To produce a concrete of sufficient strength, it is crucial to mix the constituent materials – cement, water, aggregates, and admixtures – in specific proportions. The strength of the concrete mix design is dependent on several factors. Firstly, the water/cement ratio is a critical factor. This ratio determines the amount of water required to hydrate the cement fully, which in turn affects the strength of the concrete. A lower water/cement ratio will result in a stronger concrete.
Another factor that influences the strength of the concrete mix design is the cement content. The higher the cement content, the stronger the concrete will be. However, it is essential to maintain a balance between the cement content and water/cement ratio to prevent the concrete from becoming too dry or too wet.
The relative proportion of fine and coarse aggregates is also a crucial factor in the mix design. Fine aggregates, such as sand, help to fill the gaps between the coarse aggregates and the cement paste, resulting in a more compact and stronger concrete. The proportion of each aggregate should be carefully determined to achieve the desired strength and workability.
Finally, the use of admixtures can also affect the strength of the concrete mix design. Admixtures are chemicals added to the concrete mix to enhance its properties, such as workability, durability, and strength. The type and amount of admixture used must be carefully chosen to achieve the desired strength and properties of the concrete mix design.
Factors Affecting Concrete Mix Design Strength
The following paragraphs provide explanations for the design strength variable factors mentioned earlier, drawing on various theories and specifications.
The first factor, material properties, refers to the physical characteristics of the materials used in the design. These properties include strength, stiffness, ductility, and toughness, among others. The material properties are determined by the composition of the material, the manufacturing process used to create it, and other environmental factors. In designing a structure, the material properties must be taken into account to ensure that the structure is able to withstand the forces that it will be subjected to.
The second factor, load characteristics, relates to the type and intensity of the forces that will act on the structure. These forces may be static, such as the weight of a building, or dynamic, such as the wind or seismic forces that may be experienced by a structure. The load characteristics are determined by the environment in which the structure will be located, as well as the intended use of the structure. To design a structure that can withstand these forces, engineers must carefully consider the load characteristics and calculate the necessary strength requirements.
The third factor, safety margins, refers to the additional strength that is built into a structure beyond what is strictly necessary to support the loads it will experience. Safety margins are important because they provide a buffer against unexpected events, such as an earthquake or a sudden change in loading conditions. The size of the safety margin depends on the level of risk that is deemed acceptable for the structure in question.
Finally, the fourth factor, design codes and standards, refers to the specifications that have been developed to ensure that structures are designed and built to a minimum level of safety and quality. These codes and standards are developed by professional organizations and government agencies, and they provide guidelines for engineers to follow in the design process. Adherence to these codes and standards is important to ensure that structures are safe, reliable, and durable over their expected lifespan.
1. Water/cement ratio
The strength and durability of concrete is mainly determined by the water to cement ratio (W/C ratio), rather than the amount of cement used. According to Abram’s law, the strength of concrete decreases as the water/cement ratio increases. In fact, every 1% increase in the amount of water added reduces the strength of concrete by 5%. It is important to note that a water/cement ratio of only 0.38 is needed for complete hydration of cement. While a lower ratio may increase strength, any additional water added for workability above this ratio will evaporate, leaving behind cavities in the concrete which decrease its strength and durability.
Therefore, controlling the water/cement ratio on site is crucial. Every extra liter of water added can reduce the strength of concrete by 2 to 3 N/mm2 while increasing workability by 25 mm. The permeability and durability of concrete are also strongly influenced by the water/cement ratio. The revised IS 456-2000 has limited the maximum water/cement ratios for durability considerations by clause 8.2.4.1, table 5. Thus, maintaining the appropriate water/cement ratio is vital for ensuring the strength and durability of concrete.
2. Cement content
Concrete is composed of cement as a binding agent, which provides the strength required for its durability. To ensure that concrete is able to withstand exposure to weathering agents, it is recommended that the cement content should not fall below 300Kg/m3 for reinforced concrete structures. However, the amount of cement required for a particular mix is dependent on the conditions of exposure to weathering agents. It is important to note that a higher cement content does not necessarily translate to higher strength. In fact, recent research indicates that a leaner mix with the same water/cement ratio results in better strength.
However, simply reducing the water/cement ratio does not automatically result in higher grades of concrete. Lower water/cement ratios lead to lower water content and decreased workability. To achieve a given workability, a certain amount of water is necessary, and if the water/cement ratio is reduced, the cement content must be increased to maintain workability. A higher cement content helps to achieve the desired workability at a lower water/cement ratio. Many mix design methods provide empirical relations for determining the water content needed to achieve different levels of workability. The water/cement ratio needed to achieve a target mean strength is found by interpolating from graphs provided in IS 10262 Clause 3.1 and 3.2 fig 2.
In summary, the required cement content in concrete is dependent on the conditions of exposure to weathering agents. Higher workability requires a higher cement content, while lower water/cement ratios necessitate an increase in cement content to maintain workability. While a leaner mix with the same water/cement ratio leads to better strength, reducing the water/cement ratio does not necessarily result in higher grades of concrete.
3. Relative Proportion of Fine & Coarse Aggregates
There are two types of aggregates: coarse and fine. Coarse aggregates are particles that are retained on a standard IS 4.75mm sieve, while fine aggregates are particles that pass through this sieve. In concrete, the coarse aggregate typically occupies one third of the volume, which means that a change in the coarse aggregate can affect the strength of the concrete. The proportion of fine aggregates to coarse aggregates depends on several factors, such as the fineness of the sand, the size and shape of the coarse aggregates, and the cement content.
Fineness of sand plays a crucial role in determining the proportion of fine aggregates needed to achieve a cohesive mix. When the sand is fine, a smaller proportion of it is required, while coarser sand requires a greater proportion of fine aggregate. The size and shape of the coarse aggregates also affect the proportion of fine aggregates needed. Larger aggregates have less surface area and require less fine aggregate, while flaky aggregates with more surface area require more fine aggregate. Rounded aggregates have even less surface area and require even less fine aggregate to achieve a cohesive mix.
Cement content is another factor that influences the proportion of fine aggregates required. Leaner mixes require more fine aggregates than richer mixes because cement particles also contribute to the fines in concrete. The grading of the aggregates in the concrete mix design is also important because it determines the amount of paste required to fill the voids in the mix. If the void content is high, more cement is needed to fill it. Using well-graded aggregates can help reduce the void content, requiring less paste.
4. Admixtures
Admixtures are substances that serve various purposes and are widely available in the market. They are added to concrete mixes to address specific concerns such as reducing construction costs, achieving desired properties, and maintaining quality throughout the mixing, transporting, placing, and compacting process.
To achieve strength gain in concrete, water reducing admixtures are commonly used. While reducing the water-cement ratio can compromise workability, water reducing admixtures can help attain a cement-rich mix that remains workable without the need for excess water. Ideally, a workable mix should contain 45-55% water by the weight of cement, as excess water in the pores can evaporate and create voids that weaken the concrete and lead to failure cracks. By decreasing voids through the use of water reducing admixtures, the concrete’s strength can be improved.
There are different types of admixtures available, each with its own role in enhancing the strength of the concrete mix. By understanding the properties and functions of these admixtures, engineers and builders can make informed decisions and create concrete structures that are strong and durable.
Table.1: Strength Property Gained by Different Admixture
| Sl. No | Type of Admixture | Property Gained |
| 1 | High Range Water Reducers | Decrease the water and cement content hence lower the water cement ratio.Early Strength GainResults in High strength ConcreteReduce Chloride-ion penetration |
| 2 | Accelerating Admixtures | Increase Rate of Hydration- Initial Setting of the concreteEarly Age Strength Development |
| 3 | Water-Reducing Admixtures | Reduce water-cement ratioReduce cement contentIncrease slump |