Information note to raise awareness of existing structures ... - Sétra

=Information note^ìíÜçêëW=p¨íê~=J=i`m`==g~åì~êó=OMMU=Information note to raise awareness ofexisting structures with externalprestressing protected by cement grout incontact with the tensile elements =`áîáä=båÖáåÉÉêáåÖ=píêìÅíìêÉë==OV=Several failures in external prestressing tendons protected by cement grout in contactwith the tensile elements have been recorded in France over the last few years. It shouldbe noted that since the publication of the circular n o 2001-16 of 28 February 2001, thistype of protection is in fact no longer used for external prestressing tendons.Structures potentially concerned are those of the period "early 1980s – beginning of the21 st century".The purpose of this note is to inform the various owners and construction managers ofthis phenomenon by outlining the state of present knowledge on the subject and bymaking recommendations concerning:• Investigations that can be carried out to assess the state of external prestressing.• Safety measures to be applied.• How to react in case of doubt over a tendon or in the event of a failed tendon..SummaryIntroduction ...................................................................................................................................2The state of present knowledge .....................................................................................................3Investigations to be performed......................................................................................................5Safety measures..............................................................................................................................6How to proceed in case of doubt over a tendon or when a failed tendon is discovered...............6Conclusions ....................................................................................................................................7Bibliography...................................................................................................................................7

Information Note of the Sétra – Civil Engineering Structures n° 29 2Information note to raise awareness of existing structures with external prestressing protected by cement grout in contact with the tensile elementsIntroductionExternal prestressing is a durable technique as illustratedby these two structures that were built in France in the1950s and are currently in excellent condition with theiroriginal tendons.Since the end of the 1970s, this technique has experiencedremarkable growth both in France and throughout theworld and is currently systematically used for largestructures in prestressed concrete.However, several failures in external prestressing tendons,consisting of strands and protected by cement grout that isdirectly in contact with the tensile elements, have occurredin France over the last few years (5 cases reported).Two bridges from the 1950sThis phenomenon must be taken into account in themonitoring of structures that have this type of prestressingand also in ensuring the safety of staff that need to worknear the tendons.The purpose of this note is therefore to inform the variousclients and contractors by outlining the state of presentknowledge on this subject and by makingrecommendations concerning:• Investigations that can be carried out to evaluate thestate of external prestressing.• Safety measures to be applied.• How to react in case of doubt over a tendon or inthe event of a failure.A failure in a tendon caused by corrosion

Information Note of the Sétra – Civil Engineering Structures n° 29 3Information note to raise awareness of existing structures with external prestressing protected by cement grout in contact with the tensile elementsThe state of present knowledgeThe various ways in which external tendons failFirst of all, the various ways in which external tendons fail need to be outlined. These are two case scenarios:Cases in which tensile elements are not in contact with the groutThis case concerns:• Tendons protected by a flexible product (grease, wax, etc.).• Galvanised tendons with or without a general duct.• Tendons whose strands are individually protected by HDPE ducts. These individual ducts may themselves beplaced in a general grouted ductIn this case, when a strand breaks, except if there is any severe frictionof the deviators, the elastic energy amassed during the tensioning of thestrand is freed individually and the failure occurs without too muchdamage.If there is a general duct, the strand can normally slide inside it, withoutwhipping and without modifying the state of stress in the other strands.If there is no general duct, whipping remains a possibility and collarsshould be fitted to avoid this happening.From a structural point of view, loss of efficiency is thereforeprogressive.A failure in strands not protected by groutCases in which tensile elements are in contact with the groutThese tendons are the subject of this information note.In this case, on the other hand, when a strand breaks, and because of thebond of the grout, the failed strand generally re-anchors itself inside thebundle of strands, the energy is not freed and the stress of the failedstrand is transferred to the other strands, thus increasing their stressaccordingly. If we consider that the tendons are tensioned at around70% of their breaking point, 6 strands need to be broken in a 19T15tendon, for example, for the breaking point to be reached in theremaining strands.The failure in this case is violent and it is a considerable amount ofenergy, equalling the totality of the elastic energy amassed during thetensioning of the tendon that is suddenly freed.The tendon is distorted and can whip sideways. Material (grout,fragments of concrete or wedges may be projected and the ends can beexpelled. These occurrences may endanger persons nearby.In theory, there is no risk of whipping from short tendons (under 40metres long). The risk increases with the length of the tendons and inthe first span after the anchorage.From a structural point of view, when an external tendon breaks, itloses its efficiency along the entire length. On the other hand, forinternal tendons in concrete, due to the bond of the grout whencorrectly applied, the stress can re-anchor itself on a short length and afailure of strands, or even of a tendon, has only a local effect on thestrength of the structure.Distortion of 19T15 tendons during dismantlingExpelling of the end of a 19T15 tendon duringdismantling

Information Note of the Sétra – Civil Engineering Structures n° 29 4Information note to raise awareness of existing structures with external prestressing protected by cement grout in contact with the tensile elementsGenerally speaking, the failure of one, or even two, external tendons is not likely to endanger the strength of the structure.Furthermore, experience shows that the probability of more than two tendons failing in a structure between two inspectionsis extremely low.The causes of recorded failuresRecorded failures are due to corrosion of tensile elements that are insufficiently protected because of duct grouting defects.These defects mainly result from instability of the grout during installation, possibly coupled with an inappropriate groutingprocedure.Generally speaking, these grouting defects result in defects in filling (partial or total voids, especially close to high pointsthat may be filled with water) and/or the presence on the upper part of the duct of a whitish product with the consistency ofa damp, soft paste topped by a layer of water and a pocket of air. This water comes from the grout bleeding and hasremained trapped inside the watertight HDPE duct.Susceptibility of the tensile elements to stress corrosion cracking is an aggravating factor.A detailed explanation of this process, which mainly concerns post-1980 grout with high levels of admixture, is describedin the Sétra-LCPC information note n°21 of July 1996 [1]. To remedy this condition, the circular 99-54 of 20 August1999 [2] introduced a technical grouting report issued by the CIP, based on the highly discriminating test known as the"inclined tube test". This test was used by the EN 445 standard and a new, simpler test method on a 1m tube has just beenpublished by the LCPC (French Public Works Research Laboratory), the laboratory test method n°65 "Cement groutingstability test using an inclined tube of one metre".It should be noted that since the publication of the circular n o 2001-16 of 28 February 2001[3], this type of protection is infact no longer used for external prestressing tendons.Structures potentially concerned are therefore those of the period "early 1980s – beginning of the 21 st century".The conclusions of inquiries and investigationsSo as to better understand the phenomenon and its extent, an RST (French Scientific and Technical Network) think tankwas set up and a number of inquiries and investigations were carried out on approximately fifty structures of the nationalroads network.In particular, a systematic survey of ducts was carried out by hammer to detect any filling defects. At this point, thedifference needs to be made between cases of ducts that "sound hollow" only in the upper part – which generally simplymeans that the grout has come unstuck – and tendons that "sound hollow" on the entire perimeter, which means that there isa serious lack of grout.The main conclusions are as follows:• No new tendon or strand failure was recorded on this occasion.• Very roughly speaking, the proportion of external prestressing tendons that have failed is around 1/1000.• Even though a high proportion of structures have at least one area of the duct that "sounds hollow" in the upper partwhen hit with a hammer, only a small number of structures, around 10 %, have an area that "sounds hollow" on theentire perimeter of the duct.• The presence of areas that are incompletely or un-grouted does not systematically result in the corrosion of thetensile elements.• No admixture is suspected in particular.

Information Note of the Sétra – Civil Engineering Structures n° 29 5Information note to raise awareness of existing structures with external prestressing protected by cement grout in contact with the tensile elementsInvestigations to be performedTwo levels of investigations can be performed to assess the condition of prestressing in a structure:First levelAs regards structures in which there is no evidence of failed strands, it is recommended that an initial assessment of theexternal prestressing be carried out at the latest during the next Detailed Inspection.A special chapter of the Detailed Inspection report must be dedicated to external prestressing.This assessment shall comprise:• A preliminary collection of available information regarding the grouting procedures applied during construction:Post-tensioning system and tendon type, type of duct, composition of grout (type and source of cement, type of anyadmixture(s) used), grouting method used (cold or hot), order and dates of grouting of ducts, any grouting incidentsrecorded, statements from people who checked or participated in the grouting, etc.• A detailed visual inspection of the external prestressing including the following points:a) An inspection of the ducts taking care to record the presence of longitudinal cracks, faulty connections, anddistortion (ovalisation, helix-shaped excrescence that may suggest a failed strand), etc.All along the duct, the presence of traces of grout, plugs or holes in the HDPE duct should especially belooked for (these are signs of regrouting during construction and places where these signs can be seenrepresent potential faulty grouting areas).b) Observation of the external appearance of the anchor protection caps:– presence of rust stains on the surface of the caps and on the anchor plate– identification of the position of the grouting vents that should be located in the upper part of the cap– quality of filling the grouting vents– signs of the grout leaking on the outside of the cap– etc.• An examination of the ducts using a hammer. It is recommended that the ducts are surveyed along their entirelength (except at concrete crossing points) so as to detect if they "sound hollow". Attention is drawn to the fact thatsome HDPE, manufactured a long time ago, may harden and appear to "sound hollow".This initial assessment will give an overall vision of the quality of the tendon protection and the condition of theprestressing. However, areas with whitish paste cannot be detected and the areas of the ducts in the upper part of thecrossbeams on piers or at the anchoring points cannot be surveyed with a hammer.It should be noted that these investigations particularly need to be carried out on structures already inspected, but wherecertain parts were inaccessible during the first inspection (tendons near elevated segments on piers for example), specificmeans of access (scaffolding or platforms) may be needed.Second levelIf there is the slightest doubt over the condition of the prestressing after this initial assessment (presence of an area that"sounds hollow" along the entire length of a tendon, traces of damp or corrosion, etc.) or if the administrator wishes toimprove their knowledge of the state of their structure, more advanced investigations should be performed, including:• The use of a capacitive sensor developed by the French Regional Public Works Research Laboratory at Autun andby the LCPC that enables the detection of voids in ducts without having to open them.The opening of windows in the duct without damaging the tensile elements and, if necessary, the removal ofsamples for analysis (water, whitish paste, results of corrosion, etc.). These openings of around 10 x 25cm are to bemade in the areas deemed suspect during the initial assessment. Attention is drawn to the need to reclose the ductsperfectly after taking the samples.

Information Note of the Sétra – Civil Engineering Structures n° 29 6Information note to raise awareness of existing structures with external prestressing protected by cement grout in contact with the tensile elements• The dismantling of the caps, if the operation can be performed quickly without placing excessive stress on thetendons.• Endoscopic observation in the ducts, particularly at the high points, close to crossbeams, if this is possible.• Gammagraphies.• Any other means that allows information to be gained as regards the content of the ducts (infrared thermography,etc).Safety measuresA risk study has been carried out on a structure with a failed tendon (the Saint-Cloud viaduct that links the Paris ring roadto the A13 motorway) by a specialised independent consultancy agency. In this case, the study concluded that even thoughthe risks incurred by employees carrying out repair operations on the failed tendon were acceptable in comparison with thelevel of risk in the professions in question, this risk was significant and should be minimised. Further information regardingthis repair is provided in a Sétra Civil Engineering Structures bulletin. The risk is significantly lower for the inspections andinvestigations outlined above that do not require a prolonged visit to the structure. Nevertheless, precautions should also betaken to minimise risks during these visits. These mainly involve simple common-sense measures that are moreoverapplicable to all types of visits inside the structure.Below several general measures are listed to assist in the development prior to inspections of a specific summaryintervention procedure and which could be adapted and added to according to the each structure.General measures:• In the event of failure, a tendon can whip violently. It is therefore advisable not to remain needlessly in thestructure.• If a failure occurs, the anchor may recede several metres or if the anchor does not recede, the tendon may sufferfrom substantial lateral distortion near the anchorage. Care must be taken therefore not to stand behind the anchorsin the axis of the tendon and upstream from the anchor near the tendon in the surrounding area.• Acoustic emission measurements carried out on a structure that had experienced a failed tendon have shown thatfailures in the wires that comprise the strands appear to occur for the most part when temperatures are low. It istherefore advisable to avoid visiting the structure if the external temperature fell below 0° C the week preceding thevisit.• If external signs cast doubts over the condition of the prestressing (traces of corrosion on the anchors or a bulge inthe ducts, etc.) or if a tendon failure has been observed, surveys with a hammer and second level inspections shouldnot be performed without prior consideration of risk management.How to proceed in case of doubt over a tendon or when a failedtendon is discoveredAside from the safety measures outlined in the preceding paragraph, it is not possible to issue general rules as regards howto proceed when a structure has a suspect or failed tendon.Each structure has to be studied individually. Experts need to be consulted, such as those from Scientific and TechnicalNetwork (RST) of French Public Works Department offices (Sétra, LCPC, Public Works technical research centres).Depending on the seriousness of the case in hand, various options may be envisaged, including:• Closer monitoring of the tendons.• Close supervision with acoustic monitoring. This monitoring enables the detection of failure in the wirescomprising the strands and highlights any acceleration in the situation.• The fitting of energy absorbers. These have already been tested in the above-mentioned Saint-Cloud viaduct andenable whipping to be avoided (although the risk of the anchor receding remains, as does limited buckling in thefirst span after the anchorage) and allow dismantling operations to be performed more smoothly.

Information Note of the Sétra – Civil Engineering Structures n° 29 7Information note to raise awareness of existing structures with external prestressing protected by cement grout in contact with the tensile elements• Local regrouting. Wide-reaching investigations recently carried out on several structures of the national roadnetwork have shown that the absence of grouting in a duct was not systematically synonymous with disorder in thetensile elements.• Drying the ducts to stop corrosion by blowing in dehumidified air. This technique, which is used on the majortendons of suspended bridges, has not yet been subjected to operational application on external prestressed tendons.• The replacement of all or part of the prestressing. The Sétra guide published in February 1990 "Précontrainteextérieure (External prestressing)" [4], provides information about dismantling methods. Experience of recordedfailures and dismantling carried out reveal that damage to the structure generally remains very limited (superficialsplintering of the concrete).It should however be pointed out that, at the time of writing, in the case of the few structures that have experienced tendonfailure of this type, in every case the decision has finally been taken to replace the entire prestressing.ConclusionsThe condition of external prestressed structures protected by cement grout in contact with the tensile elements can beassessed and monitored satisfactorily, following the recommendations provided above.Even though technical progress made over the past years has lead to a considerable improvement and although the risk offailure remains very low according to the present state of our knowledge, we recommend that diagnoses are carried out andthat any necessary repairs are made.Furthermore, the attention of administrators is drawn to the careful monitoring and maintenance required of all externaltendons, including those with other types of protection.Other defects resulting from insufficient maintenance that are susceptible to affect the durability of tendons also need to bedealt with rapidly (such as an interruption in ducts, exposed strands, damage to anti-vibration devices, etc.).Example of a defect that requires rapid repairBibliography[1] Coulis pour injection de conduits de précontrainte - Note d'information, Série Ouvrages d’art, n° 21 - Sétra/LCPCjuillet 1996 - Information note, Civil Engineering Structures (Série Ouvrages d’art) n° 21 - Sétra/LCPC July 1996.[2] Circulaire n° 99-54 du 20 août 1999 instituant un avis technique des coulis d´injection pour conduits de précontrainte,délivré par la commission interministérielle de la précontrainte - Circular n° 99-54 of 20 August 1999 instituting atechnical grouting report for prestressing ducts, issued by the interdepartmental prestressing commission.[3] Circulaire n° 2001-16 du 28 février 2001 relative à la conception de la précontrainte extérieure au béton - Circular n°2001-16 of 28 February 2001 regarding the conception external concrete prestressing.[4] Précontrainte extérieure – Guide technique – Sétra, février 1990 – référence : F9024 - Technical guide – Sétra,February 1990 – reference: F9024

Technical writersThierry KRETZ – Sétra / CTOAtéléphone : 33 (0)1 46 11 32 58 – télécopie : 33 (0)1 46 11 83 58mél : thierry.kretz@developpement-durable.gouv.frJean-Michel LACOMBE – Sétra / CTOAtéléphone : 33 (0)1 46 11 32 67 – télécopie : 33 (0)1 46 11 83 67mél : jean-michel.lacombe@developpement-durable.gouv.frBruno GODART - LCPCtéléphone : 33 (0)1 40 43 53 32 – télécopie : 33 (0)1 40 43 65 20mél : bruno.godart@lcpc.gouv.frTechnical informationsCentres d'études techniques de l'Équipement (CETE)NOTESétra’s information notice collection is onlyintended for the purpose of makinginformation rapidly available, which meansthat information may not be exhaustive orit may contain errors. Neither the writer northe administrative department may be heldliable for any such errors.The companies named in thesedocuments are for example only, asthey are deemed necessary for thecomprehension of the text as well asfor its implementation.This document is available and can be downloaded on Sétra website:http://www.setra.equipement.gouv.frResponsible publisher: Jean-Claude Pauc –Managing Director, SétraThe Sétra authorization is required for reproduction of this document (all oreven part).Référence : 0839A – ISSN : 1250-8675The Sétra belongsto the Scientific andTechnical Network (RST)of the French PublicWork Ministry