Precast RCC Plank and Joist

The basic concept of the precast roofing system is to optimize the sizes of the planks and joists, thereby reducing the overall volume of concrete for the construction of slab The plank and joist roof is designed as a composite Beam (joists) and Panel (planks) roof in which the precast elements are assembled in a way such that the system behaves like a T-beam.  Since the.elements are cast separately and assembled, there should be proper shear connection between them to achieve composite action for the system to behave as an integral structural unit.. Typically, the beams span the smaller dimension of the room and the panels are placed side by side on beams and the two are joined together with connectors and in-situ concrete which binds the whole system. The concrete mix of the plank and joist shall be the equivalent of minimum M20 grade concrete.
The sizing of the precast elements is based on the following factors –

  • The size of the plank is governed by the requirement of manual handling of the plank – so the weight of the plank should not exceed 50 kg, which can be lifted easily by two persons. The volume of the plank is further economized by providing a haunch (of increased thickness) at the central cross-section of the plank which is also needed to resist stresses during handling and installation of the plank.
  • The width of the beam should be sufficient to provide bearing to two successive spans of the plank, leaving adequate gap between them for the in-situ concrete needed for a monolith slab. The minimum recommended width of the beam is 150 mm – provide a 40mm bearing to both planks and a 70mm gap between them. The recommended depth of the joist is 150mm for a clear span of upto 4.5 metres. The overall depth of the joist with in-situ concrete shall be minimum 210mm, including the flange of the T-beam formed with in-situ concrete.
  • The reinforcement design of the plank and joist is done as per the IS 456:2000 for plain and reinforced concrete. The reinforcement of joist must provide for shear connectors. Triangular stirrups are therefore recommended, which project out of the joist as shear connectors.
  • In case of continuous roof over a support, proper negative reinforcement must be provided over the junction of the plank and the support.

 


Construction

Production of Planks

Mould
Steel moulds are fabricated for casting planks, as per shape and dimensions of plank (Refer construction details). The
moulds can be used to cast the planks by manual compaction on hard platform

A smooth, levelled & hard platform should be prepared for casting of planks & joists. The platform can be made with 30 mm thick 1:3:6 cement concrete, finished smooth with neat cement, laid over a base of 75 mm thick bat cement concrete.

Prepare the reinforcement cage with 6 mm MS bars as per the size of plank.

Also, the concrete platform on which the plank is cast should be coated with oil as bond release agent or covered with GI sheets or smooth wrinkle free paper.

Compaction
These mould can be mounted on a vibration table to compact concrete for casting the plank. A vibrating table can be used if
the scale of construction justifies the cast of production infrastructure, such as in a housing project.

Inside surface of the mould is coated with shuttering oil and the prepared reinforcement is placed in the would. Care is to be taken so that clear cover to the reinforcement cage from the mould is 15 mm. Cast the concrete mix and by using a plate vibrator consolidate the concrete properly.

The concrete mix should be M15 grade – 1:2:4 cement:sand:10 mm aggregate. Lay the concrete in two layers and compact each layer thoroughly. The first layer is the base concrete 30 mm thick. The second layer is the remaining 30 mm concrete which tapers towards both ends of the plank.

After about half an hour of casting, the 2 tapering member may be demanded and the remaining would can eb lifted off in 2 hours after casting depending on the weather. The demanded cast unit should be titled along its longer edge after 24 to 30 hours of casting, depending on the weather. Transport the plank in vertical position to the curing yard.

 

Modular design

 

 

Before construction begins, the house design should be based on modular size of precast planks. Ensure that, following the correct construction practice of plank and joist, the roof can be constructed with a certain number of planks and joists.

 

 

Installing Beams

Before installing plank and joists, their accurate positioning for a given room size should be ascertained. To ensure this, the center points of all joists should be marked on the wall masonry before placing them. The length of planks and their adequate bearing on walls and joists should be taken into account before marking the centre points.

  

After marking the centre points, place the joists accordingly. Joists can be placed at the defined spacing over a cement concrete bed, which can be in the form of a roof band, if needed, as per engineer’s specifications.

 

If the cement  concrete bed is not provided, concrete blocks of size 1’x9”x3” should be placed in the wall to distribute the point load of joists.

The beam can be placed in three stages after their centre points have been marked. Erect scaffolding inside the room to support movement of beam.

Prop the beam with temporary support of a steel pipe/ bamboo at every 5 feet along the beam length till the time that all planks have been placed, all joints between adjacent planks have been filled and in-situ concrete has been poured. This is needed to avoid any sudden stress which might be applied on the joist by movement of labour till the roof is complete. Also ensure that the joists are placed exactly parallel to the walls. After marking the centre points, place the joists accordingly.

 

 

In-situ Concrete

 

Before pouring in-situ concrete over planks and joists, additional nominal reinforcement is placed. 4 MS reinforcement bars of 6mm dia should be placed with every plank – 2 bars are placed on both sides of the tapering concrete portion of the plank and 2 bars are placed parallel to the joists on both sides This reinforcement satisfies requirements for a normal residential building. 

 

  

Level the top surface of walls & joists with 1:4 cement sand mortar before placing the planks. Minimum bearing of joists over walls should be 100mm and for planks should be 40 mm over wall or joist.

 

 

After laying the planks, all joints between adjacent planks should be filled with a thick paste of cement sand mortar of ratio 1:4 to fix the planks in place.

Apply cement slurry over the precast joists and in haunch portions of the planks where in-situ concrete is to be laid. Cement concrete of 1:2:4 with 12 mm and down graded aggregate is poured over the joist and over the tapered portion of planks and finished in level with the top of the plank.

This completes the roof by tying together all precast components of the roof. The in-situ concrete should be cured for at least 7 days. Do not remove the prop of the joists before the curing period is over and the in-situ concrete laid in the roof slab has attained strength. After in-situ concrete has been poured and cured, the roof can be finished conventionally with a layer of terracing and water proofing.