Fibre Reinforced Concrete: Evaluation of test methods and material development

Concrete is a structural material with excellent properties when subjected to compression, butthe abilities to resist tensile stresses are rather poor. The concrete’s tensile zone is normallyreinforced with large continuous steel bars, a combination which ensures an excellentconstruction material. Placing the re-bars generates many man-hours, which means that the reinforcement work accounts for a considerable part of the total concrete cost. An alternative to the conventional re-bars is fibre reinforced concrete. Fibre reinforced concrete is concrete reinforced with small randomly distributed discontinuous fibres instead of large unidirectional continuous steel bars. In cases where thestrength and ductility of fibre reinforced concrete is sufficient with regard to the actions, fibrereinforced concrete can be an adequate and cheaper alternative to conventionally reinforced concrete. If fibres are used together with conventional re-bars, both the total load carryingcapacity and the stiffness of the structure will increase, and the crack widths will decrease. One working hypothesis in the present thesis has been that the behaviour of fibre reinforced concrete can be uniquely described by the fibre slip, and an objective has been to investigate whether this hypothesis is correct or not. Due to the fact that the maximum fibre stress achieved during pull-out test is considerably less than the fracture stress for most fibre types,it is reasonable to assume that the fibres’ pull-out length in a real structure is identical with the crack width, because the fibres will simply loosen at the weakest side of a crack. Based on alarge number of experiments, and some simplified models which relate the experimental results to crack openings, it seems like the working hypothesis is correct. A theoretical relation between the results from energy absorption test found by two differenttest codes is established. This relation is based on the above mentioned working hypothesis,and it is shown that the theoretical relation corresponds well to an empirical relation found in the literature. A second objective has been to show that fibre reinforcement actually has sufficient strength and ductility to be used as a replacement to conventional re-bars in some types of concretestructures. A concrete called ductile high tensile strength all round concrete is developed, and this concrete shows promising properties with regard to both shear strength, bending strength and ductility. The last few pages of the present thesis deals with some paradoxes from some of the material models used to describe fibre reinforced concrete from the literature. The experiences from the experimental work, and the analyses of the results, indicate that the fibre efficiency is increased by increasing crack widths until the crack widths reaches a certain level, while most of the models suggest that fibres are most effective at smaller crack widths

Is There a Trade-Off Between Maximum Jumping and Throwing Capability in the Handball Jump Throw?

This study examined the potential trade-off in performance between maximum physical capabilities in the handball jump throw, a fundamental skill comprised of two mechanically independent tasks. Elite handball players performed jump throw actions from a force plate for each of three instructions: jump at maximum capability, throw at maximum capability, and jump and throw at maximum capability simultaneously. Jump height and throwing velocity were derived from motion capture data. When jumping and throwing at maximum capability simultaneously, no trade-off between jump height and throwing velocity was present, but rather a concurrent decline from their respective maximums. This decline could be explained by mechanical factors related to movement execution; magnitudes of directional impulses favored vertical movement for jumping and horizontal movement for throwing. However, no explanation for differences in total magnitude of impulse between instructions was evident. Due to the expertise of the participants, information processing should not be a limiting factor, leaving movement strategy as the most likely explanation for the present findings

Strength determinants of jump height in the jump throw movement in women handball players

McGhie, D, Østerås, S, Ettema, G, Paulsen, G, and Sandbakk, Ø. Strength determinants of jump height in the jump throw movement in women handball players. J Strength Cond Res XX(X): 000–000, 2018—The purpose of the study was to improve the understanding of the strength demands of a handball-specific jump through examining the associations between jump height in a jump throw jump (JTJ) and measures of lower-body maximum strength and impulse in handball players. For comparison, whether the associations between jump height and strength differed between the JTJ and the customarily used countermovement jump (CMJ) was also examined. Twenty women handball players from a Norwegian top division club participated in the study. Jump height was measured in the JTJ and in unilateral and bilateral CMJ. Lower-body strength (maximum isometric force, one-repetition maximum [1RM], impulse at ∼60% and ∼35% 1RM) was measured in seated leg press. The associations between jump height and strength were assessed with correlation analyses and t-tests of dependent r's were performed to determine if correlations differed between jump tests. Only impulse at ∼35% 1RM correlated significantly with JTJ height (p < 0.05), whereas all strength measures correlated significantly with CMJ heights (p < 0.001). The associations between jump height and strength were significantly weaker in the JTJ than in both CMJ tests for all strength measures (p = 0.001–0.044) except one. Maximum strength and impulse at ∼60% 1RM did not seem to sufficiently capture the capabilities associated with JTJ height, highlighting the importance of employing tests targeting performance-relevant neuromuscular characteristics when assessing jump-related strength in handball players. Further, CMJ height seemed to represent a wider range of strength capabilities and care should be taken when using it as a proxy for handball-specific movements

Development of a Framework for the Investigation of Speed, Power and Kinematic Patterns in Para Cross-Country Sit-Skiing: A Case Study of an LW12 Athlete

Objective: To develop a framework for the investigation of speed, power, and kinematic patterns across varying terrain in cross-country (XC) sit-skiing, and to test this framework in a XC sit-skier of the LW12 class during high- (HIT) and low-intensity (LIT) endurance training. Methods: One XC sit-skiing athlete of the LW12 class with a single above-the-knee amputation was equipped with a GNSS enabled sports watch with integrated barometry and heart rate monitoring (peak heart rate: 195 beats·min−1), and an inertial measurement unit. After a warm-up, he performed two 20-m maximal speed tests on a flat and an uphill section to determine maximal speed and power, followed by skiing 5.75 km at both LIT and HIT in varying terrain. Results: 51, 28, and 21% of the time during HIT and 53, 28, and 19% of the time during LIT were spent in uphill, flat and downhill terrain, respectively. Maximal speed in the uphill and flat section was 4.0 and 6.2 m·s−1, respectively, and the corresponding maximal power output 342 and 252 W. The % of maximal speed did not differ between the uphill and the flat section (HIT: 66 vs. 67%, LIT: 47 vs. 50%), whereas the % of maximal power output was lower in the uphill than flat section (HIT: 65 and 80%, LIT: 46 and 58%). Still, the absolute power output was slightly higher in the uphill than the flat section (HIT: 222 vs. 201 W, LIT: 156 vs. 145 W). Furthermore, cycle rate was significantly higher during HIT than LIT (60–61 vs. 45–55 cycles·min−1, across all terrains, all p Conclusions: Here, we present a new integrative framework for future investigations of performance, technical and physical demands in XC sit-skiing. In this case study, the increase in speed from LIT to HIT was due to increases in cycle rate in all terrains, while cycle length was less affected. Although the absolute power output was slightly higher in the uphill compared to the flat section both for HIT and LIT, the athlete worked closer to his maximum power output in the flat section