Types of concrete mixes

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Types of Concrete Mixes

There are four (4) types of mixes used for concrete works viz:

1. Standard mixes

2. Nominal or prescribed mixes and

3. Designed mixes

4. Designated mixes

Standard Mixes

These are mixes specified as dry weights of fine and coarse aggregate per unit weight of cement. These are based on standard tests carried out and the required values are usually tabulated. However, there are reservations for special cases.

Table 1: Standard mixes and related strengths

The materials to be used for the standard mix have been specified. Thus, when the required mix is selected from Table 1 above, the materials to produce the mix can be selected from Tables 2 and 3 below (adapted from BS 5328).

Table 2: Materials for use in standard mixes

Table 3: Mix proportions for standard mixes

Nominal or Prescribed Mixes 

These are designated in proportion by volumes based on dry material and using separate fine and coarse aggregates.

The basic nominal mixes are:

1:1:2

1:1.5:3

1:2:4

1:3:6

1:4:8

We should notice that these mixes have the ratio of fine to coarse aggregates as 1:2 by volume. This is based on the void ratio for coarse aggregate which is roughly 40%. Normally, when sand is added, the sand grains between coarse aggregate would push them apart causing more voids. Allowing 50% of sand normally ensures an adequate amount of mortar to fill voids in order to prevent honeycombing. Even though 1:2 is the common ratio of fine to coarse aggregate, it is permitted to vary it between 1:1.5 and 1:3 provided more dense and workable concrete can be produced and the sum of the volumes of fine to coarse aggregate does not change. Even though batching (measurement by volume) is used for nominal mixes, if the unit weight of the material is known, it can be converted to weight and used if it would be more suitable. Nominal mixes are used for C10 to C25 concrete based on British Standards and M10 to M25 based on Indian Standards.

Designed Mixes

This is used where the properties of the material are known by tests. The aim is to design a concrete mix to achieve desired properties in the concrete which include workability, strength, and durability. Workability depends on the aggregate: cement ratio while strength and durability depend on the water: cement ratio. Designed mixes are used in special types of work where high strength concrete is required. Tables 4 – 5 show the designed mixes for different concrete grades based on BS code, Indian code, and Eurocode and where they are appropriately sued.

Table 4: Designed mix concrete grades based on British/Indian Standards

Table 5: Designed mix concrete grades and strength based on EN 206 (Adapted from COREN mix design manual)

Concrete mix designs and compressive strength tests are usually connected with the designed mixes.

Designated Mixes

In a ‘designated concrete’, the producer is required to produce a material to satisfy the designated strength and consistency (workability) using a particular aggregate size based on the site conditions and the specifications of the owner. Designated concretes are designated as Grade 30, for example, based on the cube strength up to Grade 50, according to the application involved (see BS 5328 for more details).

Compressive Strength of Concrete

Nominal or prescribed mixes and designed mixes are usually captured in the specification of the compressive strength of concrete. Different codes have different ways by which they designate concrete compressive strength. The European and British code/standard uses the prefix ‘C’ where C represents concrete strength class. Indian standards use the prefix ‘M’ where ‘M’ represents mix. The number behind the prefix represents the characteristic compressive strength. These are the two that are commonly found in Nigeria and are often mixed up. Generally, British and Indian standard uses cube strength and differ only in the prefix while European code uses cylinder strength for compressive concrete designation. American code also uses cylinder strength.  Cube strength is determined using 150 mm x 150 mm x 150 mm cube or 100 mm x 100 mm x 100 mm cube while cylinder strength uses 150 mm x 300 mm cylinder or 100 mm x 200 mm cylinder. Table 6 shows the strength gain with age of concrete. The difference between cylinder strength and cube strength is that.

Cube strength = 1.25 x cylinder strength

Table 6: Strength gain with the age of concrete

Research has shown that design mixes have some advantages over prescribed mixes. Four (4) of these are outlined below:

1. Good quality concrete with better strength, workability, impermeability, durability, density, and homogeneity is produced with design mixes against nominal mixes.

2. More cement quantity is saved with design mixes against nominal mixes and these savings are proportional to an increase in concrete grade.

3. Unlike nominal mixes, the quality of locally available materials is strictly considered in design mixes.

4. Desired concrete properties can be incorporated in the design of design mixes but this cannot be achieved with nominal mixes.

In conclusion, it is important that concrete mix design is carried out prior to any major concrete works in order to capture all peculiarities.

Refer to BS 5328-1:1997 and BS 5328-2:1997 for more on types of concrete mixes