This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
IBS has been introduced in Malaysia since early 1960s when Housing and Local Government of Malaysia visited several European countries and evaluate their housing development program (Thanoon et al,2003) After their visit in 1964, the government had started first project on IBS and the aims is to speed up the delivery time and build affordable and quality houses. Project about 7 blocks of 17 stories flat, there are 3000 units of low-cost flat and 40 shops lot, about 22.7 acres of land along Jalan Pekeliling, Kuala Lumpur awarded to JV Gammon & Larsen and Nielsen using Danish System of large panel precast-concrete wall and plank slabs and the project was completed within 27 months from 1966 to 1968 including time taken in the construction of the RM2.5 million casting yard at Jalan Damansara (CIDB,2006;CIDB2003 and Thanoon et al, 2003).
The Industrialized Building System or IBS construction has been adopted by the Malaysia government by making it compulsory for all government projects to content 70% with IBS content, according to Y.A.B Dato' Seri Abdullah bin Hj. Ahmad Badawi, Perdana Menteri dan Menteri Kewangan Malaysia in 2008. Precast concrete is one of the elements being associated with IBS construction. The IBS enables pre-cast building components manufactured at factories, will enable cost saving and quality improvement through the reduction of labour intensity and construction standardization. Apart from these, it offers less wastage, less site materials, cleaner and neater environment, controlled quality, and lower total construction cost (CIDB, 2003).
Precast concrete means concrete which has been prepared for casting and the concrete either is statically reinforced or prestressed. Meanwhile a precast concrete element has limited size and must connect with other elements to form a complete structure. Precast concrete structure refers to the combination of precast concrete elements and the structure is able to sustain vertical and horizontal loads or even dynamic loads. The design and construction of the joints and connections is important to ensure the stability of the overall structure therefore a joint is a mechanism to take care of the forces action at the interfaces of two or more structures elements (Kim s. Elliot, 2002).
2.2 Precast Concrete System
There are 3 types of precast concrete system. The most common is the linear or skeleton (beams and columns) system. This linear or skeleton system is the combination of beams and columns which are able and strong enough to resist vertical and horizontal loads. The second type is precast panel system which normally built on ground and depends on load bearing wall to resist vertical and horizontal loads. The last system is three-dimensional or box system which normally used in industrial building and warehouse (Abraham Warszawski, 1999).
2.2.1 Linear or skeleton system
Linear or skeleton system is defined as this system use as their main structural elements columns, beams, frames, or trusses made of plain or prestressed concrete and their important features is the capacity to transfer heavy loads over large spans (Abraham Warszawski, 1999). Thus, they are used in the construction of bridges, parking lots, warehouses, industrial buildings, sport facilities and so on. They are composed or built of structural frames, spaced at equal distances, thereby creating modular "cells: that can be repeated a desired number of times in longitudinal direction or sideways (Abraham Warszawski, 1990).
Figure 1: Typical schemes of industrial linear system
Different variants of the structural scheme allow for the most convenient partitioning of the frame into connected precast elements. A rectangular frame is usually composed of two columns and a horizontal beam, connected to attain stability in the frame plane so columns maybe fixed at the bottom and the beam is freely supported by columns as shown in Figure 1 (a), which makes it easier for assembling. Columns maybe hinged at the bottom and transfer moments at the top as shown in Figure 1 (b) which does not involve foundations in the transfer of moments. Besides that, pitched frames can be composed of 2 to 4 parts which depending on their dimensions and the transportation and erection conditions as shown in Figure 1 (c) (d) (e). Single frames can be extended sideways into as many bays as necessary or upward for multistory buildings as shown in Figure 1 (f), however, attention must be paid to their lateral stability either by introduction of rigid connections at the corners, by bracing with diagonals or by attaching the multiframe to a rigid building component (Abraham Warszawski, 1999).
The roof may be supported directly on the frames when the precast slabs or Tee beams are used for this purpose and it may also be supported on a precast joist system (Abraham Warszawski, 1999).
2.2.2 Precast panel system
Probably the most widely used types of prefabricated system are those employing panel-shaped elements for floor slabs, vertical supports, partitions, and exterior walls. Unlike linear system, which is mainly used as structural framing, panel system also fulfill interior and exterior space enclose functions. These panels may be fabricated with a considerable amount of finish work such as exterior finish, thermal insulation, electrical conduits andfixtures, plumbing, door and window frames and therefore significantly reduce the amount of skilled labour on site (Abraham Warszawski, 1999). The designation of precast 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. Both vertical and horizontal panels resist gravity load and wall panels are usually one storey high. When properly joined together, these horizontal elements act as diaphragms that transfer the lateral loads to the walls (Svetlana Brzev, 2002). Figure 2 shown these panels form like a box structure.
Figure 2: A large-panel concrete building under construction (WHE Report 55, Rusian Federation)
An exterior wall that performs all these function is usually of a "sandwich" type which it is composed of several layers such as exterior finish, concrete leaves and insulation layer between them. The overall dimensions of the sandwich wall elements will depends other precast elements. Horizontal slab components are used in intermediate building floors, flat roofs, ground floors, landings and platforms (Abraham Warszawski, 1999). Slabs are usually supported along edges on bearing walls or beams as shown in Figure 3. The length of support should be at least 60mm, allowing for positioning and production tolerances and this length may be reduced to a minimum if a temporary support is provided until attaining later a monolithic connection as Figure 3 (d).
Figure 3: Room- size slab: (a) view, (b) section, (c) moment connection, (d) with temporary support
There are several advantages using precast panel system which is this system is high suitable for mass production, speed, no removal of formwork as the panels are casted at the factory and moved to the site when the concrete has gained enough strength, quality control as minimum wet work is carried out at the site, it is easy to control the quality of the material, workmanship and durability and long-term performance as precast panel system also uses the normal reinforced-concrete which has no durability and long-term performance problems (Buddhi S. Sharma,2004)
2.2.3 Three-dimensional or box systems
The three-dimensional system use, as their main building element, box units that contain concrete walls and floors. The units can be either cast in boxlike molds or assembled in the plant from panel elements. In both cases, they can contain a substantial amount of finish works such as wall and floor finish, electrical wiring and fixtures, painted and glazed doors and windows, plumbing pipes and fixtures, kitchen cupboards and so on which are made in the plant before shipping the module to an erection site. However, the size of the three-dimensional or box system modules is determined by the transportation and erection considerations (Abraham Warszawski, 1999).
2.3 Precast Materials
Materials used for precasting of concrete elements include concrete mix and its components included cement, aggregates, reinforcing steel and various admixtures (S.G. Bruggeling and G.F. Huyghe 1991) .
The currently used cements may be broken down by their composition into 4 main categories which is Portland cements made of pulverized clinker which consists essentially of tricalcium and dicalcium silicates, makes up to 90-95% of the cement content. Second, Portland cements blended with other materials such as slag, fly ash, or pozzolan, whose quantity does not exceed 35% of unit weight. Third, Blast furnace cement consisting of clinker and a large component of slag which more than 35% and Pozzolanic cement consisting of clinker and a large component of a pozzolanic material which more than 35% (Abraham Warszawski, 1999).
188.8.131.52 Ordinary Portland Cement
Ordinary Portland Cement is used in the production of most concrete components. The 28th day compressive strength of a standard mortar is made with an ordinary Portland cement, varies according to different national standards between 20 and 40 MPa. Most frequently specified mix has a 1: 3 cement and sand ratio and 0.4-0.6 water and cement ratio. Its strength after 2 to 3 days is 30 to 40% of the 28th day strength and after 7 days is 50 to 70% (Abraham Warszawski, 1999).
184.108.40.206 Rapid Hardening Cement
Rapid hardening cement attains earlier and usually higher strength than comparable ordinary cement. The different types of rapid hardening cements may be of particular interest in precasting to enable a fast turnover of molds. However, they may have various limitations with respect to their usage such as aluminum cement was found to lose a considerable amount of its initial strength under temperatures exceeding 25ËšC and humid ambient condition also may affect corrosion of reinforcement (Abraham Warszawski, 1999).
220.127.116.11 High- Strength Portland Cement
High-Strength Portland Cement attain up to 50-60 MPa after 20 days. Many of these cements develop a full strength of ordinary 20 MPa cement after 1 to 3 days (Abraham Warszawski, 1999).
18.104.22.168 Sulfate-Resisting Portland Cement
This is used in elements exposed to sulfate contact (Abraham Warszawski, 1999).
22.214.171.124 White Portland Cement
This is similar to Ordinary Portland Cement but a very low iron content. This cement is used for production of precast architectural concrete components with a required white or colored surface (Abraham Warszawski, 1999).
Aggregates are often classified according to their bulk density as lightweight is below 1100 kg/m3, normal weight is 1100-1750kg/m3 and heavyweight is above 210 kg/m3. Normal weight and some types of lightweight aggregates are used in the production of precast concrete elements. Aggregates are also classified according to their size as fine is below 5mm diameter and course is 5mm diameter and above. Normal weight aggregates (average density 1400-1600 kg/m3) are sand and gravel which are found on riverbeds and the seashore. Aggregate's attribution or characteristic for concrete use are shape, texture, cleanliness, durability, chemical stability, abrasion resistance, volumetric stability, absorption and moisture content(Abraham Warszawski, 1999).
2.3.3 Air-entraining admixtures
These admixtures increase the volume of air entrained in concrete from the regular 0-2% to 4-6%. They increase the hardened concrete's resistance to adverse frost effects and also improve the concrete mix workability. Air entrainment may reduce concrete strength, especially early strength. The maximum percentage of entrained air may reduce concrete strength by up to 10-20% (Abraham Warszawski, 1999).
2.3.4 Accelerating admixtures
Admixtures accelerate the concrete hardening process which may shorten the time for precasting. Calcium chloride is once widely used could cause accelerated development of concrete strength. However, the use of calcium chloride in precast concrete is not permitted any longer because of its corrosive influence on reinforcing steel (Abraham Warszawski, 1999).
2.3.5 Steel reinforcement
Precast concrete elements can be heavily reinforced if necessary because they are cast horizontally. BS8110 permits up to 10 per cent of the cross-section to be reinforced, although this amount is rarely used in favour of higher concrete strengths (Kim s. Elliot, 2002).
It shall be clean and free from materials deleterious to concrete.
2.4 Production technology
All precast concrete products used in the works shall be manufactured by precast concrete manufacturers and installed by installers as approved by the Superintendent Officer.
The magnitude of all loads used in the design of the precast concrete components shall be clearly stated in the drawings. Unless specified otherwise, these magnitudes shall reflect the expected live loads and the dead load from the other components including the respective precast and cast in-situ concrete components. However, these values must not be less than the recommendations which are dead loads as per British Standards BS6399 Part 1 and 2, Imposed loads as per Malaysia Uniform Building By-Laws and Wind loads as per Malaysian Standard equivalent (JKR, 2005).
The size, length and the grade of concrete used must be clearly specified in the design calculations and drawings. Load combinations shall be clearly itemised and identified to enable design checking to be carried out upon the most adverse conditions. Unless specified otherwise, the minimum grade of concrete used to manufacture the precast concrete components shall be 35N/mm2 with minimum cement content of 350 kg/m3 (JKR, 2005).
When precast concrete components are designed as prestressed elements, the flexural tensile stresses in the concrete should not exceed the Class 2 requirements, as stipulated in Clause 126.96.36.199 of British Standard BS8110 Part 1: 1997. All structural welding, if required shall be done by qualified welders using equipments and materials compatible to the base material. The work shall be done completely in the factory and no structural welding work shall be allowed on site (JKR, 2005).
188.8.131.52 Design standards
In general, all precast concrete building components shall be designed in accordance to the British Standard BS8110 Part 1: 1997 Structural Use of Concrete or the equivalent Malaysian Standard or any other equivalent internationally recognised standards. Design of large prefabricated panels shall be in accordance to the Malaysian Standard MS1313: 1993 Code of Practice on Large Prefabricated Panels. Measurements in buildings shall be standardised based on the Modular Coordination concepts as stipulated in the Malaysian Standard MS1064: 2001 Guide to Modular Coordination in Buildings (JKR, 2005).
Detail and drawings
Scheme drawings or plan shall show the plan view of relevant floors and roof together with the proposed arrangements of the horizontal precast concrete components, such as the precast concrete floor slabs and the precast concrete beams. The proposed locations of the columns shall also be clearly identified. The column-to-floor joints including the cut-outs on the precast floor slabs. In addition, the scheme drawings shall also indicate the elevations and cross-sections of the building complete with the proposed arrangements of vertical precast concrete components, such as the precast concrete columns and precast wall panels. The column-to-beam connections, including the corbels or other mechanised connectors (JKR, 2005).
Fabrication shop drawings
Fabrication shop drawings shall show all necessary dimensions and details to enable the fabrication of the precast concrete components. These details include the actual dimensions of the precast components, sizes and locations of the steel reinforcements, sizes and location of the prestressing wires or strands, concrete covers to steel reinforcements and prestressing wires or strands and details of the conduits for electrical and mechanical services. In addition to above, fabrication shop drawings shall also show the sectional properties of the components such as the cross-sectional area, moment of inertia, modulus of section, distance to neutral axis together with the designed bending and shear capacities. To ensure proper handling of the precast concrete components, all lifting and erection devices should also be clearly shown in the fabrication shop drawings (JKR, 2005).
Propping and shoring drawings
All structural precast components shall be properly stabilised during erection stages by employing suitable temporary propping, bracing and/or shoring systems. The Contractor shall propose such a system and produced design calculations and drawings to allow for safe employment of the system on site (JKR, 2005).
Certification of calculation sheets, method statements and drawings
All calculation sheets, method statements and drawings submitted to the Superintendent Officer shall be certified by Professional Engineer registered with the Board of Engineers, so that the submitted documents are produced in accordance to the accepted engineering standard. Subsequently, the Professional Engineer shall formalise his endorsements by putting his signature and professional seal onto the pages of the submitted documents (JKR, 2005).
All prestressing materials shall be either in the form of wires or strands and are produced in accordance to the British Standard BS 5896: 1980 Specification for High Tensile Steel Wire and Strand for the Prestressing of Concrete, or the equivalent Malaysian Standard or any other equivalent internationally recognised standards. If prestressing strands are used in the works, they all shall be of the low relaxation type (JKR, 2005).
2.4.4 Concrete Works
All concrete works involved in the manufacturing of the precast concrete components shall be carried out in accordance to the specification given in the Section D - Concrete Works of the JKR Standard Specification for Building Works (2005 Edition) or other later edition (JKR, 2005).
2.4.5 Fire Rating
Minimum concrete cover to the steel reinforcement or prestressing wires, tendons, cable or sheaths in any structural building components shall be as specified and in compliance with the fire resistance requirements mentioned in the Uniform Building By-Laws. The Contractor shall state clearly in the drawings submitted to the Superintendent Officer the designed fire rating of each proposed structural building component (JKR, 2005).
184.108.40.206 Good building design with fire safety measures
There are 7 considerations which included provide adequate fire appliances, fire hydrants and other facilities to assist fire and rescue personnel. Second, provide adequate fixed installation for quick and effective detection and extinguishment of fires. Third, designing and installing building services so that they do no assist the spread of fire, smoke or toxic fumes. Forth, designing and providing adequate and safe escape routes for occupants of the building. Fifth, by selecting materials for the construction which will not promote the rapid spread of fire or generate dangerous smoke. Sixth, subdividing buildings into compartments of reasonable sizes by means of fire resisting walls and floors. Lastly, designing and constructing the exterior of a building so that fire us unlikely to spread to it from another burning building (UBBL, 1984)
The concrete shall be substantially free from chipped edges, laitance and honey combs. It shall also be free from fractures and cracks and from any other defects arising from faulty materials used or from faulty methods of manufacturing. The surface of the precast concrete components shall not be coated with cement wash. No repairs shall be permitted on the precast concrete components under any circumstances. All materials, processes of manufacturing and finished precast concrete components shall be liable to inspection and approval of the Superintendent Officer. Such inspection may be performed when manufacture or delivery. All precast concrete components shall be clearly marked with the manufacturer's name or registered trademark or logo, reference number and casting date (JKR, 2005).
2.4.7 Precast manufacturing process
The Architectural drawings are used by structural engineers to design the special customized panels.
Figure 4: Architect drawing
In pre-cast production plant, the pre-cast beds or molds are configured as per the drawings supplied by structural engineering team and is prepared for concrete pouring.
Figure 5: Configured molds
Concrete is then poured into the moulds continue by hand and machine-trowelled
Figure 6: Concrete poured into mold and hand and machine-trowelled
Steel inserts are molded into the concrete during the panel construction. These are welded together at the time of the panel erection to keep the panel from moving.
Figure 7: Steel inserts are molded into concrete
Foam panels are placed in the mold to create bevelled window sills.
Figure8:Foam panel Figure9:Window sill Figure10:Bevelled window sills
Panels are fitted into the floor by inserting the vertical reinforcing bars protruding from the floor slab into large holes in the panel. These large holes are called Dross Backs
Figure 11: Dross Backs for locating panel to floor
The panels are vertically positioned on the floor slab, so that the reinforcing rods from the floor fit perfectly into the Dross Backs. Once in position, grout is pumped into the lower of the two small holes on the side of the panels, filling the tube and locking it to the reinforcing bars.
Figure 12: Panel with 2 small holes
Electrical conduits are built into the panels. The panel is drilled to reveal the conduits at point for installation of light switches and plugs.
Figure 13: Electrical conduits are built into panels at the time of manufacturing
Figure 14: The panel is drilled to reveal the conduits at points for installation of light switches and plugs
Once the concrete is set, the moulds are hydraulically tilted to vertical position for de-molding of the precast panel
Figure 15: Finished panel being hydraulically titled
Figure 16: Nearly position for de-molding of the precast panel
The panels are removed from the mould in one day and stood in the drying rack using hook. This enables the manufacturing of panels in higher volumes.
Figure 17: The precast panels are stood in the drying rack using hooks
Figure 18: The panels are removed from the mold
The panels are moved to the drying racks. The panels are then stacked in special drying racks that allow the panels to dry standing up, enabling even curing of the panels.
Figure 19: The panels are moved to the drying racks
Figure 20: Precast panels stacked at special drying racks
The panels are transported to the site in special transportation racks that are lifted and placed onto the tracks. A strip foundation or a regular foundation is laid at the site, depending on the geophysical characteristic of the site. The panels are brought in by truck and erected into place.
Figure 21: Special transportation racks that are lifted and placed onto the truck
Figure 22: A strip or a regular foundation is laid at the site
Figure 23: The panels are brought in by truck and erected into place
The roof is constructed over the vertically erected panels. Flooring is completed along with doors and window fitting. Finishing works are completed and the building is ready for occupation (Anmlex Industries PTY. LTD.,1998)
The Contractor shall inform the Superintendent Officer at least one week in advance of each launching or installation operation and submit few documents to the Superintendent Officer for acceptance which are method statement including launching systems and transportation, proposal for traffic diversion and detailed programme of each launching or installation operation (JKR, 2005).
The Contractor shall know for the transportation of the precast elements and movement of the lifting equipment to the launching site. Elements being transported would not be damaged. Temporary support for precast concrete components during transportation must be designed to withstand loads and extra forces during loading, transportation and unloading. Precast concrete components shall be lifted and supported during manufacturing, stockpiling, transporting and erection operations only at lifting or supporting points, as shown in the fabrication shop drawings, and with approved lifting devices. The Contractor shall ensure that the precast concrete components are loaded in sequence compatible with the required unloading and erection sequence on site. A certificate of test of lifting equipment shall be submitted to the Superintendent Officer together with particulars of the experiences of the operator (JKR, 2005).
2.5.2 Site access and ground preparation
Contractor is responsible for providing suitable access to the building. The ground of the launching area shall be prepared at Contractor's own cost to ensure that it is safe to carry the required load during installation or launching operation (JKR, 2005; Summer 1998).
2.5.3 Placement of precast concrete components
The Contractor is responsible for providing true level surfaces on all site placed bearing walls and other site placed supporting members. The Contractor shall also be responsible of placement and accurate alignment of anchor bolts, plates or dowel in column footings, grade beams and other site placed supporting members. All relevant shoring or propping equipments are also to be provided by the Contractor if there are any composite beams or slabs (JKR, 2005).
Precast components shall be properly aligned and levelled as required by the approved fabrication shop drawings. Variations between adjacent components shall be reasonably levelled out by jacking, loading or any other feasible methods as recommended by the manufacturer and approved by the Superintendent Officer (JKR, 2005).
2.5.5 Safety precautions during installation
Precautions shall be taken to remove any danger to the workers and general public while launching precast elements. All lifting equipment shall be designed, if the primary lifting mechanism fails, a secondary mechanism will ensure that the precast element does not fall. Upon erection, a fail-safe method shall be used to temporarily secure the precast unit until the permanent fixing arrangements are implemented. The securing systems which include providing chains between the installed precast concrete components and stable supports, connecting adjacent precast concrete components with temporary bracings between them and providing wedges or brackets to the precast concrete components. The Contractor shall inform the S.O and obtain his approval before removing any temporary work but such approval does not relieve the Contractor of his responsibilities for the safety of the work (JKR, 2005; Summer, 1998).
2.5.6 Water Proofing
220.127.116.11 Internal joints
In the dry area (living rooms, bed rooms and stores) approved bituminous based water proofing membrane shall be applied on top of the joint between the slab panels. The end of the membrane shall be at least 200mm away from the end of the precast concrete floor panel in the joint. In the regularly wet area, (bathrooms, toilets and kitchens), liquid water proofing membrane shall be applied over the entire floor area with extra 200mm minimum up the walls. Water proofing admixtures insert in the concrete screeds that are subsequently laid on top of the concrete floor topping. Additives are also added to the concrete placed in the joints to improve workability and reduce shrinkage. In either area (dry or wet), cast in-situ concrete topping of grade similar to the grade of concrete used in the manufacturing of the precast concrete components is laid on top of the waterproofing materials. When the concrete topping is designed to act as a composite with precast concrete floor panel, the minimum thickness of concrete topping shall be 60mm. In other cases, the minimum concrete topping thickness shall be 30mm only (JKR, 2005).
18.104.22.168 External joints
Both horizontal and vertical joints should be designed as 'open-drained' joints, meaning any water in the joint can flow down and discharged out on its own by gravity. Both, vertical and horizontal joints shall then be hidden by installing aluminium strips on top of the grooves and attaching one end of the strip to the wall (JKR, 2005).
22.214.171.124 Leakage test
In the regularly wet areas, 24-hour water ponding tests shall be carried out. In this test, the area to be tested shall be inundated with 50mm deep standing water for a continuous 24 hour period. After the 24-hour period is over, a thorough inspection is carried out to detect any sign of water leakage in and around the test area. The test is deemed successful when no sign of any leakage is detected (JKR, 2005).
2.5.7 Independent checker
Independent Checking Engineer shall take full responsibility of his report and recommend that it has been adequately carried out in accordance with accepted engineering practice, and to ensure the structural integrity and stability of the proposed construction. The Checking Engineer should evaluates, analyses and review the structural design in the plan and perform such original calculations with a view to determining the adequacy of key elements and verify that the key elements designed are consistent with general layout shown and in any amendments (JKR, 2005).
126.96.36.199 Scope of checker
Sufficient working drawing details and specifications shall be available to the checker to check. The checker in carrying out this is required to determine and use of the Code of Practice and Design Standards in the plans, check the design loading and method of construction, check the standards and specifications of materials to be used, ascertain the structural design concept used and identify the key structural elements, analyses all key structural elements of the building and the associated structure to be built, determine the stability of the structural frame, check structural detailing; and determine the adequacy of other aspects of the design which are peculiar to the building and the associated structures to be built and which are essential to the structural integrity of the works (JKR, 2005).
188.8.131.52 Reporting of independent check
The checker's report shall be submitted in writing to the Superintendent Officer within 2 weeks of the independent check being done. The report should specifically describe the deficiencies or potential to be which have been identified along with the relevant references to accepted standards, practices and design principles. The deficiencies should be illustrated wherever possible by marking-up the plans or with sketches, drawings and such related materials. The report may include the checkers suggestion, amendments, alternative solutions and designs for amendments and or alternative solutions. A summary of all the checkers and designers comment with or without their agreement shall be included (JKR, 2005).