Tiger Teams - Post-tensioned bridges

Asset Management Transformation Programme

Tiger Teams - Post-tensioned bridges

Outputs from Literature Review

13th October 2020

Chris Mundell

2 | August 2020Asset Management

Transformation Programme

Session Agenda

1. Purpose of the Tiger Team

2. The Problem Statement

3. Typical forms of PTSI

4. Review of NDT technologies

5. Summary and Infographic

• Link to Microsoft Forms

Questionnaire

• General ask to obtain areas of awareness of the user group

• Names and email addresses NOT captured

https://bit.ly/34QzosN

https://bit.ly/34QzosN



Priority Actions Status (1/2)

AMT Delivery: Capability Area UpdatesReview and improve our approach to managing risk for the post-tensioned bridges. Catherine Brookes & Peter Hill Breakpoint: Effective investment

prioritisation

Achievements

▪ Started to draft CHE memo, preparing pack to communicate to regions prioritisation guidance.

Capture and share good practice on management of post-tensioned bridges Pierfrancesco Valerio & Catherine Brookes Breakpoint: Capability

Achievements

▪ Sprint 1 – International good practice paper – Chris Mundell

▪ Sprint 2 – Examples of good practice for management of PTS collated from across the regions – Pierfrancesco Vallerio

Develop national skills matrix for structures to capture existing skills in the business Pierfrancesco Valerio & Catherine Brookes Breakpoint: Capability

Achievements

▪ Excel skills matrix template created, to be shared with Kate Wood and Amy Williams

▪ Engagement with HR to identify opportunities for integration with PFP/oracle system

Deliver specialist training through guidance and webinars Pierfrancesco Valerio & Peter Hill Breakpoint: Capability

Achievements

▪ Gaps in training identified (based on gaps in knowledge/ expertise around BD54 and lessons learnt from 19-20 PTSI

▪ Training mediums identified – ‘click to learn’, webinars, lessons from launch of ACS.

Review the existing centralised Operations task force to act as a forum for sharing good

practice (w. SES support)

Catherine Brookes Breakpoint: Capability

Achievements

▪ RDs in the OPT and the Senior manager’s team informed that their Teams will be asked to join their taskforce team. Invitation to go ADSTCM Malcolm Dangerfield’s meeting and Catherine will

be chairing it. 11 August

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Post Tensioned Structures – The Problem Statement

• Post-tensioned Structures are difficult to inspect!

• Quality of initial construction often unknown, particularly older structures

• Hidden ducts critical with limited visible indication of deterioration – high strength steels can fail with little visual sign through hydrogen embrittlement leading to corrosion pits, and anerobic corrosion can occur without expansive products (i.e. no spalling)

• PTSIs that have been undertaken are not often repeated

• Limited skills in the industry now for PTSIs, particularly given the 1990’s moratorium on new post-tensioned structures

• Still heavily reliant on intrusive investigations

• Typical Defects

• Voids within the ducts (most likely at high-points within the system)

• Bleed water accumulating during grout curing

• Soft grout surrounded by hard grout leading to galvanic reactions

• Incorrect placement of ducting leading to low cover

• Poor detailing of bridge joints, waterproofing or anchorage cappings



Post Tensioned Structures – Inspection Methodologies

Approach Advantages Limitations

Optical

Inspection

Human inspection is cheap and quick, can incorporate more

advanced technologies such as AI interpretation of images/footage.

Can utilise drones and high-resolution cameras for remote/difficult

access locations

Often relies on human judgement for interpretation of surface-

level indicators, and difficult access may prevent inspection.

Drone technologies currently limited by Civil Aviation Authority

restrictions on proximity to uncontrolled sites (i.e. live traffic).

Numerical

Modelling

Can be done remotely, highlighting theoretically critical details

and areas with high structural demand, and also predicting stresses

and deflections for validation/comparison in on-site measurements

and trials

Cannot be used directly to determine actual strand condition

or in-service deterioration, but may sometimes be used to

indirectly make such assessment e.g. by calibrating prestressing

force against known bridge deflections

Non-destructive

Testing (NDT)

Many systems/technologies exist for use, potentially identifying

ducts, grout and strands, and associated defects without the need

to physically interfere with the structure

All currently adopted systems have limitations and drawbacks,

with significant levels of uncertainty in the accuracy of the

results and many requiring physical access to be provided to the

structure

Intrusive Testing Provides absolute evidence of the condition of the tested

element(s), giving information on in-situ environment, material

strengths and state of corrosion

Requires access to the structural elements being tested, and

causes physical damage to the structure (typically through

sampling or boring into concrete, ducts, grout or tendons) so

cannot be used indiscriminately. Will also only give information

on a localised area, which could ill-inform assessment



Post Tensioned Structures – Non-Destructive Testing Technologies

Approach Examples

Optical Methods Direct visual inspection or optical solutions, including Digital Image Correlation utilising visible

and infra-red light spectrumsElectromagnetic

wave methods

Infrared thermography (IRT) and Impulse Radar (GPR)

Mechanical

wave/vibration

Acoustic sounding, acoustic emission (AEM), impact-echo (IE), impulse response (IR),

ultrasonic tomography, ultrasonic guided wave (GWT)Penetration

radiation

Radiography and X-ray diffraction

Magnetic methods Magnetic flux leakage

Electrochemical

methods

Half-cell potential, linear polarization resistance (LPR), Electro-impedance spectroscopy (EIS),

electrochemical noiseContact sensors Direct contract measurements, such as strain gauges, displacement transducers and load cells



NDT Common Technologies – Acoustic Sounding

• Very simple and quick

• Void detection (external or exposed ducts)

• Anchorage voids

• Cannot differentiate between voids, water and exposed grout

• Unsuitable for internal ducts

• Basic technology, involving striking of duct with an ‘impactor’ to determine internal makeup via the return acoustics



NDT Common Technologies – Ground Penetrating Radar (GPR)

• Lightweight, quick and cheap system

• Locating prestressing tendons

• Void detection

• Difficult with metallic ducts

• Cannot detect corrosion depth or duct void size

• Emission of Electromagnetic pulses that are impacted by changes in material electical conductivities (e.g. rebar, tendon ducts, voids)

• Mixed results from a number of sources

• Key use is in determining where ducts are to pinpoint for other technologies



NDT Common Technologies – Impact Echo (IE)

• Well-establish technique

• Void and crack detection

• Can be used with metallic ducts

• Requires limited access

• Grout Deficiencies

• Cannot detect strand defects

• Mechanical impact or transducer produces sonic or ultrasonic waves which are detected by an array

• Able to measure depth of cracks, air or water-filled surface openings

• Works best when there is a detailed understanding of the duct location



NDT Common Technologies – Infra-red Thermography (IRT)

• Near-surface duct location

• Exposed anchorage assessments

• Near-surface voids

• Limited use for internal ducts

• Can be either passive (using natural heat) or active (with applied heat source)

• Often impractical to apply heat sources so must utilise natural light and daily thermal cycles



NDT Common Technologies – Acoustic Emission Monitoring (AEM)

• Real-time detection of wire breaks

• Wire break location

• System accuracy questionable

• Forward monitoring only

• Costly to install/maintain

• Long-term SHM technology to detect breaking wires through the acoustic emission during wire breaks (PWBs and CWBs)



NDT Common Technologies – Radiography

• Detailed examination of duct, grout and strands

• Small exposure area

• Potentially lengthy image capture time

• Hazardous process with logistics difficulties

• X-ray or gamma radiating isotopes placed on one side of a structure, with a film placed behind to capture an image, up to a depth of approximately 500mm

• Typically images of up to 400mm square can be achieved

• Exposure times of up to an hour for deeper sections



NDT Common Technologies – Ultrasonic Tomography

• Detection of corrosion in tendons, bond quality and duct voids

• Generation of 3D images

• Portable and easy to use

• Requires flat surface

• Medical derivation, using ‘catch-pitch’ transducers and receivers

• Collection of three planar images for combination in three-dimensions



NDT Developing Technologies Technologies – Magnetic Flux Leakage (MFL)

• Detection of discontinuities and strand deterioration

• Mainly lab-based, requiring more field testing

• Application of a magnetic field to create flux lines within any steel elements using hall effect

• Can be employed ‘actively’ or ‘passively’ dependent on whether the specimen is magnetised prior to the readings



Common Technologies for Chloride Ingress

• Identification of areas of corrosion risk

• Can be monitored remotely

• Half-cell requires tendon exposure

• Local to sensor monitoring only

• Remote/wireless technologies need more robust testing

• Half Cell Potential Surveys

• Common technology used for concrete sections

• Readings taken in a grid giving potential map of area for corrosion risk

• Embedded Chloride Probes

• Wireless probes now under development from the agricultural industry for crop/soil monitoring (Netherlands)

• Embedded sensors can be as small as 10mm x 15mm

• Use of Linear Polarization Resistance (LPR) for chloride intrusion monitoring



Developing Technologies for Chloride Ingress – GPR Mapping

• Chloride mapping over a large deck area

• Impacted by dense reinforcement

• Not suitable for inclement weather

• Conductive materials hinder readouts

• Does not identify corrosion

• Swiss-developed technology, repurposing the use of GPR

• Limited availability of suppliers, including VSL



Developing Optical Technologies – Satellite Monitoring

• Wide-scale SHM possible

• Monitoring of long-term movements and loss of structural stiffness

• Forward view only

• Does not determine cause of deflections

• Need to account for short-term environmental factors

• Builds on long-term (20 year) study of post tensioned structures using hydrostatics

• Satellite images able to be optimised and processed to provide down to +/- 1mm annual movements



Developing Optical Technologies – Digital Image Correlation (DIC)

• High-res, contactless measurement of strain and displacement

• Identification of sub-surface flaws

• Research in early stages only for identification of sub-surface flaws

• DIC tracks relative pixels of images to determine sub-millimetre movements of actual surface points

• Use of DIC to map strains on a concrete surface, potentially highlighting sub-surface defects



New Construction Best Practice – Electrically Isolated Tendons (EIT)

• Ensure highest construction tolerances

• Enable PL3 monitoring

• Improve MFL system

• Cannot be retro-fitted

• Cannot easily locate void if they occur

• Swiss technology using PVC ducts with heat-shrink sleeves and couplers

• Able to measure corrosion protection during grouting to ensure fully grouted ducts

• Impedance measurements indicate if there is any loss of isolation and hence duct voids



Summary of Technologies

• Review of 40+ papers focussing on PT assessment technologies

globally, dating back to 1989

(with a focus on recent NDT technologies)

• Recurrence of similar approaches throughout literature – there is no ‘silver bullet’ with any respective technology

• Optimum approach is a blended use of technology, each playing to their strengths

• Cannot yet safely remove the need for intrusive investigations or structural analysis – NDT will work in tandem

• Much research is contradictory, with varying capabilities between papers

• Much research is limited to laboratory trials or for external ducts

• Some technologies stand out for further development and potential

use and trial

• Chloride mapping and wireless chloride probes

• Ultrasonic Tomography (MIRA already reasonably well established)

• Long term SHM through satellite imagery

• Combination of targeted SHM technologies

Asset Management Transformation Programme

Radiography

Uses: Detailed void

detection and strand

deterioration within range

Limitations: Depth of

penetration, H&S implication

of radiation, needs access to

both sides of surface

Drones (Visible Light and IR)

Uses: Visual defects and shallow

duct detection through IR, tie in

with AI for pattern recognition

Limitations: Limited use for

internal ducts and hidden defects

Embedded Probes

Uses: Chloride or moisture

monitoring with wireless

sensors

Limitations: Primarily

monitors change and sensors

only monitor local environ

Satellite Imagery

Uses: Widespread Long-

term deflection monitoring

to detect reducing stiffness

Limitations: Forward view

only, needs significant time

to be of use, issues may not

be directly PT-related

GPR Mapping

Uses: Mapping of Chloride

ingress to deck surface

Limitations: Only indicates

potential for strand

deterioration due to

presence of Chlorides

Acoustic Monitors

Uses: Detection of strand

breakages and location

Limitations: Forward view

only, with potential for false

positives and negatives

Handheld Sensors(GPR and MIRA)

Uses: GPR for duct

location, MIRA for 3D scan

of duct and voids

Limitations: MIRA

impacted by reinforcement

Digital Image

Correlation

Uses: High resolution

movement and crack

propagation (tie in with AI)

Limitations: Surface defects

only at present

Environmental Sensors

Uses: Detect deterioration of

joints via egress of moisture

Limitations: Indicates potential

for deterioration only, needs

careful placement of sensors

(e.g. bearing shelves & joints)

Boroscope

Uses: Physical validation of

NDT investigations

Limitations: Can only

investigate voided ducts,

requires physical breakout

and damage to structure

Duct & Strand Condition

Structural Health Monitoring

Chloride Mapping

Visual Condition Monitoring

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Documents

Tiger Teams - Post-tensioned bridges