Reinforced Concrete Floor : Cast in-situ vs Precast

The disadvantage of cast in-situ concrete floors are :

• The need for formwork
• The time taken for the concrete to cure before the formwork can be released for reuse and the floor made available as a working area
• The very small contrubution by a large proportion of the concrete to the strength of the floor

Before any system of precast concrete flooring can be considered , the following factors must be taken into account :

1. Maximum span
2. Nature of support
3. Weight of units
4. Thickness of units
5. Thermal insulation properties
6. Sound insulation properties
7. Fire resistance of units
8. SPeed of construction
9. Amount of temporary support required

The advantages of precast concrete floor are :

1. Economies that can reasonably be achieved through reduction in the volume of concrete , the weight of reinforcement and the size of foundations
2. Elimination of the need for formwork
3. Curing time of concrete is eliminated therefore the floor is available for use as a working platform at an earlier stage
4. Superior quality control of product is possible with factory produced components

The disadvantage of precast concrete floor when compared with cast in-situ concrete floor are :

• Less flexible in design term
• FOrmation of large openings in the floor for dust , shafts and stairwalls usually have to be formed by casting an in-situ reinforced concrete floor strip arounf the opening position
• Higher degree of site accuracy is required to ensure that the precast concrete floor units can be accomodated without any alrerations or making good

Precast concrete beam and Block floor

Advantage and disadvantage

• Service can be housed within the constructional concrete topping , or within the voids of the filler blocks
• Advantage of this floor is that two workers can safely handle the units without the need for lifting gear
• This comparatively cheap floor system provides reasonable resistance to airborne sound and resistance to fire.
• This type of reinforced concrete floor is olny suitable to be used for small spans and loads

Precast concrete beams and block floor 2

• THe T beams are placed at 270 mm centres with their ends bearing 1.2 on or into brick walls
• The hollow blocks are then placed between the beams
• The floor is finished with a layer of construction concrete topping , 50mm thick spread and levelled ready for a screeed or power floated finish
• The purpose of the constrictional concrete topping is to spread the loads on the floor over the blo ks and column and beams
• THe underside , or soffit , of the floor is covered with laster or will provide support for a suspended ceiling .
• WAll taised to acts as formwork for concrete
• 1B wall raised after concrete has set
• Hollow concrete block and between beams
• T section pre cast concrete beams
• eams built 1.2 into walls

Precast concrete Beam and block floor

• This type of reinforced concrete floor is much used for comparatively small spans and loads
• For floors that need a greater bearing capactiy , the beam or block floors can be finished with a constructional concrete topping
• The constructional concrete topping ties the beams and blocks together making a composite floor .
• Solid reinforced concrete beams generally shaped like an inverted T in section are precast in the manufactures factory
• Depth of the beams caries from 130 to 250 mm and they are90mm wide at the bottom
• The beams are made in lengths up to 6.0m
• Precast hollow blovks are made lightweight for lightness in handling and to reduce weight of the finished floor
• These blocks are usually 225mm wide , 130 to 250 mm deep and up to 225mm long

Hollow Concrete Beam Floor

• Hollow reinforced concrete beams are precast in rectangular secton
• Concrete walls of these beams are from 15 to 20mm thick with the steel reinforcement cast in the lower angles of the beam .
• the sides of the beams are indented to provide a key for the concrete topping .
• The beams are usually 355mm wide, from 130 to 205mm deep and up to 6m long
• The depth of the beam depends on the superimposed loads and the span
• Because of their length and weight , lifting gear is necessary to raise and lower the beams into place
• The beams are placed side by side with the edge joints being grounted together and their ends bearing 1/2 B on or into brick load bearing walls
• IF the ends of the beams are built into walls , the ends should be solidly filled with concrete because the hollow beam is not strong enough to bear the weight of heavy brickwork
• THe wall of the beams are made thin so that they are light in weight for transporting and hoisting into position
• When beams have been placed , brickwork is raised to act as formwork for concrete
• !b walls
• ENds of beams built in must be filled with concrete
• Constructional concrete topping 50mm thick on top of beams
• Pre-cast hollow reinforced concrete floor beams
• The thin walls of the beams are not strong enough to carry the direct weight of furniture , a layer of constructional concrete topping usually 50mm thick , is spread over the beams to distrubute point loads
• THe constructional concrete topping is an integral part of this floor systemn
• THe concrete is miced on the buildin site and is spread and levelled on top of the beams
• THe hollow beams give a flat soffit ,which can be left in its natural state or be given a skim coat of plaster
• The voids in the units can be used to house the services

Advantage and disadvantages

• The units are self-centering , therefore temporary support is not required
• THe conctruction period is considerably shorter
• Generally the overall weight of floor is less
• THey are not considered suitable where heavy points loads are encountered unless a constructional concrete topping is used to spread the load over a suitable area

ALternative sign

Hollow terracitta block floor

Advantages and disadvantages

• The main advantages of this floor are its light weight , which is generally less than comparable floors of concrete construction , and its relatively low cost.
• Advantage of this type of floor is its good resistance to damage by fire , and it is sometimes termed "fire-resisting reinforced concrete floor '
• Considerable labour is involved in placing the hollow TC blocks , reinforcement and temporary support .

Avantages and disadvantages of reinforcement concrete

1. The centering required to give temporary support to a monolithic reinforced concrete floor tends to obstruct and delay building operations
2. Such monolithic reinforced concrete floors are suitable to be sued for heavily loaded and specially designed construction and for stairs and small spans