2. INTRODUCTION TO PRECAST CONCRETE

Transcription

1 COLLEGE OF ARCHITECTURE AND PLANNING Department of Architecture and Building Sciences ARCH 436 Contemporary Building Construction Methods 2. INTRODUCTION TO PRECAST CONCRETE Dr. Mohammed Ghonim Lecture Objectives Upon completion of this lecture, the student will be able to: 1. Explain the concept of precast concrete. 2. Discuss the advantages of precast concrete. 3. Compare between precast and cast-in-place concrete. 4. List the most common precast structural and non structural units. 5. Describe the process of producing precast units. 1

2 Lecture Content Introduction. Historical Brief. Advantages of Precast Concrete. Disadvantages of Precast Concrete. Precast Vs. Cast-in-Place Concrete Precast Concrete Production. Precast Concrete Curing. Transportation & Hoisting of Precast Concrete. Precast Concrete Elements. Precast Concrete Slabs. Precast Concrete Beams. Precast Concrete Columns. Precast Concrete Walls & panels. Precast Special Elements. Introduction The concept of precast construction includes those buildings, where the majority of structural components are standardized and produced in plants in a location away from the building, and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at low cost. This type of construction requires a restructuring of the entire conventional construction process to enable interaction between the design phase and production planning in order to improve and speed up the construction. One of the key principles for achieving that objective is to design buildings with a regular configuration in plan and elevation. 2

3 Introduction Precast concrete is a construction product produced by casting concrete in a reusable mold or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place. In contrast, cast-in-site concrete is poured into sitespecific forms and cured on site. By producing precast concrete in a controlled environment which is called precast plant, the precast concrete is afforded the opportunity to properly cure and be closely monitored. Utilizing a precast concrete system offers many potential advantages over site casting of concrete, one of them the production process for precast concrete is performed on ground. There is a greater control of the quality of materials and workmanship in a precast plant rather than on a construction site. Financially, the forms used in a precast plant may be reused hundreds of times before they have to be replaced, which allow cost of formwork per unit to be lower than for site-cast production. Historical Brief Ancient Roman builders made use of concrete and soon poured the material into molds to build their complex network of aqueducts, culverts, and tunnels. Modern uses for pre-cast technology include a variety of architectural and structural applications featuring parts of or an entire building system. In the modern world, precast paneled buildings were pioneered in Liverpool, England in 1905, when the process was invented by city engineer John Alexander Brodie who designed the tram stables at Walton in Liverpool in The idea was not taken up extensively in Britain. However, it was adopted all over the world, particularly in Eastern Europe and Scandinavia. 3

4 Advantages of Precast Concrete Because precasting is done at the ground level, the cost of formwork and shoring is considerably reduced. Formwork cost reduction is also achieved through the use of standard-size elements cast in permanent forms, which are reused several times. Precasting also allows greater quality control over the strength of concrete and surface finishes, and most surface finishes are obtained more easily in a precast plant. Advantages of Precast Concrete Factory-made products: The only way to industrialize the construction business is to shift the work from the site to modern permanent factories. It means rational and efficient manufacturing processes, skilled workers, repetition of actions, quality surveillance, etc. Prefabrication has a much greater potential for economy, structural performance and durability: Optimal use of materials is obtained through modern manufacturing equipment and carefully studied working procedures. As a consequence, precast products are much more slender and present a high durability and longevity. Shorter construction time - less than half of conventional cast in-situ construction: Today the demand for a speedy return on investment is becoming increasingly important: the initially agreed construction terms have to be met. Adverse weather conditions don't influence the construction process: Prefabrication is independent of adverse weather conditions and production continues normally in wintertime when the temperature falls below 200 C. Site work stops when the temperature falls below - 5 C. 4

5 Disadvantages of Precast Concrete Precast concrete has many disadvantages, its main disadvantage is the cost of transportation, although precast members are generally lighter than corresponding site-cast members, they are still fairly heavy, transportation also limits the length and width of precast members. Another disadvantage of precasting is the need for heavier hoisting equipment at the construction site and additional safety measures that must be observed during construction. Installation and assembly at the site also introduce the need for more skilled workers compared with site-cast concrete construction. Architecturally, the most limiting factor in the use of precast concrete is the difficulty in sculpting concrete at a large scale, which is more easily realized with site-cast concrete, this is because precast elements are generally straight, with standard profiles. Precast Vs. Cast-in-Place Concrete Precast Concrete Cast-in-Place Concrete 1. Machinery & Labor Requires more machinery like trucks and cranes, in addition skilled workers and qualified contractors. Requires less machinery, and it is easier to find suitable labors and contractors. 2. Cost Lower initial cost, especially for large projects, also the formworks cost less because they may be used for hundreds of times. Higher initial cost, but as cast-in-place structures requires less maintenance, its life cycle cost may be less than precast structures, so this factor is relative. 3. Time Shorter site Construction time, precast concrete structures can be created in advance and held until the time we need them. Longer construction time due to formwork installation and removing and concrete curing time. 4. Concrete Quality We can achieve better concrete quality control in plant conditions. Therefore stronger and lighter concrete units. A number of uncontrollable factors can decrease the strength and quality of cast-in-place concrete including inaccurate mixing, weather changes, etc. 5. Weather Conditions Pre-cast concrete structures are Cast-in-place concrete is affected by poured in a controlled environment, so weather is not an influencing factor for delaying the construction progress. cold temperatures and extremely wet conditions, this can delay concrete casting. Also sever hot and arid environments requires strict treatments and control for concrete curing. 5

6 Precast Vs. Cast-in-Place Concrete Precast Concrete Cast-in-Place Concrete 6. Maintenance Precast structures need more As cast-in-place structure have fewer maintenance, they require sealant joints, so they need less maintenance. joints to prevent leakage, and they may need to be replaced every decade. 7. Recyclability Can be unfastened and removed and reused in another location with minimal damage and loss. Cast-in-place concrete structures are very difficult to be reused, steel bars only can be recycled. 8. Durability Because precast structures have many joints, they don t afford lateral forces and they require more shear walls to resist those lateral forces like wind pressure or seismic forces. Monolithic structures with fewer joints, so they are more resistant to lateral and seismic forces. 9. Sustainability Less materials consumption, more sustainable. More materials consumption, so less sustainable. 10. Flexibility Modular designs offer less flexibility in forms but more flexibility in last minute changes and extensions. Less head rooms and more projections and depths for connections and layered floor components. Cast-in-place structures offer more flexibility in design and ability to create plastic shapes. Neat structures, as the structural elements are cast together and intersected. Precast Concrete Production Concrete elements, cast and cured in a manufacturing plant, then transported to the construction site, plant casting allows increased efficiency and higher quality control. Durable, permanent steel forms are reused many times, reducing formwork costs compared to cast-in-place concrete. 6

7 Precast Concrete Production The use of high early strength cement and steam curing allow concrete members to be cast and cured in only 24 hours, controlled casting conditions and high quality forms allow for greater control of surface finishes. Transportation & Hoisting of Precast Concrete Precast members are often more difficult and costly to ship than the bulk cement and gravel ingredients used for mixing concrete on the job site. Giant cranes are needed to hoist these heavy hunks of concrete. Special care must be taken, during hoisting and placing, not to put stresses on them that are different from the ones they are designed to take in their final position in the structure. Precast concrete hollow-core slabs are loaded to a truck by a forklift. 7

8 Transportation & Hoisting of Precast Concrete The erection procedure, and in turn the design of the structure is very much affected by the weight and size of the individual components. The transport of the components is typically by truck and as such, it is imperative to be aware of allowable component size and weight. A custom-built straddle carrier moves a precast concrete section of a bridge. Precast panel lifted by a mobile crane The lifting of any precast product requires the use of special equipment and substantial planning and knowledge. The selection of crane type and size is an important ingredient to the viability of the precast structure, usually, mobile cranes are the most economical solution. Precast Concrete Elements 8

9 Precast Concrete Elements Precast Concrete Slabs Precast concrete slabs are used for floor and roof decks, deeper elements span further than those that are shallower. 9

10 Precast Concrete Slabs Solid Slabs Hollow Core Slabs Single Tee Slabs Double Tee Slabs Precast Concrete Slabs 10

11 Precast Concrete Slabs Precast concrete slabs, beams, and structural tees are one-way spanning units that may be supported by sitecast concrete, precast concrete, or masonry bearing walls, or by steel, sitecast concrete, or precast concrete frames. The precast units are manufactured with normal-density or structural lightweight concrete and prestressed for greater structural efficiency, which results in less depth, reduced weight, and longer spans. Precast Concrete Beams Structural beams, including rectangular beams, L-shaped beams, and inverted tee beams, AASHTO are used to support horizontal deck components such as double tees and hollowcore slabs. 11

12 Precast Concrete Beams Precast Concrete Columns Precast concrete columns are typically used with precast beams to form a structural frame. Because rigid joints are difficult to fabricate in a precast structural frame, shear walls or diagonal bracing are normally relied upon to stabilize the structure against lateral forces. 12

13 Precast Concrete Columns Precast columns provide support for beam and slab elements, since these elements carry mainly axial loads with little bending force, they may be conventionally reinforced without prestressing. Precast Concrete Walls & panels Precast concrete wall panels may serve as bearing walls capable of supporting sitecast concrete or steel floor and roof systems. Together with precast concrete columns, beams, and slabs, the wall panels form an entirely precast structural system that is inherently modular and fire-resistive. The lateral stability of a precast concrete structure requires that those floors and roofs that serve as horizontal diaphragms be able to transfer their lateral forces to shear-resisting wall panels. The wall panels, in turn, must be stabilized by columns or cross walls as they transfer the lateral forces to the ground foundation. All forces are transferred by a combination of grouted joints, shear keys, mechanical connectors, steel reinforcement, and reinforced concrete toppings. 13

14 Precast Concrete Walls & Panels Precast concrete wall panels are cast and steam-cured in a plant off site, transported to the construction site, and set in place with cranes as rigid components. Fabrication in a factory environment enables the units to have a consistent quality of strength, durability, and finish, and eliminates the need for on-site formwork. The precast wall panels may be conventionally reinforced or prestressed for greater structural efficiency, reduced panel thicknesses, and longer spans. In addition to the required tensile, shrinkage, and temperature reinforcement, extra reinforcement may be necessary to resist the stresses of transportation and erection. Precast wall panels may be of solid, composite, or rifted construction, window and door openings, corbels, and anchoring devices are cast into the wall panels. A variety of quality-controlled surface textures and patterns are available. Precast Special Elements Precast concrete stairs Uniquely shaped structural elements for a sports stadium Precast Concrete Pipes 14

15 Precast Special Elements Precast Concrete Tunnel Segments Precast Box Culverts Precast Concrete Fencing Systems Summary Precast construction includes those buildings, where the majority of structural components are standardized and produced in plants away from the building, and then transported to the site for assembly. Because precasting is done at the ground level, the cost of formwork and shoring is considerably reduced. Precast concrete has disadvantages, most of them are the transportation and hoisting requirements. Concrete elements, cast and cured in a manufacturing plant, plant casting allows increased efficiency and higher quality control. Precast concrete slabs are used for floor and roof decks, deeper elements span further than those that are shallower. Because precast structures lack rigid joints, shear walls or diagonal bracing are normally relied upon to stabilize the structure against lateral forces. 15

16 References Allen, EA. (2009) Fundamentals of building construction materials and methods, John Wiley & Sons. Ching, Francis (2008) Building Construction Illustrated, John Wiley & Sons. Emmitt, S.& Gorse, C. (2006) Barry s Advanced Construction of Buildings, Blackwell Publishing. Garrison, Philip, (2005) Basic Structures for Engineers and Architects, Wiley-Blackwell. Mehta, Madan et. al. (2013) Building Construction, Pearson, USA Lecture Activity Each student is required to prepare a brief but informative internet-based research about one of the topics related to this lecture (Precast Concrete); i.e., new developments, successive and local case studies or important details. The research should be presented in only one A4 sheet. 16