International Concrete Abstracts Portal

The issue of how the method of determining midspan deflection in ASTM C 1018 toughness tests influences first-crack strength, first-crack deflection, toughness indices, and residual strength factors is addressed in this paper by comparing results obtained using the method now required in the current standard, which is based on net midspan deflection determined as the nominal midspan deflection minus the average of the deflections measured at the beam supports, with corresponding same specimen results based on nominal midspan deflection only which was not explicitly excluded in earlier versions of the standard. The problem of dealing with the portion of load-deflection relationship immediately after first crack when it is unstable is discussed. The range of test specimens for which comparative data are reported includes a series of third-point-loaded 500 x 150 x 150 mm beams with three different steel fibers ranging in length from 18 mm to 63 mm; a second, smaller series of 350 x 100 x 100 mm beams allowed for assessment of the effects of beam size and fiber alignment. Fiber contents varied from 20 to 75 kg/m 3 (0.25 to 0.94 percent by volume). Also included was a series of 350 x 100 x 100 mm beams with a single type of fibrillated polypropylene fiber of length 38 to 64 mm in amounts of 0.5 to 0.75 percent by volume. The results illustrate the extent to which the ASTM C 1018 parameters I 5, I 10, I 20, R 5,10, and R 10,20 are effective in distinguishing the performance of the various fiber reinforced concretes (FRC) mixtures in terms of fiber type, geometry, and amount. The index I 5 was found to be least effective. A case is made for greater emphasis on use of residual strength factors, especially R 10,20, when employing the test to specify and control the quality of FRC.

The toughness of fiber reinforced concretes (FRC) was characterized from notched beam tests. The tests were performed under CMOD control in a servo-hydraulic machine to obtain the stable response of both the unreinforced concrete and the FRC. Several toughness measures were defined in terms of the experimentally obtained load versus crack opening (CMOD) curves. They give a better indication of the fundamental behavior of the concrete, avoid the problems associated with the approach based on the deflection of unnotched beams, and are amenable to the incorporation of serviceability considerations (for example, crack widths). The effect of specimen size on toughness was found to be significant in both the matrix- and fiber-dominated regimes of the FRC behavior. In general, toughness increases with specimen size and needs to be accounted for in the characterization. The study was conducted on beams of a 70 MPa compressive strength silica fume concrete, with and without high-strength hooked steel fibers. It was found that the incorporation of a low volume fraction (one percent) of steel fibers is sufficient to significantly decrease the brittleness of high-strength concretes.

Round-robin tests of the flexural toughness of fiber reinforced concrete were carried out using six different testing machines in five different laboratories. Six groups of beams, including a plain concrete control, two different volumes of polypropylene fibers, and three different volumes of steel fibers were tested in accordance with ASTM C 1018, with special care taken to exclude the "extraneous" deflections due to deformations at the specimen supports. The results from each laboratory were used to compute the ASTM C 1018 toughness indices I 5, I 10, I 20, I 30, and I 50 and the corresponding residual strength factors R 5,10, R 10,20, R 20,30, and R 30,50. In addition, the JSCE Toughness and Toughness Factor were also computed. It was found that, although the load vs. deflection curves were inherently quite variable, in most cases there was no significant difference among the participating laboratories, except for those mixes with a very low toughness. It was also found that the ASTM C 1018 toughness indices, particularly I 5 and I 10, did not discriminate very well between the different fiber contents or different fiber types; the JSCE parameters were rather more successful in this regard.

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.