Concrete is most widely used man-made construction material

Ingredients – Cement, Coarse aggregate, Fine aggregate, Water and some admixtures – Silica fume, Superplasticiser, Accelerator, Retarder, Pozzolana – locally available materials

User is interested now in concrete itself and not on properties of ingredients

MIX DESIGN

Grades of Concrete

Ultimate strength of concrete of 28 days

M 5 – 5 N/mm2 MPa (Mega Pascal)M 7.5 – 7.5 N/mm2 MPaM 10 – 10 N/mm2 MPaM 15 – 15 N/mm2 MPaM 20 – 20 N/mm2 MPaM 25 – 25 N/mm2 MPa

Cement

Most important constituent IS : 456 – 2000 Ordinary Portland Cement (OPC)

Grades 33, 43 and 53 (33, 43, & 53 N/mm2)

Other types : Rapid Hardening – Slag cementPozzolana – High strength – Sulphate resisting – Hyderophobic

Tests on cement Strength – Fineness – Soundness – Heat of Hydration – Setting time – Chemical component

Aggregates

Occupy 70 to 75%

Aggregates are naturally occurring rock, sand, synthetically – fly ash, slag

Classified as coarse and fineCoarse aggregate > 4.75 mmSizes – 10, 12.5, 20, 25, 40, 50, 70, 150 mmFine aggregate < 4.75 mm

Grading Curves

-20

0

20

40

60

80

100

120

0 10 20 30 40 50

SIZES OF SIEVES

% O

F P

ASS

ING

ON

SIE

VE

20MM

LOWER LT

UPPER LT

Fig. 1 Grading Curve - 20 mm Coarse Aggregate

Fig.2 Grading Curve Fine Aggregate (Sand)

0

20

40

60

80

100

120

0 2 4 6 8 10 12

SIZES OF SIEVES

% P

ASS

ING

ON

SIE

VE

ZONE - III

LOWER LT

UPPER LT

Tests on aggregate

i. Crushing strength,

ii. Aggregate impact value,

iii. Specific gravity,

iv. Flakiness,

v. Absorption of moisture,

vi. Grading curve,

vii. Deleterious constituents

Water

Clean and free from impuritiesPH value not less than 6 Range 6 to 9

Admixtures

Used to modify properties Mineral admixtures – fly ash, silica fume, rice husk, Chemical admixtures

Fresh Concrete – Workability

i. Mix to be stableii. Cohesive and mobile enough to be placed in the formiii. Amenable to proper compactioniv. To get good strength

Measure of workability

i. Slump test, ii. Compacting factoriii. Flow tableiv. Vee-Bee consistometer

Placing Conditions Degree of Workability

Slump (mm)

(1) (2) (3)

Blinding concrete;Shallow sections;Pavements using pavers

Very low Workability by compacting factor method is more appropriate than slump. A value of compacting factor of 0.75 to 0.80 is suggested.

Mass concrete;Lightly reinforced sections in slabs, beams, walls, columns;Floors; Hand placed pavements;Canal lining; Strip footings

Low 25 – 75

Heavily reinforced sections in slabs, beams, walls, columns;

Medium 50 – 100

Slip form work Pumped concreteTrench fillIn-situ piling

High 100 – 150

Tremie concrete Very High Workability by flow is more appropriate

Placing conditions – Degree of workability

Hardened Concrete

Strength – specified strength of 28 days

Factors effecting compressive strength

i. Water cement ratio

ii. Characteristic strength of cement

iii. Characteristics and proportions of aggregates

iv. Degree of compaction

v. Efficiency of curing

vi. Temperature during curing period

Durability – i. Resistance to deterioration ii. Longevity in the given environment

Influence factors – permeability, external agents, penetration and reaction

Exposure of Reinforcements due to bad construction

Mandovi Bridge after collapse

Minimum Cement content, Maximum W/C and Minimum grade of concrete for different exposure conditions for aggregate of 20 mm. nominal maximum size

Sl.No. Exposure

Plain Concrete Reinforced Concrete

Minimum Cement content kg/m3

Maximum Free Water

Cement Ratio

Minimum Grade of Concrete

Minimum Cement content kg/m3

Maximum Free Water

Cement Ratio

Minimum Grade of Concrete

(1) (2) (3) (4) (5) (6) (7) (8)

i) Mild 220 0.60 - 300 0.55 M 20

ii) Moderate 240 0.60 M 15 300 0.50 M 25

iii) Severe 250 0.50 M 20 320 0.45 M 30

iv) Very Severe 260 0.45 M 20 340 0.45 M 35

v) Extreme 280 0.40 M 25 360 0.40 M 40

NOTES1. Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions

mentioned in 5.2. The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolona and slag specified in IS 1489 (Part i) and IS 455 respectively.

2. Minimum grade for plain concrete under mild exposure condition is not specified.

The adjustments to minimum cement content for aggregates other than 20 mm. nominal maximum size is given in Table – (a).

Limitation on the water cement ratio and the minimum cement content to ensure adequate durability.

II. Standard derivation (s) of compressive strength of concrete.

III. Type and maximum size of aggregate used and grading of aggregate.

I.S. 383 – 1970

• Maximum Water – Cement Ratio

Table (a) - Adjustments to Minimum Cement Content for Aggregates other than 20 mm. Nominal Maximum Size

Sl. No. Nominal Maximum Aggregate Size

(mm)

Adjustments to Minimum Cement Content

(Kg/m3)

(1) (2) (3)

i) 10 + 40

ii) 20 0

iii) 40 - 30

Mixes are of two types

i. Nominal mix

ii. Designed mix

Nominal proportion irrespective of type, shape, size of materials for nominal mix

Designed mix – by assessing before hand the properties of materials and proportion will be made

Proportions for Nominal Mix Concrete

Grade of

Concrete

Total Quantity of Dry Aggregates by

Mass per 50 kg of Cement, to be Taken

as the Sum of the Individual Masses of

Fine and Coarse Aggregates, Kg,Max

Proportion of Fine

Aggregate to

Coarse Aggregate

(by Mass)

Quantity of

Water per

50 kg. of

Cement, Max

(1) (2) (3) (4)

M 5

M 7.5

M 10

M 15

M 20

800

625

480

330

250

Generally 1:2

but subject to

an upper limit

of 1:1 ½ and

a lower limit

of 1:2 ½

60

45

34

32

30

DESIGN MIX

Ref : i. Plain and Reinforced Concrete – Code of Practice IS 456 – 2000

ii. Recommended guidelines for concrete mix design Is 10262 – 2001

iii. Hand book on concrete mixes SP 23 – 1982

Design – As per the codes :

IS 456 : 2000 – For plain and reinforced concreteIS 383 : 1970 – Specification for coarse and fine aggregatesIS 2386 (Part 3) : 1963 – Methods of test for aggregates (Specific gravity, density, voids, absorption and bulking)IS 3812 (Part 1) : 2003 – Specification for pulverized fuel ashIS 8112 : 1989 – Specification for 43 grade ordinary Portland cement )IS 9103 : 1999 – Specification for admixtures

DATA REQUIRED :

i. Grade designationii. Type of cementiii. Maximum nominal size of aggregateiv. Minimum cement contentv. Maximum water content ratiovi. Workabilityvii. Exposure conditionsviii.Maximum temperature of concrete at the time of placingix. Method of transporting and placingx. Early age strength requirements, if anyxi. Type of aggregatexii. Maximum cement content, andxiii.Admixture, if any used – Type and condition

Characteristic compressive strength (fck)

– Strength of concrete at 28 days below which not more than 5% (1 in 20) the test results to fall.

Target strength for mix design (fck) fck = fck + t . S

s = standard deviation

t = a value dependant on accepted proportion of low results

Accepted proportion Value of “ t” of low results

1 in 5 (20%) 0.84

1 in 10 (10%) 1.28

1 in 15 (6.67%) 1.50

1 in 20 (5%) 1.65

1 in 40 (2.5%) 1.86

1 in 100 (1.0%) 2.33

Standard Deviation – calculated – when results of 30 samples availableIf such data is not available, standard deviation assumed

Grade of Concrete Standard DeviationN/mm2

M10 3.5M15M20 4.0M25M30M35M40M45 5.0M50M55

* These values are for good site control, weigh batching, regular checking

Example

For Mix M40 s = 5.0 t = 1.65

Target strength fck = 40 + 1.65 x 5.0 = 48.25 N/mm2

Mix designed for 48.25 N/mm2

Steps include

Selection of W.C. Ratio.

Selection of Water content

Calculation of Cement content.

Estimation of Coarse Aggregate proportion

Estimation of coarse and fine aggregate

Selection of Water – Cement Ratio

If data is available it can be used

Otherwise, use water cement ratio for environmental exposure conditions

Environmental Exposure ConditionsSl.No. Environment Exposure condition

1. Mild Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal area.

2. Moderate Concrete surfaces sheltered from severe rain or freezing whilst wet.Concrete exposed to condensation and rain.Concrete continuously under water.Concrete in contact or buried under non-aggressive soil / ground water.Concrete surfaces sheltered from saturated salt air in coastal area.

3. Severe Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation.Concrete completely immersed in sea water concrete exposed to coastal exposed to coastal environment.

4. Very severe Concrete surface exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wet.Concrete in contact with or buried under aggressive sub-soil / ground water.

5. Extreme Surface of members in tidal zone members in direct contact with liquid/ solid aggressive chemicals.

Water Content

Depends on size and shape of aggregates,

workability, admixtures

Increase in aggregates size reduces water

Quality of water

Nominal Maximum Size of Aggregate in mm

Maximum water content in kg

10 208

20 186

40 165

These values are for angular aggregate and for a

slump of 25 to 50 mm –

Values can be reduced by

i. for sub angular aggregates 10 Kgii. for gravel with crushed particles 20 Kgiii. for rounded gravel 25 Kgiv. for slump other than 25 mm to +3% for every 50 mm additional

25 mm slump

With water reducing or super plasticizing admixtures

decrease water content by 5 to 10% and 20% and

above

Calculation of cementitious material

Water cement ratio fixed

Water content known

Cement content in kg

Quantity in litres=

Water cement ratio

This is to be checked against minimum cement

content

Estimation of coarse aggregate• Nominal maximum size of coarse aggregate known• Zone of fine aggregate known• From the following table, volume of coarse aggregate

per unit volume of total aggregate is calculatedVolume of Coarse Aggregate vs Size of Aggregate

Sl.No. Nominal Maximum Size of Aggregate

mm

Volume of coarse aggregate per unit volume of total aggregate for different zones of fine

aggregate

Zone I Zone II Zone III Zone IV

1 10 0.50 0.48 0.46 0.44

2 20 0.66 0.64 0.62 0.60

3 40 0.75 0.73 0.71 0.69

Note : i. The volume of coarse aggregate is for saturated dry condition

ii. For pumping concrete or thickly reinforced concrete, the coarse aggregate content can be reduced upto 10%, keeping the value of slump, water-cement ratio and strength properties as it is.

iii. The values of coarse aggregate are worked out for a water cement ratio of 0.5. For increase or decrease of ±0.05 of W.C., the volume of coarse aggregate has to be decreased or increased by 0.01

Estimation of coarse and fine aggregate

• Mass of cement / cementitious material, water, and any admixtures added are known

• Volume of these materials are calculated by dividing with respective specific gravity

• One cubic metre of concrete consists of – volume of cement, volume of water, volume of admixtures and volume of coarse and fine aggregates

Volume of coarse aggregate and fine aggregate

Mass of cementitious material Mass of water = 1 – + +

Specific gravity Specific gravity

Mass of admixture 1

Specific gravity 1000

Coarse aggregate content in total aggregate is known

• Volume of coarse aggregate and fine aggregate content is known

• From this coarse aggregate and fine aggregate per unit volume are calculated

• The coarse aggregate and fine aggregate contents are calculated by multiplying the respective specific gravities and multiplying by 1/1000.

These calculated mix proportions are for Trial Mix –1

With this, the mix is prepared and slump calculated

Water content to be added or subtracted as the case may be keeping water-cement as it is and calculate the mix. This is Trial Mix – 2.

Two more mixes i.e. Trial Mix – 3 and 4 are calculated with same water content but by varying water-cement ± 10%

A graph is drawn for Trial Mix – 2, 3 & 4 between water cement and compressive strength

From this, the actual water cement ratio is determined with field trials and mix proportions calculated.