Optimizing Workability of Fiber-Reinforced Concrete
Presented By: Kamal Khayat
Affiliation: Missouri University of Science and Technology
Description: The influence of fiber type and volume on the workability and rheology of fiber-reinforced self-consolidating concrete (FR-SCC) and fiber-reinforced super-workable concrete (FR-SWC) targeted for infrastructure construction and repair, respectively, was investigated. The fibers included propylene synthetic fibers, hooked steel fibers of two separate lengths, double and triple hooked steel fibers, hybrid fiber containing crimped steel fiber and polypropylene multifilament fiber, as well as micro-macro steel fibers. Fiber volume was fixed at 0.5% for the FR-SCC mixtures and varied between 0.5% and 0.75% for the FR-SWC. A Type-G expansive agent was incorporated. A ConTec 5 rheometer was used to measure rheological properties. The investigated FR-SCC and FR-SWC mixtures had initial slump flow of 660 to 700 mm and 505 to 570 mm, respectively, and exhibited excellent passing ability and stability. Mixtures with passing ability index, evaluated using the modified J-Ring test, greater than or equal to 12 and 11 for the FR-SCC and FR-SWC mixtures, respectively, exhibited good flowability. All fibrous mixtures were stable with maximum surface settlement of 0.5% and rate of settlement at 30 min. of 0.20%/h for the FR-SCC mixtures; these values were 0.33% and 0.22%/h for the FR-SWC mixtures. The passing ability index was inversely proportional to plastic viscosity for mixtures of a given coarse aggregate content and maximum size. Good relationships between slump flow and yield stress and T50 and plastic viscosity were established for the fibrous mixtures.
Controlling Excess Paste Volume to Achieve Fresh and Hardened Performance of Fiber-Reinforced Concrete
Presented By: Jiong Hu
Affiliation: University of Nebraska-Lincoln
Description: While fiber-reinforced concrete (FRC) has demonstrated its superior mechanical, durability, and ductility characteristics compared to conventional concrete, the mix design of FRC to achieve sufficient workability is challenging. No standard proportioning method is presently available for the optimization of mix proportions to ensure the uniform fiber dispersion and optimum fresh and hardened concrete properties. In this study, a void content test of the combination of fibers and aggregates was used to determine the packing density and void content of the fiber-aggregate granular skeleton. The excess paste volume was calculated according to the void content and concrete mixture design. The paper presented a step-by-step process of using excess paste content to adjust FRC design to achieve sufficient workability and compatibility. Results from the study demonstrated that with the combination of experimental study and theoretical calculation, it is possible to design different classes of FRC for success construction and good engineering performance.
Workability and Surface Finish Properties of Fiber Reinforced Concrete
Presented By: Joshua Edwards
Affiliation: AVR Inc.
Description: Use of macro plastic fibers to reinforce concrete has attracted widespread attention from the construction industry due to the multiple benefits they offer compared to steel reinforcing mesh. When introducing synthetic fibers, a change in the finish characteristics and workability can be created. Workability, as defined in ACI 116, is the measure of how readily the concrete can be mixed, placed, consolidated and finished. This presentation covers the results of an investigation of the workability and surface finishing behavior of macro fiber reinforced concrete. The properties investigated were: ease of mixing, initial slump loss and slump loss with time, pumping, exterior broom finish and interior hard steel trowel finish. Four fiber types at two doses (4# and 7.5#) were studied in two base concretes with different water-to-cement ratios (0.45 and 0.50).
Selecting the Right Fibers for Your Application
Presented By: Marc Rached
Affiliation: BASF Corporation
Description: Not all synthetic fibers used in concrete are equal. Fibers available in the market have different architecture; they have stick, rope, or tape forms. The form and size of fibers affect the placement, consolidation, finish ability, and mechanical properties of fiber-reinforced concrete. This presentation will discuss the effect of fiber architecture on FRC performance as well as how fibers should be selected based on application and desired surface finish.
A Comprehensive Methodology to Assess the Fresh State Performance of FRC with Adapted Rheology
Presented By: Liberato Ferrara
Affiliation: Politecnico di Milano
Description: The fiber dispersing ability represents a key distinctive feature of the fresh state performance of a Steel Fiber Reinforced Concrete (SFRC) which has to be carefully assessed, also in the sight of its outcomes on the mechanical performance in the hardened state. In this presentation, with reference to a typical SFR-Self-Compacting-Concrete mix-composition (containing 50 kg/m3 hooked steel fibers 35 mm long with an aspect ratio equal to 65), different methods to evaluate the resistance to static and dynamic segregation of fibers have been discussed and the results cross analyzed also in order to address a comprehensive testing methodology and to assess its reliability. Among them, cylinder segregation test (for static segregation), channel flow test (for dynamic segregation) as well as the slump flow test: as a matter of fact, through the latter, by measuring the content of fibers in different concentric circular areas of the patty, information on dynamic segregation could be assessed. This test method represents the extension to FR-SCC of a methodology already developed for plain SCC. Results obtained from fresh state performance tests have been finally correlated to the fracture toughness properties in the hardened state, measured on specimens cast according to the same procedure employed for fresh state tests. This set of data allows a thorough evaluation of the correlation among fresh state performance, fiber dispersion and mechanical properties in the hardened state, furthermore enriching the meaningfulness, in a design-oriented perspective, of material acceptance tests for quality control in the fresh state. The influence has been finally assessed of expected tolerances in the dosage of the mix constituents (water, cement, and superplasticizers) on the performance in both the fresh and hardened state, as correlated through fiber dispersion related issues.
A New Tool and Test Procedure for Assessment of Waiting Period in Surface Finishing of Fiber-Reinforced Concrete Flatworks
Presented By: Van Bui
Affiliation: BASF Construction Chemicals
Description: Surface finishing of concrete flatworks requires a number of steps, which typically include placement, strike-off, darbying or bullfloating, edging, jointing, waiting period, floating, troweling and curing. Waiting period is a time necessary for concrete to reach initial set (stiffen) and to allow any bleed water to evaporate from the concrete surface prior to continuing finishing operations. Determination of proper waiting period is particularly critical to achieve good surface finish of fiber-reinforced concrete (FRC) flatworks. Too early or too late start of floating can cause an exposure of fibers and poor surface finish. In current practice, finishing operations are recommended to start when the water sheen is gone, and when concrete can support the finisher’s weight with only a slight surface indentation. The observation of surface indentation of the finisher’s weights is qualitative and subjective. Therefore, an investigation with a newly developed penetration pressure apparatus and test procedure was carried out at BASF laboratory and field trials to assess proper time for start of floating and troweling for FRC flatworks. Three classifications have been proposed from the studies: “Too early”, “Almost ready” and “Ready” for start of floating. The newly developed tool and procedure are useful not only in laboratory with hand-finishing or light, small equipment with a range of mix designs and different exposure conditions, but also can be applied in the field with the use of walk-behind power machine or ride-on power machine.