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Thermoplastic reinforcement for Ultra-High Performance
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Figure 1: Illustration of terms used in damage assessment. Ci is initial compliance; Cd is damaged compliance.
3 Results and Conclusions
Fig. 2 illustrates the results of impact tests for panels subjected to an energy of 16 J. The plots illustrate damage observations made for all impact energies. In general, the addition of the thermoplastic reinforcement not only significantly reduces residual deflection and change in compliance (between 80 and 95%), but also significantly reduces the variability of damage. The addition of CNF to the UHPC mix had little to no effect on performance. Perhaps most significantly, there is little to no performance difference between the thermoformed reinforcement and the vacuum-infused reinforcement. The significance is that the thermoformed approach is far simpler to implement, and is more easily scaled up to larger panel sizes and to a more rapid fabrication process.
Figure 2: Results of impact tests at 16 J: (a) residual deformation; (b) change in specimen compliance.
The research results suggest that performance gains from the thermoplastic reinforcement warrant additional development steps, including optimizing the concrete/FRP bond, and developing an appropriate material model that will allow refinement of both FRP fiber layups and processing parameters for additional performance improvements.
References
[1] Ranade, R.; Li, V.C.; Heard, W.F.; Williams, B.A.: Impact resistance of high strength-high ductility concrete, Cement and Concrete Research 98 (Supplement C) (2017) 24 – 35.
doi:10.1016/j.cemconres.2017.03. 013.
[2] Smith Gillis, R.: Development of thermoplastic composite reinforced ultra-high performance concete panels for impact resistance, MS Thesis, Civil Engineering, University of Maine, 2017.
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Innovative UHPC-NSC composite members as substitution for structural steel
Goran Vojvodic, Duc Tung Nguyen, Viet Tue Nguyen Institute of Structural Concrete, Graz University of Technology, Austria
1 Introduction
Considering that the compressive strength of UHPC nearly reaches the strength of conventional construction steel, a substitution of steel by UHPC in specific structural elements such as NSC composite members, could be possible. In this context, this contribution presents an experimental investigation on UHPC-NSC composite members, in which UHPC was used instead of conventional construction steel. As first composite member a UHPC hollow box girders were considered as a substitution for steel girders in composite superstructure of single span bridges with a span up to 30 m. Prefabricated, slender, thin-walled UHPC box girders beams are prestressed by pre-tensioning and supplemented by NSC slab as deck layer cast in place. The second member, a UHPC-NSC composite column, consisted of a precast spun NSC shell and a UHPC filled core. The spun NSC shell served both as the fire protection layer and a load-bearing element.
2 UHPC-NSC hollow box girder
The developed precast UHPC hollow box girder for composite bridge superstructures consist of thin deck slab, thin webs and thicker bottom slab. Pretension strands are mostly arranged in the bottom slab, a small number are also arranged in the deck plate, in order to prevent cracking of the deck plate during the prestressing. For simple construction and to keep the thickness of all sectional elements thin, the girders were designed without conventional reinforcement. Due to this a high amount of micro steel fibre is required in order to avoid the formation of splitting cracks due to the transfer of prestressing forces.
Fig. 1 shows as an example of construction with UHPC box girders.
Figure 1: Possible bridge construction made of several UHPC box girder (left) and corresponding girder cross section for a bridge span of 30 m (right)
In order to investigate the load bearing behavior of hollow box girder and to valid prestressing test results, a 1:2 scaled UHPC hollow box girder was cast in a precast plant and tested. A self-developed fine grain UHPC mixture with the compressive strength higher than 180 N/mm2 after 28 days without heat treatment, self-compacting properties and the Young’s modulus of approx. 54000 N/mm2 was used. The UHPC matrix tensile strength was approx. 8.5 N/mm2 and the post-cracking tensile strength was amount to 9.0 N/mm2. The steel fibers had a tensile strength of approx. 3800 N/mm2 and are structured and brass-plated to improve the bond behavior, with a maximum fibre amount of 2% by volume. The prestressing strands were 0.62”, 7-wire strands with a cross-section area of 150 mm2, steel grade 1660/1860 N/mm2 and the nominal diameter dn of 15.7 mm.
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Fig. 2 shows the tested 7 m long UHPC box girder at bearing load with corresponding cracks.
Figure 2: Specimen at bearing load (left) and corresponding bending cracks (left) from measurement with the digital image correlation system
The UHPC hollow box girder reached the bearing capacity at 950 kN when a ductile bending behaviour occurred. Clearly visible was the specimen stiffness declination at approximately 550 kN with increasing number of bending cracks. Besides the opening of bending cracks at mid span of bottom slab and webs, the shear cracks in girder webs were observed. The first shear cracks were observed at about 650 kN. No slip between strand and concrete as well at the joint between NSC and UHPC was detected, also no torsion effect through possible stiffness decline on one girder side.
3 UHPC-NSC composite column
At this innovative composite column, UHPC core is cast in a spun NSC shell and serves as the single load-bearing element in the fire design situation. The NSC shell serves primarily as fire protection layer and secondly as load bearing element in normal design situation. Besides, it is used as formwork for the UHPC core. In contrary to conventional steel-NSC composite columns, where the profiled steel core can reach its compressive strength at the maximum loading by crushing of the NSC, attentions should be paid to considerable difference in the axial strain by reaching the compressive strength between UHPC and NSC in UHPC-NSC composite columns.
Fig. 3 shows cross section of UHPC-NSC composite column and typical failures occurred.
Figure 3: Schematic cross section of UHPC-NSC composite column (left) and typical failures (right)
The experimental program had three parts. In first part, 13 UHPC columns were tested in order to investigate the effectiveness of the fibre and transversal reinforcement regarding load bearing capacity and confinement action. At the second part, 18 short UHPC-NSC composite columns were tested. The distribution of longitudinal and transversal reinforcement was varied in order to identify the appropriate reinforcement arrangement regarding confinement and ductility. At third part, 3 UHPC-NSC composite columns with larger column length were tested in order to verify applicability for building construction columns.
For a core self-developed coarse grain UHPC mixture with the compressive strength higher than 180 N/mm2 after 28 days without heat treatment and without self-compacting properties and for a shell a C 50 / 60 was used. The failure of UHPC-NSC composite columns occurred very ductile with the spalling of NSC cover.
4 Conclusions
An experimental investigation on presented innovative UHPC-NSC composite structures pointed out that both structural composite members showed excellent composite action.
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