In the fight for longevity, cracking is one of the biggest challenges that structural engineers face in a shotcrete project. Anticipating these issues is one of the cornerstones of the industry, and one of the main reasons why fiber-reinforced shotcrete has continued to gain traction over the past 40 years.
Fiber-reinforced shotcrete: How it works
Fibre-reinforced concrete is inspired in ancient building techniques such as adobe, boasting greater ductility which allows it to deform under tensile stress, as well as greater energy absorption capacity despite cracking.
Compared with steel mesh reinforcements, fiber-reinforced shotcrete also has other benefits, such as:
- a greater homogeneity of the support structure
- a more efficient rock section profile
- offering simpler application logistics
- fibers help reduce rebound and improve compaction
Fibers for shotcrete: Main types
Fibers can be categorized by size, material as well as function.
According to size and function:
-
Micro fibers (under 0.3 mm diameter)
- Used to minimize concrete shrinking-related cracking
- They also improve freeze-thaw durability, resistance to impact and abrasion, as well as helping release vapours and reducing spalling during a fire
- Typical dosage is of 1-3 kg/m³
-
Macro fibers (above 0.3 mm diameter)
- Steel and macro-synthetic fibers are considered to be structural fibers, as they help ductility and improve fracture toughness
- They help prevent the spread of cracking in set concrete
- Usual dosage for steel fibers stands at between 25-60 kg/m³
- For synthetic fibers, dosage is around 4.5-9 kg/m³
By material:
-
Metallic fibers
Typically steel
- Its main drawback is corrosion, as well as its high melting point compared to synthetics, which can cause further cracking in the shotcrete
- Steel fibers can be straight, wavy, double hooked or have flattened ends
-
Synthetic fibers:
Polypropylene is typically the most used, although there are also others made of nylon, aramide, polyester, acrylic, etc.
They show good resistance to oxide and concrete´s alkalinity, while suffering less erosion and weighing less on the crown
-
Fiberglass:
Although they may be costlier, it´s necessary to use alkali resistant fibres
-
Hybrids:
A recent study[i] concludes that steel and polypropylene fibre hybrids showed improved mechanical properties
In terms of the fiber´s geometry, the general aspect ratio between length and diameter is of great importance, offering more tenacity and resistance to cracking; but rendering the pumping potentially more complicated. Similar results can be achieved with an increased dosage.
Fiber shape will also determine its adherence to the shotcrete.
How to dose fibers?
Fiber dosage for shotcrete must be done during the mixing, preferably at the end. The mixing must be done slowly to avoid blockages and build-up. Although it´s best to add fibers in the plant itself, some clients choose to do it on-site to avoid excessive wearing out of the pipes. Synthetic fibers can be added in the plant itself, which allows for better dosing and a greater homogeneity in the final mix.
Pumping fibers: some notes
It’s key to remember that the fiber’s length must not exceed 60% of the pipe’s diameter. Similarly, the use of plasticizers is recommended as the addition of fibers may reduce shotcrete´s workability.
Find out more about the main ingredients of shotcrete in our infographic
1 O Cengiz & L Turanli, Comparative Evaluation of Steel Mesh, Steel Fibre and High-Performance Polypropylene Fibre Reinforced Shotcrete in Panel Test, Cement and Concrete Research, August 2004, http://www.sciencedirect.com/science/article/pii/S000888460400002X
