Cover to Reinforcement
I.S. : 456-1978- Code of
practice for plain and reinforced concrete lays down that reinforcement bars
shall have concrete cover (exclusive of plaster or other finish) as
follows :
- At each end of reinforcing bar not less than 25 mm, nor
less than twice the diameter of such bar;
- For a longitudinal reinforcing bar in a column, not
less than 40 mm, nor less than diameter of such bar, In case of columns of
minimum dimension of 200 mm or under, whose reinforcing bars do not exceed
12 mm, a cover of 25 mm may be used;
- For longitudinal reinforcing bar in a beam, not less
than 25 mm, nor less than diameter of bar;
- For tensile, compressive, shear or other reinforcement
in a slab, not less than 15 mm, nor less than the diameter of such bar;
and
- For any other reinforcement, not less than 15 mm, nor
less than the diameter of such bar.
The cover may be
increased by the RCC designer by inserting notes in the structrual drawings by
making stipulations in the specifications to cater for special considerations
like proximity of saline water, harmful chemicals etc.
For articles with thin
cross section like precast RCC fencing posts, certain government departments
like the MES specify concrete cover of 20 mm or twice the diameter of main
bars, whichever is greater.
Lap Splices
Length of bars supplied
by manufacturers is usually in the range of 8 to 13 m. When length of bars in
stock is shorter than the required length the bars are extended by providing
lap splices. Allowance to be made for lap splices in the calculation of
quantity of reinforcement should be:
- 24 times the bar diameter, for bars in compression.
- 30 times the bar diameter, for bars in tension.
When the estimator is in
doubt as to whether a particular bar is in compression or tension he should
allow for 30 times the bar diameter.
Normally, bars larger
than 36 mm diameter are to be extended by butt welding, but if lap splices are
permitted for such bars additional spirals of 6 mm diameter are provided around
the lapped portion.
When bars of different
diameters are spliced (as in curtailing of reinforcement) the lap length is to
be calculated on the basis of the smaller diameter of bar.
Cranked Bars
For cranked bars add 0.4
t to allow for extra length due to cranking when bars are bent to an
inclination of 450, and add 0.3 t to allow for extra length due to
cranking when bars are bent to an inclination of 300. The values of
0.4 t and 0.3 t are for each crank and thus if the bar is cranked at both ends
twice these values will have to be added. (t = vertical distance through which
bar is cranked.)
Allowance for hooks
For the purpose of
anchorage, ends of all plain round mild steel reinforcing bars including the
spliced ends are provided with hooks. Such hooks are not obligatory when using
torsteel/ ribbed/ deformed/ cold twisted bars unless particularly shown or
stipulated in drawing or specifications.
When using plain round
mild steel bars conforming to IS: 432-1960, or deformed mild steel bars
conforming to IS: 1139-1959 the minimum allowance for each hook is 9 times the
diameter of bar (but in no case less than 75 mm) rounded off to the nearest 5
mm, and works out as follows :
Diameter of bar
|
Minimum allowance for each hook
|
Diameter of bar
|
Minimum allowance for each hook
|
Diameter of bar
|
Minimum allowance for each hook
|
mm
|
mm
|
mm
|
mm
|
mm
|
mm
|
5
|
75
|
16
|
145
|
32
|
290
|
6
|
75
|
20
|
180
|
36
|
325
|
8
|
75
|
22
|
200
|
40
|
360
|
10
|
90
|
25
|
225
|
45
|
405
|
12
|
110
|
28
|
250
|
50
|
450
|
The same allowance as
given above is required for each of the two hooks provided for the purposes of
forming binders, stirrups links and the like.
Allowance for Hooks
The minimum allowance
for each hook (which in no case shall be less than 75 mm) when using the
following categories of steel is given below:
Medium tensile steel
(IS:432-1960) or (IS:1139-1959) – 11 times diameter of bar.
Cold twisted steel bars
(IS:1786-1961) ……………… - 13 times diameter of bar.
(Reference IS:2502-1963
– Code of practice for bending and fixing of bars for concrete reinforcement)
Reinforcement for
Circular Slabs
For finding length of
bars required (excluding hooks and laps) in a circular flat slab, find out a
square of the same area as the circular slab using formula is:
Side = diameter X
0.88623
Total length of bars
required for such a square slab (calculated with the same reinforcement and
spacing as that of the circular slab) will be same as total length required for
the circular slab. Allowance for hooks and laps can be added later on.
The same principle as
above can be applied for finding reinforcement required in flat walls and slabs
of curved or irregular shapes.
Two Way Reinforced Slabs
In a two way reinforced
slab, distribution or temperature bars that may be indicated in the structural
drawings are required only for the portion of the main bars taken to the top of
the slab after cranking.
In the sketch shown,
main reinforcement bars, in the innermost central rectangle (shown by dotted
lines) are at bottom in both the directions. No distribution/ temperature/
tying bars are needed in this area as the main bars in one direction can be
tied to the main bars running at right angle to them.
Similarly, in the four
rectangles formed at corners of the slab, main reinforcement bars in both the
directions will be available at top as well as at bottom for tying to each
other at intersections.
Temperature/distribution/tying
bars indicated in the schedule of reinforcement of a two way reinforced slab
are therefore required to be calculated only for the four rectangles shown
shaded in the sketch, at top, as the bent up portion of main bars need lateral
support which is provided by tying up with the temperature/distribution/tying
bars.
In a two-way reinforced
slab, depending on the design requirements, other reinforcement like extra bars
over support at top, corner bars etc., may be required. These are not shown in
the accompanying sketch.
When working out the
number of main reinforcement bars required in a slab, at the given centre to
centre spacing, it should be remembered that:
a) No bars are
required to be provided over the support (wall or beam) in the direction parallel
to the support. The first bar in the direction parallel to the
support is placed at a distance of half the specified centre to centre spacing
of the bars.
b) The number of
bars required are calculated by working out the number of spaces and adding one
for the end bar.
Binders/Stirrups/Links
a) Except where
otherwise clearly indicated in the drawings, calculation for the number of
binders/stirrups/links in columns should cater for the complete vertical length
of the main reinforcement of the column including the portion in
footing/pedestal/base and junction with beams.
b) Where a beam is
supported on columns, either terminating at, or continuous beyond the column,
no stirrups are needed for the portion of main reinforcement of the beam
entering or passing through the column.
