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.
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.
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.
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.
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.
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.