Chapter 5Concrete

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 794

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5.1 Grades

The concrete shall be in grades designated as per Table 2.

Table 2 Grades of Concrete(Clauses 5.1, 8.2.2, 14.1.1 and 35.1)
Sl No.	Group	Grade Designation	Specified Characteristic Compressive Strength of 150 mm Cube at 28 Days $N/mm^2$
(1)	(2)	(3)	(4)
i)	Ordinary concrete	M 10	10
		M 15	15
		M 20	20
ii)	Standard concrete	M 25	25
		M 30	30
		M 35	35
		M 40	40
		M 45	45
		M 50	50
		M 55	55
		M 60	60
iii)	High strength concrete	M 65	65
		M 70	70
		M 75	75
		M 80	80
		M 90	90
		M 95	95
		M 100	100
NOTES1 In the designation of concrete mix, M refers to the mix and the number to the specified compressive strength of 150 mm size cube at 28 days, expressed in $N/mm^2$ .2 For concrete of grades above M 60, design parameters given in the standard may not be applicable and the values may be obtained from specialized literatures and experimental results.

5.1.1 The characteristic strength is defined as the strength of material below which not more than 5 percent of the test results are expected to fall.

5.1.2 The minimum grade of concrete for plain and reinforced concrete shall be as per Table 5.

5.1.3 Concrete of grades lower than those given in Table 5 may be used for lean concrete, foundation for masonry walls or temporary reinforced concrete construction.

5.2 Properties of Concrete

5.2.1 Increase of Strength with Age

There is normally a gain of strength beyond 28 days.

PART 6 STRUCTURAL DESIGN — SECTION 5 CONCRETE: 5A PLAIN AND REINFORCED CONCRETE

The quantum of increase depends upon the grade and type of cement, curing and environmental conditions, etc. The design should be based on 28 days characteristic strength of concrete unless there is an evidence to justify a higher strength for a particular structure due to age.

5.2.1.1 For concrete of grade M 30 and above, the rate of increase of compressive strength with age shall be based on actual investigations.

5.2.1.2 Where members are subjected to lower direct load during construction, they should be checked for stresses resulting from combination of direct load and bending during construction.

5.2.2 Tensile Strength of Concrete

The flexural and splitting tensile strengths shall be obtained in accordance with good practice [6-5A(18)]. When the designer wishes to use an estimate of the tensile strength from the compressive strength, the following formula may be used:

Flexural strength, $f_{\mathrm{cr}} = 0.7\sqrt{f_{\mathrm{ck}}}$ N/mm²

fck

where

$f_{ck}$ = characteristic cube compressive strength of concrete, in N/mm $^{2}$ .

5.2.3 Elastic Deformation

The modulus of elasticity is primarily influenced by the elastic properties of the aggregate and to a lesser extent by the conditions of curing and age of the concrete, the mix proportions and the type of cement. The modulus of elasticity is normally related to the compressive strength of concrete.

5.2.3.1 The modulus of elasticity of concrete can be assumed as follows:

E _ {\mathrm{c}} = 5 \quad 0 0 0 \sqrt {f _ {\mathrm{ck}}}

where

$E_{c}$ = short term static modulus of elasticity, in N/mm $^{2}$ .

Actual measured values may differ by $\pm 20$ percent from the values obtained from the above expression.

5.2.4 Shrinkage

The total shrinkage of concrete depends upon the constituents of concrete, size of the member and environmental conditions. For a given humidity and temperature, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing and, to a lesser extent, by the cement content.

5.2.4.1 In the absence of test data, the approximate value of the total shrinkage strain for design may be taken as 0.0003 {for more information, see accepted standard [6-5A(19)]}.

5.2.5 Creep of Concrete

Creep of concrete depends, in addition to the factors listed in 5.2.4, on the stress in the concrete, age at loading and the duration of loading. As long as the stress in concrete does not exceed one-third of its characteristic compressive strength, creep may be assumed to be proportional to the stress.

5.2.5.1 In the absence of experimental data and detailed information on the effect of the variables, the ultimate creep strain may be estimated from the following values of creep coefficient (that is, ultimate creep strain/elastic strain at the age of loading); for long span structure, it is advisable to determine actual creep strain, likely to take place:

Age at Loading	Creep Coefficient
7 days	2.2
28 days	1.6
1 year	1.1

NOTE — The ultimate creep strain, estimated as described above does not include the elastic strain.

5.2.6 Thermal Expansion

The coefficient of thermal expansion depends on nature of cement, the aggregate, the cement content, the relative humidity and the size of sections.

The value of coefficient of thermal expansion for concrete with different aggregates may be taken as given below:

Type of Aggregate	Coefficient of Thermal Expansion for Concrete/°C
Quartzite	$1.2$ to $1.3 \times 10^{-5}$
Sandstone	$0.9$ to $1.2 \times 10^{-5}$
Granite	$0.7$ to $0.95 \times 10^{-5}$
Basalt	$0.8$ to $0.95 \times 10^{-5}$
Limestone	$0.6$ to $0.9 \times 10^{-5}$

6 WORKABILITY OF CONCRETE

6.1 The concrete mix proportions chosen should be such that the concrete is of adequate workability for the placing conditions of the concrete and can properly be compacted with the means available. Suggested ranges of workability of concrete measured in accordance with good practice $[6-5A(20)]$ are given below:

Sl No.	Placing Conditions	Degree of Workability	Slump mm
(1)	(2)	(3)	(4)
i)	Blinding concrete; Shallow sections; Pavements using pavers	Very low	See 6.1.1
ii)	Mass concrete; Lightly reinforced sections in slabs, beams, walls, columns; Floors; Hand placed pavements; Canal lining; Strip footings	Low	25-75
iii)	Heavily reinforced sections in slabs, beams, walls, columns	Medium	50-100
iv)	Slipform work	Medium	75-100
v)	Pumpable concrete	High	100-125
vi)	Trench fill	High	100-150
vii)	Tremie concrete	Very high	See 6.1.2
NOTE — For most of the placing conditions, internal vibrators (needle vibrators) are suitable. The diameter of the needle shall be determined based on the density and spacing of reinforcement bars and thickness of sections. For tremie concrete, vibrators are not required to be used (see also 12.3).

6.1.1 In the ‘very low’ category of workability where strict control is necessary, for example pavement quality concrete, measurement of workability by determination of compacting factor will be more appropriate than slump (see accepted standard [6-5A(20)]) and a value of compacting factor of 0.75 to 0.80 is suggested.

6.1.2 In the ‘very high’ category of workability, measurement of workability by determination of flow will be appropriate {see accepted standard [6-5A(11)]}.