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    Determination of Tendon Positions and Investigation of Voids in Tendon Ducts

    For Post-tensioned Concrete Structures

    Yonghao Zheng and Frank PapworthBuilding & Construction, Research & Consultancy (BCRC)

    Perth, WA, Australia

    SUMMARY

    Tendons play an important role in post-tensioned concrete structures. If the tendons are damaged during their

    service life, for example during drilling or coring into a post-tensioned structures, it may affect the performance

    of the structure and cause concern on the overall safety of the structure. Air voids in tendon duct (generally

    caused by poor grouting practice) may lead to corrosion of the tendons, and thereby reduce the effective cross

    section of the tendons, or even cause failure of the tendons during the service life of the structures.

    Advanced Non-Destructive Test (NDT) methods such as radar, impact echo and fibrescope techniques can be

    used to test and enable safe drilling, coring and rectification of post tensioned structures. Approaches for these

    specific applications are introduced and discussed in this paper. Several projects are reviewed.

    1. INTRODUCTION

    Post-tensioned concrete structures have been used extensively in the past decades in civil engineering structureswhich are often intended to have design lives of 100 years. Tendon integrity is critical to the structurescontinuing overall safety. The highly stressed steel is particularly susceptible to corrosion and each tendoncarries a high load. Hence, any damage during their service life may seriously affect the performance of thestructure.

    Failures of tendons in structures in the USA and the UK have led to major improvements in post tensioned ductgrouting procedures overseas. In Australia grouting standards have not been upgraded to the same extent and

    requirements are lower than those overseas. Grouting requirements in Australia remained at a level which led tovoidage in ducts overseas until recently so it may be that structures in Australia have voided ducts. The USAand the UK use deicing salts on their roads and hence exposure of tendons in poorly grouted ducts may be moresevere and more concerning than in Australia. However, given the susceptibility of highly stressed tendon tocorrosion, our infrastructures close proximity to marine environments, and Australias grouting standards it isinferred that the likelihood of failure of tendons would have to be possible according to AS 4360 (Table 1).Risk is conventionally assessed (AS 4360) in two dimensions as shown in Table 1.

    Table 1 : AS 4360 Risk Assessment

    Likelihood of failureConsequence of Failure

    Negligible Low Moderate Very High Extreme

    RareOnly in exceptionalcircumstances

    Negligible Negligible Very Low Low Moderate

    UnlikelyCould occur at sometime

    Negligible Very Low Low Moderate High

    Possible Might occur at sometime

    Very Low Low Moderate High Extreme

    LikelyProbably occurs inmost circumstances

    Low Moderate High Extreme Extreme

    Almost CertainExpected in mostcircumstances

    Moderate High Extreme Extreme Extreme

    Experience overseas has shown that if corrosion of tendons occurs, and goes unchecked, the consequence (e.g.bridge collapse) could be extreme. Such extreme risk is unacceptable and needs to be reduced. In the case ofexisting structures systematic inspection of structures to assess if tendon corrosion is occurring would reduce thelikelihood to rare. For new structures the likelihood can be reduced to rare by adopting US or UK groutingstandards.

    Locating tendon may also be required during upgrading of post tensioned structures. Even as built drawings willnot locate the position of tendons accurately enough to ensure they are not cut during drilling and coring.

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    Radar survey can effectively determine the position of tendon ducts in the post-tensioned concrete structure.Thus the tendon ducts can be avoided during drilling or coring operations and located for Impact Echo testing toinvestigate voids in tendon ducts. Radar and Impact Echo can be performed from one of the exposed face of thestructure, and they are truly non-destructive in nature. Coupled with intrusive fibrescope inspection detection ofand evaluation of voids in tendon ducts is possible.

    2. DETECTION OF TENDONS USING RADAR

    2.1 What is Radar (Impulse Radar, Ground Penetrating Radar)?

    Radar, which stands for Radio Detection and Ranging, uses the reflection of microwaves to measure remote

    objects. It is widely applied in many areas, i.e. military, air traffic control and car speed control by traffic police,etc. In civil engineering applications, the radar devices emit short pulses of microwaves. For this reason, the

    technique is often called short pulse radar or impulse radar. The method is most generally used for detectingobjects below ground (eg pipes) and the most common name has become Ground Penetration Radar (GPR).

    2.2 Principle Of GPR For Tendon Detection

    A very short pulse (few nanoseconds) is generated and input into the test object (ground or concrete) by atransducer in contact with it. This transducer is referred to as an antenna. The microwave travels through the testobject and energy reflected back (back-scattered) by any embedded object that has a different properties,specifically dielectric constant,, and the reflected waves are detected by a receiving antenna which is movingalong the surface together with the transmitting antenna. The received signal is processed and shown on a colourmonitor. A single pipe will appear as a hyperbola on the screen. This represents the reflected waves traveldistance reducing as the antennae approaches the object. Position and depth of the embedded object are obtainedjust by reading coordinates of the hyperbolic shape within the radar image. The method is fully described inAmerican Concrete Institute Report ACI 228.2R-98, Nondestructive Test Methods for Evaluation of Concrete

    in Structures.

    One of the main applications in civil engineering is for tendon detection (more accurately, detecting tendonducts) in post-tensioned concrete structures. Equipment suitable for tendon detection is generally a control box

    (with control unit, recorder and display units inside) and a high frequency (1000 MHz to 2000 MHz), antennaThe antenna is chosen to give the best balance between penetration depth and resolution for the testing beingundertaken.

    It is possible to detect tendon ducts below reinforcement since tendon ducts have a larger diameter and present amuch bigger hyperbolic shape than rebars in the radar image.

    2.3 Advantages And Limitations Of Radar In Tendon Detection

    Radar can be performed from one face of the structure and it is truly non-destructive. Radar can differentiaterebars and tendon ducts easily if they are embedded at a similar depth in the concrete, which is not possible forconventional micro cover meter. Recently developed 3D imaging technique and multiple array antennas make

    the tendons more visible even in some complicated arrangements.

    Radar has some limitations .If the post-tensioned slab is thick and the tendon ducts are embedded deeply,detection is difficult. The presence of additional topping on the slab with wire mesh can increase the difficulty

    for tendon detection. Detection of tendons in beams mostly depends on the geometry of the beam and thearrangement of rebars and tendons in the beam. If the tendons are embedded within the flange portion of I-beam, inverted T-beam or U-beam tendon detection becomes more difficult. Some intrusive testing might berequired for verification. Skilled and experienced personnel are required for tendon detection. Some informationon the existing structure members can help the tendon detection 1.

    3. INSPECTION OF GROUTING CONDITION OF TENDON DUCTS USING IMPACT ECHO TEST

    3.1 Needs for inspection of grouting condition of tendon ducts

    Generally, the tendons are protected by the surrounding grout, which is injected into the tendon ducts. Voids inthe grout can occur along the tendon trajectory due to blockages, improper grouting procedures, grouting

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    material problems, and construction oversight. Inadequate grouting may allow water to penetrate into the ducts,causing corrosion of the tendons leading to failure of the structures. Unfortunately, there may be little or novisual evidence that something is wrong until it is too late. In UK, a post-tensioned bridge collapsed in the 1980sas a result of corrosion of steel tendons

    2.

    Many researchers and testing engineers have been trying to explore some suitable technique to inspect grouting

    conditions of tendon ducts in post-tensioned concrete structures for the past decades. The ideal test would, ofcourse, be one that could non-destructively determine the degree of corrosion and the remaining cross-sectionalarea of each tendon along its full length. However, it does not currently exist and is unlikely to be developed inthe near future. For the time being, there are only limited techniques available for testing engineers to employ

    for this application2.

    According to the authors opinion, a combination of Radar survey, Impact Echo, intrusive testing and fibrescopeinspection is the most suitable approach for inspection of grouting condition of tendon ducts for post-tensioned

    concrete structures at this moment3.

    3.2 What is Impact Echo (IE)?

    IE is a method for non-destructive evaluation of concrete based on stress waves propagation and reflection in the

    concrete structure. ASTM Standard C 1383 - 98a describes the procedure of non-destructive measurement of thethickness of plate like structures i.e. slab, wall using IE. However, it has been widely used to evaluate concreteintegrity and concrete repair of plate like structures. It can also be used to detect voids in tendon ducts in post-tensioned slab and beam.

    3.3 Principle of IE

    A short-duration mechanical impact, produced by tapping a small steel sphere (or any other form of impact)against a concrete surface, produces low-frequency stress waves that propagate into the structure and are

    reflected by flaws and/or the opposite external surface. Surface displacement (or acceleration for certain impactecho test systems) caused by these reflected waves are recorded by a transducer (or an accelerator), located

    adjacent to the impact point. The resulting displacement (or acceleration) versus time signals are transformedinto the frequency domain, and plots of amplitude versus frequency (spectrums) are obtained. Carefully

    examining the spectrums of amplitude vs. frequency collected at the test points along tendon duct at certaininterval can provide information on the grouting condition. Like Radar technique, IE is also fully described inACI 228.2R-98.

    3.4 Advantages and limitations of IE

    Although IE was developed in 1990s in USA, it has received significant acceptance in the world. IE hasbecome one of the best tools for concrete non-destructive evaluation for civil structures where access isrestricted to one face, often giving good and encouraging results. Comprehensive research has been done and aguidebook IMPACT-ECHO: Nondestructive Evaluation of Concrete and Masonry (by Mary J. Sansalone,and William B. Streett 1997) is available.

    Occasionally, there are some conflicting reports regarding its applications and reliability. For detection of voids

    in tendon duct, its applicability depends on the geometry of a structure and the locations and arrangement oftendon ducts. Small voids in tendon ducts which are located too deep within a structure cannot be detected. Insome cases, complicated arrangement of multiple ducts, such as often occur in the flanges of concrete I-beams

    or inverted T-beams, can preclude detection of voids in some or all of the ducts. It is very difficult to detectvoids in tendon ducts at the mid-span of I-beam and U-beam as the geometry at such a portion is toocomplicated for IE.

    The requirement for skilled personnel who can provide a reliable and thorough interpretation of IE test resultscould have limited its wide application for this particular purpose.

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    4. INTRUSIVE TESTING AND FIBRESCOPE INSPECTION FOR TENDON DUCTS

    4.1 Needs for intrusive testingIntrusive testing is an essential supplementary method for Radar survey and IE test, often for initial calibrationor verification of results. Although it is low-tech, time consuming, messy and may require partial road closuresand temporary works, it is the single most useful technique available.

    Radar survey might not be able to determine the position of tendon duct in the structures with sufficient level ofconfidence in some cases, i.e. if the tendon ducts are embedded in a deep portion or the structure is heavilyreinforced at the survey area. To achieve more confident results, intrusive testing has to be called to assist the

    Radar survey.

    IE method is an indirect testing method and fully non-destructive. It is not always conclusive for inspection ofvoids in tendon ducts and needs supplemental testing to verify the outcome, often by intrusive testing. A

    combination of IE and intrusive testing allows the reliability of IE method to be assessed for a specific project3.

    4.2 Localised opening method and location

    Localised opening for inspection can be drilling, coring, or otherwise removing (hacking off) concrete. For

    bridge investigation, each bridge is unique with different type of beams. The approach for localised opening forinspection shall be different for each bridge since it much relies on the geometry of beam and the tendon profilein the beam.

    Generally, localised opening spots shall be at areas where corrosion is most likely to occur, i.e. low spots wherewater could collect, high spots where grouting may be incomplete, or joints in the case of segmentalconstruction. Unless the direction of flow during grouting is known, then both areas adjacent to the crest shouldbe suspected 2.

    4.3 Fibrescope and its application for inspection of tendon duct

    Generally, endoscope is designed to inspect regions that are otherwise inaccessible to the naked eye. Flexiblefibrescope is one of the endoscope which is often used for inspection of tendon duct (rigid borescope is another

    type of endoscope). The fibrescope is designed so that some fibers transmit light to illuminate the cavity withina structure, and the diameter of the optical bundle is only several millimeters. The operator can rotate theviewing head to allow a wide viewing angle from a single access hole. Internal condition can be viewed on adisplay of recorder or video camera that is connected to the fibrescope. A recent development that expands theflexibility of visual inspection is the small digital video camera. These cameras have optical systems with acharge-coupled device (CCD) and focal lengths. It can deliver much better resolution than traditional fibrescope.One of the examples is the videoscope from Olympus. Fibrescope method is also described in ACI 228.2R-98.

    5. REVIEW AND DISCUSSION ON SOME PROJECTS

    The detailed structural description, approach used, data interpretation and outcome of the project reviewed herecan be found in the individual reference papers listed behind.

    5.1 Investigation of voids in tendon ducts in floor slab for a new building 1

    During construction of a multi-storey industrial building, it was reported that the grouting of the tendon ductwas not carried out properly. There were no presences of grouting materials in some of the outlet hoses.Investigation to detect any voids in the tendon ducts in the slab was called.

    The thickness of the post-tensioned slab was 270mm with one way evenly spaced tendon ducts and supported bybeams. The size of the rectangular shape tendon ducts in the slab was about 70mm (width) by 20mm (height)with four 13mm diameter tendons inside. The depth of the tendon ducts were varied, generally near to the floorsurface at supporting beams and deep at mid span. Figure 1 shows a profile of typical tendon duct in an end spanslab and impact echo test points.

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    Figure 1: a profile of typical tendon duct in an end span slab and impact echo test points

    A preliminary inspection using fibrescope was conducted to inspect the presence of voids in the tendon ducts via

    some of the existing grouting outlet hoses. The inspection confirmed the presences of full voids or partiallyvoids in a few tendon ducts. All these tendon ducts were selected for further detailed investigation. IE test was

    then performed on the slab surface at an interval of 0.5m along these tendon ducts (refer to Figure 1). Theresults of IE revealed that only one tendon duct had been badly grouted with significant void inside.

    During construction stage, the questionable locations are generally known and limited to several locations.Localised opening assisted with fibrescope inspection may be enough to solve the problems in a cheaper way.

    5.2 Detection of tendons for coring for a new building 3

    M&E works were in progress after the completion of structural works for a newly built commercial building.Due to certain reason, the owner had decided to change usage of some units and had to add a washing room ineach unit. Some holes for installation of service pipes had to be drilled. The building was a seven-storey RCstructure. The most area of the floor slab was post-tensioned concrete slab with 250mm thickness and equallyspaced one way post-tensioning tendon ducts. There was a topping layer of 50mm thick chipping concrete with

    75mm spaced wire mesh inside.

    Radar survey was carried out to determine the positions of tendon ducts for coring work. A total of sixty four

    core holes were drilled. All the works inclusive of tendon detection and coring work were finished within fourworking days. The accuracy of the detection was optimistic. No tendon was hit during the coring work although

    some coring holes were near to the detected tendons.

    For this kind project where there is a layer of topping on the post-tensioned slab with wire mesh inside,detection of tendons in the slab is not an easy task. Some handy type radar sets may not be able to locate tendonsat all the locations. Carefully selection of scan areas (preferably at the areas with shallower tendon depths), acompromise between good resolution and deep penetrating depth, similar project experience of the personnelseem to be decisive.

    5.3 Investigation of voids in tendon duct of I-girder for an existing bridge

    4

    During an upgrading of a large bridge along an expressway, detection of voids in tendon ducts of I-girders wasrequired due to some signs of deterioration on the I-girder surfaces. The bridge consisted of 99 numbers of I-girders of each 30m length. Besides some prestressing strands in the bottom flange, there was a parabolicprofiled post-tensioned tendon duct in the web which was subjected to this investigation.

    A combination of Radar, IE and fiberscope inspection was adopted. Radar survey was carried out to locate

    centreline of tendon ducts. IE was used to determine voids in tendon ducts. It was mainly concentrated at thetwo quarter lengths of the I-girder at two ends where the tendon duct was embedded within web portion. Figure2 shows a cross sectional view of the I-girders.

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    Figure 2: a cross sectional view of the I-girders.

    Voids were found in some of the tendon ducts, including one completely ungrouted tendon duct. Most of voidswere continuous and concentrated toward one end. Figure 3a shows an IE spectrum from a fully grouted tendonduct while Figure 3b shows an IE spectrum from an ungrouted tendon duct. Figure 4a shows an image fromfiberscope showing the ungrouted condition of a tendon duct while Figure 4b shows an image from fiberscope

    showing partially grouted condition of a tendon duct.

    domain frequency

    reflected by tendons

    in tendon duct

    reflected by

    tendon duct

    shifted domain

    frequency

    a) spectrum from a fully grouted tendon duct b) spectrum from an ungrouted tendon ductFigure 3: IE spectrums from tendon ducts

    duct wall

    tendons grout at bottom

    portion

    empty top

    portion

    a) fibrescope image showing ungrouted condition b) fibrescope image showing partially groutedcondition

    Figure 4: Images from fibrescope inspection showing internal condition of tendon ducts

    5.4 Investigation of voids in tendon duct of U-beams for an existing bridge 5

    This is an existing bridge along a busy expressway across a road. There are a total of 48 precast post-tensionedbeams in this bridge and these beams are supported by two rows of center piers and two abutments at both ends.Each beam is 26.2m long. The section of the girder beam is U-shaped with a post-tensioned tendon ductembedded in each web with parabolic profile. Diameter of the tendon duct is 60mm. There are someprestressing strands in the bottom flange. Thus, the beam can be classified as a mainly post-tensioned precastbeam assisted by some pre-tensioned strands. Figure 5 shows the cross sectional view of the U-beam. Thickness

    of the bottom flange and web were not known. Some cracking signs were observed along the tendon position at

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    certain locations. Limited opening revealed that there were some empty tendon ducts. Thus, a full investigationwas followed up.

    Figure 5: Cross sectional view of U-beam

    Radar survey was conducted to map out the tendon duct profile for every tendon duct embedded in the webs ofU-beam. Impact echo test was performed to detect voids in the tendon duct. The P-wave speed was measured atseveral known thickness locations. The web thickness was determined using the P-wave speed measured earlier.Impact echo test was performed along tendon duct profile to detect voids in tendon ducts. Localized opening oftendon duct and videoscope inspection were carried out at some selected locations where impact echo testresults suggested doubtful internal condition. Impact echo test results revealed that the web thickness of the U-beam was not constant and it was changing along the tendon duct profile. This increased the difficulty fordetection of voids in tendon ducts using IE technique. More localised openings than expected were carried out

    to increase confident level in interpretation of IE data.

    A total of 16 tendon ducts out of 96 tendon ducts in 48 U-beams were found to be with voids. Generally, voidsfound in the tendon ducts were stretched along the ducts for several meters. One of the tendon ducts was found

    almost fully empty along the entire length of the beam.

    Figure 6a shows an internal view of tendon duct top portion showing empty condition while Figure 6b shows aninternal view of tendon duct bottom portion showing empty condition and pitting corrosion on the tendons.

    a) a view of tendon duct top portion showing emptycondition

    b) a view of tendon duct bottom portion showingempty condition and pitting corrosion on the tendons

    Figure 6: Images from videoscope inspection showing internal condition of tendon ducts

    Generally, for investigation of voids in tendon ducts for a bridge is a tough task occupying several months. Theabove two bridge projects were all successful. All the efforts put in seemed to be worthwhile since a number ofvoids were found through the investigation and were repaired. In authors opinion, it should be called whensome signs of deteriorations, such as cracking, spalling are found during a periodical inspection. Or significantvoids were found through investigation from other bridges which were constructed at the same period withsimilar beams and structures. Otherwise, huge time spent by the professional team and inconvenience caused totraffic users during investigation period might not worth it.

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    6. CONCLUSIONS

    If properly used, advanced NDT methods, such as Radar, IE and fibrescope, can often offer good solution tomeet the needs, and also provide a cost effective way for maintenance and upgrading of post-tensioned concretestructures. However, skilled and experienced personnel with thorough understanding of these techniques arerequired to get a proper test program and to interpret test data to obtain reliable results and findings.

    Nevertheless, these advanced NDT methods have been successfully used in many countries worldwide.

    ACKNOWLEDGEMENT

    The author would like to thank SETSCO Services Pte Ltd (SETSCO) in Singapore. The reference papers by theauthor listed below were published under SETSCOs name during which time the author worked with SETSCO.

    REFERENCES

    1) Yong Hao Zheng, Kee Ee Ng, John Wei Ong, Evaluation of concrete structures by advancednondestructive test methods impact echo test, impulse response test and radar survey, Proceedings,

    International Symposium on Non-Destructive Testing in Civil Engineering (NDT-CE 2003), Berlin,Germany. http://www.ndt.net/article/ndtce03/papers/v100/v100.htm

    2)

    R. T. Stain and S Dixon, Identify the techniques currently available for inspecting the tendon condition inpost-tensioned bridges, Construction RepairJan./Feb. 1994.

    3) Yonghao Zheng, NDT methods for post-tensioned concrete structures, SETSCO Technology Forum2005/ Concrete Technology Today, No. 3 Vol. 4 (2005), pages 22-23.

    4) Honggang Cao, Yonghao Zheng, Detection of voids in tendon ducts of I-girders for an existing bridge,Concrete Technology Today, No.2, Vol. 3 (2004), pages 24-29.