Concrete formwork systems
Formwork is basically a mould which is used to attain a desired shape of desired size out of concrete and other building material. As far as concrete construction is considered the false work supports the shuttering moulds. Most of the buildings which are made to stand for a long time are made of concrete and mortar. These building materials are strongest and most suitable but at the same time they are a bit tedious to deal with. They do not acquire the desired shape and need help of some sort of support or any frame which can be detached as the concrete solidifies. Actually concrete is a material without any shape. For most applications the shape matters, and concrete has to be molded or formed[Awad].
Concrete has been in use for the thousands of years. The dome of Pantheon in Rome is made of lightweight concrete, and the under face shows the moulding effect of formwork used two millennia ago. But it was only at the end of nineteenth century that the use of concrete became common, with the invention of reinforced concrete. Some of the most previous examples of concrete slabs were built by engineers of Rome [Fig 1]. Concrete is quite strong in resisting the compressive loads but has poor Tensile strength. To mold these structures, temporary scaffolding and formwork are built as per the desired shape of the structure. These building techniques were not isolated to pouring concrete, but were and are widely used in Masonry. Because of the complexity and the limited production capacity of the building material, concrete's rise as a favored building material did not occur until the invention of Portland cement and reinforced concrete[Awad].
The construction industry forms the largest single sector in any economy. In the USA for example the construction sector is responsible for 14% of the GDP (US National Statistics Bureau). Formwork is the single largest cost component of concrete building's structural frame. The cost of formwork exceeds the cost of concrete or steel and in some situations the formwork costs more than the concrete and steel combined. For some structures, placing priority on the formwork design for a project can reduce the total frame cost by as much as 25%. This saving includes both direct and indirect costs. Formwork efficiencies accelerate the construction schedule, which can result in reduced interest costs during construction and early occupancy for the structure. Other benefits of formwork efficiency include increased job site productivity, improved safety, and reduced potential for error.
Concrete is a material without shape. For most applications the shape maters and the concrete has to be molded or formed. Formwork is one of the integral parts of construction industry. This is the basic of any architectural structures on which the whole building is made[Awad]. The formwork is basically used to hold the whole structure until the whole building is capable to stand on its own. Even we use some solid iron rods in buildings but the concrete need some time to bind with that for a firm structure to stand as and in the desired shape[WSDOT:2006].
CLASSIFICATION OF FORMWORKS
Under this section of the study we will get in depth of four aspects of each and every type of the form work. These four aspects would be:
- Analysis of various components of formwork.
- Safety aspects of formwork.
- Economy aspects of formwork.
- Selection process of the material of formwork.
The formworks are broadly divided in four categories based on the material, of which it is made. This categorization is as follows:
- Timber Formwork.
- Metal Formwork.
- Plastic Formwork.
- Fiber Formwork.
This is the oldest kind and most basic formwork used in the construction industry [Fig 2 & 3]. Its moisture resistant property makes it utilizable in this industry. Though it is easy to manufacture the timber formwork but it is time consuming when formwork is to be prepared for big structures. It is relatively cheap and easy to shape as required[Awad]. It is relatively light in weight for handling, but it is of limited durability.
ANALYSIS OF VARIOUS COMPONENTS:
Sheathing is supported by horizontal members called joists or runners. Joists are made from dimension lumber spaced at constant intervals that are a function of applied loads and type of lumber[WSDOT:2006]. It is a recommended practice to round down the calculated joist spacing to the lower modular value. Joists are supported by another set of horizontal members perpendicular to the joists, called stringers. The stringers are supported by the vertical members called shores. In all wood conventional formwork [ i.e. 4 ื 4 in. or 6 ื 6 in.]. Shores are rested on heavy timbers called mudsills, to transfer the vertical loads to ground. In case where a slab on grade exits, shores are directly rested on them. Once the bottom of the beam is constructed and leveled, one side of the beam is erected first with the holes drilled in it for installing the tie roads[ISBN:2009]. Tie rods are steel rods that hold the two sides of beam together. After the first side of beam form is erected, the reinforcement is placed inside the beam and then the other side of the beam is erected. Tie rods are then inserted into all holes on both side of the beam. The tie rods' function is to resist horizontal pressure resulting from the freshly put concrete and thus keep the sides of the beams in their proper location.[Fig 4]
SAFETY ASPECTS FOR TIMBER FORMWORK:
Though Timber formwork is the basic formwork but it has some drawbacks relating to the safety which prove to be harmful for the structure to be build and the laborers. The following safety aspects must be considered while dealing with the Timber Formwork.
1. Care should be taken in presence of Moisture: The biggest disadvantage of Timber Formwork is that the timber get softens when it is wet. If the moist is too much then timber soften up to a extent that it can be poked with the help of nail tip. Though dry timber is hard enough to bear the stress developed by concrete but it is not the same when it get wet. When timber comes in contact with moisture, the methanol molecules get activated and are responsible for weak intermolecular bond and thus softening. This problem is commonly known as Wet Rot.[Fig 5]
2. Care should be taken in presence of mini organisms and micro organisms: Biggest threats to any wooden structure and article are Termite[Fig 7] and Fungus. Both of these organisms live on almost every type of wood as parasite and cause damage to huge quantity of wood. As far as Timber formwork is considered the damage occurs mainly in damp and dark regions of storage. Termites eat the wood from the inner side making it hollow and weak within. The wood thus left is so weak that it can be broken with bare hands. The damage caused by fungus is known as Dry Rot [Fig 6]. Dry-rot fungus is considered as a fatal disease and is referred as building cancer, growing through buildings destroying rapidly any timber in its path. The fungus, which thrives in moist and damp regions, will get through the brickwork to get more timber so as to cause widespread destruction of structural timbers, skirting boards and door frames and wood flooring. In short. The fungus can be explained as 'living in masonry and eating wood'. As the fungus lives in damp, unventilated conditions, it can occur in the areas of a property that are rarely taken care of, for e.g. floor voids, or beneath timber paneling, so damage may be massive prior the attack is discovered[ISBN:2009].
3. Fire Safety: We already know that wood is flammable and pron to fire if considerable precautions are not taken specially for those constructions sites which are near by to a gas station or any such location. Fires on construction sites have been recognized as a serious problem for construction companies as well as for insurers, but less of a public safety issue since there are relatively few fire deaths in such incidents. However, the obsession for ever-taller buildings, specifically in the Gulf and Far East countries has resulted in a number of significant fires during construction which have caused deaths and demonstrated the difficulties facing fire brigades when tackling such incidents. On many occasions, airways have to be used for fire extinguishing and rescue. Insurers have long had concerns about the potential for loss on tall building projects, and these fires have spread awareness that their concerns are very well founded.
Causes can involve electrical risks, smoking, hot work, overheating equipment, rubbish burning, and escapes of gases or flammable liquids - all of which are well recognized and should be amenable to normal fire safety measures[WSDOT:2006].
DIFFERENT TYPES OF TIMBER FORMWORK
- Finnish Birch
- American Softwood
- Tropical Hardwood
Sheets are normally 8ft by 4ft except for Finnish Birch, the outer grain runs in the length of the board. The first two types are produced to appropriate national standards. Hardwood ply comes from a variety of tropical countries so obtaining reliable data about them is difficult.
2. Block board:
This kind of Formwork is the most suitable formwork for high rise buildings. This system is eco friendly and proves to be more economical on long run due to good durability. In the conventional metal system, joists and stringers are made of aluminum or steel supported by scaffold-type aluminum or steel shoring. In today's construction practices, joists and stringers are made of aluminum and are supported by scaffold-type movable shoring system. This formwork is built out of prefabricated modules with a metal frame (usually steel or aluminum) and covered on the application side with material having the wanted surface structure (steel, aluminum, timber, etc.). [Fig 11]
ANALYSIS OF VARIOUS COMPONENTS:
In this system, steel joists and stringers have the advantage of supporting greater spans and fewer joists and stringers. The main problem with using steel as joists and stringers for forming concrete slabs is their heavy weight, which makes it difficult for one person to handle. A standard steel W-section is used because its wide flange makes it easy to connect stringers with shore legs. Upper apex part of the steel joist get webbed through the upper chord part of the joist and through holes present in the metal sheet formwork provided over such joists before to the filing of the concrete slab. Mixed action of open-web structure of steel joists supporting beams, girders and reinforced concrete slab[ISBN:2009]. The improvement in relation to a continuous round rod presented near the apex of each projecting web member runs parallel to the longitudinal axis of the joist and a reinforcing wire mesh draped between the rods thus giving greater spacing between the joists, whereby protruding apex parts, rods and to covered wire mesh will be enclosed in cast-in place concrete within the slab to act as shear interconnection and the state of being reinforced devices therein, to secure the beam usedto support floors and formwork together, to more intense or better the locking of the concrete slab to the protruding beam used to support apex portions, and to reinforce said concrete slab. This condition superior to an earlier condition makes optional the use of wedge members forced between such protruding joist apex portions.
SAFETY ASPECTS OF METAL FORMWORK
Though there is no need worry for any fungal or organic decay to the metal formwork but still there are same issues which must be paid attention to, relating to corrosion and careless handling. Now let's discuss these issues one by one.
1. Corrosion: The corrosion problem [Fig 13] may occur to any formwork if it is not maintained in proper way. Any part of formwork may be corroded due to direct contact with moisture. This direct contact is either because of lack of proper oiling techniques or due to standing water. If even an inch or even a centimeter is left out of oiling then it proves to be dangerous for the formwork. Many times it happens that water is leaked out and left standing. This tends to corrode the dipped part of formwork. Rusting of formwork also leave stains over the concrete unit. These stains if thick crystallize later and pose a threat to platter layer.
2. Mishandling while assembling: There have been several cases causing accidents of laborers at the site due to mishandling of the formwork. These accidents occur while lifting, if the load is not properly tied or hydraulic jack is not applied properly. Other reason responsible for on site accidents is loose screws and bolts which lead to collapsing of the erected formwork[ISBN:2009].
3. Electricity: If the site location is nearby any sort of electric network then the metal formwork should be handled with utmost care. A safe distance should be maintained and if possible any such network or device should be avoided in nearby vicinity of the site.
DIFFERENT TYPES OF METAL FORMWORK
1. Extruded Aluminum Joist: The first component of the conventional metal system is the aluminum joist. The extruded aluminum joist takes the shape of a modified beam with a formed channel in the top flange in which a wood nail strip 2 by 3 in. is inserted. The plywood deck is then nailed to the nail strip. [Fig 14]
2. Aluminum Beams (Stringers): The purpose of stringers is to transfer the loads of the surface panel to scaffold. Extruded aluminum joists can also be used as stringers[Fig 15] unless the loading is too excessive, in fact it is good practice to avoid using a mixture different beam types. Aluminum beams are commercially available for the lengths ranging from 4 to 30 ft.
3. Aluminum Scaffolding Shoring: The aluminum scaffolding [Fig 16] shoring system has been available for several years as a substitute for the steel scaffolding shoring system. The system consists of several frames connected together by cross bracing. Aluminum shoring is lighter and has load carrying capacity equal to or greater than steel shoring. Load carrying capacity of aluminum shoring can reach up to 36,000 lb.
4. Post share: A Post Shore is a single member made of steel or aluminum and support stringers. Post shores can be used to replace, or in combination with, scaffold shoring. Post shores can also be used re-shoring after striping of formwork elements.
ECONOMY OF FORMWORK
The concrete subcontractor must achieve 3 goals in his formwork workmanship:
Quality - formwork must be designed and built so that the resulting cast concrete attains the desired size, shape, position and finish.
Safety - formwork must be designed built strong enough to support all dead and live loads without blowouts, collapse, danger to workers, and risk of damage to the structure.
Economy - formwork must be designed, built and re-used in an effort to save time and money for the general contractor and the owner.
Now as far as the economical aspect of formwork is concerned let me make you aware of the fact that formwork share a considerable site budget. If we design the formwork intelligently and plan its use, then we can save a lot of money. As metal formwork is more durable and reusable as compared to the wooden formwork, it should be used. There are several advantages which make metal formwork a better option.
1. Formwork for concrete structures, By Robert Leroy Peurifoy, Garold D. Oberlender.
2. [Section 6-02.3(11)] from the WSDOT Standard Specifications, 2006
3. Formwork Hazards. Available from : http://www.workcover.act.gov.au/pdfs/hazalerts/HA43-Formwork.pdf. [30 Dec, 09]
4. Building towers over Auckland's North Shore - NZ Construction News, Volume 2, Issue 3, July 2007
5. Chicago high-rise with self-climbing formwork - Concrete Construction, July 2003, via 'findarticles.com'. Accessed 2008-06-16.
6. Handbook of Finnish plywood, Finnish Forest Industries FederDecorative plywood ation, 2002, ISBN 952-9506-63-5
7. Concrete formwork systems, By Awad S. Hanna.