Precast Concrete: Advantages and Disadvantages
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Published: Thu, 02 Aug 2018
Jump to: Advantages of Precast Concrete Construction | Disadvantages of Precast Concrete Construction
Precast concrete is a form of construction, where concrete is cast in a reusable mould or “form” which is then cured in a controlled environment, transported to the construction site and lifted into place. By producing precast concrete in a controlled environment (precast plant), the precast concrete is provides the opportunity to properly cure and be closely monitored by plant employees. There are many different types of precast concrete forming systems for architectural applications, differing in size, function and cost. The advantages of using precast concrete is the increased quality of the material, when formed in controlled conditions, and the reduced cost of constructing large forms used with concrete poured on site. It is used mainly in the construction of buildings with repetitive design and elements, such as schools and apartments.
Every construction material & system has its own characteristics which influence the layout, span length, construction depth, stability system, etc. This is also the case for precast concrete. Not only in comparison to steel, wood, & masonry structures, but also with respect to cast in-situ concrete. Precast concrete members may be solid or may contain hollow cores. Many precast components have thinner cross sections than cast in situ concrete. Precast concrete may be either normal or lightweight concrete. Theoretically, all joints between the precast units could be made in such a way that the completed precast structure has the same concept as a in-situ one. However, this is a wrong approach & one, which is very labour intensive & costly. If the full advantages of precast concrete are to be realized, the structure should be conceived according to its specific design philosophy:
“Long spans, appropriate stability concept, simple details,etc. Designers should from the very beginning of the project consider the possibilities, restrictions & advantages of precast concrete, its details, manufacturer, transport, erection & service stages before completing a design in precast concrete”.
Architectural precast concrete provides architects with an exciting medium when designing facades for a wide range of buildings, from healthcare facilities to shopping malls, commercial office buildings to sports stadiums.
1.2 History of IBS
Concept of IBS in not new and can be traced back to as early 1624. In 1624, panellised timber houses were shipped from England to the new settlement in North America. In 1851 Crystal Palace in Hyde Park, London and 1889 Eiffel Tower, Paris are the well-known examples of buildings which uses the method of precast in constructing the structure of the building itself.
1.3 Precast building system
Precast building system is divided into 3 main parts:
- Precast concrete frame
- Precast concrete wall
- Precast concrete floor
1.3.1 PRECAST CONCRETE FRAME
Precast concrete frames involve an entire structure being fabricated off-site. In addition, structural components can be supplied for incorporation into a structure on-site. Frames can simultaneously achieve both structural and decorative design requirements. A variety of mixes, colours and finishes can be used too. The precast frame is composed of precast column and beam.
Precast concrete frames are cast in the same way as precast concrete cladding, but as they are designed as structural elements they have heavier reinforcement than required for non-structural cladding. Elegant connections are required between columns and beams to transfer considerable forces without affecting the visual appearance of the frame.
1.3.2 PRECAST CONCRETE WALL
Many precast concrete walls are cast in a flat position at the building site. But they are still classified as precast, not sitecast. When the concrete is strong enough, the wall panels are lifted and placed in the desired positions for the building. This method is commonly known as tilt-up construction. Today such walls are lifted and placed with cranes, so that casting can make repeated use of single forms.
Some wall units are cast in factories, where the form, quality of materials, and finishing can be more controlled. Because of their large size and weight, these walls cannot be transported to long distance from the factory. Thus, their use is limited to a short distance from the precast factory.
Precast units for roof and floor structures are usually prestressed. However, precast wall units are usually conventionally reinforced with inert steel bars. Precast wall are used for internal & external walls, lift shafts, central cores etc. Precast wall systems are mostly used in domestic construction, both for individual housing & for apartments. The solution can be considered as the industrialized from of cast in-situ walls or classical brick or block masonry walls.
1.3.3 PRECAST CONCRETE FLOOR SLAB
Precast concrete floor slab is the most fully standardized precast concrete elements. They are used for making floor and roof slabs. The principle advantages of precast floors are speed of construction, absence of scaffolding, large variety of types, large span capacity, & economy.
Precast floors can also be classified according to their manufacture into totally & partially precast floors. Totally precast floors are composed of units, which are totally cast at the plant. After erection, the units are connected to the structure & the longitudinal joints are grouted.In some cases a cast in-situ structural topping screed is added.
Partially precast floors are composed of a precast part & a cast in-situ part. Both parts are working together at the final stage to achieve the composite structural capacity. The main totally precast floor & roof types are described hereafter.
Pre cast hollow core slab
- Most widely used type of pre cast flooring
- highly efficient design and production
Some of the advantages of using precast concrete construction are as follows:
1.4.1 Reduced Construction Time and Cost
Precast concrete construction will save valuable time and helps to reduce the risk of project delay and possible monetary losses. Precast design and production of elements can be started while the construction site is under survey or earthworks. Production are also unaffected by weather conditions due to the controlled environment of the casting area. Also, the usage of large precast panels will reduce the time taken to complete the structural works. Therefore, other trades such as painting and electrical wiring can begin work sooner.
In conventional construction method, time-consuming works such as formworks, scaffoldings and curing are needed to produce a structural element. In precast concrete construction method, structural elements are produced in manufacturing plants while other activities at the construction site proceed. When the structural elements are needed, they are immediately sent to the site and assembled continuously, forming the structural frame and enclosing the building. In precast concrete manufacturing plants, modern machineries are utilized with several technicians attending to certain production process. This greatly reduced the number of unskilled requirements.
1.4.2 High quality and aesthetical value of products
Precast products are manufactured in a casting area where critical factors including temperature, mix design and stripping time can be closely checked and controlled; and this will ensure that the quality of precast products are better than cast-in-situ concrete. A huge sum of money will be saved by not having to do rectification works. Also due to factory-controlled prefabrication environment, many combinations of colours and textures can be applied easily to the architectural or structural pieces. A vast range of sizes and shapes of precast components can be produced, providing a great deal of flexibility and offer fresher looks to the structures.
1.4.3 Cleaner and safer construction sites
Usage of precast elements eliminates or greatly reduces conventional formworks and props. Precast construction also lessens the problem of site wastages and the related environmental problems. The prefabricated products also provide a safe working platform for workers to work on. Workers and materials are also greatly reduced at the construction sites. Using Just-in-Time principles, the precast elements are kept at the factory yard until the site is ready for installation. Also, as elements are produced in the plant and mostly designed to be repetitive, minimal wastage will be experienced at both factory and construction sites.
1.4.4 Greater unobstructed span
The usage of prestressed precast solutions such as the Hollow Core slabs and Double-T beams offer greater unobstructed span than the conventional reinforced concrete elements. Having lesser beams and columns, will provide larger open space. It is very ideal for the construction of places of worship, warehouses, halls, car parks, shops and offices.
1.4.5 Lower total construction costs
All of the above simplify the construction processes and increase productivity, quality and safety. As a result, the total costs of construction are reduced. More often than not, most of the opponents of precast construction only use the costs of materials in comparing the in-situ and precast construction without realising that there are hidden costs to the traditional methods. Also a contributing factor to the low usage is the fact that Malaysia has always been able to source cheap labours from neighbouring countries. Therefore, the industry players are very reluctant to change to the new construction methods. As a result, the country is faced by problems of low quality, productivity and safety in the construction industry due to high dependency of manual labour. The country also suffers from the outflow of monies as well as social problems. If the labour supply experience sudden reduction while demand remains the same, labour costs will definitely be increased and produce more problems to the industry.
1.4.6 Increased Quality of Structural Elements
Precast concrete elements produced in plants using modern techniques and machineries. Raw materials such as concrete, sand, and reinforcement bars are under high level of quality control. Formworks used are of higher quality than those used at construction sites. This allows truer shapes and better finishes in precast components. Precast components have higher density and better crack control, offering better protection from harsh weathers and sound insulation. High density is achieved by using vibrating table or external vibrators placed on formworks. Precast concrete also provide better fire resistance for reinforcement bars. When compared to in situ concrete, this reduces the amount of long-term movement, which needs to be recognized in building design.
1.4.7 Increased Durability and Load Capacity of Structural
Elements Prestressed precast concrete components have high structural strength and rigidity, which are important to support heavy loads. This allows shallow construction depth and long span in structural components. Fewer supporting columns or walls result in larger floor space, which allow more flexibility in interior design. Dense precast and prestressed concrete components are cast-in with smooth steel, concrete or fiberglass. This result in components with smooth surfaces which resist moisture penetration, fungus and corrosion. High density concrete reduces the size and quantity of surface voids thus resisting accumulation of dirt and dust. Precast concrete components are more durable to acid attack, friction, corrosion, impact, abrasion, and other environment effects. Precast concrete structures have longer service years and require minimal repairs and maintenance.
Some of the drawbacks of using precast concrete construction are as follows:
1.5.1 High Capital Cost
A large amount of resources must be invested initially to set up a precast concrete plant. Sophisticated machineries are expensive and require heavy investment. Percast concrete is mainly used in construction of high-rise buildings and flats, which are atleast 5 storeys high. Precast concrete is also utilized in construction of housing estates where the design of houses is uniform. Other projects where precast concrete is suitable are large stadiums, halls, factories, awrehouses, airports and hangars. The scale of the construction projects using precast concrete must be large enough to ensure sufficient profit to offset the initial capital cost.
1.5.2 Sophisticated Connection Works
The behaviour of connections determines the performance of precast concrete structures. When assembling of precast concrete structures, connections between precast components must be supervised and done properly. This way, the intended behaviour of a connection (simple, semi-rigid, or rigid) can be achieved. Apart from that, a good sound insulation can be provided and water leakage problem can be avoided. Skilled and well-trained labours are required to ensure proper connection is produced during erection stages, which lead to additional cost.
1.5.3 Transportation, Handling Difficulties and Modification Limitation
Workers must be careful when handling precast concrete components to avoid damage. Precat components are manufactures in plants, which are not always situated in the area of the construction sites. Precast components must be carried from the plants to the sites using trailers. Usually, precast components are large and heavy, creating difficulties in transportation. Upon arrival at the sites, portable cranes or tower cranes will lift the precast components into place for erection. Usually, to increase the speed of construction, several cranes are used requiring large space. Proper construction planning and site management is a must. Workers be well trained to ensure that precast components are positioned and connected properly to avoid cases where the columns, beams, walls, or slabs are not well aligned, dislocated or out of plane. Precast concrete system is not flexible when future modification is taken into account. For example, the walls of a flat built using load bearing precast walls cannot be demolished for renovation purposes, as this will affect the stability of the entire precast structure.
1.6 Precast Concrete Construction in Malaysia
The usage of precast building system is gaining popularity in Malaysia because of its many advantages such as high quality structural finishes, less labour force, shorter time of completion and saves cost. In the construction industry today, the usage of cast in-situ method is not so practical and effective especially when it is in a large scale construction. Therefore, the usage of precast components is the best alternative to fulfill the needs in large scale constructions. The precast concrete concept is ideal to suit the future demands of higher specifications and performance for the commercial, industrial, civic and domestic buildings.
Precast building system or commonly known as IBS (Industrialized Building Systems) in Malaysia has many advantages if it is compared to the traditional cast in situ system. Production of precast components such as beams, columns and slabs were done in the factory thus ensuring high quality control for each component. Structures of precast building can be quickly erected on site and the delays due to concrete curing and concrete formwork removal can be reduced resulting in shorter completion time of construction. Besides that, precast system requires lesser amount of labour force.
1.6.1 Development of IBS in Malaysia
IBS in Malaysia has begun in early 1960’s when Ministry of Housing and Local Government of Malaysia visited several European countries to learn about precast concrete construction and evaluate their housing development program. After their successful visit in 1964, the government had started first project on IBS aims to speed up the delivery time and built affordable and quality houses. Precast concrete construction started in Malaysia with the production of 7 blocks of 17-storey flat, 4 blocks of 4-storey flat and 40 units of shop house opposite the Kuala Lumpur General Hospital at the intersection of Jalan Pekeliling and Jalan Pahang. This project used the Larsen-Nielson system from Denmark. The second construction project, which used precast concrete, was the construction of 6 blocks of 17-storey flat, 3 blocks of 18-storey flat and 66 units of shop house along Jalan Rifle, Penang. This project used the French’s Estior system. Since then, numerous precast structures such as high-rises, car parks, warehouses, factories, housings and retail units have been built all over the country. Some latest examples are the Telekom Headquarter in KL, townhouses in Cyberjaya, City Square in Johor Bahru, Putra Mosque in Putrajaya and Metal Pak Factory in Shah Alam.
Today, the use of IBS as a method of construction in Malaysia is evolving. Many private companies in Malaysia have teamed up with foreign expert from Australia, United States and Japan to offer precast solution to their project. In addition, more and more local manufacturers have established themselves in the market. Precast, steel frame and other IBS were used as hybrid construction to build national landmark such as Bukit Jalil Sport Complex, LRT and Petronas Twin Tower. It was reported that at least 21 of various manufacturers and suppliers of IBS are actively promoting their system in Malaysia. Nevertheless, the government of Malaysia still feels that the usage of IBS is low despite the possible potential.
From the survey conducted by CIDB of Malaysia in 2003, the usage level of IBS in local construction industry stands at 15%. The total registered IBS contractors in Malaysia stand for 1,993 in year 2007 and registered IBS manufacture in Malaysia until 2007 is 138 producing 347 IBS products available in the market.
Most of the local developed products are based on traditional materials such as reinforced concrete and the most innovative materials are based on imported technology. Previously in the RMK-7, the country intend to construct about 800,000 units of houses for its population using the IBS and 58,500 units planned for the low medium cost houses. However the achievements are disappointing with only 20% completed houses reported due to use of conventional construction method.
1.6.2 IBS as Mass Construction Method
The term IBS is often misinterpreted with negative meaning as it is always linked with industrialized buildings that were built in 1960s. These buildings are normally associated with prefabricated mass construction method, low quality buildings, leakages, abandoned projects, unpleasant architectural appearances and other drawbacks. Due to the poor architectural design, the old prefabriacted buildings have given the pbulic bad impression about precast concrete.
There have been quiet a number of cases where the use of IBS had lead to such drawbacks. For example, in the case of Pekeliling Flats in Kuala Lumpur and Taman Tun Sardon, Gelugor, Penang. These 2 early prefabricated flats were constructed in mass to produce low cost accomodation for lower income groups.
However, the design was very basic and not considering the service aspects like the need for wet toilets and bathrooms. Lacking in this design consideration leads to problems of leakage that becomes the common issue with precast buildings. In addition, in many cases the low cost housings are not maintained properly, thus contributing further to the poor image of IBS buildings.
1.6.3 Characteristics of IBS
- Industrial production of components though prefabrication.
- Reduced labour during prefabrication of components and site works.
- Modern design and manufacturing method using CAD/CAM.
- Systematic Quality Control i.e ISO 9000.
- Open Building System i.e permitting hybrid applications.
1.6.4 Why IBS
Budget Speech 2005:
“The Government is determined to ensure that every Malaysian will have access to affordable homes. During the period 1971-2003, the Government constructed 490,000 units of low-cost houses. The government intends to provide an additional 100,000 units of affordable homes to be implemented through the Industrialised Building System (IBS).This system will ensure quality, save cost, create a safer and cleaner working environment as well as reduce the dependence of foreign workers”
1.6.5 Social Problems
- Currently 75 foreign workers out of 100 construction workers.
- There are 318,000 foreign workers in Malaysian Construction Industry
- Let say one foreign worker gain RM400 per month; RM400.00 x 12 months = RM5,000 per year, 318,000 x RM5,000 = RM 1.59 Billion per year
It shows the High outflow of Ringgit
1.6.6 Types of IBS
The construction method can be classified into 4 categories:
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