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Tests show that suitably proportioned mixtures can be prepared using a drum mixer
Ultra-high-performance concrete (UHPC) is an advanced cementitious material with compressive and tensile strengths exceeding 150 and 15 MPa (21,760 and 2180 psi), respectively.1-3 UHPC mixtures are generally designed with relatively high dosages of silica fume, high-range water-reducing admixtures (HRWRA or superplasticizer), steel fibers, relatively small concentrations of small-size aggregates, and portland cement typically blended with fly ash or slag cement.4-5 Some distinguishing features of UHPC include an optimized gradation of the granular matter for realizing a high packing density, a water-cementitious materials ratio (w/cm) of less than 0.25, and a relatively high volume fraction of discrete (steel) fibers.6
Production of UHPC mixtures generally requires the use of specialty mixers or extended mixing times to ensure production of a homogenous fresh mixture. The sequencing of the components introduced into the mixer also influences the homogeneity of the fresh mixture. The high steel fiber content of UHPC can cause a high tendency for fiber balling, which can be more pronounced in large-scale production efforts. This is one reason many producers resort to the use of specialty mixers, but requiring such equipment limits the availability and raises the cost of UHPC. Figure 1 shows balls of steel fibers formed when a UHPC batch was produced in a concrete truck. Figure 2 shows balls of fibers in a core taken from a large UHPC placement in which mixing was accomplished using a concrete truck. Successful introduction of UHPC to mainstream construction applications would benefit from resolving the challenges associated with dispersion of fibers in conventional rotary drum mixers (that is, concrete trucks).7-8
Fiber balling is a challenge in scale-up of UHPC production using conventional concrete mixing practices. The selection of fiber type and volume fractions should be made to balance desired engineering properties of UHPC against the potential for fiber balling in large-scale UHPC production. The conventional approach to resolving balling of steel fibers in UHPC-by using multiple fibers bonded together with water-soluble adhesives-was found in this investigation to be ineffective. Therefore, efforts were made to select the steel fiber types with a reduced tendency toward balling that would also provide desirably high values of compressive strength. It is worth mentioning that, unlike normal- and...