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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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
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y pDisadvantages:
Different storing (climate chamber/real climate)
Cubes independent from construction
Undersized samplepressure test of a cube
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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CE
Testing with rebound hammer
Rebound HammerRebound Hammer
5
Exemplification and procedure
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound 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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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CE
Testing with rebound hammer
Rebound HammerRebound Hammer
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Assessment with table NA.2 (German national annex)
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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
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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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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)
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
CE Assessment of concrete compressive strength with
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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
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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
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer
CE Assessment of concrete compressive strength with
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)
Cape Town / Johannesburg 14./15.02.2013 - [email protected] Condition Assessment using NDT-CE – Rebound hammer