Concrete Mix design is the process of determining required and specified characteristics of a concrete mixture. Characteristics can include: (1) fresh concrete properties; (2) required mechanical properties of hardened concrete such as strength and durability requirements; and (3) the inclusion, exclusion, or limits on specific ingredients. Mix design leads to the development of a concrete specification.
Mixture proportioning refers to the process of determining the quantities of concrete ingredients, using local materials, to achieve the specified characteristics of the concrete. A properly proportioned concrete mix should possess these qualities:
- Acceptable workability of the freshly mixed concrete
- Durability, strength, and uniform appearance of the hardened concrete
Mixture proportioning is a process of selecting suitable ingredients and determining their relative proportions with the objective of producing concrete having certain minimum workability, strength and durability as economically as possible.
Mixture proportioning is to determine the most economical and practical combination of readily available materials to produce a concrete that will satisfy the performance requirements under particular conditions of use.
Factors to be Considered Mix Proportioning:
- Required workability (Cohesiveness, slump) based on placement conditions.
- Strength and durability.
- Minimize the amount of cement, Minimize w/c ratio.
- Minimum amount of water, to reduce cement content.
- do not sacrifice the quality.
Advantages of low water/cement ratio:
- Increased strength.
- Lower permeability.
- Increased resistance to weathering.
- Better bond between concrete and reinforcement.
- Reduced drying shrinkage and cracking.
- Less volume change from wetting and drying.
Compressive Strength Grading and Classes
Grade of Concrete is the classification of concrete according to its compressive strength.
There are different grades of concrete are given as M10, M15, M20, M25, M30, M35 and M40. The letter “M” denotes Mix design with proportion of materials like Cement: Fine Aggregate: Coarse Aggregate.
The Numbers represent the predetermined cube strength of 15cm cube after curing of 28 days in N/mm2.
European Standard EN206-1 Part 1: Specification, performance, production and conformity
Concrete in EN206-1 is classified with respect to its compressive strength. The characteristic compressive strength at 28 days of 150 mm diameter by 300 mm cylinders or the characteristic compressive strength at 28 days of 150 mm cubes may be used for classification.
Methods of Concrete Mix design
Most of available methods of concrete mix design are based on empirical relationships, charts, graphs developed from intensive experimental programs. Basically, they follow the same principles and only minor changes exist in different mix design methods in selecting the mix proportions.
The requirements of the concrete mix is usually identified by the general experience with regard to the structural design conditions, durability and conditions of placing. Some of the commonly used mix design methods for medium strength concrete are the following:
- Trial and adjustment method of mix design.
- British DoE mix design method.
- ACI mix design method.
- Indian standard Recommended method IS 10262-82
Calculation of Water/Cement Ratio
Figure 1 Relation between compressive strength and free-water/cement ratio
It is required to find the w/c for the 28 days compressive strength is 39 MPa. Referring to Table 1, for OPC, crushed aggregate, W/C ratio of 0.5, the 28 days compressive strength is 49 MPa. In Fig. 1 find an intersection point for 49 MPa and 0.5 W/C ratio. Draw a dotted line curve parallel to the neighboring curve. From this curve read off the W/C ratio for a target mean strength of 39 MPa. The Water/cement ratio is = 0.58.
Check this W/C ratio from durability consideration from Table B. Adopt lower of the two ratios.