1. Introduction
Shotcrete quality cannot be judged visually.
Although experienced personnel may identify obvious defects, visual assessment alone provides no reliable indication of structural performance.
For this reason, quality control in sprayed concrete relies on systematic testing and verification procedures.
EFNARC recognises quality control as an essential component of sprayed concrete execution, ensuring that the lining performs as designed throughout its service life.
2. Objectives of shotcrete quality control
The purpose of quality control is to verify that:
• The concrete delivered to site meets design intent
• The sprayed lining achieves required thickness
• Early and long-term strength targets are met
• Energy absorption capacity is achieved
• Application consistency is maintained
Quality control does not exist to assign blame. It exists to confirm performance and enable corrective action before defects become structural risks.
3. Quality control as a continuous process
Shotcrete quality control is not a single test.
It is a continuous process extending from batching through to final curing.
Key stages include:
• Pre-spray material verification
• Monitoring during application
• Post-application testing
• Long-term performance evaluation
Failure at any stage compromises the reliability of the lining.
4. Flow testing and workability control
Flow testing is used to assess the workability of wet shotcrete.
Workability governs pumpability, sprayability, and rebound behaviour.
The flow table test provides a repeatable method for evaluating consistency.
Typical procedure includes:
• Sampling after batching
• Placement of sample in flow cone
• Controlled lifting of the cone
• Repeated table drops
• Measurement of flow diameter
Typical target ranges are between 500 and 600 millimetres, depending on mix design.
5. Engineering significance of flow values
Flow values reflect internal friction and cohesion.
Low flow indicates stiff concrete, increasing pumping pressure and blockage risk.
High flow indicates excessive fluidity, increasing sloughing and reducing early strength.
Flow testing therefore provides early warning of mix instability.
EFNARC recommends maintaining consistent flow values throughout spraying operations to ensure predictable behaviour.
6. Cube testing and compressive strength verification
Cube testing verifies compressive strength development.
Samples are taken from fresh concrete and cast into standard moulds.
After curing, cubes are tested at specified ages, typically 7 and 28 days.
Compressive strength confirms:
• Cement hydration efficiency
• Water-to-cement ratio control
• Compatibility of admixtures
Although compressive strength alone does not define shotcrete performance, it remains a fundamental verification parameter.
7. Early-age strength considerations
Early strength is critical in underground environments.
Shotcrete must gain sufficient strength rapidly to:
• Support subsequent excavation
• Allow installation of reinforcement
• Maintain excavation safety
Accelerated early strength is achieved through controlled accelerator dosing.
Monitoring early-age behaviour provides assurance that the lining is functioning as intended.
8. Thickness control and probing
Design thickness must be achieved consistently.
Under-thickness reduces load capacity and energy absorption.
Thickness probing is conducted during spraying using calibrated probes or thickness indicators.
Continuous probing provides real-time feedback to the nozzle operator and ensures design compliance.
EFNARC considers thickness verification a mandatory execution requirement.
9. Energy absorption testing
Energy absorption testing evaluates the ductility of fibre-reinforced shotcrete.
Unlike compressive strength, energy absorption reflects post-crack behaviour and deformation capacity.
Panel testing involves spraying test panels under site conditions and subjecting them to controlled loading at a later stage.
Results are typically expressed in joules.
This test verifies that fibre dosage, distribution, and embedment are adequate.
10. Importance of panel preparation
Panel preparation must replicate actual spraying conditions.
Panels must be:
• Positioned vertically
• Sprayed at correct stand-off distance
• Applied using normal spraying technique
Any deviation results in unrepresentative test outcomes.
EFNARC emphasises realistic test conditions to ensure meaningful performance data.
11. Curing of test specimens
Curing has significant influence on test results.
Panels and cubes must be protected from moisture loss and temperature extremes.
Poor curing leads to reduced strength and misleading results.
Proper curing is therefore part of the quality control system.
12. Interpretation of test results
Test results must be evaluated collectively rather than individually.
A satisfactory compressive strength does not compensate for inadequate thickness or poor energy absorption.
Similarly, acceptable flow values do not guarantee correct application.
Engineering judgement is required to interpret results in context.
13. Trending and data analysis
Quality control data should be trended over time.
Trends reveal:
• Gradual deterioration in mix consistency
• Equipment performance issues
• Operator technique variation
Single test failures may be isolated events. Repeated deviations indicate systemic problems requiring intervention.
14. Corrective action procedures
When deviations are identified, corrective action must be immediate.
Possible actions include:
• Adjustment of mix design
• Recalibration of dosing systems
• Operator retraining
• Equipment inspection
Quality control without corrective action has no value.
15. Documentation and traceability
All tests must be documented and traceable.
Records should include:
• Date and location
• Mix identification
• Operator details
• Environmental conditions
• Test results
Traceability supports accountability and long-term asset management.
16. Relationship between quality control and safety
Quality control is directly linked to safety.
Inadequate lining performance increases the risk of:
• Rock falls
• Delamination
• Progressive failure
Testing verifies that the support system provides the intended level of protection.
17. Quality assurance versus quality control
Quality control measures performance.
Quality assurance ensures that systems, procedures, and training are in place to achieve performance consistently.
Both are required for professional sprayed concrete operations.
18. Engineering responsibility
Shotcrete quality is a collective responsibility involving:
• Engineers
• Supervisors
• Operators
• Material suppliers
EFNARC emphasises coordinated responsibility rather than isolated accountability.
19. Quality control as performance assurance
Quality control transforms shotcrete from a variable process into a predictable engineering system.
When applied correctly, it ensures repeatable results and long-term reliability.
20. Conclusion
Effective quality control verifies that sprayed concrete performs as designed.
It provides confidence in structural integrity and supports safe, efficient underground construction.