Aconcrete slabis a common structural element of modern buildings. Horizontal slabs of steelreinforced concrete, typically between 100 and 500 millimeters thick, are most often used to construct floors and ceilings, while thinner slabs are also used for exterior paving.In many domestic and industrial buildings a thickconcreteslab, supported onfoundationsor directly on thesub-soils, is used to construct the ground floor of a building. In high rise buildings andskyscrapers, thinner,pre-cast concreteslabs are slung between thesteelframes to form the floors and ceilings on each level.On the technical drawings, reinforced concrete slabs are often abbreviated to " r.c.slab " or simply " r.c. " .In construction , a concrete slab may beprefabricatedorin situ. Prefabricated concrete slabs are built in a factory and transported to the site, ready to be lowered into place between steel or concrete beams. They may be pre-stressed (in the factory), post-stressed (on site), or unstressed. It is vital that the supporting structure is built to the correct dimensions, or the slabs may not fit.In situ concrete slabs are built on the building site usingformwork- a type of boxing into which the wet concrete is poured. If the slab is to be reinforced, therebarare positioned within the formwork before the concrete is poured in. Plastic tipped metals, or plastic bars chairs are used to hold the rebar away from the bottom and sides of the form-work, so that when the concrete sets it completely envelops the reinforcement.

CONCRETE SLAB ONE WAY SLAB TWO WAY SLAB

TYPE OF CONCRETE SLABS STIFFENED RAFT SLAB RIBBED SLAB SOLID BLOCK SLAB

Concrete Slab

Concrete slab is known as a common structural element in building constructions. The design of solid slab is similar to the beam design but have several exception such as : Compression reinforcement is not required Shear reinforcement is not appliedA unit breadth of 1m is applied in calculation. A slab is structural element wk=hose thickness is small compared to its own length and width. It is usually used in floor and roof construction. According to the loads transferred to supporting beams and columns, slabs are classified into two types, which are : -

One-way Concrete SlabOne way slab is a beam or wall supported on two sides. Its used for medium heavy loads over short spans. Depth of this slab is approximasely 4% of span [3(76mm)] .Spans : 6 to 12(1.829 to 3.658) One-way concrete slab is where the loads distributed in one direction It supported only along its long sides The main reinforcement runs only in one direction also The types of the one-way slab are solid slab and ribbed slab On-way slab involve in control flextural cracking and deflection

Two-way Concrete Slab

Two way slab is a beam or wall supported on four side.Bay proportions should be as nearly as square as possible.The depth is slab perimeter/180 and the spans 15 to 40.

Two-way concrete slab is where the load distributed in more direction It supported all sides The main reinforcement runs in two direction The types of slab are flat slab and waffle slab system Two-way slab involve in control deformation and assure shear and flextural strength

TYPE OF CONCRETE SLABS

STIFFENED RAFT SLAB

The stiffened raft is a slab-on-ground, although it is embedded partly in-ground, with edge beams and internal beams which are poured as an integral part of the slab. The design of the beams allows for the prevention of sagging, (edge heave), and hogging, (centre heave). For stable sites (classes A and S), the internal beams are not required.Similar to the slab on the ground, but also has pre-poured concrete beams set in channels through the middle of the slab, creating a kind of supporting grid of concrete on the base of the slab. Most slabs in Australia are stiffened raft slabs. The slabs may also need additional concrete footing embedded in the soil, depending on the soil type, and are generally suited to Class M, Class H and Class E soils. The stiffened raft slab is the simplest and most common slab construction available. The stiffened raft configuration can be used on all classes of sites (except problem sites - Class P).

Stiffened raft slabs consist of:-100 mm thick concrete slab-edge beams-internal beams (except Class A and Class S sites)-Steel reinforcement. The concrete is poured in one operation.

This slab construction technique was a very popular method in the past. Newer and more advanced methods have been found but this does not mean that the stiffened ribbed slab does not have a few strengths. Below are the advantages and the disadvantages of the stiffened slab technique: Advantages. Can is used to construct building slabs in sloping and uneven ground Is most suitable for complex and technical construction processes Has great flexibility and strength even in unsuitable ground Very suitable for class M and H sites Disadvantages. Uses more concrete in comparison to the waffle raft The concrete amounts used in the slab cannot be accurately controlled and monitored Cannot be used on unstable soils such as reactive clay due to its tendency to have its beams embedded in the clay It is labor intensive as it requires extensive digging of trenches so as to support the rib structure It is much harder and technical to build as it requires elaborate planning and equipment to execute It is expensive in comparison to the waffle raft slab It has lesser strength compared to the waffle slab, coupled with higher costs making it uneconomical to implement The construction process of the stiffened rib slab can only take place in suitable weather conditions unlike the waffle slab that can be done even in wet weather. Cannot be constructed on rock surfaces that are difficult to excavate and dig.

RIBBED SLAB

Introducing voids to the soffit of a slab reduces dead weight and increases the efficiency of the concrete section. A slightly deeper section is required but these stiffer floors facilitate longer spans and provision of holes. Economic in the range 8 to 12 m. The saving of materials tends to be offset by some complication in formwork. The advent of expanded polystyrene moulds has made the choice of trough profile infinite and largely superseded the use of standard T moulds. Ribs should be at least 125 mm wide to suit reinforcement detailing.

Advantages-Medium to long spans-Lightweight-Holes in topping easily accommodated-profile may be expressed architecturally, or used for heat transfer in passive cooling.

Disadvantages-Higher formwork costs than for other slab systems- Slightly greater floor thicknesses-slower

WAFFLE BLOCK SLAB

Waffle blocks are an eco-friendly thermal acoustic slab system combined of a moulded EPS block and cold rolled steel channel to form a coffer slab system for single or multi storey buildings. The system contains a priority high strength galvanized steel rib which supports the high density EPS waffle blocks that serve as void formers. It is completed by the placement of reinforcing bars between the waffle blocks in both directions and reinforcing mesh above the blocks. Concrete is poured to fill in the channels, encapsulate the rebar and mesh to form the structure of the floor slab.The beams which are created by the concrete infill and rebar in the gaps between the waffle blocks create a monolithic, two way spanning, coffer slab structure. The void formers are left in place and act as sound and thermal insulator. Due to the structural composition of the coffer slab spans up to 11 meters can be created, which is cost saving by using less props.

System application The floor slab is designed taking into account the span and load bearing requirements combining the variables of rebar size and configuration, concrete beam depth - which can be controlled by the waffle block depth, and the horizontal floor slab and reinforcing mesh. Support wall structures are built as normal. Steel ribs are placed uni-directionally onto the supporting walls. These are propped at 1,6 meter intervals to support the wet concrete and construction loading. Steel reinforcing is fitted between the waffle blocks in both directions. Steel reinforcing mesh is placed above the blocks on EPS spacers to position the mesh near the surface of the concrete slab. If needed, formwork is placed around the perimeter of the floor area and service ducts to confine the concrete pour. Concrete is poured to complete the slab. Ceilings can be either suspended or plastered with Polyplast, propriety EPS plaster system can be supplied by us.

Waffle blocks slabs can be used in the following application areas:- double storey residential- multi storey commercial buildings- hotels- flats, duplexes and apartment buildings- shopping centers- suspended ground floor slabs- renovations

SOLID BLOCK SLAB

Solid slabs - normally made of reinforced concrete but other materials are called plates. The thickness of plate depends on span (refer span/ depth ratio for slabs BS8110 for RC design) but 200mm is common in residential houses or buildings. Other type of materials such as timber and steel can also be used to make on floor. However they may have different spans and geometrical arrangement due to properties of these materials.Solid slabs are used as a bottom mold for in-situ floor casting. It require a temporary supports. After the hollow block is stopped off as like mention before, the solid slab is made out. This is done to achieve a greater shear strength, and if the slab is supported by a monolithic concrete beam the solid section acts as the flange of a T-section. The solid slab technique is a good choice when there are very heavy liads or lits of cross piping for instant in the floor, or when extra fire resistance is required. Therefore, this slab is suitable for a heaving loading structure such as warehouse. This slab is usually made under partitions and concentrated loads.

References

-Reinforced Concrete Design; 1999; W. H Mosley, J. H. Bungey, R. Hulse; fifth Edition; PALGRAVE-Design of Structural Elements; 2004; W. M. C. McKenzie; 1st Edition; PALGRAVE MACMILLAN-http://polybuild.org/Index_files/Page819.htm-http://www.digitalcanal.com/structural/flatslabspec.htm-http://en.wikipedia.org/wiki/Concrete_slab-http://www.ehow.com/about_5533555_types-concrete-slab-designs.html

-Cope, Robert.Concrete slabs: analysis and design.Essex: Elsevier SciencePublishing., 1984. Print.

-Book :Reinforced Concrete Design By W.H Mosley , J.H.Bungey & R.Hulse.

-Book : Structure Systems Architecture Engineering Design By Robert Brown Butler.

-http://www.concrete.org/General/f302.1(04)Chap3.pdf