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City University of Hong Kong
Division of Building Science and Technology
BST21317 Construction technology 3
Group Assignment
Report submission date : 12-04-2010
Group members
Task Name Student ID.
1 Liu Pui Ying 5147 7232
2 Wong Suet Yee 5147 8007
3 Mok Wing Hui 51479711
3 Wong Wing Yee 51443820
4 Leung Wan Ki 51480980
Background about the project siteThe contract period would be 26 months with tender price of HK280 million for
this project. To identify the site environment, the project located at a busy and
crowded street in Hong Kong. There are adjoining buildings nearby. There
would be constraints in using either the precast concrete or in-situ concrete.
The proposed building is 38-storey high in both commercial and residential
development. The superstructure is separately commercial and residential
uses. There are 5-storey commercial malls and the remaining 33 floors would
be for residential use.
Now the superstructure which would be seated on 2m thick transfer plate
would be 33-storey residential house for the project development. Below the
transfer plate would be the 5-storey commercial malls. In this case,
consideration for using whether pre-cast or in-situ concrete would be only for
residential housing.
Task1
1. Pre-cast concrete
1.1 Advantage
Due to the promotion of using pre-cast concrete elements, the Building
Department allows additional floor area which lead to about 2% increase in
GFA extent of the project. This can increase the profit gain for the developer.
As long as the pre-cast concrete is made in factory and then transport to the
construction site, the quality of pre-cast concrete is ensured to be good. For
manufacturing in a factory, the cost for pre-cast concrete is higher when
compared with in-situ concrete. However, as the pre-cast concrete
manufacturers have to join the market and make a competitive price for it. The
developer can enjoy economic advantages from it. In addition to the lower
cost, it reduced time for construction by installing it directly where in-situ
concrete would be normally wait for formwork cutting and drying. So the
construction time for every single floor would be reduced.
As mentioned above, pre-cast concrete unit is manufactured in factories, the
quality of units can be ensured. So the whole unit can install immediately in
any seasons while strength of insitu concrete would be affected in rainy
seasons.
Disadvantages
Transportation
As the pre-cast concrete units are manufactured in factories, they will be
transported through long distance. During transportation, the large and heavy
prefabricated concrete units might be damaged if accidents happened.
Due to different units installed at the same time, there might be problems in
water-tightness for joining units together. The joints need to be carefully joined
to reduce the problems of leakage as Hong Kong has high rainfall every year.
The pre-cast concrete unit would require sufficient working space for
installation so that the plants can lift units to a higher level. This may be not
adoptable in congested site area which is lack of working space.
Precast concrete are mostly done for same designs. Extra time would be
required for the factory for manufacturing a specific design. It is because the
factory would have to enter new drawing to the machine and also require time
for testing of the unit in order to control the pre-cast concrete unit quality.
Therefore, the units are not that flexible at all to different building designs.
1.2 In-situ concrete
Advantages
Flexibility in different designs
In-situ concrete can provide flexibility to different design appearance as the
designed shape can be adjusted by the formwork shape.
Working space
In-situ concrete would require less working space when compared to pre-cast
concrete. It is flexible to pour into confined area.
Construction cost for in-situ concrete is acceptable since it is traditional
method and competitive market. The price for it is in steady state.
Disadvantages
Construction time would be longer in rainy seasons as the strength of
concrete would be affected.
Task22.1 IntroductionBuilding defects may occur in the different parts of the building due to the
faulty design, poor-quality materials or poor workmanship. The defects may
be different types of cracks, dampness, and risk of fire etc. to enhance a
better property management, a long-effective maintenance plan is necessary
to plan for the development.
2.2 DefectsCracks of Plastic ConcreteThe cracks will occur on two types of concrete: plastic concrete and hardened
concrete. Plastic concrete causes cracks due to loss of moisture caused by
low humidity, wind or high temperature.
Moreover, movement can be one of the reasons to cause cracking because
any movement in the ground level may seriously affect the foundation of the
entire building and it may bring out some cracks to the building such as hair
cracks on the wall, or appear on the beams etc.
This phenomenon may occur within a few hours after placing the concrete
and settlement cracks may turn out too. It will increase the rebar’s size, or
even decrease the cover of the concrete.
DampnessIt is a most common defect may occur in a building because every building is
surrounded by moisture, and it can’t be escaped.
As mention before, moisture will move into the building by rising damp from
the ground to the wall, it usually occurs near the wall edge and floor slab
especially is the junctions.
The other ways to get inside is rain penetration. We cannot control the
weather so that if there is a rain, the water and moisture will spill inside the
building from the leaking areas and cause dampness on the material.
And the third cause is condensation, inadequate heating, insulation and
ventilation is the reasons which cause condensation. Condensation may bring
out fungi and bacteria and controlling ventilation is one of the ways to solve
the defect.
Risks of FireIt may occur because of the materials are easy to burn, and cause fire to the
surface and structure of the building. When the expansion happens in the
material, movement may come and causes the cracks on the structure
surface.
If the cracking is too seriously, possibility of collapse will cause on surface or
structure which means part of the structure will fall down and cause damage
to the people.
Final, the structure may not be proper cover by the sufficient FRP materials
and expose to the outside that may easily get the risk of fire.
2.3 Maintenance PlanA well-planned maintenance plan is need to be provided to the owner of the
building for further reference. This is a programme which include lots of
information with the minimize cost and time spend.
Survey is the first requirement require within a regular period for reporting the
present state of the building condition and deterioration with the grade of
urgent to have maintenance for the defects.
For the plan, it should be included different schedules for different kinds of
building parts. The most important thing is state clear the nature of the work
which will start to work. Specification of the work descriptions and the hand
sketches or drawings with accurate dimensions measured according to the
work is needs to contain inside the plan.
As mention before, dimension is important. But the setting out of work content
and materials requirement should be mention in the plan, in order to have a
more specified description of work content and materials including
dimensions, also, it is important to mention in the plan.
2.4 ConclusionTo conclude, although there are different defects causes by natural and man-
made this may occur inside a building, those defects may cause seriously
damage for the occupiers. To provide a better living environment, a long-term
maintenance plan is major to provide with sufficient information likes
drawings, cost for the owners to have estimation of the further works.
Task33.1 IntroductionGreen building is defined as an approach to building design, construction, and
management that reduces or eliminates negative environmental impacts while
it promotes high building performance and occupant health. By using natural
sunlight during busy hours, using recycled construction materials and
designing green roofs covered with vegetation are all examples of green
building practices. Designers and engineers of green buildings take a whole-
building approach that considers how building systems react with one another
and with the natural environment. Finally, green buildings consume less
energy, save more water, generate less air pollutants, and provide healthier
indoor environments.
The practices of green building are as following:
• Integrated, whole building design choices;
• Environmentally responsible site practices;
• Energy conservation and use of renewable energy resources;
• Water conservation and reuse;
• Materials selected for environmental sustainability and human health;
• Waste reduction during construction and operations;
• Measures to ensure good ventilation and indoor air quality.
3.2 Concept of Green BuildingThe concept of Green Buildings can be envisioned as a new approach to save
water, energy and material resources in the construction and maintenance of
the buildings and can reduce or eliminate the adverse impact of buildings on
the environment and occupant. The concept of Green Buildings put forward
initiatives to encourage the design and construction of buildings with the
following features:
• Adopting a holistic life cycle approach to planning, design, construction and
maintenance;
• Maximizing the use of natural renewable resources and recycled or green
building materials;
• Minimizing the consumption of energy, in particular those non-renewable
types;
• Reducing construction and demolition waste.
By referring the concept of green building, this concept integrates the building
an environment using green practices employed with a design purpose.
3.2.1 Design of Building
3.2.1.1 Maximizing the use of natural renewable resources :- (i) Rainwater Utilization Systems
The green building by its design and shape shall not disrupt the natural water
flows, it should orient and stand just likes a tree. Rain falling over the whole
area of the complex shall be harvested in full either to replenish the ground
water table in and around the building or to be utilized in the services of the
building. The toilets shall be fitted with low flesh fixtures. The plumbing system
should have separate lines for drinking and flushing. Waste water from
kitchenette, bath and laundry shall be treated and reused for gardening or in
cooling towers of air conditioning. This system could help to minimize the
usage of water, and save water as much as possible.
3.2.1.2 Minimizing the consumption of energy :- (i) Rooftop planting
By developing a podium garden on roof of commercial building and a sky
garden on roof of residential building, the extra growing medium and
vegetation insulates the building from intense temperatures and minimizes
heat gain. According to a Canadian study, even a six-inch of extensive green
roof can reduce summer energy demands by 75 percent. A green roof's plants
remove air particulates, produce oxygen and provide shade. They use heat
energy during evaporation, a natural process that cools the air as water
evaporates from plant leaves.
(ii) Photovoltaic system
The solar energy at the top of a green building is harvested to supplement the
conventional energy. Photovoltaic system is a system using solar cells to
convert the light energy into electrical energy. Photovoltaic installations on the
building and carport roofs produce solar electricity. Although the energy
collected is not enough to support the operations of the office buildings, it can
help alleviate the heavy working load on electricity demands.
3.2.2 Construction of Building3.2.2.1 Maximize use of green building material :- According to an American study, building materials are responsible for about
20 percent of the greenhouse gasses emitted by a building during its lifetime.
Therefore, green building shall use the products that are non-toxic, reusable,
renewable, and recyclable wherever it is possible. Thus, local manufactured
products are preferable so that the collective material environment of the
locality remains a constant and the fuel for the transport of materials is saved.
(i)Green cement
Those precast concrete used in this construction could be produced by a
green method, which produce both cement and aggregate, another
component of concrete. Their method sequesters CO2 from power plant flues
and mixes the gas with sea water to produce the mineral raw materials of
concrete. For every ton of green cement Calera manufactures half a ton of fly
ash from coal plants is used apart from preventing production and emission of
CO2.
3.2.2.2 Reducing construction and demolition waste :- To reduce the impact on the environment, the waste hierarchy helps to identify
different options by ranking them in order of environmental impact. All
wastage could be eliminated at the top of the hierarchy where it is possible.
This method is shown as follow:
(i) Waste hierarchy
(a) Eliminate
Eliminating waste entirely may not always be possible, but by not creating it in
the first place reduces costs of raw materials.
(b) Reduce
Reducing the amount of waste you produce can be achieved in a number of
ways, including the amount of packing used, reduce off-cuts and rejects, send
information electronically, purchase material in bulk and use returnable
containers.
(c) Re-use
To limit extra spend of buying items in, many wastage could be re-used to
reduce waste:-
Packaging
• Packaging – boxes can be re-used many times;
• Printer toner cartridges – choose a supplier that has a returns policy so that
they can be re-filled and used them again;
• Paper – re-use paper from misprints and drafts as scrap paper in the office;
• Drums – many raw materials are delivered in drums that can be washed and
returned to the supplier or re-used on site as waste containers;
• Furniture and textiles – waste furniture and textiles may be of use to
charities or to waste exchange groups;
(d) Recycle
Recycling is an increasing requirement through legislation in order to reduce
the impact on the environment. Many items can now be recycled, speak to
your local recycling centre or waste management contractor to find out what
they are and how they should be segregated.
(e) Disposal
Disposal is the last resort when the other hierarchy options have been
exhausted. There are legal obligations that all producers and handlers of
waste need to comply with, so it is important that you contact your waste
management contractor to discuss options like recycling to make waste
disposal more efficient and save money.
Summary
To sum up, a green building shall use whole-building design to achieve
energy, economic, and environmental performance that is substantially better
than standard practice. By considering the building structure and systems
holistically, they allow us to know how they best work together to save energy
and reduce the environmental impact.
Task3
3.1 Introduction
Nowadays, the quality of environment has gotten serious. It will make a bad
impact on our society if ignoring this problem. In order to innovating the quality
of life, the government has promoted green and sustainable building development
since 2001.
3.2 Concept of Green Buildings
The definition of Green Buildings is that using environmentally technology and
resource-efficient through a whole building life-cycle which includes design,
construction, maintenance and demolition.
The objective of green buildings is that reducing the overall impact of the built
environment on human health and the natural environmen.
Also, the concept of Green Buildings put forward initiatives to encourage the
design and construction of buildings with the following features:
▪ Adopting a holistic life cycle approach to planning, design, construction
and maintenance;
▪ Maximizing the use of natural renewable resources and recycled/green
building materials;
▪ Minimizing the consumption of energy, in particular those non-
renewable types; and
▪ Reducing construction and demolition waste.
Now, we would like to apply the concept of green building in the design and construction
of the proposed building in order to show how such concept could be achieved.
3.3 Design in commercial and residential buildings
In order to eliminating the impact of buildings on the environment, sustainavle design
is necessary. Obivously, it takes advantages of using renewable resources. For
examples, solar radiation, tides and winds are the most common types of natural
resources. Also, minizing the amount of non-renewable source is another possible
solution. However, we need to consider the feasibility of those resources for the
buildings which are as follows:
3.3.1 Solar Power Supply(see Fig 1)
Photovoltaic system makes use of converter that generating the energy and then
transfer into electrical energy. The system is constructed in the form of a panel,
which consists of on the roof of building mono-crystalline silicon photovoltaic cells.
It is recommonded to use Photovoltaics becasue this system generate
clean electricity. Also, it is common used for large-scale buildings. For maintenance,
the requirement is minimal since regular inspection or repair only.
Fig 1 of Photovoltaic system
3.3.2 Water-Cooled Air Conditioning System (see Fig 21)
For the commerical malls, Water-cooled Air Conditioning Systems is sutiable
to use. As we know that the use of air conditioning in Hong Kong attributes a
large proportion of our total electricity consumption. We need to tackle as
soon as possible. A survey shows that the system can save more than 30% of
energy consumption compared with old sysem. It is not only minizied he
danage of environment, but also reducing the electricity bill.
Fig 2 Water-Cooled Air Conditioning System
3.3.3 Landscaping
Greening plays an important role in
building an environmentally friendly
society since plants absorb carbon dioxide
and release oxygen. For our case, there is
a 2m thich transfer plate. We can
landscape it as a sky garden for beautifing
the surrounding environment. Also, trees
and shrubs provide freshness, comfort and
elegance for user’s enjoyment.
3.4 Construction in commercial and residential buildings Aside form design stage, we need to concern the selection of construction
materials for environmental friendly. Besides, it is possible to reduce the
amount of energy consumption and waste so that the air quality may be
improved.
3.4.1 Renewable resources
There are a number of benefits if using recyclec or reused materials which are
a reduction in materials going to landfill, consumption of natural resources and
transport and energy costs Therefore, It is the best sloution to be reused
mterials time to time rather than discharging. Appling to the case, formwork is
the sufficient reusable system that using repeatly.
However, conerning the type of formwork materials, it is good suggestion to
use metal formwork so that getting less waste than traditional timeber
because of extendable duration.
Fig.10 Sky Garden
3.4.2 Reduction of energy consumption
The energy consumption on on-site activities and on operation of the office
building can be reduced through the use of day-lighting, solar heating,
breezes for natural cooling. Also, it shoild be switched off all plant or mechine
when they do not use. Althought it might not take a significant preformance,
we try our best to achieve the concept of green buildings.
3.4.3 Waste reduction
For traditional method, lots of waste or dusts are created during the
contruction period.
In order to minising constuction waste, it is strongly recommended to use
precast concrete or prefabrication for external facades, staircases, etc in the
residential building. It takes benefits of reduciotn of construcion time and the
amount of dust.
3.5 Summary
In order to reducing the impact of environment, it is necessary to provide
green features in the new building developments. However, we need to
concern the balance between economic and environment.
Task4Prestressing
prestressing is the process of introducing internal forces or stress into a
concrete or masonry element during the construction process in order to
counteract the external loads that will be applied when the structure is bearing
the 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 or Post-tensioning Pre-tensioning
The tension is applied to the tendons before casting of the concrete. The pre-
compression is transmitted from steel to concrete through bond over the
transmission length near the ends.
Fig.1 Stages of pre-tensioning
Post-tensioning
The tension is applied to the tendons after hardening of the concrete. The pre-
compression is transmitted from steel to concrete by the anchorage device.
Fig. 2 Stages of post-tensioning
Advantages of Using Prestressed ConcreteThe prestressing of concrete has several advantages as compared to
traditional reinforced concrete (RC) without prestressing. A fully prestressed
concrete member is usually subjected to compression during service life. This
rectifies several deficiencies of concrete. The benefits are listed.
1. Prestressing minimizes the effect of cracks in concrete elements by
holding the concrete in compression. It can reduce the steel corrosion and
increase in durability. Moreover, prestressing concrete can be higher
stiffness and increase in shear capacity, which lead to improve the
serviceability and become less deformation. In addition, it also can
improve the structural performance under dynamic and fatigue loading
2. Prestressing allows reduced beam depths to be achieved for equivalent
design strengths. Typical values of span-to-depth ratios in slabs are given
below.
Type of slab Span-to-depth ratios
Non-prestressed slab 28:1
Prestressed slab 45:1
For the same span, less depth compared to RC member. It would get the
benefit, such as reduction in self weight, more aesthetic appeal due to
slender sections and more economical sections.
3. Prestressed concrete is resilient and will recover from the effects of a
greater degree of overload more than any other structural material.
4. If the member is subject to overload, cracks which may develop, it will
close up on removal of the overload.
5. Prestressing enables both entire structural elements and structures to be
formed from a number of precast units, e.g. Segmented and Modular
Construction.
The advantages of precast construction are including rapid construction,
better quality control, reduced maintenance, suitable for repetitive
construction, reduction of formwork, and availability of standard shapes.
6. Lighter elements permit the use of longer spanning members with a high
strength to weight characteristic.
7. The ability to control deflections in prestressed beams and slabs permits
longer spans to be achieved. Reductions in deflections under working
loads can then be achieved by suitable eccentric prestressing.
8. Prestressing permits a more efficient usage of steel and enables the
economic use of high tensile steels and high strength concrete.
9. Prestressed concrete can provide significant cost advantages over
structural steel sections or ordinary reinforced concrete.
Disadvantages of Using Prestressed ConcreteAlthough prestressing has advantages, some aspects need to be carefully
addressed.
1. Prestressing needs skilled technology. Hence, it is not as common as
reinforced concrete.
2. The use of high strength materials is costly.
3. There is additional cost in auxiliary equipments.
4. There is need for quality control and inspection.
Construction Applications of Prestressing
The construction possibilities of prestressed concrete are as vast as those of
ordinary reinforced concrete. In our proposed project which is residential
development with commercial malls, the superstructure would be using
precast elements. This means that we would apply the pre-tensioning
technique for the precast unit. There are some typical applications of pre-
tensioning components in building as the following:
1. Structural components for integration with ordinary reinforced concrete
construction, e.g. floor slabs, concrete beams,columns.
2. Water tanks and reservoirs where water tightness (i.e. the absence of
cracks) is of paramount importance.
3. Construction components e.g. piles, wall panels, frames, window mullions,
power poles, fence posts, etc.
4. The construction of relatively slender structural frames.
There is some example for pretensioning components which would be
suitable for our proposed project.
Beams
Beams are typically considered structural components and
are made in one of three key shapes: Rectangular,
Inverted Tee Beams and L Beams. They are horizontal
components that support deck members like double tees,
hollow-core and solid slabs, and sometimes other beams.
Beams can
be reinforced with either
prestressing strand or
conventional reinforcing bars.
This will depend on the spans
and loading conditions.
Prestressed beams are typically
pretensioned and cast in a long-
line set up similar to that used for
double tees. Beams that are
reinforced with conventional
reinforcing bars can be cast as
individual components.
Fig. 3 Balcony beams were cast as one place with the cantilevered balcony slabs
Hollow-core slabs
Hollow-core slabs which are precast concrete units
are used predominantly for floor and roof deck
component for various structures. Besides that,
these generally are cast using a long line method
with prestressing beds, which extrudes the concrete
and creates the voids by means of either a rotating
auger or by placement of aggregate filter that is
later removed.
Fig.4 Samples of Hollow-core slabs
Fig.5 Hollow-core slabs for different shapes to form the voids within the pieces
Mullions
Mullions are thin, often-decorative pieces that fill open
space in a building façade. They are often isolated
elements forming a long vertical line, requiring them to be
cast perfectly straight to avoid any visual deformities. To
some degree, these variations can be handled by precast
concrete connections with adjustability.
They can be made in a long –line
pretensioning facility and
reinforced with prestressing
strand or conventional reinforcing
bars. Sizes and shapes (square
or rectangle) can vary to satisfy
both architectural and structural
requirements.
Solid slabs
Solid slabs are used as structural deck components similar
to hollow-core slabs. They can be made in a long-line
pretensioning facility and reinforced with prestressing
strand or cast in individual forms with either prestressing
strand or conventional reinforcing bars. They are typically
cast in the same position as used in the structure.
The top side is troweled to the desired degree of smoothness or may be
internally roughened to receive a cast-in-place concrete topping that will act
compositely and provide additional strength.
Fig.6 Eagle Gate Plaza & Office Towers
Fig.8 Sample of solid slab Fig.9 Velocity Multifamily Residential Condos
Shear Walls
Shear walls act as vertical cantilever beams, transferring
lateral forces acting parallel to the face of the wall from the
superstructure to the foundation. Typically, there are two
oriented to resist lateral loads along each principal axis of
the building. They should be designed as load bearing
panels.
Shear walls typically are cast flat in an individual form with
either prestressing strand or conventional reinforcing bars.
Fig.10 Interior shear wall system Fig.11 Exterior shear wall system
Reference:
Lecture note ”Environmental Considerations in Construction” by Raymond Li
http://www.cityu.edu.hk/CIVCAL/production/advanced/
http://www.epd.gov.hk/epd/misc/cdm/management_intro.htm
http://en.wikipedia.org/wiki/Green_building
http://www.gov.hk/en/residents/environment/sustainable/greening.htm
http://www.pci.org/resources/home_eng.cfmwww.yde.co.il/Post-Tensioned-Building.aspxhttp://www.learnerstv.com/video/video.php?video=2506&cat=Engineeringhttp://www.mwcog.org/uploads/pub-documents/yVhXWA20071213133242.pdf
http://sipilbanyumas.blogspot.com/2010/03/concept-of-green-building.html
http://businessmirror.com.ph/index.php?
option=com_content&view=article&id=22350:building-
green&catid=52:technology
http://science.howstuffworks.com/green-rooftop.htm
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