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