Construction Cost Estimating


A Study of Precast Concrete

Precast concrete is used to define individual concrete members of various shapes which are designed in a way that would fit in parts of the total structure. Before placing them in the desired position of the structure they are constructed in separate forms. Whereas, the word cast-in-situ concrete is used to define concrete members, cast or poured, in formwork set up at the final position of the structure.

The new technique called prefabrication construction of which precast concrete is a part.

Stated below are the advantages of precast concrete over cast-in-situ concrete:

• In case of casting identical members, it is advantageous for the same framework to be used.
• Superior quality of concrete can be produced as better-quality control check can be practiced in the factory or yard site.
• With this process smoother exposed surfaces can be produced which might not require plastering.
• With adequate protection in the casting yard precasting work can be carried out under all weather conditions.
• It is not difficult to make precast unit according to the required shape.
• If necessary, they can be dismantled without any breakage.
• The curing process can be completely more effectively and economically.
• Under all weather conditions constructions can be erected.
• Construction of a building can be done at a great speed.

Precast Concrete: Disadvantages

  • For handling, transportation and erection special types of equipment are required. Especially for beams, slabs, columns, etc.
  • Due to the requirement of skilled labor the cost of construction may increase.
  • During loading, transporting, and erection there is a chance of damage.
  • It is difficult to combine different units and to make them functional especially if different types of units are involved.
  • Modification in the future to the building constructed using precast units is difficult.

Precast Elements: Dimensions

Given below are the preferred dimensions for different elements of a building:

• Flooring and roofing scheme- Precast slabs along with other precast structural flooring unit:

  • Length: the normal length must be in multiples of 3M.
  • Width: the normal width must be in multiples of 1M.
  • Total thickness: The overall thickness must be in multiples of M/4.
  • Beams: (1) Length: The normal length must be in multiples of 3M. (2) Width: The normal length must be in multiples of M/4.
  • Total depth: The overall depth of the floor zone must be in multiples of M/4.
  • Columns: (1) The overall depth (which is floor-to-floor or the clear height) must be in multiples of 1M for height which is up to 2.8M, for height above 2.8M it must be in multiples of 2M. (2) Lateral dimension: The average lateral dimension or diameter of columns must be in multiples of M/4.
  • Walls: The thickness of wall on an average must be multiples of M/4.
  • Staircase: The normal width of stairs should be in multiples of 1M.
  • Lintels: (1) length: The minimum length should be in multiples of 1M. (2) The minimum width should be in multiples of M/4. (3) The minimum depth should be in multiples of M/4.
  • Sunshades Projection: (1) The minimum length must be in multiples of 1M.

The usual methods of finishes for these components are stated below:

• Molded concrete surface for design.
• Finishes are obtained by washing, tooling, grinding and grooming of hardened concrete.
• During casting laid-on finishing tiles are fixed.
• The exposed aggregates are present in-situ.
• The finishes are added in-situ.

What to keep in mind for the production of precast concrete elements.

Stated below are the factors to be considered for the effective production of precast concrete:

• Storage facilities
• Proper transport facilities.
• The erection equipment.
• The availability of raw materials.

In centrally located factory the manufacturing of precast concrete units can be done or in a site with a precasting yard set up or close to the site of project.

1. Factory Prefabrication: It is adopted in a centrally located plant to manufacture standardized components for a long period of time. The product unit works throughout the year. The production must take place under closed shed so as to avoid the effects of seasonal variations. The usefulness of the latest manufacturing technique must be considered. Furthermore, it is important to have room for the possibility of introducing enhanced technology. To make sure that continuous production is taking place work must be organized in factory-like manner with the support from a team of workers.

Factory prefabrication- disadvantages:

• Extra money spent on transportation.
• The size and shape of the prefabricated component along with road contour can lead to limited availability of transportation.
• A huge establishment is caused by the employment of organized labor and their services.

2. Site Prefabrication: The components are produced at the site or near the location of the project in site prefabrication. Such a type of precast concrete manufacturing is taken up when there is a specific job which must be completed within a short period of time.

The locally available labor force is made to use. Renting of equipment and molds take place. The work is usually carried out in an open space.

Site Prefabrication: Disadvantages

• Quality control cannot be performed due to lack of arrangements.
• Weather conditions can disrupt the work
• A high degree of mechanization cannot be adopted.

3. The process involved in manufacturing: The process involved in manufacturing consists of the main process which is supplemented by an auxiliary process and a subsidiary process.

• The main process- What involves the main process:

  • Providing and assembly of the mold.
  • Placement of a reinforcement cage in position upon the reinforced concrete work. Also, the wires are stressed in the case of prestressed concrete elements.
  • Tubes are fixed and inserted wherever required.
  • Placement of concrete in to the molds.
  • Vibration of the concrete and finishing.
  • Forms are demolded, the units are removed, stacking the products for the curing process.
  • Completion of curing. Steam curing is preferred.

• Auxiliary process: These are the activities which affects the successful functioning of the main process:

  • Mixture of ingredients in the right proportion and the manufacturing of concrete which is completed in a mixing station or by a batching plant.
  • The prefabrication of a reinforcement cage is performed in a steelyard or workshop.
  • Finishing items required for the precast units such as manufacturing od inserts are completed.
  • The finishing of the precast products.
  • Arrangement for the testing of the products are done.

• Subsidiary process:

  • Storing all the ingredients for concrete and other materials.
  • Arrangement for the transportation of cement, sand, and aggregates.
  • Transportation of the green concretes and reinforcement cages to the molding yard.
  • Finished products are transported to the stacking yard.
  • All repair works, maintenance tools, machines etc are undertaken.
  • Arrangement for steam curing process.

4. Overall management:

  • While manufacturing precast the speed and economy for it must be ensured, employment of a cyclic technological method must take place.
  • The mechanization must be improved so as to improve quality and increase productivity.
  • For the quality elements produced optimum production must take place.
  • Speed must be maintained to meet supply.
  • For people working on the job working conditions must be improved.
  • Minimum interruption of production must take place during weather changes or other such reasons.

5. Methods of Manufacturing: The standard method and the flow method are the two methods to manufacture precast concrete:

In the case of standard method, the molds are fixed and remains in one place. The various processes involved are then carried out in a cyclic order which takes place at the same place.

Whereas in the flow method, there is a movement in the precast units where they move from one place to another in a cyclic order and according to the processes involved in the work that is to be done.

Types of Precast Concrete Units

Stated below are the different types of precast concrete units:

1. Hollow Concrete Blocks: This type of precast concrete unit is manufactured in different shapes and sizes. The blocks of concrete are molded in a machine. The standard size of blocks which are used for different works are stated below:

(i) Hollow concrete blocks – 39 cm × 19 cm × 30 cm.
(ii) Hollow building tiles – 39 cm × 19 cm × 20 cm.
(iii) Hollow concrete blocks for partition walls – 39 cm × 19 cm × 10 cm.

5cm or more is maintained as the face thickness of the blocks.

For the construction of walls and columns the standard hollow blocks are used. The overall length and height of the walls must remain fixed to ensure maximum economy during construction and also to make full use of the full and half-length blocks.

To decide on the height of the door and window openings the availability of the size of hollow blocks must be considered.

For corners, sills, lintels, and jambs, wall closures, piers between the doors and windows, etc. specially shaped blocked are produced.

2. Lintel units: For windows, door openings, and passage openings prefabricated RCC lintels can be used. By using precast lintels the speed of construction indirectly increases as the curing process is eliminated.

This type is preferred for small spaces up to 2meters, usually cast in wooden molds and are cured prior to the usage.

It must be made sure that the marked face of the lintel is kept on the top or upper side while laying down the precast lintel case.

The top of all precast lintels is painted with tar or paint so that they can be distinguished from the bottom part. This is important and if the position of the reinforcement is wrong it can lead to structural failure.

3. Door and Window Frames: Prefabrication of doors and windows can take place. Through the frames of the doors or windows steel bars of about 4-40mm in diameter are run through.

For fixing hinges suitable hardwood blocks can be used. The concrete is made to vibrate using a table vibrator and it is then poured into the mold which ensures durability of the mix.

4. Roofing and flooring elements: For roofing and flooring prefabricated reinforced concrete are used instead of wooden battens. Precast reinforced and prestressed concrete trusses might be used for the sloping roof.

For the roofing and flooring of flat roofs plain precast concrete tiles are used.

5. Precast Shell Units: Plain or lightly reinforced concrete can be used in form of precast shell roofing for which a suitable frame must be made so as to cast the shell unit.

6. Sunshade Unit: With 75mm at the cantilever and tapered upwards to the depth of lintel sunshades are made. They come monolithic along with lintel.

On an average the projection will be 60-90cm. 8mm rods can be used as the main rods for the cantilever of the sunshade with a 10cm centers with 8mm distributors at 15cm centers.

A Study of Precast Concrete