A rebound hammer graph with a custom or manufacturer's conversion curve will convert the Q or R rebound index to a value of compressive strength in MPa, or hammers with data logging can convert automatically.
Conversion Curves for the Original Schmidt and Silver Schmidt Hammers are available below for users who need to make a manual conversion or learn how to read a rebound hammer graph.
The most common concrete rebound hammer form factor has a mechanism similar to a nail punch, the impact energy that is "bounced" back from striking the surface of the material is converted to a dimensionless Index, related to surface hardness.
Ideally a user will create a custom curve, breaking compressive strength test samples which have been tested with their Schmidt Hammer and creating a specific relationship for the concrete mix.
The user then either uses the graph of MPa vs Q/R Rebound Coefficient to convert the raw output, or takes the formula for that relationship and uses their hammers' Custom Material function to add an automatic conversion.
The image below is the most common manufacturer curve used in Australia, converting the rebound value R from a data set of Cylinder Sample breaks between 15-56 days of age. These samples were simple mixes of portland or blast furnace slag cement with sand gravel.
The user will find the rebound value on the horizontal axis, then find the matching point on the red conversion curve that is marked with the matching Original Schmidt alignment. They may then read the conversion from the vertical column.
The Original Schmidt Manual has additional conversion curves included. You may download here.
The image below combines the Traditional N and lower 10th percentile reference curves used with PC/ST Silver Schmidt models and the current Silver Schmidt Live OS8200 model.
The lower 10th percentile curve is derived from research work combining 2,300 data points and provides a conservative safety margin by plotting the line of best fit shown in the conversion graph such that 90% of data points are above the line.
The reference N curve is a European curve created on 28 day concrete to bridge 10-100 Mpa concrete mixes.
Both curves in the conversion graph were created using cube samples, so a Cylinder Factor should used for Australia or NZ with data logging functions or manual adjustments based upon Table 7 in EN206 used.
The form factor for the standard cylinder ranges from 0.8 to 0.87.
A custom correlation curve will provide a better relationship to the concrete mix it is created for and a more reliable and accurate MPa conversion.
To create the curve a lab will need to prepare test specimens, test the samples at pre-determined intervals as the concrete cures, chart the results on a graph and establish a line of best fit and the formula for this relationship.
The formula may then be entered into the Schmidt Hammer's data logging functions or the graph used for manual conversion.
The full process is explained in this reference document from Proceq.
In general, a Type N hammer is the correct choice for general purpose testing, Type L rebound hammers are specifically used for weak, thin or unsupported concrete elements.
A Silver Schmidt has a slight extended range of concrete strength which may be measures
The following chart shows the relative strength range of each hammer in the Screening Eagle Technology / Proceq range.
There are also specialist versions for Rock Strength, Super Low strength concrete or mortar and Paper Roll testing.
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