Introduction w

8/3/2019 Introduction w

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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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Department of Building Economics ii

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
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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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BE 2101- Structures II Alternative Slab Construction Techniques

Department of Building Economics 1

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


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


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


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.



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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.



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Figure 8: installed concrete soffit planks for easy slab system



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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.JPG


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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.

Documents

Introduction w