Fiber Reinforced Concrete – High Fiber Slabs: Extending Joint Spacing
November 04, 2019
The concrete industry has had a long-time obsession with cracks in concrete, based on the perception that floor and pavement owners are most troubled by cracking issues. And while cracks and methods to control them are certainly an important consideration in any concrete project, the vast majority of problems faced by facility owners are, in reality, more often caused by joints than random cracks. As a result, owners continue to investigate and search for methods and materials that can reduce the number of joints in floors and pavements, and thereby avoid the costly repairs and down-time associated with joint failures. To-date, those reduced-joint or joint-free methods of slab construction have centered on the use of post-tensioned slabs or shrinkage-compensating cement, both of which are very effective, yet represent a considerable up-front cost. The use of three-dimensional fiber reinforcement over the past several decades has allowed designers to effectively replace conventional non-structural temperature steel reinforcement in a wide variety of slab-on-ground applications. With the advent of higher possible dosages of fiber per cubic yard or meter of concrete, traditional joint-spacing practice and design can now be reconsidered as well and can play in important role in reducing both the short-term and long-term joint-related costs for owners.
FORTA Corporation introduced micro synthetic fibers to the U.S. market in 1978, which were designed to reduce plastic shrinkage cracking and add an element of shrinkage/temperature crack control. These products generally took the form of very fine single-filament polypropylene or nylon fibers (monofilaments) used at a low dosage of 1.0 lb./cu. yd. (0.6 kg./cu. m), and deformed net-shaped polypropylene fibers (fibrillated) typically used at 1.5 lb. to 3.0 lbs./cu. yd. (0.9 kg. to 1.8 kg./cu. m). Though it was well recognized in the laboratory that adding higher dosages would lead to considerably higher levels of shrinkage reduction, these fine-filament fibers created mixing and placing challenges due to their high surface-area characteristic.
After years of research and trials, FORTA® introduced the first of its kind macro synthetic fiber in 1999 to solve the user-friendly issues at higher dosages. The solution keys became a twisted-bundle shape to eliminate balling, a polypropylene/copolymer chemistry and long lengths to enhance strength, and a concrete-gray color and special fibril shape to accommodate finishability. For over a decade, the result has made FORTA-FERRO® the most user-friendly macro synthetic fiber in the industry, with scores of successful flatwork projects all over the world. And with these higher fiber dosages came considerable evidence of other important benefits – the dramatic reduction in shrinkage, and a resulting reduction in slab-edge curling.
FORTA’s history of real-world extended-joint practices began in the spring of 2000, where fiber length and dosage were varied in 4 identically-sized floor panels, each 30 ft. x 90 ft. (9.1 m x 27.4 m). Of the 4 panels, only one resulted in a mid-slab crack – the panel with the shortest fiber length and smallest fiber dosage. Since that time, scores of joint-stretching projects have been successfully placed across the U.S., many of which are documented in the Project Profile White Paper: “Chicago Floor Study –Warehouse Floor” (February 2009, Bartlett, IL). Also documented in the report were the background and details surrounding a unique warehouse trial study sponsored by CONCRETE CONSTRUCTION magazine, where fiber type and dosage were two of the variables observed and studied with regards to slab-edge curling. In general, the slabs with a high dosage of the FORTA-FERRO® macro synthetic fibers reported essentially zero curling over their expanse, even at panel dimensions of 38 ft. x 42.5 ft. (11.6 m x 13 m), whereas the smaller panels of 14 ft. x 12.7 ft. (4.3 m x 3.9 m) with smaller dosages of shorter fiber lengths showed noticeable evidence of curling.
It became apparent that macro synthetic fibers could play a critical role in the success of reduced-joint or joint-free slabs-on-ground, as long as good accompanying practices were also used. There are naturally many, many factors that can potentially affect a concrete slab’s success – many of which can be controlled, and a few of which cannot be controlled. However, with the proper acknowledgement of those factors and conscientious regard for complimentary practices, macro synthetic fibers have the capacity to dramatically improve the long-term durability and sustainability of concrete slabs-on-ground in a wide variety of applications by offering the opportunity to extend panel joint spacing and thereby reduce the number and volume of joints.