International Concrete Abstracts Portal

One of the possible factors inhibiting the wider application of fiber concretes is the fact that to characterize the engineering properties of fiber concrete, both cubes/cylinders and prisms have to be cast and tested, in addition to the determination of workability. This paper shows that with the use of superplasticizer, the slump test can be used to give guidance on the flowability characteristics of the fresh fiber concrete. The advantage of the slump test is that it is easy to carry it out in the field, apart from its simplicity and convenience. The paper further shows that the equivalent cube strength obtained from the broken pieces of a flexural test can adequately represent the compressive strength of fiber concretes. It is thus shown that it is possible to characterize the engineering properties of fiber concretes from only one set of prisms of about 100 x 100 x 500 mm size. Apart from first crack load, modulus of rupture, and fracture toughness properties, the prisms can be used to give additional information as appropriate, such as shrinkage and expansion, and through pulse velocity, internal microcracking. It is suggested that by rationalizing the approach to testing, it is possible to reduce not only the cost and inconvenience associated with different sizes of test specimens, but also to enhance the relevance of some of the information obtained from such testing.

Standard Test Method for Flexural Toughness and First- Crack Strength of Fiber-Reinforced Concrete (Using Beam With Third-Point Loading), was conceived to produce toughness parameters independent of the dimensions of the test specimen. This seems not to be true. Additionally, the toughness indices that are required to be reported are shown not to be sensitive to the type and amount of fibers used, thus not providing a usable value for characterizing flexural toughness. Furthermore, since the calculation of the toughness index values are directly related to the first-crack deflection measurements, a value which is difficult to determine, these values become dependent on the testing equipment used. Proposals for revision of ASTM C 1018 are presented in this paper to address these concerns.

Procedures to obtain the experimental R-curves using a compliance calibration technique are revisited in this paper. R-curves provide a convenient means to study the process of fracture and the brittle-ductile transition in materials. Single edge notched beam specimens are tested under closed loop crack mouth opening control. The procedure to obtain the R-curves using loading/unloading compliance and the residual displacements are discussed. An elastically equivalent toughness K R as a function of crack extension is defined to compare the R-curves with the available data in the literature. The developed test method is applied to fiber reinforced concrete (FRC) composites with up to eight percent by volume of short, chopped alumina, carbon, and polypropylene (PP) fibers. Significant strengthening of the matrix due to the addition of short carbon and alumina fibers was observed. R-curves in these composites are characterized by an increase in the steady state fracture toughness. In PP-FRC composites, energy dissipation due to fiber pullout increases the ascending rate of the R-curve well after the main crack has formed. The work of fracture is computed from the cyclic loading/unloading tests and the results compared with the R-curves.

Presents two test methods that can be used for evaluating durability of polymeric fibers subjected to alkaline environment present in concrete and UV light exposure. The test methods were used to evaluate three polymeric fibers: nylon, polypropylene, and polyester. Durability of the fibers in an alkaline environment was ascertained by measuring the flexural toughness of fiber reinforced concrete specimens that had been aged in lime saturated water maintained at 50 C. The UV light exposure test was conducted at a temperature of 65 C with intermittent water spray. The wet spray was used to simulate conditions in the field. Durability of the fibers was determined by measuring the retained tensile strength of the fibers after light exposure and by observing the surface characteristics of fibers under a microscope. The test results indicate that nylon and polypropylene fibers are durable in the alkaline environment present in concrete. The nylon fibers, which were light stabilized, were determined to be stable under UV light exposure. Polypropylene fibers deteriorated under UV light; the deterioration of the polypropylene single filament fibers was more rapid than for the fibrillated fibers. Hence, these fibers should not be used in applications in which the fiber contribution is needed at cracked-exposed sections.

The usefulness of fiber reinforcement in improving the cracking resistance of cement-based materials under restrained shrinkage conditions is indisputable. In fact, in many instances, this may be the sole reason of adding fibers to concrete. In spite of this general recognition, there is no universally accepted technique of demonstrating or quantifying the effectiveness of fibers under the conditions of restrained shrinkage. This paper describes a newly developed technique in which prismatic specimens with a linear restraint along the longitudinal axis are subjected to a drying environment such that conditions of uniaxial tension are generated. The specimen cracks under these conditions; if fiber reinforcement is present, the influence of fibers on the cracking pattern can be established. Results with seven types of fibers are presented. Based on the observations of the crack patterns, a "fiber efficiency factor" is proposed which appears to be an appropriate basis for characterizing the fibers.