MMC Revision Notes


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MMC Revision Notes

  1. 1. Modern Methods of Construction (MMC) MMC – a term used to embrace a range of technologies and process involving various forms of supply chain specifications Drivers for Building using MMC: 1. Shortage in Housing Supply – previously very high demand for housing 2. Skills Shortage on Sites – underinvestment in skills training leading to overall skill levels decreasing 3. Concerns about Housing Quality - perception that build quality is declining 4. Revisions to Building Regulations - higher performance levels needed to comply with ever improving Building Regs 5. Environmental Performance - increasing emphasis on environmental performance of buildings (embodied energy but initial and recurring) Different types of MMC include: Volumetric Construction Three dimensional units produced in a factory fully fitted (Modular construction) out and dropped onto prepared foundations to form a structure e.g. bathroom or kitchen PODS. Most efficient for large quantities of identical units Panellised construction Flat panel units produced in a factory and transported to site for assembly into a three-dimensional structure or to fit within an existing structure e.g. concrete wall panels, structural insulated panels (SIPS), curtain walling etc Hybrid construction Volumetric units integrated with panelised units e.g. kitchen pod as volumetric unit with the rest of the dwelling constructed using panels. Sub-assemblies and Larger components that can be incorporated into either components conventionally built or MMC structures e.g. pre-fabricated foundations, floor cassette systems, pre-assembled roof structures, pre-fabricated chimney stacks etc. Non off-site manufactured Most modern methods of construction are based off site but MMC there are also site based methods including tunnelform construction, insulated permanent formwork and aerated concrete products used to form major elements of a structure 1
  2. 2. Examples of Modern Methods of Construction: o Modular PODS (Volumetric Construction) o Steel Frame Housing (Fusion Building systems) o Permanent Formwork Systems – similar to twin wall system but lighter as formwork instead of concrete. Used to contain the placed concrete and can remain in place for life of structure. Eliminates need for striking formwork. o Surveying & IT (Use of GPS, EDM, surveying software & laser scanning) o Manufacturing & IT (CADCAM) o Prefabricated roofing systems (Smart Roof) Benefits of MMC: There are many beneficial factors associated with MMC’s, not only for the benefit of the client but also for main contractors, sub-contractors, local communities etc. Some of the main advantages to using modern methods of construction include the following: 1) Quicker on site build time/shorter programmes/reduced preliminaries: With MMC, much of the work is removed from the site and it is therefore possible to execute various activities of the project concurrently or even before the project has commenced on site. This reduces the projects construction time as the building or elements of the building can be manufactured off site while the ground and site works are taking place. MMC leads to a reduction in trades on site and a shorter construction programme which in turn leads to reduced preliminaries, overheads and a quicker return on investment for the client. 2) Reduced waste and better waste management: As production is often executed in a factory controlled environment, the waste stream can be easier to manage. Exact quantities of materials can be purchased, materials can be used more efficiently and because materials are properly stored, breakages and damage are less likely to occur. Furthermore any un-used materials can be easily collected, re-used or recycled contributing to less waste. Constant monitoring also takes place within a production plant allowing new waste management strategies to be implemented without difficulty, if necessary. Waste reduction is a very significant advantage as waste from construction is one of the principle waste streams to landfills and it has been proven that a high percentage of materials delivered to site are never even used and go straight into the waste cycle. 3) Reduction in defects and increased quality control: As you can imagine, a building site in Ireland, fully exposed to our rainy and windy climate is not exactly the perfect working environment for high quality workmanship. Construction work exposed to the elements of wind and rain proves more difficult to monitor with regard to quality control. Human error is also another significant factor which deters the achievement of high quality construction as it can prove difficult to work in extreme weather conditions. 2
  3. 3. Factory based constructions forms, engage better and safer working conditions with no interference by the Irish climate and therefore a very high standard of quality control can be achieved which includes testing, trials, checks and re-checks. For more reasons than one, factory based construction provides better working conditions than a building site and in turn produces better quality too. 4) Increased Health & Safety: Construction work carried out in a factory controlled environment is without doubt a safer working environment for all trades. Safety controls are implemented and monitored and safe working conditions are easier to meet and maintain. With off-site construction there is a significant reduction in the number of trades working on site and this proves more manageable from a health and safety perspective. Construction work on site can incorporate some very dangerous activities and in turn lead to a large number of causalities and/or fatal injuries. Construction is among the largest number of fatal injuries between all the main industries in Ireland. Statistics from 2002 to 2009 (as seen in Appendix A) show that the construction sector has been either the first or second largest contributor to fatal injuries in the past 8 years. 5) Social benefits and reduced local impacts: MMC’s and in particular off-site construction, allow local communities to benefit from the process of manufacturing away from site. The main advantage to communities is that there is much less traffic and smaller on site workforces adding to traffic congestion in the area. Furthermore due to speedier on-site programmes, noise and pollution levels will decrease and the locality surrounding the site will be disrupted for a far shorter period of time. Construction sites are only temporary employment locations and offer little or no amenities for the local communities whereas manufacturing facilities very often provide long term social services and economic benefits for the surrounding community. Manufacturing facilities are also more likely to invest in education and training for their workforce and develop a highly trained local workforce within their facility. 6) Greater efficiency in the use of resources and transport Over the years it has been noted that the use of labour, plant and materials on building sites is extremely inefficient as is not the case with factory based activities which are kept under extreme scrutiny, monitored and controlled. Re-cycling and re-using of materials is also more difficult to enforce on a building site but is easily implemented in a factory based environment. On another note, monitoring of transport patterns and schedules can be very difficult on construction sites especially if the site is condensed and compact. With off-site MMC the number of deliveries direct to the building site is reduced and deliveries to factories can be planned and controlled so that full loads can be used and transport costs are kept to a minimum. On the other hand, transport of prefabricated or modular buildings to site must be carefully planned and heavy plant and equipment necessary for off-loading and erection requires careful site management and consideration. 3
  4. 4. CASE STUDY:  Compare & Contrast a project completed in 1996 with one completed in 2006  Belgard Square v’s Sweepstakes  Similar multi storey concrete frames but constructed using different methods BELGARD SQUARE (2006) SWEEPSTAKES (1996) Structural columns All precast – both one All insitu – column boxes had to and two floors high be changes regularly and programme was determined by Advantages: speed of column construction. Reduced Waste –No Formwork Columns were construction in way Quicker to Construct that they could only incorporate an Improved quality insitu slab while precast columns are more adaptable. Central Concrete Core Used Precast Twin Walls: The Used Insitu Walls with timber twin wall system consists of two formwork. precast leaves tied together by Dis Ad: Time Consuming/Poor means of a cast in lattice truss. Finish/Large Amounts of Panel erected via crane on site Waste/Reduced H & S/Less and cavity filled insitu on site. Accurate Cladding System Techcrete SOME MMC EXAMPLES: Concrete Slab Construction: o Post Tensioned Concrete: A method of insitu that eliminates standard cut & bent reinforcement. o Involves first forming & casting the member with ducts through its length. o Ducts contain steel cables o After concrete is cured the cables are stressed to a specific length o Popular method of casting concrete floor slabs and popular on bridges Benefits:  Span further than reinforced concrete span ie reduced columns and foundations and increased open space areas  Able to accommodate irregular grids  Reduces time spent placing & fixing reinforcement  Thinners slabs made possible  Early stressing of concrete allows for early striking time 4
  5. 5. Cladding Systems: o Constructed off-site o Improvements in safety systems & reducing labour o Design has direct communications with supplier to ensure design can be completed by o Manufacturer o Most modern systems eliminated need for scaffolding and allows fixing of the cladding o panels from inside of the building o Increase in use of cladding systems because of U-Value benefits Precast Balcony Unit – Schock System: System used to eliminate problem of thermal (cold) bridging in balconies. Comprises of a rigid expanded polystyrene insulation block which makes it possible to have a layer of insulation around the whole building. Projecting reinforcement on either side ties into the floor slab or floor screed on the internal side and into the balcony slab/screed on the external side. Bathroom Pods: o Volumetric construction o Bathroom Pods have become very popular in recent years o Generally used as non-structural members o Produced as single complete units o Fully factory finished internally with service ducts on external for ease of connection and no need to open POD during commissioning o Available in timber frame, light steel frame and hot rolled steel frame or concrete frame o Only economical when ordered in large quantities/No suitable for once off house Advantages:  Strong & Durable  Suitable for many building types  Ease of transport and installation, erected by crane via hooks on top  Consistent quality, no snagging  Reduced waste  Flexible finishes and floor layout  Sealed unit so no entry required until client handover  Factory accurate/Less waste etc Sample Q3: The Construction Industry is continuously being asked to deliver projects faster & faster. As a main contractor you have been asked to propose alternative methods of construction for a proposed 16 classroom school. 5
  6. 6. In you answer you should propose alternatives to standard design and give comments on how you feel these alternatives will affect the quality of the final project. Answer: Introduction Outlining purpose of answer What is MMC? See about definition Advantages of MMC Use details as above The proposed modern methods of construction for this 16 classroom school include the use of precast wall and floor panels, precast stairs and also the use of Alltek spray on plastering to the internal walls. I believe that this will give a signification reduction on the overall programme while also adding additional benefits. It is also proposed to use 100mm wideslab flooring with a 150mm screed finish as well as precast stairs at all levels. All of the precast products are manufactured in a factory controlled environment and delivered to site ready to be lifted into place easily via a mobile or tower crane. In addition to the benefits of MMC mentioned above, precast construction offers many other benefits over insitu as follows: 1) No scaffolding required – this can save time on site and leads to further cost savings. 2) Less co-ordination needed on site – there is no need to co-ordinate the shuttering carpenters, the concrete deliveries and pump, deliver & positioning of re-enforcement etc and therefore problems and delays on site are eliminated before the arise. 3) Fair faced finish on wall panels – the precast wall panels offer a fair faced finish on one side which is ready to be painted without the need for a skim coat plaster finish. The external walls will take a cladded or external insulation finish therefore will not require any finishes as the rough side of the panel will face the external or cavity of the building. On the other hand the internal walls will require further finishing on the rough trowel side of the wall and this will be carried out using a spray on plaster finish of just a few millimetre thickness. In this case finishing works will be kept to a minimum while also eliminating the need for drying out of wet trades. 4) Electrical conduits and sockets can be cast into the precast wall panels during manufacture thus saving further time on site for chasing of walls and installation of services by mechanical and electrical contractors. Precast Wall Panels: It is proposed that 200mm solid precast wall will be used internally. These panels will be manufactured on an automatic production system therefore resulting in overall lengths and opes to within mm accuracy. The walls will carry the vertical load and act as shear walls within the structure. The Erection Process: The precast crew will work off a finished ground floor with starter bars already in place. Starter bars are vertical bars cast into the floor slab and set out, as per the positioning details 6
  7. 7. for the wall panels. The wall panels are lifted in place via a mobile or tower crane. Lifting brackets located at the top of the panels are slung with chains and the walls are set down on these starter bars which connect the building to the ground floor slab walls over bars. The panels themselves are connected to each other on the vertical side using Philips loops and threaded bar. The Philips loops are connectors at the sides of each of the solid panels and provide a fully grouted rigid connection. The threaded bar is pushed down through these loops interlocking the panels. The wall panels are connected on the horizontal with threaded bar which sits down a dowel tube in one panel and up the tube in the other. These tubes are then also filled with grout. The result of all these connections is that all elements are connected together and the building is also connected to the ground thus providing maximum stability. Electrical socket outlets and conduits can be cast in to both faces of these wall panels and the service tube allows for easy wiring of the sockets and eliminates chasing of walls by electricians at a later stage. Steel sections or additional reinforcement can also be cast into the walls at the production stage if necessary. Opening for doors, windows or services can be provided within the wall panel but extra reinforcement must be used around these opes to prevent cracking or breakage. The wall panels will be erected using a crane. Lifting brackets located at the top of the panels will be slung with chains and erected into position. M16 sockets can be pre-cast into the panel face to take diagonal push pull props on site, thus increasing erection speed and reducing damage to the panel on site. Once the wall panels are erected all joints between walls will be insitu filled with a cement and fine aggregate grout and this will provide a smooth even surface on the panels. Precast Floor Slabs: For this project it is proposed to use pre-stressed wideslab as wideslab will span up to 8m simply supported. Other options are Filigree slabs which can span 5.5m simply supported but this type of slab is not as widely used in the industry. Hollowcore flooring is another alternative but this is only really necessary for spans greater that 8m and up to 16.5m 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 wideslab will have a 100mm slab depth and a width of 2400mm 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 70mm 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 125mm screed over the 100mm wideslab will be sufficient and can incorporate services into the floor if necessary. It may be possible to cast upstands 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 7
  8. 8. 35-40N structural screed will be poured over this. The perimeter of the floor, opes, 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 powerfloat 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 opes. 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 opes must remain until it is safe to remove. Precast Stairs: It is proposed to use precast stairs throughout the school. 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 to the next floor for the precast crew as the building progresses. 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 200-300m2 of two coat Alltek application can be achieved per day. Further details of Alltek products can be seen in Appendix B. Conclusion: As can be seen through-out the above report, the use of a precast frame and a thin coat spray on plaster finish on the proposed school could produce significant reductions on the overall construction programme while also not only maintaining, but excelling general standards. 8
  9. 9. The precast frame exceeds specification as it will be 60N concrete and manufactured to a very high specification in a factory controlled environment. Tight factory production control ensures that the re-enforcement is located accurately and the panels are made to tight dimensional tolerances. Structural connections are also accurate which assists in the accurate installation of cladding, windows and other elements thereafter. Furthermore precast concrete improves structural efficiency as longer spans and shallower construction depths can be obtained using prestressed floors and/or beams. Most importantly, there will be no additional work created for the design team as the precast manufacturer produces their own in-house precast drawings for approval by the architect thus design costs do not change or increase. The Alltek plaster finish will also provide a high quality finish with all Alltek materials undergoing extensive testing before being released onto the market. Furthermore surfaces can be emulsion painted after only 24-48 hours with only a mist-coat and finish coat being required. In addition, there are other MMC’s which could be adopted on this project to further reduce the programme if necessary such as prefabricated foundations, a prefabricated roofing system, an external cladding system or an external insulation system such as Weber. Case Study: Pipers Hill Gaelscoil, Post Primary School & Sports Hall, Naas,Co Kildare. MMC Used: Precast walls, floors & stairs, Altek Spray Plastering, Weber External Insulation System Client: County Kildare VEC Main Contractor: P Elliott & Co Ltd Project Commenced Aug 2008 for opening Sept 2009 Furthermore, an 8 classroom school was built using precast walls, floors, stairs and Altek in Kinnegad, Co Westmeath by main contractor Sammon Contracting in summer 2008 with the exact model being replicated and built in Drogheda, Co Louth and Tullamore, Co Offaly that same year and further being built thereafter. 9