Civil Engineering Construction I (CBE5031) - VTCtycnw01.vtc.edu.hk/cbe5031/Precast.pdf · Civil...

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Civil Engineering Construction I

(CBE5031) 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.

Definition of Precast Concrete

1. Precast yard

• Most precast units are produced in factories or casting yards.


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

1. Precast yard


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.


2.1 Fabrication of reinforcement

Automatic cage fabrication machine


• The technique ‘prestressing’ greatly improve the strength of precast units.

• Usually, pre-tensioning is used for precasting.

• Sometimes, post-tensioning is also used for non-standard units or where curved tendons are required.

2.2 Prestressing


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

2.3 Concrete moulds

Steel Mould for Precast Staircase


External vibrators

- which mounted on the moulds reduce the labour works for compacting the concrete

2.4 Compaction of concrete

2.4.1 External vibrators


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.

2.4.2 Hydraulic pressing


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.

2.4.3 Centrifugal spinning


• 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.

2.5 Accelerated curing


• 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.

2.5.1 Steam curing (at atmospheric pressure and below 100C)

Plastic sheet cover for steam curing


• Precast units are placed into an autoclave (a pressure vessel) and steam of high pressure and temperature (about 177C and 0.8MPa above atmospheric pressure) are applied.

• Usually the 28-day strength on normal curing can be reached in about 24 hours.

2.5.2 Autoclaving (high pressure steam curing)

Autoclaving


• 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.

3. Handling of precast units


3.1 Lifting fittings

threaded sockets

lifting hooks

lifting plates


3.2 Lifting devices

tower cranes

launching girder

derrick

mobile cranes


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.

4. Application and installation of precast units 4.1 Column to foundation 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

4.2 Bolting connection

Column-foundation bolting connection Column-foundation bolting connection

(Source: R. Chudley)


• 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

4.3 Dowel and sleeve connection

Dowel and sleeve connection

(Source: R. Chudley)


• 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.

4.2 Beam to column connection 4.2.1 Simply supported joint and hinge joint

Beam-column dowel sleeve connection

Simply Supported joint and hinge joint


• 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

4.2.2 Bolting Connection

Beam to Column connection by Bolts and Brackets (Source: R. Chudley


• 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

4.2.3 Exposed reinforcement 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.

4.3 Column Splicing 4.3.1 Welding 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.

4.3.2 Dowel and sleeve connection

to be grouted

to be grouted

joint completed with

gout or dry pack

Grouted sleeve connection


• The exposed reinforcements of the columns are lapped together.

• The joint is completed with insitu concrete which provides good moment resistance.

4.3.3 Insitu concrete connection

Column Splicing with insitu concrete


• 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.

4.4 Precast Slab 4.4.1 Planks and blocks


4.4.1 Planks and blocks


• 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

4.4.2 Hollow core slab


4.4.2 Hollow core slab


• 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.

4.4.3 Double Tee Slabs

Double Tee Slabs (Source: CPCI)


• Façade Panel is the most widely used precast concrete wall in Hong Kong.

• Various installed methods had been used.

5. External Wall

Fixing of Facade Panel (Source: City University)


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

5. External Wall

Fabric reinforcement for wall construction after installing precast façade


• 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.

6. Composite Construction / Permanent Formwork


6.1 Composite Floor Slab

Solid Planks for Composite Slab

precast slab

binder

top reinforcement Insitu concrete


6.2 Composite Beam

Shell beam & Precast Slab


• 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.

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.

7.1 Advantages 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.

7.2 Disadvantages of precast concrete


• Construction of Prestressed Concretes 2nd Edt., Ben C. Gerwick, Jr. (1993), Wiley Inter. Science.

• Modern Prestressed Concrete Design Principles and Construction Methods 4th Edt., James R. Libby (1990), Van Nostrand Reinhold.

• Post-tensioning in Building, VSL

• Construction Technology Vol. 3 2nd 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

• http://www.cpci.ca/

Reference:

Civil Engineering Construction I (CBE5031) - VTCtycnw01.vtc.edu.hk/cbe5031/Precast.pdf · Civil...

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