SHOTCRETING

SHOTCRETING

K.V.SUBBA RAO

Shotcreting

•Shotcreting is a method in concrete work in which theconcrete mix is applied in layers under the pressure ofcompressed air

•. Shotcreting is done with a special unit, which consistsof a mortar gun or concrete sprayer and a compressor

Shotcreting

•A dry mix of cement and aggregate, usually sand, isprepared for shotcreting.

•The compressed air forces the mix through a hose to thenozzle, moistens it with water supplied through anotherhose, and then projects it at high velocity (130–170m/sec) onto the surface being shotcreted.

Shotcreting

•A single shotcrete layer is 10–15 mm thick and has highmechanical strength (40–70 meganewtons per sq m),density, water tightness, and cold resistance

•Shotcreting is used in building thin-walled reinforced-concrete structural components, such as shells, vaults,and tanks, and in producing the lining in tunnels. It isalso used for hydraulic insulation, for finishing the jointsof reinforced-concrete structural components, and forrepair and strengthening of concrete and reinforced-concrete components.

Shotcreting

•The shotcrete is placed layer-by-layer

•For shotcrete placing special equipment are used, whereby means of "dry" technology the mixture of cement,sand and metal batched in specified proportions issqueezed out with pressure air by means of flexible hoseto the nozzle atomizer, where the mixture is moisturizedwith water. When the "wet" technology is used, preparedconcrete is delivered to the machines

Shotcreting

•Shotcreting has high strength properties; resistance of itsextension is 10 per cent more in comparison withstandard concrete. The cover of high strength, densityand waterproofing capacity is formed because of impactlaying

•Shotcrete application allows decreasing face thicknessand out-turns; excluding the necessity of formwork andmechanizing the concrete work process totally.

Materials for shotcreting

•Cement

•Sand

•Coarse Aggregates

•Water

•Chemical additives

•Compressed Air

Materials for shotcreting

•Chemical admixtures are added for acceleration ofsetting and hardening.

•Sodium Carbonate = ½ Kg per 50 Kg bag of cement

•Sodium Aluminate = ½ Kg per 50 Kg bag of cement

•Calcium Carbonate = 1 Kg per 50 Kg bag of cement

•Alternatively, following additives can also be used:

•Super plasticizer @ 1 % by weight of cement

•Accelerator (say sodium Silicate) @ 5 % by weight ofcement

Preparation of Surface

•The following precautions are necessary in preparationof the surface before the shotcrete is applied:

•The surface should be free from lose materials. It shouldbe clean to receive the shotcrete layers.

•In case the surface is smooth it should be roughenedwith sand blasting. Otherwise the surface could becleaned with air water jet.

Preparation of Surface

•The thickness of the shotcrete layer being only 76 mmany undulations in the surface more than such thicknesswill have to be backfilled with either masonry or leanconcrete depending on the site conditions andspecifications prescribed.

Mix of shotcrete

•Cement 450 kg / m3

•Sand 1100 kg / m3 (0.70 m3)

•Coarse aggregate

•(5 mm – 10 mm size) 500 Kg/M3 (0.30 M3)

•Proportion by weight 1:2.44:1.11

Application Methodology

•Spray pneumatically the first layer of shotcrete of 38 mmthickness

•Welded wire mesh of 100 mm x 100 mm x 5 mm isnailed with the help of nails. Binding wire is used to keepthe mesh in place.

•The second and final layer of 38 mm of shotcrete issprayed pneumatically over the first layer.

Precautions to be taken

•The finished shotcrete layer is to be kept wetcontinuously for at least a week after completion.

•There would be rebound during the operations whichshall not be used for shotcrete.

•The time lag between the first layer and the second layeris to be kept as minimum as possible.-say less than 8hours.

Precautions to be taken

•After completing the shotcrete and after the mix is set,the surface should be thoroughly sounded by a hammerfor drumming areas resulting from rebound pockets orlack of bond etc., which should be cut out and replacedwith fresh shotcrete.