Part 6Structural Design

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 790

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Section 5 Concrete: 5A Plain and Reinforced Concrete

Section 5A(a) General

1 SCOPE

1.1 This Code (Part 6/Subsection 5A) deals with the general structural use of plain and reinforced concrete.

1.1.1 For the purpose of this Subsection, plain concrete structures are those where reinforcement, if provided is ignored for determination of strength of the structure.

1.2 Special requirements of structures, such as shells, folded plates, arches, chimneys, blast resistant structures, hydraulic structures, liquid retaining structures and earthquake resistant structures, covered in respective Indian Standards, have not been covered in this Subsection; these Indian Standards shall be used in conjunction with this Subsection. See good practices [6-5A(1)].

2 TERMINOLOGY

For the purpose of this Subsection, the definitions given in accepted standards [6-5A(2)] shall apply.

3 SYMBOLS

For the purpose of this Subsection, the following letter symbols shall have the meaning indicated against each; where other symbols are used, they are explained at the appropriate place:

A — Area

$b$ — Breadth of beam, or shorter dimension of

a rectangular column

$b_{\mathrm{ef}}$ — Effective width of slab

$b_{\mathrm{f}}$ — Effective width of flange

$b_{\mathrm{w}}$ — Breadth of web or rib

D — Overall depth of beam or slab or diameter of column; dimension of a rectangular column in the direction under consideration

$D_{\mathrm{f}}$ — Thickness of flange

DL — Dead load

$d$ — Effective depth of beam or slab

$d'$ — Depth of compression reinforcement from the highly compressed face

$E_{\mathrm{c}}$ — Modulus of elasticity of concrete

EL — Earthquake load

$E_{\mathrm{s}}$ — Modulus of elasticity of steel

e — Eccentricity

$f_{\mathrm{ck}}$ — Characteristic cube compressive strength of concrete
$f_{\mathrm{cr}}$ — Modulus of rupture of concrete (flexural tensile strength)
$f_{\mathrm{ct}}$ — Splitting tensile strength of concrete

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

s — Spacing of stirrups or standard deviation

w

$w_{\mathrm{d}}$ — Distributed dead load per unit area

$w_{1}$ — Distributed imposed load per unit area

\gamma_ {\mathrm{f}}

\gamma_ {\mathrm{m}}

$\delta_{\mathrm{m}}$ — Percentage reduction in moment

\varepsilon_ {\mathrm{cc}}

$\sigma_{cbc}$ — Permissible stress in concrete in bending compression

$\sigma_{cc}$ — Permissible stress in concrete in direct compression

$\sigma_{mc}$ — Permissible stress in metal in direct compression

$\sigma_{\mathrm{sc}}$ — Permissible stress in steel in compression

$\sigma_{\mathrm{st}}$ — Permissible stress in steel in tension

$\sigma_{\mathrm{sv}}$ — Permissible stress in shear reinforcement

$\tau_{\mathrm{bd}}$ — Design bond stress

$\tau_{\mathrm{c}}$ — Shear stress in concrete

$\tau_{c,\max}$ — Maximum shear stress in concrete with shear reinforcement

$\tau_{\mathrm{v}}$ — Nominal shear stress

$\Phi$ — Diameter of bar

Section 5A (B1) Materials, Workmanship, Inspection and Testing

4 MATERIALS

4.1Cement

The cement used shall be any of the following conforming to accepted standards [6-5A(3)] and the type selected should be appropriate for the intended use:

a) Ordinary Portland cement,

b) Rapid-hardening Portland cement,

c) Portland slag cement,

d) Portland pozzolana cement (fly ash based),

e) Portland pozzolana cement (calcined clay based),

f) Hydrophobic cement,

g) Low heat Portland cement, and

h) Sulphate resisting Portland cement.

Other combinations of Portland cement with mineral admixture (see 4.2) of quality conforming with relevant Indian Standards laid down may also be used in the manufacture of concrete provided that there are satisfactory data on their suitability, such as performance test on concrete containing them.

4.1.1 Low heat Portland cement conforming to accepted standards [6-5A(4)] shall be used with adequate precautions with regard to removal of formwork, etc.

4.1.2 High alumina cement conforming to accepted standard [6-5A(5)] or supersulphated cement conforming to accepted standard [6-5A(6)] may be used only under special circumstances with the prior approval of the Engineer-in-Charge. Specialist literature may be consulted for guidance regarding the use of these types of cements.

4.1.3 The attention of the Engineer-in-Charge and users of cement is drawn to the fact that quality of various cements mentioned in 4.1 is to be determined on the basis of its conformity to the performance characteristics given in the respective Indian Standard specification for that cement. Any trade-mark or any trade name indicating any special features not covered in the standard or any qualification or other special performance characteristics sometimes claimed/indicated on the bags or containers or in advertisements alongside the ‘Statutory Quality Marking’ or otherwise have no relation whatsoever with the characteristics guaranteed by the Quality Marking as relevant to that cement. Consumers are, therefore, advised to go by the characteristics as given in the corresponding Indian Standard Specification or seek specialist advice to avoid any problem in concrete making and construction.

4.2Mineral Admixtures

Mineral admixtures listed below may be used along with ordinary Portland cement. Uniform blending of the additions with the cement should be ensured. For this purpose, specific facility is required to be available at the construction site or RMC plant.

NOTE — In case more than one type of mineral admixture is to be used in the same concrete or mineral admixture is used along with blended cement, the necessary trials for suitability of concrete so made shall be done and duly approved by the Engineer-in-Charge.

4.2.1 Pozzolanas

Pozzolanic materials as given below may be used.

4.2.1.1 Fly ash (pulverized fuel ash)

Fly ash conforming to accepted standard [6-5A(7)] may be used as part replacement of ordinary Portland cement.

4.2.1.2 Silica fume

Silica fume conforming to accepted standard $[6-5\mathrm{A}(8)]$ may be used as part replacement of ordinary Portland cement. Silica fume is usually used in proportion of 5 to 10 percent of the cement.

4.2.1.3 Rice husk ash

Rice husk ash giving required performance and uniformity characteristics may be used with the approval of the Engineer-in-Charge.

NOTE — Rice husk ash is produced by burning rice husk and contains large proportion of silica. To achieve amorphous state, rice husk may be burnt at controlled temperature. It is necessary to evaluate the product from a particular source for performance and uniformity since it can be as deleterious as silt when incorporated in concrete. Water demand and drying shrinkage should be studied before using rice husk.

4.2.1.4 Metakaolin

Metakaolin conforming to accepted standard [6-5A(9)] may be used as part replacement of ordinary Portland cement.

4.2.2 Ground Granulated Blast Furnace Slag

Ground granulated blast furnace slag obtained by grinding granulated blast furnace slag conforming to accepted standards $[6-5A(10)]$ may be used as part replacement of ordinary Portland cement.

4.2.3 Precautions

4.2.3.1 For concrete made with mineral admixtures, the setting time and rate of gain of strength may be different from those of concrete made with ordinary Portland cement alone. Cognizance of such modified properties shall be taken into account in deciding de-shuttering time, rate of movement of formwork in slipform construction, initial time of prestressing, longer curing period and for early age loading. The compatibility of chemical admixtures and cementitious materials should be ensured by trials.

Concrete containing mineral admixtures generally have equal or improved finishability compared to similar concrete mixes without them. Mixes that contain high dosages of mineral admixtures, specially silica fume, can be sticky and difficult to finish.

Concrete containing mineral admixtures may exhibit an increase in plastic shrinkage cracking because of its low bleeding characteristics. The problem may be avoided by ensuring that such concrete is protected against drying, both during and after finishing.

4.2.3.2 Compatibility of chemical admixtures and superplasticizers conforming to accepted standards [6-5A(11)] with the binder system shall be ensured by trials.

4.2.3.3 Some other properties of concrete such as modulus of elasticity, tensile strength, creep and shrinkage are not likely to be significantly different. For design purposes, it will be sufficiently accurate to adopt the same values as those used for concrete made with ordinary Portland cement alone.

4.3Aggregates

Aggregates, derived from natural sources and from other than natural sources, shall comply with the requirements of accepted standards [6-5A(12)].

4.3.1 Other types of aggregates such as crushed overburnt brick or tile, which may be found suitable with regard to strength, durability of concrete and freedom from harmful effects may be used for plain concrete members, but such aggregates should not contain more than 0.5 percent of sulphates as $SO_{3}$ and should not absorb more than 10 percent of their own mass of water.

4.3.2 Heavy weight aggregates or light weight aggregates such as bloated clay aggregates and sintered fly ash aggregates may also be used provided the Engineer-in-Charge is satisfied with the data on the properties of concrete made with them.

NOTE — Some of the provisions of the code would require modification when these aggregates are used; specialist literature may be consulted for guidance.

4.3.3 Size of Aggregate

The nominal maximum size of coarse aggregate should be as large as possible within the limits specified but in no case greater than one-fourth of the minimum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all reinforcement thoroughly and fill the corners of the form. For most work, 20 mm aggregate is suitable. Where there is no restriction to the flow of concrete into sections, 40 mm or larger size may be permitted. In concrete elements with thin sections, closely spaced reinforcement or small cover, consideration should be given to the use of 10 mm nominal maximum size.

Plums above 160 mm and up to any reasonable size may be used in plain concrete work up to a maximum limit of 20 percent by volume of concrete when specifically permitted by the Engineer-in-Charge. The plums shall be distributed evenly and shall be not closer than 150 mm from the surface.

4.3.3.1 For heavily reinforced concrete members as in the case of ribs of main beams, the nominal maximum

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

size of the aggregate should usually be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum cover to the reinforcement whichever is smaller.

4.3.4 Coarse and fine aggregate shall be batched separately.

4.5Chemical Admixtures

4.5.1 Admixture, if used shall comply with accepted standards $[6-5A(15)]$ . Previous experience with and data on such materials should be considered in relation to the likely standards of supervision and workmanship to the work being specified.

4.5.2 Admixtures should not impair durability of concrete nor combine with the constituent to form harmful compounds nor increase the risk of corrosion of reinforcement.

4.5.3 The workability, compressive strength and the slump loss of concrete with and without the use of admixtures shall be established during the trial mixes before use of admixtures.

4.5.4 The relative density of liquid admixtures shall be checked for each drum containing admixtures and compared with the specified value before acceptance.

4.5.5 The chloride content of admixtures shall be independently tested for each batch before acceptance.

4.5.6 If two or more admixtures are used simultaneously in the same concrete mix, data should be obtained to assess their interaction and to ensure their compatibility.

4.5.7 The amount of admixture added to a mix shall be recorded in the production record. Redosing of admixtures is not normally permitted. In special circumstances, if necessary, additional dose of admixture may be added at project site and mixed adequately in mixer itself to regain the workability of concrete with the mutual agreement between the producer/supplier and the purchaser/user of concrete. However, the producer/supplier shall assure the ultimate quality of concrete supplied by him and maintain record of quantity and time of addition.

ct

f_{\mathrm{d}}

— Design strength

f_{\mathrm{y}}

— Characteristic strength of steel

H_{\mathrm{w}}

— Unsupported height of wall

H_{\mathrm{we}}

— Effective height of wall

I_{\mathrm{ef}}

— Effective moment of inertia

I_{\mathrm{gr}}

— Moment of inertia of the gross section excluding reinforcement

I_{\mathrm{r}}

— Moment of inertia of cracked section

K

— Stiffness of member

k

— Constant or coefficient or factor

L_{\mathrm{d}}

— Development length

L/IL

— Live load or imposed load

l

— Length of a column or beam between adequate lateral restraints or the unsupported length of a column

l_{\mathrm{ef}}

— Effective span of beam or slab or effective length of column

l_{\mathrm{ex}}

— Effective length about x-x axis

l_{\mathrm{ey}}

— Effective length about y-y axis

l_{\mathrm{n}}

— Clear span, face-to-face of supports

l_{\mathrm{n}^{\prime}}

—

l_{\mathrm{n}}

for shorter of the two spans at right angles

l_{\mathrm{x}}

— Length of shorter side of slab

l_{\mathrm{y}}

— Length of longer side of slab

l_{0}

— Distance between points of zero moments in a beam

l_{1}

— Span in the direction in which moments are determined, centre-to-centre of supports

l_{2}

— Span transverse to

l_{1}

, centre-to-centre of supports

l_{2^{\prime}}

—

l_{2}

for the shorter of the continuous spans

M

— Bending moment

m

— Modular ratio

n

— Number of samples

P

— Axial load on a compression member

q_{\mathrm{o}}

— Calculated maximum bearing pressure of soil