CSE 29300Construction Technology and Temporary Worksgy p yPrecast Construction

Professor K.F. ChungProfessor K.F. ChungD f Ci il d E i l E i iDepartment of Civil and Environmental Engineering

The Hong Kong Polytechnic UniversityHong Kong SAR, China

Scope

Characteristics of Precast Construction

Slash Construction Time and Save CostSlash Construction Time and Save Cost

Prefabricated System

Connection Methods

Quality Assurance of Prefabrication Quality Assurance of Prefabrication

Prefabrication in High Rise Buildings

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Characteristics of Precast Construction

Precast construction rises in response to the proper timesand conditions, and has the following characteristics:

Standardization

P f b i ti Prefabrication

Mechanized Construction

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Characteristics of Precast ConstructionStandardization to modular flat design

T ki h b fi f fl d i b b i i i d h i Taking the benefit of mass flat production, but bearing in mind the impactof prototype blocks, only standardization of flat units is considered.

Building skeleton components such as facades, slabs, staircases,partition walls and beams are standardized to form modular flat units.

Flat Facade Slab

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Characteristics of Precast ConstructionStandardization to modular flat design

B ildi f b i h i d b h d ki h Building fabric components such as windows, bathroom and kitchenfittings, doors, metal gate sets are standardized for factory manufacture.

Blocks are assembled using these modular flat units within layout, butoutlook of blocks can be unique and different.

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Window Bathroom Assembly

Characteristics of Precast ConstructionPrefabrication

P f b i i f i i ll h i Prefabrication of concrete components is essentially the constructionmethod which transfers some of the difficult in-situ reinforced concreteconstruction from working floor to factory. Meanwhile, the transfer is alsofrom elevated construction on site to construction on ground in factory.

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Characteristics of Precast ConstructionPrefabrication

F l d i i i f diffi l h dl li d For elevated construction, it is often difficult to handle complicatedcomponent profiles or locations which are difficult to access. Substantialfalsework and working platforms may be required. In case timberformwork is used, the workmanship may be deteriorated after repetitiveconstruction. However, for factory fabrication on ground, steel mouldscan be used, which facilitate horizontal casting of concrete and steel, gfixing.

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Characteristics of Precast ConstructionMechanized construction

M h i d i i i il h f Mechanized construction comprise primarily the use of tower crane tomove around steel formwork, concrete skips and precast components.The transportation is between ground and working floor and betweendifferent wings of working floors.

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Characteristics of Precast ConstructionMechanized construction

L l l ll f hi h l i b f k d I Large panel steel wall forms, which replace timber formwork, are used. Itwas a pioneer environmental initiative to reduce the use of timber.

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Characteristics of Precast ConstructionMechanized construction

P f d f d ki fl Precast components are transferred from ground to working floor upondelivery to site. Concrete from ready mix truck is lifted to position fromground level to working floor.

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Slash Construction Time and Save CostSlash construction time

Th l i l f f i i f ili d b h f ll i The relatively fast rate of construction is facilitated by the following:

Standardized modular flat with prefabricated components.

Re-use of large panel steel formwork.

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Slash Construction Time and Save CostSlash construction time

Th l i l f f i i f ili d b h f ll i The relatively fast rate of construction is facilitated by the following:

Fair face off-form finishes with paint to exterior and tiles/paint to interior.

Minimization of wet trade on site through prefabrication and pre-installation atfactory, and use of precast partition walls instead of blockwork partition.

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Slash Construction Time and Save CostSave cost

P f b i i ld l d i f b h i d l Prefabrication could lead to saving of both construction cost and longterm maintenance cost.

Saving in construction cost: Saving in construction cost:

Fair face off-form finishes minimizes touching up and repair, which is veryfrequent for timber construction (e.g. slab soffit and wall finishes).q ( g )

Labour cost in precast factory is much lower than that for in-situ construction, firstbecause cost is cheaper in Mainland China and second semi-skilled labour inf t i t d f kill d l b t ki fl i i dfactory instead of skilled labour at working floor is required.

In-situ architectural wet trade, such as block works partitioning, tiling, plastering,etc. are substantially reduced, mostly transferred to factory or eliminated throughetc. are substantially reduced, mostly transferred to factory or eliminated throughoff form.

Mechanized construction demands much less labour than those of conventionalf

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timber construction, particularly in terms of carpentry and temporary works.

Slash Construction Time and Save CostSave cost

S i i i Saving in maintenance cost:

Mechanization using steel formwork, ready mix concrete and tower crane canlargely reduce the amount of labour intensive type of construction thuslargely reduce the amount of labour intensive type of construction, thusminimizing human workmanship errors.

Prefabrication in factory can change the mode of construction from elevatedpositions to ground floor line production. The construction difficulty is hencesubstantially reduced and better workmanship can be assured.

Workmanship contributes significantly to the quality of built products such as the Workmanship contributes significantly to the quality of built products, such as themaintaining of consistent concrete cover to reinforcement.

Some aspects in serviceability maintenance cost are also reduced because ofp ythe prefabrication in factory, such as window leakage, water seepage at toiletfloor and so on.

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Prefabricated System

Generally, prefabricated system is composed of different components, asfollows:

Precast facades

Semi-precast slab

Precast staircase

Precast partitions

Precast tie beams

Volumetric precast bathroom/kitchen

Semi-precast wallSemi precast wall

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Prefabricated SystemPurpose of different components

P f d Precast facades

To tackle the complicated profile by casting on ground.

To cast-in windows to prevent water seepage.

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Prefabricated SystemPurpose of different components

S i l b Semi-precast slab

To avoid using substantial formwork and falsework in-situ, hence neaterconstructionconstruction.

To provide quality surface finish at soffit of slab.

To house concealed conduits within slab thickness To house concealed conduits within slab thickness.

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Prefabricated SystemPurpose of different components

P i Precast staircase

Staircase is normally confined in a limited space within the stair core. In-situconstruction is difficult and sometimes dangerous due to possible movementconstruction is difficult and sometimes dangerous due to possible movementof falsework.

Precast staircase is simple to produce in factory and easy to install on site.

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Prefabricated SystemPurpose of different components

P i i Precast partitions

Lightweight partitions are either by aerated concrete or hollow tube panel.

Lightweight partitions are to be erected after construction of structural frame,to serve as partitions but eliminate wet trade (previously by blockwork).

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Prefabricated SystemPurpose of different components

P i b Precast tie beams

These tie beams are located in elevated positions unsupported by floor slabs.Construction is difficult in terms of erection of falseworkConstruction is difficult in terms of erection of falsework.

Precast tie beams are easier to install, mainly to make the connections atsupports.

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Prefabricated SystemPurpose of different components

V l i b h /ki h Volumetric precast bathroom/kitchen

To precast a box-type structure embodies numerous pipe ducts, fittings, tiles,waterproofing membranes etcwaterproofing membranes etc.

A lot of wet trade could be transferred to the factory which is a bettercontrolled working environment.

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Prefabricated SystemPurpose of different components

S i ll Semi-precast wall

At the end gable wall or re-entrant, semi-precast wall could be used such thatwall finishes could be applied in the factory and in-situ application of finisheswall finishes could be applied in the factory and in-situ application of finishescould be avoided. This saves the setting up of scaffolding or working platform.

It has the added benefits of not requiring workers to work at height outside thebuilding perimeter.

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Prefabricated System

I d f b i ll i f ll Instead of substantially using precast concrete components, fullyprefabricated construction can be achieved in different fronts:

Prefabricated steel formwork for in situ construction Prefabricated steel formwork for in-situ construction

Precast reinforced concrete components

P f b i t d b ildi t Prefabricated building components

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Connection Methods

Th h d There are two common methods to connect precast components:

Connecting precast components with precast components by using infill groutd b k t / i f tand brackets/reinforcement.

Connecting precast component with in-situ component by using monolithicconnection with concrete and reinforcementconnection with concrete and reinforcement.

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Quality Assurance of Prefabrication

One of major concerns is the quality assurance in these remote factories.The HA has established a series of quality assurance measures asThe HA has established a series of quality assurance measures aselaborated below:

Independent consultants have been appointed to supervise the production ofthese factories, with full time staff stationed at the factories.

Each factory has to carry out internal audits at regular intervals. The independentconsultant is required to conduct monthly external audits and the HA would alsoconsultant is required to conduct monthly external audits and the HA would alsocarry out regular audits to the performance of the consultants.

A Performance Assessment Scoring System (PASS) is also imposed to theseg y ( ) pfactories, with particular focus on the workmanship of the production. Scoresobtained from the PASS would be related to the performance of the maincontractor employing the precast supplier and affect the tendering eligibility of themain contractor.

At the initial set up of the factory, it has to be accredited to ISO standards. Inaddition the supplier has to carry out a pilot production run to demonstrate its

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addition, the supplier has to carry out a pilot production run to demonstrate itscompetence before it is approved for production.

Prefabrication in High Rise Buildings

There are several important considerations to facilitate use ofprefabrication in high rise construction:prefabrication in high rise construction:

Symmetry in Layout Design

If the layout can be designed to be symmetric or as far as possible symmetric If the layout can be designed to be symmetric, or as far as possible symmetric,the prefabricated steel formwork can be rotated from one wing to another, thusavoiding the transfer of formwork to ground level.

The number of types of precast components could also be largely reduced,hence more repetitive use of steel moulds and simple logistics.

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Prefabrication in High Rise Buildings

Use of Tower Crane

T t ib t i ifi t t t t ti If th it Tower crane contributes a significant cost to construction. If the crane capacitycould be reduced, it saves the overall construction cost.

If the weight of precast components could be minimized such as the use ofIf the weight of precast components could be minimized, such as the use ofplanar facades or facades with less complicated profiles, the capacity of towercranes could be smaller.

Symmetry of layout could also greatly enhance the tower crane capacity as thereach of the crane can be optimized. Sometimes, if the block is veryasymmetric, two instead of one tower crane may be required.

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Prefabrication in High Rise Buildings

Dimensional accuracy of precast components

P t t ti ll f hi h i i th i it t fit i t d Precast construction calls for high precision, otherwise it can not fit into andinterface with in-situ construction as formwork is also prefabricated.

Where precast is connecting to precast such as non-wind resistingWhere precast is connecting to precast, such as non wind resistingcomponents, accuracy is also vital.

Generally, a maximum of 4 mm tolerance between each connection is allowed.

Transportation from factory to site

Width of precast element has to be limited to less than 2.5 m for truckptransportation.

Transportation is preferably by road, and just in time to minimize storage onsitesite.

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Conclusions

Precast construction is highly effective in building residential tower blocks with a high level of standardization.

Precast construction, after fully developed, is a fast track construction with significant savings in time and costs.g g

Precast construction relies on prefabricated elements, members as well as systemswell as systems.

Connection methods are crucial to the success of precast construction during constructionduring construction.

A high level of quality assurance in prefabrication allows its application i hi h i b ildiin high-rise buildings.

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CSE 29300Construction Technology and Temporary Worksgy p yPrecast Construction

Professor K.F. ChungProfessor K.F. ChungD f Ci il d E i l E i iDepartment of Civil and Environmental Engineering

The Hong Kong Polytechnic UniversityHong Kong SAR, China