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Chapter: Civil - Prefabricated Structures - Joint In Structural Members

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Mould for a pair of wall panels

Vertical Moulding: Best suited for panels that require a smooth surface on both sides.

Mould for a pair of wall panels:

 

Vertical Moulding:-

 

Best suited for panels that require a smooth surface on both sides.

 

Advantages :

 

1.     A large no of units can be produced in a small space.

 

2.     Concreting the units proceeds fast.

 

3.     No need to spread the concrete

 

4.     Surface finishing not necessary due to smooth mould faces.

 

5.     Heat of hydration developed is conserned and accelerates curing.

 

Single Moulds:

 

These are employed for casting volumetric elements such as sanitary units, ventilates shafts and refuse chuter. Battery moulds are employee for internal panel walls and floor panels. The swing down moulds are used for simultaneous manufacture of two wall panels.

 

Flow Line Production :

 

This is a travelling horizontal mould system in which the moulds are moved and the element from one position to the rest a series of stations, such as demoulding, mould cleaning and oiling, placing of reinforcements concreting vibration, surface finishing and curiving a stream chamber forms the part of the continuous travelling a chain and the chain can be in the horizontal plane or in the vertical plane.

 

Schematic diagram of flow line production:-

 

Moulds:-

 

1.     They should have volumetric stability to ensure dimensional accuracy.

 

2.     They can be reused a large no of times with minimum maintenance cost.

 

3.     They should be easy to handle and close tightly so that no liquid can lead out.

 

4.     They should not have adhesion to concrete and easy to clean.

 

5.     They can be sued for various cross sections shapes of the components. Wooden moulds:

 

1.  Concrete sticks more easily. to prevent this, a coating of mould oil or wood lequer is given.

2.  It can be used 30 to 40 times. The dimensions should be checked frequently.

They are used fro smaller production programmes. Steel Moulds :

 

1.     Because of the smooth surface demoulding is fairless easy.

 

2.     Indiscreminate harmmering by workmen should be avoided. Concrete moulds:

 

1.     These are used in vertical battery moulding.

 

2.     The workability is low

 

3.     These moulds enable hish degree of dimensional accuracy, but are un-suitable for making modifications.

 

4.     These moulds are stationary not often transported.

 

5.     The surface of the mould must be absolutely smooth of otherwise excessive adherion may cause difficulties in demoulding.

 

Plastic Moulds:

 

1.     Moulds made of glass fibre reinforce plastics are commonly used.

 

2.     They have the advantage of freedom of shaping and low weight.

 

3.     Demoulding is

 

4.     They are easily transportable.

 

5.     The same mould can be used 70 to 80 times without repair. Manufacture Of Precast Elements:

 

1. Wall panels :-

 

Type of moulding depends upon the constructional features and the surface finish. In the case of internal panel wall-vertical battery wall. In the case of external panel wall-horizontal moulding is usually done.

 

2. Roof or Floor Elements:

 

Depends on the type of building we construct whether residential or public building with large span. In the case of residential building the entire roof or floor is cast as a single unit. In the case of public building where long spans are encountered. The elements are east in the form of hollow core floor slabs. Trough units and ribbed slabs which are normally of PSC.

 

3. Beams and Column :

Usually horizontal moulding is done. In the case of staircases vertical or horizontal

 

1.     Placing of concrete by strips slips

 

2.     Spreading of concrete is done manually or mechanically.

 

3.     Compaction by vibrators or by vacuum process or pressing.

 

In the case of vibration it is effected by means of internal vibrations and external vibrations. Internal vibrations in the form of immersion vibrators is used in places where we have conjusted reinforcements. External vibrators ae generally used with steel moulds. Vacuum process is suitable for components with large surface area or relatively thin elements. In the case of compaction by pressing. The freshly poured concrete is subjected to a pressure of about 70 ksc. It sequences out excessive water forming a cohesive slab which can be immediately remoulded.

Curing :-              

Accelerated curing -1 cycle -(Steam curring)       

           1. Preheating period -3hrs-35C

          2.       Temp Rise Period -2hrs -35 to 8.

          3.       Period at max temp - 6hrs - 80C

          4.       Cooling period - 3hrs - 80C-45C

Accelerated curing techniques are adopted for quicker turnover. It may be in the form of steam curing or heat treatment. Hot water or hot air. The duration of stream curing cycle is influenced by the factors like type of cement, water cement ratio, size of the members, the desired strength. With proper steam curing it is possible to achieve 60% of the moist curned 28 days strength of the concrete in 24 hours. In open casting yard for steam using specially made hoods are used which are insulated and sealed to prevent excessive loss of heat and moisture. In the case of flow line product.

 

The chamber is sufficiently long to ensure that the products remain wihin the chamber for the desired time.

 

Demoulding and storage:

 

The units are first demoulded from the sides and then from the bases and the earthwork required to separate the unit from the base is more than the weight of the unit to account the adhesion of the unit to the base. Wall element are stored vertically. Floor and roof elements are stacked horizontally with & wooden strips in between two elements.

Surface Finishing Techniques:

 

a)     Surface formed in the mould.

 

b)    Mechanical treatement of surface.

 

a)     Textured surfaces are obtained by lining the mould with suitably patterned rubber linings, plastic steel or timber. Smooth surfaces are obtained by resin coated or plastic zined mould surfaces.

 

b)    This can be applied by freshly cast concrete or by the hardened concrete. In the case of freshly cast concrete. It is done by hand travelling or by rolling with a smooth steel tube on the compact concrete or by tamping with the edge of long wooden floor. In the case of hardened concrete. It is done by point tooking or by grinding the surface when soft aggregate are used.

 

Production Tolerance:-

 

By production tolerance use mean the limiting value of admissible deviations in the actual dimensions. The deviations may be caused.

 

1.

 

2.     Loose fitting of joints.

 

3.     Joints and mould sides under pressure of concrete. The limiting values are

 

1.     Length  10mm

 

2.     Width  3mm

 

3.     Thickness  3mm

 

4.     Flatness - 1/300 of the length. Planning of precast concrete works:-

Requirement of space and facilities. Space for production :

 

It is based on height and no moulds horizontal moulds -larger areas - vertical battery moulds -least amount of floor space. Height of the casting shed is based on space required. To lift and move one precast corpponent over another. The head room can be sufficiently decreased by having moulds in pits. Extra space must be provides for making the reinforcements stages cleaning and up the demoulded units.

 

Space for storage yard:

 

This depends on the daily output and the demand. The space must be sufficient to store minimum of 3 weeks production. The storage yard may be preferably aligned with the casting shed to facilitate movement of overhead cranes.

Space for facilities:

 

This depends on the type & single of ancillary facilities required namely storage of raw materials such as cement, coarse aggregate & fire aggregate, reinforcement steel. Conc batching plaint, fitters and joiness shop, Boiler and compress house. Laboratory & Office.

 

Modular co-ordination, standardization and Tolerance

 

Basic Dimensions :

 

This is the dimension between the axis defined by the dimensional grid. The dimensional grid is the two dimensional co-ordinate system of reference line defining the layout of the building.

 

Nominal or Theoretical Dimensions :

 

It is the planned dimension of the prefabricate arrived from its basic dimension and its

 

joints.

 

Actual Dimensions :

 

It is the dimension of the prefabricated when produced and its differs from the nominal dimension by the production discrepancies which are unavoidable.

 

 

 

The tolerance is the sum of acceptable positive and negative discrepancies of actual dimensions from the theoretical one. The limits of tolerance are based on the manufacturing and erection requirements.

 

Modular Co-ordination :

 

If the inter dependent arrangements based on the Pre Fabrication and System Building

 

Definition

 

Pre-fabrication means that the structure is disunited in its members and these are precast either in factories built and equipped particularly for this purpose or in temporary plants established n the site. Then the precise reinforced concrete members are shifted to the place where they are to be used, here they are hoisted, set into their final places, and assembled to form a complete structure.

 

Stages involved in pre-fabrication

 

1.     The structure divided into number of units.

 

2.     The different units are precast in permanent factories (plant fabrication) or temporary plants (site prefabrication).

 

3.     Transported to the site.

Hoisted set into their final places and assembled to

1.     Partial or total saving of material used for scaffoldings.

 

2.     Mulitple use of shuttering

 

3.     Possibility of far more accurate and better work manship.

 

4.     Working time can be shortened.

 

5.     Fewer expansion joints are required.

 

6.     Interruptions is connection can be omitted.

 

7.     The work can be carried out with a high degree of mechanization.

 

8.     Requriements in man power decrease.

 

9.     Re use of the members.

 

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