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

A number of compressive strength correlation curves have been developed for the Original Schmidt OS8000 and Silver Schmidt OS8200 hammers. The formulae for these curves may be found in the following tables.


Schmidt Hammer Conversion Curve Database


1. Original Schmidt OS8000 Type N


Curve Min R Max R Unit Form factor Equation
Proceq B (N)  15 55 N/mm2 150 mm cube Fck =1.05x( 0.01078R2 + 0.904R - 12.91)
Proceq A (N) 15 55 N/mm2 150 mm cube Fck =1.05x( 0.012516R2 + 0.706R - 7.49)

Portland Cement J

15 55 kg/cm2 150 mm cube Fck =1.05x( 0.133R2 +8.82R -122)
Early strength J   15 55 kg/cm2 150 mm cube Fck =1.05x( -0.0803R2 +20R -253)
Blast Furnace J 15 55 kg/cm2 150 mm cube Fck =1.05x( 0.2R2 +1.83R +22.4)
Averag Curve J 15 55 kg/cm2 150 mm cube Fck =1.05x( 0.0485R2 +12.3R -146)


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% 22 75 N/mm2 150 mm cube 𝑓𝑐𝑘 = 2.77𝑒0.048𝑄
Ref EU 21 62 N/mm2 150 mm cube 𝑓𝑐𝑘 = 1.8943𝑒0.064𝑄
Ref CN 20 75 N/mm2 150 mm cube 𝑓𝑐𝑘 = 0.0244𝑄2 − 0.6129𝑄 + 10
Ref RUS 20 77 N/mm2 150 mm cube  𝑓𝑐𝑘 = 0.03𝑄2 − 0.86𝑄 + 9.5


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.

A new mean value curve has been developed which is to be preferred for a best estimate if no correlation with cores is possible. This curve is not yet available in the app but can of course be entered as a custom materials curve.

Fck =3.6473*EXP(0.0475*Q)

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