11 Factors that can Affect the Strength of Concrete
Concrete is widely used building material in the
construction world. Concrete is made up of various ingredients, and of course
all of them have different role. The properties of concrete generally rely on
the mixing of concrete ingredients i.e. cement, coarse aggregates, fine
aggregates (sand), and water.
The whole world wishes their structure to be strong
and durable and for that, they always design their structure according to the
desired strength and service. Strength gives an overall indication of quality
of concrete; as it is directly related to the lifelong performance of the
concrete structure. The strength of the concrete shows the ability of the
structure to withstand various loads (i.e. Dead Load, Live Load, Earthquake
Load, Wind Load, etc..). The strength of
the concrete can be measured with the different test that are conducted on it
such as, Compressive strength, Tensile strength and Flexural strength.
Apart from the above tests there are various factors
that can also affects the strength of the concrete, highlights of those factors
are described below:
Factors
that Affect the Strength of Concrete
01.
Water/Cement Ratio
The ratio of the weight of water to the weight of
cement is called Water/Cement ratio. It is the most important factor for
gaining the strength of concrete. The lower w/c ratio leads the higher strength
of concrete. Generally, the water/cement ratio of 0.45 to 0.60 is used. Too
much water leads to segregation and voids in concrete. Water/Cement ratio is
inversely proportional to the strength of concrete. As shown in the chart below
when the w/c ratio is increased the strength of concrete gets decreased and
when w/c ratio is decreased then the strength of concrete increases.
02.
Compaction of Concrete
Compaction of concrete increases the density of the
concrete because it is the process in which air voids are removed from freshly
placed concrete which makes the concrete compact and dense. The presence of air
voids in concrete greatly reduces its strength .Approximately 5 % of air voids
can reduce the strength by 30 to 40 %. As we can see in the above chart, even
at the same water/cement ratio strength is different with different compaction
accuracies. In the fully compacted concrete, strength is higher than the
insufficiently compacted concrete.
03.
Ingredients of Concrete
The main ingredients of concrete are cement, sand,
aggregate and Water. Quality of each material affects the strength of the
concrete. All materials, therefore, should fulfil the standard criteria for use
in concrete like,
(a) Type and Quantity of Cement
The quantity of cement greatly affects concrete
strength. The higher cement content increases the tendency of shrinkage cracks
when the concrete is getting cured and hardened. Types of cement also have a
great impact on the properties of hardened concrete. According to IS 456 2000,
the minimum cement content specified ranges from 300 to 360 kg per cubic meter
of concrete for various exposure conditions and for various grades of concrete.
Maximum cement content in concrete is also limited to 450 kg per cubic meter of
concrete. The grade of cement – i.e. 33 grade, 43 grade, 53 grade will also
affect the strength of concrete. The higher the grade, the higher strength
particularly high early strength.
(b)
Types and Quantity of Aggregate
The strength of concrete depends upon the strength of
aggregates. Low quality of aggregate reduces the strength of concrete. The
quantity of aggregate also affects the properties of hardened concrete. At
constant cement content, the higher amount of aggregate reduces the concrete
strength. The shape and grading of aggregate play a major role as far as
strength of concrete is concern.
(c)
Quality of Water
Quality of water plays a significant role in the
setting and hardening process of concrete. Acidic, oily, silty, and seawater
should not be used in concrete mix. Impurities of water give an adverse effect
on the strength of concrete. Therefore, potable water is always used in
concrete mix. Particularly the impure water may lead to corrosion, carbonation
or acid attack, therefore, reduces the life of concrete.
04.
Curing of Concrete
Curing of concrete is the most essential to prevent
plastic shrinkage, temperature control, strength development and durability.
Curing provides the desired moisture and temperature at the depth and near the
surface after placing and finishing of concrete for development of strength. In
other words, curing provides sufficient water to concrete for completing the
hydration process without interruption which is important for strength
development. Commonly 7-day curing corresponds to 70 % of compressive strength.
Curing period depends on the types of cement and the nature of work. Generally,
it’s about 7 to 14 days for Ordinary Portland Cement. There are many methods of
curing like Ponding and immersion, Spraying and fogging saturated wet coverings
etc.
05.
The Shape of Aggregate
There are many shapes of aggregate like angular,
cubical, elongated, elongated and flaky, flaky, irregular androunded.
Angular aggregates are rough textured, and rounded
aggregates are smooth textured. Thus, the rounded aggregates, create the
problem of lack of bonding between cement paste and aggregate. Angular
aggregates exhibit a better interlocking effect in concrete, but the angular
aggregate contains a larger amount of voids. For this, you needed well-graded
aggregate. The shape of aggregates becomes more important in case of high
strength and high-performance concrete where very low w/c ratio is used. In
such cases, cubical shape aggregates with uniform grading are required for
better workability.
06.
Maximum Size of Aggregates
Larger size aggregates give a lower strength because
they have a lower surface area for development of gel bond which is responsible
for strength. Larger size aggregate makes concrete heterogeneous. It will not
distribute loading uniformly when stressed. Due to internal bleeding, the
problem of development of the microcracks in concrete happens when larger size
aggregates are used in concrete.
07.
Grading of Aggregate
Grading of aggregates determines the particle size
distribution of aggregates. It’s the most important factor for concrete
mix. There are three types of graded
aggregate Gap Graded Aggregate, Poorly graded aggregate and Well-graded
aggregate.
Types
of Grading of Aggregate
Well-graded aggregate contains all size of particles
of aggregate. So that, they have a less amount of voids. The use of well-graded
aggregates gives higher strength to the concrete.
08.
Weather Condition
Weather condition also affects the strength of
concrete due to different reasons. In cold climate, exterior concrete is
subjected to repeated freezing and thawing action due to the sudden change in
weather. It produces deterioration in concrete. With the change in moisture
content, materials expand and contract. It produced cracks in concrete.
09.
Temperature
With the certain degree of temperature increase, the
rate of hydration process increases in it which, it gains strength rapidly.
Sudden temperature changes create a thermal gradient, which causes cracking and
spalling of concrete. So that, the final strength of concrete is lower at the
very high temperature.
10.
The Rate of Loading
The strength of concrete increase with the increase in
the rate of loading because at the high rates of loading, there is less time
for creep. Creep produces permanent deformation in the structure at constant
loading. So that, the failure occurs at limiting values of strain rather than
the stress. In rapid loading, the load resistance is better than the slow
loading.
11.
Age of Concrete
With increase in age of concrete, the degree of
hydration would be more. Hydration process is the chemical reaction of water
and cement. Hydration produces the gel which plays a significant role in the
bonding of particles of the concrete ingredients. Therefore, the strength of
concrete increases with its age. Normally, concrete strength gets doubled after
11 years provided there are no adverse factors. The knowledge about factors
which affect the concrete strength is helpful in many ways particularly during
designing the structure, choosing material for concrete, observing precaution
for different weather conditions, choosing different methods for concreting,
aiming better life of building structures, for low maintenance of building
after construction, longer durability and better serviceability etc.
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