Updated: Oct 27, 2020
Nowadays it is not widely used in construction as in the past, but I'm sure many still remember the "wired glass". What is that? It´s glass sheets where a metal mesh is drowned. Its purpose? Prevent the glass from going in pieces when it gets broken. The mesh holds the pieces in place.
Handmade bricks of the past as well as the ones made in the poor lands even today, were and are made with the same philosophy: clay is mixed up with straw in order to have more ductility and prevent bricks from easily cracking down.
Today we achieve more concrete ductility and hit resistance adding fibers to the concrete mix.
We can summarize the fiber types available in the market in three categories:
Synthetic fibers (very common are those in polypropylene) are very thin fibers to the point that on average they develop 1000Km of total length for each Kg, they have a diameter in the order of 15-20 micron and length in the order of one centimeter.
They are easy to homogenize together with the concrete mix, creating a dense network that extends up to the surface; this network recalls, to some extent, the metal mesh of the wired glass and the straw inside the handmade bricks we talked about at the beginning. It´s like the fibers form “seams” in the concrete able to withstand and counteract the plastic shrinkage (Please read my post “Concrete shrinkage part 2”) and protect the concrete from violent hit of falling loads preventing it from going in pieces. Granted, synthetic fibers will not perform miracles! But the difference between concrete with and without them is plane to see.
Metallic fibers (of different sizes and shapes) are designed to give high ductility to concrete and counteract efficiently the hygrometric shrinkage which exerts forces way stronger than the forces exerts in plastic shrinkage. Metallic fibers are mainly used in concrete flooring both together and without the reinforcement mesh (read my post “Concrete flooring” and download my free booklet “Concrete flooring tips and tricks” signing up as a forum member, it´s free) or wherever ductility is of the utmost importance.
It is known that concrete is a rigid material, just think about what happens when a sample is submitted to the compression test: when the maximum load is applied the sample “explode” going in pieces. The same sample but with concrete designed to host fibers will break at the same load but without “explosions” and will be still able to withstand the load (even an increased load) without going in pieces.
STRUCTURAL SYNTHETIC FIBERS
They have the same function of the metallic fibers and are a good choice when it comes about outdoor structures. In fact, if some of the fibers would surface, they will never rust (that´s a concrete possibility with metallic fibers). Several types of structural synthetic fibers are available and sometimes they are even already mixed up with normal synthetic fibers in order to double up the field of action.
Whatever type of fiber will be chosen there is something that must be remembered. The concrete mix design should be specific it is not possible to simply add fibers to the mix. Why so? Because when fibers are added a new component is actually introduced and it will “demand water” so to say, meaning that its surface will be wetted at the expense of the forecasted w/c ratio. That turns out in a stiffer concrete (read my post “Workability of concrete”), hard time in mixing up all the components, balling and, of course, a horrible final result far from any expectation.
About balling: the common tendency of fibers is to stick together so it is pretty common to have “balls” of fiber or fiber not homogenously dispersed. The manufacturer therefore normally chose to have a tool by the mixing plant, a fiber dispenser, which weights and measure the precise fiber quantity at the same time separating them so to avoid any “balling”.