In-situ concrete compressive strength assessment is required whenever the actual strength of concrete in an existing structure needs to be determined — whether for structural appraisal, change of use, investigation after damage, or dispute resolution. NDTScan provides comprehensive compressive strength testing services combining non-destructive and minor-destructive methods, aligned with current UK and European standards.
Why In-Situ Strength Assessment Is Not the Same as Cube Testing
Standard cube or cylinder test results from original construction records tell you what the mix was designed to achieve in controlled conditions. They do not tell you the actual in-situ strength of concrete after decades in service, after fire, flood, freeze-thaw cycles, or chemical exposure. In-situ assessment gives you real-world data from the structure itself.
Methods Used
Rebound Hammer (Schmidt Hammer)
The rebound hammer measures surface hardness, which correlates with compressive strength. It is a Type N (indirect) test under BS EN 12504-2. Used systematically across a structure, it provides a rapid, low-cost mapping of strength uniformity and identifies zones for further investigation. NDTScan uses Proceq Schmidt OS-120 and Silver Schmidt digital rebound hammers, which provide direct carbonation correction and statistical analysis on-instrument.
Ultrasonic Pulse Velocity (UPV)
As described in BS EN 12504-4, UPV data combined with rebound hammer results in a combined method approach (also known as the SonReb method) provides improved strength prediction over either method alone. NDTScan routinely combines both where conditions allow.
Core Extraction and Testing
Where definitive compressive strength data is required, concrete cores are extracted to BS EN 12504-1 and tested to BS EN 12390-3. Core results form the primary dataset for structural assessment under BS EN 13791. NDTScan manages the complete process: locating safe coring positions (using GPR to avoid rebar and services), drilling, sealing, packing, and arranging UKAS-accredited laboratory testing.
Carbonation Depth Testing
Carbonation reduces the alkalinity of the concrete cover zone, which can lead to depassivation of reinforcement and corrosion initiation. Carbonation depth is measured during core extraction using a phenolphthalein indicator spray to BS 1881-210. Results feed directly into assessments of remaining service life and reinforcement corrosion risk.
Standards Applied
NDTScan compressive strength assessment follows: BS EN 13791 (assessment of in-situ compressive strength in structures and precast concrete components), BS EN 12504-1 (testing concrete in structures — cored specimens), BS EN 12504-2 (testing concrete in structures — rebound number), BS EN 12504-4 (testing concrete in structures — ultrasonic pulse velocity), and CIRIA Report C769 (non-destructive and minor-destructive investigation of existing structures).
When Is Compressive Strength Assessment Required?
- Structural appraisal for change of use — when imposed loading is being increased
- Post-fire or post-flood assessment — to determine the extent of thermal or chemical damage
- Dispute resolution — when construction records are missing or disputed
- Pre-purchase building surveys — for industrial, commercial, and infrastructure assets
- RAAC assessment — RAAC compressive strength is typically lower than standard structural concrete and requires specific investigation protocols
- Asset life extension planning — as part of a wider structural condition survey
Coverage
NDTScan provides concrete compressive strength testing throughout the UK from our Oxford base, with a primary service area covering a 100-mile radius: London, Birmingham, Bristol, Reading, Milton Keynes, Cambridge, Southampton, Coventry, Northampton, Swindon, Gloucester, and beyond.
Frequently Asked Questions
How many cores are needed for a valid assessment?
BS EN 13791 Approach B requires a minimum of 15 cores for a statistically valid assessment. Fewer cores (minimum 3) are permitted under the Approach A method with a confidence penalty applied. NDTScan advises on the most appropriate approach based on structural complexity and the purpose of the assessment.
What is the difference between destructive and non-destructive strength testing?
Non-destructive tests (rebound hammer, UPV) leave the structure intact but provide indirect strength estimates requiring correlation. Destructive tests (cores) provide direct compressive strength measurements but require drilling. The two approaches are used together for the most reliable assessment.
Can you assess RAAC compressive strength?
Yes. RAAC has lower density and different acoustic properties than normal structural concrete. We apply specific rebound hammer correction factors for aerated concrete and ensure the laboratory testing protocol is appropriate for RAAC core specimens.
How long does it take to get strength test results?
Rebound and UPV data is processed and reported within 48 hours of site attendance. Core results are dependent on laboratory turnaround — typically 5–10 working days for standard testing. We can arrange accelerated testing on request.