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Modern methods of construction (MMC) are transforming the way commercial developments are being built and are improving efficiency in the construction industry. Numerous methods are currently being adopted including tunnel forms, 3D volumetric construction and precast panels (MPA, 2018). Traditionally, student accommodation consists of repetitive units and thus are the ideal type of development that MMC can be used for, reducing construction time and costs (NHBC Foundation, 2006). The use of off-site construction allows quality to be controlled under factory conditions and can improve sustainability by reducing energy consumption.
This report aims to provide an overview of MMC and the methods currently available and the way in which they are implemented will be discussed. The report will also recommend a MMC that would be suitable for use on the 420-unit student accommodation block – “Corner Place” development in Glasgow. The building is fifteen floors high and is located within close proximity to a listed building on the high street. The project illustration is shown in Figure 1. Two main methods that may be appropriate for the development will be critically assessed. Both technical and resource issues will be discussed, and a final solution will be recommended to the client.
Figure 1 Project Illustration from Coursework Brief
Evolution of MMC methods
Prefabrication is defined as the manufacturing of building components in a factory or elsewhere before transportation and erection on-site (Curl, 1993). Off-site fabrication has been around since the 12th century where it was used for timber buildings (Hewett, 1980). Nevertheless, large scale adoption of MMC occurred after the Second World War where 200,000 homes were required resulting in a housing crisis, consequently there was a desperate need for housing to be delivered quickly (NHBC Foundation, 2016). Further drivers included the decrease of skilled labour and consequently an increase in labour costs, greater concerns about health and safety in construction and changes in the client demands (Nadim, 2012).
The main focus at the present time is on emerging innovative MMC which predominantly embrace off-site construction methods which are typically classified into five main categories by the NHBC Foundation (2016):
- Volumetric Construction Systems: which consists of three dimensional units which are produced in a factory where they are fully fitted before transporting to site where they are stacked on prepared foundations to form homes.
- Panellised Modular Construction Systems: are flat panel units which can be made of timber or light steel framing which are transported to site where they are assembled to form a three-dimensional structure which will fit within an existing structure.
- Hybrid Construction Systems – also known as semi volumetric: make use of both volumetric construction and panellised construction systems. This takes advantage of the positive qualities of both systems and increases flexibility as well as improving efficiency onsite.
- Sub-assemblies and Components: are larger components that can be incorporated into new or existing structures whether they are built traditionally or using MMC. For example, roof cassettes and prefabricated chimneys fitted into a house built in-situ.
- Site based MMC: the first four points were different methods of off-site construction. This method however relies on innovative methods of construction being used on-site and using traditional components innovatively. Examples include insulated formwork and joint block wood (NHBC Foundation, 2016).
Other methods include pods which are factory produced three-dimensional fully fitted units which are incorporated into the buildings super structure for example toilet and kitchen units for student accommodation (Zurich, 2014). While tunnel forms allow contractors to build large walls and slabs in a single day and is particularly suited to developments with repetitive units such as hotels or accommodation (MPA, 2018).
The advantages of prefabrication are primarily related reduced construction time, production of better quality structures and reduced impact on the local environment (Gibb & Isack, 2003). BRE believes off-site construction results in improved health and safety since the majority of construction is carried out in a controlled environment, reduced waste and the ability to manufacture unique shapes which are aesthetically pleasing (BRE, 2018). Conversely, the main perceived disadvantages are increased capital costs and resources although there will be economical benefits from reduced construction time on-site (Piroozfar & Cox, 2011). Figure 2 shows a spider diagram which depicts the perceived advantages and disadvantages of off-site construction using MMC.
Figure 2 Perceived advantages and disadvantages of MMC (Piroozfar & Cox, 2011)
Although MMC offer significant potential there are still substantial barriers that the industry needs to overcome, these include (Piroozfar & Cox, 2011):
- Lack of knowledge among designers and builds about the systems available and how to construct them. Also, some workers may be happy with the systems already in place and may resist change.
- There is currently a lack of mature and tested supply chain to support the construction process thus increasing project risks.
- High uncertainty regarding cost and performance which may result in insurers and mortgage lenders being reluctant to support projects.
- Concern from members of the public that all developments will look uniform, bland and essentially lack character.
Corner Place is comprised of 406 student residences of which 96 are studio apartments and 310 are cluster rooms. The repetitive nature of this development makes it ideal for the use of MMC. AMA research carried out a study to identify the types of MMC and the main end user sectors shown in Figure 3. It can be seen that the most commonly used MMC for student accommodation are pre-cast panels, tunnel forms and pods whilst volumetric and timber frames are less commonly used. Using hybrid construction and timber is typically limited to a maximum of 6 -10 storeys and thus they were not considered since they are not feasible methods for this project which is 15 storeys high.
Figure 3 Key MMC and End Users (WRAP, 2007)
The two methods selected that may be employed for this project are:
- Precast panels
- Tunnel forms
It must be noted that two or more MMC could be used simultaneously for example precast panels and pods. In this situation the panels can make up the superstructure and the pods could be used for kitchen and toilet unit. Pods typically have excellent stiffness and stability. However, in this report the use of MMC for the superstructure will be assessed using one MMC. Volumetric construction was also seriously considered since it has successfully been employed in similar projects such as Apex House in Wembley London, however only the two aforementioned methods will be discussed below.
Option 1: Precast Flat Panels
Precast panels have been used extensively for a wide variety of projects. Manufacturers can produce standardised using pre-existing formwork in a controlled factory environment (NHBC Foundation, 2016). One of the main advantages of using precast panels is that the manufacturing process does not require temporary supports and scaffolding. This ultimately cuts costs, improves health and safety on site and reduces waste. Precast building systems can be divided into several categories namely (Guevara-Perez, 2011):
- Large-panel systems
- Frame systems
- Slab-column systems with walls
- Mixed systems
Large panel systems are typically used for multi-storey buildings where large wall panels and floor panels are connected horizontally and vertically to form a box like structure. Usually wall panels are one floor high and any subsequent floors would require another panel which will connect to the panel underneath (Guevara-Perez, 2011). Most elements are usually linear, and economies of scale require regularly shaped buildings.
Horizontal floor elements can be either be one-way or two-way slabs and transfer lateral loads to the walls. Connections between panels are of utmost importance to ensure the structural integrity and stability of the structure. Both horizontal and vertical joints are load bearing. Vertical joints resist shear seismic forces whilst horizontal joists resist gravity and seismic loads. Joints can either be ‘wet’ or ‘dry’ (Guevara-Perez, 2011). Wet joints are filled with concrete in-situ whilst dry joints involve welding steel plate together. The type of joint to be used will depend on feasibility, practicably and serviceability.
Precast concrete has excellent sound transmission loss properties and performs better than concrete cast in-situ since quality control, curing and crack elimination issues are addressed during manufacturing (National Precast Concrete Association, 2015)
There are a number of factors which impact the dimensional planning of structures which use precast panels.
One of the major limitations of using precast panel systems is transportation. Firstly, depending on where the factory is located the panels may need to be transported large distance which is costly and not sustainable. Preferably, the factory will be local and situated in Scotland such as Lochton Precast which is the leading producer and supplier of Precast Concrete Products in the North East of Scotland.
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Engineers must consider the possibility of panels becoming damaged during transport and erection. Mackleys transport panels on their edge to reduce the risk of cracking or flexing (Mackleys, 2015). The road limitations for vehicles in the UK must also be considered. HM Revenue & Customs 2016 state that ‘The maximum vehicle weight is 44 tonnes and has up to six axles. The maximum individual truck length is 12 metres, articulated truck and trailer length is 16.5 metres and road trains are allowed up to 18.75 metres. The maximum width for all is 2.55 metres’. These restrictions will limit the size of the panels that can be transported. It must also be noted that reversing out of the site onto a main road is generally not permissible (reversing into the site is not a problem). Therefore, the vehicle used must have ample space to turn on site. A designated storage site with clear exclusion zones is a must. Panels must be stored as per the designers’ specification since incorrect storage may result in damage that may not be immediately obvious such as long term creep (Precast NZ, 2015).
Option 2: Tunnel forms
Tunnel forms are a systematic technique known to produce high quality monolithic structures with repetitive cellular compositions. It is a modern method which allows horizontal and vertical elements to be cast simultaneously using steel formwork which can be reused on other projects (Construction Tunnel Form, 2015). In-situ concrete is poured into two half tunnels to form walls and slabs. Two half tunnel L-shaped forms are used to shape the structure (Tavafoghi & Eshghi, 2013). Tunnel forms suppliers are available throughout the UK and they have previously been used for high rise student accommodation developments. Depending on the project needs, the forms are occasionally heated using hot air blowers to accelerate the curing of the concrete (Deshmukh & Shalgar, 2016). The repetitive nature of the work results in less mistakes on site because there is less chance for error (Nasvik, 2003). It reduces the number of cold-formed joints and the requirement for post construction trades such as plasterers and electricians (Deshmukh & Shalgar, 2016). Figure 4 portrays the typical slab cycle of tunnel formwork.
Figure 4 Tunnel-Form Slab Cycle (Deshmukh & Shalgar, 2016)
The structures constructed using tunnel forms are earthquake proven and provide a sound reduction of 50 decibels which is ideal to reduce sound transmission – a critical problem in student accommodation (Sawab, 2018). The resulting structure has excellent thermal mass and acoustics and is structurally stable. Formwork is made entirely from steel which can be re-used up to 1000 times which makes the process versatile and economical. Tunnel forms work well with other MMC such as framed infill panel systems (NHBC Foundation, 2006). Speed of construction with less labour is one of the main advantages of using tunnel forms as well as lower costs (Nasvik, 2003). After removing the forms, the structure will need to be back propped. This process is not necessary when dealing with precast structures and adds extra time and costs. Moreover, tunnel forms allow a greater margin of error than precast panels.
Comparisons of the Options
The availability of lifting equipment must be considered and where they are placed in relation to the listed structure and mainline railway tunnel is crucial. A large tower crane is likely to be required with adequate lifting capacity and suitable foundations must be constructed (Precast NZ, 2015). While delivering and erecting the tower crane it may be necessary to close the road lanes for safety reasons which will cause disturbance to local residents and may require prior permission from the local council. All hazards on site must be clearly marked on drawings these could include overhead powerlines.
The benefit of using tunnel forms as opposed to precast panels is that a smaller crane with lower power can be used to lift the reinforcement into place. The lower weight of the crane reduces the risk of settlement of the railway tunnel and potential damage to the listed structure. However, since the concrete is cast in-situ there will be a large number of concrete mixers coming in and out of the site throughout the construction period. Reinforcement and formwork will also need to be delivered and stored. The deliveries and concrete mixers may result in increased traffic and thus may disturb local residents and businesses. Effective logistical planning is necessary to ensure all materials are delivered to site regularly and on time.
The proposed development also has a commercial and leisure centre which will most likely have larger spans than student residences. Both tunnel forms and precast panel systems can be used to construct large spans; where tunnel forms can do a 7.5m by 15m spans and precast panels could do even larger spans but are limited by transportation restrictions. Furthermore, both proposed methods use concrete, so the materials used have a similar impact on the environment. The main difference arises from the amount of time spent on site where manufacturing methods and the distance the materials or panels need to be transported.
Welfare facilities account for a large percentage of the costs and energy consumption on site. Using precast panels will require less time to be constructed on site thus reducing the duration that welfare facilities are required. Conversely, it is likely that pre-cast panels will have to be transported larger distances. A more accurate value to determine the environmental impact, embodied and operational carbon will need to be calculated using a life cycle assessment. The availability of skilled labour in the local area must also be assessed further.
Conclusion and Recommendations.
This report discussed the evolution of MMC, the methods currently available and their uses. The strength and weaknesses of two MMC – namely precast flat panels and tunnel forms were compared to identify which method was suitable for the Corner Place student accommodation development. Based on the aforementioned arguments ‘Tunnel Forms’ are the recommended MMC for this project. The main reasons for selecting this method are:
- It produces high quality structures with time savings of up to 25% increasing efficiency and productivity.
- It is particularly suited to repetitive cellular structures and can be constructed in a staggered arrangement.
- Lower power machinery required and reduced need for storage.
- Excellent sound reduction, thermal mass and fire resistance.
- Reduced risk of impacting listed building and railway tunnel.
- Tunnel forms, concrete and reinforcement are all available locally.
- Eliminates the risk of panels getting damaged during transport since everything is constructed in-situ.
- Higher allowable margin for error with less joints which are critical for structure stability.
- Tunnel forms have been used quite extensively for student accommodation and thus lessons learnt can be acquired from previous projects.
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