Details of stress-strain curve in steel bars

The objective of stress-strain curve is to define the behavior of steel bars under loads. To develop the curve, the steel specimens are examined. A steel specimen is progressively drawn through a testing machine unless it breaks, and then stress and respective strains are recorded. The stresses are then outlined along the vertical axis, and because of these stresses, respective strains are outlined along the horizontal axis.

Several mark points exist on the stress-strain curve which demonstrate different stages for steel specimen to move through before occurrence of fracture. It is very important to recognize the stress-strain curve for getting the ability to understand the response of steel bars when exposed to loads.

When the steel specimen is exposed to load, it functions like an elastic material to indicate that the stresses and strains are proportional. When the load is raised, the specimen begins to lose its proportionality and ultimately collapses or yields. When the load is raised beyond the yield point, the steel bar undergoes stress hardening and gets the strength to resist larger stress, after it attains the fracture point.

Stress-strain Curve of Steel Bars: When the steel specimen is exposed to load, it passes through different stages ranging from elastic stage, yield point, and fracture.

Limit of Proportionality: This stage is demonstrated on the stress-strain curve from the initial point until point “A”. In this region, the stress remains low and fails to produce permanent strain. The stress and strain are relative to each other, as a result when the stress is eliminated, the steel bar would recapture its original shape.

Elastic Limit: It is situated among point “A” and “B” on the curve. When the stress on steel specimen is again raised, it will produce elastic strain. The stress and strain are not relative to each other.

Yield Point: It is the most vital point on the stress-strain curve from the design perspective. This point is indicated by letter B on the curve and treated as the failure point in the design of reinforced concrete structure. Therefore, when the steel bar attains the yield point in the reinforced concrete element, it will be treated as a failed member.

The yield point is the origin of steel plastic deformation. The stress and strain are not relative. The point B is defined as the upper yield point while the point C is the lower yield point.

Ultimate Strength: When the stress is again raised beyond yield point, strain hardening occurs that is demonstrated from point C to D, beyond which necking commences. Throughout strain hardening, the material is subjected to changes in its atomic and crystalline structure that leads to greater resistance of the material to further deformation. The maximum ordinate in the stress-strain diagram i.e. point D belongs to the ultimate strength or tensile strength.

Rupture Strength: Rupture strength stands for the strength of the material at rupture. It is also called the breaking strength. It refers to the point “E” on the stress-strain diagram.

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Common methods for concrete construction

In this civil engineering article, you will get detail information on the most common methods for developing a concrete structure.

Designing the concrete mix: The most vital part in the method is to find out the components that will form the concrete and their ratios. Several types of variables should be taken into consideration at the time of designing which range from cement type, aggregate size and type, amount of water, and mineral and chemical admixtures.

A good mix design can also lead to improper or substandard quality concrete when it is accomplished inappropriately. Based on the type of project, it is decided who will take responsibility for designing the mix. For large, publicly funded projects, a licensed civil engineer should be liable for the final design.

For residential projects like foundations and driveways it is the private contractor, who will prepare the mix design. For do-it-yourself projects it is of course the homeowner who should take responsibility to design his own mix.

To produce an exact mix design, initially, detect the properties that the fresh and solidified concrete should have and then move backwards to opt for the most inexpensive mix design that provides these properties.

Here, focus should be given on the following factors :

Supported Loads: Concrete is formed with different types of strengths, so this is considered as the gateway of the mix design. As the cost of concrete scales rather narrowly with its strength, one does not like to make the concrete stronger than it should have been.

However, if the application provides support to considerably small loads, it is generally not a good idea to indicate weak concrete since weak concrete does not have good stability. For low load applications the quality of the concrete is settled with other factors like resistance to freezing or wear resistance.

Workability: The necessary workability is mainly based on how the concrete will be arranged. Concrete can be poured, pumped, and even sprayed into place, and it will impact the desired workability. Various other factors like the shape of the molds, the rebar spacing, and the accessible equipment at the site for solidifying the fresh concrete once it is placed should also be taken into consideration.

Workability is generally settled with the slump, the tendency for the fresh concrete is to expand under its own weight when it is arranged onto a flat surface.

Environmental conditions: When the concrete is uncovered to severe conditions, then this may perfectly ascertain the required concrete quality in spite of the applied loads. In cold-weather locations the concrete should have the strong resistance capacity against freezing. Besides, it must have the ability to resist the corrosive effects of salt. Underground applications should have the capability to withstand the penetration of moisture and aggressive species from the soil. For almost any type of conditions or mode of attack, the most effective way for defense is to retain the w/c low.

Surface wear: For some applications the physical loads can erode the concrete rather than breaking it. For roads, parking garages, driveways, and industrial floors, the longevity of the structure depends on the hardness and wear resistance of the top layer of concrete.



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Effects of too much water in concrete

If additional water is present in concrete mix, the concrete can be set easily but the strength & quality of the concrete is reduced significantly. Due to additional water, the damage to concrete structures occurs in the following was :-

1. Strength Reduction 2. Drying shrinkage 3. Loss of abrasive resistance 4. Rise in permeability 5. Dusting and scaling 6. Reduced durability

1. Strength Reduction: Compressive strength is a vital characteristic of solidified concrete and it is decreased due to redundant quantity of water in concrete. The additional water will not engage in the hydration process and contain in concrete even after being solidified.

When the water is vulnerable to atmosphere, it vaporizes and produces voids in the concrete and these voids will decrease the compressive strength of concrete.

2. Drying Shrinkage: If water-cement ratio is raised, the drying shrinkage is also raised and concrete become poor in tensile strength and consequently, cracks build up on the concrete surface.

3. Loss of Abrasive Resistance: Abrasive resistance of concrete is related to its strength directly. With the rise in additional water quantity in concrete, the strength of the concrete is reduced and as a result, the abrasive resistance is also decreased.

4. Permeability: The concrete gets permeable once the additional water in solidified concrete is evaporated. The voids created due to evaporation, will consume water and transform the concrete structure permeable.

5. Dusting: The additional water in concrete mix fetches the fine aggregate to the top, therefore, after getting solidified, a fine loose powder will be arranged on the top of the concrete surface. This method is known as dusting.

6. Scaling: Because of additional water content in concrete, scaling of concrete happens. Under this situation, the top layer of the solidified concrete surface is detached. It is caused by the reaction of water with freeze and thaw effects./p>

7. Reduced durability: The above-explained effects finally result in decreasing the stability of concrete. So, it is necessary to maintain the exact water cement ration for the development of long-lasting concrete. Low water-cement ratio facilitates to produce more long-lasting concrete. By including air entraining admixtures, the longevity is enhanced with low water content.



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Types of steel bars for construction

Generally, the following four types of steel bars are utilized in concrete structure : Types of Steel Reinforcement used in Concrete Structures -

1. Hot Rolled Deformed Bars: It is extensively used in regular RCC structures. Hot rolling is performed in the mills by providing deformations on the surface i.e. ribs with the purpose of developing bond with concrete.

The stress - strain curve presents a separate yield point accompanied with a plastic stage in which strain is raised without raising the stress. It is succeeded by a strain hardening stage. It comprises of typical tensile yield strength of 60,000 psi.

2. Mild Steel Plain bars: These belong to plain bars and do not contain ribs on them. These are suitable for small projects where cost is a vital factor. Since the plain bars can’t be secured properly with concrete therefore hooks are arranged at the ends. In this type of steel, stress - strain curve also presents a separate yield point accompanied with a plastic stage in which strain is raised without raising the stress. It is succeeded by a strain hardening stage. Normal tensile yield strength is 40,000 psi.

3. Cold Worked Steel Reinforcement: Cold worked reinforcement is provided if hot rolled steel bar encounters process of cold working. Cold working is done by twisting or drawing the bars at room temperature. It properly reduces the Plastic Stage in the Stress-Strain curve, even though it provides more control on the size and tolerances of bars. As there is no plastic stage, ductility becomes lower as compared to Hot Rolled bars. It is mostly effective for the projects where low tolerances and straightness are vital factors.

The stress – strain curve does not present a separate yield point since the plastic stage is completely discarded. Yield point is obtained by drawing a line parallel to the Tangent Modulus at 0.2% strain. Yield stress stands for the point where this line overlaps the stress – strain curve.

It is defined as 0.2% proof stress. If yield stress is obtained at 0.1% strain it is defined as 0.1% proof stress. Normal tensile yield strength is 60,000 psi.

4. Prestressing Steel: Prestressing steel is applied in the form of bars or tendons which are constructed with several strands. The application of tendons / strands is very common since these can be easily placed in different profiles. Prestressing strands are built up with various wires (usually with 2, 3 or 7 wire strands). Normal seven wire strand comprises of six wires spun around the seventh wire with marginally a greater diameter, consequently a helical strand is created.

These wires are cold drawn and contain very high tensile ultimate strength (normally 250,000 - 270,000 psi). Due to extreme tensile strength, the concrete is prestressed properly even after experiencing short term and long term losses. These are applied as prestressed concrete in bridges or prestressed slabs in buildings. Prestressing steel also comes as non-bonded strands encased in PVC sheath. It is applied in Post-Tensioning of members. Prestressing strands can also be applied as Low Relaxation Strands which show low relaxation losses after prestressing. These are normally applied in prestressing members with large distances./p>

Because of cold drawing method, plastic stage in this type of steel is removed. Thus stress – strain curve does not present a separate yield point. Yield point is obtained at 0.1% or 0.2% proof stress. The design of prestressed concrete is not based on yield stress rather it relies on the ultimate strength; therefore the property of interest in this type of steel is the ultimate strength.

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Various applications of Concrete Structure Casting Remnants

The concrete applied in construction is frequently wasted in the process because of several errors in casting. The waste of ready-mix concrete is influenced by various factors like miscalculations / calculations in the jobsite, defective equipment, harmful weather, as well as inappropriate application methods resulting in wasting of materials.

Given below, some useful suggestions for utilizing the leftover concrete casting structures which can reduce the expenses of a project.

1. Applied as a practical column or lintel beam in precast.

In several enormous projects, precast methods are utilized for the wall installation work of practical columns or lintel beams. Normally, instant concrete with K-225 or K-250 is utilized. It is beneficial to utilize the structural casting residual concrete to minimize concrete waste and instant concrete purchases.

2. Applied as a cansteen

Cansteen belongs to the edge of the pedestrian, sidewalk, separator, edge of the road, park path, boulevard, and so on. It is utilized as a roadside amplifier. Normally, cansteen is measured in BQ to order the finished product. It will be better to install it in precast. Structural casting residual concrete is utilized as cansteen to reduce the budget.

3. Applied as a Car Stopper

Car stoppers belong to embankments for providing protection to the parking area to stop the moving cars. Normally, various types of materials like iron pipes, concrete and rubber are usually applied as car stoppers.

4. Applied as Paving Block

Paving blocks are mostly utilized as outdoor parking areas, jogging tracks, parks, sidewalks, home yards and pedestrians. These blocks are made precast with leftover concrete casting materials. Thus, the project costs will be curtailed significantly.



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Definition and categorisation of Pre-Stressed Concrete

Normal concrete structure contains greater compressive strength and very weak tension strength. For the purpose of removing the weakness of concrete in tension, pre-tensioning method is applied. Pre-stressed concrete belongs to the autonomous formation of permanent compressive stresses in a structure to enhance its behavior and strength toward different service situations.

Categorization of Pre-stressed concrete

Pre stressed concrete structure is categorized on the basis of their features of design and construction. The are categorized into the following three groups.

1. Location of construction
2. Level of construction
3. Method of pre-stressing

LOCATION OF CONSTRUCTION
Here, the pre-stressed structure may come as precast cast, cast in situ, composite constrictions.

PRE CAST
Precast pre-stressed components are fabricated in a plant or adjacent to the working site. Then they are conveyed to construction site and then they are constructed and set to its position.

CAST IN SITU
The cast-in-situ concrete is standard concrete that is poured into the exact formwork on the site and cured to attain the strength of RCC elements. So, the transportation and erection process are not necessary in this method.

COMPOSITE CONSTRUCTION
Composite construction is done by integrating both prestressed and cast in situ methods. As for instance, in a beam in a building structure is built up by pre-stressed concrete and the top roof slab is built up by the cast in situ method.

LEVEL OF PRE-STRESSING
Level of pre-stressing is categorized as completely pre-stressed, limited pre-stressed, partially pre-stressed.

FULLY PRE-STRESSED
In fully pre-stressed concrete members, there is not any tensile stress under the operation of working or service loads.

LIMITED PRE-STRESS
Members are pre-stressed to a limited extent in order that members may create some tensile stress under working loads but the magnitude is below the tensile strength of concrete. These members function as a completely pre-stressed member and remain uncracked under working loads.

PARTIALLY PRE-STRESSED
Partially pre-stressed structure contains both pre-stressing steel and reinforcing steel to withstand working load.

Partial pre-stressing is useful for both pre-stressed concrete and reinforced concrete. The tensile stress remains within the limit.

To get more detail, go through the following link learntocivilfield.com



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Some useful tips to determine the monthly rental value of your structure

From this construction video tutorial, we can gather knowledge on how to make the valuation of monthly rent concerning a building that has been developed on a land.

There are two types of rents like net rent and gross rent. Net rent does not include taxes associated with land or construction. Gross rent normally includes these types of taxes.

As for example, here, the value of plot is taken as 4,00,000 rupees (inclusive of all taxes).
The value of structure is taken as 4,70,000
As per government law, 5% of land value and 6% of construction value can be taken as return in one year.
Based on the above figure, the monthly rent of the building should be determined.

Therefore, the annual rent should be as follow :-
4,00,000 x 5% = 20,000/Rupees

Annual rent of structure = 4,70,000 x 6% = 28,200/Rupees
So, total annual rent will be = 28,200 + 20,000 = 48,200/Rupees
Therefore, monthly annual rent will be = 48,200/12 = 4017/Rupees

Now, if you want to find out the time period when total investment for land and structure will be recovered, apply the following formula :-
4,00,000 + 4,70,000 = 8,70,000/4017 = 216.58 months

To covert the value to year
216.58/12 = 18 years.

To get more information, watch the following video.

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