Jacketing and Collars for Concrete Column Beam Strengthening

Jacketing and Collars for Strengthening of Concrete Structures

Jacketing is the process whereby a section of an existing structural member is restored to original dimensions or increased in size by encasement using suitable materials. A steel reinforcement cage or composite material wrap can be constructed around the damaged section onto which shotcrete or cast-in- place concrete is placed.

Collars are jackets that surround only for a part of a column or pier. These are usually used to provide increased support to the slab or beam at the top of the column.

The form for the jacket consists of timber, corrugated metal, precast concrete, rubber, fiberglass, or special fabric; and may be permanent in some cases. The form must be provided with spacers to ensure equal clearance between it and the existing member.

Materials, like conventional concrete and mortar, epoxy mortar, grout, and latex-modified mortar and concrete, are used as encasement materials. For jacketing, the void between the form and the existing member is filled using pumping, tremie, or preplaced aggregate concrete.

Jacketing is particularly used for the repair of deteriorated columns, piers, and piles and may easily be employed in underwater applications. The method is applicable for protecting concrete, steel, and timber sections against further deterioration and for strengthening.

Permanent forms are preferred where protection against weathering, abrasion, and chemical pollution is desired. The collar provides increased shear capacity for the slab, and it decreases the effective length of the column. Architecturally collars are considered better than jacketing but performing the same structural function.

Before applying jackets or collars, all deteriorated concrete must be removed, cracks must be repaired, existing reinforcement must be cleaned, and surfaces must be prepared. The surface preparation improves the bond of the newly placed materials with the existing structure, which is difficult for underwater repairs. For underwater conditions, a plastic shell may be applied at the splash zone to help minimize abrasion. A drawback of jackets and collars is that they occupy space that was earlier available for other uses.

Timber, cardboard and corrugated steel forms may be used as temporary or permanent forms. Permanent fiberglass, rubber, and fabric forms have gained considerable popularity because they provide resistance to chemical attack after the repair is complete.

Jacketing used for purposes other than covering the deteriorated concrete and providing lateral confinement, such as to bear longitudinal loads, needs special considerations. The existing column may have undergone full shrinkage and most of the creep and also has elastic strains due to carried loads, whereas the shrinkage and creep of the new material has to occur.

The load transfer to jacketing is also a big issue. It is better to use jacking to release the load on the member before jacketing, to use non-shrinking materials for jacketing and to hammer steel shims at the transfer points of the jacketing after curing.

If the material used for jacketing is cement mortar or concrete, the cement content must be exactly according to the requirements; both excessive and less cement contents may be dangerous. Use clean, stable and the largest possible size aggregates. In order to reduce shrinkage, control the temperature of the materials and the immediate surroundings during placing and curing.

Use of admixtures such as plasticizers, air-entraining agents, retarders, accelerators and waterproofing admixtures is more beneficial in repair than even the ordinary construction.

Expanding mortars / concretes can be made by adding aluminum powder to the matrix to overcome the setting shrinkage and some part of the drying shrinkage. The use of iron fillings or powder can also perform this function if moisture and air are available.

In case grout is used for filling the forms, it is allowed to settle for about 20 minutes after fulfilling and then is filled to overflowing condition. The top of the jacket must be finished with pneumatically projected or hand placed concrete.

Different Types of Sedimentation Tanks used in Water Treatment

A sedimentation tank is structure in which wastewater is filled and stored for some time to remove the suspended particles present in the water. These particles may settle at the bottom of the tank and are removed by using scrapers. If the suspended particles have low specific gravity than water, they settle at the top of the tank.

Types of Sedimentation Tanks

Depending upon various factors sedimentation tanks are classified as follows.

1. Based on methods of operation

a. Fill and draw type tank

b. Continuous flow type tank

2. Based on shape

a. Circular tank

b. Rectangular tank

c. Hopper bottom tank

3. Based on location

a. Primary tank

b. Secondary tank

Types of Sedimentation Tanks based on Methods of Operation

Fill and Draw Type Sedimentation Tank

In case of fill and draw type sedimentation tank, water from inlet is stored for some time. The time may be 24 hours. In that time, the suspended particles are settled at the bottom of the tank. After 24 hours, the water is discharged through outlet.

Then settled particle are removed. This removal action requires 6-12 hours. So, one complete action of sedimentation requires 30-40 hours in case of fill and draw type sedimentation tank.

Continuous Flow Type Sedimentation Tank

In this case, water is not allowed to rest. Flow always takes place but with a very small velocity. During this flow, suspended particles are settle at the bottom of the tank. The flow may be either in horizontal direction or vertical direction.

Horizontal flow type sedimentation tank

These tanks generally in rectangular shape. They have more length twice its width. Because they need to flow more distance to settle all suspended particles. The maximum permissible velocity in this case is 0.3m/sec.

Vertical flow type sedimentation tank

The vertical flow type sedimentations tanks are generally in circular shape and flow takes place in vertical direction. Hopper bottom is provided at the bottom of the tank to dispose the collected sludge.

Types of Sedimentation Tanks based on Shape

Circular Tank

Circular sedimentation tanks are preferred for continuous vertical flow type sedimentation tanks. In this case influent is sent through central pipe of the tank and radial flow takes place.

Mechanical sludge scrappers are provided to collect the sludge and collected sludge is carried through sludge pipe provided at the bottom. But circular tanks are uneconomical as compared to rectangular tanks but they have high clarification efficiency.

Rectangular Tank

Rectangular sedimentation tanks are mostly preferred sedimentation tanks and are used widely. The flow takes place in horizontal direction that is length wise in rectangular tanks. Sometimes baffle walls are provided for rectangular tank to prevent short circuiting.

Maintenance costs are low in case of rectangular sedimentation tanks. They are also suitable for large capacity plants.

Hopper Bottom Tank

In case of hopper bottom tank, a deflector box is located at the top which deflects the influent coming from central pipe to downwards. Sludge is collected at the bottom and it is disposed through sludge pump.

Types of Sedimentation Tanks based on Location

Primary Sedimentation Tank

Primary sedimentation tank is a normal sedimentation tank in which water is stored at rest for some time and sludge collected at bottom and oily matter collected at top are removed. After primary sedimentation process the wastewater is discharged into aerobic filter where activated sludge process take place.

Secondary Sedimentation Tank

After activated sludge process the wastewater enters secondary sedimentation tank in which suspended particles contains microbes are removed and are reflected towards aerobic filter to maintain high microbe concentration in aerobic filter.



Bamboo as a Building Material – its Uses and Advantages in Construction Works

Bamboo as a building material has high compressive strength and low weight has been one of the most used building material as support for concrete, especially in those locations where it is found in abundance.

Bamboo as a building material is used for the construction of scaffolding, bridges and structures, houses.

Due to a distinctive rhizome-dependent system, bamboos are one of the fastest-growing plants in the world and their growth is three times faster than most other species of plants. They are renewable and extremely versatile resource with multi-purpose usage. Among many uses of bamboo, Housing is one of the major areas applications especially in the wake of residential shortages around the globe.

Bamboo as a building material is conventionally associated with the region of Southeast Asia and South America where climate is best suitable for its cultivation. In many of the nations, bamboo is used to hold up suspension bridges or simply make places of dwelling.

Various Structural Shapes of Bamboo as a Building Material

Bamboos are treated in such a way that they assume desired shapes and structures while they grow:

1. Squared cross-section can be obtained by compressing the growing stalk of bamboo within a square section.
2. Arch shapes of bamboo can also be created by compressing the bamboo’s growth into the desired shape. This would cost lesser than it would to get the same form with normal timber.
3. Curved and Flat shapes of bamboo are achieved through traditional techniques like applying heat and pressure.

Methods of Working on Bamboo

For a bamboo to be used as a building material, it must be worked on to create desired shape, bend and length to be used for structural or other purposes.

Following are the different works involved with use of bamboo:

1. Splitting
2. Shaping
3. Bending

Splitting of Bamboo

The bamboo canes are split into halves or quarter sections using a knife ideal for the job and setting them apart by a wedge. About four or eight segments can be acquired which are used as canes, strips or battens. Canes can be peeled to make strings and ropes up to the age of 18 months.

Splitting of a bamboo cane

Splitting bamboo with a knife frame

Shaping of Bamboo

Even though bamboos are naturally circular in form but if they are grown in a box of square shape they acquire a shape as desired.

Bending of Bamboo

Bamboos can be bent while they are freshly cut by heating them above the temperature of 150° C. Bamboo will retain this shape even after cooling and drying off.

Bamboo being subjected to bending

Preservation of Bamboo

A thorough treatment of Bamboo is required to protect it against insects and rot before it is put into use. Commonly a mixture of Borax and Boric acid are utilized for this purpose. Another procedure generally employed is to boil cut bamboo to remove the starches that draw insects.

Bamboo as a Building Material

Utilization of Bamboo for construction is achieved by a structural frame technique which is related to same approach applied in usual timber frame design and construction.

In the case bamboo, floor, walls and roof are interconnected and often rely on the other for overall stability. Bamboo has played a vital role in the growth of enterprises and the rural transformation.

Bamboo for Foundations

There is very limited use of bamboo as foundation material because when in contact with moisture laden surface they decay fast. However, this issue can be tackled to quite an extent though proper treatment using appropriate chemicals.

The various types of foundations constructed with bamboo are:

a) Bamboo which is in direct contact with ground surface.

b) Bamboo fixed to rock or preformed concrete footings

c) Composite bamboo or concrete columns

d) Bamboo piles

Bamboos are used in various shapes and forms to build foundation. Some of the common shapes of bamboos are:

a) Flattened bamboo shape which is acquired by splitting freshly cut bamboo stalks and then rolling and flattening them.

b) Bamboo mats as thin as 5-6mm or 10-15mm in size are woven according to design prerequisite. Phenolic resins are used in structures employing bamboo mats.

c) Bamboo plastic composite is a pioneering technology in which bamboo fiber as raw material is blended with plastic as the core material. These mats are highly resistant to moisture and structurally more stable.

Walls Construction with Bamboo as a Building Material

Bamboo is extensively used for construction of walls and partitions. Posts and beams are the main elements normally constructed with bamboo provide structural framework for walls. They positioned in a way to be able to withstand forces of nature. An infill is used between framing elements to add strength and stability to the walls.

Roofing with Bamboo as a Building Material

Bamboo is one of the best roofing materials and provides ample sturdiness to the structure. It is a proven shield against forces of nature or animals and are considerably light weighted which makes them easy to install. The bamboo roofs encompass purlins, rafters and trusses.

Scaffolding with Bamboo as a Building Material

Due to advantageous properties of bearing heavy load bamboos are considered as one of the highly-endorsed materials for scaffolding even for tall structures.

For the construction of scaffolding, cane extensions are obtained by lashing cane ends using several ropes. The ties are positioned in such a way that forces acting vertically downwards lodge the nodes in the lashing.

This technique has immense significance since the joints can be re-aligned in the right degree.

Advantages of Bamboo as a Building Material

The various advantages of bamboo are as mentioned below:

1. Tensile strength: Bamboo has higher tensile strength than steel because its fibers run axially.
2. Fire Resistance: Capability of bamboo to resist fire is very high and it can withstand temperature up to 4000 C. This is due to the presence of high value of silicate acid and water.
3. Elasticity: Bamboo is widely preferred in earthquake prone regions due to its elastic features.
4. Weight of bamboo: Bamboos due to their low weight are easily displaced or installed making it very easier for transportation and construction.
5. Unlike other building materials like cement and asbestos, bamboo poses no danger to health.
6. They are cost effective and easy to use.
7. They are especially in great demand in earthquake prone areas.

Disadvantages of Bamboo

Bamboos come with their own set of drawbacks such as:

1. They require preservation
2. Shrinkage: Bamboo shrinks much greater than any other type of timber especially when it loses water.
3. Durability: Bamboo should be sufficiently treated against insect or fungus attack before being utilized for building purposes.
4. Jointing: Despite prevalence of various techniques of jointing, structural reliability of bamboo is questionable.