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The Science of Shotcrete

The Science of Shotcrete

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The Science of Shotcrete

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  1. The Science of Shotcrete CIB Meeting June 5th 2012

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

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

  4. Sprayed Concrete • 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

  5. How Sprayed Concrete Works Thin layers (3–15 cm): Bridging effect • 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

  6. Dry Mix Method • 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.

  7. Dense stream system Wet Mix Method • 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

  8. Mix Design and Ingredients

  9. Critical parameters in SC • 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

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

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

  12. Combined Gradation Curve • 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. Cement and Pozzolans • Type I, Type II or Type I/II Cement • Cementitious content ~ 800lbs • Fly Ash; Class F or C • Slag • Silica Fume • Proportion similar to • how pozzolans are • proportioned in concrete

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

  15. High Range Water Reducers or “Superplasticisers” • Low water - cement ratios0.32 to 0.45 • Allows for higher slumps • High early and long term sprayed concrete strengths • Pumpable shotcrete mixes • Durability enhancement • Low dosage - cost effective

  16. Sprayed Concrete Admixtures: Hydration Stabilizer • 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 shotcreteaccelerators re-starts the hydration process and causes immediate setting

  17. Slump and Set Control withHydration Stabilizer Traditional Sprayed Concrete No control! New Flexibility with Hydration Stabilizer Take back control!

  18. Other Materials

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

  20. pH Scale 0 4 7 10 14 Safe to human body ACIDIC NEUTRAL BASIC Alkali-free accelerators Conventional accelerators Modified sodium silicate accelerators Alkali-free accelerators - human and structure friendly! 4 7 10 14 pH Scale 0 • 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

  21. Parameters Effecting Mixing and Performance of Alkali-Free Accelerator • Slump below 4in can prove difficult • Poor mixing efficiency of accelerator into stiff material • Overdosing of accelerator due to poor pump piston filling efficiency • High pulsation - layering effect • 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 Slump: >6in

  22. BackgroundFibers Outperform WWF & Rebar 3D, Discrete Reinforcement 2D, Wire Mesh Reinforcement On a single horizontal plane only Multi-dimensionally throughout entire concrete thickness

  23. Sprayed Concrete: Steel and Macro Fiber • 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

  24. Benefits of Steel Fibers 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

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

  26. Assessment of Fiber Contribution Flexural performance (ASTM C 1609) Flexural toughness (ASTM C 1550)

  27. 25 20 15 Load, kN 10 5 0 0.0 0.5 1.0 1.5 2.0 2.5 Deflection, mm ASTM C 1609 (Flexural Performance) • 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 Re,x.

  28. Panels for ASTM 1609

  29. ASTM C 1550 (Flexural toughness) • 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.

  30. Panels for ASTM 1550

  31. Application

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

  33. Pulsation results in layering Layering or Lensing

  34. Equipment set up is key elements to quality Nozzle system and set up • Air and accelerator introduction • Air volume • Air pressure Air and accelerator hose Air hose Wrong Concrete hose Accelerator 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° 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 Application Technique – Nozzle Distance 90° 1-2 m 90° 1-2 m

  37. Same concrete mix from one truck, sprayed 10 minutes apart! Sprayed by two different nozzlemen during training Application technique can be dramatic

  38. Permanent shotcrete domesNapa Valley, California . . .Nice wine too!! Questions? For More Info: Wes Morrison 571.344.3286