QA / QC

Rebound Hammer Test

Non-destructive test for in-situ concrete strength — surface hardness measured via spring-loaded hammer.

Also calledrebound hammerschmidt hammerndt rebound test
Related on InfraLens
CODES
IS 13311
Definition

The rebound hammer test (also called Schmidt hammer test) is a non-destructive method for assessing the in-situ compressive strength of hardened concrete. The test uses a calibrated spring-loaded hammer that strikes the concrete surface; the rebound number (R) is read from the device's scale and correlated to compressive strength via charts provided by the manufacturer or developed from cube tests. The Indian Standard IS 13311 Part 2:1992 governs rebound hammer testing.

Procedure: (1) Surface preparation — the test surface should be smooth, dry, and free of paint or rendering. Rough surfaces are ground smooth before testing. (2) Test sequence — at each location, take 12 readings within a 100 × 100 mm area (or as specified). (3) Calculation — exclude the highest 3 and lowest 3 readings; average the middle 6. (4) Strength estimation — use the manufacturer's rebound-strength correlation chart (provided with each device) or a project-specific calibration curve. Typical rebound numbers: M20 concrete = 24-30; M30 concrete = 30-35; M40 concrete = 35-40.

Limitations and accuracy: (a) The test measures surface hardness, not bulk concrete strength; correlation is approximate. (b) Surface condition (carbonation, moisture, finishing technique) affects rebound numbers significantly. (c) Aggregate-rich surfaces give different results than mortar-rich surfaces. (d) Calibration curves vary by aggregate type, cement type, age, and curing — site-specific calibration is recommended for important assessments. For routine quality control: ±20% accuracy is typical for in-situ strength. For detailed forensic assessment, rebound hammer is supplemented by core test (IS 13311 Part 3 or IS 516) and Ultrasonic Pulse Velocity (IS 13311 Part 1). The rebound hammer's principal value is rapid screening — finding suspect zones for follow-up detailed testing.

Typical values
Rebound number M20R = 24-30
Rebound number M25R = 28-32
Rebound number M30R = 30-35
Rebound number M40R = 35-40
Rebound number M50R = 40-45
Accuracy of strength estimate±20% typical
Where used
  • Routine concrete quality control — quick strength estimate
  • Forensic investigation of older concrete structures
  • In-situ acceptance check when cube tests are unavailable
  • Repair / renovation projects — assessing existing concrete
  • Construction monitoring — strength gain over time
Acceptance / threshold
Per IS 13311 Part 2:1992: 12 readings per test location; exclude highest 3 + lowest 3; average middle 6. Reference standard / calibration curve from manufacturer or project-specific. Results are estimates with ±20% typical accuracy; supplement with core or UPV test for important applications.
Site example
Site reality: a Pune residential project's M30 column gave rebound numbers averaging R = 28 — corresponding to ~22 MPa, well below the M30 specification. Root cause investigation: the column was tested 2 days after pour (hardened but not gained design strength). Re-test at 28 days: R = 33 corresponding to 32 MPa, passing M30. The lesson: rebound hammer is sensitive to age, moisture, and surface condition — comparing within a single batch and condition is more reliable than absolute strength estimates.
Frequently asked
What is rebound hammer test?
The rebound hammer (Schmidt hammer) test is a non-destructive method for estimating the in-situ compressive strength of hardened concrete. A calibrated spring-loaded hammer strikes the concrete surface; the rebound number (R) read from the device is correlated to strength via charts. Per IS 13311 Part 2:1992. Typical accuracy ±20%; used for rapid screening.
How is rebound hammer test performed?
Per IS 13311 Part 2: (1) prepare smooth, dry test surface; (2) take 12 readings within a 100 × 100 mm area; (3) exclude highest 3 and lowest 3; (4) average the middle 6 to get rebound number R; (5) read strength from manufacturer's chart or project calibration. Test should be on a perpendicular surface; if angled, apply correction factor.
What is the difference between rebound hammer and core test?
Rebound hammer is non-destructive and rapid (5-10 minutes per location), but estimates strength with ±20% accuracy. Core test (IS 13311 Part 3 or IS 516) is destructive — extracts a cylindrical core for laboratory compression test; gives accurate strength but costs ₹3,500-7,500 per core, takes 2-3 days, and damages the concrete. Indian forensic practice: rebound hammer for screening, core test for detailed assessment of suspect zones.
Related qa / qc terms
QA/QC Inspection
Quality assurance and control checklists/templates
Non-Conformance Report (NCR)
Report for work not meeting specification
Inspection Test Plan (ITP)
Document listing every inspection/test checkpoint for an activity with frequency, criteria, and responsible party.
Field Quality Plan (FQP)
Project-specific QC document covering all activities — test methods, acceptance criteria, sampling rate.
Hold Point
Mandatory inspection checkpoint that must be cleared before work proceeds — unlike a witness point.
Witness Point
Inspection point where the QC engineer is notified but work can proceed if QC is unavailable.
Non-Conformance Report (NCR)
Document raised when work fails to meet specifications. Includes root cause, corrective action, preventive action.
Method Statement
Step-by-step procedure for a construction activity — safety, equipment, sequence, QC checkpoints.