t0pic comparing the use of precast and c

8/18/2019 t0pic Comparing the Use of Precast and c

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T0PIC:

COMPARING THE USE OF PRECAST AND CAST IN-SITU CONCRETES IN

CONSTRUCTION INDUSTRY.

CHAPTER ONE

1.0 INTRODUCTION

1.1 BACKGROUND OF THE STUDY

Because concrete is a mixture of cement and water binds together fine and coarse particles of

inert materials known as aggregates, it is readily seen that by varying the proportions of the

ingredients innumerable combinations are possible. These combinations result in concrete of

different qualities. When the cement has hydrated, the plastic mass changes to a material

resembling stone. This period of hardening is called curing, which three things are required:

time, favorable temperatures and the continued presence of water.

To fulfill requirements it is essential that the hardened concrete have, above all else, strength and

durability. n order that the concrete in its plastic form may be readily place in the forms, another

essential quality is workability. When water tightness is required, concrete must be dense and

uniform in quality. !ence it is seen that in determining the various proportions of the mixture of

the designer must have in mind the purpose for which the concrete is to be used and the exposure

to which it will be sub"ected.

The following factors regulate the quality of the concrete: suitable materials, correct proportions,

proper methods of mixing and placing and adequate protection during curing.

1.2 STATEMENT OF PROBLEM

#xperience in design and innovative construction techniques are becoming common practice to

minimi$e pro"ect costs while maintaining or improving pro"ect quality, durability, and

workability. %ew designs are likely to combine precast with cast in&situ concrete to provide

composite action or to develop continuity.


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'recast and cast in&situ concretes in (hana, since times immemorial have been facing challenges

since the inception of technology in the construction industry. Whilst this happens, people are

ignorant or overlook the advantages and disadvantages of both precast and cast in&situ concretes.

)or that matter, artisans are not careful in decisions involving concrete works hence, the many

problems of concrete structures which sometimes make them choose the wrong material and

create so much economic hardship on the pro"ect. This pro"ect seeks to highlight the importance

of the advantages and disadvantage of both concrete types, so that construction personnel could

make informed choice about which concrete type is most appropriate for a particular component

of a building.

1.3 AIM

The aim of the study is to compare the use of precast and cast in&situ concretes for construction.

1.4 MAIN QUESTION

St!" t# $#%&'() t*) +) #, &()$'+t '! $'+t -+t $#$()t) $#+t($t# !+t(".

1./ SUBQUESTION

*$* %)t*#! *)& t# ()!$) #-+t) '#(

*$* %)t*#! ()!$) $#+t($t# $#+t

*'t t"&) #, $#$()t) +&))! & $#+t($t# &(#$)++

*$* %)t*#! *)& t# ()!$) #-+t) '+t)

1.5 OB6ECTI7ES OF THE STUDY

The overall research aim shall be achieved through the following ob"ectives:

T# !)t)(%) *$* %)t*#! *)& t# ()!$) #-+t) '#(.

T# #t) ),,)$t8) &'9 %)'+ t# ()!$) $#+t($t# $#+t.

T# !)t)(%) *$* t"&) #, $#$()t) +&))! & $#+t($t# &(#$)++.

T# !)t," *$* %)t*#! *)& t# ()!$) #-+t) '+t).

1. STUDY AREA

The study was carried out with some selected precast and in&situ concrete users in the *ape

*oast +unicipal ssembly.

1.; SCOPE OF THE STUDY


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The study is limited to advantages and disadvantages of precast and cast in&situ concretes in the

building industry. The study is therefore limited to the following areas:

*uring and protection of precast concrete.

The strength and durability of precast concrete and cast in&situ concrete.

1.


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LITERATURE RE7IE

2.0 INTRODUCTION

This chapter describes what others have already written in relation to the topic.

2.1 PRECAST CONCRETEccording to )rederick 2aina 3445/, precast refers to the process of construction in which a

concrete element is cast somewhere other than where it is to be used. The other place may be

somewhere else on the building site or away from the site, probably in a casting yard or factory.

The precast element may be pre&stressed, may be of ordinary reinforced concrete, or may even

be without reinforcement. The single precast elements may be component of a general precast

concrete system, or may serve as singular purpose in a construction system of mixed materials or

types of elements. n this pro"ect, am going to research on advantage and disadvantage of

precast and cast in&situ concrete elements and the problems encountered in designing precast

element.

'recast concrete: precast concrete framed buildings are a factory produced alternative to casting

concrete in&situ. This has several advantages in the effects of the weather. 6tructural

improvement using pre&stressing techniques is also possible. 7n site concrete production plant,

handling equipment, formwork and associated labour are unnecessary, saving considerable

construction costs.

dditionally, semi&skilled labour may be employed for the relatively simple bolted assembly.

1esign flexibility is restricted to the manufacturer-s specifications for span, height, spacing of

units, etc. but most systems incorporate sufficient variables to comply with the relatively regular

structural form expected of this types of construction.

The most common examples of precast the structural frame is in portal construction, although

precast columns, beams, and floor sections may be applied to multi&storey building.

2.2 PRE-FABRICATION OF PRECAST CONCRETE

The application of prefabrication techniques has brought a profound change in the development

of the construction industry worldwide. 6everal advantages are accrued from its use including

less time and reliance on site labour, easier site inspection, as well as greatly improved design

details and quality control. 'refabrication or precast in reinforced concrete involves a shape to

the required form, in which reinforcement is placed and concrete is then cast. The essential


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feature is that the same mould is used many times without any modification such casting is done

either in factory or at a fixed location on the site. The completed elements are finally transported

to the erection area.

period in which construction activity, production etc is reduced and conditions become worse,

construction industry has been under over&stringent pressure to heighten productivity, reduce

costs and upgrades the quality levels of constructed facilities. The need to deliver what the client

demands at the right time to the specified quality and at the affordable cost is ever increasing.

The quality of prefabricated elements is usually higher than that of the constructed in&situ

component. !owever, their site assembly is more sensitive to poor quality control, for examples,

at the "oints when connecting components. s is common to all prefabrication, most of the work

is carried out in the factory, leaving little to be done on site. This increases the likelihood of more

efficient, high quality and faster construction being achieved. s a result of the fewer tasks that

must be undertaken on site, the shorter the overall duration and the more consistent the quality.

shorter production time not only cuts down direct and overhead costs, but also allows the house

to be occupied sooner. The capability of the suppliers to produce these building components must

closely be monitored, and only those who pass the pre&qualification test are permitted to supply

the elements for the standard public housing pro"ects.

This pre&qualification must regularly be updated, hence incorporated in all building contracts.

7nly quality assured building materials and components should be used in standard housing

blocks. 8nlike the traditional methods of construction, these prefabricated components have

higher inherent quality, as well as more accurate profiles and dimensions to that fast track

construction0 will not be at the expense of poor quality. Testing requirements of the products

have to be met before they are supplied. 6ource: http:99www.google.com/

3.3.5 '2#*6T *7%6T28*T7% T#*!%8#6.

n multi&storey buildings, the intersection of structural elements can be achieved in several ways.

1ifficulties may involve the fitting of individual forms, the meeting and fitting of reinforcement

bars and the casting of concrete. When circumstance allows, the number of reinforcement barscan be reduced by using bars of larger bar diameter to reduce congestion. The basic principles to

be borne in mind are as follows:

1o not connect columns at intersections. ;ocate the connection somewhere between the

floor levels.


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do not connect facade beams at the intersection connections should be located at points

within the spans allowing the use of cantilevered section

Beams perpendicular to the facade should not terminate in the place of a column axis.

The beam end should be slightly inside in the inner faces of the columns.

3.3.3 66#+B;< 7) #;#+#%T6.

3.3.3.5 12< 66#+B;<

When concrete rests directly on concrete, the load is transferred mainly by the stiffer

constituents i.e. the pieces of coarse aggregate in the concrete. 1ry ssembly means that no

mortar is used and therefore the "oints are not water proof. 7nly compressive forces can be

transmitted but amount of lateral force can be resisted as a result of friction. The magnitude of

frictional resistance depends on the magnitude of the vertical load. simple pin connection is

sometimes used. 'in is necessary for structures which include inclined elements. The points to be

noted when using pin connections are:

void the use of two pins wherever possible. This trend to restrain the deformation

and to cause cracks.

(routing may be needed to protect a steel pin from corrosion small steel pro"ections

may sometimes be sufficient to act as pins.

6ource: http:99www.google.com/

3.3.3.3 W#T 66#+B;<

=oints of cement9mortar 54&34mm thick can be used between elements to transmit compression

and certain amount of shear. "oint with lapped reinforcement bars is able to transmit both

compressive and tensile forces in addition to some shear. Temporarily, formwork is re&used while

the "oint is cast. 'recaution should be taken to ensure that the water in the concrete does not

evaporate causing the "oint materials to suffer excessive shrinkage. 7ne disadvantage is its

delaying effect on the construction programme. Time must always be allowed for connection to

sail its necessary strength.

6ource: http:99www.google.com/

3.3.3.> T2%6'72TT7% %1 #2#*T7%


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mbrose, =. 5??5/, says, considering the transportation and erection requirements for precast

units involve the following points:

Traffic regulations limit the maximum length, weight, and si$e of individual units.

The reach and capacity of cranes and hoists imposed separate limits.

The load capacity of ;orries and trucks determine the weight and volumes which can be

carried.

When precast units are transported by road, typical maximum dimensions could be as follows:

!eight: @ A @ m.

;ength: 33m.

Transporting large box units by road can cause problem. The width of the box is generally

limited to a maximum of 3.Cm. n a case of housing unit, this restricts the room width of about

3.Dm.

3.3.@ !76T%( %1 #2#*T7% 7% 6T#

There are some mode of transport and erection as follows:

!ori$ontal transport only.

2otational transport, no hori$ontal movement e. g. walls and columns.

!ori$ontal and limited vertical movement.

Eertical and limited hori$ontal movement.

The use of "ack.

3.3.D 1#6(% *7%61#2T7% % '2#)B2*T7%.

ccording to )rederick 2aina 3445/, one of the problems in prefabrication is that, it easily gives

rise to monotony, especially when the units are used in repetitive series. This has lead to

negative perceptions about the design and aesthetic possibilities. To avoid monotony while

maintaining uniformity, especially in the case of box units, new architectural concepts are needed

for both the interior and the exterior. The transmission of load to the support is affected by

several criteria including the magnitude of the load, the span of the units, and the materials used

for supporting the construction for concrete to concrete connection.

When heavy reactions and large rotational forms must be accommodated, other materials like

plastic and rubber are used. 2ubber bearing pads have advantages of economy and little


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maintenance. 8nder compressive load, a rubber pad absorbs irregularities0 hence every part of

the concrete surfaces helps to support the load.

2.3 PRECAST CONCRETE FLOORS

ccording to 7sbourn 1erek 5?CD/, with modern developments in construction technology,

precast beamAslab units are now available with the help of which the floors can be constructed

easily and effective without the aid of any formwork. These precast units are available in about

3Dcm width, various depths, and various spans and can be supported either on walls or on rolled

steel "oists. The sides of each unit are provided with groves to form connecting "oggles for

ad"acent unit. The "oints are grouted with cement mortar using concrete guns, such floors are

economical, light weight, sound proof, and fire proof.

2.4 PRECAST ALLS

ccording to mbrose =. 5??5/, precast walls are most commonly produced by the method of

casting them in flat possible at the site. When the concrete is sufficiently strong, they are lifted

and placed in the desired position. This method is commonly described as tiltAup construction,

referring to its early development, when the walls were cast immediately next to their desired

location and placed with cranes which allow them to be cast away from the actual desired

locations. Wall units may also be factoryAcast, but the practice is more often used and is called

architectural concrete, meaning non&structural units such as those used for curtain wall

construction. critical concern for precast construction is the problem of transporting elements

to the building site.

'recast walls are typically of conventional reinforced construction. Their structural design in

general is sub"ected to the same procedures as those used for cast&in place construction. special

concern is the provisions that are required to permit lifting and handling of the units, whether

casting is done with the wall unit in a flat position. 'icking it up from this position require

development of stress conditions that are not present once the wall is in a vertical position.

2./ PRECAST CONCRETE

With the development of precast concrete constructions techniques, the precast units like floor,

slab, beams or girders, lintels etc. are being successfully manufactured by various factories. The

precast units of the designed depths are used in the floor construction. The members are made of


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such si$es as can conveniently be handled and transported. The advantages and disadvantages of

this type of construction are given as:

3.D.5 1E%T(#6.

The construction is quick.

ts resistance to fire.

ts resistance to sound.

%o time is waste for curing of the floor.

t is economical.

t has good thermal insulation properties.

t does not require formwork in construction.

3.D.3 161E%T(#6

t required fairly uniform spacing of beams throughout the structure which becomes

practically difficult.

n order that the precast members may be able to resist handling, stresses, great care has

to be taken in their construction and design.

There is wastage due to breakages in transportation of the members from factory to the

site of erection.

(ood supervision and skilled labour is required for manufacturing the units.

The forms used for casting the units are normally costly and as such the construction is

economical only when the units are manufactured on a large scale.

2.5 MULTI-STOREY CONSTRUCTION.

ccording to )rederick 2aina, precast components have successful application to building of up

to four storeys. They are suitable for schools, offices and similar commercial buildings where a

simple repetitive framework is acceptable. n order to appearance and to adapt to client

requirements, manufacturers stock a large range of components and accessories.


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2. USE OF PRECASTING.

The technique of pre casting is utili$ed in a variety of ways. 8ndoubtedly, the most widely used

precast elements are the ordinary concrete block A called a *+8 concrete masonry unit/. +ost

structural masonry is made from those units. nother widely used precast element is the tiltAup

wall unit.

This element is sit cast in a hori$ontal position, then tilted up and moved by a crane to its desired

location. The casting bed usually consists of the building floors slab on grade, resulting in a

considerable reduction inform cost. This type of construction is widely used for one storey and

low&rise commercial structures in the southern and western regions of the 8nited 6tates.

6ome of the most widely used pre&stressed elements are the flat A spanning units of hollow core

or tee form used for roof and floor construction.

These units are produced in casting factories in continuous production A processes.

6tructural systems consisting of connected components of precast concrete have been produced

in great variety. 6ome of these have been produced as patented, manufactured systems, but

mostly they have been the single, innovative product of individual designers. 1esign and

construction of precast concrete is strongly influenced by the standards and publication of the

precast concrete institute '*/.

6ource: http:99www.pdf&search&engine.com/

2.; AD7ANTAGES OF PRECAST CONCRETE

There are various reasons for considering the use of precast concrete construction. n some cases,

the choice is between precast and ordinary construction of cast in&situ concrete with elements

formed and cast at the location where they are to be used.

n other cases the choice may be between using precast elements or some other material or type

of construction. The followings are some advantages offered by pre casting generally incomparison to cast &in place construction was said by 7sbourn 1erek 5?CD/:

3.C.5 )6T#2 6T# W72F

*ast in place concrete construction usually proceeds quite slowly, requiring construction of

forms, installation of reinforcing, pouring of concrete, and hardening to sufficient strength to


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permit removal of forms, etc. #rection of precast elements is more akin to construction with steel

or timber structures, and the faster site work may be an advantage where construction time is

highly constrained. !owever, it is total building construction time that is significant not "ust the

time to get up the structure. f time cannot also be gained in other parallel construction activities,

the rapidly erected structure may "ust sit there and wait for the other pro"ect work to catch up.

3.C.3 )72+%( #*7%7+*6

)or the ordinary cast in place concrete structure, a ma"or portion of the total cost is represented

by the forming. This includes the cost of the construction, bracing and support, and removal of

the formwork. 6ome reuse of items may be possible, but the process tends to use materials up

rapidly. 'recast offers more potential for reuse of forms, even with site&cast construction. )actory

processes involve extensive reuse of forms, leading to reduction of on A site labour costs.

3.C.> 8;T< *7%T27;

'recision of detail, quality and uniformity of finishes, and uniformity of concrete properties

colour, density, compaction etc/ may be assured in factory production to a degree not possible

with site casting. !ere it is not "ust pre casting but factory conditions that are the issue. ll of this

is true if the element is standard manufactured, products sub"ect to ongoing quality control its

production. This is obviously of greatest concern for construction elements exposed to view,

especially wall components.

3.C.@ 86# 7) '2#1#6(%#1 #;#+#%T6 %1 6


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be significant. This may be a factor in the use of mixed systems of precast and cast A in A place

elements.

2.< PROBLEMS FACED ITH PRECAST CONCRETE

The following are some ma"or concerns that may be of significance in various situations:

3.?.5 !%1;%( %1 T2%6'72T%(

*oncrete construction is usually heavy0 precast elements are usually of considerable si$e, and the

combination presents a ma"or problem of handling and transporting the heavy and relatively

fragile units. 6tresses induced during handlings and erecting of factory A cast units is usually

feasible only within some reasonable distance from the factory.

3.?.3 *76T

The cost of production, handling, and transporting of precast unit is considerable. There needs to

be some considerable list of other advantages to make this form of construction generally

competitive A not so much with alternative cast A in A place concrete, but with other materials

and systems.

n the end, the total building construction cost is most significant, not "ust structural cost.

.

3.?.@ %T#(2T7%

t refers to the general problem of incorporating the precast concrete elements into the general

building construction. ma"or problem to be dealt with in this regard is the loss of some

opportunities that are present with other types of construction. nstallation of hidden items such

as wiring, piping, ducts, and housings for light switches, power out less, recessed lighting

fixtures, bath room medicine cabinets, fire hose cabinets and exist signed is made some more

difficult. With light wood frames, ordinary cast A in A place concrete, and most other types of construction precise locations of these items and the actual installation can be done during site

construction work. With precast concrete construction, provisions must be made in advance A a

procedure that is not impossible, but that does not fit with the routine operations with which most

designers and builders are familiar.


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3.?.D +J#1 64 K compared. With

conventional structural frames systems, !ybrid concrete *onstruction produces simple buildable

and competitive structures. The clients given better value and the contractor benefits from

increased off A site component manufacture, safer and faster construction and consistent

performance.

2.11 THE BENEFITS OF USING HYBRID CONCRETE CONSTRUCTION.

!ybrid *oncrete *onstruction produces simple, buildable and competitive structures. t deliversincreased prefabrication, faster construction and consistent performance. !** can achieve very

significant cost savings and can satisfy the requirements of clients most demanding of clients.

6ource: #mmitt 6tephen and (orse *hristopher 344D/

3.55.5 *76T


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lthough structural frame of a building represents about 54K of the total construction cost, the

choice of material for the frame has dramatic consequences for subsequent processes. 8ntil

recently cast in situ concrete was commonly viewed. s the most economic frame material, but

hybrid construction can offer greater, 6peed, quality and overall economy. lthough the

elemental cost of hybrid concrete )rame may be higher, total pro"ects costs are usually lower due

to time savings on site. )aster programme mean an earlier return on investment, lower interest

charges and reduced construction preliminaries.

3.55.3 6'##1

6peed of construction depends on designs which are easy to procure and construct. #ssentially

!** takes a proportion of work away from the site and into the factory, reducing the duration of

operations critical to the building programme on site.

The precast process takes place in a controlled environment, unaffected by weather. 2igorous

inspection before installation removes cause of delay on site. 6ome !** techniques can reduce

or eliminate follow Aon trades, e.g. installing ceilings and finishes. This enables even faster

programme times but requires greater co&ordination and care in detailing and protection on site.

3.55.> B8;1B;T< %1 *7%6T28*T7%.

The key advantage of !** is its buildability. Because precast and cast in A situ *oncrete are

used where most appropriate, construction becomes relatively simple and logical. The use of

!** encourages design and construction decisions to be resolved at design stage. This means,

for example, that precast elements can be manufactured, stored at the factory and delivered G"ust

A in time- to site0 they can be lifted from delivery truck to final position in a single crane

movement, eliminating the need for site storage and reducing crane book time. Traditional

formwork typically accounts for @4K of cast in A situ frame costs. t is dependent on weather and

labour. The use of !** means that percentage of the frame is manufactured by a skilled

workforce in a weatherproof factory, resulting in faster construction and other better quality.

3.55.@ 6)#T


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platforms. 7n a generally less cluttered site, if precast spandrel beams are used they can provide

immediate edge protection.

3.55.D 686T%B;T<

!** also offers the opportunity to exploit the inherent thermal mass of concrete by exposing the

soffit of precast concrete floor slabs. This fabric energy storage )#6/ of the structure can help to

control temperatures in the context of a naturally ventilated ;ow A energy building. The finish

and shape of expose precast concrete units can also be used to help with the even distribution of

lighting and to reduce noise levels than structured from other materials. )or all buildings the

operational energy *onsumption is far more significant than during *onstruction, but concrete

buildings utili$ing thermal mass can reduce this impact on the environment by minimi$ing the

need for aid A conditioning.

3.55.L 7T!#2 B#%#)T6

*oncrete is a uniquely versatile material. t is a cast material of high strength which can be

shaped to product a vast variety of structural elements.

*oncrete structures are stable, robust, fire resistant and adaptable, as well as, solid, quiet and

essentially vibration A free. The composition of concrete can be varied to produce a variety of

colour, textures and finishes, some of which closely match natural stone.

Both the structural and the aesthetic advantages of concrete can be incorporated into the design

of an !** structure. The structural versatility of concrete can be developed with !**. ;ong

spans can be achieved by using large units, or by preAstressing or post A tensioning. 'recast units

can be also be Mwelded- together.

2.12 HYBRID CONCRETE CONSTRUCTION can provide/:

faster construction

mproved safety on site.

*ost effective construction.

6imple, buildable structures.

#xcellent fire performance.

#xceptional acoustic performance.


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3.53.5 1E%T(#6

'recast quality finish for walls and soffits.

%o formwork for vertical structure.

6tructural connection between wall and slabs is by standard reinforced concrete detail:

inherently robust and, for basement-s Mwatertight-

%o permanent sealing at connections between precast units.

)lexible for casting A in items.

3.53.3 161E%T(#6

'ropping of precast required prior to sufficient strength gain of inA situ *oncrete.

The smaller dimension of the precast units can be a maximum of 3.Cm, so "oints in walls and

soffits must be dealt with: expressed or concealed.

2educed flexibility of layout as there are walls rather than columns. 'roponents of this

system claim.

Twice as fast as inAsitu concrete.

*ost neutral compared to in A situ and cheaper than full precast.

2.13 PRECAST COLUMNS AND FLOOR UNITS ITH CAST IN-SITU BEAMS.

3.5>.5 dvantages:

Eertical structure can be erected quickly, no formwork required.

'recast quality finish for columns and soffits.


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6tructural connection between precast elements is via standard reinforced concrete.

3.5>.3 1isadvantages:

'recast flooring must be temporarily propped.

6ealing between precast units is required.

2.14 CAST IN -SITU COLUMNS AND BEAMS ITH PRECAST FLOOR UNITS.

2.14.1 A!8't'9)+:

'recast floor structure can be erected quickly.

'recast quality finish for soffits.

n A situ can account for site irregularities.

2.14.2 D+'!8't'9):

'recast flooring must be temporarily propped.

6ealing between precast units is required.

2.1/ CAST IN-SITU COLUMNS AND FLOOR TOPPING ITH PRECAST BEAMS

AND FLOOR UNITS.

2.1/.1 A!8't'9)+:

'recast flooring can be erected quickly.

'recast beams support precast floor planks minimi$ing floor propping.

'recast quality finish for soffits.

)ormwork for in A situ columns can be used to prop precast beams.

6tructural connection between precast elements is via standard reinforced *oncrete.

n A situ topping to beam permits beams to be continuous over columns.

2.1/.2 D+'!8't'9)

1own stand beams need to be co&ordinate with services distribution.


18/27

'roponents of this system claim.

Twice as fast as conventional in A situ.

H"(! C#$()t)

Was also used to create a structure which allows full continuity to occur between the vertical and

hori$ontal structural elements, thus providing a stiff away framework. The combination of

elements allowed the whole frame to act as a composite structure without relying on expensive

mechanical fixings. This method of construction produces a rigid frame which is inherently

stable without the need for shear walls or bracing. !** was the natural choice of material. t

fulfilled the design criteria for a visible expression of the structure0 behind the delicate gla$ed

facades the precast column and beam structure is clearly visible, needing no further treatment

such as cladding for fire protection. n addition, by exposing the painted soffits of the concrete

floor slabs in the offices, the ventilation strategy could exploit the fabric energy storage potential

of the concrete construction.

The hybrid concrete structure consists of @>4 mm diameter precast columns and precast floor

units connected together by means of cast in A situ concrete spine beams.

#ach floor unit takes the form of a double T A section with end plates to each trough. t each

column connection the end plates are cast with a curved Mcut A out to follow part of the column

profile.

7nce the precast columns were fixed on site, the double T& section floor units were connected to

the, positioned so that the curved edge profiles trimmed the outer edge of the columns. The cast

in situ concrete spine beam was then cast between two rows of end plates stitching lower and

upper columns and ad"acent units together. Between the longitudinal "oints, loop connectors were

cast in to the units and continuous cast in A situ beam "oin the units together. The floor units are

self A finished and no screed or topping was required.

t the perimeter the same principle was used with a slightly different detail. The spine edge

beam was cast between the final rows of end plates which ran up to the inner side of each

perimeter column/ on one side and a special precast perimeter unit on the other side, which

creates a tapered edge to the ceiling soffits. The perimeter unit has a row of precast holes which

allow warm buoyant air rising up the faNade to be effectively captured and cooled by the passive

chilled beam elements above return air paths to the central atrium. The precast perimeter units

were cast with a sculpted feature where they meet the column head. They were also used at the


19/27

atrium and core perimeters cast in the same moulds with minor adaptations. 6ource:

http:99www.pdf&search&engine.com/

2.15 HAT MAKES PRECAST CONCRETE DIFFERENT FROM OTHER FORMS OF

CONCRETE CONSTRUCTION

The most obvious definition for precast concrete is that it is concrete which has been prepared

for casting, cast and cured in a location which is not its final destination. The distance travelled

from the casting site may only be a few meters, where on A site pre casting methods are used to

avoid expensive haulage, or may be thousands of kilometers, in the case of high A value added

products.

This is capable of splitting both elements unless the section is suitably reinforced. )lexural

rotations of the suspended element reduces the mating length bearing length/ creating a stress

concentration until local crushing at the top of the pillar the column/ occurs, unless a bearing

pad is used to prevent the stress concentration forming if the bearing is narrow, dispersal of

stress from the interior to the exterior of the pillar causes lateral tensile strain, leading to bursting

of the concrete at some distance below the bearing unless the section is suitable reinforced.

6ource: #mmitt 6tephen and (orse *hristopher 344D/.

2.1 PRECAST CONCRETE STRUCTURES.

precast concrete structure is an assemblage of precast elements which, when suitably

connected together, form a >1 framework capable of resisting gravitation and wind or even

earthquake/ loads. The framework is ideally suited to buildings such as offices, retail units, car

parks, schools, stadia and other such buildings requiring minimal internal obstruction and multi&

functional leasable space. The quantity of concrete in a precast framework is less than @ percent

of the gross volume of the building, and of this is in the floors. The precast concrete elements are

columns, beams, floor slabs, staircases and diagonal bracing.

6ource: )rederick 2aina 3445/

2.1; CAST-IN PLACE CONCRETE

+ost concretes for buildings are cast&in&place, that is, the wet mix is deposited and formed at the

place where the finished concrete is desired. This is now general referred to as site cast concrete,

since the location is usually at a building site. This is compared to precast concrete, which refers

to the process of casting elements and then moving them to the place they are to be used.


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*oncrete for site cast construction is typically brought to the site by the familiar concrete A

transporting mixer trucks with the large rotating barrels. The mix is prepared at a central

batching plant, where controls of the materials may be carefully monitored. !owever, the

transporting to the site, proper mixing in truck, discharging from the truck and depositing in the

forms, and handling for placement, finishing, and curing are all sub"ect to the level of

responsibility and craft exercised by the people involved. 6ite conditions in terms of accessibility

and weather can be highly critical to the work, requiring extreme measure in some situations to

control all the stages in the production process. This research is not about construction processes

or their management, but some awareness of the issues and limitations is helpful in developing

reasonable designs for concrete structures to control all the stages in the production process.

6ource: )rederick 2aina 3445/

3.5? FORMING =FORMORK>

n inherent property of concrete is that it may be made in any shape. The wet mixture is placed

in forms constructed of wood, metal, or other suitable material in which it hardens or sets. The

forms must be put together with quality workmanship, holding to close dimensional tolerances.

)ormwork should be strong enough to support the weight of the concrete and rigid enough to

maintain position and shape. n addition, formwork should be tight enough to prevent the

seepage of water and designed to permit ready removal. Timber used for forms is usually

surfaced on the side that comes in contact with the concrete, and frequently is oiled or otherwise

sealed. This fills the pores of the wood, reduces absorption of water from the concrete mixture,

produces smoother concrete surfaces, and permits the form boards to be more easily removed.

6teel formworks have advantage of being more substantial if they are to be reused. 6teel gives

smoother surfaces to the concrete, although it is almost impossible to avoid showing the "oints.

)or ribbed floors, metal pans and domes are used extensively, and columns, circular in cross

section, are invariably made with metal forms. Because the formwork for a concrete structure

constitutes a considerable item in the cost of the completed structure, particular care should beexercised in its design. t is desirable to maintain a repetition of identical units so that the forms

may be removed and reused at other locations with a minimum amount of labour.

There are no exact rules concerning the length of time the forms should remain in place.

7bviously they should not be removed until the concrete is strong enough to support its own

weight in addition to any loads that may be placed on it. lso, too early removal of forms


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introduces the possibility of excessive deflections. 6ometimes the side forms of beams are

removed before the bottom forms. When this is done, posts or shoring are placed under the

bottoms of the members to give additional support. This is called reposting or restoring. The

minimum period during which forms must remain in place before stripping is usually governed

by the local building code. 6ource: http:99www.pdf&search&engine.com/

2.20 PLACING AND FINISHING.

s soon as cement is mixed with water, a chemical action begins that eventually results in the

hardening of the concrete. The first stage of this is the wet mix, which has the character of at

thick, viscous fluid. n a short time, however, an initial hardening called set/ occurs, and the

fluid nature of the mix fades. Before the initial set occurs, the concrete must be fully placed in

the forms A a matter of only a few hours with ordinary mixes. dd up the time for loading the

trucks at the batching plant, driving the trucks to the site, emptying the trucks and moving the

mixed concrete to its desired location, and hand ling it to fill the forms and there is not much

time for leisure in the operation.

1espite the haste required, the wet concrete must be carefully handled so as not to cause

segregation, which consists of the partial separation of the ingredients. This can occur if the

concrete sits idly in its wet state, is dropped too far when deposited, or is moved around too

much in the forms before settling into place. This is a situation for careful supervision, but is

mostly up to the skills and care of the workers involved.

6urfaces of the concrete in contact with forms will derive a primary form and finish from the

surfaces of the forms. 8nformed surfaces usually the top/ may receive various treatments, the

simplest being a simple struck surface, produced by smoothing with a board or rough wood tool.

This is essentially an unfinished surface, which may be additionally treated during the initial

hardening or later. 6ource: http:99www.google.com/

2.21 CURING AND PROTECTION

*uring is the method of keeping concrete units damp to prevent excessive drying.2egardless of the care taken in proportioning , mixing, and placing, first& quality concrete can be

obtained only when due consideration and provision are made for curing. The hardening of

concrete is due to the chemical reaction between the water and cement. This hardening continues

indefinitely as moisture is present and the temperatures are favorable. The initial set does not

begin until two or three hours after the concrete has been mixed.

http://www.google.com/http://www.google.com/


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+oisture conditions can be maintained by spreading wet coverings of mats, waterproof paper or

plastic sheets over concrete, by spraying liquid curing compound on the surface of fresh

concrete, and by leaving the concrete in forms longer.

2amsey and 6leeper 5?C@/, identifies two physical conditions which profoundly affect the final

compressive strength and curing of concrete: Gthe temperature and the rate at which water used

in mixing is allowed to leave the concreteH

The optimum temperature for curing concrete is 33.C . ny great variance from this mark

reduces its compressive strength.

2.22 DESIGN AND PRODUCTION CONTROLS.

ccording to =ohn Wiley O 6ons, nc. 5??5/, 6tructural designers ordinarily document their

work in the form of a set of written computations. These computations will include a listing of

design criteria: codes and standards used, concrete strength and steel type used, design loadings

assumed, and so on. The computations are concluded by a listing of the design decision

information: required shape and dimensions of concrete elements and the positions, number, and

si$e of reinforcing bars.

n most cases some sketches are used in the computations, to indicate the arrangement of

reinforcing in member cross sections and the locations of bar cut&offs, extensions, bed points,

and so on. 6tructural computations are not ordinarily used to transmit information to the builder.

)or this purpose, it is ordinarily the practice to produce a set of contract documents: working

drawings and specifications.

2.23 MANUFACTURING ? E@TRACTION PROCESSES.

#xtraction 'rocesses.

sorting various grades of aggregate

#xtracting greywacke and argillite of the !utt hills.


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n A situ is most commonly a mixture of aggregate known as builder-s mix/ blended with

cement and clean water that is free of oils, acid etc.

ccording to 1enis Walton 5??D/, cement is a combination of limestone and silica, which is

found in some types of clay. lumina, iron oxide and magnesia are present in small quantities.

*ement is stored in a dry place such as small lockable shed.

n Willington it is sourced from quarries aggregate meanwhile is not commonly sand, gravel and

crushed stone and constitutes along the Western !utt hills and in !gaurange, where (reywacke

sandstone/ and argillite mudstone/ are sorted as aggregates and coarse sand before being

blended as builder-s mix.

3.3>.5 W!#2# +T#2;6 2# )78%1:

L%): ;ime is found throughout %ew Iealand, high quality lime from Te Fuiti and %elson is

processed for export. crystalli$ed form of lime is mined in %elson. t is used as filler and in

building construction.

S$': lso a form of sand, found in %orthland, %orth uckland and *anterbury

A%': is extracted from cola ash and oil deposits.

G"&+%: (ypsum has been buried by the accumulation of slit and dust millions of years ago.

There is no gypsum in %ew Iealand, we import ours from ustralia

3.3@ MATERIAL PROPERTIES

n&situ concrete is strong, durable, stable, readily available and relatively economic in terms of

construction and life time maintenance. t is the ideal structural material in building sites that

have difficult access.

7ther qualities that make it an ideal construction medium include:

The ability to control form and shape.

The enclosure of space and structure in one material.

The ability to form integral surface finishes.

ts compatibility with most other materials.

nd excellent acoustic and fire resistant properties.

lso admixtures and other materials can be added to serve various functions ranging from free$e

proofing to creating translucency. t should be noted that the quality of such constituents has a

considerable effect on material properties.


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2.24.1 FORMAT FINISH OF MATERIALS.

There is a range of concrete formats and finishes available to the designer 9 architect. )or a

relatively smooth but open finish, it is first necessary to screed make level/ the concrete once

poured and compacted, then Mfloat- the surface using any one of a rage of floating devices. !ence

the term Mfloated finish.- they typically have up A turned blades sitting parallel to the surface that

flatten, at high velocity, any exposed aggregate.

)or a perfectly smooth closed finish, trowelling is necessary well after the floating process, a

power trowelling machine has sharper blades that Mclose- the concrete surface. honed surface,

like the closed close surface, is perfectly smooth, but is sectioned at a depth whereby the

aggregate is visible and forms part of the surface itself.

7ther finishes are possible including the rough exposed aggregate look, which creates a non A

slip finish. !ere the top layer of cement paste has been removed to reveal the aggregate. This is

commonly achieved using bristle broom followed by water to wash off this top layer once the

concrete has firmed sufficiently.

F#(%#(

s also a popular medium in which to shape, or mould a desired surface in order to create a

patterned or stamped finish. 2ough timber boxing for instance will leave an imprint on your

finished surface. Brushing the concrete while curing the other options, while glass embedded in

the concrete itself will give the material a translucent quality.

1ifferent concrete finishes can be achieved by:

brasive processes.

*hemical processes.

+echanical processes.

*ombination processes.

C##(

s another way in which to alter finish. *oloured concrete is achieved by mixing a powered or

liquid pigment into the concrete at the time of batching. This has the advantage of providing a

consistent colour finish. )or example ;iquid *olour is a high solid liquid oxide dispersant,


25/27

produced from synthetic iron oxides available, which is normally added to ready mixed concrete

at a rate D K of the cement content in the mix.

S&('" C#$()t)

1iffers from conventional cast A in A place concrete with regard to the reinforcement and

application method and surface, resulting in a considerable range of

finishes. 6pray concrete-s flexibility in applied with compressed air added at the no$$le via a

separate hose. 6ource: http:99www.pdf&search&engine.com/

2.2/ COMMON FI@ING METHODS.

n&situ concrete is only as good as the formwork, which must cope with dynamic loading during

the placement and consolidation phases of work. 6huttering can bulge, settle and lean, all of

which have the potential to induce small variations in the structure. n addition, the concrete will

shrink as it dries. 1etailing must therefore be considered there and allow adequate tolerances.

The relative cost can be reduced significantly when the formwork is able to be re&used. !owever,

a repetitive use on the same pro"ect suggests the need for *onstruction "oints occur between

different placements of concrete.

2.25 DURABILITY AND MAINTENANCE REQUIREMENTS.

n&situ concrete is a robust material which does not require much maintenance. ts strength is

determined by three main factors0 being the quality of cement and aggregates used, the ratio of

dry Mingredients- to water during mixing, and subsequent exposure to water during the curing

process. *oncrete gains about P4 percent strength after seven days curing due to the hydration of

the tricalcium aluminates and silicates but needs a further three weeks for complete strength to

be reali$ed.

!ydration occurs when water is mixed with cement causing a hardening of the cement paste.1uring this time it will envelope and lock in position around such reinforcing as deformed bars

wired to a grid of mesh. !owever the concrete won-t reach full strength until after 3C days

curing.

*oncrete-s required strength depends on its purpose: mowing strips can be 5D+'a but any

concrete exposed to sea A spray must have 3D+'a.


26/27

t should also be noted that, admixtures any substance that is added before or during the mixing

of the concrete/ can act to reduce or augment the potential strength of the concrete mix. ;ong

term durability is compromised if the mixing, pouring, compacting and curing are not handled

carefully.

3.3L.5 W#T!#2 *7%1T7%6.

1ry but mild conditions are ideal as the presence of water is necessary to continue the chemical

reaction and increase strength. !ot days will cause the concrete-s superficial water content to

evaporate too quickly, leaving a cracked surface, while rain damage may cause the concrete to

dust after curing. Too higher water content produces a concrete that is more porous and

considerably weaker. 6ource: http:99www.pdf&search&engine.com/

3.3L.3 6T2#%(T!.

*oncrete is stronger is compression than in tension. When combined with steel mesh or rebar,

the tensile and flexural strength increases and plain concrete becomes reinforced concrete0 a

versatile and extremely strong structural material commonly used in landscape construction.

6ource: http:99www.pdf&search&engine.com/

3.3P ;)# A *


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t0pic comparing the use of precast and c

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