1 The Science of Shotcrete CIB Meeting June 5 th 2012

“ A mixture of cement and sand and water that is sprayed on a surface under pneumatic pressure” *Websters Dictionary 2

 What is Shotcrete?  Mix Design o Key Factors Water to cement ratio Sand and Stone Gradation Cement and Pozzolans HRWR Hydration Stabilizers  Other Materials o Accelerators o Fibers Steel Macro  Keys to Application 3

 Sprayed Concrete was invented in 1907, and is today widely used for rock support world wide, both in mining and tunnelling.  For a long time dry mix application was the only way of applying Sprayed Concrete, but in the seventies the wet mix method was having its breakthrough in underground works  The development in Sprayed Concrete has gone a long way since 1907, both in terms of equipment and concrete technology. Especially since the wet mix method started to get implemented, large technology steps has taken place 4

5 Fines in cracks, fissures and joints  Stiffen and strengthen the rock mass  Transfer the rock load to adjacent stable rock (shear and adhesion)  Prevent relative movements  No loosening over the time Shear resistance to blocks  Rock must cut through to fall  Sprayed concrete layer acts as a shell taking bending forces and tension when bond is low Thin layers (3–15 cm): Bridging effect How Sprayed Concrete Works

 In the dry mix method, a premix of sand and cement is fed into the hopper of a machine that with the help of compressed air convey the mix through the hose to the nozzle where water is added. 6

 For the wet mix method, sand, aggregate, cement, water and admixture are premixed in a concrete plant  Application of wet mix Sprayed Concrete is mainly performed by the use of piston pumps, that convey the concrete through the hosing system, and at the nozzle a set accelerator and air is added.  The main benefit with the wet mix method vs. the dry is; improved quality, less dust/improved working environment, less rebound, higher capacity and improved safety 7 Dense stream system

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 Sand/aggregate grading  Cement type and amount  W/C ratio  Type of Plastiscizer/Superplasticizer  Workability  Accelerator type  Hydration Control  Temperature  Accelerator dosage  Pulsation  Nozzle systems /set up  Nozzle distance  Nozzle angel 9

10  High w/c ratio gives slow setting and influences end quality  Watch the moisture content in the sand/aggregate Moisture will vary depending on where the measurement are taken Keep control  W/c ratio is critical to o Early setting and strength development o Long term strengths o Long term durability - resistance to chemical attack  W/c ratio should be less than 0.45, and preferably closer to a 0.4

11 Mixing of different fractions in the right proportions is the key Sand/aggregate grading curve influences: Water demand Workability Reactivity with Accelerator Rebound Shrinkage Durability

12  Combined gradation of aggregates should fall within these limits  Usually a 2:1 sand to stone ratio  #8 stone or gravel is primarily used

13  Type I, Type II or Type I/II Cement o Cementitious content ~ 800lbs  Fly Ash; Class F or C  Slag  Silica Fume  Proportion similar to how pozzolans are proportioned in concrete

14 Superplasticisers for very low w/c ratios and high workability Hydration control admixtures for maintaining workability from 3 to 72 hours Micro Silica And Slump Retainers Additions of steel and high performance polymer fibers, micro silica slurries Alkali-free accelerators for safety and durability

 Low water - cement ratios 0.32 to 0.45  Allows for higher slumps  High early and long term sprayed concrete strengths  Pumpable shotcrete mixes  Durability enhancement  Low dosage - cost effective 15

16 Hydration stabilizers ●Needed in almost every mix design ●Control the hydration of cement ●Maintain open time and pumpability for up to 72 hrs ●Adjust dosage according to the needs ●The addition of shotcrete accelerators re-starts the hydration process and causes immediate setting

17 Traditional Sprayed Concrete New Flexibility with Hydration Stabilizer

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 Alkali-free offers setting performance of traditional accelerators  All alkali-free accelerators promote strength and durability of sprayed concrete 19 Blisadonna Railway Tunnel, Austria

 Improved working safety  Less strength difference to base mix  Less dust and rebound  Lowered risk of ASR  Improved sulphate resistance when using standard OPC  Reduced environmental impact in hardened concrete 20 ACIDICNEUTRALBASIC Safe to human body pH Scale Alkali-free accelerators Modified sodium silicate accelerators Conventional accelerators pH Scale

 Slump below 4in can prove difficult o Poor mixing efficiency of accelerator into stiff material o Overdosing of accelerator due to poor pump piston filling efficiency o High pulsation - layering effect o AFA has a lower viscosity, and more efficiently mixed with the shotcrete at a temperature around 70 ºF than at lower temperatures  Correct set-up with air and accelerator lines and correct nozzle type is key 21

22 3D, Discrete Reinforcement2D, Wire Mesh Reinforcement Multi-dimensionally throughout entire concrete thickness On a single horizontal plane only

23  For reinforcement, direct cost of fiber is 50 – 60 % of wire mesh  Shotcrete can be sprayed in one layer  Reduces shotcrete volume due to following of irregular substrate  Better Logistics

Steel fibres  Typically 1.25 in length  0.02 inches in diameter  Type I high tensile strength (ASTM 820)  Provides uniform reinforcement  Prevents brittle failure of sprayed concrete linings  Promotes durability  Faster reinforcement method than mesh  50 to 60 % cost saving over mesh 24

Macro Synthetic Fibres o High performance replacement for mesh and/or steel fibers o Typical dose of ~ 10lbs per yard o Flexural toughness equal to steel o As cost effective or better than steel o Increased fire resistance o Reduced wear on concrete pumping equipment 25

26 Flexural performance (ASTM C 1609) Flexural toughness (ASTM C 1550)

27 This test is used to determine flexural performance characteristics of fiber-reinforced concrete; e.g., first-peak strength, residual loads and strengths, toughness (energy absorbed), and R e,x.

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29 This test is used to determine the flexural toughness of fiber-reinforced concrete (i.e., energy absorbed); this test is specified mostly for underground (UGC) applications.

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 Pulsation  Type of equipment may influence pulsation  Low workability is low filling ratio, high pulsation, reduced quality and higher cost  Integration with accelerator pump 32 A good concrete pump is not necessary a good pump for application of sprayed concrete

33 Layering or Lensing

Nozzle system and set up  Air and accelerator introduction  Air volume  Air pressure 34 Concrete hose Accelerator hose Air hose Air and accelerator hose Wrong

35  Nozzle should always point 90°to the receiving surface  For spraying onto steel arches/lattice girders exceptions are required 90° Application Technique – Nozzle Angle

36  For applications the distance should be between 1 – 2 m  Influence of nozzle distance Incorrect nozzle angle and distance have a significantly negative influence on concrete quality, such as poor compaction, strength, etc., and will dramatically increase rebound 90° 1-2 m Application Technique – Nozzle Distance

37  Same concrete mix from one truck, sprayed 10 minutes apart!  Sprayed by two different nozzlemen during training

38 Questions? For More Info: Wes Morrison