Floors

Syllabus objective: Prepare working drawing showing various types of floor and floor section. Solid, Hollow, and suspended floor, floor covering, for example, tiles, screed

Solid Concrete Ground FloorThe construction of a solid ground floor can be considered under three headings: Hardcore Blinding Concrete bed or slabHardcore the purpose of hardcore is to fill in any small pockets that have formed during oversite excavations, to provide a firm base on which to place a concrete bed and help spread any point loads over a greater area. It also acts against capillary action of moisture within the soil. Hardcore is usually laid in 100-150 mm layers to the required depth, and it is important that each layer is well compacted, to prevent any unacceptable settlement beneath the solid floor. A very common choice of hardcore material in Jamaica is marl.Blinding This is used to even off the surface of hardcore if a damp-proof membrane is to be placed under the concrete bed or if a reinforced concrete bed is specified. First, it will prevent the damp-proof membrane from being punctured by the hardcore and, second, it will provide a true surface form which the reinforcement can be positioned. Blinding generally consists of a layer of sand 25-50 mm thick or a 50-75mm layer of weak concrete (1:12 mix usually suitable) if a true surface for reinforced concrete is required.

Concrete Bed Thicknesses specified are: Unreinforced or plain in-situ (cast on site) concrete 100-150 mm thick; Reinforced concrete, 150mm minimumThe reinforcement used in concrete beds for domestic work is usually in the form of a welded steel fabric (as seen below). Sometimes a light square mesh fabric is placed 25mm from the upper surface of the concrete bed to limit the size of any cracking of the surface. In domestic work the areas of concrete are defined by the room sizes since the floors are normally cast after the walls have been erected.

Protection of floors next to the groundTo prevent the passage of moisture from the ground to reach the upper surface of the floor a suitable barrier in the form of a damp-proof membrane (DPM) is placed within the floor. The membrane should be turned up at the edges to meet and blend with the damp-proof course in the walls to prevent any penetration of moisture by capillary action at edges of the bed. The position of the DPM, whether below or above the concrete bed is a matter of individual choice. A membrane placed above the bed is the easiest method form a practical aspect and is therefore generally used. A membrane placed below the bed has two advantages; first, it will keep the concrete bed dry and in so doing will make the bed a better insulator and, second, during construction it will act as a separating layer preventing leakage of the cement into the hardcore layer, which will result in a weak concrete mix.Damp-proof courses (DPC) the purpose of a DPC in a building is to provide a barrier to the passage of moisture from an external source into the fabric of the building, or from one part of the structure to another. DPC can either be horizontal or vertical. In floor construction they are placed below the ground level to prevent the entry of moisture from the soil and; those that are placed just above ground level to prevent moisture creeping up the wall by capillary action; is sometimes called rising damp. Suitable materials for damp-proof courses: Lead, Copper, Bitumen, Mastic asphalt, Polyethylene, SlatesSuitable materials for damp-proof membranes: Polyethylene, hot-poured bitumen, cold-poured bitumen, asphalt

Suspended Concrete Ground Floor

Advantages of suspended concrete ground floor: Potential to span over sloping sites that would require a great deal of filling to make up the ground to the specified floor level Application over movable subsoils such as shrinkable clay Suitable where ventilation under the ground floor is required to dilute intrusive gases. Use of quality-controlled factory manufactured components and simple site assemblyA minimum clear void depth of 75 mm is required below these floors. However, it is usual practice to leave at least 150 mm. Ventilation of the void is advisable to dilute and prevent concentration of gases from the ground and possible leakage from piped services.

Suspended Timber Floors

This is a more expensive form of construction than a concrete floor. They are susceptible to dry rot (fungus that attacks damp timber), and can be overcome by adequate ventilation under the floor and correct positioning of damp-proof courses to keep the under floor and timber dry. The usual method of ventilation is to allow a free flow of air under the floor covering by providing air bricks in the external walls. These are sited near the corners and are approximately 2 m centres around the perimeter of the building. If a suspended timber floor is used with an adjacent solid ground floor, pipes of 100mm diameter are used under the solid floor to convey air to and from the external walls to the suspended floor. Sleeper walls placed at 2000 mm centres will give an economic joist size The width of joists is usually taken as 50 mm The usual joists depth for domestic work is 125mm The most economic layout is to span the joists across the shortest distance of the room

Detail showing the construction of a honeycomb sleeper wall allowing air to circulate

Hollow Block and Waffle Floors

Precast concrete suspended floors are generally considered to be for light to medium loadings spanning in one direction. Hollow block and waffle floors can be designed to carry heavier loadings than single spanning precast concrete suspended floors. They are in fact ribbed floors consisting of closely spaced narrow and shallow beams giving an overall reduction in depth of the conventional reinforced concrete in-situ beam or slab floor.Hollow Block FloorsThese are formed by laying over conventional floor soffit formwork a series of hollow lightweight clay blocks or pots in parallel rows, with a space between these rows to form the ribs. The main advantages of this system are its light weight, which is generally less than comparable floors of concrete construction, and its relatively low cost.

Waffle or Honeycomb FloorsThese are used mainly as an alternative to an in-situ flat slab or a beam and slab suspended floor, because they require less concrete, less reinforcement, and can be used to reduce the number of beams and columns required, with resultant savings on foundations. The floor is cast over lightweight moulds or pans, forming two-directional ribbed floor.

Floor Finishes