Relevant product: ReX 100

Concrete cover, the protective layer of concrete surrounding steel reinforcement, serves multiple essential functions. It provides corrosion protection by limiting the ingress of moisture, chlorides, and carbon dioxide. It offers fire resistance by providing thermal insulation to reinforcement during high-temperature exposure. It ensures structural performance by maintaining proper stress transfer between steel and concrete and preserving bond and durability.

Concrete design codes prescribe minimum nominal cover values to ensure durability and fire safety. These values vary depending on exposure conditions and member type.

Table 1. Minimum nominal concrete cover requirements

Code	Exposure / Condition	Minimum cover (mm)
IS 456:2000 [1]	Mild	20
	Moderate	30
	Severe	45
	Very Severe	50
	Extreme	75
ACI 318-19 [2]	Slabs & Walls (weather exposed)	40
	Beams & Columns	50
	Contact with ground	75
Eurocode 2 (EN 1992-1-1) [3]	XC1 (dry)	20
	XC3 (moist air)	25
	XD3 (chlorides, marine)	35
	XS3 (sea water)	50

The nominal cover includes allowances for construction tolerance and depends on exposure class, concrete grade, and bar size. Codes generally require additional cover, typically 5 to 10mm more, for marine or aggressive chloride environments.

While instruments with higher penetration depth (e.g., beyond 150 mm) may seem superior, such capability is often unnecessary for most reinforced concrete members, for the following reasons:

• Design relevance: Most reinforcement in slabs, beams, and columns lies within the first 20–80 mm of cover, well within the range of standard cover meters.
• Accuracy trade-off: Greater penetration often reduces resolution and increases error in locating shallow rebars accurately.
• Signal attenuation: At larger depths, the electromagnetic signal weakens, leading to false readings or multiple reflections.
• Practical objective: The primary goal of cover measurement is verifying specified cover, not detecting deeply embedded reinforcement.

Most instruments are calibrated for single-layer reinforcement, and fail to detect second or deeper layers. According to the Proceq PM8000 user manual [4], signal strength decreases with one over cover to the power of six, making shallow bars dominate the reading. The Hilti PS 300 [5] specification explicitly states that multi-layer rebars are non-detectable, and Bosch D-tect 200C Professional user guide [6] indicates that only the first conductive layer is measurable.

In retrofitted or jacketed structures, new concrete and reinforcement are added over existing members to increase strength or ductility. This creates multiple reinforcement layers, one within the original core and another within the jacket. Measurement challenges include electromagnetic interference between layers, as cover meters typically detect only the outermost steel layer. Distortion can occur due to voids or poor bond between old and new concrete layers. There is also a risk of misinterpreting deep readings as original reinforcement. Therefore, higher penetration depth can cause errors in interpretation of results.

For most reinforced concrete members, precise measurement in the 20 to 80 mm range is more valuable than deep scanning capability. Higher penetration often compromises accuracy, especially in jacketed or multi-layer configurations. Device-specific calibration and correct mode settings should be used for realistic field accuracy.

1. IS 456:2000, Plain and Reinforced Concrete – Code of Practice, Bureau of Indian Standards.
2. ACI 318-19, Building Code Requirements for Structural Concrete, American Concrete Institute, Michigan, USA.
3. EN 1992-1-1:2004 (Eurocode 2), Design of Concrete Structures – Part 1-1: General Rules and Rules for Buildings, European Committee for Standardization (CEN).
4. Proceq SA, Profometer PM8000 User Manual, 2023 – Screening Eagle Technologies, Switzerland.
5. Hilti Corporation, PS 300 Ferroscan and PS 85 Wall Scanner Operating Instructions, 2023.
6. Bosch GmbH, D-tect 200 C Professional User Guide, 2024.

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About the Authors: Dr. Sauvik Banerjee is Co-founder & Director at Nirixense Technologies, and Professor at Department of Civil Engineering, IIT Bombay. His experience includes structural health monitoring, using wave propagation and vibration-based approaches, quantitative non-destructive evaluation of structures, non-destructive testing, modelling of advanced composite structures, structural retrofitting and impact response of structures. He obtained Ph.D. degree from UCLA (Mechanical Engineering, 2003), and M.Tech. from IIT Bombay (Civil Engineering, 2001).

Dr. Siddharth Tallur is Co-founder & Director at Nirixense Technologies, and Associate Professor at Department of Electrical Engineering, IIT Bombay. His experience includes development of high resolution and low-cost sensors, high-speed instrumentation and embedded smart sensing systems. He obtained M.S.-Ph.D. degree from Cornell University (ECE, 2013), and B.Tech. from IIT Bombay (EE, 2008).

Note: This article presents the authors’ personal views and insights drawn from their expertise as faculty members at IIT Bombay. The content is intended solely for thought leadership and knowledge sharing. The views expressed do not necessarily represent those of IIT Bombay.