Concrete Mix Design as per IS Code

CONCRETE MIX DESIGN

(IS 10262 (2009): Guidelines for concrete mix design proportioning [CED 2: Cement and Concrete)

INTRODUCTION

Concrete has become an indispensable construction material. According to the present state-of-the-art, concrete has bypassed the stage of mere four component system, that is, cement, water, coarse aggregate and fine aggregate.

It can be a combination of far more number of ingredients for example, a judicious combination of ingredients from as many as ten materials. In the recent past, apart from the four ingredients mentioned above, fly ash, ground granulated blast furnace slag, silica fume, rice husk ash, metakaoline and superplasticizer are six more ingredients which are generally used in concrete produced in practice as the situation demands.

The objective of proportioning concrete mixes is to arrive at the most economical and practical combinations of different ingredients to produce concrete that will satisfy the performance requirements underspecified conditions of usc. An intcgral part of concrete mix proportioning is the preparation of trial mixes and effect adjustments to

such trials to strike a balance between the requirements of placement, that is, workability and strength, concomitantly satisfying durability requirements.

Concrete has to be of satisfactory quality both in its fresh and hardened states. Mix proportioning is generally carried out for a particular compressive strength requirements ensuring that fresh concrete of the mix proportioned to possess adequate workability for placement without segregation and bleeding while attaining a dense state.

DATA FOR MIX PROPORTIONING

1.      Grade designation

2.     Type of cement

3.     Maximum nominal size of aggregate

4.    Minimum cement content

5.     Maximum water-cement ratio

6.    Workability

7.     Exposure conditions as per Table 4 and Table 5 of IS -456

8.    Maximum temperature of concrete at the time of placing

9.    Method of transporting and placing

10.   Early age strength requirements, if required

11.  Type of aggregate

12. Maximum cement content; and

13.Whether an ad mixture shall or shall not be used and the type of admixture and the condition of use.

 Target Strength for Mix Proportioning

The concrete mix has to be proportioned for higher target mean compressive strength (f_ck). The margin over characteristic strength is

f_t = f_ck + 1.65 s

f_t is the target mean compressive strength at 28days in N/mm²,

f_{ck is} characteristic compressive strength at 28days in N/mm²,

and s standard deviation  in N/mm²

 

Standard deviation can be taken as follows

Sl.No	Grade of concrete	Assumed Standard Deviation N/mm2
1	M10	3.5
2	M15	3.5
3	M20	4.0
4	M25	4.0
5	M30 to M55	5.0

Water-Content Ratio

The water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix. A lower ratio leads to higher strength and durability, but may make the mix difficult to work with and form.

The table given below is used when only angular shaped aggregates are used in concrete as well as the slump should be 25 to 50mm.

Maximum Water Content per Cubic Metre of Concrete for Nominal Maximum Size of Aggregate

Sl.No	Nominal Max Size of Aggregate (mm)	Maximum Water Content (kg)
1	10	208
2	20	186
3	40	165

Aggregate Air content

Air content in the concrete mix is determined by the nominal maximum size of aggregate used.

Sl.No	Nominal Max Size of Aggregate (mm)	Aggregate Air Content (% of volume of concrete)
1	10	5
2	20	2
3	40	1

 Cement Content Selection

Water – cement ratio is determined as described above and quantity of water is determined. The value obtained should satisfy the minimum conditions as given in the below table. The greater of the two values is decided as quantity of cement content.

Cement Content for Plain Cement Concrete

Exposure	Plain Cement Concrete (P.C.C)
	Minimum Cement Content Kg/m3	Max Free Water – Cement Ratio	Minimum Grade of Concrete
Mild	220	0.6	–
Moderate	240	0.6	M15
Severe	250	0.5	M20
Very severe	260	0.45	M20
Extreme	280	0.4	M25

Cement Content for Reinforced Concrete

Exposure	Reinforced Cement Concrete (RCC)
	Minimum Cement Content Kg/m3	Max Free Water –Cement Ratio	Minimum Grade of Concrete
Mild	300	0.55	M20
Moderate	300	0.5	M25
Severe	320	0.45	M30
Very severe	340	0.45	M35
Extreme	360	0.4	M40

Calculation of Aggregate Ratio

For the given nominal maximum size of aggregate, we can calculate the ratio of volumes of coarse aggregate and volume of total aggregates for different zones of fine aggregates from the below table.

Nominal maximum size of aggregate	Ratio of volume of coarse aggregate and volume of total aggregate for different zones of fine aggregate
	Zone – 1	Zone – 2	Zone – 3	Zone – 4
10mm	0.44	0.46	0.48	0.50
20mm	0.6	0.62	0.64	0.66
40mm	0.69	0.71	0.73	0.75

Calculation of Aggregate Content for Concrete

We already determine the coarse aggregate volume ratio in the total aggregate volume. So, it is very easy that, 1 – volume of coarse aggregate will give the volume of fine aggregate. Alternatively, there are some formulae to find the volume of fine and coarse aggregates as follows.

Mass of fine aggregate is calculated from below formula

Similarly, mass of coarse aggregate is calculated from below formula.

Where, V = volume of concrete

W = water content

C = cement content

Gc = sp. Gravity of cement

P = aggregate ration obtained in step6

F.A & C.A = masses of fine and coarse aggregates

Gf & Gca = sp. Gravities of fine and coarse aggregates.

Trial Mixes for Testing Concrete Mix Design Strength

Based on the values obtained above, conduct a trail test by making at least 3 cubes of 150mm size as per above standards. Test that cubes and verify whether the required strength is gained or not. If not, redesign the mix with proper adjustments until required strength of cube occurs.