Shotcrete is mortar which is sprayed pneumatically through a hose onto a backing surface. For nearly a century, shotcreting has been an accepted way of placing cementitious material in a variety of applications. It is the force of the spraying action that leads to the compaction of the concrete or mortar, which then forms layers of concrete to the required thickness.
Implications of rebound
Because of the high velocity of the impacting jet, not all the concrete projected onto the surface remains in position. Some material rebounds. It has been said that rebound can waste 40 % or more of sprayed concrete applied in a project. Rebound material consists of the coarsest particles in the mix. As a result the shotcrete in situ is richer than would be expected from the mix proportions batched. This may lead to increased shrinkage.
Adfil micro and macro synthetic fibres are the answer to these and other challenges arising in projects involving sprayed concrete. Durus EasyShot macro fibres provide reliable three-dimensional secondary reinforcement. They reduce the rebound of sprayed concrete as well as shrinkage after completion of the project. As such, these high-performance fibres not only improve the economics of construction projects. They also add to durability and aesthetics to the finished concrete.
SFRS & explosive spalling
Typical end-uses of shotcrete include sprayed concrete tunnel linings. Following several tunnel fires in Europe, design engineers have had their minds firmly focused on how to prevent explosive spalling of concrete in tunnels. The phenomenon occurs when concrete is exposed to high temperatures such as those experienced during a hydrocarbon fire. The high-quality dense concrete that is associated with tunnelling projects means: in the event of a fire, moisture escaping from the heat source cannot escape quickly enough. Any voids that are present within the concrete soon become saturated. An issue also with steel fibre reinforced shotcrete (SFRS).
Passive fire protection
This is where Ignis micro fibres come in and deliver an important advantage over SFRS shotcrete. Ignis polypropylene fibres have been specifically developed to provide passive fire protection in concrete tunnel linings. When added to the mix, they will increase the permeability of the concrete during heating. They do this by melting, creating voids and reducing pore pressure. This allows steam to escape and reduces the risk of spalling.
For many years now, Ignis micro synthetic fibres have been used in tunnelling projects across Europe to reduce damage in the event of a fire. Examples include T5 at Heathrow Airport, the Westerschelde Tunnel (the Netherlands) as well as the Dublin Port Tunnel.
When will you build your first tunnel with Durus EasyShot and/or Ignis?
- Reinforcement enhances the bond of concrete and reduces rebound
- Avoids explosive spalling
- Increases speed of construction & reduces labour cost
- Improves the resistance of the concrete to plastic-shrinkage cracks
- Provides improved impact and abrasion resistance
- Optimizes the freeze/thaw resistance of concrete
- Reduces rebound during spraying