Concrete mix design is the process of selecting suitable concrete materials and determining their relative proportions with the goal of generating a concrete with the required strength, durability, and workability as economically as possible. The needed performance of concrete in two phases, namely the pliable and hardened states, governs the proportioning of ingredients in concrete. It is impossible to correctly put and compact plastic concrete if it is not workable. As a result, the property of workability becomes critical. The compressive strength of hardened concrete, which is often used as an indicator of its other attributes, is determined by a variety of factors, including the quality and quantity of cement, water, and aggregates; batching and mixing; and placement, compaction, and curing.
Materials, plant, and labour all contribute to the cost of concrete. The disparities in material costs are due to the fact that cement is several times more expensive than aggregate, hence the goal is to generate a lean mix as possible. Rich mixtures, from a technical standpoint, may cause structural concrete to shrink and crack, as well as the evolution of high heat of hydration in mass concrete, which may induce cracking.
The design mix in this investigation was grit and crushed sand, and the characteristic compressive strength was found at 3,7 and 28 days, respectively. IS:10262 was used to create the mix design.
Advantages of mix design
The goal of mix design is to produce high-quality concrete at a low cost on site.
a) Cement consumption economy With the help of concrete mix design, it is feasible to save up to 15% of cement for M20 grade concrete. In fact, the higher the concrete grade, the greater the savings. Lower cement content also means lower hydration heat, which means fewer shrinkage cracks.
b) Making the best use of available resources:The quality and amount of ingredient components are frequently limited by site limitations. Concrete mix design allows for a wide range of aggregate types to be employed in the mix. If the available materials meet the fundamental IS standards, mix design can provide a cost-effective solution. This may result in transportation cost savings.
c) Other characteristics:Form finishes, strong early strengths for early deshuttering, concrete with improved flexural strengths, concrete with pumpability, and concrete with lower densities can all be achieved with the right mix design.
Requirements of mix design
Mix design Requirement
Concrete mix design is a way of properly proportioning concrete materials in order to maximise the above-mentioned qualities of concrete in accordance with site requirements.
When delivering material for mix design to the mix design laboratory, the site engineer should include the following information: –
a) Concrete quality (the characteristic strength)
c) Requirement for workability in terms of slump
c) Additional attributes (if applicable): –
i. Delay in the initial set (to avoid cold joints in case of longer leads or for ready-mix concrete)
ii. Retention of a slump (in case of ready mix concrete)
A. Compressive strength
It is one of the most essential qualities of concrete and has an impact on a variety of other hardened concrete properties. The nominal water-cement ratio of the mix is determined by the mean compressive strength required at a certain age, usually 28 days. The degree of compaction is another aspect that affects the strength of concrete at a certain age and when cured at a specific temperature. The strength of fully compacted concrete is inversely proportional to the water-cement ratio, according to Abraham’s law.
Three factors influence the level of workability necessary. These are the dimensions of the concreted section, the amount of reinforcement, and the compaction method to be utilised. The concrete must have a good workability for the small and intricate section with several corners or inaccessible portions, so that full compaction can be achieved with a reasonable amount of effort. This also applies to the steel portions that are implanted. The compacting equipment available at the job location determines the desired workability.
D.Aggregate minimal size limit
In general, the smaller the maximum size of aggregate, the less cement is required for a given water-cement ratio, because the workability of concrete improves as the maximum size of aggregate increases. Compressive strength, on the other hand, tends to rise as aggregate size decreases.The nominal size of the aggregate should be as large as possible, according to IS 456:2000 and IS 1343:1980.
Aggregate grading and type The mix proportions for a specific workability and water-cement ratio are influenced by aggregate grading. The coarser the grade leaner, the more it can be used. A very lean mix is undesirable because it lacks sufficient finer particles to keep the concrete cohesive.
A very lean mix is undesirable because it lacks sufficient finer particles to keep the concrete cohesive. For the acceptable workability and stipulated water cement ratio, the kind of aggregate has a significant impact on the aggregate-cement ratio. The homogeneity of the grading, which can be obtained by blending different size fractions, is a crucial quality of an acceptable aggregate.
E. Quality Control
Variations in test results can be used to calculate the degree of control. The strength variances are due to differences in the qualities of the mix materials as well as a lack of accuracy in batching, mixing, placement, curing, and testing. The smaller the difference between the mix’s mean and minimum strengths, the less cement is required. The determinant
I. Proportional designations in the mix The proportions of materials in a concrete mix are commonly expressed in terms of parts or ratios of cement, fine and coarse aggregates. For example, a 1:2:4 concrete mix means that the proportions of cement, fine aggregate, and coarse aggregate are 1:2:4, or that the mix comprises one part of cement, two parts of fine aggregate, and four parts of coarse aggregate. The proportions are measured in either volume or mass. In most cases, the water-to-cement ratio is stated in mass.Factors to consider when creating a blend
The grade of concrete that specifies the required characteristic strength. The rate at which concrete’s compressive strength develops is influenced by the type of cement used.
Within the boundaries of IS 456:2000, the maximum nominal size of aggregates to be utilised in concrete may be as large as possible. The cement content must be kept as low as possible to avoid shrinkage, cracking, and creep. The size and shape of the section, the quantity and spacing of reinforcement, and the technique employed for transportation, putting, and compaction all affect the workability of concrete for satisfactory placement and compaction.
Mix Design Procedure as per IS:10262
1. Calculate the mean target strength ft based on the specified characteristic compressive strength at 28 days fck and the quality control level. ft = fck + 1.65 S, where S is the standard deviation calculated from the approximate contents table provided after the design mix.
2. Using the emperical relationship between compressive strength and water cement ratio, calculate the water cement ratio for the desired mean target, which is then compared to the limiting water cement ratio. The chosen water cement ratio is compared to the limiting water cement ratio for the durability requirements shown in the table, and the lesser of the two values is chosen.
3. Using the table, calculate the amount of entrapped air for the aggregate’s maximum nominal size.
4. From the table, select the water content, required workability, and aggregate maximum size (for aggregates in saturated surface dry condition).
5. For the concrete with crushed coarse aggregate, calculate the percentage of fine aggregate in total aggregate by absolute volume from the table.
6. For any differences in workability, water cement ratio, fine aggregate grading, and rounded aggregate, adjust the values of water content and percentage of sand as specified in the table.
7. Using the water-cement ratio and the final water content after adjustment, calculate the cement content. Check the cement against the minimum cement content specified in the durability standards, and the greater of the two numbers is used.
The findings of mix design show that crushed sand can produce concrete that is just as good as natural sand. The compressive strength achieved is comparable to that of standard blends. In fact, because to diminishing natural sand resources, the usage of crushed sand will become unavoidable in the near future. Though formed, crushed sand particles lack the spherical shape of natural sand. As a result, crushed sand requires more water than natural sand, resulting in somewhat higher cement usage. When compared to poorly sorted natural sand, crushed sand may have a lower water demand provided it is correctly graded with enough particles.
Crushed sand also allows for better gradation control as compared to natural sand. As a result, if good quality natural sand is unavailable, crushed sand may become a cost-effective alternative.