Concrete mix design is the process of selecting appropriate ingredients for concrete and determining their relative quantities to produce concrete with the required strength, durability, and workability at the lowest possible cost. The proportioning of concrete ingredients is influenced by the desired performance in both the plastic and hardened states. If plastic concrete is not workable, it cannot be placed and compacted correctly, making workability a critical property.
The compressive strength of hardened concrete, which is commonly regarded as an indicator of its other properties, depends on several factors, including the quality and quantity of cement, water, and aggregates, as well as batching and mixing, placing, compaction, and curing. The cost of concrete is determined by the cost of materials, plant, and labor. Because cement is several times more expensive than aggregates, the goal is to produce a mix with as little cement as possible.
From a technical standpoint, using rich mixes in structural concrete may cause high shrinkage and cracking, as well as the evolution of high heat of hydration in mass concrete, which can lead to cracking. In this study, grit and crushed sand were used for the design mix, and the characteristic compressive strength at 3, 7, and 28 days was determined. The mix design was performed following IS:10262.
Advantages of mix design
Concrete mix design is a process that aims to produce high-quality concrete at a reasonable cost. The term “quality concrete” refers to concrete that is strong, durable, impervious, and uniform. The mix design process takes into account various factors, including the type and quantity of ingredients, the desired strength, and the site conditions.
One of the primary goals of mix design is to achieve better concrete strength, imperviousness, and durability. The concrete must also be dense and uniform to meet the desired standards. To accomplish this, the mix design process employs a range of techniques and materials.
Another crucial aspect of mix design is to ensure economy. This involves reducing cement consumption by up to 15% for M20 grade concrete, with even more significant savings for higher grades. Lower cement content also helps reduce shrinkage cracks by decreasing the heat of hydration. In addition to saving on cement costs, mix design also enables the best use of available materials. By considering the site conditions, mix design can determine the most economical type of aggregates to use, thereby saving on transportation costs from further away.
Lastly, mix design helps achieve specific concrete properties, such as form finishes, high early strengths for early deshuttering, better flexural strengths, pumpability, and lower densities. The mix design process can provide a customized solution that meets the specific requirements of the project while being cost-effective.
Requirements of mix design
Mix design is a crucial process in the construction of concrete structures. It involves the correct proportioning of ingredients to optimize the properties of concrete according to the specific requirements of a site. To properly carry out mix design, the site engineer must provide important information to the mix design laboratory. This includes the grade of concrete, or its characteristic strength, as well as the required workability in terms of slump. Other properties may also be specified, such as retardation of initial set, slump retention, pumpability, acceleration of strength, and flexural strength. Additionally, the condition of exposure to concrete should be ascertained, with a proper investigation of soil to determine the presence of sulphates and chlorides if necessary.
The degree of control at the site must also be taken into consideration. This includes factors such as whether weigh batching or volume batching will be used, the type of aggregates to be used, whether testing of concrete cubes will be done regularly at the site, and whether the sources of sand and aggregate will be standardized or likely to change frequently. Proper supervision is also crucial to ensure the quality of concreting work, with qualified staff present to make necessary corrections such as accounting for moisture in sand and changes in material properties.
Factors affecting the choice of mix proportions
Concrete mix design is influenced by various factors that need to be considered to ensure that the desired properties of the hardened concrete are achieved. One of the most important properties of concrete is its compressive strength, which affects many other properties of the material. The mean compressive strength required at a specific age determines the nominal water-cement ratio of the mix. The degree of compaction also affects the strength of concrete, with fully compacted concrete having a higher strength. According to Abraham’s law, the strength of fully compacted concrete is inversely proportional to the water-cement ratio.
The workability of concrete is another important factor to consider in mix design. It is influenced by the size of the section to be concreted, the amount of reinforcement, and the method of compaction to be used. For narrow and complicated sections, as well as embedded steel sections, concrete with high workability is required to achieve full compaction with a reasonable amount of effort. The desired workability also depends on the compacting equipment available on site.
Durability is another important factor to consider in mix design, as it refers to the concrete’s resistance to aggressive environmental conditions. High-strength concrete is generally more durable than low-strength concrete. However, in situations where high strength is not necessary but high durability is vital, the durability requirement will determine the water-cement ratio to be used.
The maximum nominal size of aggregate is another factor to consider in mix design. In general, a larger maximum size of aggregate requires less cement for a particular water-cement ratio because the workability of concrete increases with an increase in maximum size of the aggregate. However, the compressive strength tends to increase with a decrease in the size of the aggregate. Recommended guidelines suggest that the nominal size of the aggregate should be as large as possible.
The grading and type of aggregate also influence the mix proportions for a specified workability and water-cement ratio. Coarser grading produces a leaner mix, which may not be desirable as it does not contain enough finer material to make the concrete cohesive. The type of aggregate also affects the aggregate-cement ratio for the desired workability and stipulated water-cement ratio. A satisfactory aggregate should have uniform grading, which can be achieved by mixing different size fractions.
Quality control is another crucial factor to consider in mix design. It ensures that the mix ingredients are accurately measured and mixed, placed, cured, and tested. The degree of control can be estimated statistically by the variations in test results. A lower difference between the mean and minimum strengths of the mix indicates lower cement-content required. Quality control is the factor that controls this difference.
I. Mix Proportion designations
Concrete mix proportions are commonly expressed using parts or ratios of cement, fine and coarse aggregates. These proportions indicate the relative amounts of each ingredient in the mix. For example, a mix with proportions of 1:2:4 means that there is one part cement, two parts fine aggregate, and four parts coarse aggregate in the mix. This can be expressed by either volume or mass.
The water-cement ratio, on the other hand, is always expressed in terms of mass. This ratio indicates the amount of water that is added to the mix in relation to the amount of cement. A lower water-cement ratio results in a stronger and more durable concrete, while a higher ratio results in a weaker and more porous concrete. Therefore, it is important to carefully control the water-cement ratio in order to achieve the desired strength and durability of the concrete.
Factors to be considered for mix design
Concrete is designated a certain grade to indicate its characteristic strength requirement. This helps in determining the quality and performance of concrete in various applications.
The type of cement used in the concrete mix can significantly affect the rate at which the compressive strength develops. This means that different types of cement can influence the time it takes for the concrete to reach its full strength potential.
In terms of the aggregates used in the concrete mix, the maximum nominal size can be as large as possible, but it must still adhere to the guidelines outlined in IS 456:2000. This helps ensure that the concrete has the desired properties and characteristics for its intended use.
To prevent issues such as shrinkage, cracking, and creep, the cement content in the concrete must be limited. This is because excessive cement content can lead to these types of issues, which can negatively impact the durability and lifespan of the concrete.
Finally, the workability of the concrete mix is an important consideration. It is related to various factors such as the size and shape of the section being poured, the quantity and spacing of reinforcement, and the technique used for transportation, placing, and compaction. By considering these factors, the concrete can be properly mixed, poured, and compacted to achieve the desired strength and durability.
Mix Design Procedure as per IS:10262.
To design concrete mix, several steps need to be followed. Firstly, the mean target strength (ft) can be determined by adding 1.65 times the standard deviation (S) obtained from the table of approximate contents to the characteristic compressive strength at 28 days (fck). Secondly, the water-cement ratio can be obtained using the empirical relationship between compressive strength and water-cement ratio. This ratio should be checked against the limiting water-cement ratio for durability requirements provided in the table, and the lower value should be adopted.
Thirdly, the amount of entrapped air can be estimated from the table for the maximum nominal size of the aggregate. Fourthly, the water content can be selected for the desired workability and maximum size of aggregates from the table, considering the aggregates are in saturated surface dry condition.
Fifthly, the percentage of fine aggregate in total aggregate by absolute volume can be determined from the table for the concrete that uses crushed coarse aggregate. Sixthly, adjustments can be made to the water content and percentage of sand from the table based on the workability, water-cement ratio, and grading of fine aggregate. For rounded aggregates, values are provided in the table.
Seventhly, the cement content can be calculated from the water-cement ratio and the final water content after adjustment. This cement content should be checked against the minimum cement content required for durability, and the greater value should be adopted. Finally, the content of coarse and fine aggregates per unit volume of concrete can be calculated from the quantities of water and cement per unit volume of concrete and the percentage of sand determined in steps six and seven, using the provided relations.
The given context provides a set of equations and variables used in determining the concrete mix proportions for a trial mix. The volume of entrapped air is subtracted from the gross volume of 1m3 to obtain the absolute volume of concrete (V). The specific gravity of cement (Sc), mass of water per cubic meter of concrete (W), and mass of cement per cubic meter of concrete (C) are also included in the equations. Additionally, the ratio of fine aggregate to total aggregate by absolute volume (p), total masses of fine and coarse aggregates per cubic meter of concrete (fa, Ca), and the specific gravities of saturated surface dry fine and coarse aggregates (Sfa, Sca) are also considered.
Once the concrete mix proportions have been determined using the equations, the concrete is prepared using the calculated proportions. Three cubes of 150 mm size are then cast and tested wet after 28-days moist curing to check for strength. If necessary, trial mixes with suitable adjustments are prepared until the final mix proportions are arrived at.
I. MIX DESIGN OF CONCRETE AS PER IS:10262.
Step 1: Design Stipulations Table – 1
| Grade of concrete | M 20 | M25 | M30 | M35 |
| Type of Cement | OPC/53 Grade | OPC/53 Grade | OPC/53 Grade | OPC/53 Grade |
| Maximum size of aggregate. | 30mm | 30mm | 30mm | 30mm |
| Degree of workability. | 0.8 | 0.90 | 0.9 | 0.9 |
| Water Cement Ratio | 0.5 | 0.45 | 0.42 | 0.4 |
| Cement Content kg/m3 | 400 | 400 | 400 | 400 |
| Aggregate Cement ratio | 4.3 | 4.8 | 4.8 | 4.9 |
Step 2: Test Data For Materials Table – 2
| Cement Used | OPC/53 | |
| Sp. Gravity of Cement | 3.15 | |
| Sp. Gravity of 30mm Aggregate | 2.8 | |
| Sp. Gravity of 10mm Aggregate | 2.76 | |
| Sp. Gravity of Grit Aggregate | 2.82 | |
| Sp. Gravity of Crush sand Aggregate | 2.79 | |
| Chemical admixture @0.05% by wt. of cement | Superplasticizer as per IS:1093 | |
| Water Absorption (%) | 30mm Aggregate | 1.10 |
| 10mm Aggregate | 1.42 | |
| Grit Aggregate | 3.06 | |
| Crush Sand Aggregate | 2.80 | |
Step 3: Sieve Analysis Table – 3
| I.S. SIEVE | COARSE AGGREGATE | FINE AGGREGATE | ||
| CA II % Passing | CA I % Passing | Grit % Passing | Crush Sand % Passing | |
| 40mm | 100.00 | 100.00 | 100.00 | 100.00 |
| 20mm | 29.51 | 100.00 | 100.00 | 100.00 |
| 10mm | 0.00 | 66.04 | 100.00 | 100.00 |
| 4.75mm | 0.00 | 5.93 | 94.60 | 76.80 |
| 2.36mm | 0.00 | 0.00 | 81.80 | 44.20 |
| 1.18mm | 0.00 | 0.00 | 55.00 | 32.00 |
| 600 µ | 0.00 | 0.00 | 28.80 | 15.60 |
| 300 µ | 0.00 | 0.00 | 14.40 | 6.40 |
| 150µ | 0.00 | 0.00 | 5.20 | 2.40 |
| Fineness Modulus | 7.70 | 6.26 | 3.2 | 4.23 |
Step 4: Target Strength Of Concrete Table – 4
| Grade of concrete | M20 | M25 | M30 | M35 | |
| Target strength (N/mm2) | Fck + 1.65 S | 26.60 | 28.25 | 38.25 | 43.25 |
| Characteristic compressive strength (N/mm2) | 3 days | 12 | 16 | 21 | 23 |
| 7 days | 16 | 20 | 24 | 28 | |
| 28 days | 20 | 25 | 30 | 35 | |
Step 5: Selection Of Water- Cement Ratio
| Grade of concrete | M20 | M25 | M30 | M35 |
| Water-Cement Ratio | 0.5 | 0.45 | 0.42 | 0.4 |
Step 6:Proportion Of Fine Aggregate and Coarse Aggregates Table – 6
| Cement | Grit | Crush Sand | Metal II | Metal I |
| … | 21 | 21 | 33 | 25 |
Step 7:Mix Proportions for One Cum of Concrete (SSD Condition) Table – 7
| Grade of concrete | M20 | M25 | M30 | M35 |
| Mass of Cement in kg/m3 | 400 | 400 | 400 | 400 |
| Mass of Water in kg/m3 | 200 | 180 | 168 | 160 |
| Mass of Fine Aggregate in kg/m3 | 614 | 684 | 696 | 704 |
| Mass of grit in kg/m3 | 307 | 342 | 348 | 352 |
| Mass of crushed sand in kg/m3 | 307 | 342 | 348 | 352 |
| Mass of Coarse Aggregate in kg/m3 | 1429 | 1478 | 1295 | 1271 |
| Mass of 20 mm in kg/m3 | 858 | 887 | 777 | 763 |
| Mass of 10 mm in kg/m3 | 572 | 591 | 518 | 508.4 |
| Mass of Admixture in kg/m3 | Nil | Nil | 2 | 2 |
Step 8: Mix Proportions Table – 8
| Grade of concrete | M20 | M25 | M30 | M35 |
| Cement | 1 | 1 | 1 | 1 |
| Water | 0.5 | 0.45 | 0.42 | 0.4 |
| Fine aggregate | 1.53 | 1.71 | 1.75 | 1.76 |
| Coarse aggregate | 3.57 | 3.7 | 3.24 | 3.18 |
Conclusion
The mix design results have shown that crushed sand can produce concrete that is just as good as that made with natural sand. The compressive strength of the concrete produced using crushed sand is equivalent to that of normal mixes. Due to the diminishing sources of natural sand, the use of crushed sand will likely become unavoidable in the near future. However, crushed sand particles have a different shape than natural sand particles and therefore require a slightly higher water demand, resulting in a slightly higher cement consumption. Nevertheless, if crushed sand is correctly graded with sufficient fines, the mix may have a lower water demand when compared to poorly graded natural sand. Crushed sand can also offer better control over gradation than natural sand, making it an economical option when good quality natural sand is unavailable.