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The Buildings Department established the Technical Committee on the Code of Practice for Precast Concrete Construction (TC) in March 2012 for the purpose of collecting views and feedbacks on the use of the Code of Practice for Precast Concrete Construction 2003 (the 2003 Code) from the building industry and with a view to keeping the Code of Practice in pace with the advancement in design, technology and construction practice.
This Code, Code of Practice for Precast Concrete Construction 2016 (the 2016 Code) is issued upon completion of the review by the TC, which has focused on four fronts: (a) the advancement in design and technology; (b) the experience gained and the views and feedbacks received on the use of the 2003 Code; (c) the commonly adopted local practice on precast concrete construction; and (d) necessary updates consequent upon the publication of the Code of Practice for Structural Use of Concrete 2013, the issue of Practice Notes for Authorized Persons, Registered Structural Engineers and Registered Geotechnical Engineers APP-143 and the issue of Construction Standard CS1:2010 and CS2:2012.
The contributions and efforts given by the invited members of the Technical Committee in the preparation of the 2016 Code are greatly appreciated.
This Code of Practice will be reviewed regularly. The Buildings Department welcomes suggestions for improving the Code.
First Issue : April 2016
1 General .............................................................................................................................. 1
1.1 Scope ........................................................................................................................................ 1
1.2 Definitions ................................................................................................................................. 1
1.3 Symbols .................................................................................................................................... 2
2 Design ................................................................................................................................ 3
2.1 Scope ........................................................................................................................................ 3
2.1.1 General .................................................................................................................. 3
2.1.2 Standards and codes of practice ............................................................................ 3
2.2 Planning .................................................................................................................................... 3
2.2.1 Standardisation ...................................................................................................... 3
2.2.2 Buildability ............................................................................................................. 3
2.2.3 Voids and buried conduits ...................................................................................... 3
2.2.4 Layout plan ............................................................................................................ 3
2.2.5 Compatibility .......................................................................................................... 4
2.2.6 Demolition .............................................................................................................. 4
2.3 Stability ..................................................................................................................................... 4
2.3.1 General .................................................................................................................. 4
2.3.2 Displacement ......................................................................................................... 4
2.3.3 Disproportionate collapse ....................................................................................... 4
2.4 Durability ................................................................................................................................... 5
2.4.1 General .................................................................................................................. 5
2.4.2 Shape of precast unit ............................................................................................. 5
2.4.3 Concrete cover ...................................................................................................... 5
2.4.4 Protection and maintenance of joints and connections ........................................... 5
2.4.5 Movement .............................................................................................................. 6
2.4.6 Thermal gradient ................................................................................................... 6
2.4.7 Other effects .......................................................................................................... 6
2.5 Loadings ................................................................................................................................... 6
2.5.1 General .................................................................................................................. 6
2.5.2 Demoulding forces ................................................................................................. 6
2.5.3 Handling and transportation ................................................................................... 7
2.6 Materials ................................................................................................................................... 7
2.6.1 General .................................................................................................................. 7
2.6.2 Alkali-aggregate reaction ........................................................................................ 7
2.6.3 Chlorides in concrete ............................................................................................. 8
2.7 Design considerations ............................................................................................................... 8
2.7.1 General .................................................................................................................. 8
2.7.2 Handling stresses .................................................................................................. 8
2.7.3 Early lifting of precast element ............................................................................... 9
2.7.4 Temporary stages/erection sequence .................................................................... 9
2.7.5 Lifting inserts ......................................................................................................... 9
2.7.6 Bracing design ..................................................................................................... 10
2.7.7 Design for movement ........................................................................................... 10
2.7.8 Design of ties ....................................................................................................... 11
2.7.9 Design of bearings ............................................................................................... 14
2.8 Joints and connections ............................................................................................................ 18
2.8.1 Structural connections ......................................................................................... 18
2.8.2 Joints ................................................................................................................... 20
2.9 Composite concrete construction ............................................................................................. 26
2.9.1 General ................................................................................................................ 26
2.10 Non-structural elements ....................................................................................................... 28
2.11 Ductility ................................................................................................................................ 28
2.11.1 Construction incorporating precast concrete ........................................................ 28
2.11.2 Connections in equivalent monolithic systems ...................................................... 28
3 Construction .................................................................................................................... 29
3.1 Production planning ................................................................................................................ 29
3.2 Moulds .................................................................................................................................... 29
3.2.1 Materials .............................................................................................................. 29
3.2.2 Tolerances ........................................................................................................... 29
3.2.3 Recesses, sleeves and boxouts ........................................................................... 30
3.2.4 Mould release agents ........................................................................................... 30
3.3 Cast-in connections ................................................................................................................. 30
3.4 Lifting inserts ........................................................................................................................... 30
3.4.1 General ................................................................................................................ 30
3.4.2 Lifting position tolerances .................................................................................... 30
3.5 Surface finishes ....................................................................................................................... 31
3.5.1 Types of finishes .................................................................................................. 31
3.5.2 Production ........................................................................................................... 31
3.5.3 Tiled finishes ....................................................................................................... 31
3.6 Prefabricated metal frames ..................................................................................................... 31
3.7 Pre-concreting check ............................................................................................................... 31
3.8 Concrete placing ..................................................................................................................... 32
3.9 Demoulding and lifting ............................................................................................................. 32
3.10 Curing .................................................................................................................................. 32
3.10.1 Normal curing ...................................................................................................... 32
3.10.2 Steam curing ....................................................................................................... 33
3.11 Handling ............................................................................................................................... 33
3.12 Post-concreting check .......................................................................................................... 33
3.12.1 General ................................................................................................................ 33
3.12.2 Surface finish ....................................................................................................... 33
3.13 Lifting equipment and accessories ........................................................................................ 34
3.14 Factory and site storage ....................................................................................................... 34
3.15 Transportation ...................................................................................................................... 35
3.15.1 Delivery ............................................................................................................... 35
3.15.2 Loading and storage on transporters .................................................................... 35
3.16 Erection ................................................................................................................................ 35
3.16.1 Erection preparation ............................................................................................ 35
3.16.2 Erection safety ..................................................................................................... 35
3.16.3 Erection sequence ............................................................................................... 35
3.16.4 Missing or damaged lifting inserts ........................................................................ 35
3.16.5 Rigging ................................................................................................................ 36
3.16.6 Temporary bracing and supporting structures ...................................................... 36
3.16.7 Levelling shims .................................................................................................... 36
3.16.8 Propping .............................................................................................................. 36
3.16.9 Cleansing to semi-precast slab ............................................................................ 37
3.17 Tolerances ........................................................................................................................... 37
3.17.1 Production tolerance for precast concrete components ........................................ 37
3.17.2 Erection tolerance for precast concrete components ............................................ 38
3.17.3 Volumetric precast concrete components ............................................................. 39
4 Quality Control ................................................................................................................ 41
4.1 Factory .................................................................................................................................... 41
4.1.1 General ................................................................................................................ 41
4.1.2 Testing standards ................................................................................................ 41
4.2 Site ......................................................................................................................................... 42
4.3 Special testing ......................................................................................................................... 42
4.3.1 Watertightness testing of external precast façade panels ..................................... 42
4.3.2 Load testing ......................................................................................................... 43
4.3.3 Tiled finishes ....................................................................................................... 43
1 GENERAL
1.1 SCOPE
This Code of Practice deals with the design, construction and quality control of structural and non structural precast concrete elements. The design method used in this code is the Limit State Design as given in the Code of Practice for Structural Use of Concrete. Other alternative design approaches may also be used provided sufficient justifying calculations are submitted. For bridges and associated structures, reference should also be made to the Structures Design Manual for Highways and Railways issued by the Highways Department. All design should be carried out under the supervision of a registered structural engineer or authorized person, with the execution of the works carried out under proper supervision. The requirements outlined in this code apply to both structural and non-structural members.
1.2 DEFINITIONS
For the purpose of this Code of Practice, the following definitions apply:
Back-up material Material inserted in a joint that controls the depth and back profile of the applied sealant.
Bearing length The length of support, supported member or intermediate bedding material (whichever is the least) measured along the line of support (see Figure 2.5).
Bearing width The overlap of support and supported member measured at right angles to the line of support (see Figure 2.5).
Bedded bearing A bearing with contact surfaces having an intermediate bedding of cementitious material.
Bond breaker Film or thin strip of material applied to prevent sealant adhesion to the back of a joint.
Dry bearing A bearing with no intermediate bedding material.
Elastic sealant Sealant which exhibits predominantly elastic behaviour, i.e. stresses induced in the sealant as a result of joint movement are almost proportional to the strain.
Elastoplastic sealant Sealant which has predominantly elastic properties but exhibits some plastic properties when deformed over long periods.
Equivalent monolithic system A precast concrete structural system should have strength and ductility capacity equivalent to that provided by a comparable monolithic reinforced concrete structure.
Gasket Flexible, generally elastic, preformed material that forms a seal when compressed.
Isolated member A supported member, for which, in the event of failure, no secondary means of load transfer is available.
Joint filler Compressible, non-adhesive material used to fill movement joints during their construction.
Net bearing width The bearing width after allowance for ineffective bearing and constructional inaccuracies (see Figure 2.5).
Non-isolated member A supported member which, in the event of loss of a support, would be capable of transferring its load to adjacent members.
Plastic sealant Sealant which retains predominantly plastic properties, i.e. the stresses induced in the sealant as a result of joint movement are rapidly relieved.
Plastoelastic sealant Sealant which has predominantly plastic properties with some elastic recovery when deformed for short periods.
Seal Notionally impenetrable physical barrier in contact with the components forming the joint.
Sealant Material, applied in an unformed state to a joint, which seals it by adhering to appropriate surfaces within the joint.
Sealing strip Preformed material, which may have adhesive properties, that forms a seal when compressed between adjacent joint surfaces.
1.3 SYMBOLS
For the purposes of this Code of Practice, the following symbols apply:
Gk characteristic dead load
Qk characteristic imposed load
fcu characteristic strength of concrete
fy characteristic strength of reinforcement
Other symbols are defined in the text where they occur.
2 DESIGN
2.1 SCOPE
2.1.1 General
The considerations for design and detailing of structural and non-structural precast elements including joints and connections for buildings and building works are given in this section.
2.1.2 Standards and codes of practice
Precast concrete elements should be designed and constructed in compliance with the Building (Construction) Regulations and other relevant codes of practice.
The design method specified in this code of practice for the design of precast concrete elements is the Limit State Design method. Alternative design approaches may be used provided that sufficient justifications are given. Unless otherwise specified, the design considerations and detailing requirements recommended in the Code of Practice for Structural Use of Concrete should be followed.
2.2 PLANNING
2.2.1 Standardisation
Buildings utilising precast concrete construction should be planned wherever possible to utilize standardised precast concrete elements.
Most buildings will be unique and site specific. At the conceptual design stage, a basic layout plan should be developed which achieves a balance between architectural/aesthetic requirements and a high degree of standardisation. Therefore, close collaboration amongst different design parties is essential during conceptual design to achieve the optimum standardisation.
2.2.2 Buildability
Overall planning and detailed design should aim to achieve functionality with ease of construction.
During conceptual design, consideration should be given to the following:
ease/ means of transportation and any restrictions on vehicle size;
access to and around the site;
ease of erection;
any overhead obstructions or power supplies;
size and capacity of crane available to undertake erection;
propping and/or bracing requirements;
joint widths between adjacent precast elements should be sufficient to allow safe alignment during erection and to accommodate building movement and construction tolerances;
jointing method;
structural action; and
cost of construction.
In addition, attention should be given to any special considerations affecting large sized panels particularly with regards to fabrication, de-moulding and transportation.
2.2.3 Voids and buried conduits
Where practical all voids and service openings should be preformed. Cast in/buried conduits should be placed within the reinforcement layers of the pre-cast unit.
2.2.4 Layout plan
Structural layout plans should be a complete and comprehensive set of drawings showing plans, sections, elevations and connection details of the different types of precast components used.
2.2.5 Compatibility
Whenever there are divided responsibilities for design and details in precast construction, detailed checks to ensure compatibility should be made by a designated party.
2.2.6 Demolition
Consideration should be given at planning stage to future demolition of the structure and any special requirements that are needed particularly with regards to prestressed structures.
2.3 STABILITY
2.3.1 General
The overall stability of the complete structure must be checked.
The temporary stability of the structure as well as that of the individual components during all stages of construction should be considered.
A structure comprising precast elements must possess adequate stability to resist wind load and other lateral loads. Cross walls or sway frames should be so arranged, as far as practicable, so as to provide lateral stability.
Many precast concrete structures are designed as pin jointed rather than with moment continuity as is the case with insitu concrete frames. The absence of the rigid frame means that, in the case of buildings, transverse stability is generally provided by shear walls, with floors transferring load by acting as horizontal plates. It is therefore essential to provide adequate ties between elements.
If wind load does not govern, stability should be checked for a minimum notional horizontal force acting at each floor level equals to 1.5% of the characteristic dead load between mid-heights of the storey under and above or the roof surface, as appropriate.
Consideration should be given to lateral stability during all stages of construction and erection where the behaviour of the precast elements may differ from the permanent condition. Adequate propping and bracing should be provided at all stages of construction to ensure stability is maintained at all times. A viable scheme showing how temporary stability is provided at each construction stage should be produced. The temporary works scheme should provide sufficient details including propping layouts for all stages of construction including the sequencing and timing of the dismantling of temporary works.
Particular attention should be given to stability and bracing requirements on high risk structures such as long span beams and high rise buildings.
2.3.2 Displacement
2.3.2.1 General
Structural members should possess adequate stiffness to prevent such deflection or deformation as might impair the strength or efficiency of the structure, or produce cracks in finishes or in partitions. The structure as a whole should possess adequate stiffness such that the maximum lateral deflection due solely to wind forces does not exceed 1/500 of the building height. In determining the total lateral deflection, an allowance should be included for the cumulative effects of deformation of connections (see clause 2.3.2.2).
2.3.2.2 Connection deformation
In determining the overall lateral displacement allowance must be made for slippage and deformation of connections in all structural elements. The cumulative value of deformation of connections at each level should be added to the deflection calculated from the structural analysis and the total value should comply with the limitation specified in clause 2.3.2.1.
2.3.3 Disproportionate collapse
Precast building structures should also be checked for disproportionate collapse as a result of progressive failure or the like.
2.4 DURABILITY
2.4.1 General
It is important to consider the required design life and durability of precast elements. For this purpose, the following factors are to be considered:
shape and size of the precast unit;
concrete constituents;
concrete cover;
the environmental exposure;
protection against fire;
protection and maintenance;
production;
transportation, storage and installation; and
design of joint details.
In addition to the requirements given above, the recommendations specified in the Code of Practice for Structural Use of Concrete and the Building (Construction) Regulations should be followed.
2.4.2 Shape of precast unit
The precast unit should be designed and detailed to have good drainage such that no standing pools or excessive trapped moisture would occur.
Sharp corners or sudden changes in section cause stress concentrations that may lead to cracking or spalling of concrete and therefore should be avoided. Where sharp corners or sudden changes in section cannot be avoided because of practical reasons, stress concentrations should be checked and strengthening be provided as necessary.
Buckling and instability should be avoided during lifting and erection of long slender precast units. Lifting inserts should be located to ensure that compression flange buckling would not occur, particularly during manoeuvring of precast units.
2.4.3 Concrete cover
Cover for precast elements should be no less than those specified for reinforced concrete structures.
In respect of concrete cover requirements for protection against fire, the Code of Practice for Fire Safety in Buildings should be followed, whereas for protection against corrosion, the requirements under the Building (Construction) Regulations should be adopted.
For bridges and associated structures, reference should also be made to the requirements specified in the Highways Department’s Structures Design Manual for Highways and Railways, and the most onerous requirements should be used.
The cover to all brackets and fixings etc. should comply with the minimum cover requirements specified for reinforcement.
The fire resistance of joints fillers etc. should comply with the fire resistance requirements of the precast members.
2.4.4 Protection and maintenance of joints and connections
2.4.4.1 General
To achieve durability, connections should be properly filled with suitable material to prevent corrosion, cracking or spalling of concrete.
2.4.4.2 Protection of steel
Steel, except those for temporary use such as lifting anchor, used at connections should be protected with an adequate thickness of concrete, mortar or grout. The effectiveness of bonding of concrete or grout to steel surfaces must be considered.
2.4.4.3 Protection of fixings
If sufficient concrete cover cannot be provided to protect the fixings at connections, corrosion-resisting materials such as galvanised mild steel or stainless steel shall be used.
2.4.4.4 Maintenance accessibility
The importance of the connection and its readiness for inspection usually dictate the type of protection required. Connections that are not accessible for inspection should be properly protected from corrosion.
2.4.5 Movement
To avoid concrete spalling and cracking, allowance should be made for movement (see clause 2.7.7).
2.4.6 Thermal gradient
Reinforcing steel preventing cracking of concrete should be provided in both faces of panels that are subjected to substantial thermal gradients.
2.4.7 Other effects
Indirect effects resulted from loading changes, temperature differentials, creep, shrinkage, etc can affect the behaviour of structures.
Apart from compliance with general requirements for durability, cracking and deformation, strength and stability, the following may have to be considered:
limiting the cracking and deformation arising from early-thermal movement, creep, shrinkage, etc; or
minimising restraints on structural components by providing bearings or movement joints, or if restraints are inevitable, the design should take into consideration any significant effects that may arise.
2.5 LOADINGS
2.5.1 General
The appropriate loading requirements as specified in the Building (Construction) Regulations and the Code of Practice for Dead and Imposed Loads should be complied with.
Design considerations should also be given to:
construction loads. A minimum load of 1.5 kN/m² should be used. However, due consideration should be given to any special requirements e.g. for plant loads or storage loads and the load increased accordingly;
notional horizontal load. The lateral load should be taken as not less than 1.5% of the characteristic dead load (refer also to clause 2.3.1); and
accidental loads such as earth movement, impact of construction vehicles.