Mesh reinforced sprayed concrete

[GeoKB]

Has anyone had experience using double twist mesh with sprayed concrete? If so has anyone had issues with shadowing? We are concerned that the traditional weld mesh won't be malleable enough to conform to the irregular and uneven slope face and are considering alternatives. The Sprayed Concrete Association states that mesh with apertures between 50 and 150 mm and wire diameters of less than 10 mm, the mesh is generally considered acceptable. The rock mass is weak; hence we would prefer to use mesh reinforced sprayed concrete rather than fibre reinforced concrete. I have seen reference to some tests carried out with different mesh varieties in an older thread, but haven't been able to find any further information. Thanks.

 

[Patgeotech]

GeoKB,

Not sure what you mean by shadowing? During spraying or on completion?

We used double twist mesh with sprayed concrete to stabilise rock slopes in a mountain pass. The rock was extrememly variable and the had random bedding/fracture orientation. The rock face itself was irregular, but we managed to manipulate/work the mesh on the rock surface. In some areas we drilled short rock bolts to support/pin the mesh back so that voided areas were minimised Where we encountered voids under the mesh due to abrupt changes in the rock level, we applied a thicker amount of sprayed concrete to bring the level up (not to much as we wanted to try and keep the rock slope profile and not make it look too man-made). We also incorporated rock bolts in these thicker sprayed areas as they effectively formed collars beneath unstable rock wedges and were covered by spayed concrete - we did not want the rock face to look like a pin cushion of rock bolts. The rock bolt positions were carefully chosen so that we could hide them on the rock face.

We also added oxide to the sprayed concrete and varied the quantity at times so that it did not look all the same colour. We carefully matched the oxide as close to the rock colour as we could get and then let Mother Nature do her thing from above - fine mountain wash from the crests allowed to fall/spread over the gunite - caught on the dimpling of the sprayed concrete/shotcrete. Following completion of the project, I wish I had incorporated small sprayed ledges on the rock face to collect the finer crest material and possibly provide a foothold for plants - it is amazing where plants will grow - I think this would have added to the natural look.

Due to your weak rock, I assume you are installing rock bolts in conjunction with your sprayed concrete.

We also added stainless steel fibres (20mm long) to the sprayed concrete and experienced no problems in terms of working/spraying the concrete. It all comes down to the geotechnical contractors experience with fibres I suppose.

Our few beam tests that we undertook indicated (in terms of flexural strength - thermal aspects in South Africa), our 7, 14 day tests showed very little improvement, but flexural strength improvement (mesh and fibres) in the order of 30% were recorded at 28 day strength and it is assumed that this would improve slightly with time. Two years on, the sprayed concrete (with mesh and fibres) is still going strong.

Good luck.