An Overview of Structural Health Monitoring

Systems For Concrete Structures

FUTURE CONCRETE QATAR-2014

Faris A Malhas, PE, PhD, F. ACI

Dean & Professor

School of Engineering, Science & Technology

Central Connecticut State University

Outline• Introduction

• What is SHM

• Methodology

• Sensor Technology

• SHM Testing Categories

• SHM systemC Design

• I-35 Bridge

• The Future of SHM

• ConclusionsSome material in this presentation is adopted from documents of ACI Committee 444 and ISIS- Canada

1 Introduction

• The world’s population depends on an extensive

infrastructure system

− Roads, sewers, highways, buildings

• The infrastructure system has suffered in past years

− Neglect, deterioration, lack of funding

Global Infrastructure Crisis

1 Introduction

•Approximately 25% of the 600,000 bridges in the US are either structurally deficient or functionally obsolete.

•The Federal Highway Administration (FHWA) has made it a priority to seek new methods to economically and effectively inspect and monitor bridges.

•In response to this need, structural health monitoring (SHM) has become a much discussed, but not widely implemented.

Structural Health Monitoring (SHM) canbe very helpful in serving as an alarmsystem for preventing infrastructuredegradation ………….

But what is Structural Health Monitoring?

How Can We Reduce Infrastructure

Degradation?

What is Structural Health Monitoring (SHM)

“The process of implementing a damage detection and

characterization strategy for engineering structures”

SHM Definition

Structural Health Monitoring

Non-destructive in-situ structural

evaluation method

Uses several types of sensors,

embedded in or attached

to a structure

To ensure the structural safety,

strength, integrity, and

performance

SHM Challenges

Implementation of SHM on bridges is not common as it should due to :

• Difficulties in integration of the information from sensor networks.

•Economic justification of SHM for the structure.

•The lack of a standard procedure for owners and inspectors to follow for SHM specially for bridges.

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SHM Guide: A Critical Need

The following topics should be covered in such a guide:

•Case studies with descriptions of which sensor networks have been shown to be successful

• The optimal location and number of sensors to monitor structural health.

• Guidelines for analyzing and interpreting data obtained by the SHM.

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SHM Applications

• Buildings (critical & historical)

•Bridges

• Tunnels

• Pavement

>> Practically: any critical structure

Expected Outcomes of Monitoring

• Providing answers to specific questions such as load rating.

• Addressing uncertainties related to construction processes, structural behavior or performance.

• Evaluating the effectiveness of maintenance or modification.

• Providing an objective assessment of present or future conditions.

• Detecting damage or deterioration for optimal maintenance planning.

• Evaluating the effects of hazardous events or accidents. 11

Monitoring Metrics for Concrete Bridges

Measure• Acceleration

• Strain

• Climatic Conditions

• Curvature

• Displacements

• Load

• Tilt/Slope

• Scour

Identify

Corrosion

Cracking

Strength

Location of rebars

delaminations

Increased interest in SHM is fueled by:

1. The need for…

Better management of existing structures

Monitoring of innovative designs and materials

2. The advancement of…

New sensors

Data acquisition systems (DAS)

Wireless and internet technologies

Data transmission, collection, archiving and retrieval systems

Data processing and event identification

SHM Categories

Static Field Testing:

Behaviour tests

Diagnostic tests

Proof tests

Dynamic Field Testing:

Stress history tests

Ambient vibration tests

DLA tests

Pullback tests

Periodic

Monitoring:

Includes field testing

Tests to determine

changes in structure

Continuous

Monitoring:

Active monitoring

Passive monitoring

Classification of SHM Systems

Level I

Detect presence of damage

Level II

Detect presence and location of damage

Level III

Detect presence, location and severity of damage

Level IV

Detect presence, location, severity and consequences of damage

Advantages of SHM

Advantages of SHM include…

Increased understanding of in-situ structural behaviour

Early damage detection

Assurances of structural strength and serviceability

Decreased down time for inspection and repair

Development of rational maintenance / management strategies

Increased effectiveness in allocation of scarce resources

Enables and encourages use of new and innovative materials

Ideal SHM system:

Information on demand about a structure’s health

Warnings regarding any damage detected

Development of a SHM system involves utilizing information from

many different engineering disciplines including…

Data Collection

MaterialsComputers

SensorsDamage Detection

Structures

Communication

Intelligent Processing

Monitored Structure

System Components Schematic

Sensors

(various

types)

DA system

(on-site)

Communication System

(e.g. telephone lines)

Data processing

(automatically by

computer)

Data Storage

Diagnostics

Data retrieval

(and decision

making)

Acquisition of Data

Selection of Sensors

Appropriate and robust sensors

Long-term versus short-term monitoring

What aspects of the structure will be monitored?

Sensors must serve intended function for required duration

The collection of raw data such as strains, deformations,

accelerations, temperatures, moisture levels, acoustic

emissions, and loads

Acquisition of Data

Sensor Installation and Placement

Must be able to install sensors without

altering the behavior of the structure

Sensor wiring, conduit, junction boxes

must be considered in the initial

structural design

Transfer to Data Acquisition System (DAS)

Method - Lead wire

• Direct physical link between sensor and DAS

• least expensive and most common

• Not practical for some large structures

• Long lead wires increase signal “noise”

Method - Wireless transmission

• More expensive

• Signals are transferred more slowly and are less secure

• Use is expected to increase in the future

Acquisition of Data

Data Sampling and Collection

Acquisition of Data

General Rule: The amount of data should not be so scanty as to

jeopardize its usefulness, nor should it be so voluminous as to

overwhelm interpretation

Issues:

• Number of sensors and data sampling rates

• Data sorting for onsite storage

• Sometimes: large volumes of data

Data Acquisition Algorithms

Record only significant

changes in readings

(and times that changes occur)

Record only values greater

than a threshold value

(and times that readings occur)

Acquisition of Data

Communication of Data

•Data transfer from the DAS to an offsite location

• Allows for remote monitoring

Telephone

lines

Internet

Wireless

technologies

DASOffsite

Location

Intelligent Processing of Data

Easier

Faster

More accurate

• The goal is to remove mundane data, noise, thermal, or

other unwanted effects and to make data interpretation:

Diagnostics

Converts abstract data signals into useful information about structural response and condition

• No “standards or guidelines exist for diagnostics

• Process used depends on…

Type of structure

Type and location of sensors used

Motivation for monitoring

Structural responses under consideration

CONCRETE STRUCTURAL MONITORINGPossible defects of prestressed concrete structure

27

Challenges in Concrete Structures Health Monitoring

•Concrete structures are affected by a variety of chemical, physical and mechanical degradation mechanisms such as chloride penetration, sulfate attack, carbonation, freeze-thaw cycles, shrinkage, and mechanical loading.

•Each of the following four issues: damage modeling, monitoring, data analytics, and uncertainty quantification – is a difficult challenge for a heterogeneous material such as concrete. 28

Challenges in Concrete Structures Health Monitoring

29

•Many sensor types are currently available

•Choice for SHM depends on various factors and goals

•Two behavior of concrete bridges are of particular interest to any SHM system:

•CRACKING

•CORROSION

Sensor Technology

SHM OF CRACKING

• Cracking is one of the most common damages found on bridges during inspection.

• Cracks have caused closure of bridges for repair, and even worse, collapses of bridges. Therefore proper assessment of cracking is critical in structural health monitoring.

• Various aspects of cracks such as length, width, depth, and pattern should be documented.

• It is important to document if any movement has occurred relative to observed cracking, such as shrinkage, displacement, or volumetric expansion.

Fiber Optic Sensor

•Versatile sensor, developed in the 80’s and still under intense development.

•Heavily used in SHM for Bridges, particularly in detecting cracks.

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Section: 4 Sensor Technology

Fibre Optic Sensors

Typical Optical Fibre

Assorted fibre coatings are required to protect the fibre from…

Abrasion

Protection during

handling and installation

Moisture

Weakens the fibres and

controls growth of microcracks

Concrete

Alkaline environment is

harmful to glass fibres

Outer jacket Aramid reinforcing fibres Inner jacket Fibre buffer Fibre Sensor

SHM

Intro to

ISIS Canada Educational Module 5

34

Smart Aggregate SystemIn this active sensing system, one smart aggregate is used as an actuator to generate a sweep sine signal, while the other smart aggregates are used as sensors to detect the signal response. The propagation energy of the waves will be attenuated by cracks in the concrete structure. By analyzing the sensor signal, the health status of the concrete structure is evaluated.

Smart Aggregate

Installation of smart aggregate

Maximum amplitude of acquired signal under loading and released loading.

Acoustic Emission Technique for Damage Detection

• An AE sensor composed of a thick piezoelectric element converts the mechanical energy caused by elastic waves into an electrical signal. When some cracks occur, resulting in elastic waves propagating through the target surface. These elastic waves are detected and converted to voltage signals .

• The location of damage can be identified using multiple AE sensors based on the differences in the arrival times of the AE signals.

Acoustic Emission Technique for Damage Detection

Corrosion

• Corrosion of rebars in concrete bridges is perhaps the most common form of deterioration, and is difficult to detect.

• The steel rebar in concrete is susceptible to corrosion when chloride ions enter into the concrete from de-icing salts applied to the concrete surface, or from seawater in marine environments.

• Although periodic visual inspections are performed on bridges, they cannot reveal the early symptoms of rebar corrosion within the bridge deck.

• By the time external visual evidence is seen, the damage has already occurred, and the bridge deck will need replacing.

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Corrosion Monitoring- ERE 20

Long life Reference Electrode, which can be cast into the cover concrete to check the cathodic protection and to monitor the corrosion state of reinforcing steel or predict corrosion.

Corrosion Monitoring System by S+R

Corrosion sensors (Anode Ladder). The Anode Ladder consisting of single steel anodes at different depths.

Used for monitoring of time dependent chloride ingress or carbonation progress into the concrete both for newly built and existing structures.

Corrosion Monitoring system-(RCS)

• CORROSOMETER probe can be used to evaluate the effectiveness of the cathodic protection system by detecting when chloride ingress is nearing the rebar, measuring and recording metal loss and the instantaneous corrosion rate.

Embedded Corrosion Instrument (ECI)

ECI contains a chloride threshold indicator, a temperature sensor, conductivity and resistivity sensor, a polarization resistance sensor, and an open-circuit potential sensor. ECI provides comprehensive, real-time information on structural conditions by monitoring five key factors in corrosion, which are linear polarization resistance (LPR), open circuit potential (OCP), resistivity, chloride ion concentration (Cl-) and temperature.

Other Types of Sensors Used in SHM

Load cells

Electrical resistance gauges

Vibrating wire strain gauges

Accelerometers

Linear Variable Differential Transformer

Thermocouples

LOAD

DISPLACEMENT

ACCELERATION

TEMPERATURE

STRAIN

Integrated Temperature Circuits

Linear Potentiometer

Energy harvesting systems

•The power requirement of SHM systems is especially a problem for structures in remote areas.

•The main energy harvesting sources in structures are solar, wind, and vibration.

Developer Harvesting

Method

Energy Produced Application Reference

Solarworld (SPE-

350-6)

Solar energy 9V-350 mA, produced

energy depending on

available solar energy

Suspension bridge Jang et al. (2010)

University of

Michigan

Traffic induced-

vibration

2.3 uW for 0.54 ms-2

acceleration

Suspension bridge Galchev et al.

(2011)

Laboratoire

Navier, France

Traffic induced-

vibration

0.03 mW mean power Prestressed

concrete bridge

Peigney and

Siegert (2013)49

E.H. Deployment

(a) Telegraph Road Bridge solar panels powering wireless sensors

(b) New Carquinez Bridge solar panel powering the sensor node shown in the assembly box .

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Periodic Monitoring

Periodic SHM conducted to investigate detrimental

changes that might occur in a structure

Behaviour of structure is monitored at specified time

intervals (days, weeks, months, years…)

Examples include periodic monitoring:• through ambient vibration;

• through testing under moving traffic;

• through static field testing;

• of crack growth

• of repairs

Continuous Monitoring

Monitoring is ongoing for an extended period of time

Only recently used in field applications because of high

costs and relative complexity

Real-time monitoring and data collection

1. Stored on site for analysis later

2. Communicated to remote location for real-time analysis

Usually only applied to important structures or when

there is doubt about the structural integrity

SHM System Design

Definition of SHM objectives

Sensor placement

Types of monitoring

Durability and lifespan of SHM

1. Design Issues…

SHM System Design

2. Installation Issues…

Sensor identification

Contractor education

Sensor damage during

construction

Structural changes induced by

presence of SHM system

Protection against deterioration

and vandalism

SHM System Design

3. Use Issues…

Data collection and

management

Continuity of knowledge

Dissemination of

performance results

Public awareness

SHM System Design

1. Identify the damage or deterioration mechanisms

2. Categorize influence of deterioration on the mechanical response

• Theoretical and numerical models of structure

3. Establish characteristic response of key parameters • Establish sensitivity of each to an appropriate level of deterioration

4. Select the parameters and define performance index • Relates changes in response to level of deterioration

5. Design system • Selection of sensors, data acquisition and management • Data interpretation

6. Install and calibrate SHM system (baseline readings)

7. Assess field data and adapt system as necessary

I-35W St. Anthony Falls Bridge, Minneapolis

58

I-35W St. Anthony Falls Bridge, Minneapolis

I-35W St. Anthony Falls Bridge, Minneapolis

I-35W St. Anthony Falls Bridge, Minneapolis, USA

SHM System

•The design of the system was an integral part of the overall bridge design process

•Monitoring instruments measure dynamic and static parameter points to enable close behavioral monitoring during the bridge's life span.

I-35W St. Anthony Falls Bridge, Minneapolis, USA

Sensors deployed

I-35W St. Anthony Falls Bridge, Minneapolis, USA

Sensors deployed

Computer Interface

I-35W St. Anthony Falls Bridge, Minneapolis, USA

The Future of Structural Concrete SHM

Muscle/Member Analogy:

Smart concrete have sensors

inside that provide information

about the structural members’

condition

Muscles have nerve cells

embedded in them that provide

information to the brain about

the conditions of the muscles

Smart concrete with sensors embedded inside that provide

information about the condition of the structural component

Smart Concrete

In Conclusion……………..

Butterflies and dinosaurs date from the same historical period…

Recent research leads scientists to the conclusion that

butterflies have survived because they have been

equipped with better sensors than dinosaurs, and thus

are able to adapt to environmental changes.

Should we build structures with a butterfly or

dinosaur destiny?

THANK YOU