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What is the reason for micro cracks in concrete?
vivek gami
Micro crack we found in younger concrete Reason of micro crack is Due to plastic shrinkage Due to higher temperature at site Due to improper curing Due to higher water cement ratio Due to improper cement used at particular site (like we cannot use rapid hardening cement in mass concreting) Due to prRead more
Micro crack we found in younger concrete
Reason of micro crack is
We can reduced that micro crack by adding some fibres to the concrete,
See lessWhat is meant by Theoretical Thickness in Concrete?
CB Sowmya
Theoretical thickness = (C/S Area of the section ) / (Semi perimeter in contact with the atmosphere.) Where, semi perimeter = perimeter /2 For example, Consider a 150 mm side cube, and assume these three sides are exposed to the atmosphere Then, Thus the theoretical thickness for the given cube andRead more
Theoretical thickness = (C/S Area of the section ) / (Semi perimeter in contact with the atmosphere.)
Where, semi perimeter = perimeter /2
For example, Consider a 150 mm side cube, and assume these three sides are exposed to the atmosphere
Then,
Thus the theoretical thickness for the given cube and given exposure conditions is 100 mm.
Where is it used?
Theoretical thickness is generally used in the study of creep or drying shrinkage of a structure. During dry shrinkage, the water is expelled outside of the concrete. As a result, the water travels a path. This path is expressed in terms of theoretical thickness.
See lessWhy are 6 cubes cast for testing the Compressive Strength?
INDER KUMAR YADAV
6 cubes are casted for checking the compressive strength of concrete as it is given in the code and as, In Indian standard, there are two different IS code which is defining concrete sampling frequency, those are IS 456 and IS 4926. As per is 456 Clause 15.2.2, Sampling frequency is 0–5m³ - 1 set 6–Read more
6 cubes are casted for checking the compressive strength of concrete as it is given in the code and as,
In Indian standard, there are two different IS code which is defining concrete sampling frequency, those are IS 456 and IS 4926.
As per is 456 Clause 15.2.2, Sampling frequency is
0–5m³ – 1 set
6–15m³- 2 set
16–30m³ 3 set
30–50 m³ 4 set and +1 set for every 50m³ concrete.
This cubes are to be tested in 28 days.
Additional samples should be casted if required for 1 day, 3 days, 7 days testing done.
As per IS 4926 Clause 6.3.2,
One set of samples to be casted in every 50m³ concrete and additional samples (1 day, 3 days,7 days etc) to be casted if required.
Here, 1 set sample means 3 nos of cubes.
Whatever, the sampling frequency and number of samples to be cast to be fixed mutually by both vendor and client
See lessHow to reduce Creep in a structure?
DevilAVRT
The effects of creep can be reduced through several different methods. One way to reduce creep is to lower the working temperature of the metal being used, although this is not always possible. Another method is reducing the constant load the metal has to withstand, but again, this may not be achievRead more
The effects of creep can be reduced through several different methods. One way to reduce creep is to lower the working temperature of the metal being used, although this is not always possible. Another method is reducing the constant load the metal has to withstand, but again, this may not be achievable depending on the application. Using a metal with large grains can reduce creep because less grain boundary sliding occurs. Certain metals with specific alloying element additions can avoid creep by eliminating microstructural vacancies.
See lessWhat is meant by Dry Shrinkage?
CB Sowmya
Dry shrinkage or drying shrinkage occurs due to loss of moisture in the concrete. There are different types of water in a hydrated cement paste. Dry shrinkage occurs due to the loss of capillary water. This water can be found in voids of sizes ranging from 5 nm to 50 nm in the hydrated cement paste.Read more
Dry shrinkage or drying shrinkage occurs due to loss of moisture in the concrete. There are different types of water in a hydrated cement paste. Dry shrinkage occurs due to the loss of capillary water.
This water can be found in voids of sizes ranging from 5 nm to 50 nm in the hydrated cement paste. Due to the small size of the voids, the water is held inside by surface tension or capillary tension. Due to the bond created by the surface tension, evaporation, or any other form of removal of this water causes significant volume changes to the structure. This volume changes causes dry shrinkage to the structure.
Thanks!
See lessWhy is the characteristic compressive strength fck reduced to 0.67 fck?
CB Sowmya
The characteristic strength of concrete is the compressive strength below, in which not more than 5% of the test results should fall. It is generally represented as fck. But during analysis, we reduce the characteristic strength of concrete and include a factor of safety 0.67. As a result, fck becomRead more
The characteristic strength of concrete is the compressive strength below, in which not more than 5% of the test results should fall. It is generally represented as fck.
But during analysis, we reduce the characteristic strength of concrete and include a factor of safety 0.67. As a result, fck becomes 0.67fck.
In order to understand the reason, let us look into how concrete is tested for the characteristic strength in the first place.
A given concrete mix is prepared and cast into cube molds of side 150 mm. After sufficient curing, the cubes are tested for its compressive strength. In those test results, the compressive strength below which not more than 5% of the test results fell is taken as the characteristic strength. Let’s take that as fck.
If the same design mix was cast into a cylindrical mold of 150 mm diameter and 300 mm length and given the same curing, then it would have yielded only 80% of the compressive strength of that of the cubes, i.e., 0.8 fck. This because of the side effect due to the slenderness ratio in the cylinder.
There has been a 20% reduction in strength for a mere slenderness ratio, though both the specimens were monitored in the same laboratory conditions.
Answering your question, in reality, concrete is versatile. It can be cast into a number of shapes and sizes. In order to overcome the size effect on the concrete, in design, we assume that concrete can carry only 2/3rd of its characteristic strength. Thus we reduce fck to 0.67 fck.
Thank you!
See lessHow size of aggregate affect on strength of concrete?
Tips_and_tricks
The size of the aggregate plays a crucial role in the strength of the concrete. Normally, small-sized aggregates are preferred. Larger sized aggregate reduces the strength of the concrete. This is because of the Interfacial Transition Zone (ITZ). The interfacial transition zone is the zone that is fRead more
The size of the aggregate plays a crucial role in the strength of the concrete. Normally, small-sized aggregates are preferred. Larger sized aggregate reduces the strength of the concrete. This is because of the Interfacial Transition Zone (ITZ).
The interfacial transition zone is the zone that is formed on the surface of the aggregates. When concrete is mixed, the water present in the mix coats itself on the surface of the aggregates. This thin film results in a high water-cement ratio in the vicinity of the aggregates.
Owing to the high w/c ratio, during hydration reaction, a rather porous network is formed, resulting in a weaker section surrounding the aggregates. However, as the hydration reaction proceeds, the second generation hydration gels are produced, making the zone denser but still comparatively weaker than the rest of the concrete.
This happens in all sized aggregates. But as the size of the aggregate increases, the surface area of the aggregate also increases. As a result, the ITZ increases, making the concrete weak.
See lessWhy does modulus of elasticity of concrete reduce with time?
CB Sowmya
The Modulus of Elasticity reduces with time because of the effect of creep. Creep is the phenomenon that occurs in hardened concrete over time, where for constant load or stress, deformation increases. Keeping that in mind, let us look at the definition of modulus of elasticity of concrete. It is thRead more
The Modulus of Elasticity reduces with time because of the effect of creep. Creep is the phenomenon that occurs in hardened concrete over time, where for constant load or stress, deformation increases.
Keeping that in mind, let us look at the definition of modulus of elasticity of concrete. It is the slope of the initial tangent modulus of the stress-strain curve of concrete.
The above graph shows the stress-strain curve of the concrete at an early stage.
Due to creep, for a given load/stress, deformation increases, thereby strain increases. This has been explained in the following graph.
As a result, the slope of the curve reduces and becomes like this.
As the curve descends, the slope of the initial tangent modulus of the stress-strain curve of concrete reduces.
As a result, the modulus of elasticity also reduces.
Thank you!
See less