1. Introduction
Shotcrete mix design is fundamentally different from conventional concrete design. While both materials share the same basic constituents, the mechanical and chemical demands placed on shotcrete during pumping and application require specialised consideration.
A shotcrete mix must remain stable during mixing, transport, pumping, projection, and impact. At the same time, it must develop rapid early strength once applied while maintaining long-term durability.
These competing requirements place significant demands on material selection and proportioning.
EFNARC identifies mix design as a core contributor to sprayability, rebound behaviour, and structural performance of sprayed concrete linings.
2. Fundamental objectives of a shotcrete mix design
A shotcrete mix must satisfy several engineering objectives simultaneously.
It must:
• Remain pumpable under high pressure
• Resist segregation during transport
• Maintain fibre dispersion
• Adhere to vertical and overhead surfaces
• Develop early strength after acceleration
• Achieve long-term structural capacity
Failure in any one of these areas compromises overall performance.
Unlike cast concrete, shotcrete cannot be adjusted after placement. All performance must be engineered into the mix before spraying begins.
3. Cement selection and hydration behaviour
Cement forms the chemical backbone of shotcrete.
During hydration, cement reacts with water to form calcium silicate hydrate, which provides strength, and calcium hydroxide, which influences durability.
In shotcrete, hydration behaviour is altered significantly by the introduction of accelerators at the nozzle.
Accelerators modify early hydration kinetics, causing rapid formation of hydration products and stiffening of the paste.
The cement must therefore be compatible with the selected accelerator. Incompatible combinations lead to delayed setting, flash setting, or reduced long-term strength.
For this reason, EFNARC requires mix designs to be tested using the actual cement and accelerator combination intended for site application.
4. Aggregate grading and particle packing
Aggregate grading has a dominant influence on pumpability and rebound.
Shotcrete aggregates are typically limited to a maximum size of 8 millimetres.
Well-graded aggregates create dense particle packing, reducing void content and improving cohesion.
Poor grading increases internal friction, requiring higher pumping pressure and increasing rebound.
Gap-graded mixes tend to segregate under shear, leading to paste-rich zones and aggregate-rich rebound.
A continuous grading curve is therefore essential for stable spraying behaviour.
5. Water content and strength control
Water content governs both workability and strength.
In shotcrete, excess water cannot be tolerated. While higher water content may ease pumping, it reduces strength, increases bleeding, and weakens the bond to the substrate.
The water-to-cement ratio must therefore be controlled precisely.
EFNARC emphasises that water must never be added indiscriminately on site to correct workability, as this compromises structural performance.
Workability adjustment must be achieved through admixture use rather than water addition.
6. Role of plasticisers and superplasticisers
Plasticisers reduce internal friction between cement particles, improving flow without increasing water content.
In shotcrete, plasticisers serve several critical functions:
• Improve pumpability
• Stabilise rheology
• Reduce segregation
• Maintain slump or flow
High-performance superplasticisers allow low water-to-cement ratios while maintaining pumpability.
However, overdosing can result in excessive fluidity, leading to sloughing during spraying and increased rebound.
Dosage must therefore be determined through controlled trials.
7. Silica fume and microfiller behaviour
Silica fume is commonly used in shotcrete mixes due to its fine particle size and chemical reactivity.
Its functions include:
• Increasing cohesion
• Reducing bleeding
• Improving bond to rock
• Reducing fibre rebound
• Increasing long-term strength
Silica fume fills micro-voids between cement particles, enhancing particle packing density.
It also reacts pozzolanically with calcium hydroxide, forming additional calcium silicate hydrate.
This improves durability and reduces permeability of the sprayed concrete lining.
8. Fibre reinforcement behaviour under projection
Fibres play a critical role in modern shotcrete systems.
They provide post-crack load capacity and energy absorption, allowing the lining to deform without brittle failure.
During spraying, fibres are subjected to intense acceleration and impact forces.
Poor fibre dispersion results in balling, nozzle blockage, and uneven reinforcement distribution.
Mix design must ensure sufficient paste content to coat fibres and prevent interlocking during pumping.
Both steel and synthetic fibres are used, each with different mechanical behaviour and interaction with rebound.
9. Fibre rebound mechanisms
Fibre rebound occurs when fibres fail to embed into the cement matrix upon impact.
Factors influencing fibre rebound include:
• Nozzle angle
• Impact velocity
• Paste cohesion
• Fibre length and stiffness
Steel fibres are heavier and more prone to rebound under oblique impact angles. Synthetic fibres exhibit better adhesion but require adequate paste volume.
Silica fume and cohesive paste significantly reduce fibre rebound by improving embedment.
10. Accelerator chemistry and setting behaviour
Accelerators modify cement hydration at the nozzle.
Modern alkali-free accelerators primarily influence the aluminate phases of cement, promoting rapid ettringite formation and early stiffening.
Correct accelerator performance achieves:
• Rapid adhesion
• Reduced fall-out
• Layer build-up capability
Excessive dosage disrupts normal hydration, reducing later-age strength and increasing brittleness.
Insufficient dosage leads to sloughing and loss of thickness.
EFNARC requires dosing systems to be calibrated and verified under site conditions.
11. Interaction between admixtures
Shotcrete mixes typically contain multiple chemical admixtures.
These may include:
• Plasticisers
• Hydration control agents
• Accelerators
Chemical compatibility is critical.
Certain combinations may suppress accelerator effectiveness or cause unpredictable setting behaviour.
For this reason, laboratory testing and site trials are mandatory before production spraying.
12. Early strength development and structural relevance
Early strength allows shotcrete to function as immediate support.
Strength development must be sufficient to:
• Support subsequent layers
• Allow installation of reinforcement
• Maintain excavation safety
Early-age strength is influenced by:
• Cement type
• Accelerator dosage
• Temperature
• Water content
Monitoring of early strength provides confirmation that the lining is performing as intended.
13. Long-term durability considerations
Shotcrete linings are often required to perform for decades.
Durability is influenced by:
• Permeability
• Cement hydration completeness
• Fibre corrosion resistance
• Environmental exposure
Excessive accelerator dosage, high water content, and poor curing reduce durability.
Well-designed shotcrete achieves low permeability and high resistance to chemical attack.
14. Mix design as a system, not a recipe
Shotcrete mix design cannot be reduced to a fixed recipe.
It is a system that must be matched to:
• Equipment type
• Pumping distance
• Application method
• Environmental conditions
• Operator technique
Design values must be verified through testing and adjusted based on observed performance.
EFNARC places responsibility on the contractor to demonstrate that the mix performs consistently under actual site conditions.
15. Engineering responsibility
Shotcrete performance begins with mix design.
No amount of operator skill can compensate for unstable material behaviour.
For this reason, professional shotcrete operations treat mix design as an engineering function rather than a batching exercise.