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8/3/2019 Introduction w
1/20
BE 2101- structures II Alternative Slab Construction Techniques
Department of Building Economics i
Table of Contents
1.0 Introduction ........................................................................................................................................... 11.1 An Introduction to Slab Systems ......................................................................................................... 1
2.0 Hollow Core Slab System ..................................................................................................................... 2
2.1 Methodology and Installation process ................................................................................................. 2
2.2 Advantages and Applicability of the System ...................................................................................... 3
2.3 facilities of the system ......................................................................................................................... 3
3.0 Suspended beam Slab System .............................................................................................................. 5
3.1 Methodology in constructing SBS system .......................................................................................... 53.2 Advantages and applicability of the system ........................................................................................ 6
3.3 Installation of Services ........................................................................................................................ 6
4.0 Easy Slab Pre-stressed concrete floor system ..................................................................................... 7
4.1 Methodology and installation process ................................................................................................. 7
4.2 Advantages and applicability of the system ........................................................................................ 7
5.0 Bubble-deck slab System ...................................................................................................................... 9
5.1 Technology used ion bubble deck slab system ................................................................................... 9
5.2 Properties and applicability of the system ........................................................................................... 9
5.3 advantages of Bubble slab system ..................................................................................................... 10
6.0 Construction joints in slab construction ........................................................................................... 11
7.0 Other finishing qualities in slabs ....................................................................................................... 12
8.0 Cost details ........................................................................................................................................... 13
8.1Cost details of Concrete mixes used................................................................................................... 13
8.2 Cost details of slab system components used. ................................................................................... 14
9.0 Conclusion and recommendations ..................................................................................................... 15
References .................................................................................................................................................. 16
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Table of figures
Figure 1: Hollow Cores lifted for installation ............................................................................................... 2Figure 2: Ware houses used hollow cores ..................................................................................................... 3
Figure 3: Security walls ................................................................................................................................ 3
Figure 4:installation of services .................................................................................................................... 4
Figure 5: Cross Section of a hollow Core ..................................................................................................... 4
Figure 6: Cross Section of SBS system ........................................................................................................ 5
Figure 7: Cross Section of double beam ...................................................................................................... 6
Figure 8: installed concrete soffit planks for easy slab system ..................................................................... 8
Figure 9: Bubble slab system ........................................................................................................................ 9
Figure 10: Cross section of bubble slab system ............................................................................................ 9
Figure 11: doweled post tensioned slab construction ................................................................................. 11
Figure 12: honed finish ............................................................................................................................... 12
Figure 13: Embeded Glass .......................................................................................................................... 12
Figure 14: exposed aggregate finish ........................................................................................................... 12
Figure 15: coloured abrasive finish ............................................................................................................. 12
http://f/Introduction%20w.docx%23_Toc305246431http://f/Introduction%20w.docx%23_Toc305246432http://f/Introduction%20w.docx%23_Toc305246432http://f/Introduction%20w.docx%23_Toc3052464318/3/2019 Introduction w
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BE 2101- structures II Alternative Slab Construction Techniques
Department of Building Economics iii
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Department of Building Economics iv
Tables
Table 1:Cost details on Concrete mixes ...................................................................................................... 13
Table 2: Cost details slab construction components ................................................................................... 14
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1.0 Introduction
It is true for the saying Concrete is most consuming material by human beings. It is so famous from its
different variations as it stands in present. New methods have being introduced by engineers from their
mathematical knowledge and calculations. These methods are so introduced to get the optimum use from
concrete.
In building and also in various infrastructural elements concrete is widely use. Main elements of a
building where concrete is used can be highlighted as foundations, columns, beams and slabs. All these
components of the building has to be designed to considering various criterias. Properties of concrete
have to be correctly understood before designing any structural element. Anyway these structural
elements have to forgo compressive forces, tensile forces, shear forces and forces formed due to
shrinkage and expansion. Mainly steel is used as a supportive material (as reinforcement) for concrete to
bear up these loads. Calculations must be properly done when designing these elements to avoid failures
and dangerous consequences that can occur in future.
Slabs are also designed to bear up loads which stands on top of it. Today the demand is high and quick
systems are launched. Systems like suspended beam slab system, easy slab system, Hollow Core system
are the most popular in Sri Lanka. Here it is mainly focused on these alternative slab construction
methodologies, their suitability, applicability in real life and Cost details for readersreferance.
1.1 An Introduction to Slab Systems
When designing any slab system following features are common. Even a new system being introduced
designers and engineers has to keep these features intact in order to achieve basic requirements of a slab.
Those features are base and sub base materials, preparation requirements,and vapor retarder( if required),
Concrete thickness, Concrete compressive strength, flexural strength, or both, Concrete mixture
proportion requirements, Joint locations and details, Reinforcement (type, size, and location)( if
required) Surface treatment, (if required), Surface finish, Tolerances (base, sub base, slab thickness, and
surface), Concrete curing, Joint filling material and installation, Special embedments andPreconstruction
meeting, quality assurance andquality control.
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2.0 Hollow Core Slab System
2.1 Methodology and Installation process
Installation is undertaken by highly-trained teams, eachof which can lay slabs at a rate of up to 600m a
day. A further advantage is that at no stage during the installation process is propping a requirement, andbrickwork can commence 48 hours after grouting. Cairns adds that slab soffits are so smooth there is no
need for plastering prior to painting, which is executed using dense-textured paint. Provision is also
made for down-lighting; service holes of up to 75mm in diameter can be cut on site through the hollow
sections and, when required, larger holes can be factory formed, subject to a maximum of 560mm.
Larger holes can be accommodated but require alternative designs and strengthening. The Concrete grade
used in this system is 40 to achieve the required strength of 1860 N/mm2. After the Hollow core has been
installed in a proper manner 2 inches of in-situ or ready-mix concrete is laid if further strength and
finishing quality is required.(Concrete Manufactures Association, 2002)
Figure 1: Hollow Cores lifted for installation
Source: Concrete Manufactures Association, 2002
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2.2 Advantages and Applicability of the System
Hollow core slab system is widely used in speed erection instances. It is more suitable for highly
spanning buildings above 5 m span. These hollow cores can be transported to the erection site from the
precast site and can fixed by a crane without any supports such as props. This is a great advantage from
the part of the client. Client can go for the erection in a minimal time mainly due the reduction in curing
time at the erection site.
This type of system can be used in ware houses and store rooms where storing is done to the full height.
As an example the store room can be steel-framed and supported on piledfoundations. Precast, pre-
stressed panel can be slotted into the webs of 6m steel columns. As very high forces are to be bared by
columns the finishing could be given to the hollow core itself. Here it is visible that erection is twice fast,
twice productive and half the cost is saved.
Hollow Core slabs could also be used as Security walls. These wallsare constructed with slabs measuring
4m x 1.2m. Each wall topped 3m, with the additional one metre section sunk into a foundation of
soilcrete, a mixture of compacted gravel and cement.
Figure 2: Ware houses used hollow cores Figure 3: Security walls
Source: Concrete Manufactures Association, 2002
2.3 facilities of the system
Here the facilities such as electric lights are send through the hollow core and the downward hollows can
be made so that lights could be fixed. The ancillary items can be stored inside the hollow cores and if the
downward hollows are large then those can be filled with grouting material.
Refer Cost details topic 8.0
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Figure 4:installation of services
Source: Concrete Manufactures Association, 2002
Figure 5: Cross Section of a hollow Core
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3.0 Suspended beam Slab System
3.1 Methodology in constructing SBS system
In this system four components are included. They are pre-stressed beam, Concrete masonry soffit block,
Distributed reinforcement GI weld mesh 3mm dia. 5050mm squares and structural concrete topping.
Pre-stressed beam is designed to bare a greater load than other members. Therefore it is laid with 40
grade concrete. High tensile wires are used as reinforcement in these wires. If 4mm is used then it should
include 6 wires and if 5mm is used it should include 4 wires. This shows that it is highly designed to
withstand high loads and it is clearly mentioned that workers must work standing on these beams not
other members.
Concrete masonry soffit block includes 3 voids to facilitate services and because this system is introducedto reduce weight of the slab while giving stability grade 10 concrete is used in these soffit blocks. This is
weakest part in the entire system and workers are adviced that more than two workers are not allowed on
these blocks.
Wire mesh is just laid on top of the soffit block to give added strength to the structural topping. Wire
mesh is consisting of 4mm wires welded to 5050 mm squares.
Structural Concrete topping includes grade 20 concrete laid more or less to give a finish to the surface.
The strength of topping cannot be strengthened further that lower structural components will fail.
Following Cross section shows the arrangement of the suspended beam slab system.
Figure 6: Cross Section of SBS system
Source : International Construction Consortium hand bills
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If the beam span is designed to be 5.0 m then to bare the loads and to be in stability double beam is used
as shown in the diagram below
Figure 7: Cross Section of double beam
Source : International Construction Consortium hand bills
3.2 Advantages and applicability of the system
The main feature of the system is the cost reducing effect. It is said that it reduces Cost from a traditional
in-situ concrete by 30%. This system does not require much of skilled laborers to carry out the work. No
formworks are necessary for the slab and the soffit blocks once it is placed and offers immediate working
platform for other works. On the other hand compaction becomes much more easier as the platform is
made more to a curved nature. Pre-cast elements are readily used which speeds up the process. Site
conditions are not necessary for these erections, construction could be carried out even in adverse weather
conditions
This system is more or less suitable for house hold constructions which are not spanning over 5.0 m. This
gives a great advantage to the client by saving time. The client can gain advantages for quick investment
or by giving it for even when the construction process is underway, If high loads are to bared then there is
a choice of going for double beams which will carry high loads which are impartedfrom above.
3.3 Installation of Services
Here, in this method soffit block contain 3 voids where it can be used to supply electric ducts and in some
occasions small water pipes also. The lighting can be managed by drilling through the soffit block and
fitting wiring components to it. Here the advantage is that the ease of managing ducts. In traditional
System the ducts must be laid and installed before the concrete is poured and as well as there can be
contingencies where it cannot be repairer easily. In this system most consequences are minimized. And
installations can be carried out even before the construction is laid.
Refer Cost details topic 8.0
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4.0 Easy Slab Pre-stressed concrete floor system
4.1 Methodology and installation process
In this system four components are included as by the SBS slab system.Only difference here is that soffitblock is replaced by a soffit plank. The four components are pre-stressed beam, Concrete masonry soffit
plank, Distributed reinforcement GI weld mesh 3mm dia. 5050mm squares and structural concrete
topping.
Pre-stressed beam is designed to bare a greater load than other members. Therefore it is laid with 40
grade concrete. High tensile wires are used as reinforcement in these wires. If 4mm is used then it should
include 6 wires and if 5mm is used it should include 4 wires. This shows that it is highly designed to
withstand high loads and it is clearly mentioned that workers must work standing on these beams not
other members.
Concrete masonry soffit plank includes 4mm tensiled wired reinforcement. The plank is of curved nature
specially designed to reduce weights of the structure when compared with the SBS slab system. Here
workers are free to move because grade 20 concrete is used in these planks. Curved nature of planks will
support the installation of services and also architectural features could be achieved by introducing a
ceiling finish
Wire mesh is just laid on top of the soffit block to give added strength to the structural topping. Wire
mesh is consisting of 4mm wires welded to 5050 mm squares as by the SBS slab System.
Structural Concrete topping includes grade 20 concrete laid more or less to give a finish to the surface.
The strength of topping cannot be strengthened further that lower structural components will fail.
Following Cross section shows the arrangement of the suspended beam slab system.
4.2 Advantages and applicability of the system
The advantages and applicability of easy slab system is same as the SBS slab system exept that it reduces
weight of total slab.
Refer Cost details topic 8.0
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Figure 8: installed concrete soffit planks for easy slab system
Source : International Construction Consortium hand bills
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5.0 Bubble-deck slab System
5.1 Technology used ion bubble deck slab system
Bubble-deck technology locks ellipsoids between the top and bottom reinforcement meshes, thereby
creating a natural cell structure, acting like a solid slab. For the first time a voided biaxial slab is created
with the same capabilities as a solid slab, but with considerably less weight due to the elimination of
superfluous concrete.
5.2 Properties and applicability of the system
For a Bubble-deck slab, the shear resistance is proportional to the amount of concrete, as the special
geometry shaped by the ellipsoidal voids acts like the famous Roman arch, hence enabling all concrete to
be effective. It is important that this is only valid when considering the Bubble-deck technology. Other
types of voided biaxial slabs have reduced resistances towards shear, local punching and fire. In practice,
the reduced shear resistance will not lead to problems, as balls are simply left out where the shear is high,
at columns and walls. As a Bubble-deck slab acts like a solid slab, the fire resistance is just a matter of the
amount of concrete layer. The fire resistance is dependent on the temperature in the rebars and hence the
transport of heat. As the top and bottom of the Bubble-deck slab is solid, and the rebars are placed in this
Figure 9: Bubble slab system
Figure 10: Cross section of bubble slab system
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solid part, the fire resistance can be designed according to demands. There exits different approaches and
opinions of design methods - but some general guiding principles are:
5.3 advantages of Bubble slab system
1. Saves 35% weight compared to a corresponding solid slabequal stiffness2. Simple, monolithic behaviour, uniform and continuous distribution of Forces3. Max ductile structure - increased ductility due to increased strength/weight ratio
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6.0 Construction joints in slab construction
Usage of different construction joints can be regarded as alternative to the traditional system of
construction. Construction joints are placed in a slab to define the extent of the individual concrete
placements, generally in conformity with a predetermined joint layout. If concreting is ever interrupted
long enough for the placed concrete to harden, a construction joint should be used .If possible,
construction joints should be located 5 ft (1.5 m) or more from any other joint to which they are parallel.
There are factors that are normally considered when selecting spacing of contraction joints. Those are
Method of slab design (ACI 360R), Thickness of slab, Type, amount, and location of reinforcement,
Shrinkage potential of the concrete (cement type andquantity; aggregate size, quantity, and quality; w/cm;
type of admixtures; and concrete temperature), Base friction, Floor slab restraints, Layout of foundations,
racks, pits, equipment pads, trenches, and similar floor discontinuities, Environmental factors such as
temperature, wind, and humidity and Methods and quality of concrete curing. There are construction
joints like doweled, armourd when it comes to construction.(Nawy,1992)
Figure 11: doweled post tensioned slab construction
Source Nawy,1992
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7.0 Other finishing qualities in slabs
Apart from the different slab systems mentioned above finishing of slabs also considered as a different
slab construction methodology. For a relatively smooth but open finish, it is first necessary to screed
(make level) the concrete once poured and compacted, then float the surface using any one of a range of
floating devices. Hence the term 'floated finish.' They typically have up-turned blades sitting parallel to
the surface that flatten, at high velocity, any exposed aggregate.
For a perfectly smooth closed finish, toweling is necessary well after the floating process. A power
toweling machine has sharper blades that close the concrete surface. A honed surface, like the closed
surface, is perfectly smooth, but is sectioned at a depth whereby the aggregate is visible and forms part of
the surface itself.
Other finishes are possible including the rough exposed aggregate look, which creates a non-slip finish.
Here the top layer of cement paste has been removed to reveal the aggregate. This is commonly achieved
using a bristle broom followed by water to wash off this top layer once the concrete has firmed
sufficiently. Following diagrams will show those finishes.
Figure 12: honed finish Figure 13: Embeded Glass
Figure 14: exposed aggregate finish Figure 15: coloured abrasive finish
Source: working with concrete, Arnold,2003
http://www.collaborativelandscape.org/wiki/File:Colour.JPGhttp://www.collaborativelandscape.org/wiki/File:Insitu-exposed.JPGhttp://www.collaborativelandscape.org/wiki/File:Glassconc.JPGhttp://www.collaborativelandscape.org/wiki/File:Insitu-honed.JPGhttp://www.collaborativelandscape.org/wiki/File:Colour.JPGhttp://www.collaborativelandscape.org/wiki/File:Insitu-exposed.JPGhttp://www.collaborativelandscape.org/wiki/File:Glassconc.JPGhttp://www.collaborativelandscape.org/wiki/File:Insitu-honed.JPGhttp://www.collaborativelandscape.org/wiki/File:Colour.JPGhttp://www.collaborativelandscape.org/wiki/File:Insitu-exposed.JPGhttp://www.collaborativelandscape.org/wiki/File:Glassconc.JPGhttp://www.collaborativelandscape.org/wiki/File:Insitu-honed.JPGhttp://www.collaborativelandscape.org/wiki/File:Colour.JPGhttp://www.collaborativelandscape.org/wiki/File:Insitu-exposed.JPGhttp://www.collaborativelandscape.org/wiki/File:Glassconc.JPGhttp://www.collaborativelandscape.org/wiki/File:Insitu-honed.JPG8/3/2019 Introduction w
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8.0 Cost details
8.1Cost details of Concrete mixes used.
GRADE OF CONCRETE PRICE PER M3 12.00% VAT SELLING PRICE(Rs.)
15 8500 9520 9100
20 9100 10192 9700
25 9400 10528 10000
30 9850 11032 10450
35 10300 11536 10900
40 10800 12096 11400
45 11250 12600 11850
PUMPING CHARGES PER M3 400
MINIMUM CHARGES FOR 35M3 14000Table 1:Cost details on Concrete mixes
Source: International Constructions (Pvt) Ltd, Ready-mix yard, Bokundara, Piliyandala
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8.2 Cost details of slab system components used.
DESCRIPTION UNIT WITHOUT VAT WITH VAT PRICE
Floor beams Meter 500.00 560.00
Soffit Block(525185mm) Each 65.00 72.80
Soffit Plank(550300mm) Each 100.00 112.00
Hollow Core Slab(200mm) Meter sqd. 4304.00 4820.48
Hollow Core Slab(150mm) Meter sqd. 3766.00 4217.92
Paving Slab (45045050mm) Each 195.00 218.40
R/F Paving Slab (45045050mm) Each 310.00 347.20
Paving Slab (30030040mm) Each 85.00 95.20
R/F Paving Slab (30030040mm) Each 125.00 140.00Table 2: Cost details slab construction components
Source: International Construction Consortium (Pvt) Ltd, Precast Concrete yard, Madapatha, Piliyandala.
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9.0 Conclusion and recommendations
Slab construction is now demanding a great deal to move with the present society. Each client is looking
for the best possible methods to be implemented according to his budget. Therefore the engineers and
designers more or less focus on designing slabs to bear up the compressive, tensile and shear forces
without failing or cracking. At the same time Budget is maintained at the lower rate.
From this report it was found evident that bar reinforcement system is more or less reduced and high
tensile wires are used. Where it is needed to bare high tensions high tensile wires are brought about
connected and wire strands are designed. For example Hollow core slabs are designed by this 7 wire
strands. It seem to be a good method as 7 wires will bare more distributed tension rather than one bar
reinforcement. If that one bar reinforcement fails then the entire beam must be prepared. But if one wirefails other 6 will withstand the tension.
Apart from the it is good to give a recommendation of using a plastic or polythene wire in between the
top structural concrete layer and other components to reduce moisture penetration. With this feature it is
true that most defects will overcome. As well it seems like engineers can think of methods of reducing
concrete and replace it with a more reusable material where one can de-install and use it again.
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References
Arnold, R. 2003,Working with Concrete.USA: Taunton press.
Concrete Manufactures association, 2002.Precast hollow core concrete slabs. Japan: Ishikova
publications and communications (Pvt) Ltd.
Nawy,E.G,1992. 2nd
ed,the concrete construction engineering book,USA: CRC press.
Park, R. and Gamble, W.L.,2000, 2nd
ed .reinforced concrete slabs. New York: John Wiley and sons.