To bond or not to bond...

The use of post-tensioning in building projects continues to increase, being championed, in particular, by frame contractors who wish to exploit the significant savings in construction times that can be achieved. However, there is some confusion in the marketplace as there are two systems available with conflicting advice as to which is the best suited to a particular application. This may be due to historical prejudices, the fact that some specialists only offer one of the systems, commercial considerations or just down to ignorance.

The facts are that both post-tensioning systems, bonded and unbonded, have a proven track record, are covered by recognised technical standards and can often both be appropriate for similar applications. This article intends to set out the difference between the two systems and to give guidance on the technical considerations to be taken into account when choosing between them.

Post-tensioned concrete

Common to both systems is that the concrete is post-tensioned, i.e. a force is applied to the concrete section after the concrete has been cast and has attained a certain strength. For both systems, steel cables are inserted into a duct, which is cast into the concrete section. At each end the cables (usually referred to as strands) are passed through a steel anchorage. An hydraulic jack is used to apply a force to the strand with the jack bearing against the anchorage. The difference between the systems primarily relates to the interaction between the strand, the duct and the concrete section. The principal features of each of the systems are described below.

Bonded Systems

For a bonded system, the post-tensioned strands are installed in galvanised steel or plastic ducts. The ducts are cast into the concrete section at the required profile and form a voided path through which the strands can be installed. The ducts can either be circular or oval and can vary in size to accommodate a varying number of steel strands within each duct. At the ends, a combined anchorage casting is provided which anchors all of the strands within the duct. The anchorage transfers the force from the stressing jack into the concrete. Once the strands have been stressed, the void around the strands is infilled with a cementitious grout, which fully bonds the strands to the concrete. The duct and the strands contained within are collectively called a tendon. The main features of a bonded system are summarised as follows:

* there is no reliance on the anchorages once the duct has been grouted

* the full strength of the strand can be used at the ultimate limit state (due to strain compatibility with the concrete) and hence there is generally a lower requirement for the use of unprestressed reinforcement

* due to the concentrated arrangement of the strands within the ducts, a high prestress force can be applied to a small concrete section

* accidental damage to a tendon results in a local loss of the prestress force only and does not affect the full length of the tendon.

Unbonded systems

In an unbonded system, the individual steel strands are encapsulated in a high-density polyethylene or polypropylene sheath and the voids between the sheath and the strand are filled with a rust-inhibiting grease. The sheath and grease are applied under factory conditions and the completed tendon is electronically tested to ensure that the process has been successfully carried out. The individual tendons are anchored at each end with anchorage castings and wedges. The tendons are cast into the concrete section and are jacked to apply the required prestress force, once the concrete has achieved the required strength. The features of an unbonded system and where it differs from a bonded system are:

* the tendons can be prefabricated off site

* the installation process on site can be quicker due to the prefabrication and the reduced site operations

* the smaller tendon diameter and reduced cover requirements allow the eccentricity from the neutral access to be increased thus resulting in a lower force requirement

* the tendons are flexible and can easily be curved in the vertical and horizontal directions to accommodate curved buildings or to divert around openings in the slab

* the force loss due to friction is lower than for bonded tendons due to the action of the grease

* unbonded tendon performs well in an overload situation due to the ability of the strands to increase their stress and hence support greater loads.

General considerations

It can be seen from the above that each system can offer benefits on any particular project. It is also worthwhile considering the global factors, which need to be considered in the selection of a system.

Design criteria

As previously mentioned, each system has features that affect the design. It will generally be the case that an unbonded system will result in the lowest tonnage of prestressing strand. However, it will generally require more conventional reinforcement than for a bonded system. The difference between the systems in terms of design is marginal. Design guidance is provided in the Concrete Society's Technical Report 43(1).

Speed of construction

It is generally accepted that unbonded tendons are quicker to install than bonded tendons, due to the reduced number of site operations and the fact that they can be prefabricated off site. They can also generally be stressed at lower concrete strengths. However, designs using unbonded tendons do generally require the use of more conventional reinforcement and therefore the overall effect on programme may only be marginal. It is noted that both systems offer significant time savings over conventional reinforced concrete designs, primarily for the following reasons:

* fixing time for the tendons and reinforcement is quicker than for ordinary reinforcement

* concrete volumes are reduced, thereby reducing pour times

* the slab formwork can be struck earlier as the slabs are generally self-supporting following stressing (around five days after casting).

Durability

The methods by which durability is achieved are different for each system. However, like ordinary reinforcement, a significant contribution is made from the concrete itself in providing a passive environment via the concrete cover. Unbonded tendons are afforded further protection from the plastic sheath and the rust-inhibiting grease. The live-end anchorages are protected by a proprietary non-shrink polymer-modified grout. For bonded tendons, the grout used to fill the duct is of crucial importance. It is essential that the grout is correctly installed and of the correct specification to ensure that voids are not left in the duct. A higher degree of site control is therefore required for a bonded tendon installation. With the appropriate level of site control, both systems provide equal levels of durability protection.

Post forming holes

The subject of forming holes in completed post-tensioned slabs is often one of the major factors, which is discussed when choosing between post-tensioning systems and, indeed, between post-tensioned solutions and reinforced concrete solutions. There is frequently a perception that holes cannot be formed in post-tensioned slabs but this is certainly not the case. However, it is true that a degree of care and pre-planning is required although this is equally true with conventional reinforced concrete designs.

When an unbonded tendon is cut, the post-tensioning force is lost over the full length of the tendon, which may extend over a number of spans of the structure. Conversely, when bonded tendons are cut, the grout re-anchors the strands after a small amount of slippage within the duct. Therefore, the force loss for a bonded tendon is localised to a zone either side of the cut. Unbonded tendons can be restressed after they have been cut, which is not possible for bonded tendons.

Where a hole is proposed, it is essential that the location is carefully considered to ensure that it does not comproraise the overall structural integrity in terms of bending, shear, punching shear, deflection, cracking etc. and this is true for any structure, whether it is post-tensioned or not. If a hole is formed at a critical section, i.e. through a tendon or a beam, or at a position of maximum shear or bending, then the consequential effects must be carefully considered. Under these circumstances, the strategy should be that a sensible and workable hole discipline is developed and implemented during the design, construction and use of post-tensioned buildings. This may include the identification of soft zones where the structural element can be safely penetrated. The formation of any hole should then be subject to a careful design review prior to implementation. This strategy is applicable to all structures and not only to post-tensioned buildings.

It is noted that there are numerous examples of large and small holes being formed in post-tensioned slabs using both bonded and unbonded systems. These include large openings, such as those for escalators and lifts, as well as smaller holes for service penetrations, etc.

Accidental damage

The performance of post-tensioned slabs when subjected to accidental damage is similar to that described previously for the formation of holes. For unbonded tendons, if the damage results in tendons being cut or the anchorages disturbed, then the prestress force will be lost over the full tendon length. For a bonded tendon slab the effects of damage will be more localised.

For one-way spanning unbonded slabs, the Codes of Practice require the inclusion of minimum levels of ordinary bonded reinforcement to prevent damage to any one span resulting in a progressive collapse of the other spans. Studies of instances of accidental damage to two-way spanning structures have shown that post-tensioned slabs perform well and that a limited area of damage does not result in an overall collapse of the building.

Demolition

The demolition of post-tensioned structures, like all other structures, requires careful consideration and pre-planning. It is essential that the prestress forces are released in a controlled manner and a number of techniques are available for each system. The techniques used will depend on the particular requirements of the structure being demolished. For both systems, a safe demolition method can be established in terms of cost and timescale and there is little difference in this regard between bonded and unbonded systems.

Concluding remarks

The choice between systems should be based on a thorough assessment of the merits of each, with reference to the particular requirements of the project under consideration. With correct design and procedures both bonded and unbonded systems provide efficient and safe methods of post-tensioning building structure.