# Development length formula as per IS 456

Development length is an essential concept in civil engineering that refers to the length of reinforcement required to transfer the force from the steel reinforcement to the surrounding concrete. It is crucial in ensuring that the reinforcement is effectively bonded to the concrete to resist the applied forces. “The development length depends on several factors, including the diameter of the bar and the strength of the concrete. “Another factor that affects the development length is the bond strength between the steel reinforcement and the surrounding concrete.”

Properly understanding development length is essential for designing reinforced concrete structures to ensure their safety and stability. Engineers calculate the development length to ensure that the reinforcement will provide the intended strength and reinforcement to the structure. “Insufficient development length can cause the reinforcement to fail to transfer forces to the concrete effectively. This can ultimately lead to structural failure.”

## What is the development length?

To develop the full tensile strength of the reinforcement, one must embed the reinforcement in concrete for a minimum length known as the development length. This is necessary to ensure that the reinforcement can resist the applied loads. This should happen without pulling out of the concrete or causing concrete failure.

Either pull-out or splitting failure modes typically control the length. In pull-out failure, the force applied to the reinforcement exceeds the pull-out strength of the concrete. This generally causes the reinforcement to pull out of the concrete. In splitting failure, the force applied to the reinforcement causes the concrete to crack and split. This can lead to the failure of the reinforcement.

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## Significance and functions

This is a critical concept in reinforced concrete structures that ensures the effective transfer of forces and prevents premature failure. It is important for the safety and stability of structures and is a crucial factor in their design and construction. The main function is as follows.

### Transfer of applied forces

Ensuring effective bonding of the steel reinforcement to the surrounding concrete is the purpose of the Development length in reinforced concrete structures. This allows it to transfer the applied forces to the concrete.

### Prevents structural failure:

Basically, the proper bonding of the reinforcement to the concrete prevents premature failure of the structure. This could otherwise result in catastrophic consequences.

### Important for design

Properly understanding Develop length is critical for designing reinforced concrete structures. Engineers must calculate the length to ensure that the reinforcement provides the intended strength and reinforcement to the structure.

Basically, an insufficient development length can lead to the reinforcement not being able to transfer the forces to the concrete effectively. However, this results in premature failure and instability.

### Structural safety and stability

Generally, this is crucial for the safety and stability of reinforced concrete structures. The failure to effectively bond the reinforcement to the concrete would result in the inability to transfer the applied forces. However, this can lead to structural failure.

## Factors determining Development strength

Several factors influence the required development length to fully develop the tensile strength of reinforcement in concrete, including

• Reinforcement properties: The strength and diameter of the reinforcement significantly impact the required development length. Generally, high-strength reinforcement with a larger diameter will require a longer D length to develop its full strength.
• Concrete properties: The strength, stiffness, and thickness of the concrete member where we place the reinforcement are crucial factors. However, a higher concrete strength requires a longer d length, while a thicker concrete section may require a shorter length.
• Bond strength: The bond strength between the reinforcement and concrete is critical in determining the development length. However, the bond strength depends on various factors. This includes the surface condition of the reinforcement, the degree of deformation, and the quality of the concrete surface.
• Environmental conditions: Environmental factors such as humidity, temperature, and exposure to corrosive agents can affect the bond strength between the reinforcement and concrete. In such cases, we may require a more extended development length.
• Load conditions: The type, magnitude, and direction of the load applied to the reinforcement significantly influence the development length required. Generally, Higher loads require a longer D length to prevent the reinforcement from pulling out of the concrete.
• Design codes and standards: Design codes and standards typically provide guidelines for determining the minimum development length required for different types of reinforcement and loading conditions. However, these guidelines may vary depending on the specific code or standard used.

## Development length as per IS 456

Basically, Clause 26.2.1 of the Indian code for the design of reinforced concrete structures (IS 456:2000) provides the formula to calculate the development length of reinforcement bars in tension. Basically, we require the length of the reinforcement bar to transfer the stresses between the reinforcement and the surrounding concrete.

The formula for calculating the D length (Ld) of a reinforcement bar with a diameter of D, embedded in concrete with a grade of M, and subject to tension, is as follows:

Ld = (0.87 fy A / 4τ_bd) + (0.2 √fc) …Equation 1

where:

• fy is the characteristic strength of the reinforcement in N/mm²
• A is the area of the reinforcement in mm²
• τ_bd is the bond stress between the reinforcement and the surrounding concrete in N/mm²
• fc is the characteristic compressive strength of concrete in N/mm²

The first term in Equation 1 represents the basic development length, which is the minimum length required for the reinforcement to fully develop its strength. The second term represents the additional development length due to the curvature of the bar.

It is worth noting that the code also provides alternative methods for calculation, such as the empirical equations given in Table 5 of the code. However, Equation 1 is the most widely used method for calculating the development length in India.

It is important to note that these calculations are based on certain assumptions and simplifications, and the actual development length required may vary based on the specific design requirements and site conditions.

## Development length as per IS 456 for columns, footings and beams

The dev. length of rebars is the minimum length required for the effective transfer of forces from the steel reinforcement to the surrounding concrete. However this ensures that the reinforcement is properly bonded to the concrete, preventing premature failure of the structure.

## Development length as per codes

The development length of a reinforcing bar, or rebar, is the minimum length of the bar that must be embedded or overlapped with concrete to ensure proper transfer of stresses between the concrete and steel. This is a critical design parameter, and it is determined based on various factors such as the strength of the rebar, the strength of the concrete, and the design requirements of the structure.

Here are the formulas as per some commonly used codes:

### ACI 318-19 (American Concrete Institute)

Ld = [(φ x Fy x As) / (4 x Fc’^(0.5))] x (1.3 for deformed bars, 1.7 for plain bars)

where: Ld = development length in inches

φ = strength reduction factor (0.7 for deformed bars, 0.8 for plain bars)

Fy = yield strength of rebar in ksi

As = area of rebar in square inches

Fc’ = specified compressive strength of concrete in psi

### BS 8110-1:1997 (British Standard)

Ld = [(1.2 x σst x As) / (0.87 x Fy x (1 + (200/d))^(0.5))] x (1.4 for deformed bars, 1.7 for plain bars)

where: Ld = development length in mm

σst = stress in rebar at yield in N/mm2

As = area of rebar in mm2 Fy = characteristic yield strength of rebar in N/mm2 d = diameter of rebar in mm

IS 456:2000 (Indian Standard)Ld = [(0.87 x fy x As) / (4 x τbd x fck^(0.5))] x (1.2 for deformed bars, 1.6 for plain bars)

where: Ld = development length in mm

fy = characteristic strength of rebar in N/mm2

As = area of rebar in mm2 τbd = design bond stress in N/mm2

fck = characteristic compressive strength of concrete in N/mm2

It is important to note that the development length calculation may vary based on the specific requirements of the structure, and it is recommended to consult the appropriate code for accurate and up-to-date information.

# FUSION BONDED EPOXY COATING-TOTAL OVER VIEW

Fusion bonded epoxy coating is also known as F B E coating and F B E powder coating. F B E coating is a thermosetting polymer powder coating used for protecting reinforcement steel used for concrete works in coastal areas. F B E coating acting as a barrier in preventing oxygen, chlorides, and other corrosion elements from reaching the surface of steel thereby prevents corrosion. The composition is based on a specially selected range of Epoxy resins and hardeners.

The effectiveness of a corrosion control coating mainly depends on the ability to perform long term and essential that it should stay bonded to the substrate during the entire life span of the structure.

## ADVANTAGES OF FUSION BONDED EPOXY SYSTEM

The coating poses a strong adhesion to the coated surface.
The coating has tremendous chemical and mechanical resistance in all climatic conditions and poses high mechanical impact strength.

PERFORMANCE PROPERTIES OF FUSION BONDED EPOXY

a) F B E poses low permeability to water vapour and gas pressure.

b) F B E is abrasion-resistant and can be handled and conveyed without damage.

c) F B E protected steel poses strong resistance to corrosive soil, saltwater, microorganisms, etc

d) Have an ageing resistance against heat and have very good dielectric properties.

The application process of Fusion bonded Epoxy is different from the conventional method of epoxy applications.

## APPLICATION PROCESS OF FUSION BONDED EPOXY ON REBAR

Resin and hardener in powder form are applied over the steel and heated to a particular temperature. A thin continuous film is formed fusing the powder and this process is FUSION BONDING.

This process is not irreversible. Once the process is complete it cannot be returned to its original form anymore.

There are four application methods used for fusion-bonded coatings. They are fluidized bed process, electrostatic bed process, electrostatic spray process, and plasma spray process. The electrostatic spray process is the most commonly used process of applying powders.

## ELECTROSTATIC SPRAY PROCESS- METHODOLOGY OF APPLICATION

### a) SURFACE PREPARATION

Surface preparation is the most important activity which is required to bond a coating to the rebar surface. Shot blast cleaning is the best & effective method that can remove mill contaminants, mill scale, rust, etc, and make the surface rough. Shot blast cleaning provides an anchor of 40 -100 milli micron depth on the steel surface and increases the effective surface area of steel. The rebar are blast cleaned to a near white metal finish as per Swedish standards.
In the blast cleaning process, a blast wheel uses a centrifugal to propel the abrasive (grits used for blasting) in a controlled direction speed and quantity. Normally two blast wheels are provided at the top and 2 at the bottom of the cabinet over the top and bottom of the rebar pass line. The location of the blast wheel shall be in an angle where the total area of steel shall be covered.

### b) PRE -HEATING

Heating can be done in various methods, but induction heating is the commonly used method for preheating. Induction heating is the process of heating metal by electromagnetic induction and developing eddy currents. Eddy currents generated within the metal and resistance induces Joule heating of the metal. For preheating using induction heating, the steel is passed through a high-frequency AC magnetic field which heats steels to the temperature required for the Fusion Bonded Epoxy application. The typical application temperature ranges between 225 degrees to 250 degrees.

### c) FUSION BONDED EPOXY COATING

The heated rebar is passed through a powder spray booth where the dry epoxy powder is sprayed through several spray nozzles. Immediately when powder leaves the spray gun, an electrical charge is imparted to the particles. These electrically charged particles are attracted to the grounded steel surface providing even coating coverage. When the dry powder hits the hot steel, it melts and flows into the anchor profile (i. e., the microscopic peaks and the valleys on the surface) and covers the ribs and deformations of the rebar making good bondage with steel.

The heat also initiates a chemical reaction that causes the powder molecules to form complex cross-linked polymers that give the material its beneficial properties. Coating thickness in the range of 50 to 150 micrometers is usually obtained, even though lower or higher thickness ranges might be specified, depending on service conditions.

The molten powder is converted to a solid coating, once the ‘gel time’ is over, which occurs within a few seconds after the coating application. The resin part of the coating when subjected to heat undergoes a process known as cross-linking. This process is known as ‘curing’ in hot conditions. Complete curing is achieved either by the residual heat on the steel or by the help of additional heating sources. Full curing can be achieved in a minute or two depending upon the coating system used.

### d) WATER QUENCHING

The coated steel after hot curing is passed through a water tunnel and subjected to water spray for cooling the bars.

### e) INSPECTION AND TESTING

FBE coated reinforcement bars are tested as per the relevant standard (IS 13620 – 1993). Checks for thickness are also carried out. All batches are laboratory tested for chemical resistance, short spray, resistance in continuous boiling water, abrasion resistance and impact resistance, etc. Any weak patches observed shall be marked and touched up before loading in a rack area. They are bundled in the rack area for transporting to destinations.

## DISADVANTAGES OF FUSION BONDED EPOXY.

Fusion bonded epoxy coatings can affect the bond strength between coated rebars and concrete. Apart from that, the coating process is factory based and the rebar has to be handled and transported to the destinations. These double handlings may damage the coatings. So handling of bars has to be done with utmost care to minimize damage to the coatings.
The performance of the FBE coated rebars is dependent on the coating quality. Patching the defective areas may not be as effective as factory coating and this may act as a corrosion initiator in the later stage because due to the lack of cathodic protection systems.

Corrosion cells accumulate in the damaged area of the reinforcement bars leads to the delamination of the epoxy coating followed by rusting.

FBE coated reinforcement bars undergo degradation on long term exposure to sunlight. FBE coated reinforcement bars shows generally poor alkali resistance.