July 1993 DESIGN MANUAL FOR ROADS AND BRIDGES VOLUME 3 HIGHWAY STRUCTURES: INSPECTION AND MAINTENANCE SECTION 1 INSPECTION PART 3 BA 50/93 POST-TENSIONED CONCRETE BRIDGES PLANNING, ORGANISATION AND METHODS FOR CARRYING OUT SPECIAL INSPECTIONS SUMMARY This Advice Note outlines the organisation and planning procedures for carrying out Special Inspections of post-tensioned concrete bridges. It includes advice on the application of general and specialised methods of inspection and testing. INSTRUCTIONS FOR USE This is a new document to be incorporated into the Manual. 1. Remove existing contents pages for Volume 3 and insert new contents pages dated July 1993. 2. Insert BA 50/93 into Volume 3, Section 1. 3. Archive this sheet as appropriate. Your attention is drawn to Interim Advice Note 3, which has been issued by the Highways Agency for use on trunk roads and motorways in England. Click here to view this Interim Advice Note.
POST-TENSIONED CONCRETEBRIDGESPLANNING, ORGANISATION ANDMETHODS FOR CARRYING OUTSPECIAL INSPECTIONS
SUMMARY
This Advice Note outlines the organisation andplanning procedures for carrying out SpecialInspections of post-tensioned concrete bridges. Itincludes advice on the application of general andspecialised methods of inspection and testing.
INSTRUCTIONS FOR USE
This is a new document to be incorporated into theManual.
1. Remove existing contents pages for Volume 3and insert new contents pages dated July 1993.
2. Insert BA 50/93 into Volume 3, Section 1.
3. Archive this sheet as appropriate.
Your attention is drawn to Interim Advice Note 3,which has been issued by the Highways Agency for
use on trunk roads and motorways in England.Click here to view this Interim Advice Note.
Post-tensionedConcrete Bridges
Planning, Organisation and Methods for Carrying Out SpecialInspections
Summary: This Advice Note outlines the organisation and planning procedures forcarrying out Special Inspections of post-tensioned concrete bridges. Itincludes advice on the application of general and specialised methods ofinspection and testing.
POST-TENSIONED CONCRETEBRIDGES: PLANNING,ORGANISATION AND METHODSFOR CARRYING OUT SPECIALINSPECTIONS
Contents
Chapter
1. Introduction
2. Organisation
3. Procedures
4. Methods of Inspection
5. Stress Conditions
6. Structural Inspection
7. Internal Inspection
8. Reporting of Inspection
9. References
10. Enquiries
Annex A Sample Pro-formas
Volume 3 Section 1 Chapter 1Part 3 BA 50/93 Introduction
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1. INTRODUCTION
Background
1.1 Post-tensioned concrete bridges areparticularly vulnerable to corrosion and severedeterioration where internal grouting of tendon ducts isincomplete and moist air, water or de-icing salts canenter the ducting system. The ingress of water and sainto tendon ducts is most likely at joints in segmentalconstruction, other construction joints and anchoragesat the ends of members.
Existing post-tensioned concrete bridges with groutedtendon ducts are to be examined in a Special InspectiProgramme over a five year period.
1.2 A series of Standards and Advice Notes isbeing prepared on the Special Inspection ofPost-tensioned Concrete Bridges. This Advice Noteforms part of the series. BD 54 (DMRB 3.1.2), ThePrioritisation of Special Inspections, dated January1993, has been published. Further documents will dewith other aspects of the Special Inspection Programmie Assessment, Strengthening, Repair and Monitoring.
Scope
1.3 This Advice Note is intended to assist theProject Manager responsible for carrying out SpecialInspections of Post-tensioned Concrete Bridges,hereafter called Special Inspections. It applies to alltypes of bridges, any part of which has beenconstructed using post-tensioning techniques withtendons contained in ducts which may or may not begrouted.
1.4 This document outlines the organisation andplanning procedures which are necessary forundertaking Special Inspections. It includes advice onthe application of general and specialised methods ofinspection and testing. Guidance is also given oninspection routines, reporting requirements andappraisal of inspection reports.
1.5 The methods and procedural requirements sedown here are generally advisory. However, some aremandatory since they are procedures required in otheDepartmental Standards.
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Implementation
1.6 This Advice Note is intended to helpMaintenance Agents (MA's) in the implementation the programme of Special Inspections as mentionBD 54 (DMRB 3.1.2).
sIt is desirable for one organisation to be responsibcarrying out all three phases of the Special Inspe
work on a structure (excluding testing which will bcarried out by a specialist testing firm) it should i
the Overseeing Department at the earliest opportun
The names of specialist testing firms should besubmitted for approval to the Overseeing Departmin the usual way.
1.7 The general procedures for inspections anmaintenance records for use in England are descin TRMM 2/88, Trunk Road and Motorway Struct- Records and Inspection. In Wales, WOTRMM 2
If a MA considers that it is unable to carry out all of the
applies and in Northern Ireland BE 4/77 NIRS applies.Further advice on Principal Inspections, SpecialInspections and Testing of Concrete HighwayStructures is given in Departmental Advice Note BA 35(DMRB 3.3). Departmental Standard BD 27 (DMRB3.3) applies in England, Wales and Northern Ireland. In Scotland advice on inspections and testing is givenin Technical Memorandum SB1/78 (DMRB 3.1).
1.8 For use of this Advice Note in NorthernIreland, the Overseeing Department shall be considereto be the Roads Service Headquarters and theMaintenance Agent shall be considered to be theappropriate Roads Service Divisional Office.
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2. ORGANISATION
Inspection Management
2.1 The Project Manager for a Special Inspectionshould be a senior Chartered Engineer with specialistexperience of post-tensioned bridge design andconstruction methods. The Project Manager will beexpected to have wide experience of bridge inspectiotesting and monitoring procedures. He should beresponsible for the preparation of the programme for Special Inspection and obtaining all necessary approvfrom the Overseeing Department and the preparationthe final report.
Inspection Team
2.2 The delegation of responsibility within theinspection team should be the responsibility of theProject Manager. Members of the inspection teamshould have a sound basic knowledge of the design opost-tensioned concrete structures, stressingprocedures, grouting techniques and the operation ofstandard sampling and test equipment. Specificexperience available within the inspection team shoulinclude the supervision of specialist methods ofinspection for detection of reinforcement corrosion,voids in tendon ducts and corrosion of tendons,methods for concrete removal and instrumentationtechniques.
Special Techniques
2.3 A variety of highly specialised techniques mabe considered for the site investigations in a SpecialInspection. Such techniques already exist for thedetection of voids in ducts and the corrosion of tendoand the determination of the stress conditions in theconcrete and steel, although there is a need to recogntheir limitations. It is essential that the proposed use all specialised techniques should be clearly identifiedthe programme for the site investigation and approvashould be obtained from the Overseeing Department advance of preparing any contract documents. Techniques in this special category should be basedupon fundamental research and calibration tests, butdue to their complexity, the interpretation of the datarequires extensive experience of the method. Therefore, all site staff operating these techniques
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,2.4 The Project Manager should make a suitaand sufficient assessment of the risks for the par
l situation and inspection techniques involved,f consulting the Environmental Health Officer, the
Health and Safety Executive, the Police, and other
should be described in the programme and fullevidence of their relevant experience called for as parof the tendering process.
Safety
interested parties, where appropriate. The assessmenshould cover the risks to the workforce and the public,with particular attention given to pedestrians, vehiculatraffic, waterway and railway travellers as appropriate.
Where the assessment identifies particular problems fhealth and safety, a detailed statement should beprepared covering the procedures it is proposed toadopt to mitigate the risks. The statement should beapproved by the Overseeing Department in advance othe works starting.
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3. PROCEDURES
General
3.1 A Special Inspection should be carried out inthree distinct phases. The basic steps in the twoplanning phases are illustrated in Figure 1. Each phasof the planning process is completed by a report, whicreviews the findings, defines the work to be carried ouand outlines the technical plan for the next stage of theSpecial Inspection.
The general procedure for a site investigation isillustrated in Figure 2. In many cases, it may besufficient to carry out only the initial testing of theinvestigation. Where problems are encountered andadditional testing is considered necessary, then furtheinvestigation should be undertaken.
If at any time during the Special Investigation there isimmediate concern for public safety, the ProjectManager should consider the actions that can be taketo maintain an appropriate level of public safety such amonitoring, traffic measures, close bridge to traffic oremergency propping given in paragraph 3.8. Therespective representatives of the OverseeingDepartment should be notified and approval sought fosubsequent actions.
Preliminary Desk Study
3.2 A preliminary desk study forms Phase 1 of aSpecial Inspection. It is required in order to determinethe fundamental design and construction details and toreview the previous inspection and maintenancerecords for the bridge. This information is essential sothat construction details can be verified and previouslyrecorded deterioration or repairs can be checked durina preliminary site inspection.
3.3 The basic design details required prior to apreliminary site inspection should include the type ofdeck, mode of articulation, degree of redundancy in thdeck and the structural dimensions of the primarymembers. Information on the type of post-tensioningsystem, location of individual tendons and endanchorage positions is also needed in advance, sinceobserved surface defects may be related to the internaprestressing details.
The construction records should be examined todetermine the method and sequence of construction,
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stressing and grouting and the type of deckwaterproofing. These details may indicate the possiblocations and reasons for defects in the bridge.
The original specification for the grout and concretemixes should provide information on the expectedcement content, water/cement ratio and compressivestrength. Where possible, the construction recordsshould be consulted to ascertain the type of cement,sand and aggregate used in the grout and concretemixes, the curing times and the age of the sections athe time of stressing. Information on any additives, aentraining agents or cement replacements will beparticularly relevant. These details may indicate thelikely permeability, durability and relative performancof the various sections in a bridge.
Where insufficient design or construction records exisappropriate provision will need to be made in thesubsequent site investigation for the determination ofall material properties, section geometry, theprestressing system and reinforcement details at critisections. In this context, the term critical section isused to describe areas at high risk from water ingresareas where yield points may form in a potentialcollapse mechanism. Therefore, in general, criticalsections will include end support regions, midspanareas, regions over intermediate supports and any foof construction joint transverse to a post-tensionedcable and duct.
3.4 It is very important that all previous inspectioreports should be reviewed to identify the knowndefects and separate them from more recent problemwhich may be identified for the first time in the courseof the Special Inspection. Particular attention shouldbe paid to the purpose of any previous repairs to thebridge.
Details of any previous monitoring, reference points odatum readings may be valuable during the preliminainspection. Similarly, information on the materials antechniques employed in carrying out previous repairswill be very helpful when judging their performanceand relative value in future repairs of the same nature
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Pha
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Pha
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Preliminary Desk Study
Design Basis Construction Details
Identify vulnerable areas
Previous Inspections Maintenance Records
Report on the
Objectives of Preliminary Inspection
Confirmation ofConstruction Details
Areas ofDeterioration
Preliminary Site Inspection
Is there a need for an
Assessment identified
at this stage?
Yes
Yes
No
No
Carry out "As-Built"
Assessment
Is there a need to take
immediate action
StructuralMonitoring
Traffic Measures
Close Bridgeto Traffic
EmergencyPropping
Report on Technical Plan,Recommendations and
Objectives forSite Investigation
Prepare Contract
Documents
FIGURE 1 - BASIC STEPS FOR PLANNING A SPECIAL INSPECTION
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Site Investigation
Corrosion DetectionSampling
Void DetectionInternal Examination
Is FurtherTesting Required?
No
Yes
Yes
No
MaterialTests
VoidsTendons
StressConditions
LoadTesting
FurtherTests?
Project Manager's Report
FIGURE 2 - GENERAL PROCEDURES FOR A SITE INVESTIGATION
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A preliminary site inspection forms the basis of Phase of a Special Inspection. The objectives of thepreliminary inspection should be clearly identified inthe Report, arising from the preliminary desk study. These objectives should be based upon the availabledesign and construction information and the previousmaintenance history of the bridge.
Preliminary Site Inspection
3.5 The initial aim of the preliminary inspection isto verify the type of bridge construction, form ofarticulation, geometry of the sections and locations ofall construction joints. However, the primary purposeof this visual examination is to identify any areasshowing signs of distress and plan an appropriateinvestigation programme to determine the causes andconsequences of the deterioration (Technical Plan). The faults detected at this stage are likely to becracking, deflections, water leakage and staining,breakdown of bearings and expansion joints, steelcorrosion and losses of concrete section.
All signs of deterioration should be examined to asseswhether they warrant inclusion in the subsequent siteinvestigation. A high priority should be given to areasadjacent to critical sections, as defined in Para 3.3. Particular attention should be paid to these areas durinthe inspection and an initial appraisal made of the riskof a sudden mode of collapse and the need to takeaction to maintain an appropriate level of public safetyAdvice on possible actions and the need to involve theOverseeing Department is given in paragraph 3.8.
Consideration of Structural Form and Condition
3.6 Most forms of in-situ post-tensionedmonolithic construction carry little risk of suddenstructural collapse. Solid slabs and voided slab decksrepresent the safest form of construction. Monolithicbeams with or without composite slabs and monolithicforms of box construction are all unlikely to collapsewithout prior warning. Providing there are no built-inplanes of weakness arising from construction joints,there is a low probability of all the prestressing tendonacross a deck failing at specific transverse sections.
In comparison with monolithic construction, all typesof segmental bridge decks have a higher probability ofa sudden mode of collapse. Many forms of segmentalconstruction have been used for both simply supportedand continuous bridge decks. The basic distinctionsthat can be made between them relate to the directionof the joint, the joint material and the width of the joint.
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A classification system for segmental forms of deconstruction is defined in Paras 3.9 to 3.14, to asProject Manager with an appraisal of possible mofailure.
3.7 A variety of concrete bridge decks, bothreinforced and prestressed have been constructed
tied-down principle. A common form of this type ofconstruction is the cantilever and suspended spandecks, using half joints to support the suspended spaThe cantilever sections are stabilised by an anchor sispan, which can be tied down by vertical post-tensioning if the side span has insufficient self-weightto counter-balance the suspended span. Similar tied-down side spans have also been formed in continuoubridge decks, where the end spans could go into uplifunder certain loading conditions. This category ofstructure, regardless of the form of deck construction,carries a high risk of a sudden mode of collapse, if thevertical post-tensioning should fail.
3.8 Where a bridge is considered to be in a highrisk category and the preliminary inspection confirmsthe presence of deterioration at critical sections (asdefined in Para 3.3), the Project Manager shouldimmediately consider the actions that can be taken tomaintain an appropriate level of public safety. Therespective representative of the Overseeing Departmshould be notified and approval sought for subsequenactions.
Consideration may be given to installing monitoringequipment prior to the full site investigation. The maipurpose of the monitoring equipment would be toprovide an early warning system to detect non-linearbehaviour and imminent collapse. It should be notedhere that, in general, deflection monitoring is not anappropriate method for detecting incipient failure insegmental or tied-down bridge decks. Specific adviceon monitoring requirements is given in Para 8.8.
In extreme cases, it may be necessary to close thebridge to traffic, until further information can beobtained from the site investigations. Alternatively, itmay be sufficient to reduce the traffic loading andintroduce emergency propping measures, whilstdetailed information is gathered and more permanentsolutions are sought.
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Classification of Segmental Bridge Decks
3.9 The need to maintain an appropriate level ofpublic safety leads to a system of classification forsegmental post-tensioned bridge decks. The broadcategories of segmental decks in Table 1 are intendedto illustrate the degree of risk of a brittle mode offailure associated with various types of structures. Where the risk is high, special monitoring and testingprocedures should be considered for the siteinvestigation. Sudden failure is more likely wherethere is no secondary reinforcement across the joints.
3.10 A variety of segmental bridge decks have beenconstructed without any form of composite action. Inthe extreme case of simply supported segmental beamit is necessary to consider monitoring methods toprovide a reliable warning of imminent failure. Acombination of specialist techniques can be applied, buthe technical approach needs very careful planning andconsiderable experience. Longitudinal cracks mayindicate that tendons have severed and re-anchored, athese cracks should be investigated and monitored withsuitable instrumentation.
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3.11 The probability of a sudden mode of collapseis reduced when simply supported segmental beamstransversely connected to form a grillage. There hasbe a degree of load sharing and severe corrosion oflongitudinal prestressing tendons along a potentialfracture line before a failure can take place. Nevertheless, the risk is high if exposure conditions asevere and a general loss of prestress allows water tpenetrate more easily into the joints. It should be notthat fracture lines across a grid may not be straight, sany long-term monitoring system has to be plannedaccordingly to take this into account.
3.12 Simply supported box girder bridge decksconstitute another form of beam grillage in terms offailure mode and the probability of a rapid failuremechanism is still high. Therefore, the monitoringprocedures considered for this form of deck should bsimilar in principle to a beam grillage deck.
DECK JOINT DIRECTION ELEMENT TYPE RISK OFTYPE BRITTLE MODE
OF FAILURE
Simply supported Transverse Beams Very high(non-composite)
Longitudinal and transverse Beam grillage High
Transverse Box girders High
Longitudinal Monolithic beams with Very lowtransverse prestressing
Simply supported Transverse Composite beams with Medium(composite) in-situ top slab
Transverse Composite beams with Lowin-situ top and bottomslabs
Continuous Transverse Composite beam and Lowslab, and box girders
Table 1 - Classification of Segmental Bridge Decks
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3.13 Composite decks generally represent a muchsafer type of bridge structure, since the presence of ain-situ slab connecting precast segmental units provia better degree of redundancy if isolated tendons shofail. Moreover, the slab helps to protect the beamsfrom the ingress of chlorides.
Where a simply supported composite deck is alsoformed with an in-situ bottom slab, the formation of acentral hinge is less likely. The presence ofuntensioned reinforcement should be sufficient tospread the risk of failure between adjacent segmentsIn addition, the bonded untensioned steel will alsoprovide a degree of gradual yielding, so that a regulamonitoring procedure should be adequate to detect tonset of any failure mechanism.
3.14 In general, continuous forms of segmentalpost-tensioned decks carry a lower probability of asudden mode of failure. Due to continuity over theintermediate supports, it is necessary for a mechanisto form before a collapse can occur. Therefore, at letwo complete decks sections must yield in an end spof a continuous deck. Internal spans require threehinges to form a mechanism so a collapse condition more unlikely to develop, but it may still occur withouvisible warning.
Where continuous bridge decks are formed from aseries of beam segments with a composite reinforcedconcrete top slab, there is normally a significantamount of bonded untensioned reinforcement crossinthe transverse joints. Hence, there should be anadditional reserve of strength in the vicinity of the pieand a sudden type of collapse is less likely.
Site Investigation
3.15 The technical plan and particular objectives fthe site investigation should be defined in the Reportfollowing the desk study and the preliminary siteinspection. The general procedure for a siteinvestigation is illustrated in Figure 2. In many cases,may be sufficient to carry out only the initial testing othe investigation. Where problems are encountered additional testing is considered necessary, then furthinvestigation should be undertaken.
The main purpose of the site investigation is todetermine the existing conditions at all critical sectionso that a realistic appraisal of the residual strength ofthe bridge can be made. Sufficient numericalinformation will be required in order that a presentcondition load assessment of the structure can be
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undertaken.
The areas selected for detailed investigation shotake into account the form of deck, construction
and the type of deterioration already detected. Particular attention is necessary in selecting the mo
the presence of voids in ducts and the conditionsgrout, tendons and anchorages. Detailed guidancthe relative merits and application of various methis given in Chapter 4.
The Project Manager should prepare an associatschedule of standard sampling and testing to depresence of corrosion activity and assess the co
of the concrete and reinforcement in the vicinity of
material sampling, laboratory and site testing shacarried out by specialist testing firms or laboratorapproved by the National Measurement Accredita
Service (NAMAS) for the type of laboratory testirequired, or by equivalent accreditation bodies omember states within the European Community
The testing procedures shall be carried out inaccordance with the recommendations given in Cl
7.3.2/7.3.3 of Departmental Advice Note BA 35/90
(DMRB 3.1) should be referred to for testingprocedures. These standard tests on the bridge
be complementary to previous principal inspectiomay not be required if sufficient information onmaterial properties has recently been obtained.
3.16 Consideration should be given to any fur
appropriate techniques which are required to establis
critical sections under investigation. All routine
(DMRB 3.3). In Scotland, Appendix 3 to SB1/78
material tests, the need to widen the search for voidstendon ducts or the corrosion of tendons. An interimreport may be required to summarise the initial resultsfrom the detailed investigation and producerecommendations for any additional testing that may deemed necessary.
Depending upon the structural form of the bridge, theresults of previous load assessments, or the perceiverisk of a sudden collapse, it may be necessary to chelocal stress conditions in the steel or global stressconditions in the concrete. Where load distribution inthe deck depends upon residual levels of transverseprestress, it may also be appropriate to applyincremental load testing techniques. However, it isvery important that the maximum applied loading isrestricted to serviceability levels to avoid damage to tstructure.
The introduction of such specialist testing wouldrequire technical approval from the Overseeing
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Department. The organisational requirements forhighly specialised techniques are detailed in paragraph2.3, while the principles of these methods and theirlimitations are described in Chapter 5.
3.17 The results of Phase 3 of a Special Inspectionshould be reviewed in the Project Manager's Report. Inaddition, the structural condition, risk assessment andmonitoring requirements should be summarised to forma basis for future management of the bridge. Thequantitative information gathered during theinvestigation should be recorded in a suitable form forfuture reference and recommendations made forsubsequent load assessment of the structure. Furtherguidance on the requirements for this report is given inChapter 8.
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Volume 3 Section 1 Chapter 4Part 3 BA 50/93 Methods of Inspection
4. METHODS OF INSPECTION
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General
4.1 Methods of inspection for post-tensionedconcrete bridges range from a visual inspection tocomplex non-destructive and semi-destructive methodsThe methods adopted in a site investigation shouldcommence with a simple visual examination androutine surface and material tests under Phase 3 of aSpecial Inspection. Progression to the more complexmethods of the Special Inspection may be justified ifthere is evidence of tendon corrosion and a risk ofsudden failure in a structure. However, it should berealised that serious corrosion of the tendons can occuwithout any visual evidence.
An indication of general corrosion of the reinforcementin the concrete, may be taken as indicative of thepotential for corrosion occurring in the prestressingsteel. Therefore, the methods of determining corrosionrisk, as outlined in Departmental Advice Note BA 35(DMRB 3.3), (SB1/78 in Scotland) provide a valuableprecursor to the use of other inspection techniques. Inparticular, high concentrations of chloride ions increasethe probability of tendon corrosion. Therefore,chloride ion content of the concrete should also bedetermined.
Non-destructive testing can be used to assist in thedetection of voids in the tendon ducts. If no voids arefound this does not preclude the possibility of corrosionoccurring. However, in fully grouted ducts anycorroded and broken wires will quickly re-anchor andthe risk of full loss of prestress should be reduced. Anassessment of the potential for a sudden mode ofcollapse should be undertaken, as outlined inparagraphs 3.6 to 3.14, and the necessity for furtherinvestigations should be determined.
If voids are found and the conditions within theconcrete are conducive to corrosion of the steel theninternal examination of the tendons should beundertaken. The method for gaining access to thetendon duct should be chosen considering the positionof the duct and the degree of damage that will becaused. In all cases, drilling holes should be carried ouwith the agreement of the Project Manager and utmostcare must be taken to ensure that the tendon is notdamaged.
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4.2 The visual examination of post-tensionebridges should be carried out according to the ad
given in Chapter 6. The inspection should be carrout in such a way as to identify actual and poten
areas of distress. As such, the inspections shouperformed by persons with experience ofpost-tensioned structures as specified in paragrato 2.3.
Prestressed bridges are normally designed to avoid
cracks can have serious durability implications anindicate a loss of prestress. Cracks along the lintendon ducts may be indicative of corroded and bwires or tendons. Such cracks may be formed b
bursting forces that are generated as a broken wirand then re-anchors.
Signs of general corrosion on the surface of theconcrete may be indicative of conditions within theconcrete which are conducive to corrosion of the
tendons. The presence of any water leakage throug
Detailed advice on the possible cause and interpof visual defects is given in Chapter 6.
Visual Examination
cracks in the concrete. As such, the development of
the deck should be recorded and the source located.
Void Detection
4.3 The detection of voids in post-tensioning ductis important in isolating potential areas where corrosioof the tendon may occur. The methods of detection cbe non-destructive and a guide to the use of suchtechniques is included in BS 1881: Part 201. Determining the position of any voids, prior to aninternal examination to ascertain the condition of thetendon, should restrict the degree of damage caused the structure. However, the only certain method ofdetermining the tendon condition is by exposing it forvisual inspection.
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Endoscope
4.4 The endoscope is an optical instrument whichenables inaccessible places, such as the interior ofpost-tensioning ducts, to be inspected. Access to theduct may be gained by the drilling of 25mm diameterholes through the concrete. Where possible, a holeshould then be made in the duct using a hand-heldchisel. The number and position of the holes drilledwill depend upon the cable profile and the ease ofaccess for drilling. Holes should be drilled as close tothe anchorages as possible, at midspan and at eitherside of high points where voids are most likely to haveformed.
The tendon can be visually inspected by viewingthrough for example either a rigid tube with reflectingprisms or a flexible tube with a fibre optic system. Themajor limitation of this technique is the difficulty ofaccess for the drilling of holes into the duct. Drillingshould be carried out only with the agreement of theProject Manager. The Project Manager should thenensure that there is close supervision of the drillingoperation by a suitably experienced member of theInspection Team to prevent damage to the tendon.
An endoscope survey carried out using small diameteinspection holes drilled at carefully chosen locationsoffers a reliable method of checking the condition oftendons. However, the tendon condition can only beinspected locally to the drilled hole and thereforecorrosion elsewhere could go undetected.
Pressure-vacuum Testing
4.5 The techniques of pressure and vacuum testinenable the volume and continuity of voids and leakageinto a duct to be determined. Access has to be gaineto the top of the duct by drilling 25mm diameter holesthrough the concrete. A small hole should then bemade in the duct using a chisel. The number andposition of the holes drilled will depend upon the cableprofile and the ease of access for drilling. Holes shoube drilled as close to the anchorages as possible, atmidspan and at either side of any high points wherevoids are most likely to have formed.
The continuity of any voids found is determined byevacuating each hole in turn and measuring anypressure change at the remaining holes. The volumethe voids is estimated by using a water gauge connecto the evacuated holes and measuring the height ofwater drawn up a perspex tube. Leakage out of theduct is measured by pressuring it and measuring theinput flow rate required to maintain a set pressure.
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4.6 The method of radiography utilises gammrays or high energy X-rays as an energy source wwill penetrate the concrete. Photographic film, plaon the opposite side of the concrete from the sourwill be exposed to varying degrees of radiation
under examination. Thus, steel shows up on thenegative as lighter than concrete because its highdensity impedes the radiation to a greater extent, wvoided areas show up as darker images.
A limitation on the use of this technique is that it isessential to have access to both sides of the conc
Errors can arise in the estimation of the volume of thevoids if there is leakage from the duct. In addition, thepressure within a partially grouted duct after it has beeevacuated is not uniform. The major limitation of thistechnique is the difficulty in making holes into the ductparticularly at the end anchorages where the amount obursting steel makes drilling difficult.
In all cases, drilling holes should be carried out withthe agreement of the Project Manager and utmost caremust be taken to ensure that the tendon is not damageNo information can be gained on the condition of thetendon using this method of testing alone. Therefore aendoscope survey, as described in paragraph 4.4, or aopen visual examination should also be undertaken.
Radiography
depending on the variation in density of the material
The thickness of concrete penetrated is limited to500mm with a gamma ray source. High energy X-raysare more suitable for examining concrete thicknessesup to 1m. Current developments include, for example,a Betatron radiation generator which can be applied tobridges.
This non-destructive method is the most direct meansof providing pictorial evidence of the interior of theconcrete. However, the technique is not able to detectcorrosion sites with any degree of accuracy althoughbroken wires and cables can be detected.
General guidance on the use of radiography is given inBS 1881: Part 205. However, the extensive safetyprecautions and highly specialised equipment used inradiography makes it essential that this work is onlycarried out by experienced radiographers withknowledge of working with concrete. The Health andSafety Executive should always be consulted beforeradiographic methods are employed.
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Other Methods
4.7 The surface penetrating radar system works othe principle of the reflection of short duration pulsesfrom interfaces between materials with differentdielectric constants such as bars, voids and ducts. Unfortunately, although radar can be used to accuratelocate the position of metallic tendon ducts, the ductsmask any reflection from within. Therefore, thistechnique is not at present able to determine tendoncondition or deficiencies in the grout in structures withmetallic ducts. This limits its use considerably.
4.8 The impact-echo method is currently anexperimental technique that relies on the propagation a stress pulse through the concrete, which is reflectedby internal discontinuities or external boundaries. Thepulse is introduced into the concrete by a mechanicalimpact on the surface and the reflected waves aremeasured at the surface by a receiving transducer.
The major limitation of this technique appears to bethat the boundaries of a defect cannot be determinedprecisely since points within the object will interactwith the propagating waves. Although the method hasbeen widely researched, it has not yet been fully provefor general applications. The specialised equipmentand difficulties in interpreting the results makes itessential that this type of work is performed byexperienced operators.
4.9 The method of reflectometry utilises therelationship between the electrical parameters anddefects in tendons. A high frequency signal is input atone end and received at the same end. Changes inimpedance are used to indicate the type of deterioratioin terms of a reduction in tendon section and voids inthe surrounding grout.
Although this technique has been validated and usedelsewhere it has yet to be proven in practice in theUnited Kingdom. The major limitation of thetechnique is the need to have access to one end of thetendon. Where there are a number of faults in thetendon, the first fault encountered may mask the other
The specialised equipment and difficulties ininterpreting the results makes it essential that this typeof work is performed by experienced operators. Anyexposure of the end anchorages of the tendons must bcarried out with the utmost care and under thesupervision of the Project Manager.
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Internal Examination
4.10 Once voids and potential corrosion ofpost-tensioning tendons have been identified, the mdirect way of establishing the degree of damage is
an internal examination of the duct. There are a nof ways by which access can be gained topost-tensioning ducts in order that an internal
examination can be carried out. The degree of damcaused to the structure will depend upon the meth
exposure chosen. Any exposure of the tendon shobe carried out under the direction of the Project
the tendon is not damaged.Manager and utmost care must be taken to ensure that
Percussive Methods
4.11 Closely supervised percussive methods, suchas hand-held and small machine mounted impactbreakers, provide an effective method for exposingpost-tensioning ducts. The damage caused to theconcrete section may be irregular and strict controlmust be exercised over the exposure of the ducts. Exposure of the tendon should be carried out using ahand-held chisel to ensure no damage is caused. Thesmethods should be limited to the exposure of easilyaccessible tendons in order to limit the level of damagecaused. In addition, microcracking of the exposedconcrete surface layers is likely and subsequent repairprocedures will need to take this into account.
Diamond Core Drilling
4.12 Diamond core drilling offers an effective wayof exposing post-tensioning ducts. However, the use othis technique relies upon the operative knowing whenthe core drill has made contact with the duct, in order tostop drilling. An alternative is the use of a drill with anautomatic cut-out which should stop once the duct isreached. A hand-held chisel should be used to exposethe tendon. Every effort should be made to ensure thathe cooling water does not penetrate the duct.
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High Pressure Water Jetting
4.13 High pressure water jets can be used to exposlengths of post-tensioning ducts and then only afterrotary drilling of pilot holes have established theprecise location. Every effort should be made to ensurethat the water cannot penetrate the duct. Therefore, ifleaks in the ducts are suspected this method of exposushould not be used.
Cutting of the concrete using this method is likely toproduce irregular shaped holes. Although water jettingwith a grit additive offers better control for concretecutting, it is particularly dangerous since it can also cutthrough the prestressing steel. Hence, it should not bepermitted for this purpose.
Grit Blasted Holes
4.14 Dry grit blasting can be used to form accessholes in the concrete in a similar manner to highpressure water jetting. The use of a vacuum pump tocarry away the debris negates the problems caused bydust and fly back of material. Caution has to beexercised when using this technique since the duct andtendon may be damaged. However, the lack of watermakes this a more attractive option in particularsituations.
Material Testing
4.15 During the internal examination, grout samplesfrom the duct should be tested to determine the degreeof carbonation which has occurred and whether it hasbeen contaminated with chlorides. If the grout is foundto be wet, contaminated with chlorides or carbonatedthen there is a risk of local corrosion occurring.
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5.1 Determination of steel stresses and concretestresses at critical sections may be used to give anindication of the local and general levels of residualprestress. Sufficient measurements should be taken ensure confidence in the results. The current live loadperformance and behaviour of the deck in the long-termay be checked using a carefully controlled load testand monitoring of the structure at critical positions.
Steel Stresses
5.2 The current levels of stress in individualprestressing tendons can be determined directly by hodrilling using electrical resistance strain gauges. Thetechnique relies on the principle of stress-relief and isan adaptation of the centre-hole technique. Thediameter of the hole drilled may be 1-3mm, dependingupon the form of the prestressing tendon. Thecentre-hole method gives a very accurate picture of thparticular wires drilled, but the results must then beextrapolated to the whole tendon. The stress measureis the total stress, it is necessary to subtract locked-inmanufacturing stresses. Steel stress measurements ogrouted tendons represent conditions for about 1m oneither side of the test location and a representativenumber should be taken if reliable average values arerequired.
The centre-hole technique, although partiallydestructive, may be used on a bridge in service.Alternatively, the residual prestress may be determineby a complete release of stress in selected wires orstrands. Several wires in a tendon may be cut byclosely controlled use of a hack-saw if access ispossible. The main benefits of this method arecheapness and direct measurements of the residualpre-strain in individual wires or tendons, it is necessarto think very carefully before cutting any wires on abridge in service.
Concrete Stresses
5.3 Specialist methods have been developed toprovide in-situ measurements of concrete stresses usinstrumented coring and slot-cutting techniques. Theresults obtained from such stress relief methodsrepresent the total stress conditions in the concrete.
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Cores of 75-150mm diameter may be drilled into theconcrete and the stress release determined usingcarefully selected strain gauge patterns.
A likely lower-bound valve for the concrete modulusmay be determined from compression tests on theconcrete cores. Additionally a hydraulic jackingsystem may be inserted into the hole produced by thecoring process, to provide an in situ determination ofthe elastic modulus. This form of test provides anin-plane measurement of the modulus and represents aupper-bound composite value created by thesurrounding prestressing tendons, reinforcement andconcrete.
The main advantage of the instrumented coringtechnique for the assessment of existing levels ofconcrete stress is that principal concrete stresses can bdetermined in both magnitude and direction. Althoughtotal concrete stresses are obtained, the likelymagnitude and presence of self-equilibrating internalconcrete stresses due to secondary effects can also bedetermined during coring.
The slot-cutting technique for concrete stressdetermination may be carried out in several ways. Narrow slots, 300-500mm in length, can be producedusing a diamond saw mounted on a travelling rig andcooled by water. Strain measurements taken across thslot on either side of the slot can be converted intostresses using laboratory calibrations and the localelastic modulus determined by hydraulic jacking orcompression tests from a complementary core test.
An alternative method of slot cutting employs amounted air-cooled diamond saw in conjunction withvery thin semi-circular flat jacks which are used torestore the stress state uni-axially. The main advantagof this form of slot-cutting technique using pressurecompensated jacks is that a value of elastic modulus isnot required to be known.
The concrete core and slot-cutting methods give auseful guide to the overall concrete stress levels at agiven point. Where parallel beams are connected by insitu concrete or mortar joints, the slot-cutting methodscause minimal damage to the structure and can provida measurement of the effect transverse prestress acrosthe longitudinal joint. Both coring and slot cuttinghave a stress raising effect and required careful makinggood to prevent ingress of water and road salts.
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Cutting of the reinforcement in a structure should beavoided. Apart from local damage to the structure,highly stressed reinforcement may cause debondingand local micro-cracking at the concrete surface. Anystrain gauge measurements on the surface are thereforlikely to be invalidated.
The various methods have different levels of sensitivityand the selected techniques should be appropriate to thanticipated levels of concrete stress and the accuracyrequired in the determination. It is very important torealise that concrete stress measurements do notprovide a means for identifying local strand failures ina section. Therefore, the proper use of concrete stressmeasurements is to provide an indication of the globallevels of residual prestress in a deck.
Load Testing
5.4 The main purpose of load testing any form ofbridge deck is to determine the effective levels oftransverse load distribution and correlate the structuralanalysis with the assessed structural action. Verysignificant reserves of strength can be present in somebridge decks due to secondary effects such asmembrane forces, edge stiffening, end restraint andcomposite action.
Local damage caused by the failure of a small numberof prestressing tendons cannot, in general, bedetermined by load testing. However, the globalbehaviour of a bridge deck can be verified by acarefully selected pattern of loading. It is veryimportant that the magnitude and positions of theapplied loading is maintained at levels representingserviceability conditions but no higher. The loadingshould be applied in controlled increments and the deckresponse monitored by an appropriate pattern of straingauges in order that the test does not cause any damagto the bridge.
Deflection monitoring and dynamic stiffnessmeasurements are generally not appropriate for loadtesting and are unlikely to detect the onset of non-lineabehaviour in segmental structures. Load testing shouldnot be used to assess the deterioration in shear strengtof a post-tensioned bridge.
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6.1 The main objective of the structural inspectionis to determine the cause and extent of deterioration fthe purpose of assessing structural integrity. Theresults of this inspection may also form the basis forremedial measures and strengthening works in thefuture.
The inspection should be carried out by experiencedinspectors who should systematically record theserviceability problems and be able to interpret thesignificance of the faults. Particular attention should bpaid to determining the presence of, and reason for, acracks and the location of water leakage through thedeck. The conditions surrounding all end anchoragezones, half joints and construction joints should alsoreceive special examination.
Cracking
6.2 The location and direction of cracks are often valuable first indication of the present condition of astructure. Useful advice on diagnosing non-structuralcracking may be found in the Concrete Societypublication `Non-structural Cracking of Concrete -1992'. Typical cracks found in prestressed concretestructures are summarised in Table 2, together with thpossible causes to which they may be attributed. Aparticular crack can be the result of a combination ofseveral defects.
It is important to identify the source of cracking, sinceit will lead to a more precise determination of the actustructural damage. Cracks which appear to be criticato the structural integrity of the bridge should bemonitored at regular intervals to check the developmeof further deterioration. Such information will benecessary for the future condition assessment of theresidual strength of a section.
6.3 Transverse flexural cracks may be anindication of a significant loss of prestress or tendonfailure in the midspan or intermediate support region oa deck. However, the force in a fractured tendon ismainly transferred to adjacent tendons and it is likelythat the only significant damage will be in the re-anchorage zones on each side of the fracture. Thedamage to the concrete section will depend primarilyupon the grouting in the duct and the amounts of
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surrounding shear reinforcement.
The presence of significant bursting stresses at afracture point may be indicated by longitudinal surfacecracks along the line of a broken tendon. Where atendon fails in the region of an end anchorage,structural damage may take several forms.
Local splitting along the line of a fully grouted duct islikely to be well contained, since the areas of shearreinforcement and end anchorage bursting steel shoube more than adequate. However, tendons at the enof a deck are often inclined and there may be asignificant reduction in the contribution to shearcapacity, leading to inclined shear cracks in the websbeams.
6.4 Inclined shear cracks in webs may also occurin bending moment cross-over regions in continuousbridge decks, where prestressing tendons for sagginghogging moment regions may be anchored. Aquantitative assessment of damage in this type ofregion may require both concrete stress measuremenusing coring techniques and direct measurements ofsteel stress in the local shear reinforcement.
6.5 Severe structural cracking can also occur inprestressed bridge decks as a direct consequence ofvehicle impacts. Apart from the local crushing at thepoint of impact, it is possible for secondary shockwaves to generate large scale longitudinal cracking inthe bottom flanges of beam and slab decks. Diagonashear cracks may also develop in the webs of beamswhich are supported on the lateral restraint bearings.
Water Leakage
6.6 The bridge deck drainage system should be tsubject of a thorough visual examination. All gullies,downpipes and manholes should be checked todetermine whether the system is working effectively. Ideally, checks should be carried out during intervals heavy rain and subsequent dry periods. Careful noteshould be made of the influence of the carriagewaysurface condition, including cracks or pot-holes. Thelocation of any surface ponding on the deck should berecorded and related to midspan and intermediatesupport regions.
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6.7 Surface water on the deck may enter thefootways and any central reservation areas along thekerblines. Water and de-icing salts may remain trappin these areas or penetrate into service bays, serviceducts or voids within the deck construction. Water maalso flow towards the ends of the deck and remaintrapped against upstands formed at the deck expansijoints.
The introduction of drainage holes into the soffit of adeck may permit trapped water to escape. Strict safeprecautions should be exercised in drilling anyexploratory drainage holes, since the trapped water mbe alkaline and, in extreme cases, may cause severeburning of the skin or damage to eyes. Specific advicon draining voids within bridge decks is given inChapter 10 Departmental Advice Note BA 35 (DMRB3.3).
6.8 During the detailed inspection, all signs ofwater leakage through cracks in the deck slab,construction joints, expansion joints and half jointsshould be systematically recorded, along withcomments relating to the cause and source of the waSimilarly, all surface leaching should be recordedtogether with any signs of discolouration which mayindicate the presence of internal rusting or othercontaminants.
Deflections
6.9 The effect of broken tendons in grouted orpartially grouted ducts is unlikely to produce anyvisible or measurable deflections. Broken tendons donot constitute a significant loss in stiffness in a beam deck cross-section, since the adjacent tendons will taup the force released, producing only a local change steel and concrete strain. Where the tendon ducts inbeam are ungrouted, broken tendons will release theiforce along the entire length and the resulting loss inprestress can produce a significant change indeflections and end rotation.
6.10 The presence of hogging deflections in themidspan region of a post-tensioned bridge deck isnormally indicative of a satisfactory level of residualprestress. Where sagging deflections are observed, suggests there may be excessive losses in prestress to creep, shrinkage or temperature effects. Additionasources of prestress losses may be due to a largenumber of in-situ joints between precast segments orthe use of concrete with a low elastic modulus.
6.11 It is important that the deflected shape of abridge deck should be recorded, particularly for long-
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span structures. The profile of the edge beams athe soffit, the bottom flange or the parapet stringmay form a convenient reference line. A carefulof the temperature conditions should be taken a
future deflection measurements recorded under scircumstances. In deep sections, such as box-gi
decks, it may be necessary to record temperatugradients at the same time.
Concrete Spalling
6.12 Concrete spalling may occur for a variety ofreasons and careful note should be taken of the locatand orientation of all surface spalling. The mostcommon cause is likely to be corrosion of ordinaryreinforcement producing surface delaminations. Corrosion of prestressing tendons may or may not beexpansive, depending upon the supply of moisture anoxygen and the type of iron oxide formed. If corrosionof the tendons has caused splitting of the concretesurface layers, the cracking is likely to be more deeprooted compared to that caused by corrosion ofordinary surface reinforcement.
Large-scale concrete spalling may be observed wherewater filled voids in tendon ducts have frozen. Localspalling of the concrete surfaces may also occur due stress concentrations arising from bursting stresses,misfit between segments or misalignment of tendonducts across joints between precast units.
Steel Corrosion
6.13 Normal methods for detecting the potential focorrosion and the presence of excessive amounts ofchloride should be sufficient. These tests andprocedures are fully described in Clause 7.3Departmental Advice Note BA 35 (DMRB 3.3),(SB1/78 in Scotland). No general non-destructiveprocedures currently exist for detecting corrosion inprestressing tendons and serious corrosion of thetendons may exist without any visual signs of distress
STRUCTURAL LOCATION CRACK POSSIBLE CAUSESECTION DIRECTION
Soffit End of span Longitudinal Bursting stresses. (beam or slab) Lack of end block
reinforcement. ASR in concrete.
Midspan Longitudinal ASR in concrete.
Transverse Loss of prestress.
Broken tendons.
Excess live load.
Web End of span Diagonal Shear stresses.
Longitudinal ASR in concrete.
Loss of prestress.
Duct flotation.Broken tendon. Frozen water in ducts.
Web Over support Vertical Loss of prestress.(cantilever/continuous beam)
Top flange Midspan Transverse Differential shrinkage.(T-beam/box beam)
Over support Longitudinal ASR in concrete.
Transverse Differential shrinkage.
Broken tendon.
Loss of prestress. Excess live load.
Table 2 - Typical Cracks in Prestressed Concrete Sections(Not an exhaustive Table)
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Construction Joints
6.14 The type of construction joints within a bridgestructure should be identified and recorded in separatcategories. The condition of joints between precastunits and the materials used to complete the jointshould be carefully noted. In-situ joints betweenprecast units may be formed with dry packed mortar,concrete or epoxy resins. Plain construction jointsbetween deck pours may be simple dry joints and thejoint direction could be vertical, horizontal, stepped orinclined. The lengths and widths of all joints should brecorded.
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6.15 All construction joints in post-tensionedstructures represent a potential plane of weakness. Concrete sections may act in a partially-crackedmanner, when compressive stresses across the joint between 1-2 N/mm . All joints should be examined2
throughout their length to search for signs ofmicrocracking or water-staining. The start and finishof any cracking or water penetration may be significanand should be recorded in detail. Any areas causingconcern should be noted and marked for potentialmonitoring in the future.
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Deck Bearings
6.16 The inspection of the bearings should becarried out with the objective of confirming themovements of a bridge structure are occurring asintended without damage to the deck, the fixings or thsub-structure. The parapet fencing and carriagewaysurfacing should be checked for signs of movement orotation at both ends of a bridge and at all intermediasupports. Similarly, all bearings should be examinedfor signs of movement and rotation, irrespective oftheir intended design function. The condition of thematerials in the bearings and all forms of deterioratioshould be noted.
The integrity of any fixings to the bearings should benoted and any local failures recorded. Holding downbolts may be required to carry occasional uplift forcesin some forms of bearing and the performance of thebearings should be observed under live load.
General loss of prestress in continuous or semi-continuous post-tensioned structures may lead tosignificant increases in the axial forces carried by uplbearings or tied-down post-tensioned anchors.
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7.1 The methods of inspection described inChapter 4 for internal examination of post-tensioningsystems may be used alone or in combination,according to the type of problem being investigated. The method employed will be particularly dependentupon site access, safety and the type of structure.
Duct Positions
7.2 The first priority in performing a detailedinternal examination is to establish the actual locationof tendon ducts at each section under investigation. During construction, tendon ducts may float upwardsbetween tie-points. Typical vertical movements can be25-50mm, but displacements exceeding 100mm are nouncommon in some forms of construction.
Duct displacements are a potential cause of seriousdamage during investigations involving any form ofhole drilling. Tendons may be hard against the bottomof a duct at any position due to duct flotation. Therefore, drilling holes into the bottom of a tendonduct at midspan to use an endoscope is unlikely toprevent the tendons being damaged. Allowance shouldalso be made for horizontal displacements of tendonducts.
Grout Integrity
7.3 The detection of voids within a grouted tendonduct may be achieved using the non-destructive andsemi-destructive methods described in paragraphs 4.3to 4.9. However, a full inspection of the void can onlybe made by internal examination using the variousmethods described in paragraphs 4.10 to 4.14. Themethods adopted for exposure of the grout should bechosen to allow the condition and properties to beexamined in an undisturbed state and samples to beremoved in sealed containers. Supervision of theexposure and removal of specimens is of paramountimportance.
It is important to determine the in-situ moisturecontent, colour and composition of the grout. Wheretendon ducts are partially filled, a cement grout is likelyto be white rather than grey due to the effect ofcarbonation. A grout may be completely dry and solid
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within a duct or it may be a wet paste or shattered andbroken in appearance. The composition of the grout,alkalinity, chloride content and the presence ofadditives should all be determined from dry samplesremoved from a duct.
Tendons
7.4 The exposure of post-tensioned tendons maybe achieved by using the methods described inparagraphs 4.10 to 4.14. It is essential that the entireprocess of concrete removal, opening of internal ductsand removal of grout should be closely supervised at atimes by experienced inspectors.
In order to examine tendons in an undisturbed state anto avoid damage to the prestressing steel, the groutaround the tendons should be carefully removed byhand methods. The type and size of the tendons in thduct should be confirmed and the presence of any formof surface corrosion or pitting carefully recorded. Theposition of the tendon within the duct and the packingof wires or strands in a tendon should be noted, sincethis may provide useful evidence of duct displacementduring construction.
7.5 The repair of all access holes made for theinternal examination of tendon ducts requires verycareful consideration at the time of planning theinspection. Priming of the exposed steel may be anadvisable precaution, depending upon the condition anthe local environment. Shrinkage compensated repairgrouts and mortars should be adopted for filling holesand consideration given to the addition of a surfacerepair coating as a further precaution against the ingreof water and chlorides. Specific advice on local repairto post-tensioned structures will be given in theDepartmental Advice Notes on Strengthening, Repairand Monitoring. General guidance is given inDepartmental Standard BD 27 (DMRB 3.3), Materialsfor the Repair of Concrete Highway Structures.(Not applicable for use in Scotland).
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Anchorages
7.6 Exposure of post-tensioned anchorages locatedat the ends of decks, or similarly restricted areas suchas deck half-joints may be carried out using carefullyselected water jetting or grit blasting methods. Theexposure procedure should be a gradual process, whicallows visual examination or the use of endoscopetechniques at various intervals.
Full supervision of the exposure should be undertakenby experienced inspectors and the work carried out byspecialist operators, in accordance with therequirements of Chapter 2. In particular, all personnelshould be made aware of the requirements of paragrap2.4 and avoid standing behind the anchor plates duringwater or grit blasting operations.
The areas selected for exposure at an anchorage may behind the anchor plates and immediately adjacent tothe end block zone. Removal of concrete within thebursting zone of the anchorage should be avoided,since this localised region is by definition, likely to behighly stressed. Particular care should be taken not tofracture the end anchorage plate or end wedges of thepost-tensioning system.
The condition of the end anchorage zone and anchorplate should be recorded and fragments of thesurrounding concrete or mortar protection should beexamined and removed for laboratory examination todetect the presence of chlorides. Repairs to the endanchorage areas should follow the advice outlined inparagraph 7.5 and the recommendations given in theDepartmental Advice Notes on Strengthening, Repairand Monitoring.
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Volume 3 Section 1 Chapter 8Part 3 BA 50/93 Reporting of Inspection
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Preliminary Desk Study
8.1 This report should contain the findings from thepreliminary desk study described in Chapter 3. It shouinclude a summary of the essential design, constructionand maintenance details, presented in a systematic forin an Appendix to the main report. The conclusions frothis report should contain the objectives for thepreliminary inspection.
The design details should include four principal groupsinformation:
a) List of available drawings.b) Form of construction.c) Type of concrete.d) Prestressing and reinforcement.
A sample proforma, FORM A1, for recording the designinformation is included in Annex A.
The construction records should be summarised on aseparate proforma, under four main headings:
a) List of record drawings.b) Form of construction.c) Type of concrete.d) Construction information.
A sample proforma, FORM A2, for recording theconstruction information is shown in Annex A. Thedifferences between design and actual construction shbe carefully noted.
The maintenance history of the structure should also bsummarised on a separate proforma, FORM A3, includin Annex A. The information is required under two maiheadings:
a) List of previous reports.b) Summary of previous defects.
The number of pages required will depend upon the sizof the structure and the extent of the problems detecteduring previous inspections.
Preliminary Site Inspection
8.2 This report should contain the findings from thepreliminary site inspection described in Chapter 3. Anyvariations in the construction details or new areas of
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A8, in Annex A. Where stress conditions areinvestigated, the results should be collected under fseparate headings:
serious deterioration should be identified and recorded ian appendix to the main report, in a similar format to themaintenance history. The main text of the report shouldinclude an initial evaluation of the potential risk of asudden collapse and the need for regular monitoring. Tconclusions should form the basis of the technical plan fthe site investigation.
The inspection data should be grouped into two mainheadings:
a) Amendments to constructioninformation.
b) Principal areas and defects requiring siteinvestigation.
A sample proforma, FORM A4, for recording the basicinformation from the preliminary inspection is included inAnnex A.
Site Investigation
Project Manager's Report
8.3 This final report should include the results fromall previous routine material tests and tests conducted todetect the risk of reinforcement corrosion, additionaltesting and specialist tests that may be carried out in thefinal phase of the Special Inspection. The results fromany further standard tests should be incorporated in thestandard proforma A5-A7, and should supplement theinformation previously gathered in the Preliminary SiteInvestigation. The results of the Special Inspectionshould be fully discussed and reviewed.
The site investigation records should be summarised onthree separate proforma, FORMS A5, A6, A7 in AnnexA. The detailed information provided should be underthree main headings:
a) Tests for corrosion risk - FORM A5.b) Concrete material tests - FORM A6.c) Results from internal examination -
FORM A7.
8.4 Where special methods are introduced todetermine in-situ stress conditions, the basic informationshould be summarised on the standard proforma, FORM
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a) Temperature conditions.b) Steel stresses.c) Principal concrete stresses.d) Secondary concrete stresses.
Where possible, estimates should be made of the preslosses in a structure. The results from any load testingshould be fully discussed and interpreted in relation to condition of the structure and the in-situ stressmeasurements.
Structural Condition
8.5 The overall condition of the structure should befully appraised following the detailed inspection and sitinvestigation. Particular attention should be drawn to telements of the structure which have suffered fromcracking and water leakage.
The significance of defects in the prestressing systemshould be carefully assessed in terms of long-termdurability and the consequences of structural failure. Where voids are discovered in the grouting, the alkalinand the presence of any moisture or chlorides should bconsidered in order to assess the potential for corrosiothe tendons. The possible benefits of injecting furthergrout into voided areas should be reviewed in terms of potential improvement in durability and ultimate strengt
Broken wires or strands from several tendons may notrepresent a significant loss in strength in some forms obridge deck. Broken tendons are likely to re-anchor ovshort distances of 1-2m, but this depends upon themagnitude of the force released, the integrity of the groand the adjacent shear reinforcement. Therefore, the rdistribution of forces in the region of a broken tendonshould be carefully reviewed in terms of these basicparameters and the condition of adjacent tendons.
Risk Assessment
8.6 The initial risk assessment carried out after thepreliminary site inspection should be completely reviewand due account taken of all defects, particularly atcritical sections. The structural consequences arisingfrom all broken and corroded tendons should be carefuexamined. Where grouting of tendon ducts is voided,structural failure may occur at a critical section, even ifthe tendons are fractured at other locations.
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The risk of failure at all critical sections should beassessed individually. Potential collapse mechanismsinvolving failures at various critical sections should beconsidered, bearing in mind the present condition an
tress future.
the
and the measurement of temperature conditions shoe carefully planned according to the location of the fauhe and the type of construction.
Where joints in segmental structures occur at critical
proposals should be prepared for regular monitorinthese circumstances, it is necessary to establish davalues for the in-situ concrete stresses adjacent to t
ity location and to install a strain gauge monitoring syste across the joint. A programme for regular monitorin of interpretation of the results should be prepared in o
provide advance warning of significant changes inthe structural behaviour.h.
fer
ute-
potential for further deterioration at each section in the
Monitoring Requirements
8.7 The type and extent of the defects in a structuremay require the introduction of frequent inspections andregular monitoring of critical sections. Where any formof monitoring is considered, the frequency of readings
locations or there are known defects in the vicinity,
Recommendations for Load Assessment
8.8 Details of all major cracks, losses of concretesection and corroded reinforcement and broken tendonsshould be correlated so that the effect on serviceabilityand ultimate capacity of sections can be appraised. Cracks in joints between precast segments may relate togeneral loss of prestress and subsequent reduction in lodistribution behaviour within a deck. Recommendationsshould be prepared to provide guidance on the likely losin section properties and reductions in strength of allsections suffering from deterioration.
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Volume 3 Section 1 Chapter 8Part 3 BA 50/93 Reporting of Inspection
cts
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Preliminary Recommendations for BridgeManagement
8.9 Future management of the structure should bedetermined according to the extent of the existing defeand the potential for future deterioration. The proposedmanagement of the structure should also be influencedthe results of the risk assessment and the monitoringrequirements.
Recommendations should be made on the frequency atype of future inspections. Where serious defects existfurther corrosion of tendons is inevitable, suggestionsshould be made for the introduction of repairs,strengthening works or forms of replacement.
Summary Report
8.10 A summary report should be included in theProject Manager's Report. This report should contain abrief description of the structure, the prestressing systeand the scope of the Special Inspection. The principalresults arising from the preliminary desk study,preliminary site inspection and the site investigationshould be presented.
The main conclusions arising from the SpecialInvestigation should include summary statements on thstructural condition, risk assessment and futuremonitoring requirements. Recommendations should bemade on the effects of deterioration on section strengthfuture management of the structure and the need forremedial measures or the possibility of replacement.
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Volume 3 Section 1 Chapter 9Part 3 BA 50/93 References
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9. REFERENCES1. TRMM 2/88, Trunk Road and MotorwayStructures - Records and Inspection (for use in England).
2. Technical Memorandum (Bridges) SB 1/78 -The Inspection of Highway Structures (DMRB 3.1)(for use in Scotland).
3. WOTRMM 2/88 Welsh Office Trunk Roadand Motorway Structures - Records and Inspections(for use in Wales).
4. Technical Memorandum (Bridges) BE 4/77NIRS - The Inspection of Highway Structures (for use in Northern Ireland).
5. BS 1881: Part 201: Guide to the use ofnon-destructive methods of testing hardened concrete.
6. BS 1881: Part 205: Recommendations for theradiography of concrete.
7. Concrete Society "Non-structural Cracking ofConcrete - 1992"
SB 1/78 Inspection of HighwayStructures [and AmendmentNo 1 dated July 1990](for use in Scotland only)
Volume 3: Section 3: Repair
BD 27 Materials for the Repair ofConcrete Highway Structures(DMRB 3.3)(Not applicable for use inScotland)
BA 35 Inspection and Repair ofConcrete Highway Structures(DMRB 3.3)
Volume 3 Section 1 Chapter 10Part 3 BA 50/93 Enquiries
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10. ENQUIRIESAll technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:-
Head of Bridges Engineering DivisionThe Department of TransportSt Christopher HouseSouthwark Street P H DAWELondon SE1 0TE Head of Bridges Engineering Division
The Deputy Chief EngineerRoads DirectorateThe Scottish Office Industry DepartmentNew St Andrew's House J INNESEdinburgh EH1 3TG Deputy Chief Engineer
Head of Roads Engineering (Construction) DivisionWelsh OfficeY Swyddfa GymreigGovernment BuildingsTy Glas Road B H HAWKERLlanishen Head of Roads EngineeringCardiff CF4 5PL (Construction) Division
Assistant Chief Engineer (Works)Department of the Environment forNorthern IrelandCommonwealth HouseCastle Street D O'HAGANBelfast BT1 1GU Assistant Chief Engineer (Works)
Volume 3 Section 1Part 3 BA 50/93 Annex A
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SAMPLE PRO-FORMAS
FORM A1 - Design Details
FORM A2 - Construction Details
FORM A3 - Maintenance Records
FORM A4 - Preliminary Site Investigation Record
FORM A5 - Site Investigation Record - Tests for Corrosion Risk
FORM A6 - Site Investigation Record - Concrete Material Tests
FORM A7 - Site Investigation Record - Results from Internal Examination
FORM A8 - Site Investigation Record - In Situ Stress of Conditions
Volume 3 Section 1Annex A Part 3 BA 50/93
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DESIGN DETAILS FORM A1
Name of Bridge: National Grid Ref:
Bridge Number: STKEY:(England Only)
Location of Design Details: Date of Design:
LIST OF DRAWINGS
Title Drawing Number Date
FORM OF CONSTRUCTION
Number of Spans: Span No.:
Angle of Skew: Length:
Curved on Plan: Width:
TYPE OF Classification Compressive StrengthCONCRETE
