By Ir Dr Eddy W T LAU
The challenges of adopting modular integrated construction
Modular prefabricated housing was first mentioned as a government policy in the Chief Executive's 2017 Policy Address. In September 2018, the Development Bureau further published a booklet titled Construction 2.0, which outlines the changes demanded by the construction industry including innovation, professionalisation and revitalisation. One of the innovations being promoted is modular integrated construction (MiC), which refers to the off-site manufacture of volumetric building modules with architectural finishes, building services and even furniture in place. It is in response to issues Hong Kong faces today, like the shortage of site labour supplies and the needs for providing more quality housing units. This article looks at the details of relevant technology, its advantages and challenges of adoption particularly in the Hong Kong market.
MiC is not a new concept in Hong Kong. The very first exemplar of MiC construction dates back to the year 1985, when the HSBC headquarters was built. At that time, pre-fabricated plant rooms and service pods were adopted. In the housing sector, the Housing Authority began to adopt volumetric precast bathrooms and kitchens in two public housing sites in Kai Tak in 2009. Currently, MiC is being applied to some government and institutional projects including staff quarters, researcher and student residences, as well as NGO-led social housing units.
Details of the technology
In a typical MiC project, the architect divides the floor plates into different modules to facilitate their productions. Usually, one module is used for a small studio flat. Two or three modules are used for a larger-sized apartment. A general rule is to locate the joints at the partition walls so that touching-up work in the future would be limited to the doorway locations. The modules are designed to resist vertical loads over the full height but not lateral forces. Nevertheless, the cast in-situ lift/stair cores perform the latter function. A typical design layout of a MiC building is shown in Figure 1.
The relevant regulations on road transport need to be considered when designing a module. Generally, a module width of less than 3.8 m to 4.5 m would be sensible, whereas the length would be from 5 m to 8 m. The modules are usually made of structural steel frames and dry walls, there are plasterboards with insulation sandwiched in between. For slabs, they can be made of either reinforced concrete or steel joists plus insulation sandwiched between boarding. Module weights vary from 10 tons to 20 tons. Lifting can be done by a 100-ton or 200-ton mobile crane (see Figure 2). If a tower crane is used, the lifting radius will need to be restricted. This means that more cranes will need to be erected on each building site. Besides, the crane would need to be free-standing as reinforced concrete (RC) facades are usually not available for tying back.
Unlike conventional construction, manufacturing accuracy of +/-5 mm is typically achieved in the modules while installation accuracy of +/-10 mm is necessary. It is because eccentricities on load-bearing walls must be minimised. From this, developers and architects should pay attention to the selection of builders/assemblers as it would be unlike traditional construction where broad tolerances are allowed.
Developers also need to pay attention to building warranties as not all insurance companies and mortgage providers would accept unconventional construction methods. In the UK, high-rise modular construction systems are assessed by the Steel Construction Institute (SCI). The SCI website shows a list of assessed companies and systems. This gives warranty providers and financial institutions added independent assurance of the technical soundness of MiC system products.
The advantages of MiC
Time saving, productivity and quality assurance are the main factors driving the adoption of MiC. For example, when site preparation and piling are being carried out, the manufacture of modules can take place simultaneously, resulting in significant shortening of the construction programme. Module fabrication is unaffected by weather; workers are more productive in a factory environment, and deliver better quality products. Besides, there will be less pollution on site and reduced disturbance to the locality.
In the case of Hong Kong, MiC will change our traditional heavy-weight RC structures to light-weight ones. It has a potential of reducing embodied carbon/energy. It may also improve the thermal insulation of building envelopes, thus saving energy use in air-conditioning. However, it will reduce the building's thermal mass. Heavier thermal mass stores heat in the structure, which will dissipate at night. It is beneficial to buildings that are unoccupied at night or buildings in climate zones where diurnal temperature variation is high. Since the latter two conditions do not generally apply to Hong Kong, the reduction of thermal mass would not lead to any major disadvantage.
The challenges to MiC
When using MiC, the construction cost could be higher as the walls and slabs will become double-layered once two modules are assembled, resulting in the use of more materials. The double-layer walls will also consume internal floor spaces, so the Buildings Department of the HKSAR Government needs to consider giving gross floor area concessions to the developers in order to provide incentives for them to adopt MiC.
There is another major disadvantage of using MiC. With the manufacture of flats advanced to an early stage, it may be impossible to implement late design changes. In Hong Kong, private developers sometimes need to make last-minute change to the flax mix, that is the proportions of one-bedroom, two-bedroom and three-bedroom flats, in response to the change to the property market. MiC makes the developers' changes impossible.
The major advantage of MiC is fast-track construction. Earlier completion has a higher potential of monetary advantage if the flats are built to rent (B2R). While the B2R development model is common in the UK market, it is rare in Hong Kong. In the past, when the property market was on an upward trajectory, selling the flats earlier would not be advantageous to the developer. However, owing to the government's introduction of vacant property rates, some developers are now converting a certain amount of their stock of properties to short-term or long-term rental units, making the development partially B2R. It is still uncertain whether a full B2R market will gradually develop in the future. If this occurs, it will be beneficial to the adoption of MiC.
In a typical MiC project, more than 60% to 70% of the elements are usually made in the factory as only the lift/stair cores and the podium are built in-situ. It is questionable whether the builder/assembler will still be the "main contractor" (MC) as the majority of the construction costs would be spent on one specialist sub-contractor. So, in some cases, the design-build model led by the module supplier, or the construction management model, with all participating companies playing on a level field, would be adopted. The traditional MC model will most probably be more feasible if the module supplier is a subsidiary company of the builder. Whichever model is used, a problem is still in presence today. Currently, only a few proprietary MiC systems have been pre-accepted by the Buildings Department. Therefore, the situation may restrict the competitiveness of tendering if MiC is specified in a tender. A possible solution is that the government and related statutory bodies should take the lead in adopting MiC. Once a sizable demand for MiC prevails in Hong Kong, more MiC factories or companies should be able to emerge.
Hong Kong's dense skyline has always been dominated by high-rise structures with repetitive housing units occupying a major portion. This makes the city a perfect place to test and implement the MiC technology. While a number of factors may hinder the adoption of MiC by private developers, this does not prevent the public sector from adopting it extensively. On the private side, if a B2R market is gradually formed in the future, using MiC will become more attractive as it enables private developers to generate rental incomes earlier.
About the author: Ir Dr Eddy W T Lau is a Fellow of the HKIE. He works as the Head of Green Labelling at the Hong Kong Green Building Council and has a research interest in sustainable construction.
Figure 1: Layout of an MiC building
Figure 2: An MiC building under construction