Precast Concrete

Precast Concrete

- a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place.

Advantages of Precast Construction

1.     Mass production of standardized  and repetitive units.

2.     Least labor cost per unit due to mechanized series of production

3.     Use of unskilled labor.

4.     Less construction time

5.     Better quality control and higher strength of concrete.

6.     Construction free from the effects of weather conditions.

7.       Very rapid speed of erection

8.       Entire building can be precast-walls, floors,beams,etc.

9.       High quality because of the controlled conditions in the factory

1.   Prestressing is easily done which can reduce the size and number of the structural members.

Disadvantages of Precast Construction

1.       Very heavy members

2.       Camber in beams and slabs

3.       Very small margin for error

4.       Connections may be difficult

5.       Somewhat limited building design flexibility

6.       Because panel size is limited, precast concrete can not be used for two-way structural systems.

7.       Economics of scale demand regularly shaped buildings.

8.       Need for repetition of forms will affect building design.

9.       Joints between panels are often expensive and complicated.

10.   Skilled workmanship is required in the application of the panel on site.

11.   Cranes are required to lift panels.

Types of Precast Structure

1.     Wall Panels

-       this type of precast structure has numerous designs depending upon the architectural requirements. The common shapes produced for one to four storey high structures are sections having a width of up to 2.4 meters. They are used as curtain wall attached to columns and beams or sometimes as bearing walls.

The different types of wall panels are:

a.     FLAT TYPE

b.    DOUBLE TEE TYPE

c.     RIBBED TYPE

d.    WINDOW OR MULLION TYPE

-       to improve the thermal insulation of the panel, foam glass, glass fibers or expanded plastic is inserted between two layers of lightweight concrete adequately bonded and interconnecting the two layers to act as one unit. Stresses in handling and erection of the member is more than that of the finished field structure, hence, control of cracking is of great consideration.

2.     Roof and floor Members

-       are made in wide variety to suit the different conditions such as span, mafnitude of load, fire ratings and appearnce.

a.     Flat slab

- is usually 10 cm thick but sometimes as thin as 7 cm. when used on several continous span having widths that ranges from 1.20 m. to 2.40 m. with length of up to 11 m.

                b.      Hollow Plank/ Hollow Core slab

– is a lightweight member that covers a longer span made by extrusion in a special machine with thickness that ranges from 10 to 20 centimeters and widths from 0.60 m. to 1.20 m.; used on roofing having spans from 5 m. to 10m. and on floors with 3.50 m. to 7.00 m. span which could be augmented to 9.00 meters when 5 cm. Topping is applied to act monolithically with the hollow planks.

c.       Double Tee

- are the most widely used shapes for longer span having depth from 4-6.50 m. generally used on roof having span up to 18 meters when a topping of at least 5 cm. Is applied to act monolithically with the precast members. It could be used on floor up to 15 m. span depending upon the load and deflection requirements.

d.      Single Tee

– is used for roofing having a span of up to 30 meters and more, The flange of the tee constitute the floor or roof slab.

3.       Precast Beam

-       The shape of precast beam depends pon the manner or framing and the various shapes are:

a.       Rectangular beam – where the floor and roof members are supported on top of the beam.

b.    Ledger Beam –is designed to reduce the height of the floor and roof construction.

c.     L-beam – to provide bearing, the beam is designed in a form of L.

d.    AASHTO Bridge Girder – was named after  the Association of State Highway and Transportation Officials.

4.       Precast Column

-       sizes are from 30 cm to 60 centimeters.

-       columns are continous up to 4 storey wherein corbels are used to provide bearing for the beam.

-       Is sometimes used directly to support double tee floor members without the use of intermediate members.

Posted in: Precast Concrete