Synthetic fibers have been around for 25 years, and are acknowledged as being effective at controlling plastic shrinkage cracking in concrete and can be beneficial to help concrete resist abrasion and impact. A new generation of fibers, though, has now been developed that serves to control crack widths from thermal forces and shrinkage cracking, replacing secondary or temperature reinforcement.

The newer fibers, called macrofibers or structural fibers, have demonstrated their ability to control crack widths, particularly in slabs on ground. However, many concrete contractors are unfamiliar with how the old fibers (now referred as to microfibers) differ from the new macrofibers, and are unfamiliar with the capabilities and benefits available with these new fibers.

In general, the differences between the fiber types are few, but very meaningful. Dan Biddle, of Forta Corp., points to the 4 C's of structural or macrofibers—Chemistry, Content, Configuration and Correct length—to get at the differences.

Chemistry simply refers to what the fiber is made of. Microfibers originally were made from all sorts of things, including polyester, nylon, and steel. Their purpose was to build up early tensile strength to resist shrinkage stresses that increase as the concrete sets. They didn't necessarily have to last forever; their job was finished once the concrete hardened.

However, macrofibers have to last as long as the concrete, so polypropylene (also referred to as polyolefin) tends to be most popular choice. It doesn't absorb water and is resistant or inert to alkali.

Synthetic “structural” fibers in concrete.
Synthetic “structural” fibers in concrete.

Content refers to the amount of fibers placed in a mix. The dosage of the first-generation synthetic fibers is 0.5 to 1.5 pounds per cubic yard to be effective in reducing plastic shrinkage cracks. Structural fiber dosages up to 20 pounds per cubic yard, though, are not uncommon. So, the suggested dosage, as found on a technical data sheet, is a dead giveaway whether a fiber is intended to control plastic shrinkage cracking or can provide something more.

Macrofibers can function as secondary reinforcement, but they actually help with everything beyond plastic shrinkage cracking, says Mike Mahoney, materials engineer for Euclid Chemical. This includes drying shrinkage and thermal cracking—and they may also be effective for increasing the flexural strength of the concrete. The term “structural” fibers, however, isn't universally accepted. For example, the American Concrete Institute does not recognize any tensile contribution from synthetic fibers in the design of concrete structures.

Pouring a precast structure with only macro-synthetic fibers as the reinforcement.
Pouring a precast structure with only macro-synthetic fibers as the reinforcement.

Yet, unlike wire mesh or rebar, macrofibers are added directly into the mix and thus do not need to be placed into position. They don't affect the mix design because they don't absorb water. However, macrofibers can affect the slump of the mix, which might require the use of superplasticizers. As always, it is best to consult the manufacturer for mixing, placing, and finishing procedures.

Configuration refers to the shape of the fiber. The two main shapes are monofilament or fibrillated. Monofilament fibers are single discrete fibers, though some have bulbous ends that act as an anchor in the concrete. Fibrillated fibers, which disintegrate into a smaller network of fibers, tend to provide a better anchor than standard monofilaments. Some manufacturers have developed combinations of monofilaments and fibrillated fibers; others blend macrofibers and microfibers together.

Last but not least, correct length is important. First-generation fibers (microfibers) are typically ¾ inch in length, but macrofibers tend to be longer, usually 2 inches, sometimes longer. That's where the term macrofiber comes from.