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Wednesday, August 8, 2007

SHOTCRETE



METHOD STATEMENT FOR SHOTCRETE


GENERAL
Shotcrete or gunite is a cement motar applied by spraying which contains no coarse aggregate. It can be sprayed over mesh, rock bolts and ribs to form part of integrated support system. The support systems maintain close contact with the rock or ground surfaces to reduce ground movements to a minimum.

MATERIALS

  1. The cement to be used shall be Type 1 Portland cement.
  2. Fine aggregate shall comply with MS 29 : 1971. The fine aggregate shall be washed.
  3. Wire mesh – BRC A4
  4. Water/cement ratio ranges from 0.35 to 0.50.

MACHINERIES AND EQUIPMENTS
  1. GENERATOR
  2. AIR COMPRESSOR
  3. CONCRETE MIXER
  4. GUNITING MACHINE
  5. WATER PUMP
  6. STAGING

APPLICATION

  1. Shotcrete or gunite is defined as concrete or mortar pneumatically applied at high velocity on to a surface.
  2. The cement for shotcrete shall conform to the requirement of BS 12. Type 1 Portland Cement will be used.
  3. Fine aggregate (sand) shall clean and free from silt.
  4. Fabric reinforcement (wire mesh) shall be hard-drawn steel wire with 7mm diameter wire (BRC A4) complying with the requirements of BS 4483:1983.
  5. Mix proportion for cement and sand shall be 1:4. Water/cement ratio ranges from 0.35 to 0.55. The concrete shall have minimum compressive strengths of 10Mpa at 7days and 20 Mpa at 28 days.
  6. Suitable air compressors shall be used to provide sufficient air volume to pump the water and the mortar to the discharge nozzle for shotcreting.
  7. Loose materials on slope surface shall be cleared before carry out shotcreting process. All rebound and loose material shall be removed by air jets or other suitable means from the surface of each layer as work proceeds.
  8. Surplus water shall be removed by air jet. If any water flowing from or across the existing surface it shall be diverted by pipes, hoses or other approved materials.
  9. The fabric reinforcement (wire mesh) shall be securely fastened with surface dowel bars at 3m centers and support with spacers to provide sufficient clearance between fabric reinforcement and the slope surface.
  10. 50mm diameter uPVC weep holes are then installed at 1.2m centers in each direction.


SHOTCRETING or GUNITING PROCESS - DRY MIX

  1. Cement and sand are thoroughly mixed by concrete mixer.
  2. Cement-sand mixture is fed into a guniting machine.
  3. The mixture is metered into the delivery hose by a feed wheel or distributor.
  4. Thus material is carried by compressed air through the delivery hose to a special nozzle. The correct pressure in a dry – mix shortcreting process is normally around 480 – 550 Kpa. Water is introduced to the nozzle with pressure and intimately mixed with the other ingredients. Water is added at the nozzle at a pressure of about 410 Kpa.
  5. The mortar is jetted from the nozzle at high velocity onto the slope surface.
  6. The spray should be perpendicular to the slope surface to minimize the rebound.
  7. No mix shall be used more than 2 hours after any water has been added. Rebound material shall not be reused.
  8. To avoid sagging and bleeding, shotcrete shall be built-up successive layers, each layer not exceeding 50mm thickness. The mortar shall cover the fabric reinforcement to a depth of at least 30mm.

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