2. SYLLABUS
STUDIES ON FRESH CONCRETE : Workability:
Definition, factor affecting workability, measurement of
workability: Slump test, compacting factor test – segregation
– bleeding – steps of manufacture of concrete: batching,
mixing, transporting, placing, compacting – curing of concrete -
Factors affecting strength of concrete: water / cement ratio,
maturity of concrete, micro –shrinkage of concrete and factors
affecting it.
3. SLUMP
Slump is a measurement of concrete's
workability, or fluidity. It's an indirect
measurement of concrete consistency or
stiffness
5. FACTORS AFFECTING THE
STRENGTH OF CONCRETE
1. Quality of Raw Materials:
Cement: Provided the cement conforms with the
appropriate standard and it has been stored correctly
Aggregates: Quality of aggregates, its size, shape, texture,
strength etc determines the strength of concrete. The
presence of salts (chlorides and sulphates), silt and clay
also reduces the strength of concrete.
Water: frequently the quality of the water is covered by a
clause stating “..the water should be fit for drinking..”.
6. 2. WATER / CEMENT RATIO:
Lower w/c ratio- Less water, More cement, Less
workability
Less workability- Reduced compaction-Reduced strength
High w/c ratio- Higher spacing b/w cement grains-Less
strength-More voids
7. W/C RATIO
The relation between water cement ratio and strength of concrete
is shown in the plot as shown below:
8. 3. Coarse / fine aggregate
ratio
Compressive
strength
Water cement
ratio
Water demand
Surface area of aggregate
9. 4. Aggregate and cement
ratio
• Sand volume high- Surface area will be higher-
Cement is higher-water is low- Less workability-
Reduced strength
• Type of cement
• High cement content-shrinkage
11. Aggregate and cement
content
1. For a given workability an increase in the proportion
of cement in a mix has little effect on the water
demand and results in a reduction in the water/cement
ratio.
2. The reduction in water/cement ratio leads to an
increase in strength of concrete.
3. Therefore, for a given workability an increase in the
cement content results in an increase in strength of
concrete.
12. 5. Age/maturity of concrete
• The degree of hydration - the age of concrete
• No drying / hydration- @ Low temperature
• Concrete maturity indicates how far curing has
progressed
• Maturity is the relationship
between concrete temperature, time, and
strength gain
13. THE MATURITY METHOD
• The maturity method, often simply referred to as
“maturity,” is a way of evaluating new concrete’s in-place
strength by relating time and temperature measurements to
actual strength values.
• Maturity is calculated by tracking changes in fresh concrete
temperature over time.
• Since each concrete mix has its own strength-maturity
relationship, we can use maturity to estimate the strength of that
mix at any moment after placement.
• When we know the maturity of a certain concrete, we can
use that concrete’s specific strength-maturity relationship to
make a reliable estimate of its strength.
14. 6. Compaction of concrete
• Inadequate compaction - reduction in strength.
• 10% trapped air in the concrete- the strength
will fall down in the range of 30 to 40%.
15. 7. Temperature
• The rate of hydration reaction is temperature dependent.
• If the temperature increases the reaction also increases.
• This means that the concrete kept at higher temperature will
gain strength more quickly than a similar concrete kept at a
lower temperature.
• However, the final strength of the concrete kept at the higher
temperature will be lower. This is because the physical form of
the hardened cement paste is less well structured and more
porous when hydration proceeds at faster rate.
• This is an important point to remember because temperature
has a similar but more pronounced detrimental effect on
permeability of the concrete.
16.
17. 8. Relative humidity
• If the concrete is allowed to dry out, the
hydration reaction will stop.
• The hydration reaction cannot proceed without
moisture.
• The three curves shows the strength
development of similar concretes exposed to
different conditions.
18.
19. 9. Curing
It should be clear from what has been said
above that the detrimental effects of storage of
concrete in a dry environment can be reduced
if the concrete is adequately cured to prevent
excessive moisture loss.
