Shotcrete has introduced key improvements in the construction industry thanks to its versatility as well as cost-reduction, but the pneumatic application needed to project and compact the shotcrete implies inherent challenges such as rebound.
Rebound is the part of the shotcrete that does not adhere to the surface during application, causing some material to ricochet.
The loss of material and subsequent clean-up can slow down work on site while increasing costs, so it´s important to minimize it.
What factors influence rebound?
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Application stage:
Rebound is greater at the start of the application, even though the creation of a cushioning layer diminishes the issue
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Surface inclination
Estimates vary between 2-5% on floors or slabs, up to 5-10% on sloping or vertical walls, or 10-15% on overhead work.
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Steel mesh reinforcements
Keys to avoid rebound while shotcreting
Away from an adherence issue, rebound also implies a lack of internal cohesion in the mix, causing the fresh concrete to fracture and fall off the surface.
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The right equipment
The advanced technology components of mechanized shotcrete equipment guarantee greater projection precision, therefore reducing the rebound rate compared with manual dry-mix spraying.
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The correct application technique
It’s vital to keep a distance of 1-2 m between the nozzle and the surface: too short a distance will create a greater amount of rebound, while an excessive one will prevent the shotcrete from compacting.
The application angle is also of vital importance: the nozzle must be kept at a 90° angle to the surface, and the shotcrete must be applied in slow circular movements to guarantee a homogeneous result.
Other factors such as the thickness of the applied layer and the speed of projection are also important.
- A good shotcrete mix with the right consistency
- Several companies sell shotcrete additives specifically designed to reduce rebound
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The nozzleman: the determining factor
Ultimately, the key factor to avoid rebound will always be the nozzleman, as an incorrect application can cause greater loss of material, and even compromise the safety of all personnel on site.
Check out our traning courses for nozzlemen: training for operators, nozzlemen training courses, and Putzmeister’s preventive maintenance and trouble-shooting course.
Source:
- Marc Jolin, J.-D. Lemay, N. Ginouse, B. Bissonnette, É. Blouin-Dallaire, THE EFFECT OF SPRAYING ON FIBER CONTENT AND SHOTCRETE PROPERTIES, Engineering Conferences International, 2015
http://dc.engconfintl.org/cgi/viewcontent.cgi?article=1013&context=shotcrete_xii
- Gilbert Gedeon, INTRODUCTION TO SHOTCRETE APPLICATIONS, Continuing Education and Development, Inc. 1993
http://web.iitd.ac.in/~tanusree/courses_files/02Standard%20Practice%20for%20Shotcrete.pdf
- MacDonald, Ballou, Biddle: CASE HISTORIES USING SYNTHETIC FIBER REINFORCED CONCRETE, Engineering Conferences International, 2009
http://dc.engconfintl.org/cgi/viewcontent.cgi?article=1002&context=shotcrete
