Pre-cast RC Structures - Notes
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(2) Module 1 Types of RC Prefabricated Structures, Long wall and cross wall large panel buildings, One way and two way prefabricated slabs, Framed buildings with partial and curtain walls, single storey industrial buildings with trusses and shells, Crane, Gantry systems. Module 2 Functional Design Principles: Modular coordination – Standardization - Disuniting, Diversity of prefabricates – Production – Transportation – Erection - Stages of loading,codal provisions- Safety factors - Material properties - Deflection control Lateral load resistance - Location and types of shear walls. Module 3 Floors, Stairs and Roofs: Types of floor slabs – Methods of Analysis and design example of cored and panel types and two-way systems - Staircase slab design - Types of roof slabs and insulation requirements - Description of joints, behaviour and requirements - Deflection control for short term and long term loads - Ultimate strength calculations in shear and flexure. Module 4 Walls: Types of wall panels - Blocks of large panels – Curtain partition and load bearing walls Load transfer from floor to wall panels – Vertical loads Eccentricity and stability of wall panels –Use of Design curves -Types of wall joints, their behaviour and design – Leak prevention, Joint sealents, sandwich wall panels. Module 5 Industrial Buildings: Components of single storey industrial sheds with crane gantry systems - Design aspects of R.C. Roof Trusses - Roof panels R.C. Crane - Gantry Girders - Corbels and columns and Wind bracing..
(3) MODULE-1 1.1 Prefabrication-General Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site and transporting complete assembles to the construction site where the structure is to be located. Prefabricated building is the completely assembled and erected building of which the structural parts consist of prefabricated individual units or assemblies using ordinary or controlled materials. Prefabricated construction is a new technique and is desirable for large scale housing programmes. 1.2 Principles 1) To effect economy in cost 2) To improve in quality as the components can be manufactured under controlled conditions. 3) To speed up construction since no curing is necessary. 4) To use locally available materials with required characteristics. 5) To use the materials which possess their innate characteristics like light weight, easy workability, thermal insulation and combustibility etc. 1.3 Need for Prefabrication Prefabricated structures are used for sites which are not suitable for normal construction method such as hilly region and also when normal construction materials are not easily available. PFS facilities can also be created at near a site as is done to make concrete blocks used in plane of conventional knick. Structures which are used repeatedly and can be standardized such as mass housing storage sheds, godowns, shelter, bus stand security cabins, site offices, fool over bridges road bridges. Tubular structures, concrete building blocks etc., are prefabricated structures 1.4 Advantages Speed of construction, owing to the ability to begin casting components for the superstructure while foundation work is in progress. Precast concrete components can also be cast and erected year-round, without delays caused by harsh weather; Aesthetic flexibility, due to the variety of textures, colors, finishes and inset options that can be provided. Precast is extremely plastic and can mimic granite, limestone, brick, and other masonry products. This allows it to blend economically with nearby buildings finished with more expensive materials; Design flexibility, resulting from the long-span capabilities to provide open interiors;.
(4) . . Durability, which allows the material to show minimal wear over time and resist impacts of all types without indicating stress; Energy efficiency, due to the material’s high thermal mass. This is enhanced by the use of insulated panels, which include an insulated core; Environmental friendliness, as seen in its contributions to achieving certification in the Leadership in Energy & Environmental Design (LEED) program from the U.S. Green Building Council (USGBC); and High quality, resulting from the quality control achieved by casting the products in the plant. Plants certified by PCI undergo stringent audits of their quality procedures, ensuring the quality of fabrication in these facilities.. 1.5 Disadvantages Careful handling of prefabricated components such as concrete panels or steel and glass panels is required. Attention has to be paid to the strength and corrosion-resistance of the joining of prefabricated sections to avoid failure of the joint. Similarly, leaks can form at joints in prefabricated components. Transportation costs may be higher for Voluminous. Prefabricated sections than for the materials of which they are made, which can often be packed more efficiently. Large Prefabricated Structures require heavy-duty cranes & Precision measurement and handling to place in position. 1.6 Precast Concrete Construction Precast concrete consists of concrete that is cast into a specific shape at a location other than its in service position. The concrete is placed into a form, typically wood or steel, and cured before being stripped from the form, usually the following day. These components are then transported to the construction site for erection into place. Precast concrete can be plant-cast or site-cast. Precast concrete components are reinforced with either conventional reinforcing bars, strands with high tensile strength, or a combination of both. The strands are pretensioned in the form before the concrete is poured. Once the concrete has cured to a specific strength, the strands are cut (detensioned). As the strands, having bonded to the concrete, attempt to regain their original untensioned length, they bond to the concrete and apply a compressive force. This “pre-compression” increases load-carrying capacity to the components and helps control cracking to specified limits allowed by building codes.. Precast components are used in various applications and projects of all types. Key components include: Wall panels, which can include an inner layer of insulation and be load supporting if desired;.
(5) . . Spandrels, which generally span between columns and are used with window systems in office buildings or in parking structures; Double tees, so named due to the two extending “stems” perpendicular to the flat horizontal deck. These tees are often used for parking structures and buildings where long open spans are desired; Hollow-core slabs, which are long panels in which voids run the length of the pieces, reducing weight while maintaining structural strength; Columns and beams, including columns and a variety of beam shapes; Bridge components for both substructure and superstructure designs, including girders in a variety of shapes, box beams, and deck panels; Piers, piles, caps and other supporting components for bridges.. 1.7 Types of Prefabrication Elements The system of prefabricated construction depends on the extent of the use of prefabricated components, their material, sizes and the technique adopted for their manufacture and use in building. The various prefabrication systems are outlined below. 1) Small prefabrication 2) Medium prefabrication 3) Large prefabrication 4) Off-Site prefabrication system 5) Open prefabrication system 6) Large panel prefabrication system 7) Wall system 8) Floor system 9) Stair case system 10) Box type system 11) Frame system 1. Small Prefabrication: The first 3 types are mainly classified according to their degree of precast elements using in that construction. For example, brick is a small unit pre-casted and used in buildings. This is called as small prefabrication. That the degree of precast element is very low. 2. Medium Prefabrication: Suppose the roofing systems and horizontal member are provided with precast elements. These constructions are known as medium prefabricated construction. Here the degree of precast elements are moderate. 3. Large Prefabrication: In large prefabrication most of the members like wall panels, roofing/flooring systems, beams and columns are prefabricated. Here degree of precast elements are high. 4. Off-Site (Factory) Prefabrication.
(6) One of the main factors which affect the factory prefabrication is transport. The width of road walls mode of transport vehicles are the factors which factor the prefabrications which is to be done on site or factory. Suppose the factory situated at a long distance from the construction site and the vehicle have to cross a congested traffic with heavy weighed elements the cost in-situ prefabrication is preferred even though the same condition are the cast in site prefabrication is preferred only when number of houses are more for small elements the conveyance is easier with normal type of lorry and trailors. Therefore we can adopt factory (or) OFF site prefabrication for this type of construction. 5. Open Prefabrication System This system is based on the use of the basic structural elements to form whole or part of a building. The standard prefabricated concrete components which can be used are, a) Reinforced concrete channel units b) Hollow core slabs c) Hollow blocks and battens d) Precast plank and battens e) Precast joists and tiles f) Cellular concrete slabs g) Prestressed / reinforced concrete slabs h) Reinforced / prestressed concrete slabs i) Reinforced / prestressed concrete columns j) Precast lintels and sunshades k) Reinforced concrete waffle slabs / shells l) Room size reinforced / prestressed concrete panels m) Reinforced / prestressed concrete walling elements n) Reinforced / prestressed concrete trusses The elements may be cost at the site or off the site. Foundation for the columns could be of prefabricated type of the conventional cast in situ type depending upon the soil conditions and loads. The columns may have hinged or fixed base connections depending upon the type of components used and the method of design adopted. There are two categories of open prefabricated systems depending on the extent of prefabrication used in the construction as given below. i. Partial Prefabrication Open System The system basically emphasizes the use of precast roofing and flooring components and other minor elements like lintels, sunshades, kitchen sills in conventional building construction. The structural system could be in the form of in-situ frame work or load bearing walls. ii. Full Prefabrication Open System In this system, almost all the structural components are prefabricated. The filler walls may be of bricks or of any other local materials..
(7) 6. Large Panel Prefabrication System This is based on the use of large prefabricated components. The components used are precast concrete large panels for walls, floor roofs, balconies, stair cases etc. The casting of the components could be at the site or off the site. Depending upon the context of prefabrication, this system can also lend itself to partial prefabrication system and full prefabrication system. Hence construction is a time consuming labor-intensive process. Builders need to bring together all of the necessary materials and skilled workers to complete the project successfully within a given time frame one way to make the process easier is by using prefabricated components. Such as pre-built walls (or) larger wall panels.. The simple way of classification of precast wall panel is based on their size or the materials of which they are made. They can be classified According to size as small and large or as narrow vertical stirrups or as broad horizontal bands. The material that are used for precast wall panel are bricks, hollow clay blocks, normal density concrete light weight metal gypsum plastic & timber. Generally materials that are locally available or which can be easily obtained are used for the production of precast wall panels. Due consideration is also given to the structural and physical properties of the materials in their selection particularly in respective of their strength, thermal and sound insulation properties and relative cost. Another classification of precast concrete wall which is especially application to prefabricated construction is based on their function and location in the building. They can also be distinguished for their cross sectional characteristics. As regards their location the wall panels may be classified as exterior or interior location walls. Depended on their function they may be either structural (load bearing) or non-structural (non-load bearing) elements. They may be of solid ripped sandwich hollow core, or composite construction they can be either prestressed or conventionally reinforce. In large panel construction the load bearing wall may be laid out either perpendicular to the longitudinal axis of the building (cross wall system) or parallel to it ( spine wall system). A.
(8) mixed system consists of cross wall and spine wall system. In most Vertical load carrying elements transfer their loads directly to the foundation without an intermediate frame.. 7. Wall System Structural scheme with precast large panel walls can be classified as 1) Cross wall system 2) Longitudinal wall system Cross Wall System In this system the cross walls are load bearing walls. The facade walls are non-load bearing. This system is suitable for high rise buildings. Longitudinal Wall System In this system, cross walls are non-bearing, longitudinal walls are load bearing. This system is suitable for low rise buildings. A combination of the above systems with all load bearing walls can also be adopted.. Precast concrete walls could be Homogeneous walls: The walls could be solid or ribbed. Non-homogeneous walls: Based on the structural functions of the walls, the walls could be classified as a. Load bearing walls b. Non-load bearing walls c. Shear walls a. Load bearing wall: Precast load bearing walls provide an economical solution when compared to the conventional column/ beam/ infill wall system. The primary advantages are speed of construction and elimination of wet trades. In adopting the wall thickness, structural adequacy is not the sole consideration. Other factors to be considered include: • Connection details for supported beams and slabs. • Sound transmission and fire rating. • Joint details at panel-to-panel connections. • Possible future embedded services, which could reduce the concrete area available..
(9) Based on typical layouts and building configurations, a thickness of 180mm is recommended for the precast panels used for party walls.. b. Non-Loading Bearing Wall Curtain Wall Curtain wall is a non-load bearing concrete wall construction that protects covered and/or conditioned interior spaces from the outside environment. Often designers consider aluminum-framed walls of glass or thin in-fills of metal or other materials as curtain walls. Sandwich Walls Insulated sandwich wall panels can be strictly architectural, strictly structural, or a combination of both. The difference between typical panels and insulated sandwich wall panels is that the latter are cast with rigid insulation "sandwiched" between two layers of concrete. The insulation thickness can vary to create the desired thermal insulating property ("R" value) for the wall.. . The structural behavior is either: Composite in which the Wythes are connected using ties through the insulation that fully transfer loads. The structural performance is then based on the full thickness of the panel;.
(10) . Non-Composite in which the Wythes are connected using ties through the insulation, which limits performance to the individual capacities of each Wythe. Whether the panel is composite or non-composite depends on the configuration and material used for the ties. Insulated sandwich wall panels can be designed to be loadbearing and support floor and roof components. They make an ideal structural element for this purpose, typically by casting a thicker interior wythe to provide the necessary support. They can also be non-loadbearing to complete a façade. Finishes: As with typical wall panels, the panels are cast in a flat orientation, so the form side is typically the side that will be exposed to view in the final construction. This face can be made with virtually any type of finish. GFRC panels allow for great aesthetic details and extensions such as cornices, due to the manufacturing process. The back face is typically troweled smooth, but is not left exposed. The back-up systems are often used to attach drywall and/or other finish materials. Typical widths: 4 to 15 ft. Typical heights: 8 to 50 ft. Typical thicknesses: 1.5 to 3 in. Precast Non-Load Bearing Facade Wall Typically, the wall panels for the front and rear elevations are non-load bearing facade elements. Support of these panels is achieved by any of the following methods: • The facade panel is connected to main load bearing walls and is designed to carry its own weight between supports. • The facade panel is connected to the floor slab or beam, which is then designed to provide support to the wall. These panels will typically be designed for vertical loads due to self-weight and an allowance for floor loads, if applicable, in addition to horizontal loads due to external wind pressures. A typical panel thickness of 120mm is proposed on the basis of strength considerations and to accommodate window fixings and profiles around the window perimeter.. Facade panels will often require three-dimensional architectural features, such as hoods, sills and ledges. In cases where there is a reasonable degree of repetition, customized.
(11) moulds can be produced, enabling these features to be economically incorporated into the panels. As an alternative, when repetition is limited, it will be most economical to cast the façade panel flat and subsequently add the features, manufactured separately using materials such as precast concrete, GRC, Aluminum or steel. 8. Floor System Depending upon the composition of units, precast flooring units could be homogeneous or non-homogeneous. 1) Homogeneous floors could be solid slabs, cored slabs, ribbed or waffle slabs. 2) Non-homogeneous floors could be multilayered ones with combinations light weight concrete or reinforced / pre stressed concrete with filled blocks. Depending upon the way, the loads are transferred the precast floors could be classified as one way or two way systems. One Way System One way system transfers loads to the supporting members in one direction only. The precast elements of this category are channel slabs, hollow core slabs, hollow blocks and hollow plank system, channels and tiles system, light weight cellular concrete slab etc. Two Way Systems Transfer loads in both the direction imparting loads on the four edges. The precast element under this category are room sized panels two way ribbed or waffle slab system etc... Typical Flooring / Roofing system. 9. Stair Case System Stair case system consists of single flights with inbuilt risers and treads in the element only. The flights are normally unidirectional transferring the loads to supporting landing slabs or load bearing walls. 10. Box Type System In this system, room size unit are prefabricated and erected at site. This system derives its stability and stuffiness from the box limits which are formed by four adjacent walls. Walls are joined to make rigid connections among.
(12) themselves. The box unit rest as plinth foundation which may be of conventional type of pre-cast type. 11. Frame System Precast frames can be constructed using either linear elements or spatial beam column sub-assemblages. 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.. Typical Precast Beams.
(13) Typical Precast Columns 1.8 Industrial Building Industrial type building (Workshops, warehouses, etc.) is governed by laws differing from those controlling housing building. Prefabrication in situ of the main load-bearing beams and other secondary members (trusses, floors, etc.) is by now of common use in any construction yard for the erection of a factory or an industrial building. Quite often the construction Company purchases the main beams and other load-bearing members directly from specialized firms expressly equipped for an industrial type production. This tendency is mentioned here because it is probably destined to assert itself even more in the presumable development of the building industry which will convert the construction companies into concerns for the assembly of industrially prefabricated structural elements. 1.8.1 Components of Industrial Building (Single-Storey).
(14) The roofs of single storey shed type industrial buildings maybe constructed by purlins with covering of roofing slabs or corrugated asbestos cement sheet method. These are the most popular forms of roof covering used in central Europe. This is not surprising considering the simplicity of manufacture of purlins and the availability from stock of factory made lightweight roofing slab and panels. The structural system of the purlins maybe a) Freely supported beam b) Cantilever girder.
(15) c) Continuous girder The connections of purlins over the support are designed only to absorb a limited bending moment. Normal purlins spans between 5 and 10m.The purlins are spaced at intervals of 2m to 3m. Roofing members are classified as, Reinforced planks: Reinforced planks made of hollow tiles. The reinforced planks with longitudinal circular holes. Thickness of these tiles is 60mm, 80mm & 100mm & the width is 200mm & length is vary from 360mm to 400mm. On the upper side one longitudinal groove is provided. Reinforcement is placed into these grooves which are subsequently filled with cement mortar. In this way, roofs of length 2 to 3m & thickness of 60 to 100mm & width 200mm can be constructed. The end tiles resting on the support are provided with 3.11mm dia stirrups protruding from the tile. There are kept together over mortar of 40mm thickness & in further concreting of joint is completed. Light weight concrete roofing members: Light weight concrete roofing members play a role in addition to space bordering & load bearing in heat insulation. The thickness varies from 7.5 to 25cm for reinforcement of light weight concrete roofing members, welding nets is used. Steel reinforcement is given additional coating to prevent any corrosion care is taken to give good bonding of reinforcement with concrete. The unit weight of these members is 750kg/m3& width of 50cm.Its varies from 1.75mm to 6m.precast members can be made either in usual way using lightweight materials. Sand as aggregate & combination of high strength concrete. The top & bottom layer of about 2 to 3cm thickness is provided with high strength concrete. Its consists of prestressed 2.5mm dia embedded in these layers. The middle portion is made with light weight concrete. Small reinforced concrete roofing members: The Small reinforced concrete roofing members is essentially precast simply supported ribbed concrete slab width varying from 450 to 120cm & length varying from 2 to 4m. Purlins: Purlins are usually solid web members. For long span they maybe lattice girders or trussed beams. Freely supported purlins are designed as parallel flanged or fish- belly members. Purlins designed as cantilever girders (articulated girders) are usually parallel flanged members. The cross section of purlins is generally rectangular but it can also have trapezoidal, T, L and I shape. The c/s features depends on the spans of purlins and on the slope of the roof. The purlins for flat roofs are usually rectangular T- section or (prestressed concrete)..
(16) T-section members for steeply sloped roofs if the purlins are loaded also bi axial bending L-section and the approximate spans associated with them for a purlins spacing of 3m are indicated for the flat roofs. The dimensions relate to freely supported purlins. Precast purlins can be simply supported or cantilever beams & for the bearing of loads beyond these weight simply supported purlins can be transformed into continuous beams. It is very simple & easy to place. For cantilever purlins placing of hinges should be determined in a manner to develop positive & negative moments equal to each other. This can be arrived by placing the hinges @ 0.145 from the support where I is the spacing between the supports.. Purlins section with associated spans for a purlins spacing of about1.25m in the case of steeply sloped roofs with corrugated asbestos cement sheet are indicated. The L-section is popular with British Firms channels section purlins have been developed by among others professor VON HALASS. They are convenient to manufacture with the legs of the channel upwards whereby very thin webs can be produced. This type of purlins maybe conventionally reinforced or by prestressed, also they may be freely supported or be continuous over several spans. In case of L- section purlins usually only the flange of the section is supported The fish belly girder is very favorable with regard to material requirements and the pattern of forces in the girder, but it has the disadvantages of being rather unsatisfactory. From the point of view of architectural aesthetics when it is used it is generally designed as a reinforced concrete purlins. Large reinforced concrete roofing members: Large reinforced concrete rest on the main girders. These are generally used for large hall structures & these are most advanced type of precast structures. Members are manufactured corresponding to spacing of the frame length of about 6 to 10m & width of 1.3 to 1.8m. As they are most supported on main girder purlins are not required. Four kinds of members exist: 1. Normal members..
(17) 2. Intermediate members. 3. Members with cornice. 4. Members with gutter & eves border. Shell Roof: The shell structure can have ribs in the centre & provided with curved membrane like roof. There are many industrial structure are built by precast members with shells. The thickness of shell varies from 2 to 10cm.Some precast shell, are produced with dimensions which are very difficult to transport. To avoid such difficulty large size shells are precast near to the resting or construction place. The transportable or small size shell members can be precast in factories & these are transported to the site. Examples: Barrel shells, Saddle or hyperboloid shells. Cupola or parabolic shells. The advantage of shells is that it provides large column free area for the monolithic construction. The cost of shuttering & scaffolding is very high but if manufactured in a precast factory in large scale. The production cost can be considerably reduced. Type of Shell Constructions a).Single barrel Shell Structure The structure above is a single barrel with edge beams. The shell has been allowed to project beyond the edge of the stiffener in order to show the shape of the shell. Stiffeners are required at columns. They do not necessarily have to be complete diaphragms but may be arches with a horizontal tie. The thickness is based on design of a slab element, the thickness of the barrel shell is usually based on the minimum thickness required for covering the steel for fireproofing, plus the space required for three layers of bars, plus some space for tolerance. If these bars are all half inch rounds, a practical minimum would be 3 ¼ inches. Near the supports the thickness may be greater for containing the larger longitudinal bars. If more than one barrel is placed side by side, the structure is a multiple barrel structure & if more than one span, it is called as multiple span structure. b).Multiple barrel Shell Structure This structure shows a multiple barrel with vertical edge beams at the outside edges. The stiffeners have been place over a roof. The advantage of having the stiffeners on top is that there are no interruptions to the space inside the shell so both the inside appearance & the utility are better. The movable formwork may be used which will slide with little decentering lengthwise of the shell. The multiple span structure should have an occasional expansion joint to reduce shrinkage & thermal stresses. This can be accomplished by cantilevering half the span from each adjacentstiffener. A small upturned rib placed on each side of the joint & accordion type sheet metal flashing is arranged to prevent roof leakage..
(18) The maximum spans for this type shell are again limited by the geometry off the cross section .Assuming the maximum width of barrel to be 50 feet & maximum end slope to be 45deg, the rise would be about 14 feet, the maximum span would be in the order of 150 feet. c).North light shells This type of shell structure is used to provide large areas of north light windows for factories requiring excellent natural lighting. The windows may be slanting or may be vertical. The member at the bottom forms a drainage trough with the curved shell & materially assists in stiffening the structure. The effective depth of the shell is not the vertical distance between the two ends but is merely represented b the depth if the shell is laid flat with the ends of the circle on the same horizontal line. The spans for the north light shell must be rather small in comparison to the vertical depth of construction. The edges of adjacent shall should be tied together by concrete struts serving as mullions between the window glazing. d).Long barrel shell Long barrel shell obtained hen the semicircle or a segment of same is translated along the longitudinal axis. Generally used for shed for industrially purpose & buildings for large column free areas. Generally the prefabricated barrels off sizes 3.5 to 5m & 10m long with edge beams having thickness of 60mm.The thickness of the shell should not be more than 40mm.The dimension of these members were finally limited by the load carrying capacity of the available hoisting machines using the girder system built of precast prestressed trusses with parallel chords, areas having a span of even more than 15m can be cover with barrel shell. 1.9 Materials Used Prefabricated building materials are used for buildings that are manufactured off site and shipped later to assemble at the final location some of the commonly used prefabricated building. The materials used in the prefabricated components are many. The modern trend is to use concrete steel, treated wood, aluminum cellular concrete, light weight concrete, ceramic products etc. While choosing the materials for prefabrication the following special characteristics are to be considered. foundations ermal insulation property.
(19) 2.0 Characteristics of Materials. of foundations..
(20) MODULE-2 2.1 Modular Coordination Modular coordination is a concept of coordination of dimension and space, in which building components are dimensioned and positioned in a term of a basic unit or module, which is also known as 1M and which is equivalent to 100 mm. MC is internationally accepted by the International Organization for Standardization (ISO) and many other countries as well as Malaysia. A module is a unit of measurement and it means Standardized and easily fit components. Generally the word Module is derived from Latin Word ‘MODULUS’ meaning a small dimension. MC is the International system of dimensional standardization in building. The smallest Module is generally used to coordinate Position and Size of Components, Elements and their Installations. Smaller Dimensions should be more clearly distributed than larger dimensions. Modular coordination was first explored as an aid to design shortly after the introduction of prefabrication in the construction industry in the industrialization. It was conceived as a further step in the development of systematic design and construction of the building. This subject has been discussed and attempted in an actual building experiment in practically every developed country. Modular coordination was first studied in Singapore in the early seventies. The housing and development board implemented the concept in 1973 in the new generation flats. Prefabrication and standard components were subsequently introduced. Modular blocks and bricks were introduced in 1983. There are merits to extend the use of modular coordination in other components as well. 2.2 Objectives The principle objectives of modular system is to provide practical and coherent solutions for coordination of the position and dimensions of elements, components and space in building design. This process can contribute to increase design freedom and improved balance between quality and cost in manufacture and construction. 2.3 Principle of Modular Coordination The main purpose of Modular Coordination is to achieve the Dimensional Compatibility between the Building Dimensions, Span or Spaces and the Size of Components and Equipment’s by using related Modular Dimensions. Modular Coordination generally provides the easy grasped layout of the positioning of the building components in relation to each other and to the building and facilitate collaboration between planners, manufactures, distributors and contractors..
(21) 2.4 Advantages of Modular Coordination To facilitate collaboration between building designers, manufactures, distributors and contractors. To permit the use of building components of standard size to construct the different types of building. To optimize the member of standard sizes of building component. MC increases the speed of construction. Benefits through the increase use of computer aided design and drafting. Reduction in manufacturing and installation cost. MC minimize the wastage of materials, time and manpower in cutting and trimming on site Facilitate prefabrication. MC improved the balance between Quality and Cost. 2.5 Disadvantages of Modular Coordination Uniformity. Can lead to problems when modules are linked because link must thoroughly test. It is difficult to manufacture to produce components based on mm tolerance. 2.6 Principle of Modular Coordination 1. Basic Module 2. Modular Dimension 3. Planning Module 4. Placing of Components 5. Modular Grid. 1. Basic Module The fundamental module used in modular coordination the size of which is selected for general application to buildings and components..
(22) 2. Modular Dimension Traditionally designers have trained to use simple whole number ratios. Modular coordination provides a sound basis for an ordered selection of dimensions and accommodates a proportional flexibility that satisfies the needs of architectural aesthetics. 1) The planning grid in both directions of the horizontal plan shall be: a. 3M for residential and institutional buildings b. For industrial buildings, 15M for spans up to 12m 30M for spans between 12m and 18m and 60M for spans over 18m The center lines of load bearing walls shall coincide with the grid lines. 2) In case of external walls, the grid lines shall coincide with the center line of the wall 50mm from the internal force. 3) The planning module in the vertical direction shall be 1M up to end including a height of 2.8m, above the height of 2.8m it shall be 2M. 4) Preferred increments for sill heights, doors, windows etc. shall be 1M. 5) In case of internal columns, the grid lines coincide with the centre lines of columns. In case of external columns and columns near the lift and stair wells the grid lines shall coincide with centre lines of the column in the top most storey or a line in the column 50mm from the internal face of column in the top most storey. 3. Planning Module and Placing of Components There are different methods of locating components within dimensional frame mark of the building the distribution is made between load bearing walls, slab components vertically and horizontally. The placement of components either made on axial to the boundary planning. 4. Modular Grids To simplify the design process a mesh of lines, which have preferred space dimension, are plotted in three directions for all types of buildings. A rectangular coordinate reference system in which the distance between consecutive lines is the basic module or a multi-module. This multi-module may differ for each of the two dimensions of the grid. Basic Modular Grid The fundamental modular grid, is that in which the intervals between consecutive parallel lines is equal to the basic module, smallest planning grid. Multi - Modular Planning Grid In addition to the basic modular grid, multi-modular grids in which the intervals between consecutive lines are a multi-modular may be used..
(23) Type of Modular Grid There are different types of grid patterns which are used to locate the positions and dimensions of building spaces components are A.Continuous grid Where all dimensions in either direction are based on one increment only. B. Superimposed grids When the modular grid of 100 mm increment is superimposed on a multi-modular grid. C. Displacement of grid or tartan grids Where there is a homogenous and repetitive relation between at least two basic increments. Eg:- 1M +2M (or) 3/2 M + 3M D. Interrupted grids (or) neutral zones Where there are non-modular interruptions of grids neutral zones are created to cope with the economics of building design. 2.7.Modular Coordination Design Rule: Basic Module 1M= 100mm Structural Grid 3M (1M as the second preference) Horizontal Multi-Module 3M (1M as the second preference) Vertical Multi-Module 1M (0.5M as the second preference) Doors Multiples of 1M (width and height) Windows Multiples of 1M (width and height) Sub-modular increment 0.5M and 0.25M Planning modules for main dimensions of framework especially the span (horizontal dimensioning) are shown in figure..
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(25) 2.8 Notation and Symbols. 2.9.Tolerance It is the sum of acceptable positive and negative discrepancies of actual dimensions from the theoretical one. The limits of tolerance are based on the manufacture and erection requirements. Types of Tolerance: 1. Manufacture Tolerance (T) • Deviation caused by shrinkage, creep and temperature changes. • Also due to Loadings. • Positive and negative manufacturing tolerances are assumed equal to T/2(mm) 2. Erection tolerance • These are the limits of deviation of the positioning in the assembly of the prefabricates..
(26) •. The position tolerance are normally defined by five components namely, deviation in positioning of the prefabricates in x,y,z directions ( x, y, z) and deviation in positioning with respect to another prefabricate and the deviation in the verticality of the elements. Tolerance as per IS: 15916-2011.
(27) 2.10 Standardization Standardization is to the creation and use of guidelines for the production of uniform interchangeable components especially for use in mass production. It also refers to the establishment and adoption of guidelines for conduct to global marketing the term is used in describe the simplification of procurement & production to achieve economy. It is the extensive use of components, methods or processes in which there is regularity, repetition and a background of successful practice (e.g. standardisation of the dimensions of components such as doors and windows, uniform standards for certain common materials such as steel and concrete, etc.). The construction industry can improve its efficiency through wider use of components of standardised dimensions and standardised processes. To maximize these benefits, we need to factor in the use of standardised components at the design stage to ensure compatibility in design and to facilitate the manufacturing process. The industry will also need to act together in order to achieve the necessary economy of scale. Standardised processes and practices provide much greater predictability about what is performed, by whom, how and when and the possible outcomes. Wide adoption of standardised processes and practices across the industry would facilitate integration among industry participants, minimize development efforts, and promote learning sharing.. Typical Standard Precast Concrete Sections 2.10.1 Advantages of Standardization 1) Easier in design as it eliminates unnecessary choices 2) Easier in manufacture as there are limited number of variants. 3) Makes repeated use of specialized equipment in erection and completion 4) Easier and quicker..
(28) 2.10.2 Factors Influencing Standardization 1) To select the most rational type of member for each element from the point of production, assembly, serviceability and economy. 2) To limit the number of types of elements and to use them in large quantities. 3) To use the largest size of the extent possible, thus resulting in less number of joints. 4) To limit the size and number of prefabricate by the weight in overall dimension that can be handled by the handling and erection equipment and by the limitation of transportation. 5) To have all these prefabricates approximately of same weight very near to the lifting capacity of the equipment 2.10.3 Types of standardization 1. Generic standardization - where an element or process is by its nature standard and is usually recognized as such worldwide. E.g. steel, concrete, cement or plaster. International standards (ISO etc.) seek to rationalize standards internationally. 2. National standardization - where some items are standard for a country or group of countries, such as the European Union. The dimensions of a household brick would be an example of national standardization. National standards (BSI etc) seek to rationalize such items or processes into standards that are practices throughout the country. 3. Client standardization - where a particular client defines certain elements, processes or procedures in their business. 4. Supplier standardization - where a supplier, or in some cases a whole product or materials sector, stipulates that certain components, sub-assemblies, or even whole products are standard. 5. Project standardization - where a project team will decide to standardize certain procedures or building elements. For example, Quality Assurance procedures, column sizes, dimensional grids or module sizes. 2.11 Codal Provision (IS-15916: 2010) 2.11.1 Materials Use of materials for plain and reinforced concrete shall satisfy the requirements of IS 456. Connections and jointing materials shall be in accordance with 9.3. While selecting the materials for prefabrication, the following characteristics shall be considered: a) Easy availability; b) Light-weight for easy handling and transport; c) Thermal insulation property; d) Easy workability; e) Durability; f) Non-combustibility; g) Sound insulation;.
(29) h) Easy assembly and compatibility to form a complete unit; j) Economy; and k) Any other special requirement in a particular application. 2.11.2 Plans and Specifications The detailed plans and specifications shall cover the following: a) Such drawings shall describe the elements and the structure and assembly including all required data of physical properties of component materials. Material specification, age of concrete for demoulding, casting/erection tolerance and type of curing to be followed. b) Details of connecting joints of prefabricates shall be given to an enlarged scale. c) Site or shop location of services, such as installation of piping, wiring or other accessories integral with the total scheme shall be shown separately. d) Data sheet indicating the location of the inserts and acceptable tolerances for supporting the prefabricate during erection, location and position of doors/windows/ventilators, etc, if any. e) The drawings shall also clearly indicate location of handling arrangements for lifting and handling the prefabricated elements. Sequence of erection with critical check points and measures to avoid stability failure during construction stage of the building. 2.11.3 Components The dimensions of precast elements shall meet the design requirements. However, the actual dimensions shall be the preferred dimensions as follows: a) Flooring and Roofing Scheme — Precast slabs or other precast structural flooring units: 1) Length — Nominal length shall be in multiples of 1 M. 2) Width — Nominal width shall be in multiples of 0.5 M. 3) Overall thickness — Overall thickness shall be in multiples of 0.1 M. b) Beams 1) Length — Nominal length shall be in multiples of 1 M. 2) Width — Nominal width shall be in multiples of 0.1 M. 3) Overall depth — Overall depth of the floor zone shall be in multiples of 0.1 M. c) Columns 1) Height — Height of columns for industrial shall be 1 M and other building 1 M. 2) Lateral dimensions — overall lateral dimension or diameter of columns shall be in multiples of 0.1 M. d) Walls Thickness— The nominal thickness of walls shall be in multiples of 0.1 M. e) Staircase Width — Nominal width shall be in multiples of 1 M. f) Lintels 1) Length — Nominal length shall be in multiples of 1 M..
(30) 2) Width — Nominal width shall be in multiples of 0.1 M. 3) Depth — Nominal depth shall be in multiples of 0.1 M. g) Sunshades/Chajja Projections 1) Length — Nominal length shall be in multiples of 1 M. 2) Projection — Nominal length shall be in multiples of 0.5 M. 2.11.4 Design Considerations The precast structure should be analyzed as a monolithic one and the joints in them designed to take the forces of an equivalent discrete system. Resistance to horizontal loading shall be provided by having appropriate moment and shear resisting joints or placing shear walls (in diaphragm braced frame type of construction) in two directions at right angles or otherwise. No account is to be taken of rotational stiffness, if any, of the floor-wall joint in case of precast bearing wall buildings. The individual components shall be designed, taking into consideration the appropriate end conditions and loads at various stages of construction. The components of the structure shall be designed for loads in accordance with IS 875 (Parts 1 to 5) and IS 1893 (Part 1). In addition, members shall be designed for handling, erection, Ties and bearings. Handling stresses Precast units should not be inflicted with any permanent damage arising from their handling, storage, transportation and erection. Consideration should be given during design to: Loads on erected elements at construction stage; Design considerations should also be given to: Construction loads. A minimum load of 1.5 kN/m² should be used. However, due consideration should be given to any special requirements e.g. for plant loads or storage loads and the load increased accordingly; Notional horizontal load. The lateral load should be taken as not less than 1.5% of the characteristic dead load; Accidental loads such as earth movement, impact of construction vehicles. Demoulding, storage, transportation and erection of precast units on site; Temporary stages/erection sequence The critical loading for precast elements is often not the permanent condition but can occur during the construction phase and, hence, the temporary condition may govern the design of elements. Consideration should be given to the loading imposed on precast elements during each phase of construction. Examples of such cases are as follows: Precast sections of composite elements which are required to support self-weight plus construction load prior to casting of an in-situ topping; Lower precast floor slabs or precast stair flights which support propping to upper levels during installation; and Bearing or halving joints which support higher temporary construction loads because of back propping to upper levels..
(31) The design should also take into consideration that the structural action and framing might be different during the temporary stages resulting in higher stresses in individual members. Design of Ties The types of ties to be provided for stability and interaction between precast units are as follows:. Types of tie in structural frame 1. Peripheral Ties At each floor and roof level an effectively continuous tie should be provided within 1.2 m of the edge of the building or within the perimeter wall. The tie should be capable to resisting a tensile force of Ft equal to 60 kN or (20 + 4N) kN whichever is less, where N is the number of storeys (including basement). 2.Internal Ties These are to be provided at each floor and roof level in two directions approximately at right angles. Ties should be effectively continuous throughout their length and be anchored to the peripheral tie at both ends, unless continuing as horizontal ties to columns or walls. The tensile strength, in kN per metre width shall be the greater of. where (gk + qk) is the sum of average characteristic dead and imposed floor loads in kN/m2 and lr is the greater of the distance between the centre of columns, frames or walls supporting any two adjacent floor spans in the direction of the tie under consideration. The bars providing these ties may be distributed evenly in the slabs or may be grouped at or in the beams, walls or other appropriate positions but at spacing’s generally not greater than 1.5 lr..
(32) 3. Horizontal ties to column and wall All external load-bearing members such as columns and walls should be anchored or tied horizontally into the structure at each floor and roof level. The design force for the tie is to be greater of, a) 2 Ft kN or ls × Ft × 2.5 kN, whichever is less for a column or for each meter length if there is a wall. ls is the floor to ceiling height, in meter. b) 3 percent of the total ultimate vertical load in the column or wall at that level. For corner columns, this tie force should be provided in each of two directions approximately at right angles. 4. Vertical ties (for buildings of five or more storeys) Each column and each wall carrying vertical load should be tied continuously from the foundation to the roof level. The reinforcement provided is required only to resist a tensile force equal to the maximum design ultimate load (dead and imposed) received from any one storey. Bearingfor Precast Units Precast units shall have a bearing at least of 100 mmon masonry supports and of 75 mm at least on steel or concrete. Steel angle shelf bearings shall have a 100 mm horizontal leg to allow for a 50 mm bearing exclusive of fixing clearance. When deciding to what extent, if any, the bearing width may be reduced in special circumstances, factors, such as loading, span, height of wall and provision of continuity, shall be taken into consideration. 2.11.5 Joints The design of joints shall be made in the light of their assessment with respect to the following considerations: a) Feasibility — The feasibility of a joint shall be determined by its load carrying capacity in the particular situation in which the joint is to function. b) Practicability — Practicability of joint shall be determined by the amount and type of material required in construction; cost of material, fabrication and erection and the time for fabrication and erection. c) Serviceability — Serviceability shall be determined by the joints/expected behavior to repeated or possible overloading andexposure to climatic or chemical conditions. d) Fire rating — The fire rating for joints of precast components shall be higher or at least equal to connecting members. e) Appearance — The appearance of precast components joint shall merge with architectural aesthetic appearance and shall not be physically prominent compared to other parts of structural components. 2.11.6 Design Requirements for Safety againstProgressive Collapse Prefabricated buildings shall be designed with proper structural integrity to avoid situations wheredamage to small areas of a structure or failure of singleelements may lead to collapse of major parts of the structure..
(33) The following precaution may generally provide adequate structural integrity: a) All buildings should be capable of safely resisting the minimum horizontal load of 1.5 percent of characteristic dead load applied at each floor or roof level simultaneously. b) All buildings shall be provided with effective horizontal ties, 1) Around the periphery; 2) Internally (in both directions); and 3) Columns and walls. c) All buildings of five or more storeys shall be provided with vertical ties. In proportioning the ties, it may be assumed that no other forces are acting and the reinforcement is acting at its characteristic strength. Normal procedure may be to design the structure for the usual loads and then carry out a check for the tie forces. 2.12. Stages of Loadings There were two stages of loadings. In the first stage, the structure is loaded with the Ultimate Superimposed Design Load. In the second stage, going beyond the Ultimate Superimposed Design Load, the structure is loaded with a point load in the center, which was increased until the structure failed. In this case, the test is stopped before catastrophic failure because of space limitations of the test setup. 2.13Stages of Prefabricated Concrete Product.
(34) 2.14 Materials Concrete: The most common grade of concrete for precast is M30 to M60. The type of concrete depending upon the structural requirements. The code specify SCC, Light weight aggregate concrete and Cellular Concrete. Steel: Generally High tensile hot rolled ribbed bars are used for precast reinforced construction. Diameter of steel varies from 6mm to 40mm.. 33.
(35) 2.15 Methods of Prefabrication Site prefabrication: In this scheme, the components are manufactured at site near the site of work as possible. This system is normally adopted for a specific job order for a short period. The work is normally carried out in open space with locally a valuable labour force. The equipment machinery and moulds are of mobile nature. Therefore there is a definite economy with respect to cost of transportation. This system suffers from basic drawback of its non-suitability to any high degree of mechanization. It has no elaborate arrangements for quality control. Plant prefabrication: Factory prefabrication is restored in a centrally located plant for manufacture of standardized components on a long form basis. It is a capital intensive production where work is done throughout the year preferably under a covered shed to avoid the effects of seasonal variations high level of mechanization can always be introduced in this system where the work can be organized in a factory like manner with the help of constant team of workmen. The basic disadvantage in factory prefabricated, is the extra cost in occurred in transportation of elements from plant to site of work sometimes the shape and size of prefabrication. Semi-mechanized The work is normally carried out in open space with locally available labour force. The equipment machinery used may be minor in nature and mouldsare of mobile or stationary in nature. Fully-mechanized The work carried out under shed with skilled labour. The equipment’s used are similar to one of factory production. This type of precast yards will be set up for the production of precast components of high quality, high rate of production. 2.16 Process of Manufacture The various processes involved in the manufacture of precast elements are classified as follows: Main Process It involves the following steps. 1) Providing and assembling the moulds, placing reinforcement cage in position for reinforced concrete work, and 2) Fixing of inserts and tubes where necessary. 3) Depositing the concrete in to the moulds. 4) Vibrating the deposited concrete into the moulds. 5) Demoulding the forms. 6) Curing (steam curing if necessary) 34.
(36) 7) Stacking the precast products. Secondary (Auxillary) Process This process is necessary for the successful completion of the process covered by the main process. 1) Mixing or manufacture of fresh concrete (done in a mixing station or by a matching plant). 2) Prefabrication of reinforcement cage (done in a steel yard of workshop) 3) Manufacture of inserts and other finishing items to be incorporated in the main precast products. 4) Finishing the precast products. 5) Testing the precast products. 2.17 Production Methods The term production of systems is describes a series of operation directly concerned In the process of making or more apply of moulding precast units on the face of it there are very many techniques since almost every type prefabricates requires a specific series of operation in its production. These techniques however may be grouped into three basic method of production. These are 1. Stand Method 2. Flow Method a) The ‘Stand Method’ where the mouldsremainstationary at places, when the various processes involved is carried out in a cyclic order at the same place. b) The ‘Flow Method’ where the precast unit under consideration is in movement according to the various processes involved in the work which are carried out in an assembly-line method. The various accepted precasting methods are listed in below Table (given in IS: 15916-2011) with details regarding the elements that can be manufactured by these methods.. 35.
(37) 2.18 Quality Control At Factory Ordinance (“BO”) and the approved plans. to conditions stipulated by the BO and in the approved plans. e precast concrete works. “QAS”) provided by the manufacturer satisfies the purpose in that the manufacturer has made adequate provisions ensuring the production of the precast elements complies with the provisions of the BO and the approved plans. with the QAS prepared by the manufacturer in application for consent to commence work. ’s stream) to supervise the precast concrete production works at a frequency of not less than once a week. g book recording details of the supervisory personnel and details of the production, inspection, auditing and testing carried out for the production of the precast units. 36.
(38) book at the site office. elements at least once every month. Prepare audit reports for submission. s.. to carry out regular technical audits of the factory and the production of the precast units at a minimum frequency of once per month. to conditions stipulated by the BO and in the approved plans. tor (T3 TCP under the RC’s stream) to provide continuous supervision of the precast concrete production works. Provide supervisory personnel at the factory and an inspection log book recording details of the supervisory personnel and details of the production, inspection, auditing and testing carried out for the production of the precast units. Make sure the log book is available for inspection at all time by keeping the log book at the site office. d production of the precast units at least once every month by the Authorized Signatory of the RC. Prepare and submit the audit reports to the AP/RSE for endorsement and onward submission for record purposes. Duties of the manufacturer lements shall be manufactured by a factory possessing an ISO9000 quality assurance certification. subsequently to make application for consent to commence works. aintain the quality of the manufacturing of the precast elements. to carry out regular technical audits of the factory and the production of the precast elements at a minimum frequency of once per month. The QAS shall cover but not be limited to the following items: -bars, finishes and building services provisions. such as the frequency and standards adopted for the equipment used for the cube compressive strength test. employed and demoulding details. such as details of the curing procedure and associated controls. 37.
(39) by the independent parties employed by the manufacturer or the RC. quality assurance scheme and in accordance with the specification and the approved plans. Where Authorized Person (AP), Registered Structural Engineer (RSE) Registered Contractor (“RC”). 2.19 Quality Assurance System 1. Organization Chart 2. Casting yard set up of yard Number of moulds with estimated production rate Machinery employed 3. Production procedures (casting and transportation of the precast units within the yard). 4. Quality control procedures on materials and check points for Concrete re-bar Couplers finishes Building services installations 5. Quality control procedures on production and check points. Approved plans used. Shop drawings used. moulds assembling. re-bar fixing. Couplers fixing/welding work. Finishes and building services installation. concreting work. curing. 6. Calibration of testing equipment (responsible parties, frequency, and standards) 7. Testing of precast units such as dimensional check, cover meter test, pull-out test for tiled finishes, bonding test for building services installation, etc. 8. Concrete repair procedures. 9. Handling of non-compliant precast units with corrective/preventive action. 10. Inspection forms. 38.
(40) 11. Identification system of the precast units. 12. Audit by independent parties. 2.20 Construction Methodology 1. Production Planning Generally, the production cycle is one day for a non-complicated precast element. In planning the production of precast elements, time of construction of each floor is a key factor in estimating the number of precastingmoulds. For example, in a project consisting of 15 precast façades per storey and a working cycle of 6 days per storey, the number of mould required is 3. A storage area in the precast yard should be sufficient to accommodate precastelements delivered to the construction site and extra precast elements in caseof emergency delivery. Example of a working schedule for production planning is shown in below figure.. Working schedule for precast unit. For the production of precast elements, the precast manufacturer requires about or at least 1.5 months for manufacturing the moulds and 0.5 month for production of the precast units. Therefore, the precast shop drawing (showing geometrical size) should be consolidated at least 2 months in advance of the scheduled date of delivered to site. All the above should be allowed for in addition to the time required for approval and consent by government or client. Embedded items including window frames, E/M pipe sleeves and openings in precast elements should be delivered to the precast yard before production.. 39.
(41) Unlike traditional in-situ construction, this requires coordination and approval of embedded items at an early stage. For projects with a large number of precast elements, time required for the above would be much longer and has to be taken into account. 2. Moulds 2.1 Materials Moulds can be made of any suitable material including steel, timber, glass reinforced concrete or a combination of these. The selection of the mould materials will depend on the several factors highlighted in the Code. Locally, the steel mould is the most common type owing to its robustness and precision. In general, the steel plate thickness adopted for mould design and fabrication varies from a minimum of 4.5mm to 6.0mm, which can be used over 100 times with proper care and maintenance. Material for moulds depends on the number of repetitions, required surface finish, quality and shape complexity of precast elements. • Steel moulds are preferred owing to its robustness and precision. • Minimum 8mm thick steel plate can be used for 500 repetitions. • Minimal number of demoulding parts of mould helps to ensure good maintenance of dimensional accuracy during production to facilitate easy assembly and dismantling. • Adjustable moulds for greater flexibility and variety in production of precast elements. 2.2 Tolerances To enhance cost competitiveness, adjustable moulds should be adopted where possible, for greater flexibility and variety in the production of precast concrete elements. 2.3 Recesses, sleeves and boxouts Moulds should be designed to allow for appropriate placing and compaction of the concrete. Adequate numbers of braces, ties and struts should be provided for proper casting and hardening of the concrete.. 40.
(42) Applying mould release Agent. Steel mould. Combined use of steel and timber mould. 3. Cast-In Connection Three basic types of in-situ concrete connections commonly used in precast construction are A thin topping layer is cast to form a composite member, typically used with floor units such as hollow-core and double-Tee. It also acts as a leveling screed and may not be mechanically connected to the unit. Longitudinal shear due to bending is transferred by bonding and is also a function of the roughness of the interface. Composite construction is such that the in-situ concrete is a major component of the structural member. A typical example is a beam-shell where the precast unit forms the soffit and sides of the beam and contains the longitudinal reinforcement or prestressing wires and the shear steel. This type of construction allows continuous members to be easily formed by placing negative reinforcement in the in-situ concrete over supports. Simple spans are usually propped until the in-situ concrete attains sufficient strength to carry the dead weight on the composite section.. where beam or column continuity is required as in earthquake-resistant construction. Bond. 41.
(43) length of the bars being lapped dictates the length of the splice. It may be necessary to connect large main bars by welding.. Use of thin topping on Slabs and Beams. Cast-in connection on Beams and Columns 4 Lifting Inserts 4.1 General Three common types of lifting inserts used in precast concrete are: Reinforcement bar with omega “Ω” shape lifting insert. It is used in thin precast elements, such as a precast partition; and precast elements of shallow depth, such as a semi-precast slab. Lifting anchor with bulky head with U bars reinforcing the bottom head. Lifting anchor with eye for reinforcement bar to pass through. Lifting capacity of lifting inserts depends on the material strength of the insert and, more important, the strength of surrounding concrete. Clear instructions must be specified on concrete strength requirements for lifting, especially for the first lifting out of the mould. 42.
(44) 4.2 Lifting position tolerance If the lifting anchor is offset substantially when compared with the drawings, the centre of gravity of the lifting point may be offset substantially from the centre of gravity of the precast element. This may cause the precast element to become out-of-balance and incline during lifting (i.e. not vertical in the lifting stage) making it difficult to handle, especially during installation. 5. Prefabricated Metal Frames Prefabricated frames such as windows should be protected to avoid damage by fresh concrete.. 43.
(45) Prefabricated frames. 6. Pre-Concreting Check Prior to concreting, the condition of the mould shall be inspected since it directly influences the quality of the precast concrete product: Themould form shall be level and the flatness of the base of the mouldand the squareness and stiffness of the mould form examined and the mould kept free from spillage. Manufacturers shall ensure that the dimensions of the mould are within the tolerances specified in accordance with Code. Themould shall be clean and free from debris (e.g. from the previous precasting operation). Form oil or release agent and retarder shall be applied to the surface of the mould to be in direct contact with concrete. They shall be applied in accordance with the manufacturer’s instructions. Over application may lead to puddling on the concrete surface. Reinforcing bars and cast in items such as lifting inserts, window frames and earthing lugs shall be fixed only after preparation of the mould: Rebar size, spacing, lap length and cover requirement shall be checked in accordance with the approved drawings and within the tolerance limits. Lifting inserts shall possess adequate length of embedment to prevent damage during lifting. A sufficient number of spacers, chairs and supports shall be properly placed and secured to achieve the required concrete cover during casting. Window frames shall be installed and fixed in place according to the approved shop drawings, and with full electrical continuity to the earthing lugs and façade. Reinforcing bars and all cast in items shall be clean and free from contamination by mould oil and cement grout before casting, as this may lead to poor bonding to the concrete.. 44.
(46) Checking of rebar, lap length, cover and lifting inserts before concreting. 7. Concrete Placing Density, uniformity and surface quality of precast concrete products depend on the workability of concrete, placing and the compacting procedures used during the production process. The workability of a concrete mix is presented in detail in the Concrete Code handbook. Attributes which relate to measuring the workability of concrete are as follows: Consistency depends on the degree of dryness and wetness of the concrete mix. Deformability is a measure of workability for low to medium workability concrete. Flowability is a measure of workability of high workability concrete. Passing ability measures the ability of a concrete mix to pass through narrow gaps. Segregation resistance or cohesiveness relates to the potential separation of some ingredients due to free falling and sliding along surfaces during the placing of concrete. Factors affecting workability include size and shape of aggregate, mix proportions, cement content, admixtures used and concrete temperature during placing and compaction. Procedures 45.
(47) and precautions for placing concrete are detailed in the Concrete Code 2013. Vibration and compaction of concrete is the principal method forconsolidating concrete. Fresh concrete must beproperly vibrated so that once hardened, its strength and durability are fully realized. Studies have shown that proper vibration enhances compressive, tensile and flexural strength and resistance to deterioration by increasing the density of concrete and eliminating voids, honeycombing and entrapped air due to poor placement of concrete. The use of vibration tables, external form vibrators, and surface vibrators are examples of external vibration/compaction techniques that are applicable in precast concrete production. Form vibrators shall be mounted on the form to induce vibration in the mould, which is then transmitted to the concrete. The number and locations of external vibrators used shall be strategically planned to best distribute their impact. Surface vibrators are installed on the concrete surface, exerting their effects at the top surface and consolidating from top down. They are used mainly in precast slab construction. Vibration tables are used to vibrate the frame that supports themould and are usually used for elements cast in small moulds. The table is isolated from the ground with springs or neoprene isolation pads to prevent undesirable vibrations affecting other production processes. Proper vibration and compaction shall be carried out, in particular in congested areas with a lot of steel reinforcement. When all air, entrapped in pockets and voids has been released, vibration is considered sufficient. This is demonstrated when air bubbles cease to emerge at the concrete surface.. (a) Using a vibration table. (b) Using an external vibrator. 46.
(48) Concreting and compaction of concrete. 8.Demoulding and Lifting A precast concrete product shall only be demoulded and lifted when the designed compressive strength of the concrete has been achieved. This can be assessed by compression tests on cubes cured under the same environment as the precast element itself. The minimum concrete strength at which a precast element can be lifted from the mould shall be based on the calculated concrete stresses at the lifting points, stresses caused by the transfer of prestressing forces or handling, the anchorage length of inserts and the type of precast element. To overcome additional suction and frictional forces during demoulding, the minimum concrete strength for lifting may be higher than the recommended value specified in the Code. It depends on the design and shape of the mouldand the precast element. Flat mould suction increases in the presence of water and can be relieved by first lifting one edge of an element gently. Frictional forces are induced by contact and bonding between concrete and the verticalsides of the mould. To reduce friction, the mould shall be designed with adequate draw or removable sides or vibrated gently while lifting one edge of the member. The Code also suggests the use of a high quality demoulding agent to reduced suction and frictional forces. Embedded hardware, threaded inserts, dowel connectors and removable sections of the moulds are usually attached to the mould with bolts, pins and clamps. Before demoulding, all bolts and pins connected to the mouldshall be loosened and all clamps removed. Side forms, window capping shall also be detached from the element. Failure to remove all bolts and pins is a common cause of failure of lifting insert and the formation of cracks on the concrete surface.. 47.
(49) Detaching the mould from a precast element.. Sequence of lifting of a precast façade (from top and left to right).. 48.
(50) 9.Curing Curing has four major objectives: To maintain a suitable environment for new concrete to produce as much gel as possible so as to develop its full strength potential and reduce its permeability for better protection of the steel reinforcement from corrosion To avoid damage by plastic cracking and early age thermal cracking To avoid damage by shock vibrations due to nearby activities To avoid damage by premature loading caused by movement of adjacent parts of the structure Therefore, the scope of curing includes: Moisture control to prevent premature drying out of the concrete mix due to solar radiation and wind that may lead to plastic shrinkage cracking of the free surfaces not in contact with mould surfaces. Thermal control to prevent large temperature rises and drops, whichcould cause serious thermal cracking problems. Vibration control, which is particularly important if the precast plant is located on or near a construction site, or adjacent to any activities involving vibration. Movement and deformation control, required if the mould might move during the curing process. Curing of precast elements is usually achieved firstly by accelerated curing followed by a normal curing process (i.e. sprinkling water and keeping the elements moist with a curing membrane). Steam curing, described in the Code, is a subset of accelerated curing. The chemical cement hydration reaction takes place more rapidly with increased curing temperature and results in greater early strengths and efficiency in the production of precast products. In practice, elevated temperatures and addition of moisture during the steam curing process are both used in order to accelerate the rate of strength gain. The following explains the stages of a steam curing process: Stage 1 – Fresh concrete in the mould is allowed to achieve its initial set before putting the concrete in contact with steam or hot air. Steam is applied within a suitable enclosure that permits free circulation of the steam. Precautions shall be taken to prevent moisture loss from the concrete. Stage 2 – The precast element is heated to a maximum temperature of 700C at a heating rate usually within 100C per half hour. A curing temperature exceeding 700C may result in delayed ettringite formation which is detrimental to concrete strength. Gradual increase in temperature ensures a small thermal gradient between the surfaces and the interior of the concrete element. Stage 3 – Temperature and pressure of the environment are maintained for a sufficient duration, depending on the thickness and shape of the section. Stage 4 – The temperature is lowered at a rate not exceeding the rate of heating and the pressure is normalized. Low pressure steam curing refers to steam curing as mentioned in the Code and 49.
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