FACTORS EFFECTING THE STRENGTH OF CONCRETE:
Concrete strength is affected by many
factors, such as quality of raw materials, water/cement ratio, coarse/fine
aggregate ratio, age of concrete, compaction of concrete, temperature, relative
humidity and curing of concrete.
QUALITY OF RAW MATERIALS
Cement: Provided the cement conforms with the
appropriate standard and it has been stored correctly (i.e. in dry conditions),
it should be suitable for use in concrete.
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.”. This
criterion though is not absolute and reference should be made to respective
codes for testing of water construction purpose.
Water / Cement Ratio
The relation between water cement ratio
and strength of concrete is shown in the plot as shown below:
The higher the water/cement ratio, the
greater the initial spacing between the cement grains and the greater the
volume of residual voids not filled by hydration products.
There is one thing missing on the graph.
For a given cement content, the workability of the concrete is reduced if the
water/cement ratio is reduced. A lower water cement ratio means less water, or
more cement and lower workability.
However, if the workability becomes too
low the concrete becomes difficult to compact and the strength reduces. For a
given set of materials and environment conditions, the strength at any age
depends only on the water-cement ratio, providing full compaction can be
achieved.
Coarse
/ fine aggregate ratio
Following points should be noted for
coarse/fine aggregate ratio:
o If
the proportion of fines is increased in relation to the coarse aggregate, the
overall aggregate surface area will increase.
o If
the surface area of the aggregate has increased, the water demand will also increase.
o Assuming
the water demand has increased, the water cement ratio will increase.
o Since
the water cement ratio has increased, the compressive strength will decrease.
Aggregate
/ Cement Ratio
Following points must be noted for
aggregate cement ratio:
o If
the volume remains the same and the proportion of cement in relation to that of
sand is increased the surface area of the solid will increase.
o If
the surface area of the solids has increased, the water demand will stay the
same for the constant workability.
o Assuming
an increase in cement content for no increase in water demand, the water cement
ratio will decrease.
o If
the water cement ratio reduces, the strength of the concrete will increase.
The influence of cement content on
workability and strength is an important one to remember and can be summarized
as follows:
1.1. 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.
Age
of concrete
The degree of hydration is synonymous
with the age of concrete provided the concrete has not been allowed to dry out
or the temperature is too low.
Age
|
Strength in percentage
|
|
Ordinary
cement
|
Rapid hardening cement
|
|
7 days
|
35%
|
65%
|
28 days
|
60%
|
90%
|
3 months
|
85%
|
95%
|
1 year
|
100%
|
100%
|
In theory, provided the concrete is not allowed to dry out, then it wil always be increasing albeit at an ever reducing rate. For convenience and for most practical applications, it is generally accepted that the majority of the strength has been achieved by 28 days.
Compaction
of concrete
Any entrapped air resulting from
inadequate compaction of the plastic concrete will lead to a reduction in
strength. If there was 10% trapped air in the concrete, the strength will fall
down in the range of 30 to 40%.
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.
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 show the strength development of similar concretes
exposed to different conditions.
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.
Frost
The frost causes
disintegration of concrete and as such strength, durability and impermeability
are reduced. Resistance to frost action depends upon the structure of the pores
in the concrete.
Entrained air
The entrained
air in concrete is due to incomplete compaction. It has the effect of reducing
the strength of concrete. With 1% of entrained air, the strength of concrete is
reduced by 5%. It also increases permeability of concrete.
No comments:
Post a Comment