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### What are the formulae used in the compressive strength correlation curves in the OS8000 and OS8200 rebound hammers?

The formulae used in the compressive strength correlation curves in the OS8000 and OS8200 rebound hammers can be found in the Schmidt Hammer Conversion Curve Database below:

1. Original Schmidt OS8000 Type N

Curve

Min R

Max R

Unit

Form factor

Equation

Proceq B (14 - 56 day mean)

15

55

N/mm2

150 mm cube

Fck = 1.05 x ( 0.01078R2 + 0.904R - 12.91)

Proceq A (7 days mean)

15

55

N/mm2

150 mm cube

Fck = 1.05 x ( 0.012516R2 + 0.706R - 7.49)

Portland Cement J

15

55

kg/cm2

150 mm cube

Fck = 1.05 x ( 0.133R2 +8.82R - 122)

Early strength J

15

55

kg/cm2

150 mm cube

Fck = 1.05 x ( -0.0803R2 + 20R - 253)

Blast Furnace J

15

55

kg/cm2

150 mm cube

Fck =1.05 x ( 0.2R2 + 1.83R + 22.4)

Averag Curve J

15

55

kg/cm2

150 mm cube

Fck = 1.05 x ( 0.0485R2 + 12.3R - 146)

Characteristic strength R

10

95

N/mm2

150 mm cube

Step function (see below)

The characteristic strength curve is based on the reference table for the screening test provided in the Annex to EN13791. This curve provides a safe estimate with <5% probability of over-estimating the strength.

R value - Strength (N/mm2)
20 to 29 - <10
30 to 32 - 10
33 to 34 - 15
35 to 37 - 20
38 to 39 - 25
40 to 42 - 30
43 to 46 - 37
47 to 48 - 45
49 to 50 - 50
51 to 52 - 55
53 to 56 - 60
57 to 59 - 67
60 to 64 - 75
65 to 68 - 85
69 to 75 - 95

Impact angle conversion is applied to the measured R value before calculation of the compressive strength according to these curves. The impact angle correction is based on this formula:

Rcorr=R +(-a0+R*a1)*sin(angle)

 a1+ 0.08 a0+ 7.1 a1- 0.043 a0- 4.3

Take a0+ and a1+ for angles >= 0°, a0- and a1- for angles < 0°

# 2. Silver Schmidt OS8200 Type N

 Curve Min Q Max Q Unit Form factor Equation < 10% (old) 22 75 N/mm2 150 mm cube fck = 2.77e^0.048Q Ref EU (old) 21 62 N/mm2 150 mm cube fck = 1.8943e^0.064Q Ref CN 20 75 N/mm2 150 mm cube fck = 0.0244Q^2-0.6129Q+10 Ref RUS 20 77 N/mm2 150 mm cube fck = 0.03Q^2-0.86Q+9.5 Mean strength 10 75 N/mm2 150 mm cube fck = 3.6473e^0.0475Q Characteristic strength Q 10 95 N/mm2 150 mm cube Step function (see below)

No angle correction is required for measurements with the Silver Schmidt.

Note! The Ref EU curve was determined with very high quality concrete and gives very high strengths.

The characteristic strength curve is based on the reference table for the screening test provided in the Annex to EN13791. This curve provides a safe estimate with <5% probability of over-estimating the strength.

Q value - Strength (N/mm2)
10 to 33 - <10
34 to 39 - 10
40 to 44 - 15
45 to 48 - 20
49 to 51 - 25
52 to 55 - 30
56 to 59 - 37
60 to 61 - 45
62 to 63 - 50
64 to 65 - 55
66 to 67 - 60
68 to 70 - 67
71 to 72 - 75
73 to 74 - 85
75 to 80 - 95

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