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Speedy construction- FORMS AND MATERIALS

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Speedy construction- FORMS AND MATERIALS

  1. 1. SPEEDY CONSTRUCTION QUES-1) WHAT DO YOU UNDERSTAND BY SPEEDY CONSTRUCTION? Ans-1) SPEEDY CONSTRUCTION: In general concrete construction, we have seen that whatever the elements cast (beams, columns, slabs etc.) Require time for setting. This is a long drawn process necessary for hardening of concrete. However, nowadays in large scale projects, newer methods of faster construction are being explored. Fast and efficient construction of the concrete core of a building is essential to maintain phased progress on other parts of the building. Formwork is one of the most important factors in determining the success of a construction project in terms of speed, quality cost and safety of work as it accounts about 40% of the total project cost of the structure. To minimize the costs the contractor needs to complete the project as soon as possible and the client wants the building to use the building as early as possible for the intended purpose. In building construction the most efficient way to speed up the work is by achieving a very short floor cycle. That directly depends on the selected form work type for the construction. Some of the methods to provide speedy construction are: • Members are cast on ground and then lifted to save on the time and money used for shuttering. Ex: lift slab construction • Precast concrete blocks are used. • Such members are used which do not require elaborate formwork. Ex: flat slab • Ready mix concrete (rmc) 1) LIFT SLAB CONSTRUCTION : Lift slab construction is a method of constructing concrete buildings by casting the floor or roof slab on top of the previous slab and then raising the slab up with hydraulic jacks, so being cheaper and faster as it does not need forms & shores as it is needed for cast-in-place slabs. Lift-slab construction can be more economic than conventional construction when the building is vertically uniform, such as for hotels, apartment buildings, and dormitories, and where the slab designs are repetitive. After the slabs have cured long enough to reach a prescribed strength, powerful hydraulic jacks mounted on top of the columns lift the slabs into their respective positions. A console connected to each hydraulic jack synchronizes the number of turns of the check nuts to assure that the concrete slab is being raised the same amount at all points.  Lift slab can be used for heights up to about 16 stories.  Economical column spacing ranges from 22 to 32 feet.  Columns may be pipe, tubes or wide flange sections; concrete columns may be used in 3- to 4- story buildings.
  2. 2. ADVANTAGES OF LIFT SLAB CONSTRUCTION:  Lift-slab construction method becomes more advantageous in buildings with similar floor plans throughout the height of the building and where flush slab may be desired.  This method eliminates the need for redundant formwork as only shuttering required on the edges, therefore casting concrete slabs is the simplest stage in whole constriction process of lift- slab method.  Lift-slab method may be employed with ribbed slabs not only flat slabs with some compromise of the ease of casting.  Another advantage is reduced handling and hoisting of materials and supplies that can simply be placed on top of the slabs and lifted with them. MAJOR LIMITATIONS OF LIFT SLAB CONSTRUCTION:  This method not to be use for multistorey building, only use for 15- 16 storey building.  No large span slab are constructed in this type of construction. 2) READY MIXED CONCRETE: Ready mixed concrete, or rmc as it is popularly called, refers to concrete that is specifically manufactured for delivery to the customer's construction site in a freshly mixed and plastic or unhardened state. Concrete itself is a mixture of portland cement, water and aggregates comprising sand and gravel or crushed stone. In traditional work sites, each of these materials is procured separately and mixed in specified proportions at site to make concrete. Ready mixed concrete is bought and sold by volume - usually expressed in cubic meters. Rmc can be custom-made to suit different applications ADVANTAGES OF RMC: Ready mixed concrete is manufactured under computer-controlled operations and transported and placed at site using sophisticated equipment and methods. Rmc assures its customers numerous benefits:  Uniform, consistent and assured quality of concrete  Flexibility in concrete design mixes  Easier addition of admixtures  Faster and speedier construction  Reduced inventories, material handling and storage of raw materials at sites  Savings in labour requirements, labour costs and supervision of labour  Reduced wastage of materials
  3. 3. QUES-2) WHAT IS IN-CAST SITU CONSTRUCTION? DEFINE CROSS WALL AND BOX WALL WITH SKETCHES. ANS-2) CAST-IN SITU CONSTRUCTION: The cast-in-situ concrete is standard concrete which is poured into the specific formwork on the site and cured to get the strength of RCC elements. In-situ is most commonly a mixture of aggregate (known as builder's mix) blended with portland cement and clean water, that is free of oils, acid and others. In situ techniques are often more labour-intensive, and take longer, but the materials are cheaper, and the work is versatile and adaptable.  Column, slab etc. elements are cast on site and hence it is difficult to control mix, placement and curing in cast-in-situ concrete.  More labours are required.  In situ concrete construction is slow as gaining of strength requires time.  Weather condition can delay the casting work.  In situ concrete is a cheaper form of construction for small structures.  Less maintenance cost.  More resistant to earthquake and wind forces. CROSS WALL & BOX WALL : •In Box Frame construction, the walls and slabs are cast monolithically in the form of boxes with the open ends of the boxes as cladded main external walls. •The lateral front and rear walls do not participate in the load bearing structure. •The load of floors and slabs are carried by internal walls perpendicular to the principal axis of building called as Cross walls and hence also called as Cross Wall Construction. Origin of Cross Wall Construction : In traditional buildings, the timber joists were arranged in the direction of shorter spans and were often supported on cross walls or partition walls rather than columns or lateral external walls. MERITS OF CROSS WALL CONSTRUCTION:  Structural & non-structural elements can be standardized and can be fabricated, resulting in speeding up of erection.  The cross- walls result in economy of labor, material and space.  The cross wall being internal are not affected by weather provided the designed thickness affords a reasonable standard of sound insulation, no additional thickness is called for, as the problem of rain penetration does not rise. DEMERITS OF CROSS WALL CONSTRUCTION:  The limitation of planning which results when the cross walls are maintained at maximum intervals of about 5.5m.  It is clear that a building would be structurally unsound if it consisted of a number of detached cross walls extending through several storeys without lateral or longitudinal ties or supports, especially when the cladding is the light weight type.  Renovation work is most difficult in such construction.  Doors need to come in the middle third of the wall. cross wall
  4. 4. ECONOMIC ACHIEVEMENT IN BOX FRAME CONSTRUCTION : •As lateral external walls are non-load bearing, so, In contemporary cross wall construction, the external walls are of relatively light cladded construction, which is resistant to rain transmission and is adequately insulated •Maximum economy is achieved if cross walls are properly planned to be same distance apart through out the length of the building. this results in :  Standardization of certain elements.  Panels of light weight cladding.  R.C.C. concrete floor having same imposed load.  Form work can be used again and again.  Erection is expedited. USES:  These are used in buildings where floor plans are typical like hospitals, hotels etc.  This type of construction is most suited to building types that are upto 5 storeys high.  The spacing of cross walls varies from 3-5.5 m  Used at the site where speedy construction is done. POINTS TO TAKE CARE WHILE CONSTRUCTING CROSS WALL :  Care must be taken to ensure that the end of the cross walls are sealed to prevent the admission of rain.  The damp proofing of the cross wall is done with a fibrous asphalt felt or D.P.C.  To prevent the admission of rain through any defective joints in the terracotta facing, the end of the wall may be given two good coats of bituminous paint or other water repellent emulsion cross wall.
  5. 5. QUES-3) WHAT IS THE REINFORCEMENT AND WHAT ARE THE TYPE OF REINFORCEMENT AND THEIR USES? EXPLAIN WITH SKETCHES. ANS-3) Concrete is very high in compressive strength, but it is low in tensile strength. Thus, when only the compressive loads are acting on the concrete surface then there is no need of using reinforcement in it. But where tensile forces are also involved, as in, beams and slabs, there is a very risk of its failure when plain concrete is used. Steel, however, as we know, has a very high tensile strength (and also as high compressive strength. Hence when the two (concrete and steel) are combined together, a material of construction is obtained that is capable of withstanding all the three types of forces likely to act upon a structure: compressive loads, tensile stresses, and shear forces Such a material is known as Reinforced Cement Concrete. There are mainly 4 types of steel reinforcement used in concrete structures: 1. HOT ROLLED DEFORMED BARS: This is the most common type of reinforcement for regular RCC structures. Hot rolling is done in the mills which involves giving it deformations on the surface i.e. ribs so that it can form bond with concrete. The stress - strain curve shows a distinct yield point followed by a plastic stage in which strain increases without increase in stress. This is followed by a strain hardening stage. It has typical tensile yield strength of 60,000 psi. 2. MILD STEEL PLAIN BARS: These are plain bars and have no ribs on them. These are used in small projects where economy is the real concern. As plain bars cannot bind very well with concrete hence hooks have to be provided at the ends. In this type of steel too stress - strain curve shows a distinct yield point followed by a plastic stage in which strain increases without increase in stress. This is followed by a strain hardening stage. Plastic stage in Mild Steel Bars is even more pronounced than Hot Rolled Deformed Bars. Typical tensile yield strength is 40,000 psi. 3. COLD WORKED STEEL REINFORCEMENT: When hot rolled steel bar undergoes process of cold working, Cold worked reinforcement is produced. Cold working involves twisting or drawing the bars at room temperature. This effectively eliminates the Plastic Stage in the Stress-Strain curve, although it gives more control over the size and tolerances of bars. Due to removal of plastic stage it has lower ductility than Hot Rolled bars. Its use is specific to projects where low tolerances and straightness are a major concern. The stress – strain curve does not show a distinct yield point as plastic stage is entirely eliminated. Yield point is determined by drawing a line parallel to the Tangent Modulus at 0.2% strain. Yield stress is the point where this line intersects the stress – strain curve. This is known as 0.2% proof stress. If yield stress is determined at 0.1% strain it is called 0.1% proof stress. Typical tensile yield strength is 60,000 psi. 4. PRESTRESSING STEEL: Prestressing steel is used in the form of bars or tendons which are made up of multiple strands, however, tendons / strands are more frequently used as these can be laid in various profiles, which is a primary requirement of prestressing steel. Prestressing strands are, in turn, made up of multiple wires (typical 2, 3 or 7 wire strands). Typical seven wire strand consists of six wires spun around the seventh wire which has a slightly larger diameter, thus forming a helical strand. These wires are cold drawn and have very high tensile ultimate strength (typically 250,000 - 270,000 psi). Their high tensile strength makes it possible to effectively prestress concrete even after undergoing short term and long term losses. These are used in prestressed concrete in bridges or prestressed slabs in buildings. Prestressing steel is also available as non-bonded strands encased in PVC
  6. 6. sheath. It is used in Post-Tensioning of members. Prestressing strands are also available as Low Relaxation Strands which exhibit low relaxation losses after prestressing. These are typically used in prestressing members with large spans. QUES-4) HOW READY MIX CONCRETE IS DIFFERENT FROM THE CONCRETE PREPARED AT SITE? ANS-4) Ready mixed concrete is certainly an advancement over the age-old site mixed concrete. The benefits of RMC in terms of quality, speed, life-cycle cost and environmental friendliness are overwhelmingly superior to those of site mixed concrete. Following brief comparison illustrates this vividly: Factors Hand/Manual/Site Mix Ready Mix Concrete Concrete Consistency Fairly difficult to achieve consistency since it is manually mixed at the site. Concrete mortar is consistent since it is computerized machine produced. Manpower Requirement Requires more labor force both skilled and unskilled for mixing the mortar and shifting it, laying and compacting. Skilled labor required only to lay and compact the concrete as per the design. Concrete Mortar Mix Ratio Respective structural Engineer/ Architect will provide the details of Concrete Grade whether it is M20, M25, M30 etc. and the thickness of the concrete mould. Respective structural Engineer/ Architect will provide the details of Concrete Grade whether it is M20, M25, M30 etc.. and the thickness of the concrete mould. Material Management Requires proper estimation the material sand availability at the site ahead of schedule. Material procurement not a concern but proper concrete estimation needs to do. However, the RMC agency should confirm the materials used in the mortar which may include Fly ash, GGBS, Chemicals etc.. Time Spent Requires approximately 50% more time compared to RMC. Requires approximately 50% less time compared to Site Mix. Site Condition Requires enough space to accommodate materials and site mix machinery. Requires a road width of minimum 30 feet for smooth maneuvering of the RMC carrier vehicle. Quality Monitoring the concrete mortar mix ratio through the process is critical. Owner should obtain the cube test report from the RMC Company. Quality control on Raw Materials Quality consistent materials is very critical to achieve the concrete compression strength required. Choose certified RMC suppliers who manufacture according to IS standards and will provide cube test reports. Laboratory test results for concrete Normally not tested for site mixes. The owner can opt for cube testing by any third party testing laboratories. Testing to be done by RMC manufacturer and no additional charges are levied. Pricing Site mix is generally cost effective compared to RMC. RMC is costlier compared to site mix but it is very convenient. QUES-5) WHAT IS THE ROLE OF FORMWORK IN SPEEDY CONSTRUCTION? DEFINE ITS TYPES. ANS-5) The use of formwork is so important in the process of construction. This is an impermanent or stable molding into which concretes are poured. The use of formwork is assisted with scaffolding in order to support it. This is used to provide the exact formation to a building; hence, this should be made with caution and should be done only by professional formwork contractors. The following are the key reasons why formwork is highly important for any project -
  7. 7. 1. Time Management - Good formwork systems help to minimize floor to floor construction cycle time, thereby achieving substantial indirect cost saving for the project. 2. Safety - Modern formwork systems provide designed solutions against all superimposed loads thereby ensuring structural safety. Also they integrate proper access and working platform arrangements in the scheme to ensure workmen safety. 3. Quality - Surface finishing of any concrete is solely contributed by quality & type of formwork used. Superior surface finish imparted by Aluminium formwork, Tunnel form eliminates the need of plastering for any RCC surface. 4. Project Planning - Proper knowledge of formwork management facilitates on-time mobilization & demobilization of formwork resources, effective set movements, minimize idling, maximize repetitions and minimize cost. It also helps in correct time & cost inputs for tendering process. 5. Human Resource - Formwork activities requires specialized skill sets for all levels of personnel involved. Hence proper skill assessment, training & effectiveness tracking are continual mandatory needs to maximize labour productivity. 6. Capital Management - Substantial quantum of Formwork materials owned by any construction company fall under asset category and travels from one project to the next. Hence accounting, physical verification, loss mitigation needs to be practiced meticulously to avoid unnecessary substantial cost impacts. TYPES OF FORMWORK: 1) Conventional Type of Formwork: This is the most traditional type of formwork and this uses timber, bamboo, masonry and carpentry to complete construction. Low initial cost, low experience factor and low weight are some of the advantages while high floor cycle, poor finish, and high labor requirement are the disadvantages of this formwork type. This formwork type is still in practice in two – three storey building construction projects. 2) Fiberglass formwork: The use of fiberglass as a material to make forms for concrete work has increased rapidly within the past two years. Among the reasons are: – Fiberglass forms provide a means of producing a concrete surface that is architecturally acceptable without rubbing and grinding. – Special patterns and designs can be readily molded into the material. – Large areas or sections may be made without joints or seams. – When repeated usage is possible, fiberglass is often the most economical form material. 3) Plastic Formwork: It comes as a individual structural elements like columns and walls. Various sized columns and walls are available in the market. The formwork is very easy to install and uninstall and has a very high repetitive nature. The ease of application makes it very useful where faster completion of work is a criteria. 4) Fiber-reinforced polymer (FRP) formwork system: FRP formwork is used where unique configuration required for the arches and columns or any other structural parts. The FRP formwork pieces were produced using the hand layup method using molds fabricated by means of a computer numerical control (CNC) machine. On site, the size and weight of the formwork required the use of forklifts and scissor lifts for assembly. 5) Aluminium Panel System Formwork: System formwork has prefabricated modular components with casting panels. The system formwork can suit the required shape of concrete structure. The speedy and quality construction is the biggest advantage in this type while high initial cost is the main disadvantage and hence this is not economical to use in low-rise buildings. But this is the most economical form of formwork type to be used in
  8. 8. high-rise building construction when it is having few (more that 10) typical storeys as it can be made in a single storey complete formwork set form and a faster floor to floor construction can be achieved. QUES-6) WHAT ARE THE JUMP SYSTEM AND SLIP SYSTEM? ANS-6) JUMP FORM SYSTEM:  Generally, jump form systems comprise the formwork and working platforms for cleaning/fixing of the formwork, steel fixing and concreting. Jump form, often described as climbing form.  It is suitable for construction of multi-floor vertical concrete elements in high-rise structures, such as shear walls, core walls, lift shafts, stair shafts and bridge pylons.  It is a highly productive system designed to increase speed and efficiency while minimizing labor and time. TYPES OF JUMP FORMWORK  Normal jump/climbing form  Guided-climbing jump form  Self-climbing jump form PROCESS EFFICIENCY :  Fast construction can be achieved by careful planning of the construction process. Crane availability is critical for normal jump form.  Self-climbing formwork cuts down the requirement for crane time considerably. By allowing the crane to be used for other construction work this may reduce the total number of cranes needed on site.  The formwork is independently supported, so the shear walls and core walls can be completed ahead of the rest of the main building structure. This can help to provide stability to the main structure during its construction and can have the beneficial effect of taking the jump form core off the project critical path.  High quality surface finishes are possible.  The highly engineered nature of jump form systems allows quick and precise adjustment of the formwork in all planes.  A small but skilled workforce is required on site.  It is easier to plan construction activities due to the repetitive nature of the work. SLIP SYSTEM: • Slip form construction, or continuously formed construction, is a construction method in which concrete is poured into a continuously moving form. Basically, this method involves the continuous placing of concrete in a shallow mould having the same plan as the building to be constructed. • This rigid mould, or "slip-form" as it is called, forms the working deck which is jacked slowly upwards at a controlled rate until the required elevation is reached.  • Method of vertically extruding a reinforced concrete section and is suitable for construction of core walls in high-rise structures – lift shafts, stair shafts, towers. Overview In the slip form method, paving is done using a special-purpose slip form paver, without installing framing. With this method, concrete structures with the same cross-section are finished continuously using a steel mold. This system is used for ordinary concrete slabs and continuous reinforced concrete slabs.

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