Posteck Prestressing
Where qualities meet technical excellence

Where qualities meet technical excellence

Q: What are the pros and cons of post-tensioned slab against precast slab system?

A: Post-tensioned slab and precast slab are all time competitors. Their benefits might depend on the availability and skill of the builder. Some benefits of post-tensioned slab over precast slab are listed as follows: 


(Pros)

+ Be able to construct without supporting beams.

+ Provide less structural depth

+ Be able to construct for very large span say 15m+, while it is very difficult for precast construction to operate without large crane.

+ Structure are casted continuously that the slab could be thinner.

+ It does not require much space for transporting concrete to upper floors.

+ Cost might be more effective in terms of material and labor. 


Some known disadvantages of post-tensioned slab are 

(Cons)

+ Require setting up formwork for the whole construction floor.

+ Require special method of pumping or shooting concrete.

+ Construction time might be a bit slower, but this commonly does not affect to overall project time.

+ Require larger amount of workmanship.


However, post-tensioned slab are quite more general and it does not require large equipment to install. Moreover, it depends less on outsource precast manufacturer and require less labor skill. Nowadays, post-tensioned slabs have been proven to be one of the most efficient methods for concrete building construction.


Last Updated: 11.09.2016

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TextAloud: IVONA Kimberly22 (Created: 9/10/2016 11:45:29 PM)
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Q: What is shear band reinforcement?

A: Shear band is an alternative punching shear reinforcement to column-supported slab systems. The Shear band system is a high tensile and high ductility thin metal strip and differs from all existing systems. The system can be placed on top of all other reinforcement, with minimal loss of cover. Shear band brings a simple construction solution against conventional stirrup and probably easier and cheaper to manufacture than welded stud rail. Posteck Prestressing has been preparing a series of testing program of this alternative punching shear device before bring them to the actual construction projects.



Source: https://www.sheffield.ac.uk/ci/research/concrete/sb


Last Updated 11.09.2016

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TextAloud: IVONA Salli22 (Created: 9/11/2016 1:15:07 AM)
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Q: What are major sources of stress loss in post-tensioned concrete design? 

A: Stress losses are the reduction of tensile force in stressed tendon that affects to both compressive force and balanced load. For bonded system, this losses are not constant along tendon's length. Losses can be classified as short and long term according to elapsed time to occur:


Short term losses are due to (1) friction along length of tendon, (2) wedge seating.

Long term losses are due to (1) elastic shortening, (2) stress relaxation of tendon, (3) creep and (4) shrinkage. Restraints due to connecting stiff elements are also important, which must be considered in design of post-tensioned members.


Posteck Prestressing has developed PT-LOSS , an integrated cad environment software for computing stress losses. For more information about losses in post-tensioned concrete, please visit our blog


Last Updated: 11.09.2016 


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TextAloud: IVONA Kimberly22 (Created: 9/10/2016 10:57:31 PM)
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Q: What is the relationship between cube and cylinder compressive strength of concrete? Why does cube strength always be greater?

A: Due to difference in geometry and hence their confinement condition are different. During testing program, almost all part of cube are subjected to lateral compressive stress formed by diagonal compression strut, while part of cylinder specimen is not subjected to lateral compression. Commonly, cube strength is 1.25 times greater than cylinder strength. While this ratio is not constant, it can be used as practical guideline.


Updated 11.09.2016  

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TextAloud: IVONA Joey22 (Created: 9/10/2016 10:59:57 PM)
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Q: I have noticed some surface cracks just after concrete placement. What are the causes and how to remedy them?
A: Visible surface cracks might occur within just an hour after concrete has been placed. This cracks are formed when concrete is in its plastic state due to excess rate of evaporation. This so-called "Plastic Shrinkage" is commonly found when concreting in hot and/or windy environment. To reduce plastic shrinkage crack, some recommended 
approaches by PCA can be applied as follows:
+ modifying concrete mixture by reducing fine constituent, 
+ protect finished concrete surface by plastic sheet, 
+ and to spray concrete surface during setting. 

From our experience, combination of the latter approaches work efficiently. For more information about plastic shrinkage, please visit our blog.

Last updated 10.09.2016     
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TextAloud: IVONA Salli22 (Created: 9/10/2016 11:27:38 AM)
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Q: What is integrity tendon and what is the function of bottom integrity bar?
A: Whenever possible, at least 2 strands of integrity tendon should pass through column core in both directions to enhance post punching shear strength of slab. If this is not possible, ACI318-11 and ACI 352.1 recommend integrity rebar as continuous bottom bar passing through column in place of integrity tendons. There are some different in bottom rebar defined by both design recommendations. It is a matter of choice of the designer to choose which one to follow. 

Last updated 10.09.2016 
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TextAloud: IVONA Kimberly22 (Created: 9/10/2016 10:13:10 AM)
-0:38
Q: What is punching shear?
A: Punching shear is the most critical issue for column-supported slab. Both reinforced and prestressed concrete slab without beams are susceptible to punching shear failure. The components that contribute to punching shear strength of slab are as follows: concrete shear strength, tie dowels or tendons, vertical component of rebar or any shear reinforcement devices. Calculation of punching shear must consider the effects of gravity shear and transfer moment due to shear action. Posteck Prestressing has conducted punching shear tests as well as developed a specialized software PTPUNCH for automatic design of punching shear reinforcement. For more information about punching shear, please visit our blog.

Source: https://www.sheffield.ac.uk/ci/research/concrete/sb

Last Updated 10.09.2016 
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TextAloud: IVONA Kimberly22 (Created: 9/10/2016 10:09:34 AM)
-0:45
Q: What is ePosteck system?
A: ePosteck is our e-based central management system. ePosteck is the new concept to centralize the information for each operation units (designer, factory and site staffs) as well as to share them with clients. With ePosteck, all information used for the projects are up to date and 100% synchronized. Major benefits are apparent from less coordination efforts to progress track up as well as service enhancement.

Last updated 04.08.2016

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TextAloud: IVONA Kimberly22 (Created: 9/4/2016 11:18:52 AM)
-0:33
Q: How does edge reinforcement work during stress transfer? 
A: Stressing tendons is one of most critical states in post-tensioned concrete construction. For thin slab with inadequate bearing strength at load-transfer point, formation of stable or unstable cracks can visibly occur. In many severe cases, top slab bursting could happen where stressing process must be postponed for slab repair.
Edge rebar a.k.a. U-bar works as part of anti-bursting reinforcement together with PT-Helix. Its function is to strengthen the zone behind anchorage. When tensile stress exceeds concrete strength, edge reinforcements will act as ties to prevent unstable crack to propagate and hence minimize the risk of local zone failure. This new concept of anti-bursting rebar complies with AS3600. Based on our experience, this details have been successfully used for million sq.m. of PT slab. It has simplified the laborious work done by contractor against traditional system as well as provided cost-saving solution. Edge reinforcement with PT-Helix is currently the most efficient anti-bursting system for PT slab. For more information about our anti-bursting system and researches, please visit our blog page "HOW ANTI-BURSTING REINFORCEMENT WORK?".

Last updated: 04.08.2016

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TextAloud: IVONA Joey22 (Created: 9/4/2016 10:21:48 AM)
-1:16
Q: What is progressive collapse and how to prevent it?
A:   A building undergoes progressive collapse when a primary structural element fails, resulting in the failure of adjoining structural elements, which in turn causes further structural failure (wikipedia). Column-supported slab is a type of structure that is prone to punching shear failure. A single failure caused by punching shear can lead to a progressive collapse event.
Design to prevent progressive collapse is commonly accomplished by providing complete secondary load paths to support the primary load resisting system upon local failure. A local failure when occurs will be restricted to a controlled area not to expand to the whole building.

At Posteck, we take serious consideration by providing safe and redundant design to alleviate such progressive collapse condition that may occur during construction stage when concrete strength is low.  For more information regarding safe design, please contact our engineer to assist.

Last updated: 05.10.2014
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Q: How does modern analysis technique help our engineers to design a PT slab?
A:  As we realize the needs of modern building design to fit with functional and aesthetic requirements, simple plan building with regular column layout are not always the best solution for most project owners. With power of modern numerical technique implemented by us, the boundary of restriction is nearly eliminated. Nowadays, slab performance under service condition can be testified before the slab will actually be built. Numerical method reveals any defect or weak zone where additional tendons or rebars may be added in order to control cracking and slab deflection. Our clients can be assured that the designed slab will perform well under required occupancy. 

Last updated 21.09.2014 
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Q: What are delayed strips and how do they work?
A:  For particular types of building plan where stiff elements like core walls inhibit slab to shortening, delayed strip (a.k.a. pourstrip) can be introduced to reduce restraint to shortening cracks. Delayed strips are usually located at the mid or quarter point of the span. Pourstrips should remain free as long as possible, but in practice they are properly shored for 30-60 days to allow significant slab movement at early age.


Last updated: 20.06.2014
Photo is courtesy of K. Dirk Bondy & Bryan Allred
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Q: How does vertically curved tendon help to improve slab performance?
A: After stressing, vertically curved tendon will generate distributed transverse loads or so-called equivalent loads other than compression to slab. With proper profiling, gravity loadings are counteracted by post-tensioning. This hence reduces slab deflection and stresses at service condition. In typical design, 70-125% of slab selfweight will be balanced by post-tensioning after all losses. Post-tensioned slab is less susceptible to cracks and excessive deflection under service condition.
   Equivalent loads do not involve in strength calculation, but for indeterminate structures, the secondary moment due to support reactions must be considered in ultimate moment & shear force calculation. With post-tensioning, slab can be built to be longer span and less weight.

Last updated: 21.09.2014
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Q: What are the differences between bonded and unbonded PT slab system?
A: Bonded and unbonded system are quite different in many aspects. Major advantanges of each system are listed as the following:
Unbonded system:
- Provides greater available lever arm and drape
- Applicable to very thin slab
- Reduces friction losses
- Strand and shealth are come into piece
- Grouting is not required
- Shorter construction time for tendon placement
- Generally cheaper
- Enhance structural integrity by catenary action

Bonded system:

- Develops higher ultimate flexural strength
- Does not depend upon the anchorage after grouting
- Localize the effects of damage
- Simple technique for demolishing or providing future opening in slab

As Bonded system is only a common practice in Thailand, Posteck currently supply only Bonded system for both PT slab and beam constructions.

Last Updated: 22.06.2014
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-1:04

Q: Can post-tensioned slab be used with heavy duty industrial floor?
A: Absolutely it can!

PT slab offers major advantages for industrial slab with heavy duty load over conventional RC slab or precast plank.

Major benefits are listed as following:

  • Save tremendous amount of rebars and labor
  • Fast construction & minimal maintenance cost
  • Continuous floor without unsightly control joints
  • High durability to impact and wheel loads from forklifts


  • With bonded system, it is possible to penetration or even strand cutting.
  •  Provide support for machineries
  • Subjected to less noise and vibration


For more information, please contact our engineer to assist.


Last Updated: 26.06.2014

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