technical workshop condition assessment using classical ... · surface, carbonation, direction of...

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Technical Workshop

Condition Assessment using classical and modern Non-Destructive Testing methods

Assessment of concrete compressive strength in situ

Alexander Taffefrom

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from

in cooperation withSascha Feistkorn

SVTI (Switzerland)

Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer

CEOutlineOutline

Options to assess the concrete compressive strength

- Assessment with the rebound hammer

- Combination rebound hammer/cores

- Combination rebound hammer/cubes

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Summary


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CE Testing with rebound hammer (NDT-CE) Non destructive assessment in structures

Disadvantages:

Indirect assessment of compressive strength

Options to assess concrete strength IOptions to assess concrete strength I

Indirect assessment of compressive strength

Only information about near-surface areas

Influence of carbonation

Cubes/cores in combination with NDT First: non destructive assessment in structures

testing with Silverschmidt rebound hammer

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(rebound hammer; ultrasonic pulse velocity; pull-out force)

Second: taking cores in same test locationsDetermine the compressive strength of cores

Application when large construction

Determination of ultrasonic pulse velocity


CE Cores without NDT When existing structures;

Number of cores depends on amount of concrete/test purpose

Options to assess concrete strength IIOptions to assess concrete strength II

p p

Disadvantages:

Destructive test

Not possible in highly stressed areas

Cubes without NDT When new constructions; mostly 3 retain sample

example of taking a core

4

y pDisadvantages:

Different storing (climate chamber/real climate)

Cubes independent from construction

Undersized samplepressure test of a cube


3

CE

Testing with rebound hammer

Rebound HammerRebound Hammer

5

Exemplification and procedure


CERebound number RRebound number R

Rebound number R = distance of the mass after the impact on the surface

Advantages: consistent with the standard, large number of measurements (pairs of test results R/core test)

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reference: Proceq


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CERebound value QRebound value Q

„Q“-value Q=vR/v0; Ratio of velocity vR and v0

measured shortly before and shortly after the impact

Advantages: „Q“-value independent f di ti f t t ( it ) dfrom direction of test (gravity) and friction, lower variation

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reference: Proceq


CE Compressive strength: Capacity of a material to withstand axially directed pushing/compressing forces

Basic principle IBasic principle I

Schenck 4 MN compressive strength test machine

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Hardness: Mechanical resistance of a specimen against mechanical indentation through a harder material swing hammer


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CE

Relationship

HARDNESSCOMPRESSIVE STRENGTH

Basic principle IIBasic principle II

Principle of testing with the rebound hammer

THE HARDER THE MATERIAL

THE LESS

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THE LESS

MECHANICAL ENERGY WILL BE ABSORBED


CE

Device:

Rebound Hammer (spring-loaded steel hammer measuring the rebound distance)

Calibration Anvil (hardness of minimum 52 HRC a mass of (16±1) kg Ø~150 mm)

Non destructive assessment of compressive strength with rebound number (EN 12504-2)

Device Device forfor NDTNDT

Calibration Anvil (hardness of minimum 52 HRC, a mass of (16±1) kg, Ø~150 mm)

Grinding Stone (medium-grain texture; silicon carbide stone)

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Rebound testing in structures Verification on the Anvil

GrindingStone


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CE Test conditions:

Temperature within a range of 10 °C - 35 °C

First step: Verification on the calibration anvil

Second step: Rebound testing

Procedure of rebound testingProcedure of rebound testing

Plunger has to impact perpendicularly to the surface

Increase the pressure on the plunger until the hammer impacts

Recording of rebound number and test direction

Use a minimum of 9 readings for one test location

Minimum distance of 25 mm between two impact points; no impact point within 30 mm of an edge

Examination of each impression on the surface after impact; when an

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Examination of each impression on the surface after impact; when an impact has crushed/broken through a near-to-surface void; discount the result

Third step: Verification on the calibration anvil

Result: Median of all readings as a whole number


CE

Test region = bridge beam

Selection of test regionsSelection of test regions

Test location; n ≥ 9

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Test region: One or several structural elements or precast concrete componentsassumed or known to be from the same population A test region contains several test locations


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CESelection of test locationsSelection of test locations

Test location: Limited area selected for measurements used to estimate one test result, which is to be used in the estimation of on-site compressive strength

At l t 100 thi k d fi d ithi t t

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At least 100 mm thick and fixed within a structure Shall be approximately 300 mm x 300 mm Avoid test locations with honeycombing, scaling, rough texture or high porosity - Considering type of surface, type of concrete, moisture condition of the surface, carbonation, direction of test, movement of the concrete under test Grind heavily textured or soft surfaces or surfaces with loose mortar


CE≥ 300 mm≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

36, 38, 38, 39, 40, 40, 41, 43, 45

≥ 300 mm≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

≥ 300 mm≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

34, 35, 35, 38, 41, 41, 41, 42, 46

33, 35, 38, 38, 40, 42, 42, 45, 47

rough 36, 36, 37, 39, 39, 40, 41, 41, 42

40, 41, 39

Example Example –– rebound testrebound test

≥

≥ 300 mm≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

≥ 300 mm

≥ ≥

≥ 300 mm≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

≥ 300 mm≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

≥ 300 mm ≥ 300 mm

39, 40, 40, 42, 43, 44, 44, 45, 46

38, 38, 38, 41, 42, 44, 46, 46, 47

35, 36, 36, 39, 40, 40, 40, 42, 59

R-num

bers

thr

g

43, 42, 40

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≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

≥ 25≥ 25≥ 25≥ 25

≥ 30

0 m

m≥

25

≥ 2

5≥

25

≥ 2

5

22, 42, 44, 44, 45, 45, 48, 48, 49

39, 39, 40, 41, 43, 43, 45, 45, 46

36, 41, 41, 43, 44, 44, 45, 46, 46

incre

asin

g

com

pacting

45, 43, 44


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CE

Testing with rebound hammer

Rebound HammerRebound Hammer

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Assessment with table NA.2 (German national annex)


CE

table NA.2 – rebound numbers and comparable compressive strength (german national annex) - DIN EN 206-1

compressive strength classminimum median

for each test location

scale division scale division

minimum median for each test region

median above all medians of the test locations

has to greater than…

Exemplification Exemplification -- interpretationinterpretation

median of every test location

has to be greater than…

Assessement with table NA.2 (german NA); Carbonation depth ≤ 5 mm!

assessedassessed compressive strength classcompressive strength classC25/30

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39 40 40 41 42 43 43 44 45


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CE

Cores and NDT CE

Testing with rebound hammer and cores ITesting with rebound hammer and cores I

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Combination of non destructive and destructive testing according to EN 13791


CETwo alternatives (indirect methods)

according to EN 13791

Testing with rebound hammer and cores IITesting with rebound hammer and cores II

Alternative 1:

Determination of an own specific curve between the in-situ

Alternative 2:

Shift the basic curve by the amount ∆f

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compressive strength and the rebound tests results

Direct comparison with cores by the use of an own specific curve

Indirect comparison with cores by calibrating the basic curve


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CE Determination of an own specific curve

Alternative 1 Alternative 1 –– specific relationship Ispecific relationship I

Testing on structures

Determine rebound numbers Rm on at least 18 test locationsDetermine rebound numbers Rm on at least 18 test locations in according to EN 12504-2

Taking cores in the test locations (18 cores) in according to EN 12504-1

Compressive strength fc, core, is of cores

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18 pairs (Rm; fc, core, is)

Best-fit curve by regression analysisCape Town / Johannesburg 14./15.02.2013 - [email protected]

Condition Assessment using NDT-CE – Rebound hammer

CE

is=

fm

(n),

is–

1,48

*s

is=

fis

,low

est+

4

N/m

m²

Alternative 1 Alternative 1 –– specific relationship IIspecific relationship II

rebound number R

x concrete 1, 2 and 3

third degree polynomial regression

Best-fit curve has to be shifted downwards

f ck,

f ck,

ism

inf c

k,i

f isin

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90 % of the strength values are expected to be higher thanthe estimated value = lower ten percentile of strength

Very extensive only for large construction; rarely


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CE Shifting the basic curve to the appropriate level

Testing on structures

Determine rebound numbers Rm on at least 9 test locations

Alternative 2 Alternative 2 –– shift the basic curve Ishift the basic curve I

Determine rebound numbers Rm on at least 9 test locationsin according to EN 12504-2

Taking cores in the test locations (9 cores) in according to EN 12504-1

Compressive strength fc, core, is

of cores

2325,1 Rf R

5,3473,1 RfR

für 20≤R≤24

für 24≤R≤50

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9 pairs (Rm; fc, core, is)

Shift the basic curve by ∆fRebound number R in accordance with EN 12504-2


CE

Determine the difference δf for each value (δf = difference in in-situ strength between the measured value on the core and the value given by the curve)

Shift the basic curve by ∆f

Alternative 2 Alternative 2 –– shift the basic curve IIshift the basic curve II

a ue o t e co e a d t e a ue g e by t e cu e)

Calculate the mean δfm(n) and the standard deviation s of the differences

Amount ∆f = δfm(n) – k1 · s (k dependent on the =

fm

(n),

is–

1,48

*s

= f

is,lo

wes

t+

4

in N

/mm

²

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(k1 dependent on the number of paired tests)

Shift the basic curveby the amount ∆f Principle for shifting the curve


f ck,

is=

f ck,

ism

inf c

k,is

=

f isi

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CE

Cubes and NDT CE

Testing with rebound hammer and cubes ITesting with rebound hammer and cubes I

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Combination of non destructive and destructive testing according to EN 13791 German NA


CESpecific line W Specific line W -- overviewoverview

Determination of an own specific line W(correlation between compressive strength of cubes fcube - rebound number Rm)

At least 10 cubes of 150 mm edge length

First step:

Determination of rebound number Rm

Second step:

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example of specific line W

Determination of compressive strength fc

Calculation of regression lineCape Town / Johannesburg 14./15.02.2013 - [email protected]

Condition Assessment using NDT-CE – Rebound hammer

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CE

Manufacture of cubes

Intention:

Specific line W Specific line W -- cubescubes

Variation in compressive strength (at least 20 and at most 30 N/mm²)

a) Modification of the effective water content by identical concrete mix and the same age of concrete

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b) Modification of the age of concrete by identical concrete mix

At least 10 cubescubes with the same concrete formula


CE

Determine the rebound number Rm

Loading the cubes with 2,5 N/mm²

Specific line W Specific line W –– rebound number Rrebound number R

Testing with the rebound hammer (25 mm between two impact points; no impact point within 30 mm of an edge)

Testing on opposite faces of one cube

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Median Rm of at least 9 readingsDetermination of the „Q“-value on a cube


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CE Determination of compressive strength

Cube compressive strength according to EN 12390-3

Specific line W Specific line W –– compressive strengthcompressive strength

Loading rate 0,6±0,2 N/mm² s

Compressive strength = indicated peak load [kN]

10 pairs (R ; f ) for linear

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10 pairs (Rm; fc) for linear regression

Calculation of the regression line according to EN13791 German NA Schenck 4 MN compressive strength test machine


CESummary Summary -- OverviewOverview

Assessment of concrete compressive strength

Assessment with the rebound hammer (NDT) in accordance with EN 13791 (German national annex)accordance with EN 13791 (German national annex)

Combination rebound hammer + cores + own specific curve in accordance with EN 13791

Combination rebound hammer + cores + basic curve in accordance with EN 13791

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in accordance with EN 13791

Combination rebound hammer + cubes + own specific line in accordance with EN 13791 (German national annex)


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CE Assessment of concrete compressive strength with

Summary Summary –– NDT CENDT CE

Rebound hammer (NDT CE) according to EN 13791 with table NA.2 (German national annex)

At least 9 readings in each of at least 9 test locations for one test region

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Median of every test location and median above all medians

Carbonation depth ≤ 5 mm



Summary Summary –– NDT CE + cores + own curveNDT CE + cores + own curve

Rebound hammer + cores + own specific curve according to EN 13791

at least 18 pairs (Rm; fc, core, is)

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regression analysis

shift the own specific curve to the lower ten percentile of strength


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CESummary Summary –– NDT CE + cores + basic curveNDT CE + cores + basic curve

Assessment of concrete compressive strength with

Rebound hammer + cores + basic curve according to EN 13791

at least 9 pairs (Rm; fc, core, is)

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shift the basic curve by ∆f

∆f dependent from compressive strength difference between cores and basic curve



Summary Summary –– NDT CE + cubes + own lineNDT CE + cubes + own line

Rebound hammer + cubes + own specific lineaccording to EN 13791 (German national annex)

at least 10 cubes of 150 mm edge length

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rebound number Rm und concrete compressive strength fc

specific line W = regression line of pairs (Rm, fc)


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