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Lecture#5

WATER & ADMIXTURES

Role of Water,
Three main purposes of using water in concrete
a
...
To provide consistency or fluidity
c
...

i
...


Bound Water - chemically combined in solid products (hydrates) or physically bound
to solid product

For complete hydration of PC paste, only 40-45% water by mass of PC is required
(w/c = 0
...
45)
 For w/c > 0
...
40, some PC will remain un-hydrated (WR often added to coat &
lubricate un-hydrated PC particles with water – this improves strength)

PC & WATER – RATE OF REACTION
 Initial reaction – must be slow enough to allow for concrete transportation &
placement

 In contrast, rapid hardening is desirable once concrete has been placed & finished
...
Shehroze Ali

Page 1

PC HARDENING - INITIAL & FINAL SETTING

Mixing Water- Is it drinkable?

Water Influence (Concern?)
For a fixed set of raw materials used in mix design &curing conditions employed on a
construction site, hardened concrete quality is greatly influenced by amount of water used
in relation to amount of PC
...
This in turn dilutes the “glue” holding capacity
of concrete giving rise to many disadvantages

Mixing water Impurities - Effect on Concrete

1
...
Reduces strength
3
...
Increases corrosion of rebar
5
...
Reduces durability

Properties of Concrete- Engr
...
Increases workability of fresh concrete
2
...
Increases amount of shrinkage in concrete
4
...
Increased compressive & flexural strengths
2
...
Increased weathering resistance (durability)
4
...
Reduced drying shrinkage & cracking
6
...
4 - 0
...
40 – 0
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Shehroze Ali

Page 3

ADMIXTURES
Definition
Materials other than PC, water & aggregates that modify two properties of concrete:
1
...
Hardened properties
temperature rise

– strength development,

freeze-thaw

resistance &

 Admixtures are often added to mix before or during the mixing stage

ADMIXTURES – CLASSIFICATION
Admixtures are generally characterised into three main groups:
1
...
Mineral admixtures
3
...

2
...

4
...

6
...

8
...

10
...
Reduce cost of construction (admixture itself is more expensive but only used in very
small quantities)
2
...
Maintain quality during mixing, transporting, placing & curing in adverse weather
conditions
4
...
g
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Shehroze Ali

Page 4

SUPERPLASTICISER ADDITION – BEFORE & AFTER AFFECT

FRESH CONCRETE / PLASTIC CONCRETE– REQUIREMENTS
Usually concrete mix is designed to achieve any sort of desired properties while generally
following are the seven basic requirements what a fresh mix should meet:
1
...

2
...
Flowbility – required to flow into formwork & around rebar (congested
reinforcement formwork)
4
...
Compactibility – required for compacting fresh concrete in place with ease
6
...
Finishability – finishing surface with

HARDENED CONCRETE - REQUIREMENTS
Following are the keen parameters of hardened concrete:
1
...

3
...

5
...
Shehroze Ali

Page 5



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