Does a Fibre Bond to the Cement Interface in Concrete
It is has become increasingly popular to reinforce concrete with small, randomly distributed fibres. Their main purpose of these fibres is to increase the energy absorption capacity and toughness of concrete, as well as increase tensile and flexural strength of concrete. Fibres of various shapes and sizes produced from steel, plastic, glass, and natural materials are currently available to the concrete industry; however, for most structural and nonstructural purposes, steel and plastic fibres are the most commonly used.
When the concrete fibre is mixed into fluid concrete, some of the paste volume of the concrete mix saturates the surface of the concrete fibre. As the concrete cures and hardens, the fibre is mechanically anchored into the hardened paste. This phenomenon is similar to the action of the cementitious paste hardening around grains of sand and rock. Where the sand, rock, and fibre become an effective part of the concrete composite.
The sand and rock have the benefit of an irregular shape. The irregular shape helps create that locking and mechanical bond in the concrete composite and therefore, helps create the environment for load transfer from paste to sand and rock.
Concrete fibres, which are often time’s steel or plastic, can be smooth. A smooth fibre will not offer the same type of irregular shape as the grains of aggregate to help form the mechanical bond. Instead of an efficient load transfer, a smooth fibre might experience ‘fibre pull-out.’
What is Fibre Pull-Out
Fibre pull-out refers to the action of a fibre that has not mechanically or chemically bonded to the hardened matrix of concrete. During the application of service loads, if the fibre is engaged through the paste, the lack of bond at the interface between the fibre and the hardened will render the fibre useless. The fibre will be pulled from the hardened matrix and offer little to no resiliency to the tensile loads it was originally intended to absorb.
How to Prevent Fibre Pull-Out
To prevent the ‘fibre pull-out’ effect, manufacturers commonly employ the functionalization methods to the fibre. Functionalizing is recognized as the process of adding new features or properties to the fibre by changing the surface chemistry or structure of the fibre to make it more conducive to concrete hardened properties.
- Coating the fibre with some an inorganic or organic material that reacts with the components of cement hydration. This method focuses on a chemical bond to establish a stronger interface and greater resistance to fibre pull-out.
- Damaging the fibre increases the absorption potential at the fibre surface creates an environment that forces the fibre to imbibe some of the fluid paste. As the cementitious paste cures and hardens it effectively anchors the fibre into the concrete. While this method compromises some of the structural integrity of the fibre it does so with the objective of gaining a greater interaction between the fibre and concrete
- Shaping the fibre through manufacturing methods can be used with steel and plastic fibre alike. Where the principle of shaping is similar to the previous damage principle but done so with a specific design concept and accuracy.