Civil Engineering Construction I (CBE5031)

Transcription

1 Civil Engineering Construction I (CBE5031) Precast Concrete 1

2 Definition of Precast Concrete Precasting means casting a concrete member at a place other than where it will be used and then moving it to the place where it will be installed. 1. Precast yard Most precast units are produced in factories or casting yards. 2

3 1. Precast yard Fundamental factors that contribute to the success of a factory/casting yard for the precasting include: proximity to the place where the precast unit will be installed good access such as road, rail or pier sufficient area for the storage of materials, bending and fabrication of steel reinforcement, casting, curing and storage of finished products land price or rate of rent of the casting yard availability and cost of materials and labour supply availability of plants such as batching plants and lifting facilities 3

4 2. Techniques to improve the production on of Precast Concrete 2.1 Fabrication of reinforcement Reinforcement fixing is labour intensive on site. For precasting, mechanization is possible for the fabrication of reinforcement because of mass production. Cutting, bending and fixing the reinforcement can have high degree of automation. The reinforcement can also be fixed by spot welding. Normally, welding for T-bars is not permitted on site as poor temperature control on welding lowers the strength of high tensile steel. 4

5 2.1 Fabrication of reinforcement Automatic cage fabrication machine 5

6 2.2 Prestressing The technique prestressing greatly improve the strength of precast units. Usually, pre-tensioning is used for precasting. Sometimes, post-tensioning is also used for nonstandard units or where curved tendons are required. 6

7 2.3 Concrete moulds Steel moulds are usually used for precasting which have the following advantages: easy assembling and demoulding durable - can be reused up to a thousand times for percasting works hard and smooth surfaces of the moulds can be cleaned easily and give good concrete finishes Steel Mould for Precast Staircase 7

8 2.4 Compaction of concrete External vibrators External vibrators - which mounted on the moulds reduce the labour works for compacting the concrete 8

9 2.4.2 Hydraulic pressing Hydraulic pressing can be employed to compact low slump concrete of small precast units, such as paving blocks and concrete drainage pipes. The units can also be demoulded immediately without breaking. 9

10 2.4.3 Centrifugal spinning Centrifugal spinning In the production of some precast concrete pipes and prestressed tubular pile (e.g. Daido Pile), the centrifugal spinning process effectively compacts the zero slump concrete. It produces a uniform hollow tube without the need of a void former. 10

11 2.5 Accelerated curing An increase in the curing temperature of concrete increases its rate of development of strength. It reduces the curing time hence reduces the cycling times of concrete moulds and prestressing bed. 11

12 2.5.1 Steam curing (at atmospheric pressure and below 100 C) Steam curing is normally applied in special chambers or in tunnels through which the concrete members are transported on a conveyor belt. Alternatively, portable boxes or plastic sheet covers can be placed over precast members; steam is supplied through the connections of flexible hose. Plastic sheet cover for steam curing 12

13 2.5.2 Autoclaving (high pressure steam curing) Precast units are placed into an autoclave (a pressure vessel) and steam of high pressure and temperature (about 177 C and 0.8MPa above atmospheric pressure) are applied. Usually the 28-day strength on normal curing can be reached in about 24 hours. Autoclaving 13

14 3. Handling of precast units Since precast concrete unites are bulk and heavy, lifting equipments are required for the lifting. Lifting fittings should also be cast into units for easy handling. 14

15 3.1 Lifting fittings lifting hooks lifting plates threaded sockets 15

16 3.2 Lifting devices tower cranes launching girder mobile cranes derrick 16

17 4. Application and installation of precast units 4.1 Column to foundation connection Pocket connection A pocket is reserved in the foundation. The column is set into the reserved pocket in the foundation and the spaces between the column and the socket is filled with cement grout. 17

18 4.2 Bolting connection The main bars of the precast column are connected to the steel base plate / channels by welding. The precast column can then be connected to the foundation by bolting Column-foundation bolting connection Column-foundation bolting connection (Source: R. Chudley) 18

19 4.3 Dowel and sleeve connection Grout-sleeves are cast into a precast unit. The sleeves fit over reinforcement projecting from the mating part. The sleeves are grouted and the gap between the units is filled with dry-pack or non-shrink grout Dowel and sleeve connection (Source: R. Chudley) 19

20 4.2 Beam to column connection Simply supported joint and hinge joint The beam usually sits on a corbel or column head. For a heavy structure, it is important to place a resilient pad, commonly called a bearing, between the two structural components to transfer the load uniformly and to prevent localized stress. If the horizontal translation is restrained by a dowel, it becomes a hinge joint. Beam-column dowel sleeve connection Simply Supported joint and hinge joint 20

21 4.2.2 Bolting Connection For bolting connection, steel brackets have to be shop welded to the main bars of the precast units. The precast units can then be connected to the main structure on site rapidly by bolting 21 Beam to Column connection by Bolts and Brackets (Source: R. Chudley

22 4.2.3 Exposed reinforcement connection The exposed reinforcements of the precast column and the precast beam are lapped together. The joint is then completed with insitu concrete. The joint provides good moment resistance. It is also called composite moment connection 22

23 4.3 Column Splicing Welding connection A steel plate is fixed to the end of each precast column by welding to its main reinforcements. The columns are butted against each other and the joint is completed by butt welding. 23

24 4.3.2 Dowel and sleeve connection Grout-sleeves can be cast into a precast unit, then the sleeves fit over reinforcement projecting from the mating part. The sleeves are grouted and the gap between the units is filled with dry-pack or non-shrink grout. to be grouted joint completed with gout or dry pack to be grouted Grouted sleeve connection 24

25 4.3.3 Insitu concrete connection The exposed reinforcements of the columns are lapped together. The joint is completed with insitu concrete which provides good moment resistance. Column Splicing with insitu concrete 25

26 4.4 Precast Slab Planks and blocks A precast slab can be formed by placing long planks at suitable centres supported on main beams or loading bearing walls The intermediate spaces are then filled with smaller block units to complete the slab. Normally, a structural topping is not required but the upper surfaces of the units are usually screeded to provide the correct surface for the floor finishes. This method eliminates the requirement of falsework during the construction period. 26

27 4.4.1 Planks and blocks 27

28 4.4.2 Hollow core slab used for most building floor or roof systems. voids reduce the dead load of the slab and the material cost. The web resembles I-beam sections which provide efficient moment resistance. Prestressed hollow core slabs are available. This means long spans, shallow depth and the ability to carry heavy loads are easily accommodated. Hollow core slabs may be simply supported on beams or load bearing wall. Steel dowels can be provided to resist hogging moment at the end supports 28

29 4.4.2 Hollow core slab 29

30 4.4.3 Double Tee Slabs Double tee slabs are prestressed. Double tee slabs can be used for most applications requiring a long span floor or roof system (10m to 30+ m) and/or additional load carrying capability. Double Tee Slabs (Source: CPCI) 30

31 5. External Wall Façade Panel is the most widely used precast concrete wall in Hong Kong. Various installed methods had been used. Fixing of Facade Panel (Source: City University) 31

32 5. External Wall The prevailing installation method: Erection of the façade panel with temporary plumbing guide Fixing of reinforcement of adjoining walls lapping with the dowel of the façade panel Shuttering of wall formwork and casting of concrete Fabric reinforcement for wall construction after installing precast façade 32

33 6. Composite Construction / Permanent Formwork Precast units are placed underside to serve as formwork for concreting. They also become integral parts of the permanent structure. The major advantage of using permanent formwork is that it eliminates or minimizes the temporary works such as formwork and falsework. there is no need for stripping. 33

34 6.1 Composite Floor Slab top reinforcement Insitu concrete binder precast slab Solid Planks for Composite Slab 34

35 6.2 Composite Beam Shell beam & Precast Slab 35

36 7.1 Advantages of precast concrete The units can be mass-produced and are therefore cheaper. Cost of formwork can be reduced. Easier to fix reinforcement and place concrete which to be done on ground and under cover. Units can be cured by accelerated techniques. The quality of units can be strictly controlled. 36

37 7.1 Advantages of precast concrete Units can be cast before the site becomes available hence the construction time can be reduced. Temporary supports such as falsework and scaffolding are reduced to minimum. Precast units can be structurally load tested if required. Precast units can be pre-tensioned. Precasting produces less construction waste than insitu works and therefore more environmental friendly. 37

38 7.2 Disadvantages of precast concrete Uneconomical if only a small number of units are required. Waterproofing of joints may be expensive. The transportation of long units may be difficult. Cranes may be required to load and unload the units on site. 38

39 Reference: Construction of Prestressed Concretes 2 nd Edt., Ben C. Gerwick, Jr. (1993), Wiley Inter. Science. Modern Prestressed Concrete Design Principles and Construction Methods 4 th Edt., James R. Libby (1990), Van Nostrand Reinhold. Post-tensioning in Building, VSL Construction Technology Vol. 3 2 nd Edt., R. Chudley (1991), Longman. Civil Engineering Construction IV Vol. 4, S.A.R Jufri & R.J. Wellman (1992), Hong Kong Polytechnic. Precast Concrete Material, manufacture, properties and usage, M. Levitt (1982), Applied Science Publishers Recommended Practice for Erection of Precast Concrete, PCI Erectors Committee (1983), PCI 39