the project was found to be cohesive and homogeneous, indicating a uniform consistency throughout. The slump of the mix was measured at 120mm, which was within the acceptable range for the project. A total of nine cubes were casted and tested for their compressive strength at both 7 and 28 days. The average compressive strength of the cubes at 7 days was recorded as 52.07 MPa, while at 28 days it was 62.52 MPa. This result was found to be greater than the required strength of 58.25 MPa, and as a result, the mix was accepted for use in the project.
Percentage strength of concrete at various ages
Concrete is known to become stronger over time. In fact, as it ages, its strength increases. This is demonstrated by a table that shows the strength of concrete at various ages in relation to its strength at 28 days. It is clear from the table that concrete gains strength as it ages beyond the 28-day mark.
| Age | Strength per cent |
| 1 day | 16% |
| 3 days | 40% |
| 7 days | 65% |
| 14 days | 90% |
| 28 days | 99% |