Fiber Reinforcing for Concrete

Traditional black steel reinforcement in concrete structures is industry standard for nearly all applications. Wire mesh is commonly used for crack control and edge spalling, but can be difficult to place accurately in thin slabs or complex formwork, and its benefit in preventing cracking is often not at the areas where it is needed most - corners, edges, and fine details.

Fiber reinforcement is accomplished by adding up to 2% by volume of short strands (typically 1/2" to 2-1/2") of glass fiber reinforcing strands. Individual fiber trand diameter ranges from 0.005 to 0.05 inches. Aggregates used are typically smaller, and cement content is increased. Unit weight of the concrete is also decreased. When added to the concrete mix the strands disperse evenly throughout the concrete and function to control micro-cracking due to temperature or shrinkage before it propagates through concrete and causes more severe cracking.

Fiber reinforcement does not replace structural reinforcing in concrete, though it does increase flexural strength up to 2-3X that of traditional concrete, as well as improving impact resistance. Raw glass fiber suffers from degradation due to the alkalinity of cement in concrete, which raises concerns for long term durability if not addressed. Alkali-resistant glass fibers have been used for decades to alleviate this concern.

Common applications include pre-cast concrete architectural details, underground utility structures, pre-cast cladding panels, and topping slabs over pre-cast concrete structural members and slab on metal decking. In pre-cast applications, the fiber inproves the ability of the cast object to be removed from formwork without chipping, spalling, or form adhesion. The cost of fiber additives is relatively low, and the benefits often make it a smart investment to save headaches down the road.

When finishing concrete with fiber reinforcement the fibers will protrude from the concrete surface and leave a sparse "fuzzy" look up-close. This is not noticeable from afar, and can be addressed by burning the fibers off or grinding/polishing the floor if it is to be left exposed. In appearance-critical applications fiber use should be considered carefully due to these concerns. But in most applications where the fiber is cast into formwork, concealed underground, or will be covered with a finish material, it has benefits that help make it worth considering.

AGB 11/2004