What is the Rebound hammer test?
The rebound hammer measures the surface hardness of concrete to provide an on-site, non-destructive, convenient and rapid assessment of compressive strength for new or existing concrete structures.
How does the Rebound hammer test work?
With a defined energy, a striker impacts the concrete and rebounds according to the hardness of the concrete.
Tables of conversion (calibration curves), make it possible to deduce the compressive strength from the value of rebound called rebound number/index.
What is the Rebound hammer test used for?
Deterioration process | Defects | Control of repairs |
Applicable to all deterioration processes where the compressive strength evaluation is useful | To detect zones with different concrete strengths | In-situ quality control |
How do I carry out a Rebound hammer test?
To carry out a rebound hammer test effectively to BS EN 13791:2019 the following procedure must be followed:
Ensure a 150 mm diameter, flat test surface, devoid of loose mortar and moisture. Avoid testing frozen concrete or directly over steel with less than 20 mm cover.
Hold the hammer perpendicular to the surface, recording the orientation to the nearest 45 degrees, noting positive for upward and negative for downward orientations.
Press the instrument until impact, lock the plunger, and record the rebound number.
Take 10 readings per area, maintaining specific distances between impact points and from edges. Disregard readings from air void impacts and adjust for gravity effects using calibration charts.
Calculate the rebound number based on the average of the 10 readings, removing any outliers beyond 6 units from the initial average, and recalculate the average. Discard sets with more than 2 outliers over 6 units, testing new locations within the area.
To estimate concrete strength using a rebound hammer, calibration curves initially suggested a correlation between rebound numbers and concrete strength in general, though later studies, such as by Malhotra and Carino (2004), indicated the need for establishing specific relationships for accurate in-place strength evaluation.
Figure 3 shows the variation in calibration curves derived based on different concrete mix designs and highlights the importance of using a bespoke curve where possible in order of achieve the most accurate test results (Jedidi, M. 2020)
BS EN 13791:2019 recommends taking at least 9 cores from various locations with different rebound numbers to establish this relationship. The derived correlation is only valid for similar orientations and conditions as the test locations
What equipment and expertise are required for Rebound hammer testing?
The rebound hammer test is extremely popular in the UK due to its simplicity and effectiveness. Introduced by Proceq in the 1950s with their Original Schmidt Hammer, it has become a standard in assessing concrete strength properties globally for quick, in-situ evaluations of concrete hardness and strength across various construction and maintenance projects.
Several commercial systems are available from brands such as Proceq or Stanley with digital measurement and direct compressive strength display through programmable conversion software.
The SonReb method combines rebound hammer tests with ultrasound measurements to accurately evaluate concrete elements. The rebound hammer measures surface hardness, while Ultrasonic Velocity Transmission (UVT) assesses internal uniformity and density, offering a comprehensive assessment of concrete's condition and strength more accurately than using either technique alone.
What are the advantages of Rebound hammer testing?
Quick, cheap and easy to use.
Lightweight and portable.
Provides instant results.
Allows investigation of large areas.
Can assess the uniformity of concrete strength across a structure.
Perfect for preliminary assessments.
When used in SonReb method provides full picture of element strength.
What are the disadvantages of Rebound hammer testing?
Requires bespoke calibration.
Accuracy of single results is limited.
Results can vary with presence of aggregate, reinforcement etc. near the surface.
User error can easily result in anomalous results.
How accurate is Rebound hammer testing?
The local value of rebound is the average of at least 9 measurements in the same vicinity. Deducing the strength requires a calibration. If a standard calibration curve is used, the error can be very large. The correct calibration requires some cores from the same structures.
After calibration, the standard error is about +/- 5MPa.
What are the limits of Rebound hammer testing?
Evaluation of “internal” concrete (not close to the surface) is impossible.
Ancillary information
Maturity of test: >10 years
Qualification & interpretation : Inspector
Service disruption: No
Preliminary works: Yes
Time consumption Low (< one hour)
Cost Low
Access to element 1 face
References and further information