Options to assess the concrete compressive strength. - Assessment with the rebound hammer. - Combination rebound hammer/cores

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1 Technical Workshop Condition Assessment using classical and modern Non-Destructive Testing methods Assessment of concrete compressive strength in situ Alexander Taffe from in cooperation with Sascha Feistkorn SVTI (Switzerland) 1 Outline Options to assess the concrete compressive strength - Assessment with the rebound hammer - Combination rebound hammer/cores - Combination rebound hammer/cubes Summary 2 1

2 Options to assess concrete strength I Testing with rebound hammer (NDT-) Non destructive assessment in structures Disadvantages: 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 (rebound hammer; ultrasonic pulse velocity; pull-out force) Second: taking cores in same test locations Determine the compressive strength of cores Application when large construction testing with Silverschmidt rebound hammer Determination of ultrasonic pulse velocity 3 Options to assess concrete strength II Cores without NDT When existing structures; Number of cores depends on amount of concrete/test purpose p Disadvantages: Destructive test Not possible in highly stressed areas Cubes without NDT When new constructions; mostly 3 retain sample Disadvantages: Different storing (climate chamber/real climate) Cubes independent from construction Undersized sample example of taking a core pressure test of a cube 4 2

3 Rebound Hammer Testing with rebound hammer Exemplification and procedure 5 Rebound 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) reference: Proceq 6 3

4 Rebound value Q Q -value Q=v R /v 0 ; Ratio of velocity v R and v 0 measured shortly before and shortly after the impact Advantages: Q -value independent from direction of test t (gravity) and friction, lower variation reference: Proceq 7 Basic principle I Compressive strength: Capacity of a material to withstand axially directed pushing/compressing forces Schenck 4 MN compressive strength test machine Hardness: Mechanical resistance of a specimen against mechanical indentation through a harder material swing hammer 8 4

5 Basic principle II Relationship HARDNESS COMPRESSIVE STRENGTH Principle of testing with the rebound hammer THE HARDER THE MATERIAL THE LESS MECHANICAL ENERGY WILL BE ABSORBED 9 Device for NDT Non destructive assessment of compressive strength with rebound number (EN ) 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) Grinding Stone (medium-grain texture; silicon carbide stone) GrindingStone Rebound testing in structures Verification on the Anvil 10 5

6 Procedure of rebound testing Test conditions: Temperature within a range of 10 C - 35 C First step: Verification on the calibration anvil Second step: 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 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 11 Selection of test regions Test region = bridge beam Test location; n 9 Test region: One or several structural elements or precast concrete components assumed or known to be from the same population A test region contains several test locations 12 6

7 Selection 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 least 100 mm thick and fixed within a structure t 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 13 Example rebound test , 38, 38, 39, 40, 40, 41, 43, , 40, 40, 42, 43, 44, 44, 45, , 42, 44, 44, 45, 45, 48, 48, , 35, 35, 38, 41, 41, 41, 42, , 38, 38, 41, 42, 44, 46, 46, , 39, 40, 41, 43, 43, 45, 45, , 33, 36, 35, 37, 38, 39, 38, 39, 40, 40, 42, 41, 42, 41, 45, , 36, 36, 39, 40, 40, 40, 42, , 41, 41, 43, 44, 44, 45, 46, 46 R-numbers thr rough increasing compacting 40, 41, 39 43, 42, 40 45, 43,

8 Rebound Hammer Testing with rebound hammer Assessment with table NA.2 (German national annex) 15 Exemplification - interpretation Assessement with table NA.2 (german NA); Carbonation depth 5 mm! table NA.2 rebound numbers and comparable compressive strength (german national annex) - DIN EN compressive strength class median minimum of every median test location for each test location has to scale be greater division than median minimum above all median medians of for the each test test locations region has scale to greater division than C25/30 assessed compressive strength class

9 Testing with rebound hammer and cores I Cores and NDT Combination of non destructive and destructive testing according to EN Testing with rebound hammer and cores II Two alternatives (indirect methods) according to EN Alternative 1: Determination of an own specific curve between the in-situ compressive strength and the rebound tests results Direct comparison with cores by the use of an own specific curve Alternative 2: Shift the basic curve by the amount f Indirect comparison with cores by calibrating the basic curve 18 9

10 Alternative 1 specific relationship I Determination of an own specific curve Testing on structures Determine rebound numbers R m on at least 18 test locations in according to EN Taking cores in the test locations (18 cores) in according to EN Compressive strength f c, core, is of cores 18 pairs (R m ; f c, core, is ) Best-fit curve by regression analysis 19 is = f m(n),is 1,48*s is = f is,lowest + 4 f ck, f ck,i f ck,is min Alternative 1 specific relationship II N/mm² f is in x concrete 1, 2 and 3 third degree polynomial regression Best-fit curve has to be shifted downwards rebound number R 90 % of the strength values are expected to be higher than the estimated value = lower ten percentile of strength Very extensive only for large construction; rarely 20 10

11 Alternative 2 shift the basic curve I Shifting the basic curve to the appropriate level Testing on structures Determine rebound numbers R m on at least 9 test locations in according to EN Taking cores in the test locations (9 cores) in according to EN Compressive strength f c, core, is of cores f R 1,25 R 23 für 20 R 24 1,73 R 34, 5 für 24 R 50 f R 9 pairs (R m ; f c, core, is ) Shift the basic curve by f Rebound number R in accordance with EN Alternative 2 shift the basic curve II Shift the basic curve by f 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) Calculate the mean δf m(n) and the standard deviation s of the differences Amount f = δ fm(n) k 1 s (k 1 dependent on the number of paired tests) f ck,is = f m(n),is 1,48*s f ck,is = f is,lowest + 4 in N/mm² f is i Shift the basic curve by the amount f f ck,is min Principle for shifting the curve 22 11

12 Testing with rebound hammer and cubes I Cubes and NDT Combination of non destructive and destructive testing according to EN German NA 23 Specific line W - overview Determination of an own specific line W (correlation between compressive strength of cubes f cube - rebound number R m ) At least 10 cubes of 150 mm edge length First step: Determination of rebound number R m Second step: Determination of compressive strength f c Calculation of regression line example of specific line W 24 12

13 Specific line W - cubes Manufacture of cubes Intention: 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 b) Modification of the age of concrete by identical concrete mix At least 10 cubes cubes with the same concrete formula 25 Specific line W rebound number R Determine the rebound number R m Loading the cubes with 2,5 N/mm² 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 Median R m of at least 9 readings Determination of the Q -value on a cube 26 13

14 Specific line W compressive strength Determination of compressive strength Cube compressive strength according to EN Loading rate 0,6±0,2 N/mm² s Compressive strength = indicated peak load [kn] 10 pairs (R m ; f c ) for linear regression Calculation of the regression line according to EN13791 German NA Schenck 4 MN compressive strength test machine 27 Summary - Overview Assessment of concrete compressive strength Assessment with the rebound hammer (NDT) in accordance with EN (German national annex) Combination rebound hammer + cores + own specific curve in accordance with EN Combination rebound hammer + cores + basic curve in accordance with EN Combination rebound hammer + cubes + own specific line in accordance with EN (German national annex) 28 14

15 Summary NDT Assessment of concrete compressive strength with Rebound hammer (NDT ) according to EN with table NA.2 (German national annex) At least 9 readings in each of at least 9 test locations for one test region Median of every test location and median above all medians Carbonation depth 5 mm 29 Summary NDT + cores + own curve Assessment of concrete compressive strength with Rebound hammer + cores + own specific curve according to EN at least 18 pairs (R m ; f c, core, is ) regression analysis shift the own specific curve to the lower ten percentile of strength 30 15

16 Summary NDT + cores + basic curve Assessment of concrete compressive strength with Rebound hammer + cores + basic curve according to EN at least 9 pairs (R m ; f c, core, is ) shift the basic curve by f f dependent from compressive strength difference between cores and basic curve 31 Summary NDT + cubes + own line Assessment of concrete compressive strength with Rebound hammer + cubes + own specific line according to EN (German national annex) at least 10 cubes of 150 mm edge length rebound number R m und concrete compressive strength f c specific line W = regression line of pairs (R m, f c ) 32 16