User talk:Shakthi0

GUNITING-PRACTICAL ASPECTS

Definition: Guniting is the process of spraying dry mix cement/sand mortar pneumatically at high velocity on soil or rock surface of slopes / walls.

Picture: the forum vijaya mall, Chennai-india

The main differences between guniting and shotcrete are:-

Guniting: Guniting consists of dry cement (350-600kg/m3), sand (< 4mm size) and water; comparatively smaller machines and compressor; applicable for thinner section and smaller output (0.5m3-2m2/hr); more rebound or wastage (20-40%) especially on near vertical surface. Shotcrete: Shotcrete can be dry or wet mix, with aggregate size up to 15mm with more economic cement content (300-450 kg/m3), requires very large machines but higher output (2-10m3/hr); lower rebound or wastages (<10%); more applicable for very thick section up to 400mm or more such as tunnel lining.

Structural strength of gunite depends on the proportions of the mix ingredients and water content (W/C ratio usually 0.45-0.50 to control shrinkage and crack problems). Due to unavoidable change of water content and mix and rebound problems, a skilled nozzle-man is required to adjust the water intake, orientation and distance of guniting nozzle from the surface, etc. so as to produce quality gunite which should be consistently of high density and high strength with even thickness.

Important QC tests are control panel test to check the quality of gunite mix and skill of the nozzleman, and coring test (at about one core per 150 to 300 m2) to check the quality, thickness and strength of gunite sprayed. The defective gunite that exhibits segregation, honeycombing, lamination, porous voids or sand pockets, etc shall be carefully hacked off and regunited.

Factors that can affect the quality of gunite with respect to strength, density, finishes, durability, etc are:-

- Composition of the mix ingredients, water and additives, grading of sand; too much fines and water may cause lumpy and choking/blockage in the conveying hose.

- Type and design of nozzle

- Impact speed of the sprayed material (depending on air pressure / air blow and speed and Distance / height of the conveyed material, capacity and characteristics of pump or rotor engine).GI pipe may be used to reduce blockage and choking in hose when height is too high or steep.

- Surface hardness of receiving surfaces (more rebound for harder surface)

- Reinforcement wire mesh, weep holes, spacer blocks, etc

- Distance of the nozzle from the surface (usually 0.6m – 1.5m) and at near perpendicular direction

- Angle of spraying

- Capability and experience of the nozzle man Picture: the forum vijaya mall, Chennai-india

Surface Preparation

Just before guniting, the surface shall be cleaned by compressed air or sand blasting to remove all loose particles, debris, leaves, etc. The surfaces shall also be firm, and reasonably dry and even. Wet surface after raining should be covered by tarpaulin to keep dry before guniting to avoid debonding. Surface with seepage shall be dried up by longer and closer weep holes or horizontal drains.

Except at nail head, nominal reinforcement (BRC A5 or A7) is sufficient to reduce shrinkage crack if the W/C ratio is less than 0.5. Too much reinforcement wire-mesh will cause debonding which can lead to creation of voids, excessive shear or bending on soil nails head or dowels, resulting in slipping or cracking.

Loss of gunite due to rebound, surface over-break or surface unevenness (resulted from erosion or improper excavation) can be 100% to 200% of the design quantity and is a significant factor in costing.

In guniting, the material travels at very high speed to slope surface to create impact to get high density. The lighter particles are carried towards the receiving surface with higher speed than the heavier or coarse particles. Initially, upon striking the surface, rebound or blow away of 50% of all particles may take place. Finally a cushion of cements and water is built up and subsequent rebound will be reduced to 10-40% at near vertical surface. If the wire-mesh is not firmly fixed to the nail heads, dowels or weep holes, spacers, the wire-mesh will vibrate and cause more rebound and voids in gunite.

Rebound and dust (cement and fines) can be reduced by high grade nozzle and experienced nozzle-man. To avoid debonding and creation of voids in gunite. Nozzle man should not step on wire mesh. Improper bonded gunite will crack and have a reduced life time. Rebound shall not be reused and will cause pores and localized weakness.

Measures to reduce rebound: mix ingredients thoroughly mixed, higher cement content, slightly more Fines, smaller maximum sand size, finer grading, inclusion of some fly ash or silica fume, etc. Too Much fines will cause slumping and choking in the conveying hose.

Air Requirements The compressor shall maintain a constant supply of clean, dry, oil free air to maintain the required steady nozzle flow/velocity. Capacity of compressor required depends on the type of equipment, its conditions and mode of operation, type, length, size and height of hose from the position of compressor to the location of guniting, density of gunite, bends and kinks. GI pipe needs lower capacity of compressor and vertical portion of the hose should be replaced by GI pipe, if there is excessive drop in pressure. For 35mm to 50mm diameter hose and typical compressor of 750-850 cfm x 100-300 psi, pressure shall be increased by 35kPa for each 15m hose length to 35kPa added for each 8m height increased. Curing to keep gunite wet for at least few hour or avoid hot sun by covering with tarpaulin sheet should be carried out.