## 4 Hybrid Fiber Composition  
### Steel--Steel Hybrid Fiber  
#### 4.1.1 Physical Characteristics  
The effect of straight steel fibers with lengths of 6 mm and 13 mm and a diameter of 0.2 mm is presented in [135]. The fibers differed only in length. For a 2% content of straight fibers in the UHPC mixture, no significant differences in flow were observed depending on the percentage content, unlike in the case of hooked-end fibers. As the content increased to 1.5% of 13 mm fibers, the flow of the mixture decreased by 3.3% compared to a 0.5% addition of 13 mm fibers. These results align with [136], where the influence of straight two-length steel fibers was investigated. The highest fluidity was obtained for a content of 0.5% 13/0.2 mm fibers and 1.5% 6/0.16 mm fibers, with an increase of 4.2% compared to an addition of 2% of 13 mm fibers. The negative effect of fibers is mainly due to three reasons: (1) the fiber size is significantly longer than the aggregate, resulting in a much higher contact surface between the matrix and the fibers than for the aggregate alone, (2) the granular skeleton structure is disrupted in the presence of rigid fibers, (3) the shape of steel fiber, which often includes deformations to improve anchorage in the matrix [156,157]. The fibers were observed to align with the flow of the fresh mixture. However, longer or deformed fibers increase internal friction during mixing, reducing the flowability of the UHPC. Their geometry disrupts the mobility of particles and may require additional admixtures to maintain workable consistency. Short fibers can cause the rotation of long fibers opposite to the flow direction, which explains why the flowability of hybrid mixtures is higher than that of a single type of steel fiber.  
The addition of hooked-end steel fibers (length/diameter = 30/0.5 mm) in the amount of 0.5 to 1.5% to UHPC, in combination with 13/0.2 mm straight steel fibers, in a 2% total fiber blend, required increasing the amount of high range water reducer (HRWR) from 25% to 150% [139]. Despite this, the flow time increased by up to 90%. The sample containing up to 1.5% steel fibers (straight, corrugated, hooked end) exhibited the lowest flowability when hooked-end fibers were added, indicating a negative impact of this type of fiber on flow properties [158].  
In the case of a UHPC mixture with 0.5% hooked-end steel fibers (13/0.2 mm) and 0.5% straight fibers (19.5/0.2 mm), no reduction in fluidity was observed compared to a mixture with 1% straight steel fibers (19.5/0.2 mm) [140]. However, increasing the fiber content to 0.75% led to a decrease in fluidity. Furthermore, longer hooked-end fibers (25 mm) reduced the flowability of the mix more than shorter fibers (13 mm), suggesting that short fibers created a denser particle skeleton, which decreased the flow parameters. Ensuring proper fluidity is crucial for the usability of the concrete mixture, as low fluidity can affect fiber dispersion and lead to reduced strength. The results were consistent with [159], where a 40% reduction in flowability was observed in a hybrid UHPC mixture with 1% straight steel fibers (6/0.16 mm) and increasing amounts of hooked-end fibers (30/0.38 mm) from 0.5% to 2%.  
Table 4 summarizes the workability studies of the UHPC mixes containing hybrid steel fibers.