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1. What are the benefits of hollow core slab/ Plank?

Hollow core slabs provide economical, efficient floor and roof systems. Structurally, a hollow core slab provides the efficiency of a pre-stressed member for unsurpassed load capacity, span range, and deflection control. In addition, the grouted slab assembly provides a basic diaphragm for resisting lateral loads. Excellent fire resistance is another attribute of the hollow core slab system. Used as floor-ceiling assemblies, hollow core slabs have the excellent sound transmission characteristics associated with concrete.

2. How much would hollow core slab/ plank cost for my project?

Hollow core is an engineered product and each project has its unique design requirements that have to be met to achieve a proper design. For more detail Refer our PDF on websites.

3. How far can I span a hollow core slab?

Under typical residential loading conditions & Based on 4.7 KN/m2 (100PSF) design load: -150mm slab/ plank can span 18 to 20 feet, 200mm slab/ plank can span up to 27 to 30 feet, 250mm slab / plank can span up to 35 to 38 feet, 300mm slab / plank can span up to 38 /42 feet and 380mm slab / plank can span up to 45 to 50 feet. For more detail refer our Hollow core slab specification / load tables as a guide. One should consult an Architect & Structural engineer, but the load table give by us can be used as a rule of thumb. Residential loading is 2.15 KN/m2, Offices are 4.788 KN/m2 General assembly spaces are 4.788 to 7.18 KN/m2, & Garage floors are generally 2.39 to 2.89 KN/m2.

4. How is the hollow core erected?

Qualified skilled persons should erect hollow core slab with the help proper safely handling lifting devices and Cranes tonnage capable to lift it. The slabs are to be positioned and connected to the structure in accordance with pre-caster’s drawings and details. The slabs are erected one at a time with slings and spreader bars specifically suited for hollow core.

5. How do I achieve widths other than 1200mm?

Slab / Plank can be split into two to achieve any size as per design width. Just because the slab/plank is produced in 1200mm wide sections, does not mean that you must design in a 1200 mm module. The saw cut edge of the split is generally hidden over the adjacent non-load bearing wall or above the drop ceiling.

6. Where & How can I cut holes for plumbing etc. in the slab /plank?

Care should be taken that the pre-stressed strand is not cut while making openings. Generally, the openings should be lined up in the core or void area of the slab /plank or situated to minimize the cutting of the pre-stressing strands. All holes should be placed exactly as per shop floor drawings, so that the openings can be properly engineered.

Now Cutting of small openings / holes can be core drilled or with a zip saw, though larger openings are cut longitudinal to cores with concrete saw. It is important to cut the holes only after the plank has been erected and the keyways are grouted & cured. If the openings are the entire width of the plank 1200mm, they can be framed off with a steel header. Rectangular and round openings can be core drilled by concrete drill etc. Once again, care should be taken to reduce the number of strands that are cut. Openings may be provided in hollow core systems by forming openings in the plant, by installing steel headers, by shoring and saw cutting, or by cutting after a deck is installed and grouted. In laying out openings for a project, the least structural effect will be obtained by orienting an opening parallel to a span, or by coring small holes to cut the fewest pre-stressing strands.

An opening greater than 600mm x 600mm should be cast into 2.5 meter wide hollow core slab / plank. This would usually occur if the plank were a wet-cast product. The opening could be plant or field cut in an extruded or dry-cast product. If the size of the opening exceeds the safe limitations of cast in the use of a structural header is used to create the required opening.

Openings less than 600mm can be cast in or field cut in the plank providing that all of the location and size is provided to the pre-caster prior to manufacturing. The general contractor must coordinate with all trades involved to get information to the hollow core manufactured during the shop drawing phase so that additional reinforcement can be designed into the plank to carry the required design loads. An opening smaller than 10" square cannot be cast into a plank due to production limitations.

Please do not cut pre-stressing strand in the plank without the approval of the hollow core manufacturer.

7. Can I utilize the Core / voids to run electrical and mechanical conduit through the slab?

Electrical and ductwork can be considered. This will require early coordination with the producer. In theory, hollow core plank may be used for electrical and mechanical runs when properly coordinated for alignment. However, in reality this is cost prohibitive. Co-ordination with other trades as well as schedule pressures often limits the use of hollow core for an internal conduit system. The most efficient design detail is to build the hollow core slab ends into the wall. This butt joint includes grout filled into the cores. In addition to the proper alignment of the voids, they must also be plugged to prevent the flow of grout material in the core. If the grout flows more than 100mm back from the bearing wall, the area where access is needed becomes solid.

In order to snake the conduit through the slab / plank, the contractor needs to drill up into the core, preferably in the space between the dry wall and furring. This is typically where the wire is run and will be hidden from view. Another deterrent is fastening the electrical box into the core. If not securely anchored, the box will not carry the weight of the light fixture so toggle bolts must be installed for that purpose. Also, the electrical wire should be encased in metal tube through the length of the cores.

8. How do I connect hollow core?

At end bearing walls specified size rebar extends from the key way joints and laps with the wall or bond beam reinforcing. At walls parallel to plank spans, bent bars are placed into notches or pockets cut into the side of the plank and then lapped or anchored to the wall or bond beam reinforcing.

9. How do I connect to steel?

Standard industry practice is to weld alternating ends of the slab /plank to the steel beam. This welded connection provides sufficient stiffness to brace the steel beam and reduce the un-braced beam length. These plates can also transfer lateral loads from the slab system to the lateral supporting system for the building.

10. How does the non-load bearing curtain wall connect to the Hollow core slab/plank?

These systems can be connected using expansion anchors or screws directly into the slab /plank or by welding to embed plates, which can be provided on the top or bottom surface of the slabs.

11. What is Camber?

It is the upward deflection created by the forces of the pre-stressed strands located below the center of gravity of the plank. This pre-stressing force is a function of the project-required loads and spans. Camber is inherent in all pre-stressed precast products & cannot be designed to an exact number.

Slabs / Planks with dissimilar lengths, strand patterns or openings adjacent to each other will have camber differences. The benefits of camber and pre-stressed concrete products is that it allows for longer clear spans and higher load carrying capabilities with shallower depths of section than what could be achieved with conventional building materials.

12. Can I put a column right on the Hollow core slab /plank?

Structural engineers do need to analyse each case of unsupported loads and will give you a specific recommendation on your project. Not all columns, point loads, and wall loads need to be supported from underneath. Plank is surprisingly strong and in some cases can support many types of loads.

13. How do I get started in my hollow core design? What primary factors should I take into consideration for my project?

One Need to know your loads and fire rating. The primary consideration in developing a framing scheme using hollow core is the span length. They can help you optimize the span length and slab thickness. There are also many other things to consider when planning your hollow core project. For more detail refer our Hollow core spec. chart / Load tables as a guide.

Hollow core plank are typically considered with a span-depth ratio of 45 for floors and roofs when fire endurance, openings or heavy or sustained live loads do not control the design.

One should also take into consideration factors, which affect the slab thickness selection for a given span. Heavy superimposed loads would require a lower span depth ratio. For example, heavy partitions, or a large number of openings typically mean higher load capacity requirements.

Once slab thickness and spans are selected, the economics of the layout becomes important. Hollow core plank ends can be cut at an angle but it is more efficient to have the bearing perpendicular to the span so square cut ends should be used whenever possible. Again, consult your Architect / structural engineer for assistance with your bay spacing and to minimize partial width slabs. While it is desirable to have the plank dimensions fit the bay size or slab module 1200 mm, subsequently can cast filler pieces to use as partial width slabs.

The Architect / structural engineer would typically provide information on the drawings showing the span directions, loading requirements, connection information, fire resistance requirements, topping requirements (e.g. leveling course or composite topping) and of course all the openings you will need with their location and sizes indicated.

14. Should hollow core plank be used in an exterior application like a balcony?

Answer is No. As the water will enter into the Cores of hollow core slab & reduce the strength of slab. Hollow core plank is designed for interior applications; if your project will be open to the environment for extended periods of time make sure you consult your Architect for sealing the cores with Grout etc.

15. How can I finish off the underside of the plank?

In India 90% of residential & commercial building are made with false ceiling, so one need not to worry for finish. However the plank underside can be finished to provide a durable, maintenance free ceiling for the room below with a suitable medium to heavy textured / POP or acoustical material. There are also flat paints available on the market that can be applied for a matte finish.

16. How do I finish off the top of the plank prior to carpet?

The plank can be finished with a 50mm structural concrete topping. You can also use a concrete leveling material or latex finishes. The type of topping for each application depends on the finished flooring material specified. When planning for your topping slab allows for camber, elevation of door openings; and building conditions, which have, slab/ plank 90 degrees to each other.

17. What is the specification for structural topping?

Where structural topping is required, a minimum 50mm concrete topping supplied and installed by the general contractor or others must be bonded to the plank. Topping must be designed for a 28-day strength of 4,000 psi. When placing concrete topping, precautions must be taken to thoroughly clean the plank and thoroughly wet the top surface. Topping must be thoroughly cured. It is further recommended that wire mesh be used in the concrete topping. Mesh size to be determined by architect/engineer. Note that 50mm minimum thickness topping is measured at mid-span (high point of camber) and additional concrete may be required at the perimeter to level the floor.

18. How do I determine the fire resistance of hollow core plank?

A fire rating is dependent on equivalent thickness for heat transmission, cover over the Pre-stressing strand for strength in high temperatures and end restraint. A typical 200 mm thick plank has a 1.5 hour fire rating. For a higher rating a concrete topping would be applied or spray-on fire resistant material can be added to the underside of the plank.

19. What type of grout should be specified?

When water, sand and cement are mixed together without coarse aggregate, the result is called grout, mortar or dry pack, depending on consistency. These grout materials have various applications: sometimes only for fire protection; or cosmetic treatment; other times to transfer loads through horizontal and vertical joints. Except for special situations, keyway grout is normally a sand and (Portland) cement mixture in proportions of 3:1. The amount of water used is a function of the method used to place the grout but will generally result in a wet mix so keyways may be easily filled. Rarely is grout strength required in excess of 2,500 psi for vertical load transfer. Non-shrink grout is not required for satisfactory performance of hollow core slab systems and is cost prohibitive.

20. How do I anchor hangers in the bottom?

Anchors can be drilled and sleeved into the bottom of the plank. Different connectors can withstand varying degrees of pullout force. Drilling through the plank and bolting completely through the plank can achieve heavier hanging loads.

21. What happens when water enters the cores?

During erection the hollow core plank is washed down to prep for the grouting process. Water is also used in saw cutting and core drilling openings in the slabs. In order to remove the water before it becomes a problem in the finished structure, drill small weep holes in the bottom of the core (typically done in the field). This will allow the water to drain in the field during these construction stages. Then fill the holes with a grout.

22. Is it common practice to weld alternate ends of the hollow core plank? When is this practical?

Connections are required in hollow core slab systems for a variety of reasons. Most are for localized forces. Connections are an expense to a project and, if used improperly, may have detrimental effects by not accommodating volume changes or other movements that occur in a precast structure. Connections may develop forces as they restrain these movements. In specifying connections, the actual forces must be addressed. If no force can be shown to exist, the connection should not be used.

The precast engineer often makes recommendations to the engineer-of-record regarding industry common practices. Typically, for buildings over 100-feet in length, with hollow core set on structural steel, the precast floor system would require only one hard connection. This is accomplished by specifying a welded connection at opposite or alternating ends of the plank. The entire slab is then grouted in the normal manner. This allows the thermal movement in the floor slab to occur without damage to the embedded weld plates. This practice makes sense for a structural steel frame, but not for hollow core bearing on masonry, nor total precast concrete systems. The practice is to avoid over-restrained systems for large footprints.

It is always a good idea to discuss your concerns with the precast manufacturer before production and construction begins.

23. What advantages are there to precast beams and columns?

Precast beams and columns are a cost effective way of providing a fire resistant concrete system that can be used for parking garages, offices, dormitories or almost any construction project. A complete precast concrete beam, column, and hollow core plank can be used in place of any other structural system.

24. What advantages are there to a precast wall and slab/ plank system?

Precast walls and plank (or in conjunction with beams and columns) can provide the entire structural shell of most any building. Wall bearing with precast walls is a fast and economical means of designing a load bearing wall system. The precast walls are ready for textured paint or paper and backer. Also, the electrical conduit and boxes can be cast into the walls during the pre-casting of the units.

25. Can Hollow core be cantilevered?

Yes, hollow core plank can be cantilevered. This will require a special design and it is best to consult with the manufacture. Precast Hollow core cantilever planks used for balconies are not recommended a solid slab would be a better solution for this condition.

26. Why is concrete so heavy?

In fact, concrete is actually 31% of the weight of structural steel. Usually Concrete weighs 2400 Kgs. / cubic meter & steel is 7840Kgs/ cubic meter. In office structures, a completed concrete structure (floors, walls, roof, beams, columns etc.) could be as much as 19% lighter than a composite steel frame and concrete slab building.

27. How much plank can be install in one day?

One can install hollow core slab /plank at the rate of 250 sq. meters per day per team. This rate depends on site access, crane and other factors.