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The Malaysian construction industry is undergoing a transitional change from an industry employing conventional technology to a more systematic and mechanized system. This new system is now known as the Industrialized Building System (IBS). This new method of construction can increase productivity and quality of work through the use of better construction machinery, equipment, materials and extensive pre-project planning. This study becomes very necessary since there is yet no organized body, which can provide the necessary information on the building cost comparison between the conventional system and industrialized building system in Malaysia's construction industry. This study also addresses the building cost comparison of the conventional system and industrialized building system of formwork system. It provides the details building cost between the conventional system and the formwork system and indicates which of the two is cheaper. The data were collected through questionnaire survey and case study, which consisting of institutional buildings. Through the statistical test't-test' it is shown that there is a significant difference in cost saving for the conventional system as compared to the formwork system (industrialized building system).
The Malaysian construction industry is undergoing a transitional change from an industry employing conventional technologies to a more systematic and mechanized system employing the latest computer and communication technologies. This is vital for the future health of the industry, given the trend towards global competition and the advent of the k-economy.
The Industrialized Building System (IBS) has been introduced in Malaysia since the 60's by the use of precast concrete beam-column elements. Since the demand of building construction has increased rapidly, it is necessary to innovate a construction method, which speeds up the building construction process. To sum-up, in general, the IBS is a methodology whereby a local construction industry is driven towards the adoption of an integrated and encouraging key players in the construction industry to produce and utilize pre-fabricated and mass production of the building at their work sites. This will help to enhance the efficiency of construction process, allowing a higher productivity, and quality, time and cost saving.
The construction cost of a building using precast components should be assessed in its overall context. The traditional method of costing by material quantities with a fixed factor for labor cost can lead to incorrect estimation. For example, if labor usage is halved, this will more than compensate for a 10% material increase. More importantly, there is saving in time. Also, if properly designed and executed, precast can lead to much better quality of work. The overall cost impact of precast has therefore to take all these factors into consideration. With the rising costs of labor and less assurance of dependable skilled manpower, the trend is that precast construction will become increasingly competitive compared to cast-in-place construction?
PRECAST CONCRETE CONSTRUCTION
Every construction material & system has its own characteristics which to a greater or less extend influence the layout, span length, construction depth, stability system, etc. This is also the case for precast concrete, not only in comparison to steel, wood, & masonry structures, but also with respect to cast in-situ concrete. Theoretically, all joints between the precast units could be made in such a way that the completed precast structure has the same monolithic concept as a in-situ one. However, this is a wrong approach & one, which is very labour intensive & costly. If the full advantages of precast concrete are to be realized, the structure should be conceived according to its specific design philosophy:
- Long spans, appropriate stability concept, simple details, etc. Designers should from the very outset of the project consider the possibilities, restrictions & advantages of precast concrete, its detailing, manufacturer, transport, erection & serviceability stages before completing a design in precast concrete.
Precast concrete system enables faster programmed times - not affected by weather or labour shortages. Improves buildability - early enclosure of dry envelope enables follow-on trades to start sooner. Produces a high standard of workmanship in factory conditions - reduces potential for accidents, addresses on-site skill shortage. Has a high quality finish that can be left exposed - concrete's thermal properties can be exploited in low-energy buildings.
- Keep water out
- Prevent air leakage
- Control light
- Control radiation of heat
- Control conduction of heat
- Control sound
- Resist wind forces
- Control water vapor
- Adjust to movement
- Thermal and moisture expansion or contraction
- Structural movements
- Resist fire
- Weather gracefully
- Easy to install
Architectural precast concrete provides architects with an exciting medium when designing facades for a wide range of buildings, from healthcare facilities to shopping malls, commercial office buildings to sports stadiums.
Precast concrete provides:
- Complete thermal protection
- Continuous air/vapour barrier
- Effective rain screens
- Superior lifespan
- Reduced construction schedule and on-site labour
- High quality control standards
- Numerous finish options and colours
CATEGORIES OF PRECAST BUILDING SYSTEMS
Precast buildings constitute a significant fraction of the building stock in the republics of the former Soviet Union and Eastern European countries. Depending on the load-bearing structure, precast systems can be divided into the following categories:
- Large-panel systems
- Frame systems
- Slab-column systems with walls
- Precast concrete floor
The designation “large-panel system” refers to multistory structures composed of large wall and floor concrete panels connected in the vertical and horizontal directions so that the wall panels enclose appropriate spaces for the rooms within a building. These panels form a box-like structure (see Figure 1). Both vertical and horizontal panels resist gravity load. Wall panels are usually one story high. Horizontal floor and roof panels span either as one-way or two-way slabs. When properly joined together, these horizontal elements act as diaphragms that transfer the lateral loads to the walls.
Depending on the wall layout, there are three basic configurations of large-panel buildings:
- Cross-wall system. The main walls that resist gravity and lateral loads are placed in the short direction of the building.
- Longitudinal-wall system. The walls resisting gravity and lateral loads are placed in the longitudinal direction; usually, there is only one longitudinal wall, except for the system with two longitudinal walls. Two-way system. The walls are placed in both directions.
Thickness of wall panels ranges from 120 mm for interior walls to 300 mm for exterior walls. Floor panel thickness is 60 mm. Wall panel length is equal to the room length, typically on the order of 2.7m to 3.6 m. In some cases, there are no exterior wall panels and the façade walls are made of lightweight concrete. A typical interior wall panel is shown in Figure 2.
Panel connections represent the key structural components in these systems. Based on their location within a building, these connections can be classified into vertical and horizontal joints. Vertical joints connect the vertical faces of adjoining wall panels and primarily resist vertical seismic shear forces. Horizontal joints connect the horizontal faces of the adjoining wall and floor panels and resist both gravity and seismic loads.
Depending on the construction method, these joints can be classified as wet and dry. Wet joints are constructed with cast-in-place concrete poured between the precast panels. To ensure structural continuity, protruding reinforcing bars from the panels (dowels) are welded, looped, or otherwise connected in the joint region before the concrete is placed. Dry joints are constructed by bolting or welding together steel plates or other steel inserts cast into the ends of the precast panels for this purpose. Wet joints more closely approximate cast-in-place construction, whereas the force transfer in structures with dry joints is accomplished at discrete points.
Precast frames can be constructed using either linear elements or spatial beam-column subassemblages. Precast beam-column subassemblages have the advantage that the connecting faces between the subassemblages can be placed away from the critical frame regions; however, linear elements are generally preferred because of the difficulties associated with forming, handling, and erecting spatial elements. The use of linear elements generally means placing the connecting faces at the beam-column junctions. The beams can be seated on corbels at the columns, for ease of construction and to aid the shear transfer from the beam to the column. The beam-column joints accomplished in this way are hinged. However, rigid beam-column connections are used in some cases, when the continuity of longitudinal reinforcement through the beam-column joint needs to be ensured. The load-bearing structure consists of a precast reinforced concrete space frame and precast floor slabs. The space frame is constructed using two main modular elements: a cruciform element and a linear beam element. The cruciform element consists of the transverse frame joint with half of the adjacent beam and column lengths. The longitudinal frames are constructed by installing the precast beam elements in between the transverse frame joints. The precast elements are joined by welding the projected reinforcement bars (dowels) and casting the concrete in place.
Slab-Column Systems with Shear Walls
These systems rely on shear walls to sustain lateral load effects, whereas the slab-column structure resists mainly gravity loads. There are two main systems in this category:
- Lift-slab system with walls
- Prestressed slab-column system
Precast columns are usually two stories high. All precast structural elements are assembled by means of special joints. Reinforced concrete slabs are poured on the ground in forms, one on top of the other. Precast concrete floor slabs are lifted from the ground up to the final height by lifting cranes. The slab panels are lifted to the top of the column and then moved downwards to the final position. Temporary supports are used to keep the slabs in the position until the connection with the columns has been achieved. In the connections, the steel bars (dowels) that project from the edges of the slabs are welded to the dowels of the adjacent components and transverse reinforcement bars are installed in place. The connections are then filled with concrete that is poured at the site. Most buildings of this type have some kind of lateral load-resisting elements, mainly consisting of cast-in-place or precast shear walls, etc. In case lateral load-resisting elements (shear walls, etc.) are not present, the lateral load path depends on the ability of the slab-column connections to transfer bending moments. When the connections have been poorly constructed, this is not possible, and the lateral load path may be incomplete. Another type of precast system is a slab-column system that uses horizontal prestressing in two orthogonal directions to achieve continuity. The precast concrete column elements are 1 to 3 stories high. The reinforced concrete floor slabs fit the clear span between columns. After erecting the slabs and columns of a story, the columns and floor slabs are prestressed by means of prestressing tendons that pass through ducts in the columns at the floor level and along the gaps left between adjacent slabs. After prestressing, the gaps between the slabs are filled with in situ concrete and the tendons then become bonded with the spans. Seismic loads are resisted mainly by the shear walls (precast or cast-in-place) positioned between the columns at appropriate locations.
Precast concrete Floor
The principle advantages of precast floors are speed of construction, absence of scaffolding, large variety of types, large span capacity, & economy. Precast floors can also be classified according to their manufacture into totally & partially precast floors. Totally precast floors are composed of units, which are totally cast at the plant. After erection, the units are connected to the structure & the longitudinal joints are grouted. In some cases a cast in-situ structural topping screed is added. Partially precast floors are composed of a precast part & a cast in-situ part. Both parts are working together at the final stage to achieve the composite structural capacity. The main totally precast floor & roof types are described hereafter.
CONVENTIONAL IN-SITU CONSTRUCTION
Conventional Construction Method
Conventional construction encompasses traditional forms of structural load-bearing elements; typically composed of concrete, brickwork and structural steel. We are well-versed in all forms of conventional construction and have substantial in-house capacity. The majority of our commercial and unique residential products to date have utilized conventional methods of construction. A number of designers that we have worked with tend to express the structural elements of the construction, from exposed beams, cantilevered slabs and stairs, to exposed structural steelwork. This requires a high degree of accuracy as well as a high level of workmanship; both of which are easily attained using our in-house skills. Conventional building method is defined as components of the building that are pre-fabricated on site through the processes or timber or plywood formwork installation, steel reinforcement and cast in-situ. Conventional buildings are, mostly built of reinforced concrete frames. The traditional construction method uses wooden formwork. It is much more costly for construction, which includes labor, raw material, transportation and low speed of construction time.
Cast-in-situ Construction Method
This system is suitable for a country where unskilled labor is limited. There is no heavy machinery or high technology involved. The system is technically applicable to almost all types of building. Formwork is used as a mould, where wet concrete, is poured into a temporary system. The temporary system also acts as a temporary support for the structures. The objective of in-situ method is to eliminate and to reduce the traditional site based trades like traditional timber formwork, brickwork, and plastering and to reduce labor content. A carefully planned in-situ work can maximize the productivity, speed and accuracy of prefabricated construction. Cast in-situ method uses lightweight prefabricated formwork made of steel/fiberglass/aluminum that is easily erected and dismantled. The steel reinforcement is placed within the formwork as they are being erected and concrete is poured into the mould. When the concrete is set according to the required strength the mould is dismantled. The workers can be trained easily to erect the moulds and set the steel reinforcement. Its advantages over the traditional construction method are, its low skill requirement, can be quickly constructed, maintenance is low, structure is durable and cost can be less.
In-situ method is to eliminate and reduce the traditional site based trades like traditional timber formwork, brickwork, plastering and to reduce labor content. Carefully planned in-situ work can maximize the productivity, speed and accuracy of prefabricated construction. The formwork system is based on the combination of pre-fabrication and in-situ conventional construction, which features the utilization of permanent concrete for elements instead of conventional timber formwork.
Differences Between In situ and Precast Construction Method
Precast construction method only use semi-skilled workers and don't use skill or unskilled- worker in construction process. Economies are generated through reduced requirements for formwork, access scaffolding and less reliance on wet trades. Reduced on-site supervision by the main contractor is also a saving. So for precast construction method, labours are not use 100 percent for making formwork, access scaffolding, and handle wet concrete. Due to speed of construction, gives earlier return on investment, freeing up the project critical path and allowing earlier completion. It is estimated that a precast structure takes up to 20% less time to construct than a similar cast in situ structure, using labour can be late because of rest time and energy of a labour. For quality and accuracy, precast construction methods will more quality n accuracy than in situ. This ensures that reinforcement bars are accurately located and that clients receive high quality products manufactured to controlled dimensional tolerances. Precast method delivers a high performance product with a quality appearance. Have a high quality finish that can be left exposed - concrete's thermal properties can be exploited in low-energy buildings because all of the equipment are made up in factory.
In situ construction method use skilled worker, semi-skilled worker, and un-skilled worker in construction process. Labour can amenable to almost any design and labour can alteration the design in last minute construction process. Design can proceed as the structure is built because labour takes a time to build. Construction can proceed independently of weather conditions because workers still can proceed the work even though whether are not good. Construction process is easily used for two way structural systems because labour used two ways structural system in construction process. It is not necessary to pay for crane on site because of labour can take instead of crane functions.
Environmental and manufacturing conditions at a precast concrete plant are easily monitored. The production of precast concrete elements takes place under controlled conditions in enclosed factories. This makes the control of manufacturing, waste, emissions, noise levels, etc. easy compared with the same processes at a building site. The raw material consumption is similar for similar qualities of concrete, whether the production takes place in a factory, at a ready-mix plant or at a building site. The raw material waste in precast concrete production is very small.
The process of preparing mild steel reinforcement may be the same for a precaster as for a contractor at a building site, except that precasters will usually have less waste. This results in better utilization of the steel and less consumption of natural resources. Surplus materials are generated during the production of precast elements. Much surplus material is recyclable, and consists mainly of hardened concrete with or without reinforcement, steel reinforcement and pieces of structural steel, plywood and other wooden materials, fresh concrete (from production and washing of equipment), slurry from the sawing of concrete, insulating materials (mineral wool and polystyrene), oil etc. from machinery and paper and other packaging materials.
The amount of surplus material varies between factories and different types of production. Studies in the Scandinavian countries have shown that the magnitude is typically about 100 kg of surplus material per m3 of concrete produced. About 40% of the surplus material is fresh and hardened concrete and about 45% is wastewater from washing equipment and sawing slurry generated during hollow core slab production. It is possible to collect and sort different types of surplus materials in precast plants. Excess materials that can be recycled and reused include steel, wood, insulating materials, oil, paper and other packaging materials. Wood can be sorted out, cut and used as industrial firewood, or used for other construction purposes.
Buildings are constructed with traditional cast in-situ concrete, using timber formworks. Building that timber formwork was the major contributor to construction waste, accounting for 30% of the total identified waste. Wet trades, such as concreting, masonry, plastering and tiling on-site were considered as the second major waste generator, accounting for 20% of the total on-site waste generated. A recent study demonstrated that the ‘off-cuts from cutting materials' were a major cause of wastage during construction. Waste also arises as a result of design concepts and decisions.
Case studies in Sri Lanka
In the construction industry, it is well known that there is a relatively large volume of material being wasted due to a variety of reasons. The problem of material waste on construction sites is not an isolated issue and is of environmental concern. Therefore, waste minimization has become an important issue in the construction industry. The aim of this research was mainly to identify the pre-cast contribution to the construction waste minimization in the Sri Lankan construction industry, through a comparison of material waste arising from pre-cast, ready-mixed and site-mixed concrete.
Data were collected from 27 building construction projects and three concrete elements: slabs, beams, and columns, were considered to quantify construction waste. To compare the wastage due to pre-cast involvement with other types, three categories of building projects were used, including projects using pre-cast concrete elements, in situ concrete elements - site mix, and in- situ concrete elements - ready mix.
The data for the study were collected from 27 multi-storey housing constructions projects, of which, seven projects used pre-cast construction and 20 projects used in situ construction. The wastage was compared between the basic materials used for three types of concrete elements are columns, beams and slabs. In this study, material wastage includes waste arising from manufacturing process at the factory level to the site level. For instance, material waste of pre-cast and ready-mixed concrete were quantified considering the waste arising from manufacturing process at factory level to usage at construction site. However, it was identified that waste during the transportation of ready-mix concrete and pre-cast elements is negligible. Further, waste of pre-cast elements at the site level was also noted as almost zero and, hence only the factory level waste was considered for the analysis. Techniques of material reconciliation were used to analyse the waste of ready-mix concrete and pre-cast elements at the site level, while work studies were used to quantify the waste of site-mixed concrete at the site level and wastage of ready-mix concrete and pre-cast concrete at the factory level.
Pre-cast concrete waste
The mean wastages of cement, sand and metal amounted to 5.34 per cent, 13.86 per cent and 7.62 per cent respectively showing the lower values compared with the material wastages in the other two situations (Table III). Further, it was shown that there is a noticeable difference in the generation of material waste between pre-casts and in situ (Figures 1 and 2). The main reason behind this may be due to the negligible wastes arisen during transportation and installation at the site. The pre-cast concrete elements transported to the site were stored unit wise by during transportation had been minimized and identified as zero. Since pre-cast elements were supplied according to the required length, waste arising during installation of elements was at a minimum level and waste occurring due to over ordering of materials was also eliminated. Further, the pre-cast elements were produced at factories under proper supervision using steel moulds which can be formed of different sizes. Therefore, the wastage of materials during manufacturing also reduced to a considerable amount.
Site-mixed concrete waste
In site-mix concrete, the mean wastages of cement, sand and metal amounted to 14.39 per cent, 25.70 per cent and 16.11 per cent respectively showing higher values compared with the material wastages in other situations (Table III). This large quantity of wastage was identified due to the lack of supervision, inaccurate mixing methods, inappropriate type of equipment used, poor storage of materials and poor quality workmanship and this led to higher waste of materials in ways of excess cement being used to accelerate the curing process, excess concrete being used due to the breaking of form work, higher waste in transit and handling of metal and sand and excess concrete being used in uneven surfaces (e.g. - attached concrete column).
Ready-mixed concrete waste
The mean wastages of cement, sand and metal amounted to 6.61 per cent, 22.31 per cent and 13.01 per cent respectively showing the higher values than material wastages of pre-cast concrete and lower values than material wastages of site-mixed concrete (Table II). Although there was lower wastage at the factory level, the overall wastage of ready-mixed concrete showed higher values. The main reason behind this is the excess ordering of materials, large quantity of concrete remains in pump car and pump pipe and poor quality workmanship at the site level such as breaking of formwork.
Case studies in Hong Kong
The questionnaire survey revealed that the construction activities were closely related to the amount of waste generated. Timber formwork is the major contributor to construction waste. The wet trades associated with finishing work such as screeding, plastering and tile laying are identified as the second major set of waste generation processes in the construction of buildings. Concrete work and masonry work are the next most significant groups. Site activities need to be emphasised in order to reduce building waste. In general, it was estimated that about 5-10 per cent of materials ended up as waste on building sites.
Pecaform foundation formwork is made by laminating a layer of polyethylene to each side of a high tensile steel wire mesh. This combination creates a material that is both light and structurally strong, making it very easy to handle. It can be used for constructing ground beams, pile caps, footings, curved structures, ribbed and waffle slabs. The formwork is cut-to-size and bent to shape at a factory and arrives at site ready for installation. There is no need to strip the formwork after the concrete has cured. Very little waste is produced. A clean and neat site can be obtained in the foundation stage.
Large panel formwork compared with traditional timber formwork, metal panel system formwork has several advantages. The use of large panel formwork can save time and labour in erecting, striking and re-erecting the formwork as the panel is handled as one unit. It can also produce a concrete surface much neater than conventional timber formwork and the surface essentially needs no additional applied finishes for levelling. So far, steel is the most popular material used for the formwork and the reusability of steel formwork can be as high as 100 times, and therefore much formwork waste is reduced. On-site waste audit also indicated that large panel formwork was effective in reducing concrete waste generated by loss in concreting and broken formwork, which usually accounts for about 30 per cent of the total concrete waste Prefabricated steel reinforcement system is tailor-made in plants where steel bars are mechanically cut, bent and fabricated. The completed systems are then transported to the sites for use. During the manufacturing process, the steel bars are cut and bent in a more systematic and accurate manner so that the wastage is kept to a minimum.
Precast cladding is a new construction method for tiling works in the corridors of public housing projects in Hong Kong. The cladding panels are manufactured in the precast factory. The production processes are: place wall tiles onto the steel mould face down, pour lightweight concrete onto the back of the tiles, and demould after hardening. The tiles are thus cast integrally with the lightweight concrete and no traditional tile fixing by cement mortar or adhesive is required. At the construction site, the positions for holding brackets will be set out, and cladding panels levelled and fixed. The advantages of precast cladding panels are: reduced requirements on skilled labour with better end product quality, less wastage of raw materials and waste generation as wet trades on site such as plastering and tiling are eliminated, flexible time control as no setting and curing time are needed on site, and a cleaner and safer working environment.
Machinery sprayed plaster was originally used in civil engineering applications. It now replaces the traditional cement mortar in some building projects. The major difference between the mechanised plaster and the traditional cement mortar is that the former is mixed and applied by means of a mechanised operation whilst the latter is applied and trowelled smooth by hand. The use of machinery sprayed plaster has the benefits of high productivity, low labour demand and less waste.
Precast bathroom is an innovative feature in the construction of public housing. The wall, floor and ceiling of the bathroom are prefabricated with concrete as a whole unit and finished with tiles in the factory. PVC sleeves are also left in the structure for the future fixing of drainpipes. It is then transported to the site and installed into the final position of the building structure with the help of a crane. The only work required on site is to install the sanitary fittings, connect plumbing and electricity wiring to the building mains. Wet trades are avoided on site and this reduces material damages and wastage.
Pre-cast external wall panels. Traditionally, external wall panels in high-rise residential buildings have been constructed in-situ with concrete panels finished with mosaic tiles. Pre-casting external wall panels enables panels to be pre-finished with the mosaic tiles attached together with windows and louvres installed. Building services provision can also be included. The finishes are fixed in the factory, production is not affected by the weather, a cleaner safer production environment results in stronger bonding and less tile wastage.
Pre-fabricated fibre glass water tank. This type of pre-fabrication is an example of saving construction waste by materials substitution. All residential housing units of the New Harmony type include water storage on the roof of the building; the normal design solution is a cast in-situ concrete tank. An alternative to the construction of the cast-in-situ concrete water tank on the roof of the building is the adoption of a pre-fabricated fibre glass tank.
Pre-fabricated internal wall panels e.g. dry wall partitions, and gypsum boards may be used to replace concrete or brick walls which are then finished by plastering in-situ. This has the potential to make significant savings in construction waste because a typical residential building would have some 452m2 of internal walls per floor and typically there would be 32 or 40 floors per building. The adoption of this form of construction has minimal impact on the design tasks because the pre-fabricated internal walls simply replace the concrete or brick walls without affecting the overall architectural design. There is some impact on the design approach for selected design tasks because regard has to be taken of the penetration and fixing details through and onto the pre-fabricated internal wall panels.
In the building industry, there are low-waste technologies like precast units, reusable formwork, drywall, spray plastering, tubular scaffolding and steel hoarding, which can reduce waste generation in the construction processes. In general, these low-waste building technologies facilitate quicker construction, produce better quality work and require less labour. In addition, there would be cost saving if mass production is required or the construction materials can be reused. The use of low-waste building technologies would be an effective means to reduce waste generation.
Concrete is used in either a ‘precast' or an ‘in-situ' state. Precasting is the process whereby concrete is cast into elements (units) prior to their integration into a structure. From the precasting process comes precast concrete formed in the image of its mould to be used as an element of architectural design, whether structural, functional or decorative. Whereas, concrete which is cast in place, on site, is called in situ concrete.
Precast concrete covers all factory-made concrete products from mass produced blocks, paving and roof tiles, to massive customized bespoke units. Examples of precast concrete are numerous and as it will be seen from the following description, ‘precast concrete' needs to be an applied term for it to have a useful descriptive meaning. Precasting may take place as either an on-site or a factory operation.
Thus, quality of precast concrete can be controlled and manipulated better than conventional in situ construction method with several reasons. The reasons will be further discussed as in below. Those precast companies work with specialist fixing teams to install their products. This guarantees precise, reliable workmanship ensuring that the quality of service from precast is maintained after the products leave the factory. This is due to the reason that the precast products are produced in factories under strictly controlled conditions. The factory environment has a steady temperature, regular shift patterns and a dedicated workforce. This can say that all high quality products can be made every day, regardless of the weather. And in order to maintain and increase the workforce in the precast industry, workforce who is highly skilled are attracted with good benefits and the safety of this industry has improved also. It can be shown through the safety record with accidents 65% down in seven years since 2000.
As the precast industry keeps a keen eye on the national and international standards, this ensures that customers receive the best quality products which are compliant to all relevant standards. As well as specific product standards, many manufacturers also comply with ISO 9001 and ISO 14001. Besides, precast concrete made in factories always give great results. This is because the factory - controlled procedures is specific and can produce many precast concrete once. Repetition of individual units can be achieved with confidence whether there is one, ten or 100. With the re-using of a mould will make both environmental and economic sense. Thus, every additional cast saves materials, energy and time, and it can prevent moulds going to recycling or to disposal prematurely. Besides, the moulds used are long lasting, they can be stored to allow later replication, and no matter additional units is required one day, one week or one year later or not.
Moreover, precast concrete are weather proof. It is resistant to rain penetration and wind-blown debris. Only concrete and masonry walls can provide this kind of protection. In a study of exterior wall systems, Texas Tech. University's Wind Engineering Research Center found that only concrete wall systems were proven to withstand 100% of all known hurricane-force winds, and over 99% of tornado-force winds. Concrete can also withstand many winters of freeze-thaw cycles, unlike other materials that can deteriorate quickly with such regular exposure to climatic changes. In damp, exposed or harsh environments, other materials struggle to match the performance of concrete.
In addition, precast concrete is also resistant to the effects of climate change. Predictions of higher winds, more driving rain, tropical-style deluges and flash floods, and more incidences of localized windstorms in the UK are of concern for homeowners and businesses. Precast concrete and masonry products offer better protection against these possible effects of climate change
Because they are robust, durable and have structural integrity.
Also, precast concrete does not rust. It is resistant to corrosion and can therefore be used with confidence, even in very aggressive environments. For example, precast concrete piers are resistant to the inter-tidal anaerobic attack experienced in some marine environments. Furthermore, tight quality controls in factory production mean that cover levels for rebar are guaranteed in any application, and in most cases this prevents even minor unsightly rust patches developing. It also resistant to chemical attack. For vehicle hard-standing, aircraft standing aprons and other paved areas, concrete paving blocks are an ideal choice because they are resistant to fuel and oil spills. Their use makes sense economically and environmentally because pavement repairs are localized, using less materials and causing less disruption.
Besides, precast concrete is tough and durable and resilient in the face of intense pressure. For example, used for underground pipes, precast is resistant to jetting and on roads or other paved areas it provides a durable surface against the rutting caused by traffic.
In addition, due to the high quality of precast and the fact that it does not erode or rot make the task of cleaning up after a flood very straightforward. This can be a difficult time for people struggling to come to terms with the devastation that floods can bring, so precast brings a welcome respite.
It also has impressive whole life value. The strength of concrete increases with years. Other materials can deteriorate, experience creep and stress relaxation, lose strength and/or deflect over time. However, studies have shown that precast concrete products can provide a service life in excess of 100 years.
Besides, precast concrete is noncombustible. The component in precast are naturally fire protected, because they will not burn. They will eliminate the messy and time-consuming fireproofing required for a steel structure and subsequent repairs caused by other trades. Besides, concrete does not lose its structural capacity nearly as quickly as steel, which is now a significant consideration as witnessed in the attacks on the World Trade Center and the towers' subsequent collapse. Other materials besides concrete and steel are flammable and/or do not perform well in elevated temperatures. Fiberglass begins losing structural integrity at 200 F. HDPE begins to melt at 266 F. It is same like many other concrete and masonry materials, precast concrete will not melt in high temperatures. This means that there is no need for protective paints or special insulation and finishes can be viewed just as the designer intended. Concrete will not drip molten particles in a fire, and this helps protect human life by providing safe escape routes and preventing fire spread.
Moreover, precast concrete does not leach. This is because precast concrete is chemically inert product. It will not leach out any harmful chemicals in use. Thus, it is safe to be used in distribution of drinking water and store or transport harmful fluids securely.
Every construction material & system has its own characteristics which to a greater or less extend influence the layout, span length, construction depth, stability system, ect. This is also the case for per-cast concrete, not only in comparison to steel, wood, & masonry structures, but also with respect to cast in-situ concrete. Theoretically, all joints between the pre-cast units could be made in such a way that the completed pre-cast structure has the same monolithic concept as an in-situ one. However, this is a wrong approach & one, which is very labour intensive & costly. If the full advantages of pre-cast concrete are to be realized, the structure should be conceived according to its specific design philosophy.
For long spans, they need appropriate stability concept, and simple details. Designers should from the very outset of the project consider the possibilities, restrictions & advantages of pre-cat concrete, its detailing, manufacturer, transport, erection & serviceability stages before completing a design in pre-cast concrete.
STRUCTURAL PRE-CAST CONCRETE
- Productivity on site
- Pre-cast design concept
“Productivity is a process of continuous improvement in the production/supply of quality output/service through efficient, effective use of inputs, with emphasis on teamwork for the betterment of all”. It is an attitude that seeks the continuous improvement of what exists. It is a conviction that one can do better today than yesterday, and that tomorrow will be better than today. Furthermore, it requires constant efforts to adapt economic activities to ever-changing conditions, and the application of new theories and new methods. It is a firm belief in the progress of humanity.
Pre-cast concrete construction has managed to remain the least understood of the major forms of multi-storey building construction. This is partly due to the fact that trainee engineers are not exposed to the design requirements and benefits of pre-cast concrete as part of their education. This limited amount of training and hence questioning of pre-cast design within the consulting engineering profession has resulted in structural pre-cast concrete often being overlooked. Over the last 10 to 20 years manufacturers have promoted their in-house design capabilities and refined their products to widen the use of structural pre-cast concrete. The result is a closing of the gap between complex building design and the perceived limitations of using pre-cast concrete. Structural engineers and architects are now beginning to appreciate the benefits.
Pre-cast structures have been shown to be extremely cost effective, durable, stable, and of the highest quality and strength. Design, due to its specialized nature, often remains with the manufacturers and their personal engineers. This paper is designed to give further insight into the benefits of structural pre-cast concrete design and construction, with particular emphasis on skeletal frame structures.
2.1 Preliminary concepts
Probably the most critical aspect of successful pre-cast design is the preliminary evaluation. Each design concept is evaluated to determine the most economical and efficient construction method. The design should be considered as a complete pre-cast system and not simply as numerous individual components connected together.
The structural options to be considered include:
- Panelized structural envelope: This type of structure generally comprises structural pre-cast walling and/or exposed spandrel beams and panels with the floor system spanning between walls.
- Structural pre-cast skeletal frame: This type of structure incorporates a structural pre-cast frame of columns and beams with a pre-cast floor system. The frame can be either moment resisting or pin jointed with lateral loads carried by shear walls.
- Hybrid structure: This term has been coined in Europe to describe combination structures that incorporate pre-cast concrete combined with other structural materials. For example a structure with pre-cast flooring supported on a steel frame or masonry walls.
- Combination structure: As the name suggests a building combining more than one of the above systems.
The first task, in pre-cast design, is to establish the most economical geometric layout by minimizing the number of components. The cost of components should be considered, with the objective being to minimize the number of high cost components, and maximize the number of low cost components. The optimum solution is generally found in a rectangular grid. The most economical solution, unlike traditional construction techniques, is usually found with the pre-cast floor system spanning the longest dimension. This is due to the fact that with pre-cast floors and in particular hollow core systems the cost penalty of increasing the slab span from 8m to 12m is virtually insignificant. On skeletal frame structures this allows supporting beams to span the shorter dimension and the beam depth to be minimized. It must be stressed, however, that the most economical solution is very much project defined and must be evaluated on a job-by-job basis.
While the types of components that can be produced and effectively used within this form of construction are only limited by the imagination, one of the significant reasons that pre-cast has become such a successful form of construction has been the ability to standardize a number of components. Standardization is often misconstrued as modulation. Standardization typically refers to construction techniques and section types rather than a specific unit. For example, two completely different structures can be designed and erected using standardized products simply by adjusting beam depths, column lengths, wall panel positions and different floor systems. One of the most significant reasons for the evolution of the following concepts has been the consideration of labour verses material costs. This has resulted in an emphasis on simplicity and repetition, even at the expense of material costs, to ensure labour productivity is maintained at the optimum level.
Pre-cast columns can be produced as either multi-storey corbelled columns or single floor to floor elements. They may be either pre-stressed or reinforced. In the Kuala Lumpur market it has been found that the single floor to floor column is the most economical. Single storey reinforced columns are simple to design, detail and construct. Once loads and bending moments are established the design process is the same as a standard reinforced in-situ column. Eccentric loading due to erection requirements and localized effects at the top and bottom of the column should be taken into account in the design. Extra reinforcement is usually provided at the top and bottom of the column, these additional ties act as anti-splitting reinforcement Other important factors that must be considered in designing include the required beam bearing on the column. When considering this bearing area, it must be remembered that due to the corner chamfer and backing rod to dam the high flow grout approximately 30 to 40 mm is lost around the perimeter. The base connection is generally analyzed as a pin joint and due to connection details the columns tend to be conservatively sized, manufactured with high strength concrete, and reinforcement typically limited to four corner bars with nominal ties. This approach results in extremely simple components that can easily be mass produced.
Pre-cast beam details have been developed with simplicity and practicality in mind. Typically they are an inverted Tee profile and are designed as pre-stressed or partially pre-stressed. This type of component is designed as continuous for imposed loads in its final form, while being simply supported during the erection phase. They are also designed so that no propping is required during erection of the supported floor. The pre-cast floor components sit directly on the ledge of the inverted Tee. With floor to floor height columns the beams are able to sit directly on top of the columns. Dowels protruding from the columns pass through ducts within the end of the beams to provide the pin joined connection. This allows the connection between beams and columns to be very simple and eliminate the need for difficult corbelled or mechanical shear type connections. One of the most critical design cases for the beams and the beam to column connection is the design for torsion loading. During the erection phase it is inevitable that at some stage the beam will be loaded on only one side causing the beam to â€žroll? on the column particularly if the column is narrower than the beam. The beam column connection must be designed and detailed to resist this torsional load. While the use of this type of one way skeletal structure is a very simple and effective method of construction, it does require a slightly increased overall beam depth compared to slim line profiled beams or a traditional band beam system. The end result is generally that it is more economical to increase the overall building height than to reduce beam depths.
IN-SITU CONCRETE FRAME STRUCTURE
1. Concrete frame process
A concrete frame construction process consists essentially of three main stages, as shown in Figure 1.
- Conception : outline planning authorization
- Detailed Design : details planning authorization
- Construction Preparation : construction authorization
- Built : handover authorization
- Operation & maintenance
The stages are, in turn, further sub-divided into a series of phases comprising a number of specific tasks. Passage between phases and stages is controlled by a series of formal review meetings to authorize progress to the next stage. Supporting activities (e.g. project management, health and safety, information management, task scheduling, etc.) are handled in a series of support processes that underpin the construction process. Before a process can be improved it must be mapped and understood.
Understanding the current process
To develop an understanding of the current process the following activities are recommended:
- Identify the separate tasks and how they link to other tasks.
- Monitor people, their methods of working, movements and interaction with others.
- Monitor flow of information and materials, into and out of the site and between people and tasks.
- Maintain a site diary, one page per day for main events.
- Video the site overall and the individual tasks.
- Create a site reference grid to locate all movements and to provide a reference for the videos. Make the grid visible by using coloured tapes on the floor.
- Construct process charts: Gantt charts and flow charts for activities within each task.
It is not suggested that all these techniques are used. Particular circumstances at each site will determine which are relevant.
Improving the current process
3.1 The key activity in the improvement process is the identification of waste. Waste is any part of the process that does not add value to the concrete frame construction. It includes:
- Waiting and queuing.
- Scrap and rework.
- Over-manning and inefficient working.
The main areas for improvement are likely to be:
- Logistics and supply chain management.
- Resource allocation.
- Operational methods.
3.2 Improving the following areas can also help reduce waste:
- People motivation.
- Use of multi-skilled labour.
- Information management.
Activities that do not add value and that can be eliminated must be removed. Those which are needed to support the process should be optimised to utilise minimum labour and consume minimum time.
Implementing the improved process
The process improvements should be implemented at all three stages of the construction process as described below.
- Provide secure information links between designers and the contractor and ensure that there are no misunderstandings.
- Brief the construction team about the importance of adopting a process approach.
- Set targets for the improvements.
- Brief suppliers to ensure deliveries are made just in time, so that they are neither cluttering the site, nor holding up the process.
- Plan information management.
- Consider introducing information technology, e.g. bar-coding of supplies, secure Internet communication between stake-holders.
- Organize systems for scheduling and define work methods. Fix project review points and targets for each review.
- Determine a policy for conflict resolution.
- Ensure the improved process is followed.
- Identify and record where the process can be further improved and continually make these improvements.
- Involve suppliers in process improvement.
- Feed back findings continually to professionals, site staff and other interested parties.
- De-brief the team and categorize and document the lessons learnt.
- Review the optimized process and further improve it.
- Consider further applications of information technology to underpin the process improvement.
- Measure the improvements in terms of man hours, quality and delivery time.
- Try to gauge other supplementary improvements, e.g. morale, lessons learnt, etc.