PRECAST TO LAST S C Lam, K C Chung and S W Sham Hong Kong Housing Department Abstract: The Hong Kong Housing Authority (HA) pioneered the precast concrete construction in high-rise residential buildings in the early 90s by successful introduction of planar precast elements, i.e. precast façades, semi-precast floor slabs and precast staircases, which altogether amount to some 18% of the total concrete volume of a building. Seeing the immense potential of precasting in building construction to enhance quality and sustainability, the HA has been dedicated to bringing greater benefits to the public through more precasting. To increase the percentage of precasting, to precast structural walls (given that a standard housing block is a wall-slab structure) and to leap from planar to volumetric precast construction has thus become the prime and foremost task. The HA has recently made a breakthrough in this area and developed an Enhanced Precast and Prefabrication System (EPPS) for the construction of standard high-rise housing blocks. The EPPS contains a lot of innovative initiatives in prefabrication and precast construction that include the key precast structural walls and volumetric precast construction as well. With the EPPS, the precast concrete volume will substantially increase to 60% of the total concrete volume of a building. Evolution of the EPPS has satisfactorily progressed stage by stage from design development, a site mock-up to the present stage that the precast buildings are under construction. INTRODUCTION Hong Kong s building industry has been continually striving to keep up with the latest developments around the world. It is always ready to explore new technologies and try out innovative construction methods. In recent years, the HA has been actively pursuing the mechanized precast concrete system in its public housing developments with an aim to satisfy customers need, upgrade built quality, improve construction safety, enhance environmental protection and increase cost effectiveness. Back in the early 90 s, the HA pioneered the precasting technique in high-rise residential buildings by introduction of precast façades, semi-precast floor slabs and precast staircases. Over the past decade or so, improvements have been made to both the design and construction of these precast elements. Now, these precast elements amount to some 18% of the total concrete volume of a building and the façades construction cost has substantially dropped to about half of that when they were first used. The resultant benefits in quality, safety, environmental protection and cost-effectiveness are quite noticeable.
However, the HA is not complacent with this level of precasting and is dedicated to bringing greater benefits to the public through more precasting. Drawing on overseas experience, consultants expertise and in-house knowledge, the HA has recently developed an Enhanced Precast and Prefabrication System (EPPS) for an existing standard block type that comprises innovative precast structural walls and volumetric precast construction. This is a breakthrough achievement as the precast concrete volume will substantially increase to 60% of the total concrete volume. The HA chose the Kwai Chung Flatted Factory Redevelopment Project as a pilot to try out the EPPS. Evolution of the EPPS has satisfactorily progressed stage by stage from design development, construction of a mock-up on site to the current stage that the precast buildings are under construction. THE PROJECT The 2-hectare Site is surrounded by two residential estates - Kwai Chung and Kwai Hing Estates - with some industrial developments to the north. It is for a housing development that comprises 2 no. 41-storey New Harmony 1 blocks (NH1) and 1 no. 36-storey New Harmony Annex 5 block, providing 1,983 flats for a population of approximately 6,346 persons and a total domestic GFA of 90,518 m². Figure 1 Site Location Plans There are very severe site constraints - the Mass Transit Railway (MTR) protection zone (two MTR tunnels running underground at the west of the Site) and a cable reserve zone to the east. Ample open space thus provided renders the Site particularly
suitable for this pilot project where on-site precasting of the crucial precast structural walls is mandatory to ensure close site supervision and quality control. THE ENHANCED PRECAST AND PREFABRICATION SYSTEM (EPPS) The EPPS is developed for the construction of the two standard 41-storey NH1 blocks with prime consideration not to change the standard structural layout to facilitate construction (as the contractors are very familiar with standard blocks the layout, the flat size, the architectural details, building services, etc. they can easily adapt most of their existing temporary works, construction machinery, construction sequence by wing and logistics in the standard block construction to this pilot project and focus their attention on the new precast items). It should be noted that the EPPS is not only restricted to the standard NH1 blocks; the various precast initiatives can also be adapted individually or in combination to other non-standard structural layouts. Figure 2 Key Plan of Precast Components In addition to the existing precast elements such as precast façades, semi-precast floor slabs and precast staircases, the EPPS contains the following new initiatives:- Innovative Precast Structural Walls - Welded Wall - Semi-precast Wall Innovative Volumetric Precast Components - Precast stair-core - Precast lift core - Precast bathroom - Precast bathroom-cum-kitchen
(1) Precast Structural Walls The precast structural walls are crucial to structural safety of the standard block which is a wall-slab structure. Unlike precast floor slabs, façades and staircases which carry the load of one single storey and therefore concern local structural safety only, the precast structural walls are to carry all the gravity loads and wind loads from the floors above and therefore impinge on global structural safety. Based on structural consideration, waterproofing requirements, buildability and cost factors, two types of precast shear walls are developed, namely Welded Wall and Semi-precast Wall. Welded Wall is mostly used for external walls and specially designed with multiple waterproofing features. As compared to conventional insitu construction, this precast wall type can do away wall form and falsework and external working platform, not to mention that it will enhance the quality of the finished work and speed up the construction on site. Joining upper and lower precast walls of this type is by welding of steel couplers that are cast in each end of the walls, i.e. structural steel sections at the two ends of the counterparts are welded together for load transmission. This connection neither requires special materials such as proprietary couplers nor special skills as qualified welders are commonly available in the market to carry out high-quality jointing work. Quality-wise it is comparable to proprietary couplers; cost-wise, it is more favourable. Besides, it has several more advantages: on-site quality control tests can be performed, and its price and supply position can be better controlled in contrast to proprietary couplers that have only limited suppliers. Figure 3 Welded Wall
Figure 4 Waterproofing Features at Base of the Welded Wall Semi-precast Wall is specifically designed for the gable end walls, furthest away from the tower crane that is usually located near the center of the block. Considering the normal lifting capacity of those tower cranes commonly available in the market, i.e. no need to upgrade the crane, the weight of the precast components must be properly controlled. Therefore, this wall type adopts a semi-precast design with an external precast wall panel and an internal in-situ concrete portion. It should be noted that the external precast panel is used as formwork for concreting the internal in-situ portion of the wall. The external precast panel is securely bolted onto the falsework at the internal side making use of the prefabricated threaded sockets cast in the stiffeners of the precast panel; thus the external wall form, falsework and working platforms are saved. Such semi-precast construction can also avoid undesirable vertical joints if smaller wall panels are adopted as an alternative to control the weight since these joints at external walls certainly require expensive waterproofing measures. Figure 5 Semi-precast Wall
(2) Volumetric Precast Construction The breakthrough achieved in the precast structural walls has enabled it to explore and develop volumetric precast construction. Of the four types of volumetric precast components as mentioned in Section 3 above, the last two precast bathroom and bathroom-cumkitchen are elaborated below. Figure 6 Precast Bathroom-cum-kitchen The innovative precast bathroom and bathroom-cum-kitchen have three special features. Firstly, they integrate intelligently with the adjoining shear walls, i.e. there will not be any double walls. Secondly, most of the architectural works are done in the precast factory, i.e. better quality control and less wet trades at working floor. Thirdly, a unique fibre reinforced polymer floor pan is used to replace the traditional waterproof membrane at the base of the bathroom and bathroom-cum-kitchen. It allows us to dispense with double floor slabs. As opposed to other proprietary products, more usable space can be provided with less construction costs because no double walls or double floor slabs are used. The floor pan is a seamless machine-moulded pan encased into walls, which will prevent water leakage and associated concrete spalling problems. This will mean much less maintenance and in turn minimize nuisance or disturbance to residents. Besides, as most finishing works are prefabricated in factory, better quality is ensured and an overall shorter construction period can be achieved. For precast stair-cores and precast lift shafts, apart from saving of external working platforms, precast construction can achieve better quality by minimizing defects such as stepped joints commonly found in conventional cast-in-situ construction method.
THE SITE MOCK-UP In view of the limited experience of local building contractors in the EPPS, the HA tried out the new system by constructing a two-storey one-wing mock-up structure at the Site before starting the main building project. By simulating the process of building the standard block with the EPPS, the HA could check out all the details involved in the production, installation and testing. During the course of the mock-up construction, the HA worked closely with the contractor in identifying problems, proposing improvements and developing feasible and practical solutions. The final, improved design was then let out for construction. STRUCTURAL DESIGN PHILOSOPHY OF THE EPPS (1) Structural Framing The NH1 block is a reinforced concrete building that comprises basically slabs and walls and has to take up high wind loads. It is geometrically symmetrical with aspect ratio (height to width) less than 4. Design of the precast structural wall, in particular the connections between floors, depends very much on the amount of dead load acting on it. When there is sufficient dead load, the wall will still be fully in compression under the wind load combination. However, when the dead load is not sufficient to offset the wind-induced uplift, part of the wall is under tension then. Therefore, a more accurate load run-down is required. Current tributary area method commonly used in high-rise reinforced concrete building design may not be suitable for the purpose. Instead, finite element analysis was adopted; floor slabs and walls of the entire building were mesh-modeled. Further, for a more accurate assessment, construction stage analysis was also performed. The loading distributions in partially completed structures, built up floor by floor, due to gravity loads (mainly the self-weight) was carefully studied. Significant differences in member forces were found in columns and walls at low level and lintels at top few floors when compared to results obtained from conventional analysis of the whole building based on an elastic model. (2) Precast Structural Walls The precast design was carried out to British Standard BS8110 with reference made also to precast standards and practices adopted in Singapore, New Zealand and USA. As a pre-requisite to the precast concrete design, conscious effort was made to ensure the overall stability and safety at all stages of the building construction, particularly the structural continuity when precast elements were connected. Much attention was paid also to the design of local effects and detailing of connections between precast units and between precast and in-situ concrete. In addition, in consideration of the special design and locations of the joints, the behaviour of the precast walls was compared with that of cast-in-situ ones. Finite element analysis was used to find out whether precast walls and cast-in-situ walls could attain comparable stiffness and to check for presence of local weak spots that might cause local cracking of concrete and decrease the monolithic behavior of the precast wall. With increasing loads, concrete stress was allowed to increase up to a threshold value. Beyond this level, either local failure
or cracking would occur. Although cracking in concrete is a very complex mechanism, it can be generally visualized to start at highly stress concentrated points. Mesh elements at these points were deleted from the analysis model. Similarly, supports were released in the model where tension reactions occurred which, in practice, no continuity of reinforcement through the joints would be provided and the grouting material assumed not to take any tension. Original continuum of the finite element model was disturbed since discrete voids were formed when some mesh elements were deleted. The transformed model was subject to the same loads again in further analyses. New set of results (stresses and reactions) was obtained and the trimming exercise was carried out again. The iteration process repeated until, after several iterations, the model converged to an equilibrium state. Results were then compared with those obtained from the control cast-in-situ models for deviation in stiffness of precast walls. Any significant deviation was considered in the analysis of the superstructure. (3) Volumetric Precast Bathroom Among the many volumetric precast components, volumetric precast bathroom is used as a representative to illustrate the structural design philosophy behind. The precast bathroom is integrated with a precast structural wall, i.e. one of its walls is a structural wall. The provision of a joint at top of the non-structural walls on the other three sides, to be sealed up only at a later stage, defines a distinctive load path that the self-weight and imposed load are taken up by the integrated precast structural wall. It should be noted that premature sealing the joint may cause cracking to the non-structural walls since part of the imposed load will be transferred through the walls and accumulated at low floors. CONSTRUCTION OF THE BLOCK For the purposes of close monitoring and better quality control, in this pilot project, the HA has made it mandatory to have on-site manufacture of the precast structural walls. The set up of the on-site fabrication yard takes into consideration the logistics for production, the storage and erection of the precast elements with an aim to minimize double-handling in internal transportation of the precast elements, which is costly. Considering that there is less construction activities at working floor as compared to conventional cast-in-situ construction method, it is evaluated that the normal 6-day construction cycle could be shortened to four or five days when the contractor becomes familiarized with the EPPS. The faster construction rate is one of the major benefits of the system. BENEFITS OF THE EPPS The EPPS has a number of major benefits. (1) Effective Quality Control Better quality is ensured. As precast components are mass produced by moulding in factories, quality control is easier and construction errors are lesser.
Under the EPPS, not only can aesthetic finishes be applied to the precast components, but also aluminum windows and building services devices can be installed in advance. These will greatly reduce potential damage to the materials arising from on-site installation while improving durability and cutting down future maintenance costs. The use of the seamless polymer floor pans also effectively solve water leakage problems in bathrooms and kitchens, in turn reducing repair and maintenance costs and also minimizing nuisance or disturbance to the residents. (2) Enhanced Site Safety The EPPS is much safer as it can reduce a large amount of work at height in the construction site such as reinforcement fixing, formwork erection and concrete pouring. (3) Greater Efficiency & Shorter Construction Time As the precast components are factory-made, site work will only involve simple assembly of the components and a small amount of concrete pouring work. Site progress is thus sped up. It is estimated that the current construction cycle of typical floor can be shortened to four or five days. It is estimated that a normal 28-month building project (normally adopted for HA housing projects comprising 41-storey blocks) can be shortened by at least five months when the industry is familiar with the EPPS. Besides, the works progress is also less susceptible to inclement weather, which helps to ensure timely completion of a construction project. (4) Better Environmental Protection The mechanized production of precast components in factories is much more environmentally friendly when compared to the conventional insitu construction. It will reduce both material wastage and construction waste. Also noteworthy is that the EPPS can substantially cut down the use of temporary external working platforms. At the same time, as less concrete pouring work is carried out on-site, noise, air pollution and other associated nuisance to nearby residents can be minimized. (5) Increased Cost Effectiveness All these benefits, when added together, can bring about a substantial saving in the overall construction and maintenance costs when the EPPS is widely used in public housing developments. CONCLUSIONS Actualisation of the EPPS is not an easy task dealing with the existing conventional construction practice and challenge of more precasting, putting down ideas and concepts on the drawing boards and finally turning the blue-prints to construction reality. It is indeed a result of HA s commitment to public benefits, persistent pursuit of sustainability, creative articulation of overseas experience and local practice, and concerted efforts of the HA and the Contractor. As the EPPS has so many valuable potentials, we should explore its application in part or in full. This will certainly contribute to the quality and sustainability of public housing developments and help to meet the ever-increasing expectation of the public.