Jacketing is a method of encasing an existing structural member with new material. It is a structural retrofitting and strengthening technique where the size of an existing section is either increased or restored to its original size using a thin layer of concrete or some other suitable material like carbon or glass fibres, or plates.
This jacket (a thin layer of additional material) increases the load-bearing capacity of the structural member. The technique is used for the repair of columns, beams, and walls. The additional reinforcement is wrapped around the section in consideration and into this shotcrete or in situ concrete is placed.
This additional layer of reinforcement and concrete increases the strength of the concrete and prevents the section from further deterioration.
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Need of Jacketing
Jacketing in Civil Engineering is required to increase the strength of structural members like beam-column slabs etc. Sometimes due to the exposed environment strength of these structural components reduces over time. Another reason may be poor construction technique or the quality of the material.
If the structural component like a column is built but due to negligence it doesn’t develop the desired strength, then the strength may be increased by this method. Other reasons may be wrong design, due to some event like an earthquake, fire explosion, or change in structure use.
- When the load of the structure is increased and existing columns can’t take that load.
- When the design is faulty.
- When poor quality material is used.
- When the wrong construction practice is followed. e.g. steel bars are bent from their original axis.
- When structure component is deteriorated due to weathering agency.
- Damage to column due to flood, earthquake, fire, etc.
Advantages of Jacketing in Construction
- Increases the strength and displacement capacity of a structure.
- The shear strength of the column is increased.
- Certain type of jacketing (steel, FRP) doesn’t increase the overall section of the concrete structure.
- The seismic capacity of the column is increased.
Types of Jacketing
Various methods of jacketing are available nowadays. Some of these methods are discussed in detail in the post.
Reinforced Concrete Jacketing
In reinforced Concrete Jacketing, additional longitudinal and lateral steel reinforcement is tied around the old structure and formwork is placed. The space between the old member and newly laid formwork is then filled with either the normal or modified concrete. the use of normal concrete is discouraged as concrete has a tendency to shrink and may not form a bond with old concrete.
Thus micro concrete is used which doesn’t shrink. This addition of reinforced concrete as a jacket increases the flexural strength, ductility, and shear strength of the column. Effective jacketing is easy to achieve in circular columns but in square columns, it is difficult to achieve effective jacketing.
Vertical bars in the mid-region of each face of the column are susceptible to buckling, but the corners are effectively confined. To address this, a hole is drilled through the column, and cross-links are installed as shown below
Benefits of Reinforced Concrete Jacketing
- Commonly used/easily available material.
- Easy process, every engineer can do it.
- Increase strength and ductility.
- Reinforce cement can take any shape.
Drawbacks of Reinforced Concrete Jacketing
- It is expensive, time-consuming and labour intensive due to the use of formwork.
- Cross-sectional size increased which will change the stiffness and seismic demands.
- An increase in ductility is small because the use of concrete increases brittleness.
Steel Jacketing Method
In the steel Jacketing method, the angle section is wrapped around the corners of the column, and the steel flat is joined using structural pouring glue in-between angle sections. to enhance its load-carrying capacity.
The steel jacketing method increases the load-carrying capacity and increases the ductility and rigidity of the column without much increase in the size of column. It can be used in places where the size of the existing column cannot be increased but load-carrying capacity, ductility, and rigidity need to be increased.
The steel is placed on the column and joined using structural pouring glue. Steel jacking increases the cross-sectional area of steel under tension and compression.
Steel Jacketing for Piers
Steel Jacketing was developed for circular columns. Two half-shell of steel plate is wrapped around the column and welded at the site. A gap of 50 mm is provided between the jacket and column. The space is grouted with cement grout. For rectangular columns, elliptical confinement is used and the space between the column is filled with unmodified concrete.
A rectangular steel jacket around a rectangular column is not recommended. The rectangular jacket is fully capable to restrain the shear but the flexural ductility is not developed except at the corners. Thus rectangular jacketing is avoided.
Advantages of Steel Jacketing
- It is a ductile and commonly used material.
- Good confinement, increase both strength and ductility.
Drawbacks of Steel Jacketing
- Expansive and labour intensive.
- Rusting/corrosion can occur.
- Heavyweight.
- Change in cross-sectional size cause change in stiffness and seismic demand.
Fibre Reinforced Polymer (FRP) Jacketing.
Fibre Reinforced Polymer (FPR) jacketing is one of the most used retrofitting techniques. It has a high strength-to-weight ratio, good impact resistance, highly resistant to corrosion. It has a great advantage because it causes minimum alteration to the structure.
The reinforcement corrosion is a big problem in old buildings, to strengthen the corroded columns fibre reinforced polymer is used. It significantly increases the shear strength. Fibre-reinforced polymer technology is divided into two types fibre cloth reinforcement technology and fibre plate reinforcement technology.
Carbon fibre has high strength and is light in weight. After jacketing the appearance of the structural component is practically not changed and at the same time weight of the structure is not increased much. The FRP Jacketing process is very simple, short, and high efficiency.
Carbon fibre has a high elastic modulus. It performs better in temperature cracks and rust expansion cracks. It does not require external connections like bolts and anchors and there is no damage to the existing structure. Carbon fibre reinforced is very thin, and when the double or triple layer is pasted there is no influence of shear moment so the reinforcement effect is better.
Carbon fibre cloth material can be used in narrow spaces due to its flexibility and weldability. the process is easy and thus saves a lot of time
Advantages of FRP Jacketing
- Easy laying and fast installation speed.
- It is corrosion-resistant.
- Very less modifications to the structure and aesthetics don’t change.
- High strength to weight ratio and high durability.
- Enhancement in strength and ductility.
Drawbacks of FRP Jacketing
- Material is costly but easy installation reduces the overall cost.
- low efficiency due to debonding.
- Can perform poorly in high temperature and wet environments.
- The increase in strength is relatively less.
Glass Fibre Reinforced Polymer Jacketing
Glass Fibre-reinforced Polymer Jacketing is corrosion-resistant, non-magnetic and possesses good resistance to chemical attacks. Glass Fibre is mixed in the concrete during construction, but now Fibre Reinforced Polymer composite can be laid externally with much ease.
This easy laying property makes it the most popular used retrofitting technique. This high-strength, chemical-resistant glass fibre is embedded in concrete. Fibre takes load while surrounding concrete keeps it in the desired location.
Near Surface-mounted Fiber Reinforced Polymer Concrete
In the Near Surface Mounted technique, grooves are first cut into the concrete cover of the structure and Fibre Reinforced Polymer is bonded therein with groove filler like epoxy paste or cement grout.
The advantages of the NSM technique are better resistance to corrosion, an increase in speed of installation and reduced groove size due to high tensile strength and corrosion-resistant.
Advantages of near-surface mounted fibre reinforced polymer concrete
- Less prone to debonding.
- Minimum disturbance to aesthetics and geometry of the structure.
- less prone to accidental damage due to impact due to protection by the concrete cover.
- enhances strength considerably.
Drawbacks of near-surface mounted fibre reinforced polymer concrete
- Material is costly but easy installation reduces the overall cost.
- More labour intensive than Fibre-reinforced polymer concrete but less labour intensive than RC jacketing.
- ductility doesn’t increase much.
Shape Memory Alloy (SMA) Wire Jacketing
Shape memory allow has been used for retrofitting of bridges and buildings. This SMA has potential for seismic applications due to their mechanical behaviour such as shape memory effect and superelastic behaviour. They are more useful than FRP and other retrofitting techniques due to no need for adhesive, and very easy installation.
Advantages of Shape Memory Alloy Wire Jacketing
- Very fast installation
- Doesn’t require adhesive
- no danger of peeling off
- highly elastic and durable
- increases strength and durability
Drawbacks of Shape Memory Alloy Wire Jacketing
- it is costly.
- composite action with concrete is missing.
- strength doesn’t increase much
Hybrid Jacketing
As the name suggests Hybrid Jacketing is a combination of two or more retrofitting methods/materials for enhancing the strength of the column. The use of two or more than two methods draws advantages from both methods. The Hybrid Jacketing method requires no curing time and is less expensive than FRP Technique.
It is lighter in weight than Steel or Concrete jackets and installation is less labour-intensive.
Advantages of Hybrid Jacketing
- Fast installation.
- the minimum change in geometry and aesthetics of the structure.
- highly durable
- Significant increase in both strength and ductility.
Drawbacks of hybrid jacketing
- Costly material.
- labour-intensive as compared to FRP.
Conclusion
Jacketing is the retrofitting technique for increasing the strength and seismicity of the building member. Various types of jacketing methods are available like Reinforced concrete jacketing, steel jacketing, fibre-reinforced jacketing, Near Surface-mounted Fiber Reinforced Polymer Concrete, Shape memory alloy wire jacketing and a Combination of any two of the above methods.
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