BC - Precast Concrete.pdf

Chair of Building Construction
2017
Prepared by: Habteselassie Dejene (MSc in CoTM)
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PRECAST CONCRETE CONSTRUCTION

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1. What – definition , unique features
2. When – Historical overview
3. Why – Advantages and disadvantages/challenges
4. How many –
a. Precast building elements: Foundation, walls, slab, beams, stair, column
b. Types of precast building systems
- Frame systems, Large-panel systems, Slab-column systems and Mixed systems
CONTENT

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 Pre-fabrication - Production in advance
 Precast concrete is a construction product produced by casting concrete in a
reusable mold or "form” mostly in a factory
DEFINITIONS

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 Precast construction – construction of buildings where the majority of
structural components are standardized or modularized and produced in
plants in a location away from the building site, and then transported to the
site for assembly.
DEFINITIONS

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Transport
Assembling
Produce individual elements

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- The history of prefabrication in the west begins with Great Britain‟s global
colonization effort.
- In the sixteenth and seventeenth centuries, settlements in the Colonies
required a rapid building initiative.
- Since the British were not familiar with many of the materials in abundance in
the colony countries.
- Components were manufactured in England and shipped by boat to the
various locations worldwide.
HISTORICAL OVERVIEW

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- The earliest of such cases recorded was in 1624, when houses were prepared
in England and sent to the fishing village of Cape Anne in what is now a city in
Massachusetts.
HISTORICAL OVERVIEW

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1. Modular or dimensional coordination:
- Module is a standard unit of size used to coordinate the dimensions of a
building and components, At an international level, 100 mm is accepted as the
basic module „„M“
- Enables the coordination between the different parts that make up the total
structure/construction which are supplied from widely separated sources.
- Building components will be multiples of basic module
FEATURES

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1. Modular or dimensional coordination: reasons for module adaptation
- Reduce time of production and installation of components,
- Achieving repeatability and
- Able to construct building at lower cost.
FEATURES

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2. The degree of prefabrication: varies from single elements like column,
beam, slab, ….. to room sizes called capsules.
- It is affected by the available production capacity & transportation mechanisms
FEATURES

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Stair Triple story high column
FEATURES

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Completed in 1972 the Nakagin Capsule Tower was the worlds first capsule architecture
built for actual use. Kisho Kurokawa's tower was meant to draw residents from the
suburbs back to central Tokyo
FEATURES
Nakagin Capsule Tower Interior

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3. Industrial production: The components are manufactured by industrial
methods: Standardization, Mechanization, Automation and Mass production
principles.
FEATURES

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3. Industrial production: standardization
- Mechanization: an effort to move standardization to greater economies of scale, use of
advanced mechanical machinery, that reduce human labor.
- Mass production: the concept is to produce as much of the same thing in order to bring
down the cost of a single item.
- Automation: The development of digitally informed manufacturing machinery via
computer numerical control and CAD/CAM software.
- Mass customization working to maximize the benefits of mechanization and automation
production methods, reducing labor costs, but works to preserve the benefits of variability
and customization in the output.
FEATURES

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4. Demands transportation mechanism : transportation of these heavy
structures from factory to site is one key aspect of prefab construction system.
- Large low bed vehicle and cranes ( for loading, unloading and assembly ) are
needed.
FEATURES

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5. Prestressing: describe the process of introducing internal forces (stress) into
a concrete during the construction process in order to counteract the external
loads that will be applied when the structure is put into use (known as service
loads).
- These internal forces are applied by tensioning high strength steel, which can
be done either before or after the concrete is actually placed.
- Pre-tensioning: the steel is tensioned before concrete placement
- Post-tensioning: the steel is tensioned after concrete placement
FEATURES

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6. Joining mechanisms: very simple details
a. Dry joints: are constructed by bolting or welding together steel plates or other
steel inserts cast into the ends of the precast elements for this purpose.
b. Wet joints: are constructed with cast-in-place concrete poured between the
precast elements. To ensure structural continuity, protruding reinforcing bars
from the panels (dowels) are welded, looped, or otherwise connected in the
joint region before the concrete is placed.
FEATURES

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7. Anchoring, lifting points: metals or steel inserts casted at some logical
points of the precast elements for lifting purpose.
- Has to be located in consideration of the center of mass
- A minimum of two lifting points are needed to have a proper balanced
movement.
FEATURES

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 Does precast system have advantages?

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 There are three main advantages of precast construction:
I. Time
II. Cost
III. Quality
ADVANTAGES OF PRECAST BUILDING SYSTEM

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I. Time advantage: a short construction period helps to meet tight deadlines.
1. Increased paralleling of activities: Fabrication of elements during
permitting or site preparation or foundation preparation saves time. More
scheduling advantages or options.
2. All-Weather Construction: precast components can be produced and
erected in summer conditions, regardless of weather conditions.
3. Fast and smooth erection:
- No or minimal adjustments and surprises of details and shapes.
- Formwork and scaffolding construction is eliminated.

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I. Time advantage: ….. continued
4. No Curing period: the concrete has already attained its strength before
erection in the production factory/site.
5. Minimal on site finishing work:
- Pre-cast concrete elements have smooth surface that do not require
plastering because of the precise factory formworks.
- Any kind of desired surface finish will be applied in the production process
6. Minimum transaction or one stop shopping:

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II. Cost advantage:
1. Shorter construction period: Time is money in construction.
- Minimized overhead cost: additional cost due to prolonged construction
period
- Functionality of the building in a short period: income generation

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II. Cost advantage: …continued
2. Minimal resources = Minimum cost.
- Minimal waste since elements are produced in controlled factory
environment. There is a lot of wastage during site cast construction.
- No plastering is required.
- Perfect and sufficient designs that have the prescribed size especially
thickness of concrete layer.
- The steel reinforcement is used exactly to requirement, optimally used.

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II. Cost advantage: …continued
3. Minimized formwork cost: many elements are produced with a single
formwork or mold. [material & labor cost minimized ]
4. No need of shoring and scaffolding: the structures are self supporting.
5. Low maintenance cost: Precast structures require less maintenance
6. Bulk purchase of materials by the producers.

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III. Quality advantage:
1. Highly precise and simple details:
- Produces a high standard of workmanship in factory conditions - reduces
potential for accidents, addresses on-site skill shortage.
- This production technology is extremely precise when it comes to measuring.

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II. Quality advantage: …continued
2. Unusual shapes: increased design freedom in terms of shape
- Concrete mouldability helps designers to copy classical details like keystones
and capitals or match the finish of materials like weathered stones.

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3. High quality and versatile surface finish: provides architects with options
for different color, texture or any intended surface finish.
- Smooth due to being produced in a factory molds: plastering is not required
thus, it can be left exposed or painting the concrete surface is enough.
- Any kind of color or texture can be created in a consistent manner.

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4. Structural efficiency and reliability:
- The usually prescribed thickness of the fixed concrete layer can be reduced.
- The steel reinforcement can be built exactly to requirement and therefore
used optimally.
- The quality can be checked before a unit is erected into the structure.
- Advanced technologies used in the precasting plants create an improved
quality product (i.e. reduced tolerances, thinner sections, engineered
solutions) compared with cast-on-site concrete.

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5. Environmentally friendly: helps the environment by conserving natural
resources and reducing wastes and the environmental impacts of new
construction.
- Optimum use of materials or resources
- Precast concrete units can entirely be re-used reinstalled in the same building
or even transported a short distance and used in a comparable structure
- It minimize noise and dust etc. at a site.

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 There are some disadvantages or challenges in precast construction
1. Larger groups of buildings from the same type of prefabricated elements tend
to look monotonous.
2. Less flexible in design concept than purpose made structures, it is less
customized
3. Connection between the precast concrete units can present problems.
4. Transportation costs may be higher for large or heavy prefabricated sections
5. Large prefabricated sections require heavy-duty cranes and precision
measurement and handling to place in position.
DISADVANTAGES OF PRECAST BUILDING SYSTEM

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 There are some disadvantages or challenges in precast construction
6. Employment opportunity is lost:
- Local jobs may be lost, if the work done to fabricate the components being
located in a place far away from the place of construction.
DISADVANTAGES OF PRECAST BUILDING SYSTEM

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1. Foundation: transmit the load to the ground.
- There are two types of pad foundations a socket type and a pad with a short
starter foundation column
PRECAST BUILDING ELEMENTS

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2. Precast columns: provide support for beam and slab elements.
- Since these elements carry mainly axial loads with little bending force, they
may be reinforced without prestressing.
- Are usually square in shape multistory in height.

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3. Beams and girders: provide support for slabs.
- The projecting reinforcing bars will bond with concrete cast on site.
- Width: ½ of depth.
- Depth/Span: 1⁄15 of span for light loads and 1⁄12 of span for heavy loads for
rectangular, inverted tee, and L-shaped beams.
- Projecting ledgers on inverted-tee and L-shaped beams are usually 15cm
wide and 30cm deep.

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4. Precast slabs: used for floor and roof decks.
- Deeper elements (toward the right below) span further than those that are
shallower (toward the left).
Increasing depth / thickness - increasing span

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Solid flat slab :
- Width: Varies
- Thickness: 8 - 20cm
- Depth/Span: 1/40 of span
- Span: up to 8m
Double tee slab:
- Width: 244 cm, 305cm
- Thickness: 30 - 81cm
- Depth/Span: 1/28 of span
- Spans up to 30m
Hollow core slab:
- Width: 61, 122, & 244cm
- Thickness: 20, 25 & 30cm
- Depth/Span: 1/38-40 of span
- Spans 7.6m, 9.75m & 12.2m
Single tee slab:
- Width: 244 cm, 305cm
- Thickness: 91, 122cm
- Depth/Span: 1/26 -28 of span
- Spans up to 32m

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Double tees supported by wall panels

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Ribbed slab
- This slab construction system, introduced
by the Low-cost Housing Project, avoids
formwork, reduces requirements of skilled
manpower and time.
- The system has two major components:
the pre-cast beam and the slab HCB.
- The production of the slab HCB is done in
the same way as production of wall HCB
PRECAST BEAM
SLAB HCB

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5. Precast walls: used for space enclosure, to support lateral & slab loads .
- Precast concrete wall panels may be solid, hollow, or sandwiched (with an
insulating core).
- Wall panels can be ribbed, to increase their vertical span capacity while
minimizing weight, or formed into other special shapes.

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6. Stair: Stairs provide immediate access to working levels as construction
proceeds, no need for temporary stair for the construction purpose.
- Precast concrete flights may be casted with or with out landing

Prefabrication
EiABC Habteselassie D.

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 Depending on the load-bearing structure, precast systems can be divided into
the following categories:
1. Frame systems
2. Large-panel systems
3. Slab-column systems with walls
4. Mixed systems
TYPES OF PRECAST BUILDING SYSTEM

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1. Frame Systems: is constructed using either linear elements
- Linear elements, columns and beams carry the loads are preferred
because of the difficulties in forming, handling, and erecting spatial elements.
- No load bearing wall
- Connections : usually beams are seated on corbels at the columns

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1. Frame Systems: Example Column-to-Column Connection
- Metal bearing plates and embedded anchor bolts are cast into the ends of
the columns.
- After the columns are mechanically joined, the connection is grouted to
provide full bearing between elements and protect the metal components
from fire and corrosion.

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1. Frame Systems: Example Beam-to-Column Connection
- Beams are set on bearing pads on the column corbels. Steel angles are
welded to metal plates cast into the beams and columns and the joint is
grouted solid.

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1. Frame Systems: Example Slab-to-Beam Connection
- Hollow core slabs are set on bearing pads on precast beams. Steel reinforcing
bars are in inserted into the slab keyways to span the joint.
- The joint is grouted solid.
- The slab may remain untopped, or topped
with 6 -8cm of cast in place concrete.

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1. Frame Systems: Site cast Concrete Toppings
over precast Slabs
- Greater floor strength and stiffness
- Greater fire resistance
- Greater acoustic isolation
- Allow easy integration of electrical services into
floor system
- Create a smoother, flatter floor surface.

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2. Large-panel systems: composed of large wall and floor concrete panels
connected in the vertical and horizontal directions so that the wall panels
enclose appropriate spaces for the rooms within a building.
- The loads are carried by the panels
- No Column, No beams
- Thickness of wall panels ranges from 12 cm for interior walls to 30 cm for
exterior walls.

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3. Slab-Column Systems with Shear Walls: The slab-column structure
resists mainly gravity loads
- The structural elements are column and slab, there is no beam
- The systems rely on shear walls to sustain lateral load effects.
- There are two main systems in this category:
a. Lift-slab system with shear walls
b. Prestressed slab-column system

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3. Slab-Column Systems with Shear Walls:
a. Lift-slab system with walls: the floor slab and the flat roof are cast one over
the other at ground level at around columns, stairs and lift cores.
- The whole slab is cast monolithically, is designed to span continuously
between and across points of support/columns, minimizes thickness of slab.
- the slab is lifted by jacks , operating on the top of each column, which lift a
pair of steel rods attached to each lifting collar in the slab being raised.
- a central control synchronizes the process for a uniform lift from all directions.

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a. Lift-slab system with walls
- Lifting collars: they are cast into each slab around each column providing a
means to lift the slab and also providing shear reinforcement they are fixed to
columns by welding shear blocks to plates welded b/w column flanges and to
the collar after the slab has been raised in position.
- Connections to concrete columns are made by welding shear blocks to end of
steel channels cast into the column and by welding the collar to the wedges.

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- Uses horizontal prestressing in two orthogonal directions to achieve continuity
- The precast concrete column elements are 1 to 3 stories high.
- The reinforced concrete floor slabs fit the clear span between columns.
- After erecting the slabs and columns of a story, the columns and floor slabs
are prestressed by means of prestressing tendons that pass through ducts in
the columns at the floor level and along the gaps left between adjacent slabs

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- After prestressing, the gaps between the slabs are filled with in situ concrete
and the tendons then become bonded with the spans.
- This technology has been used in Yugoslavia during the last 40 years under
the proprietary name, “IMS Building System,” and it can be found in other
countries, such as Cuba, the Philippines, and Egypt.
- The kality prefab factory produce such kind of prefab elements.

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Exemplary structures: Helen tower

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RECENT TECHNOLOGY
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RECENT TECHNOLOGY
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2017
The Art of Precast Concrete: Tone Texture Form
David Bennett
As a building material, precast concrete allows a wide range of sculptural forms and
design options. By treating the surface in various ways, fascinating textures and fine
finishes can be produced. This book provides a systematic overview of the variety of
applications for such concrete elements throughout Europe.
Author David Bennett provides in-depth information on newly developed, especially
lightweight forms of concrete such as GRC (Glass-Fibre Reinforced Concrete), Ductal and
CRC (Compact Reinforced Composite). A selection of some 24 projects which are of
particular significance, are documented in detail and provide a wealth of inspiring
design ideas. The appendix comprises an overview of the building practices in the
individual European countries and the availability of concrete elements. Amongst the
buildings documented are the Scottish Parliament Building in Edinburgh by Enric
Miralles Benedetta Tagliabue, the Synagogue in Dresden by Wandel, Hoefer, Lorch +
Hirsch, and the Mexican Embassy in Berlin by Gonzlez de Len.
Birkhauser September 2005

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THANK YOU !

BC - Precast Concrete.pdf

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