INTRODUCTION TO CONCRETE

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Nov 25, 2013 (3 years and 9 months ago)

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Concrete Notes


Page
1


INTRODUCTION TO CONCRETE


SPECIFYING CONCRETE

Specifications require concrete to be of so many MPa which indicates its

strength after 28 days.

In
ordering concrete on a construction job you would also indicate the

required maximum aggregate size
and slump.


TYPES OF CONCRETE

Basically two broad types:
-

a)

Plain concrete



(
un
-
reinforced

concrete)

limited uses


examples
-

footpaths, driveways ,

curbing and
channelling
steeping blocks, garden edging


b)

Reinforced concrete


multiple uses


examples
-

reinforced

driveways for multiple houses,
retaining walls, footings, building foundations, bridge piles and capping slabs, abutments


Plain Concrete

Plain concrete is un
-
reinforced. It has l
imited uses. Regardless of what you are using it for
, contraction
joints sho
uld be

provided t
o overcome shrinkage and expansion problems.


Reinforced Concrete

Reinforcing consists of ste
el bars (rods) or mesh placed i
n concrete to

control shrinka
g
e and to provide strength i
n

tension (also i
n compression

where necessary)

. There ar
e tw
o types of reinforcement used i
n concrete.

a)

Mild

steel (or high yield steel ) i
n the form of plain round or

deformed bars

b)

High tensile wires (or cable or bars) which are used in prestressing concrete


Typical deformed Reinforced Concrete Bar

The Purpos
e
of Reinforcement


Concrete is relatively strong
i
n compression but
has littl
e resistance to

forces which tend to pull it apart
or shear it. To counteract these tensile

and shear forces
i
n a beam or slab we reinforce the concrete
wit
h appropriate

amounts
of reinforcing steel. These will
be longitudinal bars (or wire/cabl
es

and so on
for prestressed concrete) and ties or stirrups.

The figure below shows how a beam behaves under loading.

Concrete Notes


Page
2




Reinforcement provides tensile resistance to support heavy loads


Th
e figure below shows the longitudinal reinforcement placed adjacent to the

side
i
n tension and the
stirrups closest together at zones of shear, both of

which will resist the forces tending to crack the
beam.


Concrete Notes


Page
3


In columns where both the con
crete and s
teel are strong in compression,
the

steel needs to be
prevented from buckling. For this reason ties/stirrups

are important in columns.



TYPES OF CONCRETE CONSTRUCTION


1.

Mass Concrete


This
is mainly used
i
n water retaining structures and earth

retaining structures.

It is a large block of
concrete used to provide mass to resist forces

tending to overturn them.

Although mass concrete is
plain (un
-
reinforced) it may contain some

reinforcement around openings.


CONCRETE GRAVITY DAM


Concrete Notes


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4




CONCRETE GRAVITY WALL








Concrete Notes


Page
5


2.

In
-
sit
u Concrete


Concrete which i
n
its plastic form

is placed in its fina
l position

in the structure is termed
i
n
-
sit
u concrete.





3.

Pre
-
cast Concrete


C
oncrete which has been placed i
n it s final form either on site or i
n

a
pre
-
casting yard and i
s t
hen
transported and installed in it
s final

position. Appl
ications include


fl
oor

sl
abs, beams, columns etc
.


Beams

and
Box Beams








Wall Panels
Columns and Piles








Concrete Notes


Page
6


Twin Tees Uni Tees










Typical Pre
-
cast Panels


4.

Ti
lt
-
up Sla
bs

Are cast on the floor slab and then lifte
d to a vertical position by

crane t
o be
incorporated with in
-
si
t
u
concrete columns.

Examples
-
swimming pools, buildings etc




Figure 1a


Tilt
-
up Slabs

Lift Slabs


Are cast on the ground floor of a mul
ti
storey buil
ding ,
all

the floors

are

cast on top o f each other lik
e a
multi
-
layer s
andwich. After they

have been cured and have reached the required strength they are
jacked

up on steel columns to the
i
r respective level. When finally fixed i
n

position the outer walls would
be

placed either in concrete or i
n some other material.

NOTE : T
o ensure slabs don't bond together a bond breaking fluid may cover each successive slab to be
lifted.


Concrete Notes


Page
7





Figure 1b


Lift Slabs


1.

Slip Formed Concrete

This method of construction is similar to an extrusion process. Plastic

concrete is placed or pumped int
o
forms and the forms act as a continuous

moving die to shape the concrete.


The rate of movement of the forms is regulated so that the forms leave

the concrete after i
t is strong
enough to retain it
s shape and support

its own wei
ght.

Appl
ication
s

a)

Vertical

Structures
-

sil
os
, chimneys, centre core

of some lif
t slab buildings.

b)

Horizontal Structures
-

tunnel inverts, canal

Li
nings and concrete highways
.

Vertical forms are generally moved by jacks
,

which move on smooth steel

rods embedded
i
n the
hardened concr
ete.

Horizontal slip

forms generally move on a rail system and are pulled
into place by
a winch.



REINFORCING STEEL


Steel for concrete reinforcement is covered by a new Joint Standard AS/NZS 4671 : 2001.


Steel Grades


There are two grades of steel for c
oncrete reinforcement
-

grade 300 and grade 500. Pacific Steel
produces Reinforcing Bars in Grade 300 & Grade 500 to comply with the new joint standard ASNZS 4671,
Class "E", which has been written specifically to suit the New Zealand construction requirem
ents. The
bars carry a unique identification mark.


Welding


Grade 300 reinforcing steel can be welded but needs care and the use of low
-
hydrogen electrodes for
best results. Grade 500 is weldable only with special procedures including preheating and post
heating
with controlled cooling, under the supervision

of a qualified technician or metallurgis
t.
Because of these
requirements,
grade 500 i
s
generally considered not weldable.

Concrete Notes


Page
8


Tack welding t
o assemble stirrups t
o main bars
fo
r columns

and
beams, although
once popular, should
never be done.


Surface

To work together, steel and concrete rely on frictio
n between

the two materials to prevent independent
movem
ent. This

friction is generally known as
bond
.
N
ew steel is covered with

a thin skin of mill scale
tha
t

is
light blue

i
n colour.

Steel can be used sa
tisfactorily

with tightly adhering mil
l

scale, but there is a danger th
a
t as surface rus
t
develops, the

mil
l
scale wil
l loosen.

If
, as of
ten happens, steel is exposed t
o the weather for a few

months, a light su
rface rus
t develops, and the
mil
l scale flakes

off. So long as the surface rus
t is
only
light
, the
steel wil
l

be in the best condition to d
evelop a good bond with the concrete.

H
eavily r
usted
stee
l mu
st be cleaned with mechanical wir
e

brushes or some other

proces
s to remove loose or flaky

rust
, and

if severe pitting has developed, the stee
l should not be used

at al
l or should be down
-
grade
d
for use in less critica
l areas.


Plain Round Bars


Plain round bars are stil
l

extensively used, but mostly i
n

grade 30
0. Plain round bar is
best for making
stirrups b
ecause

it
is easier t
o obtain consistent accuracy from bending machines

with thi
s material.


Deformed B
ars

The picture below

show
s

the appearance of typica
l
deform
ed

reinforcing bars
. The
deformations are
act
ually raised ribs
,

usually two lo
n
gitudina
l

ribs and a pat
t
er
n of transverse ribs

at regular intervals.
These ribs provide a positiv
e mechanical

rip
t
o the

concrete in addition to the frictiona
l bond, and

they
ensure a better transfer of force between stee
l and concrete
. For thi
s reason, grade 500 is very seldom
produced in plain

round form and is certainly
not carried in stock by the stee
l

merchants or fabricators
.




Grade Identification (before 2001)


The system used in New Zealand is to omit two transverse ribs at regular intervals on the grade 380 bars
only. The maximum spacing of these

identification marks (missing ribs) is 1.3 metres, which will apply to
the largest size bar. Smaller sized bars will have a lesser distance between identification marks. Figure 3
shows the identification on a grade 380 bar.

The minimum standard stock leng
th of a bar is 6 metres, but most sizes are available on 9 metre lengths,
and the larger sizes are sometimes stocked in lengths up to 12 metres. You must take the precaution of
checking availability when detailing bars with lengths close to, or greater tha
n 12 metres.





Concrete Notes


Page
9


Welded Mesh or Welded Reinforcing Fabric


Fabric reinforcement is used extensively in wall panels and floor slabs, whe
re large areas are to be
covered

with light

to moderate am
ounts of steel. Reinforcing fabric is made in

New Zealand by
H
urricane Wire Products
. T
he fabric is

often known as HRC, CRC, or GRC fabric.







Figure 4
-

Welded reinforcing fabric


Figure 4
(above)
shows part of a

sheet of fabric, Note that

every wire crossing is

electrically welded.

Mesh can be made from galvanised steel wire; if protection from corrosion is

required, the sheets can
be hot
-
dip galvanised after manufacture.
Although mesh is described in m
etric terms, it retains many
of
its old im
perial characteristics. The reference number is

generally a three
-
figure number, in which the
first digit is the spa
cing of the longitudinal wires in inches, the second digit is the spacing of the
transverse wires in inches, and

the third di5it is the wire size in Imperial Standard Wire Gauge.


SITE WORK


Formwork and Placing


Formwo
rk is constructed to contain
-
the fres
h concrete in the shapes required. For small and medium
-
sized constructions, the details of formwork construction are often left to the contractor and his
tradesmen, but on larger commercial and industrial projects,
the design and detailing of form
work is
done in the drawin
g

office.
After construction of the form
work, or sometimes concurrently with it, the
reinforcing steel is placed in position. Steel for columns and beans is very often re
-
assembled in an
adjacent area, and a whole asse
mb
ly of main bars an
d

stirrups is placed in the fo
r
mwork by crane. Soft
black tie wire is used to bind every intersection of one bar with another. This is most
im
portant to
ensure that the reinforcing steel stays in its cor
rect position and is not dislodg
ed during pouring of
concrete or the su
bsequent vibration and com
paction.







Concrete Notes


Page
10


Cover


Cover is the shortest distance from the surface of the concrete to the surface of the reinforcing steel.
Cover is necessary
for the complete encasem
ent of the steel, so that complete
bond be
tween the two
m
aterials can be obt
ained

for the transfer
of load. The minimum
, cover must be greater than the largest
sized aggregate used in the concrete. Cover is necessary for fire rating. Steel buckles under heat and
concrete stops buckling.


Cover is
also required to protect the reinforcing steel from the corrosive effects of the weather, ground
water, or to whatever liquid, or substance the concrete surface is exposed.


Spacers


Spacers are use
d t
o support the assembled reinforcin
g steel

at the correc
t distance from the formwork.
There are several

different
methods f
o
r doin
g

this,
Some options are ;




Using
moulded plasti
c

spacers

that clip on t
o the reinforcing



Using
formed
-
wire

support
s

or

bar chair
s”



Using a
pre
-
cast concrete block of

appropriate th
ickness.



Pouring
Conc
rete


Reinforcing steel is placed and the concrete

poured

Bri
e
fly
, the methods used fo
r
pouring

concrete are
-

a)

Barrow
-

manual

b)

Bucket
-

manual

c)

H
oist and chute

d)

Skip (or bottom
-
discharge bucket) and crane

e)

Conveyor bel
t

f)

Pumping

g)

Direct
from agitato
r truck, and

h)

Tremie pipe
-

for placing concrete under water or in

deep narrow confines.


Note :
Concrete t
hat is t
o be
pumped
requires special characteris
t
ic
s

to make it flui
d
enough t
o go
through the pump and through the

following pip
es.


Comp
action


After pouring,

the concrete must be compacted to ensure tha
t

it completely fills the form
work,
completely bonds with the

reinforcing steel
,
and that there are no voids within the m
ass.

The usual
method is to vibrat
e the concrete by the use of

im
mer
sio
n
vibrators
.


Surface Finish


Most concrete these days is lef
t with the surface as it is when

the formwork has been removed. Many of
the modern surface

finishes

on walls
are much cheaper than p
lastering
.

Polished floors are a very
popular choice of inte
rior design for commercial and domestic buildings.


Concrete Notes


Page
11


STANDARD TE
RMS


a)

Cover

-

the distance between the outside of the
structure
and outside of
steel
.





b)

H
ook/Bend

-

The shape to which the end

of the bar is

bent to give it bond, usually a standard
shape.

-

See table


c)

Stirrup

-

Made of plain round bar they bind the main

reinforcement in structural beams.


d)

Deformed Bar

-

mild steel, sometimes in high tensile
.


e)

Plain Round

Bar

-

mild steel



f)

Lap/Splice

-

The distance for which the ends of two bars

are lapped

lo
ngitudinally when the total
length of a bar is

greater than the manufacturing length, or

convenient

handling length has been
sp
ecified.

Lap distance is normally 40d

for plain round bars and 32d

for deformed bars and 48d
for high tensile bars. For bars

of d
ifferent diameter
-

use the largest value.


g)

Lap/Splice



distance
For lap or splice length see

table
.






Note :
Typic
al bar lengths are 6,7.5,
9,11
m


Concrete Notes


Page
12




h)

Cranking
(Bar remains on same centreline)



i)

Mark
-

The label or ta
g g
iven to a particular size and

sh
ape of a bar to

identify it and indicate its
lo
cation

in the structure.


j)

Starters

-

A bar projecting through a construction joint

which is to lap with other bars placed
within the next

concrete pour.


k)


Mesh

-

welded st
eel grid and label
s by size of grid


l)

T
rim
m
er Bar

-

Additional reinforcing placed around an

opening e.g. lift wells, stairway, windows.
-

Bars must be

shown in full.


m)

Formwork

(boxing, shuttering)


n)

Costing
-

Quantities

1.

Excavation
(m
3)

$..............

2.

Hardfill
(m
3)


$..............

3.

Concrete (
m
3
)


$
..............

4.

Reinforcing (kg

or tonnes)

$
...............

5.

Boxing (m
2
)

$
..............


TOTAL $..............+ 40% labour




DI
ME
NSIONS

-

Wh
ere possible, one unit only should be used throughout the

job, i.e drawn in millimetres.

-

The symbols for the units selected

may be discarded in

accordance with the following rules.

a)

whole numbers indicate m
m

b)

decimalised expressions to 3 d
ecimal

places always indicate

m
etres

c)

all other dimensions are followed by their unit symbols

e.g.
mm



SCALES

(
1:200,
1
:100, 1:50, 1:20)
-

most common

(1:10, 1:5
, 1:2, 1:1
)
-

m
ainly used for
exploded
(or blown up)
detailing







Concrete Notes


Page
13



METHOD

OF CALLING UP REINFORC
ING BARS


Bars sh
all be labelled in the following

way


NEW ZEALAND STANDARDS FOR CALLING UP STEEL


GENERAL


This Section sets out requirements for drawings for structures, made from particular

materials. For
composite structures, discretion should be used

in selecting the

provisions

applicable to the principal
material used in the particular structure.


REINFORCED AND PRESTRESSED CONCRETE


1.

CONCRETE DRAWINGS

Concrete drawings should clearly show the dimensions and shape of the structural element

or elements

depicted. The classification and designation, size, shape, extent and location of

all reinforcement shall
also be clearly shown. Depending on the
c
omplexity of the element,

the detail drawing may show both
the concrete outlines and reinforcement on the sa
me view

or provide separate views, or drawings for
each.

Concrete Notes


Page
14


NOTE: For concrete beams, depth is specified first. For strip footings, width is specified first.


2.

NOTATION FOR REINFORCEMENT


GENERAL


Reinforcement shall be specified by the classification and desi
gnation of bar and reference

number of
mesh (see AS/NZS 4671). Where a schedule is prepared in conjunction with the

drawings, a reference
number for that schedule should be given on both schedule and

drawing.

Reinforcement shall be
specified on the view of

the structural element in which the

rein
forcement will be first placed.
For
example, where a bar is placed in a slab and extends

into a wall it shall be specified on the plan of the

slab.


BARS


Bar reinforcement shall be specified by the number of bars,
type, size, spacing and location,

and, if
applicable, by a shape code and bar mark. Spacing is normally specified at right

angles to the bar
direction and any variation should be fully detailed, e.g. for skewed bars.




One of the following notations shall

be used:

i.

The following information concerning reinforcing bars shall be given on the drawing:



The information concerning the example presented should be written as follows:

If bar marking is used ..............................................
.......
19
-
D500N16
-
L
-
23
-
200T.

If bar marking is not used ...................
..............................
......
D. 5OONl6
-
L
-
200T.


ii.

The following information concerning bundles of reinforcing bars shall be given on the drawing:

Concrete Notes


Page
15



The information concerning the example

presented should be written as follows:

a)

If bar marking is used ...........................................
.............

5 x 3D500N36L
-
27
-
400B.

b)

If bar marking is not used ............................................
.......
5 x 3D500N36
-
L
-
400B.


The preferre
d notation order is presented in (a) and (b). If a different notation order is used,

a legend
shall be provided. Delimiters, such as x, 1 or
-

shall be used if there are

consecutive numbers in the
notation.

For complex structures it may be necessary to spe
cify the bar mark number, type, size,

shape, location
and also the number of bars or the bar spacing, or both, as appropriate.


NOTE: The use of bar marks, shape codes and bundle marks on engineering drawings is optional.


Each set of 'identical' bars in a

structure should be given the same bar mark.


A group of bars in the same placing zone may
be regarded as being 'identical”

if they have

the same
type, size and bent shape. They can, however, have a varying length if supplied in

a set for a tapered
sectio
n.



REINFORCEMENT SCHEDULE LENGTHS


A range of bending shapes for reinforcement is given in Tables 3.1 (A) and 3.1 (B). The

scheduled length
shall be taken as L. Where L is not specified in Table 3.1(A), the

scheduled length shall be the sum of the
out
-
to
-
out dimensions, 'A', 'B', etc.








Concrete Notes


Page
16



PROCEDURE TO DRAW UP REINFORCED CONCRETE DRAWINGS


1.

D
raw

the outline of the

reinforced concrete
section.


2.

Work out

where the cover is require
d
, and

draw in the covers
using a construction layer. (offset
from the outli
ne of the concrete)


3.

Now draw in the reinforcing bars that are placed across

the section
, up against the cover
.



NB: R
einforcing bars are
normally
diagrammatic

only
, However you may be asked to draw the
actual thickness
in your Structural Assignment
.


4.

Use

the method indicated on the
sketch on
Page 1
3 of these notes

to draw the bars and to c
all
up
the

bars in the structure
. This sketch also indicates how to show the bar limits.


5.

Draw bar bending schedule and calculate quantities.


ARRANGEMENT OF DRAWINGS IN

A SET

Drawings should be collated in the following sequence (first drawing to last)
-

(a) General notes. (b) General arrangement drawings. (c) Footings. (d) Columns.

(e) Framing plans. (f) Elevations. (g) Cross
-
sections. (h) Details.


In a multi
-
storey bui
lding, drawings for each storey should be grouped as follows:




Floor plan.



Elevations



Beams.



Sections.



Details


NOTE: A drawing may incorporate more than one of the above.


REINFORCEMENT ALLOWANCES


DIAMETER

AREA

MASS PER
METRE

minimum lap

mm

mm

kg

mm

6.
5

33

0.260

208

10

79

0.617

320

12

115

0.884

390

16

201

1.578

520

20

314

2.466

670

24

452

3.536

910

28

616

4.834

1.100

32

804

6.315

1.350

40

1257

8.868

1.680


DEDUCTION FOR RIGHT HAND BEND


Diameter of Bar



6.5

10

12

16

20

24

28

32

40

Deduction





16

25

30

40

50

60

70

80

100

Concrete Notes


Page
17



Concrete Notes


Page
18


COMMON REINFORCED CONCRETE ABBREVIATIONS


The

following standard abbreviations ore
recommended, but

if there is any risk of confusion or

ambiguity, then
the

words shall be written in full.

When abbreviating by
the use of i
nitials

e.g., Bench Mark
-

8.9.' ALWAYS use capitals,

otherwise
abbreviations are to be in similar printing style to the rest of the lettering

on the sheet.



Abbreviation

Meaning

Alt.

Alternate

B.H.

Bench Hark

B.

Bottom

C.I.

Cast Iron

C.

Central

CRS

Centres

C/C

Centre to centre

CL

Centre Line .

Chkd .

Checked

Col.

Column

C.J.

Construction Joint

M3

Cubic metre

D.P.C.

Damp Proof Course

D

Deformed Reinforcing Bar

Dia.

Diameter

Dwg

Drawing

E.F.

Each Face

E.W.

Each Way

Ext.

External

F.F.

Ext
ernal Far Face

F.F.L.

Finished Floor Level

F.S.

Full Size

Galv.

Galvanised

G

Gauge

G.L.

Ground Level

H.D. bolt

Holding Down Bolt

Hor.

Horizontal

Int.

Internal

I .D.

Inside Diameter





Concrete Notes


Page
19


Abbreviation

Meaning

I.L.

Invert Level

L.A.R.

Lap at Rand
om

M.H.

manhole

Mk

Mark

Max.

Maximum

m

Metre

M.S.

Mild Steel

mm

millimetre

Min.

minimum

N.F.

Near Face

N. L.

No Lap

N.T.S.

not to Scale

NO.

Number

O.D.

Outside Diameter

0/0

Outside to Outside

O/A

Overall

P.C.D.

Pitch Circle Diameter

P.C.

Pr
e
-
cast Concrete

P.S.C.

Pre
-
stressed Concrete

Rad
.

Radius

R. C.

Reinforced Concrete

Reinf.

Reinforcement

Spec.

Specification

cm3

Square Centimetre

S.S.

Stainless Steel

Std.

Standard

S.W.G.

Standard Wire Gauge

St.

Starter

Str.

Straight

S.L.

Struc
tural Level

U.T.S.

Ultimate Tensile Strength

V.L.

Varying Length

Vert.

Vertical