Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

CONCRETE AS BUILDING MATERIALS
IN CONSTRUCTION

1

PART II : FRESH CONCRETE

2

FRESH CONCRETE
Introduction
vFresh concrete is that stage of concrete in which
concrete can be moulded and it is in plastic state
...


vAnother term used to describe the state of fresh
concrete is  consistency , which is the ease with which
concrete can flow
...


v The quality is strongly affected by the methods of
mixing, transporting, placing , and curing
...


v Batching by weight is more accurate than batching by
volume , since weight batching avoids the problem
created by bulking of damp sand
...
Mostly it is used
in terms of bags
...
035m3)
...


7

Batching and mixing cont…
ØA gauge box is used for batching of fine and coarse
aggregate by volume
...
It should be
completely filled with aggregates
...


ØGenerally capacity of a gauge box is equal to the volume
of one bag of cement (i
...
35 litres)
...
5m

9

10

Batching and mixing cont…
Ø For hand mixing, the following operation are
performed

§ Measured quantity of sand is spread evenly on platform
§ Spread the measured quantity of cement on this sand
and mix it till the colour of mixer is uniform

11

Batching and mixing cont…
Ø Spread the measured quantity of coarse aggregate on
the platform with sand and cement
...
5m3 to 9m3
...

v Specifications usually require a minimum of 1 minute
of mixing for stationary mixers of up to 0
...
75m3 capacity
...
Finally, the remainder of the water is added to
the mixer
14

Factors affecting mixing time
v Type and condition of mixers
v Speed of rotation
v Size of the charge
v Nature of the materials
• Mixing time is not standard for each batch For
example, a drum mixer with a small diameter creates a
greater
velocity
than
a
drum 
mixer with a large diameter, therefore, the mixing
time would be decreased
...

15

Non tilting concrete mixer

Tilting concrete mixer

Mobile concrete mixer

Pan type concrete mixer

16

Transporting Concrete

Transporting Concrete

v The transferring of concrete from the mixing plant to
the construction site

v The main objective in transporting concrete is to
ensure that the water-cement ratio , slump or
consistency , air content, and homogeneity are not
modified from their intended states
...


18

Transportation of concrete cont…

vAt a central or portable ready-mix plant, concrete
should be discharged from a truck mixer or
agitator truck within two hours
...


vAll delays must be avoided in order prevent
honeycombing
...
And you
must have all the right equipment ready
...
And you will also
require pokers, external vibrators or vibrating beams
to compact the concrete after placing
22

Placing, Compacting and Curing of Fresh Concrete

• Place the concrete carefully in a series of layers of
roughly equal depth – normally 300-500 mm – and
compact each layer with a vibrator
...

• With deep walls and columns, don’t let the concrete
free-fall more than about one metre unless the mix
has been designed to be dropped
...

Ensure that poker hoses and power lines are long
enough to reach the work
...
If nothing is
done to get rid of them, the resultant blowholes or
honeycombing weaken the finished concrete and make it
less able to resist water penetration
...

 In general, concrete in the truck has an air content of
around 20% and this needs to be reduced to between 2%
and 5%, depending on the mix, by vibration
...

 There are three main types of compacting equipment –
poker vibrators, external vibrators and vibrating beams
...

Ø Since the poker must be able to fit easily between the reinforcement,
the spacing will sometimes decide what size of poker is needed
...

Ø Generally the spacing should be around four to six times the poker
diameter
...

Ø Try not to let it touch the formwork or reinforcement
...

Once air bubbles stop rising, take the poker out and re-insert at the
next position, making sure the areas overlap
...

Ø External vibrators are extremely noisy, so wear
ear protectors
...

Ø Watch out for grout loss through joints, loose
vibrators and formwork instability
26

Placing, Compacting and Curing of Fresh Concrete
Vibrating beams
ØAlthough slabs up to 100 mm thick can be tamped by
hand, a vibrating beam drawn across the surface will do
the job much better
...

With deep slabs and heavily reinforced slabs, start by
compacting with pokers and finish with a vibrating beam
...

ØOne pass should be enough, as further passes may bring
excess cement laitance to the top

27

Placing, Compacting and Curing of Fresh Concrete

When is compaction complete?
Ø Compaction is complete when bubbles stop rising
to the surface
Ø When using a poker, you can also tell by the fact
that the sound of the poker stops changing
...

ØWell cured concrete is stronger, more resistant to
chemical attack and traffic wear, and more watertight
...

ØMost curing is done by using polythene sheeting or a
sprayed membrane
...

ØCuring should always start before the concrete begins to
dry out
...

• It should be placed on a slab as soon as the concrete
is hard enough to prevent surface damage or
marking
...

• If possible, water-spray the concrete the next day
and reseal the polythene
...

 Be careful when selecting a suitable product, as some
prevent subsequent adhesion of screeds or applied
finishes
...
If the surface has dried, dampen it
before spraying
...
With slabs, a portable walkway
may be needed to avoid damage to the concrete surface
...

36

Troweling
 Creates smooth, hard, dense

surface
 Exterior concrete should not
be troweled because:
-it can lead to a loss of
entrained air caused by
overworking the surface
-troweled surfaces can be
slippery when wet
...


v Setting of concrete is based or related to the setting of
cement paste
...


39

Workability
 Workability is often referred to as the ease with
which a concrete can be transported, placed and
consolidated without excessive bleeding or
segregation
...


 Air-entraining agent is very effective in reducing the
bleeding
...


 Rich mixes are less susceptible to bleeding than lean
mixes
...


 A good concrete is one in which all the ingredients
are properly distributed to make a homogeneous
mixture
...


 Paste separating away from coarse aggregate
...


45

Causes of segregation:
• Dropping concrete from a considerable height,
• Concrete should always be placed direct in the position
in which it is to remain and must not be allowed to flow
or be worked along the form
...

• Use of coarse aggregate whose specific gravity differs
appreciably from that of fine aggregate would lead to
increased segregation
...
Slump test
2
...
Vebe test

47

1
...


51

Compacting Factor Test: Used for low workable concretes
...

53

54

THE END

55



---