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
$f_{\mathrm{d}}$ — 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

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.