Multiple components are combined to create concrete. This heterogeneous material hardens into a mass that resembles stone. Engineers are extremely concerned about concrete because of its widespread use in the construction industry. A concrete engineer has to be familiar with its qualities in order to participate in its many uses. We have given you an overview of some of the characteristics of concrete in this post.
Concrete comes in the following varieties of strength:
a. Shear strength; b. Tensile strength; c. Flexural
strength; and d. Compressive strength
Strength in Compression
Bangladesh uses two different kinds of test specimens: cubes
and cylinders.
Concrete cube specimens of the required size, often
measuring six inches in cube, are formed in steel or cast iron molds. Standard
cubes and cylinders are tested at set ages, usually 28 days, with further tests
frequently done at 1, 3, and 7 days. The standard cylinder specimen of concrete
is 6 inches in diameter and 12 inches in height, and it is cast in a mold that
is typically made of cast iron. Under a testing equipment, the specimens'
crushing strength is evaluated. Crushing strength numbers obtained from cube
tests are typically 20–30% higher than those obtained from cylinder testing.
A cylinder specimen's strength is equivalent to
three-quarters of a cube specimen's strength, according British standard.
Age's impact on the strength of concrete
Concrete becomes stronger over time. Within 28 days,
ordinary cement concrete achieves over 70 to 75 percent of its ultimate
strength, and over 90 to 95 percent over the course of a year. In many cases,
it is preferable to evaluate a concrete's compatibility well in advance of the
28-day test findings being made public. In the absence of precise information
about the ingredients used to make concrete, it is reasonable to infer that the
28-day strength will be 1.5 times greater than the 7-day strength. Experiments
have indicated that the extrapolation of 28 days' strength from the 7 days'
strength is quite reliable, with the ratio of the 28 days to 7 days' strength
for concrete made with regular Portland cement typically falling between 1.3
and 1.7, and most results exceeding 1.5; the figure below illustrates the rate
of strength gain of the various types of cement concretes.
Tensile power
Tension is not strong in concrete. Ordinary concrete has a
tensile strength that is between 7 and 10 percent greater than its compressive
strength.
Flexural power
The tensile strength of plain concrete virtually entirely
determines its flexural strength. Experiments, however, demonstrate that the
modulus of rupture is far larger than the tension strength.
Simple strength
It is the actual deciding element for short columns'
compressive strength. Between around half of the compressive strength for rich
mixtures and about 0.8 of the compressive strength for lean mixtures,
concrete's average strength in direct shear varies.
Adaptability
The degree of compaction has a major impact on the strength
of concrete at a particular mix proportion. Therefore, it is essential that the
mix's consistency allow for the concrete to be finished, placed, and
transported with enough ease and without separating. A concrete is considered
workable if it meets these requirements.
The following variables can affect how workable concrete is:
water content, mix proportions, aggregate size, shape, grading, surface
texture, use of additives, use of supplemental cementitious materials, time,
and temperature.
The Slump test is typically used to infer indirectly how
workable a concrete mix is.
Flexible Characteristics
Even at low loads, there is a noticeable permanent setting
in concrete, which makes it not entirely elastic for all loading scenarios. At
any loading stage, the deformation is not proportionate to the stress.
Concrete's elastic qualities change depending on the mixture's richness and the
degree of stress. The age of the concrete also affects them.
Sturdiness
The ability of concrete to endure the conditions for which
it was intended, without degrading over time, is known as durability.
Concrete's internal agents or external agents originating from the environment
can both contribute to its lack of durability.
There are three types of causes: mechanical, chemical, and physical.
While mechanical causes are mostly related to abortion,
physical causes are caused by the effect of frost and variations in the thermal
characteristics of the cement paste and aggregate.
Lack of permeability
The durability of concrete may be negatively impacted by
components dissolved in solution, such as when aggressive liquids (acids)
assault the material or when Ca(OH)2 is being leached out. Concrete's
susceptibility to moisture and freezing temperatures is significantly
influenced by its permeability. Steel will corrode in the case of reinforced
cement concrete due to air and moisture penetration. This causes the steel's
volume to rise, which causes the concrete to crack and spall. Concrete
permeability is crucial for hydraulic and liquid-retaining constructions;
Separation
Segregation is the tendency for the coarse aggregate grains
to separate from the concrete mixture. When the concrete mixture is excessively
moist and lean, it increases. Additionally, it rises when quite large,
coarse-textured material is utilized. The following are some ways to prevent
the segregation phenomena.
i. Including tiny air-entraining agents in the mixture.
ii. Limiting the quantity of water to the absolute minimum.
iii. Every operation, including handling, positioning, and
consolidation, needs to be carried out with caution.
iv. It is not appropriate to let concrete fall from great
heights.
Bleeding
Bleeding is the term used to describe the tendency of water
to rise to the surface of freshly placed concrete. Sand and cement particles
are carried by rising water to the surface, where they solidify to produce a
scum layer known as laitance. The following actions can be taken to check for
bleeding in concrete.
i. Increasing the amount of cement; ii. utilizing more
finely ground cement; iii. Making sure the mix is appropriately designed and
utilizing the least amount of water; iv. Using less air entraining agent; v.
Increasing the finer part of fine aggregate
Fatigue
When flexured, plain concrete shows signs of fatigue. The
endurance limit of concrete, whose value is dependent on the number of stress
repetitions, indicates the concrete's ability to withstand flexure. The maximum
permissible flexural working stress in concrete pavement design is 55% of the
modulus of rupture.
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