Design of Concrete Structures Introduction

IMPORTANT CODE

Table- Permissible Limit for Impurities in the water

(IS 456:2000)

Impurity	Maximum permissible limit ( mg/l)
Organic	200
Inorganic	300
Sulphates (as )	400
Suspended matter	2000
Chloride as Cl	2000- plain Concrete 500- Reinforce concrete work

IMP TEST ON CEMENT

1- Consistency- Vicat apparatus

2- Initial and final setting time- Vicat apparatus

3- Soundness test- Autoclave test

4- Specific gravity- Le chatelier flask

5- Fineness by specific surface- Blaine air permeability test

Concrete is graded according to its compressive strength.
A grade lower than M20 should not be used in Reinforced cement concrete.
For prestressed concrete, the minimum grade is M40 for pretension work, and M30 is used in the case of post-tension work.
For foundation, the Minimum grade of Concrete is M35.
The water-cement ratio affects the compressive strength of concrete.

IMPORTANT CRITERIA USED IN RCC

Comparison of workability by various methods

Minimum cement contents and maximum w/c ratio for durability

Exposure conditions

Environment	Exposure condition
Mild	Concrete surfaces protected against weather or aggressive conditions, except those situated in the coastal area
Moderate	Concrete surfaces sheltered from rain or freezing whilst wet Concrete exposed to condensation and rain Concrete continuously underwater Concrete in contact or buried under non-aggressive soil/groundwater Concrete surfaces sheltered from saturated salt air in the coastal area
Severe	Concrete surface exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation. Concrete completely immersed m seawater. Concrete exposed to the coastal environment
Very severe	Concrete surfaces exposed to seawater spray. corrosive fumes or severe freezing conditions whilst wet concrete in contact or buried under aggressive subsoil groundwater
Extreme	The surface of members in the tidal zone. Members in direct contact with liquid/solid aggressive chemicals

NOMINAL COVER

It is minimum clear cover required for the outermost layer of steel reinforcement.

Minimum Nominal cover

Member	Mild (mm)	Moderate (mm)	Severe (mm)	Very severe (mm)	Extreme (mm)
Slab	20	30	45	50	75
Beam	25	30	45	50	75
Column	40	40	45	50	75
Foundation	40	50	50	50	75

CHARACTERISTIC STRENGTH OF CONCRETE (f_ck)

1-The strength below which 5% of test result are expected to fail is called characteristic strength of concrete.

2- f_ck value gives compressive strength at 28 days after casting.

If f_m is targeted mean strength and f_ck is characteristics strength, then

f_m = f_ck + 1.65 × ∝

∝ = standard deviation

Value of standard deviation

Grade	standard deviation (∝)N/mm² )
M10- M15	3.5
M 20-M25	4
M 30 & Higher	5

ACCEPTANCE CRITERIA FOR CONCRETE

As per IS code 456:2000

1- The avg. strength of four non overlapping consecutive test should not be

f_av ≥ (f_ck + 0.825 × ∝) or f_avg ≥ (f_ck + 3) {whichever is more}

2- For individual test result

ITR ≥ (f_CK─ 3)

3- Test result should be obtained after testing on at least three cubes. The difference in each test block strength and average strength should not be more than 15%.

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