Post-­‐Tensioning...  Economical  Engineering    

AKHIL  MITTAL  

 

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Table of Contents    

Introduction ............................................................................ 4 What is Post-Tensioning? ...................................................... 5 How does Post-Tensioning affect the concrete? .................. 6 Process of Post-Tensioning .................................................... 8 The Benefits of Post-Tensioning Systems in Buildings: .... 10 Post-Tensioning at Waterfront ........................................... 11 How to carry out Post-Tensioning ...................................... 13 Costs ...................................................................................... 16 Annexure ............................................................................... 17 Conclusion ............................................................................ 18 Bibliography ......................................................................... 19  

 

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Introduction Over the summer of 2011, I worked as an intern with a real estate developer, Kunal

Corporation on a project site called “Waterfront Tower” located in Mumbai. The tower is

proposed to be 158meters high with 25 residential flats, 9 floors of car parking and a floor for

a swimming pool and gymnasium. The developers have kept the building modern using latest

technology from different parts of the world including a 9 floor automated tower parking

system from Germany. The building includes a sewage treatment plant as well as rain water

harvesting.

Being a part of a family that pioneered in real estate, I have developed a keen interest to work

in this field. The Indian real estate market has undoubtedly been on a high growth path over

the last 5 years. Currently, the market holds the ninth position among the retail markets in the

world, growing at a rate of 30 percent per annum. India is currently one of the most attractive

destinations for Foreign Direct Investment (FDI) in real estate. All these factors influenced

my decision in pursuing this field as one of my possible career options.

Needless to say that I learned a lot during my stint at Waterfront Tower, what grasped my

attention was a new technology, recently introduced to the Indian real estate market called,

“Post-Tensioning.” At Waterfront Tower, the developers decided to use this unique method

of reinforcing concrete in their car parking floors and girder level floor. I was able to research

and understand the Post-Tensioning method, as well as learn how this technique would be

beneficial to the building. I decided to take the initiative along with professional engineers on

site to research on this technique and provide my sample to the builder.

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What is Post-Tensioning? Post-Tensioning is a method of reinforcing concrete, masonry and other structural elements.

It is a method of Pre-Stressing. Pre-Stressed concrete is made up of internal stresses (forces)

included into it during the construction phase with the purpose of counteracting the

anticipated external loads that it will encounter during its lifecycle.

There are two methods of Pre-Stressing:

1. Pre-Tensioning: This method consists of stressing and reinforcing the inside of large

steel buttresses, and then casting the concrete around the reinforcement. This method

can only be done at a precast manufacturing facility. The completed pre-stressed

concrete is then taken to the job site and assembled.

2. The other method of Pre-Stressing is called Post-Tensioning. Instead of stressing the

reinforcing inside of the large steel buttresses at a manufacturing plant, the high

tensile cables are installed on the construction site after the contractor forms up the

slabs or constructs the walls and columns. The cables are housed in sheathing or ducts

that prevent the steel from bonding to the concrete so that it can be stressed after the

concrete cures (hardens).

While the end result of both these methods are almost the same, the only benefit of the post-

tensioning method is that the builder can get the freedom to construct the members (walls,

columns, slabs) in any shape or configuration. In pre-tensioning, the process is limited to

standard shapes and sizes that can be transported from the manufacturing unit to the

construction site.

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How does Post-Tensioning affect the concrete? Many would think that the technique of post-tensioning only helps the steel to get pre-

stressed and tensioned; however this technique gives its advantages to concrete as well.

While the steel element gets tensioned, the concrete gets compressed. Compression is that

force that crushes while tension is that pulls something. Concrete in general is very strong in

compression and weak in tension while steel counteracts it by the strength of its tension. Thus

with the technique of providing tension to steel, we are compressing the concrete and making

both forms of material in its strongest form. The result is stiffer concrete that has more

capacity to resist tensile forces.

Due to heavy load on concrete slabs or other factors, concrete floors can often bend. This

bending can often result in high tensile forces that can cause the concrete to form a crack and

can be very dangerous for any building. In such cases, the use of reinforcing concrete is very

beneficial. As steel has the capacity to resist tensile forces, it is embedded in the concrete at

the highest tension unit it can withhold and thus allowing the tensile forces to be handles by

the reinforcing steel.

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There is another way of reinforcing the concrete; this is known as draped profile. A draped

profile is an elevated concrete slab that has post tensioning strands routed through a high

point over the slab’s supports, and through a low point in between those supports. In this

case, optimum efficiency is obtained because:

• the concrete is compressed

• the post-tensioning reinforcing is

creating an uplift force in the

middle of the spans, where it is

needed the most

• the post tensioning forces is located

in the tension zones

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Process of Post-Tensioning Post-Tensioned reinforcement consists of very high strength steel strands. These steel strands

are known as tendons. The strands are used in horizontal applications like foundations, slabs,

beams and bridges; bars are used in vertical application like walls and columns. The pre-

stressing steel is housed in a sheath or duct to allow it to move as the tensioning force is

applied after the concrete hardens. A typical steel strand used for post-tensioning has a tensile

strength of 1861 N/mm2 four times more than that a typical non-pressed piece of reinforcing

that has a tensile strength of 413 N/mm2. Strands normally have a diameter of 12.7mm or

15.2 mm (0.60 in) and are stressed to a force of 228 N/mm2 using a hydraulic jack.

The jack is supported by one of the anchors that is embedded in the concrete and pulls the

steel to a preset force. The steel stretches as it is tensioned, and it is locked into place using

an anchoring component that forms a mechanical connection and keeps the force in the strand

for the life of the structure. After the tendons have been stressed and engineering approval

given, they are cut off and permanently sealed. A free flowing cementitious grout is then

pumped into the ducts to lock the cables in position permanently. This is classified as bonded

post-tensioning. Bonded post-tensioning is generally used in flat slab situations where the

maximum tendon eccentricity allows the slab depth to be kept at a minimum. The ducts or

sheathing that houses the pre-stressing steel provides a layer of corrosion protection. Multiple

pieces of pre-stressed steel in a duct is termed as multi-strand tendons; while a single pre-

stressed steel strand is known as a mono-strand tendon.

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Anchorage blockouts (Casting) with a bearing plate

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The Benefits of Post-Tensioning Systems in Buildings:

• It provides longer spans, which reduces structural floor depths, resulting in larger

column-free areas

• Thinner concrete sections

• Stiffer walls to resist loads

• Stiffer foundations to resist the effects of shrinking and swelling soil

• Simpler stripping of formwork

• Fast construction cycles are envisaged

• Positive deflection control

• Improved constructability

• More architectural freedom

• Less material handling on site and storage

• Less labour needed

• Less environment impact as reduced concrete and steel volumes are needed

Therefore, post-tensioning helps the building structure to withstand more weight and

protects it against natural calamities like earthquakes etc. The large amount of material

that is saved also helps in reducing the cost for the developer.

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Post-Tensioning at Waterfront At Waterfront Tower, the technique is used in the 9 floors of car parking, as well as the girder

level floor which forms a second foundation to the building and makes it stronger to sustain a

high rise load. The total area of post-tensioning used is 1656m2 in the podium level for car

parking. Below are the details of the breakup used as provided by SF-MGC Systems, the

company that designed and implemented Post-Tensioning at Waterfront Tower.

Above mentioned is a list of specifications of materials that was needed for the post-

tensioning done at Waterfront Tower.

Sr.No. Item Specification/ requirement Remarks

1 Nominal diameter of Strand 12.7 mm2 Nominal Steel Area 98.7 mm2

3 Nominal Mass 0.775 KG/M4 Nominal UTS 1860 N/mm2

5 Minimum Concrete grade M-406 At Transfer 25 Mpa7 At 28 days 40 Mpa8 Water-cement ratio for grouting 0.45

9 Grout Admixture

SIKA INTRAPLAST,500 GRAM/BAG(50KG)

10 Grout Pressure 75 PSI 1 KN =1450 PSI 11 Stressing pressure to Strand 5500 PSI

12Nominal pressure after stressing i.e. elongation 0.65

13Cement Cube test of grouting at 7 days 17 N/mm2

100*100*100 mm3 (size of the cube mould)

14Cement Cube test of grouting at 28 days 30 N/mm2

100*100*100 mm3 (size of the cube mould)

15 Jacking Load 139 KN16 Jack Ram 37.09 mm2

Specification / requirement of materials

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Below are the specifications of the materials used for the post-tensioning slabs and girder at

Waterfront Tower.

Sr. No. Quantity UNIT

1 176.4 M

2 18 No3 90 No4 139.44 M5 18 No6 36 No7 2.5 Kg8 5 Bag

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Tendon - Each tendon contains 5 no. of PC strand ( 6 x 11 + 12 x 9.2 ), total no.of tendons = 18Anchorage blockoutsWedges20mm x 70mm galvanised flat duct

Post Tensioning work of a slab (a typical podium floor) at Waterfront Tower

Consumables Materials required for PT work of slab- ONE END LIVE

Ordinary Portland Cement - 53 Grade Other materials : Tie bars,sealent, chairs, masking tapes etc

Bearing plate 5-sGrout ventGrout additive (anti-shrinkage chemical)

Sr. No. Quantity UNIT

1 1584 M

2 12 No3 144 No4 132 M5 12 No6 24 No7 3.5 Kg8 7 Bag9

Bearing plateGrout ventGrout additive (anti-shrinkage chemical)Ordinary Portland Cement - 53 Grade Other materials : Tie bars,sealent, chairs, masking tapes etc

20mm x 80mm galvanised flat duct

Post Tensioning work of a Girder (G-7) at Waterfront Tower

Consumables Materials required for PT work of Girder-7, ONE END LIVE

Tendon=12 no., Each tendon cotains 12no. Strands of length11 m , hence total no. of strands= 12 x12 = 144Anchorage blockoutsWedges

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How to carry out Post-Tensioning

As seen in the picture above the process of post-tensioning occurs in systematic steps to make

sure that the process is smooth and the steel and concrete reaches the optimum level of

efficiency.

1. To begin with the first step, the formwork/ shuttering of the slab is put in place as per

the rendered architect and structural drawings.

2. The labourers then put the re-bars as per the detailing of RCC plan (Fe500 for the

podium slab).

3. As seen above, the G.I. (galvanized iron) Conduits of size 20mmX70mm flat ducts

are placed horizontally in position as per PT drawing. (Annexure A)

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4. The above representation shows the ends of the tendons that are tied together with the

main reinforcement bars

5. The tendons are then fixed together with the help of an anchorage block as shown

below

6. The concrete is then poured on the formwork over the reinforcement bars.

7. The concrete is left to cure/ harden for 3 days, after which a cube test is performed

which requires an average compressive strength of 25N/MM2

8. The bearing plate (shown in Annexure B) is then fixed at the end of Section 6-6 to

stress the steel tendons.

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9. As the conduits have empty spaces in between them, there is a process called grouting

that takes places at this stage which fills the voids with cement slurry between the

strands.

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Costs One of the important benefits of post-tensioning is the capacity to reduce the load of steel

bearing beams and columns and thus reducing the costs significantly.

For example, a typical podium cost at Waterfront Tower comprised of the following

elements:

At the time of discussions on the implementation of post-tension, the builder realised that

every podium would save approximately 20% of the steel cost and therefore the

implementation of post-tensioning seems beneficial to the building.

S.No. Description Unit Rate (Rs) Quantity Amount (INR) Amount ($)A Concrete Work1 P/Laying RMC M30 grade concreting CUM 6449 13.92 89,744.28 1,909.45 2 P/Laying RMC M50 grade concreting CUM 8343 247.15 20,61,972.45 43,871.75 3 EPCO KP 200 @ KG/Cum KG 214 522.13 1,11,736.25 2,377.37

Total 22,63,452.98 48,158.57 B Formwork4 P/Fabrication erection fixing in position, deshuttering Sq. M 493 1384.03 6,82,328.76 14,517.63 C Reinforcement5 P/Cutting, Bending in position of reinforcement bars MT 17717 41.59 7,36,920.90 15,679.17 6 Steel provided MT 31625 41.59 13,15,410.25 27,987.45

Total 20,52,331.15 43,666.62 D Post Tension Work

As per Area SM 726.95 176.56 1,28,346.66 2,730.78 Total 51,26,459.55 1,09,073.61 VAT @ 5% 2,56,322.98 5,453.68 Service Tax @ 4% 2,05,058.38 4,362.94 2% Education Cess on Service Tax 4,101.17 87.26 1% Higher Education Cess on Service Tax 2,050.58 43.63 Grand Total Cost for Podium Slab 55,93,992.66 1,19,021.12

WATERFRONT TOWERCost of a Typical Podium Floor

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Annexure My stint at Waterfront Tower also included regular meetings with the architect, M/s Talati

and Panthaki Pvt. Ltd along with the structural engineer, Vora and Associates Pvt. Ltd.

I had the opportunity to learn how to read and understand architect rendered drawings on

AutoCAD, a CAD software that has a 2D and a 3D design and drafting application. The

software is used by the architects and structural engineers for drawings.

Enclosed are a few annexure that I worked on along with the architects and engineers.

• Annexure A – depicts the laying of tendons on the PT slab. Each tendon has 5 strands

placed at a distance of 1060mm apart.

• Annexure B – shows the front cut view of an anchorage blockage bearing plate and

how it is attached to the tendons that are protruding out.

• Annexure C – depicts the drawing of the girder level plan with the size of each

column and beam. The girder floor is one of the most important slabs of the building

as it forms a second foundation to the building.

• Annexure D – portrays how the tendons are placed in the girders.

• Annexure E – shows the area used for post-tensioning in the car parking floor.

• Annexure F – shows the area that would have been completed with regular RCC if

post-tensioning hadn’t been completed. Here you can see the two beams (shown in

dotted lines) that would have been constructed if there was no post-tensioning. The

beans would have obstructed the height of the slab thus reducing the height of

each floor. As this was a car parking slab, the builder wanted to have a clear height so

that large cars can fit easily.

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Conclusion The advent of post-tensioning in the real estate market has re-defined the construction

industry. It has not only led to a reduction in the amount of material but also helped increase

the profitability for developers.

By reducing the height of beams, it allows the builder to construct more floors within the

permissible height. Post-tensioning tendons provide a tensile strength that is four times that of

normal reinforcement steel bars. This helps make the foundation of the building much

stronger.

By using post-tensioning, the developer saved 20% of the steel cost in the podium and girder

level slab, where the technology was implemented. Working with the engineers on site as

well the Malaysian company, SF-MGC, I managed to learn a lot about this technology. It also

helped increase my interest in the field of construction and enabled me to think out of the box

and explore new ideas for the construction of the building.

Researching on the technique of post-tensioning taught me that something as simple as

tensioning the strands after the slab is casted, could make a big difference to the building

structure.

 

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Bibliography • Innovative Post-Tensioning Partners – The brochure for SF-MGC

• www.sf-mgc.com

• What is Post-Tensioning, Evaluation and Certification Service, LLC , http://www.builderspt.com/wp-content/themes/builderposttension/basic_post_tension.pdf