MIXDESIGN OF CONCRETE LAKSHMI RAVI

CHANDU KOLUSU

OBJECTIVES OF MIX DESIGN

To find a combination of constituents that would give concrete of properties complying with certain specifications economically.

Properties are selected depending on the application and expected performance.

WHAT IS MIX DESIGN?? The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required strength, durability, and workability as economically as possible, is termed the concrete mix design

TYPE OF MIXES Nominal Mix: The mixes of fixed cement-aggregate ratio which ensures adequate strength

are termed nominal mixes. These mixes offer simplicity and under normal circumstances, gives a

specified marginal strength. But variability of mix ingredients causes variation in strength.

TYPE OF MIXES Standard mix: The nominal mixes of fixed cement-aggregate ratio (by volume) vary widely in

strength and may result in under- or over-rich mixes. Those mixes which ensure a minimum compressive strength are known as

standard mixes. IS 456-2000 has designated the concrete mixes into a number of grades

as M10, M15, M20, M25, M30, M35 and M40

TYPE OF MIXES Standard mix M- mix Number- 28 day characteristic compressive strength in MPa.

Grade of Concrete Mixproportion

M10 1:3:6

M15 1:2:4

M20 1:1.5:3

M25 1:1:2

TYPE OF MIXES Design Mix: Most rational approach to the selection of mix proportions This approach results in the production of concrete with the

appropriate properties most economically

FACTORS AFFECTING MIX DESIGN Compressive Strength

Most important property Influences many other properties of concrete

Workability Degree of workability required depends on:

Size of section

Amount of reinforcement

Amount of compaction

RECOMMENDED SLUMP VALUES

(IS 456:2000)Use Slump value

Shallow section Very low

Mass concrete, lightly reinforced beam

25-75

Heavily reinforced beam 50-100

Pumped concrete, tremie concrete

100-150

FACTORS AFFECTING MIX DESIGN

Durability: High strength concrete more durable than low strength concrete. Depends on exposure conditions.

Maximum nominal size of aggregate Larger aggregate size -higher workability Higher aggregate size-low strength.

FACTORS AFFECTING MIX DESIGN

Grading and type of aggregate Type of aggregate influences aggregate cement ratio.

Quality control Degree of control can be estimated by studying variation in test results.

METHODS OF MIX PROPORTIONING

Arbitrary proportion Fineness modulus method Maximum density method Surface area method Indian Roads congress ,(IRC 44 method. High strength concrete mix design Road note No.4 method (Grading Curve Method) ACI committee 211 method DOE method Mix design for pumpable concrete Indian Standard Recommended Method IS 10262:2009

ACI METHOD Introduced in 1944- later revised to include air entraining agents in 1954.

1970- developed as a standard method of concrete mix design- ACI 211.1 of 1991

ACI 211.1 OF 1991 Assumptions : Fresh concrete of given slump and containing well graded aggregate

will have a practically constant total water content ( regardless of variations in w/c ratio and cement content).

Dry volume of coarse aggregate per unit volume of concrete depends on its maximum size and fineness modulus of the fine aggregate.

Irrespective of the compaction, a defnite % of air remains which is inversely proportional to maximum size of aggregate.

IS METHOD( IS 10262: 2009) Introduced in the year 1982. Later modified to confirm with the requirements of IS 456:2000

DATA FOR MIX PROPORTIONING

Grade designation Type of cement Maximum nominal size of aggregate. Minimum cement content. Maximum water-cement ratio. Workability.

DATA FOR MIX PROPORTIONING

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

Maximum temperature of concrete at the time of placing.

Method of Transporting and placing. Early age strength requirements, if required. Type of aggregate. Maximum cement content. Whether admixture used or not.

IS METHOD( IS 10262: 2009)

1. Target Mean Strength Concrete mix is designed for certain higher strength than characteristic

strength.

f’ck = fck + 1.65 σ

f’ck= target mean strength at 28 days in N/mm^2

fck = characteristic compressive strength at 28 days in N/mm^2 .

σ = standard deviation in N/mm^2.

CALCULATION OF STANDARD DEVIATION(a) Standard deviation based on test strength

of sample The total number of test strength of samples required to constitute an acceptable record for calculation of standard deviation shall be not less than 30.

CALCULATION OF STANDARD DEVIATION

(b) Standard deviation from Table 1 of IS 10262:2009

Grade of Concrete Assumed Standard

deviation(Mpa)M10, M15 3.5M20, M25 4.0M30 and above 5.0

Note: Above values correspond to the site control having proper storage of cement; weigh batching of all materials; controlled addition of water, regular checking of all materials. aggregate grading and moisture content; and periodical checking of workability and strength. Where there is deviation from the above, values given in the above table shall be increased by 1 N/mm2

2. SELECTION OF WATER CEMENT RATIO

Water cement ratio selected based on laboratory or site trials. Relationship between free water cement ratio and 28 day strength should

be established for the actual materials used. Not possible always due to lack of data and time.

RELATION BETWEEN W/C RATIO AND COMPRESSIVE STRENGTH AS PER IS 456:2000

As per IS 10262:1982,w/c ratio is selected from ABCDEF curves relating strength of concrete and w/c ratio.

2. SELECTION OF WATER CEMENT RATIO:

w/c ratio selected should be checked against limiting w/c ratio for durability( from Table 5 of IS 456:2000)

Lower of two values should be adopted

TABLE 5 OF IS 456

3. SELECTION OF WATER CONTENT The quantity of maximum mixing water per unit volume of concrete

may be determined from Table 2 of IS 10262:2009. For angular aggregate and 25 – 50 mm slump range.

Nominal maximum size of aggregate(mm)

Maximum water content (kg)

10 20820 18640 165

3. SELECTION OF WATER CONTENT Water content should be increased by 3% for

every additional 25 mm slump.

Modification of water content for aggregates other than angular.

Reduce water content by 5 to 10% for plasticizers and by 20 % for superplasticizers.Type of aggregate Water content reduced

by( kg)

Sub angular 10Gravel with some crushed particles

20

Rounded gravel 25

4. CALCULATION OF CEMENT CONTENT The cement and supplementary cementitious material content per unit

volume of concrete may be calculated from the free water-cement ratio

quantity of water per unit volume of concrete.

The cement content so calculated shall be checked against the minimum content for the requirements of durability(IS 456:2000)

Greater of the two values should be adopted.

TABLE 5 OF IS 456

5. ESTIMATION OF COARSE AGGREGATE PROPORTION

Nominal maximum size of aggregate

Volume of coarse aggregate per unit volume of total aggregate for different zones of fine aggregate for w/c ratio of 0.5

Zone IV Zone III Zone II Zone I10 0.50 0.48 0.46 0.4420 0.66 0.64 0.62 0.6040 0.75 0.73 0.71 0.69

5. ESTIMATION OF COARSE AGGREGATE PROPORTION

Volume of coarse aggregate is modified by -/+ 0.01 for every +/- 0.05 change in water cement ratio.

6.ESTIMATE OF FINE AGGREGATE FRACTION

Proportion of all other ingredients except CA and FA is determined.

Find out the total aggregate volume for 1 cubic meter of concrete.

From this amount of fine aggregate is found out.