Rebound Hammer by Schmidt

Schmidt Hammer Test on Concrete

Schmidt Hammer Test on Concrete

Schmidt Hammer Test on Concrete: In 1948, Ernst Schmidt, a Swiss inventor, invented a test hammer for determining the hardness of concrete using the rebound principle. The hammer was designed and thoroughly tested at the Swiss Federal Materials Testing and Experimental Institute in Zurich as a result of his work. By 1986, about 50,000 Schmidt rebound hammers had been sold worldwide.

Principle of Schmidt Hammer Test

Schmidt rebound hammers are primarily used to determine the surface stiffness of concrete, with no clear theoretical association between the strength of concrete and the hammer’s rebound number. However, empirical relationships between strength properties and the rebound number have been formed under some limits. Additionally, Kolek10 attempted to create a relation between the hammer rebound number and the Brinell hardness.

Description of Schmidt Hammer

Figure 1 illustrates the Schmidt rebound hammer. The hammer weights about 1.8 kg and is ideal for laboratory and field use.

Figure 1: Schmidt Hammer

Figure 2 depicts a schematic cutaway view of the rebound hammer. The outer body, plunger, hammer mass, and main spring are the primary components. Additionally, there is a latching device that secures the hammer mass to the plunger rod and a sliding rider that measures the hammer mass’s rebound. The rebound distance is expressed on an arbitrary scale ranging from ten to one hundred. The rebound distance is expressed as a “rebound number” that corresponds to the rider’s location on the scale.

A cutaway schematic view of the Schmidt rebound hammer.
Figure 2: A cutaway schematic view of the Schmidt rebound hammer.

Testing Procedure

To prepare the instrument for testing, press the plunger against the concrete and slowly move the body away from the concrete. This extends the plunger from the body and engages the hammer mass with the plunger rod through the latch (Figure 2A).

Maintaining the plunger parallel to the concrete wall, cautiously advance the body toward the test object. The key spring attaching the hammer mass to the body is extended as the body is moved (Figure 2B).

When the body reaches its max, the lock immediately releases, and the hammer mass is propelled toward the plunger tip by the energy contained in the spring (Figure 2C). The mass collides with the plunger rod’s shoulder and bounces up.

The slide indicator moves with the hammer mass during rebound and tracks the rebound distance (Figure 2D). The plunger is locked in the retracted position by pressing a button on the side of the body, and the rebound number is read from the scale.

The measurement may be performed horizontally, vertically up or down, or at some angle in between. Due to the differing influence of gravity on the rebound as the test angle changes, the rebound value for the same concrete will be different, necessitating the use of separate calibration or correction maps.

Correlation Procedure

Correlation curves for each hammer have been produced by the manufacturer using standardized cube specimens. However, using these curves is not advised because the material and testing conditions which differ from those used to calibrate the instrument. The following is a standard correlation technique.

  1. Prepare a series of 150 300-mm cylinders* that cover the strength spectrum expected on the job site. Utilize the same kind of cement and aggregates as those that would be used on the job. Cure the cylinders in a regular moist-curing room,** maintaining the curing time constant with the control age defined in the field.
  2. After capping, position the cylinders in a compression testing device with an initial load equal to around 15% of the ultimate load to restrain the specimen. Verify that the cylinders are moist on the surface and in SSD condition.
  3. Take 15 hammer rebound measurements, five on each of three vertical lines 120 degrees apart, against the side surface of each cylinder in the middle two thirds. Attempt to avoid checking the same location twice. Take five readings on each of the four molded faces of cubes, avoiding checking the same area twice.
  4. Calculate the rebound number for the cylinder under test by averaging the readings.
  5. Repeat this process for the remaining cylinders.
  6. Compression test the cylinders to failure and graph the recovery values against the compressive strengths.
  7. The least squares approach is used to fit a curve or a graph.

Figure 3 illustrates a standard curve formed by Zoldners for limestone aggregate concrete. This curve was created using data from 28-day experiments conducted with various concrete mixtures.

Schmidt Hammer Test on Concrete
Figure 3: Relationship between 28-day compressive strength and rebound number for limestone aggregate concrete obtained with Type N-2 hammer.

The calibration curves in Figure 4 were collected by researchers in four different countries. It is important to note that some of the curves deviate significantly from the hammer’s supplied curve.

Schmidt Hammer Test on Concrete
Figure 4: Correlation curves obtained by different investigators with a Schmidt rebound hammer Type N-2. Curve by Greene was obtained with Type N

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