Tests have a vital role to play in the mixing of shotcrete, with on-site sampling and testing essential to ensure the sprayed concrete offers the best possible quality and performance.
The two tests most frequently done on fresh shotcrete are the flow table and the slump tests, which essentially determine the consistency and workability of the material before spraying. They can both be carried out on site with relatively inexpensive materials, and a simple methodology.
The optimal consistency and workability of the shotcrete will help reduce pulsations in the concrete pump, homogenize the fiber and accelerant content throughout the mix while improving the adherence of shotcrete onto the sprayed surface.
How does the process work?
Watch this video for a step-by-step breakdown of the flow table test, in accordance with European regulation UNE-EN 12350-5:2009.
The process is as follows:
- Find a flat and level surface
- Clean and dampen the table surface and cone
- Place the cone in the middle of the table
- Secure it in place by standing on the foot-pieces
- Fill the bottom half of the cone with the first layer of shotcrete, then tamp ten times with the tamping rod
- Apply the second layer and repeat
- Use tamping bar to ‘skim off’ any extra shotcrete, leveling it with the top of the cone. Clean off any spillage while waiting for 30 seconds.
- Carefully lift off the cone
- Slowly lift the table top by the handle until the upper stop, then drop
- Repeat this cycle to a total of 15 drops, each cycle should take around 4 seconds
- Measure the largest dimension of the concrete spread vertically and horizontally, in parallel with the table edges
What do you need?
You need the following to do the test:
- Flow table (essentially a flat plank measuring 700 mm x 700 mm with a handle, a hinge allowing it to be lifted from one side by a maximum of 40 mm and an upper stop),
- A metallic conical mould cut at the top and bottom (height of 200 mm, with a diameter of 130 mm on the top and 200 mm at the base) with foot pieces,
- A tamping rod measuring around 200 mm, plus a handle
Putzmeister Underground recommends that the shotcrete should deliver a flow value or total spread diameter of between 500-550 mm, which is also echoed by industry body EFNARC.
Failure to deliver within those parameters means the shotcrete is not optimal and therefore inappropriate for spraying.
- European Committee for Standardisation (CEN), “Testing Fresh Concrete: Part 5: Flow Table Test”, Publicado 2009
- EFNARC, “Specifications & Guidelines for Self-Compacting Concrete”, Page 21, Published February 2002, Last accessed 23/08/2016
- Shotcrete, “Shocrete Testing Around The World”, Lars Balck, Published Spring 2014, Last accessed 23/08/2016
- Shotcrete, “Shotcrete Testing: Who, Why, When and How”, Charles S. Hanskat, Summer 2011, Last accessed 23/08/2016
- Civil Engineering Dictionary: “Tests on Concrete”, Last accessed 23/08/16
- “Tests for checking quality of concrete”, theconstructor.org, último acceso 23/08/2016