Fibre-reinforced concrete (FRC) contains fibrous material which is suitable for increasing the strength and durability of concrete. Using fibre can assist with durability, strength, cost savings and faster construction, especially as concrete is the most used building material globally. The idea behind the concept of adding fibre to concrete is to increase bonding in the concrete so when the fibres are evenly mixed and distributed, they increase the strength of the concrete and maintain its structural integrity.
Fibre additions to concrete mix is not a new concept, but has rather evolved with the advancement of technology. Horsehair used to be the additive when making mud bricks in ancient times to assist with strengthening. Asbestos later become a major additive, but once the health risks were discovered, this product was rejected and changed to what we know today, a variety of fibres. The construction industry continues to carry out research and development in this area, so the variety and methods are bound to change.
What types of fibre are used in reinforced concrete?
Glass fibre, natural fibre and synthetic fibre are some examples of fibre used to enhance the strength and performance of the concrete, pending the application. We can break down fibre sizes into two formats: (a) micro and (b) macro.
These range between 15 mm and 30mm, but there have been cases of larger sizes for specialised projects. Engineers prefer fibres longer than 20mm, but once again, this depends on both the user and the project, as longer fibres could ‘ball up’ in the mixing, depending on the quality and process during their curing process. Some projects even call for a blend of fibres, with a laboratory making the final determination of the composition. In cases like this, the application determines the most suitable combination of strengths and advantages of each fibre.
Fibre reinforced concrete reduces the potential of cracking during the curing time and ultimately lowers maintenance costs. One of the leading companies offering fibre-reinforced concrete solutions, the Sika Group, states that FRC increases energy absorption, reduces dangerous spalling (the process of hydration absorption creating splits or small break-offs) at high temperatures and increases its inherent strength. Conversely, normal concrete is permeable and susceptible to cracking and spalling.
Studies have shown that adding 4% fibres to concrete more than doubles flexural strength. This product also provides improved corrosion protection, fire resistance and extended service life.
Advantages of fibre reinforced concrete
High-rise buildings and similar structures require high tensile strength performance from their building concrete. There are three levels of fibre-enriched concrete:
- Non-reinforced concrete is found at the lowest performance level. Adding higher concentrations of fibres is part of the normal formula to minimise plastic shrinkage cracking (cracking that occurs as soon as concrete is poured and for the first 24 hours).
- Next, engineers would recommend using fibre reinforcement as an alternative for mesh, as the strength values are far higher and more able to hold the crack together should it occur.
- Adding fibre in high concentrations makes for concrete with maximum strength, in place of, or in addition to, steel reinforcement.
Besides higher performance, other benefits include:
- Less cracking from shrinkage in the early stages of curing.
- Better cohesion when applying fresh concrete.
- Improved load capacity.
- Increased resistance to abrasion and wear.
- Less expensive than rebar (steel rods or wire mesh added to concrete to make FRC).
Applications suited to FRC
- Shotcrete (gunite or sprayed concrete)
- Storage structures
- Polished concrete
- Elevated decks
- Underground construction
FRC is also suited to aircraft runways, tunnel lining and slope stabilisation, dams and hydraulic structures. Technology for concrete is gaining momentum, with one of the recent developments being ultra-high performance concrete.
BLEND Plants provide the opportunity to produce FRC with low tolerance levels. The batching equipment can be customised to add both admixture tanks and emulsion tanks and hoppers. These additions are then added to the computer management system and once the user has inserted the desired mixture settings, the plant will extract the material required to create the specified type of fibre-reinforced concrete.
In 2016, tests were completed at the University of Milan in Italy with steel fibres using Blend Plants machinery. The results confirmed the advantages of using a Blend Plant to produce a batch of FRC. Under normal circumstances using standard industry equipment, transfer mix is not normally more than 30 kg/cubic meter of steel fibres. Using Blend Plants machinery, 80 kg/cubic meter of steel fibre inclusion was achieved. The end result is a superb homogeneous structure, or distribution of the steel fibre in the end product.