Cement Concrete Mix Design

The concept of concrete mix design can be resolved to a large extend from its name itself to the primary level. In a macro level of definition, cement concrete mix design can be stated as the process of selecting the suitable ingredients of concrete and determining their relative amount with the objective of producing a concrete of such grade which satisfies the required standard of the designer and condition. Some of the most relevant and necessary standard that are kept in mind while cement concrete mix design are durability, workability, strength and as economical as possible, but the proportion of different ingredients that are used in the cement concrete mix design are governed by the required performance of concrete in two states, namely the plastic state and hardened state. If we consider the different properties of concrete than the property of workability is one of the most important among all it is because if the plastic state of the cement concrete mix design is not workable, it cannot be property, place and compacted.

Cement Concrete Mix Design

The compressive strength of hardened concrete which is generally considered to be an index of its other properties depends on many factors, e.g. quality of cement, water, and aggregate, batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of material arise from the fact that the cement is several times costly than the aggregate, thus, the aim is to procedure as lean a mix as possible. The cement concrete mix design relates to many other things such as it is related to the cost of material required for producing a minimum mean strength called characteristic strength that is specified by the designer of the structure. This depends on the quality control measures, but there is no doubt that the quality control adds to the cost of concrete.

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Types of Cement Concrete Mix Design

Nominal mixes

These types of cement concrete mix design offer simplicity and under normal circumstances have a nominal margin of strength above that specified. Due to the variability of mix ingredient the nominal concrete for a given workability varies widely in strength. IS 456-2000 has recommended that minimum grade of concrete shall not be less than M20 in reinforced concrete. The code allows nominal mixes for RCC work of M20 or lower. The practice of nominal mixes must be conducted under the permission of the engineers.

Standard mixes         

The nominal mixes of fixed cement –aggregate ratio vary widely in strength and may result in under or over-rich mixes, for this reason, the minimum compressive strength has been included in many specifications. These mixes are termed as standard mixes. IS 456-2000 has designated the concrete mixes into a number of grades as M10, M15, M20, M25, M30, M35 and M40. In this designation, the letter M refers to the mix and the number to the specified 28-day cube strength of mix in N/ sq. Metre. The mixes of grade M10, M15, M20, M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3), and (1:1:2) respectively. According to IS 456-2000, all cement concrete mix design above M20 Grade for RCC work must be of design mixes.

Designed mixes

In these mixes, the performance of the concrete is specified by the designer, but the mix proportion are determined by the designer but the proportions are determined by the producer of concrete, except that the minimum cement content can be laid down. The designing process is a trial and error method right proportion of ingredient is sought to be determined so as to achieve the targeted mean strength which is kept somewhat higher than the characteristic compressive strength of the concrete. The designing process in most of the major project is usually carried out through reputed laboratories. IS 10262-2000 is the relevant Indian Standard stipulating guideline for concrete mix proportioning.

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Factors Affecting the Choice of Mix Properties and Cement Concrete Mix Design

As we have mentioned that the designing of the cement concrete mix designing is done according to the requirement of the compressive strength, workability, durability and economy. Thus, the various factors affecting the cement concrete mix design are briefly discussed below:

  • Compressive strength – It is a well-known fact that concrete is superior in resisting the compressive forces, so while designing the Grades of concrete the required compressive strength of the concrete is kept in mind. This is because it is the most important property that determines other properties of the cement concrete mix design in a hardened state. Any sample of concrete gains its 90% of the strength in 28 days, which determines the nominal water-cement ratio of the mix, and the rest of the strength is gained with time.
  • Workability – Workability is one of the physical parameters of cement concrete mix design which affects the strength, compaction, and durability of the designed concrete. In an easy and conventional manner, the workability of concrete can be stated as the ease with which the concrete can be placed and compacted homogeneously i.e. without bleeding or segregation. The property of fresh concrete which is indicated by the amount of useful internal work required to fully compact the concrete without bleeding or segregation in the finished product.
  • Durability – The durability of cement concrete mix design is the duration till which a prepared cement concrete mix design will meet all the required standards with the loading. In other words, the durability of concrete is its resistance to aggressive environmental conditions. The durability requirement in some cases will be determined by the water-cement ratio to be used. This is because in some cases high strength concrete are avoided even though it would provide the required amount of durability, but in such cases, the cement concrete mix design with varying water-cement ratio is used in order to attain the required strength and durability.
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