Fiber distribution in ready-mix concrete

Abstract: While manufacturing fiber reinforced concrete, it is of utmost importance that quality and homogeneous fiber distribution are ensured. To verify that the fibers are evenly distributed in ready-mixed concrete, SS-EN 206 has introduced requirements for continuous control of fiber distribution. The requirement for test frequency was originally the same as for compressive strength. Since this frequency requirement would result in an unreasonably large workload, it has been opened for national adjustments. In order to proceed with this work, there is a need to develop documentation and investigate eventual impact on fiber distribution of the timing and the way the fibers are added to concrete. The purpose of this thesis is to study the possible impact of when and how fibers are added to the concrete. Based on the results, prepare a basis for recommendations for routine description. The hypothesis is that there are no measurable differences regardless how and when the fibers are added to concrete. The correctness of the foregoing is fully confirmed. Samples from real insitu steel fiber reinforced concrete deliveries were extracted at the beginning, middle and end of the discharge. The steel fibers were, thereupon, separated from the fresh concrete and washed. Thereafter, lab tests were performed; steel fibers of each sample were dried in a special oven and then weighted by a high accuracy weight scale. The rule of proportions was subsequently used, in order to compare the final result with the client’s order and study the deviation from the average. The results have shown that the variation between the three different methods of adding the fibers, namely, in the central concrete mixer plant, in the concrete mixer truck after concrete is filled, in the concrete mixer truck at the same time that concrete is filled, as well between start, middle and end of the discharge is negligible.  A questionnaire survey, focusing on different aspects about how a concrete company handles steel fiber reinforced concrete, was additionally performed in order to provide supplementary data to the literature study and the testing process. The initial hypothesis was once again verified; the differences between the methods used to produce steel fiber reinforced concrete as well as when the fibers are added to the concrete mix are insignificant, according to the surveyed companies.  Preliminary investigation, continuous control as well as careful procedures for addition and mixing are recommended for a more reasonable test frequency. Preliminary investigation includes a detailed documentation process from the manufacturer for adding and mixing fibers, before the concrete containing fibers is manufactured; fiber parameters (shape, length and type), duration of mixing, how fibers should be added, type of mixer that should be used. Continuous control includes tests that should be conducted if fibers are added to the concrete mixer truck and the acceptance criteria shall be according to B.5 in SS-EN 206.