t0pic comparing the use of precast and c
TRANSCRIPT
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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$ing floor propping.
'recast quality finish for soffits.
)ormwork for in A situ columns can be used to prop precast beams.
6tructural connection between precast elements is via standard reinforced *oncrete.
n A situ topping to beam permits beams to be continuous over columns.
2.1/.2 D+'!8't'9)
1own stand beams need to be co&ordinate with services distribution.
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'roponents of this system claim.
Twice as fast as conventional in A situ.
H"(! C#$()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
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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,
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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.
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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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