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Wednesday, January 11, 2017



4.1. Precast Floor Slabs:
            For this project it is proposed to use pre-stressed wide slab as will span up to 8m simply supported. This is sufficient for the development as the maximum floor span is around five and a half meters. Other options are Filigree slabs which can span 5.5 m simply supported but this type of slab is not as widely used in the industry. Hollow core flooring is another alternative but this is only really necessary for spans greater that 8 m and up to 16.5 m and the building is in excess of seven or eight floors and a lighter weight option is required. It will be necessary to prop the floor at 600 centres using acro type props with timber girders running horizontally from prop to prop.
            For this structure, the proposed wide slab will have a 100 mm slab depth and a width of 2400 mm as standard will be used where possible. This shallow floor will give maximum floor-to-floor height resulting in no loss of space. It will also provide a smooth soffit finish, which can be used as a finished surface in areas where suspended ceilings are not specified i.e. the apartments. The floor slab will sit approx 70 mm into the wall panel and the slabs will also be lifted in by crane via projecting lifting hooks on the surface of the slab. A 125 mm screed over the 100 mm wide slab will be sufficient and can incorporate services into the floor if necessary. It may be possible to cast up stands on the precast walls instead of shuttering the perimeter for screeding. A393 mesh along with tie steel will be placed on the slab and a 35-40 N structural screed will be poured over this. The perimeter of the floor, opens, stairwells etc will have to be shuttered to house the screed and prevent spillages. The concrete will be pumped in using a concrete pump and all floors will receive a power float finish.
            The floors will be connected to the wall panels via U bars which are wrapped around the treaded bar projecting from the panel to connect the wall directly above.  Handrail will also be necessary around the perimeter and large opens. There will be a plastic leg on the handrail posts that can be left in the screed afterwards. The perimeter handrail will be moved up floor by floor as screeding progresses but handrail around opens must remain until it is safe to remove.

Fig 4.1 Precast Floor Slabs
4.2 Precast Stairs:
            It is proposed to use precast stairs from basement to fourth floor. The stairs are prefabricated off site and are cast in a steel mould for manufacture. The use of precast stairs means that while they can be erected by crane with the wall and floor panels, they also allow access up the building for the precast crew as the building progresses. 

Fig 4.2 Precast Stairs
4.3 Internal Finishes:
            It is also proposed that a thin coat spray on plaster system will be used on the rough side of the precast walls and also on the ceilings. The material recommended for this is a thin spray on plaster known as Alltek which is supplied by International Coating Products (ICP) and who many suppliers and applicators across Ireland, the United Kingdom and Europe. The Alltek product is packaged in standard 25kg bags and comes in the form of a white powder which comes from fine graded marble. Alltek is applied in two coats and is suitable to be sprayed onto fair faced concrete surfaces, gypsum boards, smooth plastered surfaces etc. The Alltek Red Label can also be sprayed over the Alltek Blue Course plaster which is used on rough surfaces.
            Alltek also supply dry fillers used to fill joints in panels, the filler is just mixed with water on site and applied to the joint. Once the joints are dried and sanded down, the walls are then ready to receive the plaster spray coat. The Alltek red label is poured into the spray machine and the applicator then commences spraying the Alltek onto the prefilled surface. The second coat of spray can only be applied once the first has fully dried out. Alltek can be applied in a flat or textured finish and in several pastel shade thus reducing painting and decorating costs. Between 200-300 m2 of two coat Alltek application can be achieved per day.


            It is my belief as the contractor that by adopting the above mentioned modern methods of construction on the South Cumberland Development we can significantly reduce the program duration for this project. Based on figures from one precast manufacturer, Alcrete Ltd, 1450m2 of precast walls can be produced in their factory in just one week. As can be seen from the below chart, this is more walls that needed for the entire development. Hence production of precast walls off-site will automatically reduce the program and even taking into account the lead in time required by the manufacturer, signification time savings can be achieved here. It must also be noted that lead in times for precast manufacturers has reduced significantly due to lack of workload in this economic downturn.
            Further reductions in program duration can be achieved through erection on site of the precast elements. Further figures from Alcrete show that 325 m2 of double walls and 450 m2 of solid walls can be erected per week by a 5 man crew. In addition, 280 m2 of wide slab can be erected per day by a 4 man crew thus showing how the frame can be erected in just a number of weeks. 
Furthermore with no scaffolding, drying out time, erection or dismantling of shutters/pans etc needed, the precast frame is less weather dependent than in-situ construction forms and is therefore less likely to experience delays or set backs on site. Once erection of the frame has been completed additional time savings can also be achieved in the finishes such as plastering works, mechanical and electrical works, installation of cladding etc.


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