Proportioning concrete involves selecting the appropriate quantities of cement, sand, coarse aggregate, and water to achieve the desired strength and quality of the concrete. The proportioning of coarse aggregate, cement, and water must be carefully chosen to ensure that the resulting concrete possesses specific properties.
One of the critical properties that fresh concrete must possess is workability, which allows it to be easily placed in the formwork. Additionally, the concrete must be dense and strong, making it water-tight and durable. The cost of materials and labor used to create the concrete must also be minimized.
Determining the proportions of cement, aggregates, and water to achieve the desired strength requires either designing a concrete mix, known as design mix concrete, or adopting a nominal mix, known as nominal mix concrete. While nominal mix can be used, design mix concrete is typically preferred.
To ensure consistency and quality, the concrete for each grade must be analyzed separately to determine its standard deviation. This analysis helps to ensure that the concrete consistently achieves the desired strength and properties.
Where,
The given context defines the term “deviation” as the difference between an individual test’s strength and the average strength of a set of n samples. The variable n represents the number of sample test results that are being considered. In other words, deviation is a measure of how far apart the results of an individual test are from the average of a group of tests.
To further clarify, let’s say we have conducted n tests and obtained their respective strengths. We can then calculate the average strength of these n samples. The deviation of an individual test strength would then be the difference between its strength and the average strength of the n samples. This deviation value can be positive or negative, depending on whether the individual test’s strength is higher or lower than the average strength of the n samples.
Overall, deviation provides insight into the variation within a set of test results. It can help us identify outliers or unusual results and understand how much the individual test results differ from the average of the group.
Methods of Proportioning Concrete
Arbitrary Method of Proportioning Concrete
The proportion of cement, sand, and coarse aggregate can be expressed as 1: n: 2n by volume. However, for achieving very high strength, the proportions may vary. In such cases, the ratios may be 1: 1: 2 or 1: 1.2: 2.4.
For normal works, the proportions may be 1: 1.5: 3 or 1: 2: 4. These ratios are suitable for achieving standard strength requirements.
For foundations and mass concrete works, the proportions may be 1: 3: 6 or 1: 4: 8. These ratios are suitable for situations where high strength is not the main requirement, but rather durability and stability are the primary concerns.
Recommended Mixes of Concrete
According to the Indian Standard code of practice for reinforced concrete, IS 456:2000, it is advised that only concrete with a minimum grade of M20 should be used in reinforced concrete construction. This means that grades lower than M20 should not be used in any RCC work. This is a crucial guideline to ensure the strength and durability of the structure being built. Therefore, it is important to follow the recommended grade of concrete to ensure the safety and longevity of the structure.
| M10 | 1 : 3 : 6 |
| M15 | 1 : 2 : 4 |
| M20 | 1 : 1.5 : 3 |
| M25 | 1 : 1 : 2 |
Fineness Modulus Method of Proportioning Concrete
The term “fineness modulus” is a measure that indicates the average size of particles in a quantity of aggregates. It is calculated by adding the percentage of weight of material retained on a series of sieves, and then dividing that number by 100. A higher fineness modulus indicates coarser aggregates.
There are different sieves used for different types of aggregates. All aggregates are tested using sieves with openings of 80 mm, 40 mm, 20 mm, 10 mm, and Nos. 480, 240, 120, 60, 30 and 15. Coarse aggregates are tested using sieves with openings of mm, 40 mm, 20 mm, 10 mm, and No. 480. Fine aggregates are tested using sieves with openings of Nos. 480, 240, 120, 60, 30 and 15.
The proportion of fine aggregate to combined aggregate by weight is an important factor to consider in the testing of aggregates.
The given context is a mathematical equation that includes a variable P representing the desired fineness modulus for a concrete mix containing both fine and coarse aggregates. The equation is not in paragraph form, but I can rephrase it in a sentence or two to provide additional context.
In concrete mixing, the fineness modulus is a measure of the particle size distribution of the aggregate used in the mix. It is calculated by adding the cumulative percentages of aggregate retained on each of a specified series of sieves, and dividing the sum by 100. The resulting number is the fineness modulus.
The variable P in the equation represents the desired value for the fineness modulus in the concrete mix. This value will depend on the specific application and properties desired for the finished product. The equation is used to determine the appropriate proportions of fine and coarse aggregates to achieve the desired fineness modulus.
= fineness modulus of fine aggregate
= fineness modulus of coarse aggregate.
Minimum Void Method
When it comes to creating a concrete mix, it is important to ensure that the right amount of each component is used in order to achieve the desired result. In particular, the amount of sand that is used should be enough to fill all of the gaps in the coarse aggregate. Similarly, the amount of cement that is used should be enough to fill all of the gaps in the sand. By doing this, a mix can be created that is both dense and has a minimal amount of voids.
In practice, the amount of fine aggregate that is used in the mix is typically around 10% more than the amount of voids in the coarse aggregate. This helps to ensure that all of the gaps are filled and that the mix is as dense as possible. Additionally, the amount of cement that is used is typically around 15% more than the amount of voids in the fine aggregate. This ensures that there is enough cement to fill all of the gaps in the sand and create a strong and durable mix. By following these guidelines, it is possible to create concrete mixes that are both effective and reliable.
Maximum Density Method:
(Not very Popular)
The context given describes a situation where a box is being filled with a combination of fine and coarse aggregates. In this scenario, there are certain parameters to consider, such as the maximum size of the coarse aggregate (referred to as D), the percentage of material finer than a specific diameter d (by weight), and the maximum size of the fine aggregate.
To determine the optimal combination of aggregates, various proportions of the fine and coarse materials are tested, and the proportion that yields the heaviest weight is chosen. This approach ensures that the mixture of aggregates used in the box is optimized for maximum weight and density.
Overall, this context highlights the importance of carefully considering the characteristics of different aggregates and their combinations when seeking to optimize the weight and density of a mixture. By taking into account factors such as maximum size and percentage of material finer than a certain diameter, it is possible to create a combination of aggregates that is ideally suited to the intended application.
Water – Cement Ratio Method of Proportioning Concrete
Abram’s Law states that the strength of well-compacted concrete with good workability is primarily dependent on the water-cement ratio. A lower water content results in a stiffer paste with greater binding properties, leading to increased strength. Conversely, a higher water content increases workability but lowers the strength of the concrete. To determine the optimum water-cement ratio for a given compressive strength, graphs and expressions based on various experiments can be used. If the amount of water is less than the optimum amount, the strength of the concrete may decrease, and a decrease of 10% below the optimum amount may not ensure complete cement setting.
Practical values for the water-cement ratio in reinforced concrete structures include 0.45 for 1:1:2 concrete, 0.5 for 1:1.5:3 concrete, and 0.5 to 0.6 for 1:2:4 concrete. Concrete that has been vibrated by efficient mechanical vibrators requires less water cement ratio and therefore has more strength.
There are thumb rules for deciding the quantity of water in concrete. The weight of water is calculated as either 28% of the weight of cement plus 4% of the weight of the total aggregate or 30% of the weight of cement plus 5% of the weight of the total aggregate. It is important to note that if the water-cement ratio is less than 0.4 to 0.5, complete hydration of the cement may not occur.