What is concrete mix design?

Concrete mix design is defined as the appropriate selection and proportioning of constituents to produce a concrete with pre-defined characteristics in the fresh and hardened states.

Moreover, concrete mixes are designed in order to achieve a defined workability, strength and durability . Finally, this article presents factors affecting the choice of concrete mix design.

Basis for selection and proportioning of materials 

  • The structural requirements of the concrete
  • environmental conditions
  • The job site conditions, especially the methods of concrete production, transport, placement, compaction and finishing
  • The characteristics of the available raw materials

The various factors affecting the choice of concrete mix design are:

1. Compressive strength of concrete

  • Concrete compressive strength considered as the most important concrete property. It influences many other describable properties of the hardened concrete.
  • The mean compressive strength (fcm) required at a specific age, usually 28 days, determines the nominal water-cement ratio of the mix.
  • ISO 456-200, British Standard, and Eurocode utilize the term mean compressive strength which is slightly greater than characteristic compressive strength. However, ACI Code do not use such term.
  • Other factors which influences the concrete compressive strength at given time and cured at a specified temperature is compaction degree.
  • Finally, it is demonstrated that, concrete compressive strength of fully compacted concrete is inversely proportional to the water-cement ratio.
Means compressive strength vs characteristic compressive strength

Fig.1:Means compressive strength vs characteristic compressive strength

compressive strength of concrete

Fig.2: compressive strength of concrete

2. Workability of concrete

  • Concrete workability for satisfactory placement and compaction depends on the size and shape of the section to be concreted, the amount and spacing of reinforcement, and concrete transportation; placement; and compaction technique.
  • Additionally, use high workability concrete for the narrow and complicated section with numerous corners or inaccessible parts. This will ensure the achievement of full compaction with a reasonable amount of effort.
  • Frequently, slump test values used to evaluate concrete workability.
  • Lastly, ACI 211.1 provides slump test values for various reinforced concrete sections which ranges from 25 mm to 175 mm.
Workability of concrete

Fig.3: Workability of concrete

Self compacting concrete, high workability concrete

Fig.4: Self compacting concrete, high workability concrete

3. Durability of concrete

  • The ability of concrete to withstand harmful environment conditions termed as concrete durability.
  • High strength concrete is generally more durable than low strength concrete.
  • In the situations when the high strength is not necessary but the conditions of exposure are such that high durability is vital, the durability requirement will determine the utilized water-cement ratio.
  • Concrete durability decreases with the increase of w/c ratio.
Durability of concrete

Fig.5: Durability of concrete

4. Maximum nominal size of aggregate

  • Reinforcement spacing controls maximum aggregate size.
  • Aggregate size is inversely proportional to cement requirement for water-cement ratio. This is because workability is directly proportional to size of aggregate
  • However, the compressive strength tends to increase with the decrease in size of aggregate. smaller aggregate size offers greater surface area for bonding with mortar mix that give higher strength.
  • IS 456:2000 and IS 1343:1980 recommends that the nominal size of the aggregate should be as large as possible.
  • Finally, in accordance with ACI code, maximum aggregate size shall not exceed minimum reinforcement spacing, bar diameter, or 25mm.
Maximum aggregate size

Fig.6: Maximum aggregate size

5. Grading and type of aggregate

  • Aggregate grading influences the mix proportions for a specified workability and water-cement ratio.
  • The relative proportions between coarse and fine aggregate in concrete mix influence concrete strength.
  • Well graded fine and coarse aggregate produce a dense concrete because of the achievement of ultimate packing density.
  • If available aggregate, which obtained from natural source, does not confirm to the specified grading, the proportioning of two or more aggregate become essential.
  • Additionally, for specific workability and water to cement ratio, type of aggregate affects aggregate to cement ratio.
  • Lastly, An important feature of a satisfactory aggregate is the uniformity of the grading that achieved by mixing different size fractions.
aggregate grading types

Fig.7: aggregate grading types

6. Quality Control at site

  • The degree of control could be evaluated by the variations in test results.
  • The variation in strength results from the variations in the properties  of the mix ingredients, in addition to lack of control of accuracy in batching, mixing, placing, curing and testing.
  • Finally, the lower the difference between the mean and minimum strengths of the mix lower will be the cement-content required. The factor controlling this difference is termed as quality control.