condition assessment of structures · ndt objectives ndt methods are extremely valuable in...

1

Rak-43.3301 Repair Methods of Structures I (4 cr), Fahim Al-Neshawy & Esko Sistonen, Autumn 2015

Condition Assessment of Structures(Visual inspection & non-destructive testing of structures)

16.09.2015


2Course outline

IntrinsicExtrinsic

ChemicalPhysicalBiological

Initiating

Leads to

Assessed

Visual inspectionNon-Destructive TestingDestructive Testing

CrackingScaling

SpallingPopouts

Delaminationetc.

Principles forrepair and protection

DeteriorationProcess

VisibleDamage

Conditionsurvey

Repair

Factors CausingDeterioration

Conditionevaluation


3

Outlines

• Main objective of condition assessment

• Deterioration / Building materials

• Condition assessment

– Condition Survey Planning

– Visual inspection

– Non-Destructive testing (NDT)

• detection of cracks/ voids/ delamination etc.

• corrosion assessment, location and diameter ofreinforcement and cover thickness

• strength estimation of concrete --- to be continued in the next lecture ---

4


Main objective of condition assessment

Main objective of condition assessmentare to place the building into one of thefollowing three categories:

i. The building has not shown anysigns of distress and It satisfiesall the safety and serviceabilityrequirements according torelevant Codes of practice, henceno action is needed towardsrepairing.

ii. The building is seen to bedeficient (or distressed) but it canbe repaired and strengthened tosatisfy the Codal safetyrequirements or performancecriteria set by the user.

iii. The building is badly damaged. Itis to be demolished and a newbuilding may be built, build backbetter.

Main steps of condition assessmentwill bea) To record the damage if any, and find

out the causes for distressb) To assess the extent of distress and

to estimate the residual strengths ofstructural components and thesystem including the foundation

c) To plan the rehabilitation andretrofitting/strengthening of thebuilding

5


Deterioration / Building materials (1/2)

http://www.vtt.fi/inf/pdf/technology/2012/T33.pdf

6


http://www.vtt.fi/inf/pdf/technology/2012/T33.pdf

Deterioration / Building materials (2/2)

7


Condition assessment planning

Preliminary investigation:1. Review of relevant

documents2. Visual inspection, with

documentation of defects3. Field and laboratory testing4. Preliminary analysis and

evaluation

Is furtherinvestigation

required?

Detailed investigation:1. Review of additional

documents and data source2. Additional field

observations, and field andlaboratory testing

3. Detailed analysis andevaluation

YES

NO

Isrepairingrequired?

YES

NO

Identify and analysis repairoptions

Identify special conditionsto further considered (e.g.maintenance, planning

Finalreport

8


Review of plans and relevant documents

• To review documents from designand construction process as well asinspection and maintenance reportsis in general the easiest way ofgathering data about the structure tobe assessed.

• It has to be assured that the revieweddocuments are correct.

• Loads can be usually determined fromcurrent loading codes andenvironmental conditions may beobtained from inspection reports.

• Resistance properties like materialand structural properties anddimension can be obtained from:– Construction specifications -

Codes– As-built drawings--architectural,

structural, mechanical, andfoundation plans

– Construction documents (e.g.material delivery documentation)

– Documentation of performance,defects, maintenance, andchanges (Alterations)

– Reports of earlier inspection andmaintenance.

9


VISUAL INSPECTION

10


Scope of Visual Inspection

• Prior to the starting of visual inspection,the structural engineer is to obtain a setof the building’s structural layout plansfrom the building owner.

• The availability of the structural layoutplan will help the structural engineer to:

a) understand the structural system andlayout of the building;

b) identify critical areas for inspection;c) identify the allowable imposed loads,

in order to assess the usage andpossibility of overloading; and

d) verify if unauthorised addition oralteration works that affect thestructure of the building have beencarried out.

http://www.bca.gov.sg/PeriodicStructuralInspection/others/PSI_PE.pdf

Fig. Lahdensivu, J.

11


Visual Inspection Tools and Instruments

• Simple tools and Instruments like:– Camera– Magnifying glass– Binocular– Gauge for crack width

measurement– Chisel and hammer are usually

needed.– Pocket knife, screwdriver– Occasionally, a ladder or light

platform/scaffold tower can beused for access to advantage.


12


Scope of Visual Inspection


A visual inspection is generally carriedout of:a) the condition of the structure of

the building§ to identify the types of structural

defects§ to identify any signs of structural

distress and deformation§ to identify any signs of material

deteriorationb) the loading on the structure of the

building§ to identify any deviation from

intended use, misuse and abusewhich can result in overloading

c) any addition or alteration worksaffecting the structure of thebuilding§ - to identify any addition or

alteration works which can result inoverloading or adverse effects onthe structure

• If there are no signs of any structuraldeterioration or defects, the visualinspection should suffice and unlessthe structural engineer otherwiseadvises, no further action needs tobe taken

13


Visual inspection report (example)


1. General Information of the Building• address, usage of the building,

maintenance history etc.

2. Structural System of the Building• reinforced concrete, prestressed

concrete, steel, etc

3. Date and Scope of the Inspection4. Survey of addition or alteration works

to building structure5. Survey of signs of structural defects,

damages, distress, etc.6. Survey of exposure to aggressive

environment7. Conclusions on the structural

condition8. Sketches, plans and photographs

14


Examples of typical defects found by visual inspection

Erosion of Brick Face Efflorescence Brick Spalling/Delaminating

15


Examples of typical defects found by visual inspection

http://www-pub.iaea.org/mtcd/publications/pdf/tcs-17_web.pdf

Concrete CrackCrack and Spall of

Concrete Around SteelMember

DelaminatingConcrete

Over Reinforcement

16


Non-Destructive testing (NDT)on

reinforced concrete structure

17


NDT Objectives

NDT methods are extremely valuable inassessing the condition of structures,such as bridges, buildings, elevatedservice reservoirs and highways etc.• The principal objectives of the NDT /

PDT of concrete in situ is to assess oneor more of the following properties:– In situ strength properties– Durability– Density– Moisture content– Elastic properties– Extent of visible cracks– Thickness of structural members

having only one face exposed

– Position and condition of steelreinforcement

– Concrete cover over thereinforcement.

– Reliable assessment of theintegrity or detection of defects ofconcrete members even when theyare accessible only from a singlesurface.

18


NDT Advantages and Disadvantages

Advantages

• Access to hidden items – “seethrough walls”

• Better investigations with NDT• Rapid accumulation of data• Generally less expensive than

destructive testing• Minimize interruption of building

services• Evaluation and quality assurance

Disadvantages

• More than one test method may berequired

• Environmental conditions mayeffect ordistort results

• Construction details & buildingcomponents may effect results

• Some conditions cannot bedetermined witha reasonable degree of accuracywithout destructive testing

19


Typical situations where non-destructive testing is needed

• Quality control of pre-cast units orconstruction in situ

• Monitoring of strength development inrelation to load application or similarpurpose

• Location and determination of theextent of cracks, voids, honeycombingand similar defects within a concretestructure

• Determining the concrete uniformity,possibly preliminary to core cutting,load testing or other more expensive ordisruptive tests

• Determining the position, quantity orcondition of reinforcement

• Increasing the confidence level of asmaller number of destructive tests

• Determining the extent of concretevariability in order to help in theselection of sample locationsrepresentative of the quality to beassessed

• Confirming or locating suspecteddeterioration of concrete resulting fromsuch factors as overloading, fatigue,external or internal chemical attack orchange, fire, explosion, environmentaleffects

• Assessing the potential durability of theconcrete

• Providing information for any proposedchange of use of a structure forinsurance or for change of ownership.

http://www-pub.iaea.org/mtcd/publications/pdf/tcs-17_web.pdf

20


NDT Methods for Specific Distresses

Distresses Condition Assessment Methods

Air Pockets andHoneycombing

Chain Dragging, Ground Penetrating Radar, HammerSounding, Impact-Echo, Ultrasonic, Visual Inspection

Alkali-Silica Reaction Visual InspectionChloride-InducedCorrosion Half-Cell Potential, Rapid Chloride Permeability, Resistivity

Cracking Impact-Echo, Ultrasonics, Visual Inspection

DelaminationChain Dragging, Coring, Ground Penetrating Radar,Hammer Sounding, Impact-Echo, Infrared Thermography,Ultrasonics

Popouts Visual InspectionPotholing (Caving) Visual InspectionScaling Visual InspectionSpalling Visual Inspection

21


NDT for detection of cracks/ voids/delamination etc.• Hammer sounding• Chain dragging• Ground penetrating radar• Impact-echo• Ultrasonic pulse velocity• Radiographic testing• Crack width measurement

22


NDT - Sounding

• A qualitative evaluation of concrete canbe easily obtained by just sounding it(i.e. tapping it) with a hammer.

• When the hammer is struck on goodconcrete, a ringing sound is created.

• On areas where delaminations orcracks occur, the striking of thehammer produces a drum-like sound.

• The limitation of this method is that itcannot detect defects that exist deep inthe member.

• Also, defects lying under overlays arealso difficult to find.

23


NDT - Chain Dragging

• The objective is to detect regions wherethe sound from dragging the chainchanges from a clear ringing sound(sound deck) to a somewhat mute andhollow sound (delaminated deck).

• Chain drag is a relatively fast methodfor determining the location of adelamination

• Chain Dragging is normally used onlarge concrete surface areas, such asbridge decks

• The method typically rely on theexperience of the inspector todifferentiate the relative sounds ofsimilar materials

24


NDT - Ground Penetrating Radar (GPR) [1/4]

• RADAR Radio Detection andRanging.– Detect target in free space– Determine the range

• Ground-penetrating radar (GPR) isa geophysical method that usesradar pulses to image thesubsurface.

• GPR finding and detecting buriedobject.

• GPR can be used in a variety ofmedia, including rock, soil, ice,fresh water, pavements andstructures.

• Civil engineering applications– Probing into soil to detect pipelines

and tanks– Cavities– Thickness determination– Locating reinforcement– Identifying deterioration– detect objects, changes in

material, and voids and cracks

http://www.foundationperformance.org/pastpresentations/gehrig_paper_march2004.pdf

25



Components of GPR:• Transmitting and receiving unit• Control unit• Display unit• Power supplies

26



• An EM pulse is sent through an antenna,penetrating into the surveyed material.

• A portion of the energy is reflected back tothe antenna when an interface betweenmaterials of dissimilar dielectric constant isencountered.

• The amount of reflected energy at aninterface is governed by:

• where1,2 is the reflection coefficient

r1 and r2 are the dielectric constants.

21

212,1

rr

rr

eeee

r+

-=

27


• The thickness of a layer is given by:

• where– di is the thickness of layer i,– ti the total travel time through that

layer,– C is the speed of light

r,i the dielectric constant of thelayer

Typical Dielectric Constants:

ir

ii

Ctd

,2 e=


28


NDT - Impact-echo [1/3]

P

S

RR

Impact• Based upon evaluation of stress wavesgenerated by an elastic impact on aconcrete surface

• Originally developed at CornellUniversity and NIST by M.Sansalone and N. Carino

• Impact Echo can be used to measurethe thickness of slabs, plates, columnsand beams, and hollow cylinders.

• It can also be used to determine thelocation and extent of flaws such ascracks, delaminations, voids,honeycombing and debonding in plain,reinforced and post-tensioned concretestructures.

P – Compression wavesS - Shear wavesR - Rayleigh waves

By: Guy Rapaport, Ramboll

29


Procedure• Impact Echo testing consists of

measuring both the time record andfrequency spectrum associated with amechanical impact on the surface of astructure.

• As stress waves propagate through thestructure, they reflect off internal andexternal boundaries and causeperiodic displacements on the surface.

• These motions are monitored by atransducer and digitized.

• The waveform is transformed into thefrequency domain, so that theperiodicity of stress-wave arrivals canbe accurately determined.

• As part of Impact Echo testing, directmeasurements of compression (P-)wave velocity are also made.

• Given the P-wave velocity and thearrival period (or frequency), the depthsto internal flaws or external boundariesare calculated.


30


The fundamental equation of impact-echo is d = Vp/(2f),where

d is the depth from which the stress waves are reflected (the depth of a flawor the thickness of a solid structure),

Vp is the wave speed,f is the dominant frequency of the signal


31


Ultrasonic Pulse Velocity

• Ultrasonic waves are verysimilar to light waves in thatthey can be reflected,refracted, and focused.

• Reflection and refractionoccurs when sound wavesinteract with interfaces ofdiffering acoustic properties.

• Ultrasonic reflections from thepresence of discontinuities orgeometric features enablesdetection and location.

http://www.theconcreteportal.com/nde.html

32


Ultrasonic pulse velocity in concrete (UPV) ASTM C597

• In the UPV method, the velocity of apulse traveling through concrete ismeasured and correlated to its stiffness

• The velocity of the pulse increases withthe stiffness of the concrete, butdecrease with increasing density.

• Wave attenuation increases whenconcrete becomes denser, because ofabsorption of energy.

• the UPV can be used in three modes –direct, semi-direct, and indirect.

• The direct mode, or the through-transmission mode, is the most reliable,but needs access to both sides of thematerial.

http://www.olsoninstruments.com/pdf_downloads/catOI_upv.pdfhttp://www.theconcreteportal.com/nde.html

33




• The pulse velocity can be determinedfrom the following equation.

=where

V = pulse velocity (km/s),L = path length (cm),T = transit time(µs).

• Based on this technique, the velocity ofsound in a concrete is related to theconcrete modulus of elasticity.

( )

where,E = modulus of elasticity,r =density of the concrete

34




35


Radiographic testing

• Radiography can be used to obtainpermanent image of surface and sub-surface (embedded) discontinuities

• The same discontinuities can beradiographed again after a period ofservice life and the radiographs can becompared to measure the change inthe size and shape of the discontinuity.

• Radiographic testing of the structuralreinforcement in reinforced concreteelements is carried out with the use ofan apparatus containing sources ofgamma or X rays.

• The classical inspection arrangementconsists of a radiation source at oneside of the object to inspect and an X-ray film at the other side.

• There exist also radiation methods forone-side inspections, using thebackscatter effect.

• These methods need long inspectiontimes and are limited to single layers ofreinforcement near the surface.

http://www.ndt.net/article/ndtce03/papers/v020/v020.htm

36



• The selection of the sourcedepends on the wall thickness,which shall be inspected


37



• Stereo radiography is used forquantitative measurement of depth anddiameters of steel reinforcements.

• It needs two different source positions.• The reconstruction can be done by a

graphical back projection or acomputerised one.

• The simple graphical backprojection issufficient for so called simplestructures.

• This is usually some separated steelparts or one layer of steel bars.


Principle of stereo radiography

38



Results• flat bar in the corner

pillar is shifted vertical• flat bars are led around

round bar runningvertical for the anchor

Radiation technique arrangement of the measuring point

39


Crack Width Gauge• Align the Crack Width Gauge where the

calibration and the crack are the samewidth.

• Record the width, length and location ofthe crack.

Crack width measurement

Measuring Magnifier• Crack widths are normally limited to 0.2

mm or 0.3 mm in concrete structures.• Measuring Magnifier device enables

accurate determination of whethercracks exceed this limit.– Magnification 10x– Measuring range 20 mm x 0.1 mm– Field of View 32mm

40


Crack width measurement

CRACK DETECTION MICROSCOPE• Measure crack widths in concrete.• Consisting of a high definition

Microscope connected to an adjustablelight source which provides a well-illuminated image under all workingconditions.

• The image is focused by turning theknob on the side of the microscope andthe eyepiece can be rotated through360 degrees to align with the directionof the crack being examined.

• The 4mm measurement has a lowerscale divided into 0.2mm divisions.

• the maximum crack widths should notexceed 0.3mm which is 15 divisions onthe scale for most types ofenvironment.

• Specification:– Magnification x 40– Measuring Range 4 mm– Divisions 0.02mm

41


NDT for corrosion assessment, location anddiameter of reinforcement and cover

thickness

• Cover meter• Half Cell Potential test• Concrete Resistivity test

42


Cover meter test

• Cover meter test is used to assess thelocation and estimate the diameter ofreinforcement bars and concrete cover.

• Principle:– based measurement of change of

an elctromagnetic field caused bysteel embedded in the concrete.

• Equipment:– profometer comprise a search

head, meter and interconnectingcable.

• The concrete surface is scanned, withthe search head kept in contact with itwhile the meter indicates, by analogueor digital means, the proximity ofreinforcement

43


Cover meter test

• The cover meter applies a currentpulse

• The instrument measures theamplitude of the induced current, whichdepends on the orientation, depth, andsize of the bar.

• The search head is directional andmaximum signal is obtained when thebar is aligned with the long axis of thesearch head.

• The pulse-induction technique isuniquely stable, is not affected bymoisture in concrete or magneticaggregates, and is immune totemperature variations and electricalinterference.

Cover Master, Germann Instruments:http://germann.org/products-by-application/half-cell-potential/covermaster

44


Example of the cover meter test results

http://www.proceq.com/fileadmin/documents/proceq/products/Concrete/Profometer_5_/English/Proceq_Brochure_Profometer_5__E.pdf

45


Half-Cell Potential Method (ASTM C 876-91)

• Then generally readings taken areat grid of 1 x 1 m for slabs, wallsand at 0.5 m c/c for Column,beams

Principle:• The electrical activity of the steel

reinforcement and the concrete leadsthem to be considered as one half ofweak battery cell with the steel actingas one electrode and the concrete asthe electrolyte.

• The electrical potential of a point on thesurface of steel reinforcing bar can bemeasured comparing its potential withthat of copper - copper sulphatereference electrode on the surface.

• Practically this achieved by connectinga wire from one terminal of a voltmeterto the reinforcement and another wireto the copper sulphate referenceelectrode.

http://theconstructor.org/concrete/corrosion-potential-assessment/2969/

46


Half-Cell Potential Method (ASTM C 876-91)

The results affected by:• Degree of humidity in concrete

– More negative potentials result forconcrete with higher degree ofsaturation.

• Stray currents– The presence of stray currents will

significantly affect themeasurements of the half-cellpotential.

• Oxygen content near thereinforcement– The lack of oxygen near the

reinforcement results in morenegative potentials

• Microcracks– Localized corrosion can be

generated by microcracks, whichalso modify the concrete resistivity,consequently affecting thecorrosion potential measurement

47


Example of the Half Cell Potential test result

MeasuredPotential

Ecorr values Corrosion Condition

mV vs. SCE<-426 <-500 Severe corrosion

<-276 <-350 High (>-90% risk ofcorrosion)

-126to

-275

-350to

-200

Intermediate corrosionrisk

>-125 >-200 Low (10% risk ofcorrosion)

CSE = Copper / Copper sulphate electrode,i.e. the potential values are stated with therespect to CSE

http://theconstructor.org/concrete/corrosion-potential-assessment/2969/

48


Example of the Half Cell Potential test result

http://www.pcte.com.au/corromap-half-cell-potential-corrosion-rate-and-resistance-mapping

49


• The Galvanostatic PulseMeasurements technique (GPM) wasfirst used in the field in 1988.

• It provides a solution to interpretationproblems found when the half cellpotential methods is used in someenvironments, e.g. in wet concrete.

• estimation of corrosion rate as well,which means how much reinforcementsteel is being dissolved per year.

• The GalvaPulse™ is a rapid, non-destructive polarization technique forthe evaluation of reinforcementcorrosion rate as well as half-cellpotentials.

typically been used in connection with:– Swimming pools– Bridges– Balconies– Parking houses

Galvapulse - Surface Corrosion Rate System

http://www.pcte.com.au/galvapulse-surface-corrosion-rate-system

50


Advantages• Estimation of the corrosion rate in the

reinforcement can be made in less than10 seconds.

• Reliable evaluation of reinforcementcorrosion also in wet, carbonated orinhibitor treated concrete.

• Half cell potential and electricalresistance of the cover layer are given.

• Lightweight electrode / hand heldcomputer and easy to operatesoftware.

• Durable Guard Ring system forfocusing the current field to thereinforcement.

• Measurements possible on uneven andcurved surfaces.

• Measurement results in Excel-formatare easily transferred to PC for furtherprocessing and presentation.

Threshold values

Galvapulse - Surface Corrosion Rate System

51


Concrete Resistivity test

The Measurement Principle• measure the electrical resistivity of

concrete or rock in a non-destructivetest.

• A current is applied to the two outerprobes, with the difference measuredby the two inner probes.

• In concrete material with high electricalresistivity the corrosion process will beslow compared to concrete with lowresistivity in which the current caneasily pass between anode andcathode areas

http://www.rbmltd.co.il/img/ckFiles/files/ProductsPDF/Proceq/resipod_SF_E_2013_06_19_low.pdf

Wherea is probe spacing [cm]V is measured potential [V]I is the current applied [A]

52


Concrete Resistivity test result

http://www.electrochemsci.org/papers/vol2/2010001.pdf

• The electrical resistivity of concrete wasproposed as an effective parameter toevaluate the risk of reinforcing steelcorrosion, particularly when corrosion isinduced by chloride attack

• The resistivity measurement is a usefuladditional measurement to aid inidentifying problem areas or confirmingconcerns about poor quality concrete.

• Measurements can only be consideredalong side other measurements.

• Reinforcing bars will interfere withresistivity measurements.

53


Limitation• It is difficult to measure resistivity in

very close reinforcement• Carbonation may affect the

resistivity• It cannot be used where ambient

change in temperature is there.• Experience operator is required to

handle this equipment.

Concrete Resistivity test

54


Summary

Lecture summary

• Main objective of condition assessment

• Deterioration / Building materials

• Methodology of condition assessment

– Condition Survey Planning

– Visual inspection

– Non-Destructive testing (NDT)

• detection of cracks/ voids/delamination etc.

• corrosion assessment,location and diameter ofreinforcement and coverthickness

Next lecture– Tests for strength estimation of

concrete– Destructive and laboratory

testing on reinforced concretestructure

– Mold inspection– Inspection of masonry walls– Inspection of rendered facades

condition assessment of structures · ndt objectives ndt methods are extremely valuable in...

Documents