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CONCRETE MIX DESIGN

Concrete is the second largest material consumed by human beings after food and water as per WHO. It is obtained by mixing cement, fine aggregate, coarse aggregate and water in required proportions. The mixture when placed in forms and allowed to cure becomes hard like stone. The hardening is caused by chemical action between water and the cement due to which concrete grows stronger with age. The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, proportion of the mix, the method of compaction and other controls during placing, compaction and curing.

Concrete possess a high compressive strength and is usually more economical than steel and is non corrosive which can be made with locally available materials. Hence concrete is used widely in all present-day constructions. The concrete is good in compression and bad in tension. Hence liable to be cracked when subjected to tensile load. In situations where tensile stresses are developed concrete is strengthened by steel bars forming a composite construction called Reinforced Cement Concrete (RCC).

Mix Design

Design of concrete mixes involves determination of the proportions of the given constituents namely, cement, water, coarse aggregate and fine aggregate with admixtures if any. Workability is specified as the important property of concrete in the fresh state. For hardened state compressive strength and durability will be considered

Methods of concrete mix design

The mix design methods being followed in different countries are mostly based on empirical relationships, charts and graphs developed from extensive experimental investigations.

Following methods are in practice

1. ACI Mix design method

2. USBR Mix design method

3. British Mix design method

4. Mix design method according to Indian standard

Since ACI Mix design method is an originator for all other methods, including Indian standard method, wherein every table and charts are fully borrowed from ACI, so we follow the ACI Mix design method in practice

Factors to be considered in Mix design (as per SP23-1982)

The design of concrete mix will be based on the following factors.

(a) Grade of concrete: This gives the characteristic strength requirements of concrete. Depending upon the level of quality control available at the site, the concrete mix has to be designed for a target mean strength which is higher than the characteristic strength.

(b) Type of cement: The type of cement is important mainly through its influence on the rate of development of compressive strength of concrete as well as durability under aggressive environments ordinary Portland cement(OPC) and Portland Pozzolona cement (PPC) are permitted to use in reinforced concrete construction.

Table1 GRADES OF CONCRETE

M60 60

M55 55

M50 50

M45 45

M40 40

M35 35

M30 30

M25 25

M20 20

MI5 15

MIO 10

Specified Characteristic Compressive Grade Designation Strength In N/mm2 At 28 Days Curing

Note:

In the designation of a concrete mix M refers to the mix and the number to the specified characteristic compressive strength of 15 cm cube at 28 days curing expressed in N/mm2. MI5 and less grades of concrete may be used for lean concrete bases and

simple foundation for masonry walls. Grades of concrete lower than M20 shall not be used in reinforced concrete structure as per IS 456-2000. Grades of concrete lower than M30 shall not be used in pre stressed concrete structure

c) Maximum nominal size of aggregate: It is found that larger the size of aggregate, smaller is the cement requirement for a particular water cement ratio. Aggregates having a maximum nominal size of 20mm or smaller are generally considered satisfactory.

d) Minimum water cement ratio: The minimum w/c ratio for a specified strength depends on the type of cement.

e) Workability: The workability of concrete for satisfactory placing and compaction is related to the size and shape of the section to be concreted.

Out line of mix design procedure

(a) Arrive at the target mean strength from the characteristic strength required.

(b) Choose the water cement ratio for target mean strength.

(c) Arrive at the water content for the workability required

(d) Calculate the cement content

(e) Choose the relative proportion of the fine and coarse aggregate

(f) Arrive at the concrete mix proportions

Target mean strength:

Considering the inherent variability of concrete strength during production it is necessary to design the mix to have a target mean strength which is grater than characteristic strength by a suitable margin

ft = fck + k x s

Where

ft = Target mean strength

fck= Characteristic strength

k = a constant depending upon the definition of characteristic strength and is derived IS from the mathematics of normal distribution

S = Standard deviation of the particular mix which is available in IS 456-2000

The value of k is equal to 1.65 as per IS 456-2000 where not more than 5% of the test results are expected to fall below the characteristic strength.

 

By: Dr. B.N. KRISHNASWAMI

Former Faculty,

DEPT OF CIVIL ENGG

NIT, TRICHY ORGANISED

CIVIL ENGINEERS & ARCHITECTS ASSOCIATION,

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