International Concrete Abstracts Portal

This CD-ROM consists of 14 papers that were presented at ACI conventions in Charlotte, NC,and Denver, CO, in 2006. Selected examples of FRC applications highlighted in this special publication include slab-on-ground, jointless slabs, thin section composites, prefabricated modular housing elements, concrete buried structures, concrete infrastructure repair, fire-resistant concrete, decorative concrete, and shotcrete. Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-268

The structural use of steel fibers as the only principal reinforcing has been developed and refined for the last 15 years. Total replacement of traditional rebar is now common in applications like suspended slabs resting on a pile grid which spans from 3 m (10 ft) to 5 m (17 ft) each way. Generally, the span-depth ratio of the slabs in such applications ranges from 12 to 25. Although most of these slabs use the ground as a form only, some of them have been cast in elevated conditions without any contact with the ground to ensure total independence in the event expansive clay or gas hazards are present or could be present. More recently, steel fiber reinforced concrete has been used in suspended elevated slabs with a span-depth ratio equal to 30, and spans from 5 to 8 m (17 to 26 ft) length. The present article reviews the concrete mix design, the type of steel fiber, the dosage rate needed, the hardened concrete testing method based on current standard documents and round indeterminate panel slab tests. An example of steel fiber-reinforced concrete elevated slab is given and the design method is outlined in detail.

The case history presented in this paper describes a small sized project for design and construction of a macro synthetic fiber reinforced concrete (SnFRC) residential driveway with an average grade of 17%. The study highlights risks and benefits of choosing this material for this project. Five almost-equal lengths of SnFRC sections were placed in two groups 11 months apart. The delay between placements allowed for some experience to better analyze and determine if this was the best solution given the customer’s performance criteria and the difficult construction conditions. The design included a high cementitious content mixture with small aggregate, synthetic macro fibers, and air-entraining admixture. The resulting driveway was constructed down hill and has performed well in spite of minimal surface preparations and no jointing or saw cuts in the overlay. Some small cracking has occurred but has been of no consequence because the concrete has been held together by the synthetic fibers. For the construction of this residential driveway utilizing synthetic fiber reinforced concrete, the performance criteria was met, the construction schedule was on time, and the construction costs were significantly lower.

The effects of fresh state properties on the fiber dispersion characteristics of fiber-reinforced composites (FRCs) were studied by quantifying fiber segregation. Fresh state properties of concrete mixes were varied using different combinations of a plasticizing agent and viscosity modifier. A self-designed parallel-plate rheometer was used to obtain rheological parameters. Vibration was applied to the specimens and vibration times were varied to understand the effects of vibration on fiber segregation. An electrical characterization method, alternating current-impedance spectroscopy (AC-IS), was used to quantify fiber segregation in specimens. The relationship between fresh state properties and fiber segregation was evaluated.

Water loss from concrete results in volumetric shrinkage, which is significant at early ages. This shrinkage is particularly pronounced when the surface to volume ratio is large of the placements. Fibers, especially synthetic fibers are known to reduce cracking induced due to restrained plastic shrinkage. However, few studies have been conducted to monitor the early-age shrinkage of fiber reinforced concrete (FRC) using embedded sensors in the field. This study involved developing crack resistant FRC material in the laboratory using the bonded overlay technique developed at UBC and using it for a field project. Results from a plain concrete slab-on-grade section and a high volume fly-ash placement were used for comparison with fiber-reinforced concrete (FRC). Three sections were cast using synthetic fiber and their performance was monitored by reading strain signals from embedded sensors. Both traditional (electrical) and state of the art optical sensors were used. Optical sensors registered low strain values due to lack of bond with concrete. On the contrary, traditional electrical sensors clearly demonstrated the reduction in strain in FRC when compared to plain and fly-ash concrete. Specimens were cast on site for conducting tests in the laboratory. In addition, nondestructive tests were conducted on-site for monitoring performance of the slabs. These results are also presented in this paper.