Patent Description:
Structural elements or tensioning elements of civil engineering structures such as bridges or columns need to be maintained or replaced from time to time in order to ensure their safety. These pre-tensioned structural elements or tensioning elements needs to be detensioned in a controlled manner before being demounted and replaced completely.

Patent document <CIT> discloses a method for replacing a tensioning element of a large civil structure such as a suspension bridge with towers in the structure thereof. A number of mobile temporary hangers are placed on the upper cable to be replaced, the cable to be replaced is located in the lower portion of these hangers and the cable is removed from the anchors thanks to the mobile temporary hangers that are connected to upper cable. These hangers can be moved towards the deck of the bridge in order to remove the cable to be replaced. This solution is complicated to set up. Moreover, it is only suitable to detension to certain types of structural elements or tensioning elements such as the one described therein.

A "window cutting method" is also known in the art for the detensioning of structural elements or tensioning elements of a civil structural. This de-tensioning method consists of providing several windows along the tendon (tension element), where the protecting sheathing and grout surrounding the strands is removed. The tendon (tension element) is then cut by gas torch or remote-controlled diamond wire strand by strand in one or two windows. As the strands are cut, the force in the remaining strands increases and eventually rupture due to overstress. The sudden rupture of the remaining strands leads to a high residual force release which cannot be fully controlled and might in the process of sudden release cause an impact to the surrounding structure. The impact of the sudden residual force release may be reduced by providing additional damping; however, the force release is not fully controlled and there remains a degree of uncertainty to the implications to the surrounding structure. The above description covers the usual case of a tendon (tension member) formed by a set of individual strands, but it is also applicable to other types of tendon (tension member), e. tendon composed of wires or ropes.

Patent document <CIT> discloses another known detensioning system that relies substantially on so-called improved filled materials injected in the cavity of clamps for load transfer. It is however completely silent on the internal structure and the form of the clamps.

Therefore, there remains a need to find an improved system and improved method for a fully controlled de-tensioning method and complete force release of the tension element to enable the replacement of structural elements and/or tensioning elements that are being pre-tensioned or pre-stressed.

In the present invention, it is proposed that the tendon force of the structural element or the tensioning element is transferred to the stressbars of a bracket system (e.g. presently described detensioning system) clamped to the structural element or tensioning element (e.g. tendon) which are being pre-stressed, with the objective to reduce the tendon force in at least a portion of a structural element or tensioning element. The tendon is then cut in said portion which has been detensioned, for example in between two activated clamping devices. Once the tendon has been cut, the restrained stressbars of the detensioning system can be released in a controlled manner by detensioning the stressbars for instance through hydraulic cylinders, which in turn will release the tendon force over the full tendon length.

The detensioning system according to the present invention can be realised through at least one clamping device. Nevertheless, it is also foreseeable that the clamping device is provided on both sides of the portion of the structural element or the tensioning element to be detensioned. Thanks to the detensioning system which comprises at least one clamping device, wherein the clamping device provides a substantially elongated, annular cavity comprising a gradually declining diameter along a longitudinal axis of the elongated annular cavity shaped by at least one peak and one groove on the internal surface profile of the clamping device, the detensioning system and particularly the clamping device according to the present invention can be built more compact compared to other clamping system, for instance a clamping system comprising a cylindrical annular cavity, as the clamping device according to the present invention allows higher gripping force to the tendon.

Moreover, the clamping device according to the present invention may comprise for instance one, or more preferably with two half conical clamps (e.g. machined high-strength steel sections), which may then be bolted together to form a conical stressing clamp, comprising at least one substantially annular cavity. Apart from the two halve (or "U-shape") conical clamps, the clamping device may also comprise other mechanical components such as sealing plates, bolts, gasket seals, hydraulic cylinders and etc. to form a functioning clamping system. The detensioning system further comprises stressbars to allow a portion (e.g. between two clamping devices) of the tendon force to be transferred thereto before detensioning of the remaining length of the tendon is taken place. Prior to the installation of the detensioning system such as the clamping device, the sheath and/or grout around the surface of the tendon may be chiselled away partially or completely so as to increase the friction (or the grip) of the clamping device to the tendon (when the inner element is introduced to the annular cavity). One or more inner elements can be introduced to the substantially elongated, annular cavity provided by the conical clamping device. The inner element can be a preform element or can be formed by a filler hardening material such as a grout.

A first aspect of the invention is to provide a detensioning system for detensioning a portion of a structural element or a tensioning element, comprising.

Wherein the detensioning system is characterized in that the clamping device has an internal surface profile, arranged to form a substantially elongated, annular cavity surrounding the structural element or the tensioning element to be detensioned, comprising an empty space for receiving one or more inner elements to be introduced therein, wherein the substantially elongated, annular cavity comprises a gradually declining diameter along the longitudinal axis of the elongated annular cavity shaped by at least one peak and one groove of the internal surface profile of the clamping device.

A second aspect of the invention is to provide a detensioning method for detensioning a portion of the structural element or a tensioning element, comprising the following steps:.

According to a third aspect of the invention, it relates to a use of the detensioning system according to present invention in detensioning a portion of a structural element or a tensioning element.

According to some embodiments, each of the clamping device is formed by at least two half clamps, wherein the two half clamps are joined together by mechanical fixing means, wherein one or two clamping devices are provided to each side of the portion of the structural element or the tensioning element to be detensioned. This allows the clamping device to be installed and de-installed easily.

According to some embodiments, the clamping device comprises a surface profile, arranged to form a substantially elongated, annular cavity having multiple segments, defined by multiple grooves and peaks, wherein each of the segment of the annular cavity comprises a gradually declining diameter along the longitudinal axis of the elongated, annular cavity. This allows an even higher gripping force to the tendon.

According to some embodiments, the substantially elongated, annular cavity comprises one or more segments, wherein each segment of the annular cavity comprises at least one channel for introducing of the filler hardening material and at least one channel for venting of the filler hardening material, wherein the filler hardening material is introduced through the channel into the substantially elongated, annular cavity formed by the clamping device, such that the one or more inner elements that is hardened comprises a reversed impression of the internal surface profile of the clamping device, or comprises a similar impression as the surface profile of the annular cavity. This allows the filler hardening material to be introduced into the cavity.

According to some embodiments, the substantially elongated, annular cavity comprises one or more segments, wherein the segments are provided in a trapezoidal or a conical form, or a wedge when viewed from a longitudinal section, so as to increase the longitudinal force transferred between the structural element or the tensioning element to be detensioned and the clamping device, through the one or more inner elements. The different forms of the segment can be chosen based on the actual need and depends on the situation.

According to some embodiments, the substantially elongated, annular cavity comprises one or more segments, wherein each segment of the annular cavity is provided with same or different volumetric capacity for accepting the filler hardening material for the formation of the one or more inner elements such that each segment of the one or more inner elements comprises the same or different amount of filler hardening material.

According to some embodiments, the substantially elongated, annular cavity comprises a periphery with edges and/or a substantially linear periphery. In other words, the annular cavity formed by the clamping device may not have corners (round) or may have one or more edges and corners.

According to some embodiments, the substantially elongated, annular cavity comprises one or more segments, comprising a surface profile defining between two and ten segments or at least two, three, four, five, six, seven, eight or more segments, defined by multiple grooves and peaks, wherein each of the segment is provided in a conical shape, or a wedge when viewed from a longitudinal section. It has been found that generally the more the number of segments, the higher the gripping force to the tendon.

According to some embodiments, the substantially elongated, annular cavity comprises one or more segments, wherein each segment of the annular cavity is provided in form of a trapezoid or a cone, or a wedge when viewed from a longitudinal section, wherein the wedge angle is provided in between <NUM>° and <NUM>°, preferably between <NUM>° and <NUM>°, between <NUM>° and <NUM>°, between <NUM>° and <NUM>°, between <NUM>° and <NUM>°, between <NUM>° and <NUM>° or preferably at around <NUM>°.

According to some embodiments, the one or more inner elements is made by a filler hardening material, which is a grout such as an epoxy-based or a concrete-based grout. Filler hardening material is advantageous over such as pre-forms as the shape is only formed upon the hardening of the hardening material.

According to some embodiments, one or more displacement sensors are provided either in between the clamping devices located on at least one side of the portion of the structural element or the tensioning element to be detensioned, or to the far end of the clamping devices to a fixed reference.

According to some embodiments, strain gauges are provided to the detensioning system, for instance the strain gauges are mounted on the surface of the portion of the structural element or tensioning element to be detensioned, in the filler material and/or at fixings between two half clamps.

According to some embodiments, a cutting machine such as a diamond wire cutting machine is provided and set up before stressing operation begins.

According to some embodiments, the clamping device configured to form one or more annular cavities comprises two half clamps, mechanical fixing means such as high strength bolts, debonding agent, sealing plates, sealant, stressbars and/or instrumentation for monitoring purposes such as through strain gauges, displacement sensors and/or video camera.

According to some embodiments, the detensioning method according to the present invention further comprising one or more steps of.

By "about" or "approximately" in relation to a given numerical value, it is meant to include numerical values within <NUM>% of the specified value. All values given in the present disclosure are to be understood to be complemented by the word "about", unless it is clear to the contrary from the context.

The indefinite article "a" or "an" does not exclude a plurality, thus should be treated broadly.

To this end, it is disclosed that the stressbar can be replaceable with other similar component or element having similar function. For instance, instead of stressbar, one or more strands or ropes can be used instead of the stressbars.

The term "structural element" as used herein refers to a basic component of a building structure which forms a structural frame building structure such as beams, pillars, roof terraces, slabs, columns, girders and/or other structural members and connections.

The term "tensioning element" as used herein refers to an element which carries tension and no compression. The tensioning element may be provided to such as bridge cable in order to support the main deck where the traffics flow. The tensioning element described herein may be for instance a tendon.

The inventors of the present invention propose a detensioning system and a method for a controlled de-tensioning of the structural elements or tensioning elements (e.g. tendons) which have previously been pre-tensioned, involving one or more clamping devices of the detensioning system espoused herein. The clamping devices are mounted for example in between the cutting location, and both the clamping devices are stressed via stressbars against one another for instance to <NUM> % of the tendon force before cutting the tendon. The detensioned tendon section (in between the clamping devices) can then be cut by for instance a diamond wire cutting machine, and subsequently the tendon can be de-tensioned in a controlled manner by de-tensioning the stressbars with hydraulic cylinders (e.g. jacks) until the entire tendon force of the tendon is released.

The structural elements or the tensioning elements to be repaired or replaced e.g. tendon, in the present context are usually pre-tensioned at a very high tension. For instance, the tendons can be stressed to at least <NUM> kN or more. Before cutting the aged or damaged tendon, the tendon force has to be transferred so that the tension of the tendon can be detensioned in a controlled manner before the tendon can be safely cut. In doing so, at least one clamping devices is arranged on each side of the structural element or the tensioning element to be detensioned.

In case a detensioning system comprising one or more clamping devices are used to detension the tendon, annular cavity which is generally a cylindrical in shape formed by the clamping device may be suitable to be used in the detensioning operation. However, the cylindrical annual cavity has lower gripping force compared to the clamping device in the present invention, wherein the substantially elongated, annular cavity comprises a gradually declining diameter along the longitudinal axis of the annular cavity. This in turn allows a smaller and more compact clamping device as well as detensioning system. This is advantageous when limited space is available for the replacement work to be carried out.

According to the gist of the present invention, the detensioning system comprises at least one clamping device <NUM> arranged on at least one side of the portion 20a of the structural element or the tensioning element to be detensioned, wherein the clamping device <NUM> having an internal surface profile, arranged to form a substantially elongated, annular cavity 55c surrounding the structural element or the tensioning element to be detensioned, comprising an empty space for receiving one or more inner elements <NUM> to be introduced therein, wherein the substantially elongated, annular cavity comprises a gradually declining diameter along the longitudinal axis of the elongated annular cavity 55c shaped by at least one peak and one groove of the internal surface profile of the clamping device <NUM>.

The inner element may be a preform element which is placed within the annular cavity of the clamping device (e.g. clamped by the clamping device), or may be a grout or a filler material to be introduced into the space within the annular cavity formed by the clamping device such that a filler hardening material having a reverse impression of the formwork can be formed encircling the structural element or the tensioning element to be detensioned.

Figure 1A shows a representative of a civil structural i.e. bridge where the bridge structure comprises one or more tendons <NUM> to be replaced that are being installed within a box girder anchored at one <NUM> or both ends. In order to replace or to repair the tendon, workers may enter into the box girder or the duct through a manhole <NUM> to install the detensioning system <NUM> according to the present invention. In this example, the detensioning system <NUM> can be used for example to detension tendon having tendon size <NUM>-<NUM> (<NUM> strands, <NUM>") or <NUM>-<NUM> (<NUM> strands, <NUM>") with a tendon force of about <NUM> kN, where the terminal end of the tendon may be abutted to an abutment.

<FIG> shows a general overview of the detensioning system according to an embodiment of the present invention. In this embodiment, the detensioning system <NUM> comprises only one clamping device <NUM> on one end and another end of the tendon is anchored at a fixed structure <NUM> such as a diaphragm or similar. The clamping device <NUM> is installed to be surrounding a tendon <NUM> such that a portion 20a of the tendon <NUM> can be detensioned in a controlled manner by transferring the tendon force to stressbars <NUM>, thereby reaching a nominal force at the portion 20a. One or more hydraulic cylinders <NUM> are involved in this operation.

<FIG> shows another embodiment of the detensioning system <NUM>, wherein one clamping device <NUM> is arranged on each side of the portion 20a of the tendon <NUM> to be cut. This embodiment is suitable for structural element or tensioning element with a pre-tensioned tendon force of about <NUM> to <NUM> kN. For instance, two hydraulic cylinders <NUM> (on each clamping device <NUM>) and two stressbars <NUM> are employed in this embodiment. Each of the clamping device <NUM> may be subject to <NUM> kN, restrained by the two stressbars <NUM> which are diagonally placed, and each of the stressbars may be stressed to <NUM> kN so as to allow the reduction of the tendon force in the portion 20a between the two clamping devices <NUM> to a nominal force before being cut.

<FIG> is similar to the <FIG> with the exception that a total of four clamping devices <NUM> and four stressbars <NUM> are employed in the detensioning system <NUM>. In this example, the detensioning system <NUM> is suitable to be used for example for tendon size <NUM>-<NUM> (<NUM>", <NUM> strands) or <NUM>-<NUM> (<NUM>", <NUM> strands), having a higher pre-tensioned tendon force of about <NUM> kN.

In the above exemplified embodiments, once the set-up of the detensioning system <NUM> is in place, the tendon force (of the tendon to be replaced <NUM>) in the portion 20a can be transferred to the detensioning system <NUM> such to the stressbars <NUM> with the help of for instance hydraulic cylinders <NUM> (e.g. centre hole jacks) mounted on one or both sides of the clamping devices <NUM>.

Thanks to the conical clamping device <NUM> which comprises an elongated and annular cavity 55c, wherein the cavity 55c comprises a gradually declining diameter along a longitudinal axis X of the annular cavity (<FIG>), the gripping force exerted by the clamping device <NUM> in such a configuration is better than the gripping force exerted by a substantially rectangular annular cavity. For this reason, the clamping device <NUM> having such internal surface profile with at least a peak and a groove can be provided in a more compact size compared to a clamping device having a substantially rectangular annular cavity. It has been found that such clamping device <NUM> gives an optimum performance in terms of coefficient of friction between the clamping device <NUM> and inner element <NUM>, as well as between the inner element <NUM> and the tendon <NUM>.

<FIG> illustrates a further embodiment of the invention, wherein the annular cavity 55c can be provided to comprise more than one segment. In this embodiment, three segments <NUM> shaped by multiple grooves <NUM> and peaks <NUM> (see also <FIG>) of the internal surface profile of the clamping device <NUM> are shown. The inventors of the present invention found out that when a six segmented annular cavity 55c is provided to the clamping device <NUM> (<FIG>), an even higher gripping force can be achieved. Such clamping device <NUM> can therefore achieve similar gripping force exerted by the clamping devices shown for example in <FIG> but having much smaller size. This is advantageous when a limited space is available on site.

As the clamping devices <NUM> are designed to comprise an annular cavity 55c comprising a gradually declining diameter along a longitudinal axis X of the annular cavity, the gripping force of such a clamping device <NUM> having a unique internal surface profile (e.g. wedge shape profile when viewed from a longitudinal section) is higher, therefore tendon force in the portion 20a can thus be effectively reduced to a nominal force through the smaller size clamping device <NUM>, as shown in the examples demonstrated in the <FIG>. The inventor found out while the number of segments <NUM> can be provided ranging between one to ten, a six segmented annular cavity 55c provided by the clamping device <NUM> seems to be the most optimal.

<FIG> demonstrate an example of the clamping device <NUM> according to yet a further embodiment of the invention which is applicable to all previously described embodiments. The two half clamping devices 120a, 120b, when aligned on top of each other, form a conical clamping device <NUM> having an elongated, annular cavity 55c. The clamping device <NUM> comprises an internal surface profile with at least one peak <NUM> and groove <NUM> such as to form at least one segment <NUM>. In this example, the clamping device <NUM> comprises six segments <NUM>, defined by multiple grooves <NUM> and peaks <NUM>. In other words, the elongated, annular cavity 55c comprises six gradually declining diameters along a longitudinal axis X of the annular cavity 55c (or the clamping device <NUM>).

A number of bolts may be used to serve as a mechanical fixing means <NUM> to tighten the two half clamps 120a, 120b when the two half clamps 120a, 120b are aligned. In this connection, it is disclosed herein that each of the segment <NUM> may comprise at least one channel <NUM> for injecting of the filler hardening material <NUM> (not shown in <FIG> but in <FIG>) and at least one channel <NUM> for venting (shown in <FIG> and <FIG>). To this end, it is reiterated that the channel <NUM> for introducing the filler hardening material <NUM> and the channel <NUM> for venting of the filler hardening material <NUM> are indistinguishable from each other. In other words, these vent channels <NUM>, <NUM> can be used for any of the roles and therefore, the roles are exchangeable although the channel provided facing bottom tends to serve as a channel <NUM> for injecting filler hardening material <NUM> while the channel provided facing above tends to serve as a channel <NUM> for venting filler hardening material <NUM>.

<FIG> illustrates an exploded view of the clamping device <NUM> of the detensioning system <NUM> according to the embodiment as illustrated in the <FIG>, wherein two clamping devices <NUM> are provided to flank on each side of the portion 20a of the tendon <NUM>. In this example, the clamping device <NUM> may have a diameter of approximately <NUM>, wherein the functioning clamping device <NUM> may comprise two half clamps 120a, 120b ("U-shape") which are mechanically assembled together with other mechanical components (e.g. sealing plates, bolts, gasket seals, hydraulic cylinders and etc.) to form a functioning clamping device <NUM>. The annular cavity 55c can be occupied by one or more inner elements <NUM>.

It is disclosed herein that the inner element <NUM> may be a preform which has a pre-determined shape, wherein the one or more inner elements <NUM> is first provided to surround the tendon <NUM>, subsequently surrounded by the two half clamps 120a, 120b. Alternatively, the inner element <NUM> may be formed by injecting a filler hardening material <NUM> or a grout through the one or more injecting channels <NUM>. The inner elements <NUM> formed by the filler hardening material generally comprises a reverse impression of the internal surface profile of the clamping device <NUM>.

<FIG> illustrates a representative inner element <NUM> introduced to the elongated, annular cavity 55c according to the embodiment as disclosed in the <FIG>. In this embodiment, several repeated declining diameters along the longitudinal axis X of the annular cavity 55c defined by multiple grooves and peaks on the internal surface profile of the clamping device forms a multiple segments <NUM>. As can be seen in the <FIG>, the annular cavity 55c comprises six conical segments <NUM>, wherein the conical segments <NUM> having without vertices (or vertex) are connected longitudinally to form the inner element <NUM>. In other words, each of the segment <NUM> comprises a flat circular surface, a curved surface and two edges, wherein one of the edges has a larger diameter than another. The inner element <NUM> may be a preform which can be introduced into the annular cavity 55c, or the inner element 55cc may be formed through the injection of the filler hardening material <NUM> or a grout.

The inner element <NUM>, which are inter-connected longitudinally, when viewed from a longitudinal view, resembles a six wedged shape "teeth" as illustrated in the <FIG>. As explained above, it is foreseeable that in every segment <NUM> (or "tooth"), a injecting channel (or a grout vent) <NUM> for introducing filler hardening material <NUM> and a venting channel <NUM> (or grout vent) for venting of the filler hardening material <NUM> may be provided at the low or the high point, respectively, of the clamping device <NUM> (cf. Moreover, it is found that for example when the wedge angle α is provided in between <NUM>° and <NUM>°, or between <NUM>° and <NUM>° or most preferably at around <NUM>°, it gives the most optimum gripping force. When the stressbars <NUM> are being stressed, the "teeth" or segments <NUM> (formed by the multiple peaks <NUM> and grooves <NUM> on the internal surface profile of the clamping device <NUM>) are drawn into the inner element <NUM>, which in turn activates lateral confinement of the clamping device <NUM> by developing compression diagonal to the pre-stressed tendon <NUM>. Therefore, an optimum force can be transferred from the clamping device <NUM> to the tendon <NUM>. Similar force transfer is achieved when the pre-tensioning tendon <NUM> is cut.

<FIG> shows an over-simplified clamping device <NUM> in a longitudinal section (cross-sectional view), wherein one or more inner elements <NUM> are provided surrounding a tendon <NUM>. The inner element <NUM> occupies the empty space of the annular cavity 55c provided by the clamping device <NUM>. In this example, the annular cavity comprises six conical segments <NUM>. <FIG> is a closed-up view of the region H shown in the <FIG>, wherein the clamping device <NUM> comprises an internal surface profile defined by multiple grooves <NUM> and peaks <NUM>, forming multiple segments <NUM>. Each of the segment <NUM> comprises a gradually declining diameter along the longitudinal axis X of the annular cavity 55c. When viewed from a longitudinal section as shown in the <FIG>, each of the segment <NUM> of the annular cavity (or equivalent to the inner element <NUM>) comprises a wedge angle α, as shown in the <FIG> and <FIG>. When more segments <NUM> are provided to the annular cavity 55c, the value of wedge angle α are usually higher.

<FIG> is a schematic representation demonstrating how the tendon force can be transferred. In this embodiment, one clamping device <NUM> is provided in between the portion 20a to be detensioned, wherein the clamping device <NUM> comprises an annular cavity 55c with multiple segments <NUM>, defined by multiple grooves <NUM> and peaks <NUM> on the internal surface profile of the clamping device <NUM>. Two stressbars <NUM> are seen in the <FIG>, whereby they are stressed P against each other with the hydraulic cylinders <NUM> until the portion 20a of the tendon force in between the two clamping devices <NUM> is completely released (transferred to the stressbars <NUM>). Thereafter, as shown in the bottom figure, the portion 20a of the tendon can be cut, followed by detensioning the tendon force T of the rest of the tendon in a controlled manner.

<FIG> shows a close-up view of the clamping device <NUM> according to an embodiment of the present invention, wherein the clamping device may comprise two half clamps 120a, 120b and sealing plates <NUM>. The inner element <NUM> may be formed from a filler hardening material <NUM> or a grout introduced through injecting channel (not shown in figure but the venting channels <NUM> are shown). Stressbar hydraulic cylinders <NUM> are mounted on the respective stressing chairs <NUM> which in turn are bearing against the clamping device <NUM>. To this end, it is disclosed that the hydraulic cylinders <NUM> maybe provided directly to the clamping device <NUM>. The portion 20a is a representative location where the tendon <NUM> can be cut once the installation of the detensioning system is ready.

<FIG> illustrates a transverse section (or cross-sectional view) of the detensioning system <NUM> from the location X shown in the <FIG>. Two half clamps 120a, 120b are provided to form the clamping device <NUM>, wherein said two half clamps 120a, 120b may be provided to tighten the two half clamps 120a, 120b through a mechanical fixing means <NUM> e.g. bolts. The clamping device <NUM> comprises a substantially annular cavity 55c, wherein one or more inner elements <NUM> can be introduced into said cavity, surrounding the tendon <NUM>. Four stressbars <NUM> are provided to the detensioning system <NUM> such that when the hydraulic cylinders <NUM> are actuated, the tendon force is transferred temporarily to the stressbars <NUM>.

The filler hardening material <NUM> or grout for forming the one or more inner elements <NUM> may be a high strength epoxy grout (e.g. Sikadur) or a high strength cement-based grout (e.g. Ductal).

For instance, an epoxy-based (e.g. Sikadur) may be used as a filler material <NUM> for the mould <NUM> formation. Such filler material <NUM> are particular suitable to be used for certain applications as it achieves the following properties:.

Alternatively, Ductal or ultra-high-performance cement-based grout with early strength development may be used to form the inner element <NUM>. The ductal or ultra-high-performance cement-based grout is based on the following properties:.

The de-tensioning system <NUM> according to the present invention comprises clamping devices <NUM> as well as stressbars <NUM>. Suitable stressbars are for instance high strength bars (Grade <NUM> diameter <NUM>) as they comprise good properties as follows:
Type VSL CT Stressbar (Bar Grade <NUM>), nominal tensile strength <NUM>,<NUM> MPa; Alternatively, SAS Stressbar (Bar Grade Y <NUM>/<NUM>, nominal tensile strength <NUM>,<NUM> MPa).

A number of hydraulic cylinders <NUM> (or bar jacks) may be used in the present invention and not limited to certain jacks. For instance, double-acting hollow core bar jacks (also called centre hole bar jacks) are mounted on top of a stressing chair <NUM>, which in turn is bearing against the back of the clamping devices. The bar jacks are used to stress as well as to de-tension the stressbars <NUM> after cutting of the tendon 20a. Ideally, the bar jacks should be calibrated before use. Suitable bar jacks may have properties as follows:.

As explained, the tendon force is able to be transferred temporarily to the stressbars <NUM> of the detensioning system <NUM>, through the clamping devices <NUM> which is clamped to the tendon <NUM>, as it allows the tendon force in between the clamping devices <NUM> (the portion 20a) to be reduced to a nominal force before the portion 20a can be cut. After cutting the tendon <NUM>, the stressbars <NUM> of the detensioning system <NUM> are released and detensioned in a controlled manner through the hydraulic cylinders <NUM>, which in turn will release the tendon force over the full tendon length.

Prior to the installation of the clamping devices <NUM>, the sheath and/or grout of the tendon may be chiselled away to increase the friction. After installation of the clamping devices <NUM>, the space between the tendon <NUM> and clamping device <NUM> (which is the annular cavity 55c) is filled with the inner element <NUM> (e.g. filler hardening material <NUM>). The internal surface profile of the clamping device <NUM> comprises at least one groove <NUM> and peak <NUM>, forming at least a segment <NUM>, wherein the segment <NUM> comprises a gradually declining diameter along the longitudinal axis X. Such conical annular cavity is advantageous compared to cylindrical annular cavity as it not only enhances the gripping force of the clamping device, it also prevent the displacement of the inner element in the longitudinal axis direction.

According to a most preferred embodiment, the annular cavity 55c comprises six segments <NUM> connected longitudinally, wherein each of the segment <NUM> are provided in a conical shape (but without the vertex or the tip region). The wedge angle α can be provided in between <NUM> ° and <NUM> °, while <NUM> ° being the preferred wedge angle.

Once the defect or aged tendon is identified, the HDPE around the surface of the tendon <NUM> where the clamping devices is to be placed may be firstly removed, followed by chiselling away the grout in order to expose the core of the strands E (<FIG>). In this connection, all loose grout particles can be removed with e.g. air pressure. Thereafter, lower half of a clamping device 120a can be positioned next to the tendon to be cut <NUM> having a part of an exposed external surface E, on two steel support (<FIG>), followed by installing three strain gauges <NUM> at the internal of lower part of the clamping device <NUM>. <FIG> demonstrate two different views of the clamping device <NUM>, left picture being viewed from a terminal end while the right picture being viewed from a lateral end.

Approximately three strain gauges <NUM> may then be installed at the lower half of the inside of the clamping device 120a (cf: 129i of <FIG>) which comes out of the grout vent <NUM> (or each grout vent comprises between one and three strain gauge). Subsequently, a debonding agent <NUM> (e.g. Lithium spray WD <NUM> or similar) can be applied on the internal surface of the clamping device <NUM> (<FIG>). Afterwards, compressible rubber gasket <NUM> can be installed on the flanges of the lower half of the clamping device 120a (<FIG>), followed by applying mechanical fixing means <NUM> e.g. bolts (<FIG>).

Once the preparation works of the lower half of the clamping device 120a is completed, similar preparation works can be repeated to the upper half of the clamping device 120b, for instance by applying the debonding agent <NUM> and the strain gauge <NUM> as explained (<FIG>). Once the preparation works on the upper half of the clamping device 120b is completed, it can be lifted using chain blocks CB to place and coordinate the position of both the upper and lower half clamps 120a, 120b (<FIG>), and followed by installing mechanical fixing means <NUM> e.g. bolts to form a complete conical clamping device <NUM>. The bolted two half clamps 120a, 120b are aligned and realigned in order to ensure that the clamping device <NUM> is centred to the tendon <NUM>. Afterwards, sealing plates <NUM> can be installed, bearing against HDPE duct, and are then bolted to the clamping devices <NUM>. Additional silicone may be applied to seal the sealing plates <NUM> against the HDPE duct. Finally, an air tightness test can be carried out with compressed air to check for leakage.

The filler material <NUM> such as grout can be prepared and introduced through a lower grout vent channel <NUM> while another channel <NUM> can be seen in the <FIG> for venting purpose. The filler material <NUM> may be pumped through the lower grout vent channel <NUM> with a vacuum assistance. A minimum of <NUM> bars pressure can be used for example to confirm whether the clamping device <NUM> is fully grouted. After the grouting process, each top grout vent channel <NUM> can be checked for complete filling of the filler material <NUM>.

Once the injection of the filler material <NUM> is completed, stressbars <NUM> (diameter <NUM>) are installed from one end to the other end of the clamping device <NUM> (in case one clamping device <NUM> is provided on each side of the tendon <NUM> to be cut as shown in <FIG>), or overlapping with the intermediate anchorages at inner clamping device (in case two clamping devices <NUM> is provided on each side of the tendon <NUM> to be cut as shown in <FIG>). <FIG> shows an example where two clamping devices <NUM> are installed on each side of the tendon <NUM> to be cut. Thereafter, other components of the clamping devices such as bearing plates, spherical washers, bar nuts can be installed, followed by installing stressing chair <NUM> at the back of the clamping device <NUM>, bearing against the stressbar bearing plate. The bolts can be torqued to (e.g. <NUM> kN) each, after grout has been cured for at least <NUM> hours. The torque force is preferably applied in two stages, first being <NUM> kN and the second being <NUM> kN. Finally, displacement sensors can be installed for example in between the clamping devices <NUM> to measure the distance the clamping devices <NUM>, and/or installed against rear of clamps to a fixed reference.

The tendon may then be cut by a diamond wire cutting machine <NUM> for instance, as illustrated in <FIG>. In order to minimize the exposure of the tendon <NUM> after stressing the clamping devices <NUM>, the diamond wire cutting machine <NUM> is preferably set up before the stressing operation. The diamond wire cutting machine <NUM> may be set up to enable remote controlled operation away from the tendon <NUM>. The cutting operation may be monitored by video camera.

Under such set up, the tendon <NUM> can be cut remotely with the diamond wire cutting machine <NUM>. After the tendon <NUM> is cut, the force is fully transferred to the stressbars <NUM> and the clamping devices of the detensioning system <NUM>. Subsequently, the stressbars <NUM> are released by retracting the centre hole bar jacks. The centre hole bar jacks can be remotely operated. For instance, a retracting of <NUM> of the bar jacks allows the tendon to be fully de-tensioned.

To this end, it is disclosed that for example two type of instrumentations can be installed for monitoring purposes. The first type of the instrumentation is the strain gauges <NUM> where they are being installed to monitor the stresses of the clamping devices <NUM>. As an example, three strain gauges <NUM>, each may be installed at the lower and the upper half of each clamping device 120a, 120b. The strain gauges <NUM> are mounted on the inside of the clamp and are connected with a wire through the grout vent <NUM>.

The second type of the instrumentation is the displacement sensors where they are installed to monitor the movement of the clamping devices during the stressing and de-tensioning operation. In this case, the position of the clamping devices <NUM> to a fixed reference and the gap between the two clamping devices <NUM> can be monitored with displacement sensors.

Claim 1:
A detensioning system (<NUM>) for detensioning a portion (20a) of a structural element or a tensioning element, comprising
- At least one clamping device (<NUM>) arranged on at least one side of the portion (20a) of the structural element or the tensioning element to be detensioned;
- Two or more stressbars (<NUM>) arranged to connect between an anchored structure and the clamping device (<NUM>) which is arranged on one side of the portion (20a) of the structural element or the tensioning element to be detensioned, or arranged to connect between the clamping devices (<NUM>) arranged on both sides of the portion (20a) of the structural element or tensioning element to be detensioned;
- Two or more stressbar hydraulic cylinders (<NUM>), wherein each of the stressbar hydraulic cylinder (<NUM>) is either mounted directly on the clamping device (<NUM>), or on a stressing chair (<NUM>) which in turn is bearing against the clamping device (<NUM>) located on at least one side of the portion (20a) structural element or the tensioning element to be detensioned;
characterized in that the clamping device (<NUM>) has an internal surface profile, arranged to form a substantially elongated, annular cavity (55c) surrounding the structural element or the tensioning element to be detensioned, comprising an empty space for receiving one or more inner elements (<NUM>) to be introduced therein, wherein the substantially elongated, annular cavity comprises a gradually declining diameter along the longitudinal axis of the elongated annular cavity (55c) shaped by at least one peak and one groove of the internal surface profile of the clamping device (<NUM>).