Obtaining Reliable Compressive Strength Estimations with a Rebound Hammer

Measuring the in-situ compressive strength of concrete in final structures is an essential but challenging process. Coring combined with non-destructive rebound hammer measurements provides the most efficient and comprehensive overview of structural strength. This article outlines how to use and maintain a rebound hammer correctly, enabling reliable strength estimations

Concrete is one of the most widely used construction materials. In-situ strength testing ensures that concrete has cured correctly, meets the specified characteristic strength and is strong enough to meet safety requirements and regulations. However, measuring concrete compressive strength in-situ can be challenging.

Coring, which involves cutting cylinders of concrete from the final structure and measuring their strength using a compression testing machine, is considered the gold standard for compressive strength testing. Although coring provides accurate strength measurements, there are obviously limitations as to how many cores can be removed from a structure. Additionally, in many locations coring is not possible at all. As a result, combining coring with non-destructive measurements is the most efficient way to obtain a comprehensive overview of the in-situ concrete strength.^1-3

Rebound hammers provide non-destructive estimations of concrete strength

Rebound hammers, also known as Schmidt hammers, provide non-destructive in-situ concrete compressive strength estimations. They use a spring rebounding method to measure the surface hardness of concrete, which can be correlated with compressive strength to provide reliable strength estimates.³

Rebound hammer surveys are used to identify the best locations for coring. Several guidelines including RILEM and EN13791 recommend this approach. The results are then correlated with the core strength results to provide a thorough assessment of the entire concrete structure. Several regions such as China, Japan and Germany allow compressive strength estimation using a rebound hammer without coring. In this case, the strength estimate provided is intentionally conservative to avoid over-estimation.

Are estimations of concrete strength provided by rebound hammers accurate?

In some circles, and especially wherever concrete coring and destructive testing has been provided as a service to end-customers, there is resistance to using rebound hammers for in-situ compressive strength estimations due to misperceptions regarding their reliability or accuracy.

In fact, research has repeatedly shown that rebound hammers can be used confidently to provide reliable estimates of compressive strength – if they are used correctly. Namely, inaccurate strength estimations are usually caused by misuse of the hammer or a misinterpretation of its measurement results. Inaccurate compressive strength estimations from rebound hammers can then be caused by:^3,4,6,7

• Failing to use the rebound hammer in accordance with the standards
• Variations or human error during impact measurements
• Errors during measurement validation and/or rebound number calculation
• Failing to calibrate the rebound hammer properly
• Failing to use an appropriate rebound correlation curve

Classical, fully mechanical Schmidt hammers are therefore subject to operator-dependence, something that can nowadays be mitigated thanks to digital technology. The Original Schmidt Live rebound hammer and app from Proceq were conceived to make it simple to work in accordance with the standard and to reduce the risk of errors. As a result, you can be confident that Original Schmidt Live will provide accurate, reliable estimates of concrete strength every time.⁸

Reducing operator variations and error when using a rebound hammer

To prevent errors caused by variations during impact measurements, Original Schmidt Live automatically detects and corrects for the impact angle.

What’s more, the app records all the impact measurements automatically, eliminating the risk of operator error. Finally, the app automatically validates each series of measurements according to the chosen standard and informs the user if the series is valid or invalid.^8,10,11

Calibrating a rebound hammer

Standards require that the calibration of rebound hammers must be checked periodically to ensure they are generating accurate rebound numbers.

Forgetting to calibrate a rebound hammer can result in inaccurate measurements. Original Schmidt Live keeps track of hammer calibration and gives a warning when verification is required, so you will never wonder if you have forgotten to calibrate your hammer again. The verification feature in the app allows a test anvil to be selected, and automatically compares measurements against the calibration value, making verification easy.^11,14

Developing a rebound correlation curve

As referenced in the “In-Place Methods to Estimate Concrete Strength” report by ACI Committee 228, when using a rebound hammer, the energy absorbed by the concrete is related to the strength-stiffness relationship of the concrete. A low-strength, low-stiffness concrete will absorb more energy than a high-strength, high-stiffness concrete and will result in a lower rebound number.

Therefore, it is possible for two concrete mixtures to have the same strength but different stiffnesses, and so there could be different rebound numbers even if strengths are the same. Equally, it is possible for two concretes with different strengths to result in the same rebound numbers if the stiffness of the low-strength concrete is greater than that of the high-strength concrete.¹⁶

There is no such thing as a “one-size-fits-all” rebound correlation curve, and manufacturer curves should not be expected to give accurate compressive strength estimates. ^3,4,6,7,15

All the major standards recommend developing rebound correlation curves by removing cores from the final concrete structure, then measuring their compressive strength using a compression testing machine and correlating the measurements with their rebound numbers. Although this method is time-consuming and more expensive than using rebound hammer surveys alone, it provides the most accurate and comprehensive overview of compressive strength.⁴

The custom materials function of Original Schmidt Live make it very simple to enter and use correlation curves derived in accordance with the standards. This guarantees a reliable compressive strength estimate you can be confident of using.^11,14

Calibration curve recorded with Original Schmidt Live.¹⁴

Original Schmidt Live ensures accurate strength estimations

The features of Original Schmidt Live rebound hammer and app from Proceq make the process of obtaining accurate strength estimations foolproof. As a result, you can be confident that Original Schmidt Live will provide accurate, reliable estimates of concrete strength every time.^8,10,14

References and further reading

1. ‘Reinforced concrete structure: Non-destructive in situ strength assessment of concrete’ — Pucinotti R, Construction and Building Materials, 2015.
2. ‘In-situ Concrete Compressive Strength of Residential, Public and Military Structures’ — Mazılıgüney L, Yaman Öİ, Azılı F, 8th International Congress on Advances in Civil Engineering, 2008.
3. ‘Testing of Concrete in Structures: Fourth Edition’ — Bungey JH, Grantham MG, CRC Press, 2014.
4. ‘Reliability of Rebound Hammer Test in Concrete Compressive Strength Estimation’ — Sanchez K, Tarranza N, International Journal of Advances in Agricultural & Environmental Engineering, 2014.
5. ‘BS EN 13791:2007 Assessment of in-situ compressive strength in structures and pre-cast concrete components’
   https://shop.bsigroup.com/ProductDetail/?pid=000000000030018199
6. ‘Case study of strength evaluation of structural concrete using rebound hammer test’ — Gupta PK, Gupta N, Singh A, The Indian Concrete Journal, 2015.
7. ‘Reliability and significance of Schmidt Hammer readings in assessing the strength and variability of concrete’— Abdelaziz GE, El Mohr MAK, Ramadan MO, Engineering Research Journal, 2003
8. ‘Original Schmidt Live – most advanced concrete strength and uniformity test hammer’ https://www.proceq.com/product/original-schmidt-live/
9. ‘BS EN 12504-2:2012 Testing concrete in structures. Non-destructive testing. Determination of rebound number’
10. SL Video’ https://buffer.com
11. ‘Original Schmidt Live Presentation’
    https://www.proceq.com/uploads/tx_proceqproductcms/import_data/files/Original%20Schmidt%20Live%20Cue%20Cards.pdf
12. ‘Original Schmidt: Operating Instructions’ https://www.proceq.com/uploads/tx_proceqproductcms/import_data/files/OrginalSchmidt_Operating%20Instructions_Multilingual_high.pdf
13. ‘Silver Schmidt: Operating Instructions’ https://www.proceq.com/uploads/tx_proceqproductcms/import_data/files/SilverSchmidt_Operating%20Instructions_English_high.pdf
14. ‘Original Schmidt Live: Quick Start Guide’https://www.proceq.com/uploads/tx_proceqproductcms/import_data/files/Original%20Schmidt%20Live%20Quick%20Start%20Guide_high.pdf
15. ‘Calibration and Reliability of the Rebound (Schmidt) Hammer Test’ —Brencich A, Cassini G, Pera D, Riotto G, Civil Engineering and Architecture, 2013.

This information has been sourced, reviewed and adapted from materials provided by Proceq.

For more information on this source, please visit Proceq.