What Is Post Tensioning?

WHAT IS POST TENSIONING?

Post tensioning is a technique for reinforcing concrete. Post-tensioning Post-tensioning tendons, which are restressing steel cables inside Galvanized steel  ducts or sleeves  , positioned in the forms before the concrete is placed  Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete.

Post-tensioning is a form of restressing. Restressing simply means that the steel is stressed (pulled or tensioned) before the concrete has to support the service loads.

Although post-tensioning systems require specialized knowledge and expertise to fabricate, assemble and install,

The concept is easy to explain. Imagine a series of wooden blocks with holes drilled through them, into which a rubber band is threaded. If one holds the ends of the rubber band, he blocks will sag. Post-tensioning can be demonstrated by placing wing nuts on either end of the rubber band and winding the rubber band so that the blocks are pushed tightly together. If one holds the wing nuts after winding, the blocks will remain straight. The tightened rubber band is comparable to a post-tensioning tendon that has been stretched by hydraulic jacks and is held in place by wedge-type anchoring devices.

ADVANTAGES OF POST-TENSIONING

There are post-tensioning applications in almost all facets of construction. In building construction, post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender, dramatic elements. Thinner slabs mean less concrete is required. In addition, it means a lower overall building height for the same floor-to-floor height. Posttensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors. This reduces the foundation load and can be a major advantage in seismic areas. A lower building height can also translate to considerable savings in mechanical systems and façade costs.

Longer Spans

Give chance to use  longer spans in elevated members, increase the flexibility of use for the structure and can result in higher rental returns.

Overall Structural Cost

  • Thinner concrete member sizes; reduction in concrete is approximately 20%
  • Rebar in floor elements is reduced by 60% to 75%
  • Decreased dead load reduces rebar and concrete in columns and foundations

Shrinkages and cracks

Reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are needed. Cracks that do form are held tightly together.

Early Formwork Release

The earlier stripping of formwork and reduced back propping requirements enable faster construction cycles and quick re-use of formwork. This increase in speed of construction is explained further in the next section on economics.

APPLICATIONS FOR PT

Elevated Slabs in Buildings

Typical suspended slab applications, Car parks, Apartment buildings, Commercial office space, Hotels, Retail centers, Hospitals, Stadiums, Exhibition & convention centers, Educational institutions such as schools & universities, Vertical load transfer structures, Tanks & silos, Storage facilities

Slab on grade for industrial projects and roads

Bridge girders and deck

Concrete water tanks are often post-tensioned to reduce crack width and leakage.

WHY SHOULD I BE INTERESTED IN POST-TENSIONING?

The advantage of post-tensioning is flexibility of design, low material cost, It also will be able  Thinner PT slabs and early strength stressing gives a faster floor cycle time, allowing the structure to progress fast  It is used for bridges and stadiums where there are complex curves, elevations and grade changes. In buildings , post-tensioning allows long clear spans and a highly creative architectural approach. In commercial buildings space and light can be maximized by construction of large column-free spaces. PT will need fewer joints and thus reduced joint maintenance and helps to increase durability. In tanks and silos post-tensioning can provide crack-free concrete.

PROCESS FLOW OF PROJECT

It starts with a discussion

When you get in touch with SPAN TECH. ,our designer  will visits you to discuss and understand the requirements of the project. This information will be used for the further process.

The following are the processes which undergoes till the final execution at site.

Over to SPAN TECH

Once your drawings reach us, we create a database keeping all your requirements in mind. We try to make feasible and best solution that meets your requirements as fast as we can.

Feasibility check

Through our experiences gained from different variety of projects, we have determined to deliver quick, quality and best solutions. We understand the importance of the customer requirements. That is why we have committed ourselves to bring out the best feasible outcome for our customers.

Design and Estimation

As soon feasibility is checked, we prepare a preliminary design based on the structural configuration and feasibility. This inputs are used to derive the quantity estimation for PT Elements and other items.

Final design for construction

SPAN TECH  uses the latest design software which is continually updated to reflect current national and international construction codes of practice. SPAN TECH   prepares a set of drawings like forming plan, reinforcement plan and PT drawings. This set of drawings are send to the site for further implementation of the system.

Final review

SPAN TECH   assures its clients by issuing a Certificate of Stability after conducting gravity analysis and design of PT. This will affirm the stability and quality of the PT members to provide intended function under the gravity condition.

Product assembling

SPAN TECH   delivers outstanding post-tensioning solutions and products. The drawing displays the layout of PT cable and its accessories. This give the details of the BOQ and based on this, the final product is prepared for the respective project.

On site activities

SPAN TECH   keeps the safety on site as one of the top priority. SPAN TECH   is committed to maintain its quality both in design and at site. The installation and stressing activities at project site are followed by number of inspection and quality assurance formalities.

VALUE ENGINEERING

Our employees have gained experience and proficiency from variety of projects across all over Egypt and gulf region . SPAN TECH  employees are able to bring out the latest ideas and have the skills to work on different new technologies from market. Using value engineering methodology, SPAN TECH can determine the most advantageous to meet the specific requirements of a customer.

Working Standards

SPAN TECH  have resolute, professional engineers having in-depth knowledge of reinforced and pre-stressed concrete and possess a considerable understanding of the design of structures.

Designing

SPAN TECH, designing is done to create safe, refined, durable , cost-effective structures which are also sustainability.

Drawing

The detailing of specific elements are provided in the form of drawings along with the design .We implement graphical representation of the elements, so that it can be easily understood and grasped by site engineers and contractors.

Operations

SPAN TECH follow the construction method statement which follow the requirements of national and international specification

CONSTRUCTION METHODOLOGY

1.  Form work set up.

  • Form work shall be constructed so as not to restrain elastic shortening, deflection or cambers resulting from application of the pre-stressing force. Form work shall be constructed to resist all construction loads including reactions which may occur during stressing
  • The forms shall be sufficiently rigid to prevent displacement of the tendons beyond the tolerance

2. Tendon Installation  

A. Set up of pre-stressing tendon components

  • Uncoil and lay strand in position.
  • Lay duct into position and connect ducts with duct couplings.
  • Tape all duct joints.
  • Insert or push the strand into the duct with one strand at a time.
  •  All tendons should be placed in the location and profile as per approved and shop drawing.
  • Set up bar chairs and profile according to the location given on shop drawing
  • Vertical deviations in tendon location should be kept to ±¼ in.(6mm) in concrete with depths up to 8 in.(200mm), to ±3/8in.(9.5mm) in concrete depths between 8in. (200mm) and 2ft. (610mm), and ±1/2 in. (12.7mm)in concrete depths over 2ft. (610mm).
  • Horizontal location of tendon is typically not as critical. However, avoid excessive wobble (unintended curvature) in tendon
  • High and low points are the most critical locations; as moth curve should be maintained between high and low point.
  • Alternatively empty PT ducts are installed in location and after concreting and hardened. PC strand are pushed into ten don duct by strand pusher prior to stressing operation.

b. Set up of stressing anchorage components

  • Mark at slab edge according to the approved Post Tension drawings to identify the anchorage bearing plate exact place after marking make hole at anchor place
  • Install anchorage bearing plate to formwork
  • Support and fix the anchorage bearing plate into position using chair
  • Provision and fix grout vent into anchor
  • Seal all connections of components with tape.
  • Provision and fix  anchorage reinforcement

c. Set up of DE anchorage components

  • The onion (bulb) dead end type will be used.
  • The strand for each tendon will be formed with onion dead end from the factory by  means of a  special hydraulic jack 
  • Anchorage bursting reinforcement at DE will be fixed into position.

3. Concreting  

  • Pouring concrete shall be done when the steel reinforcement and post-tensioning components are completely installed.

  • Concrete mix proportions shall be so adjusted that no segregation shall occur.

  • Concrete shall be compacted specially around anchored
  • It is recommended to make marking below tendons on form work, to be reference to the exact place of tendons after Castin
  • Tendons shall be observed during concretin Any misalignment of tendon shall be rectified.
  • After concreting is complete the concrete should be cured as appropriate until the strength has attained 30N/mm2prior to stressing operation.

4. Stressing

A. Personnel and Safety

  • Restressing tendons are the backbone of the structure. When properly stressed, they will prevent the structure from cracking and deteriorating But, a badly stressed tendon looks exactly like a properly stressed tendon. Therefore, the only way to ensure proper stressing is to have an experienced crew present during all stressing operations.
  • Erection and maintenance of all scaffolding and safety barriers including access plat form of min. width of1.2matall stressing locations.
  • Stressing should be considered a basically unsafe operation. People operating the equipment and taking measurements should never Stand behind a live jack. This is also true at the dead-end of the strand: never stand behind the anchor of at end on being stressed. Although it does not happen often, tendons do break, wedges do let go and large forces are released in a split second, making jacks jump and propelling tendons out of an anchorage.

b. Preparation for stressing

  • Edge forms and stressing boxes should be removed as soon as possible to allow easier removal of plastic pocket former and cleaning of anchor cavity while concrete is still green.
    • Remove pocket former
    • Clean out anchor cavity of the anchor
    • Place anchor head.
  • Install the wedges evenly; make sure that the orientation of wedges is such that the seating plunger of the jack engages all three wedge section The unique feature of EPT (INSIGHT POST TENSION) Post- tensioning system lies in its special wedge locking procedure. The wedges always remain in contact with the strands during the stressing operation. As the pressure in the jack is released, the wedges automatically lock in the conical holes of the anchor head.
    • Cleanup strand prior to stressing operation.
    • Mark the tendon tail with spray paint for elongation measurement reference, at a constant reference dimension from the slab edge or from anchor head. If the tendons are double end stressed, it is important that both ends are marked before stressing is commenced.

c. Stressing equipment

    • Stressing equipment’s i.e. jack and pump should be set at a given operating pressure as conforming to the calibration document.

d. Stressing Options

  • In members where early stressing is not desired stressing hall be commenced for all Post Tension anchorages with 100% of the jacking force when concrete strength has attained 75% of the specified strength fcu (Min. 30N/mm2) at transfer stage.
  • In members where early stressing is desired, to reduce the risk of early shrinkage cracking initial stressing shall be commenced for all Post Tension anchorages with 25% of final pre-stress force, and is carried out as soon as the concrete achieve strength of 10 N/mm2.Final stressing shall be commenced for all Post Tension anchorages with 100% of the jacking force when concrete strength has attained 75% of the specified strength fcu (Min .30 N/mm2) at transfer stage

e. Stressing the tendons

  1. Commence stressing only after concrete strength has attained the specified strength by testing sample as specified.

2. Stressing load on each strand shall be 15 Ton (80% of the ultimate load).

3. Stressing should be done according to a sequence.

  • In beam and slab construction, stress slabs first; followed by beam tendon
  • In tow-way slab systems, stress distributed tendons first; followed by banded tendons.
  • In one anchor the following sequence should be

4. Elongation should be observed and recorded as soon as the stressing is complete

  • It has been common Practice to accept variations between the on- site and the calculated measurement of ±10% between the on-site and calculated extension
  • After the elongation has been approved the strand should be cut by abrasive disc.

  • Sometimes a wire will break in a tendon. If only one or two wires break, it may be a situation that of relatively little concern. For instance, when one wire breaks only 1/7th of a strand’s capacity has been lost. On a multi-strand tendon this will be much smaller proportion. Very often wire breaks will be within the anchor flare cone, possibly at the back of the wedge plate .Most specifications

    Allow up to 2% of the wires to be broken. However, persistent wire breakage should be investigated and action taken to change procedures or equipment to avoid or significantly lessen the problem.

    The cause of wire breakage should always be determined. Some possible causes are: overstressing, poor strand, bad wedges, or high friction points in the duct. Overstressing and high friction points show up when the stressing records are carefully examined.

5. Grouting

a. Purpose of grouting

  • Cement grout is chemically basic and provides a passive environment around the post-tensioning bars or strands. In addition. In the free lengths of external tendons the principal role of the grout is to provide an alkaline environment inside the polyethylene duct .Never the less, complete filling of the duct with grout is essential for proper protection.

b. Advantages of IAC-EPT Special additive ad mixture

  1. Higher strength and adhesion.
  2. Improved workability and fluidity.
  3. Increased cohesion, reduced bleeding and segregation.
  4. Protects against corrosion of pre-stressed wires.
  5. Prolonged setting time.

Cement for grout

  • The primary constituent of grout is ordinary Portland cement(Type I).

Water for grout

  • The water-cementitious material ratio should be limited to a maximum of 0.45 to avoid excessive water retention and bleed and optimize the hydration process.
  • Temperature of the water shall be between (8:32)°c.


Sequence of grouting

  • After the final stressing completed and the elongation been approved the strand should be cut by abrasive disc.
  • Anchorage should be capped with concrete or suitable capping material.
  • Grouting operation shall start, any time but mot more than 4 weeks from final stressing.
  • Practically the additive admixture (VitacreteAG3) or equivalent should be added after water/cement.
  • A mixing time of 4-5 min should be sufficient.
  • After grouting is complete grout vent shall be cut flat by means of abrasive disc.