PSI - Issue 42

Martina Drdlová et al. / Procedia Structural Integrity 42 (2022) 1382–1390 Martina Drdlova/ Structural Integrity Procedia 00 (2022) 000 – 000

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Table 3 Results of impact tests

Designation/ Parameter

PC MESH

HPFRC HPFRC 3

PC

PC PU

PC 3

PC 3 PU

PC DR

UHPFRC UHPFRC 3

Impact toughness (kJ/m 2 ) Spalling (%wt)

20.5

94.27

13.4

19.6

86.5

99.2

16.5

71.4

41.03 >115.0

0.8

1.3

0.3

0.0

5.1

0.0

1.1

3.9

0.4

0.0

The inclusion of reinforcement in the form of bars and mesh nets significantly increased the impact toughness of the tested elements. When a mesh reinforcement was incorporated, the PC specimens' impact strength rose from the (average) value of 13.4 kJ/m 2 to 71.4 kJ/m 2 . Incorporation of the ribbed reinforcing steel bars brings further increase to 86.5 kJ/m 2 . This shows that the mesh, although introducing more reinforcement globally into the element, provides a lower effect in terms of increasing the toughness and resistance of the element to impact loading compared to the rebar reinforcement. This is probably related to a certain stress concentration along the transverse reinforcement. During the impact, the transverse reinforcement may act as an impactor on the longitudinal bars and initiate their rupture. Similar behaviour has been observed on test bodies with a high-value matrix by Bibora (2019) and Popovic (2018) and should be considered when designing the impact-resistant protective elements. Comparing the impact toughness of HPFRC 3 and PC 3 specimens shows that the value obtained for the PC 3 specimen is close to that of the high-value matrix test body (HPFRC 3). This may be related to the fact that the matrix consisting of the lower strength concrete (and without fibre reinforcement) was crushed in the vicinity of the impact of the hammer and partial debonding of the concrete from the reinforcement occurred. Being not supported by mass of concrete, the reinforcement was prone to deformation; the deformation process absorbed the part of the impact energy. Similar results were also achieved in previous research by Drdlova et al. (2020) on 250x100x500 mm specimens tested on a drop tower, where similar impact toughness results were achieved for rebar-reinforced samples with matrices of significantly different strength. However, a significant difference was observed in the fragmentation of both test bodies - while specimen PC 3 showed a high level of fragmentation, specimen HPFRC 3 fragmented only to a limited extent and with significantly smaller fragments. The combination of rebar reinforcement and polyurea (PC 3 PU specimen) results in a higher impact toughness value compared to the high-value HPFRC 3 matrix specimen. It was clearly demonstrated, that similar impact resistance capacity can be achieved using different reinforcement combinations, which is crucial for designing the material composition with the optimal performance/cost ratio. UHPFRC 3 showed the highest impact toughness of all tested samples. Failure limit has not been achieved; the energy required to fracture the sample exceeded the measuring device range. Thus, the impact toughness is recorded in Table 3 as >115.0. The strength of the matrix influences the overall impact toughness to a high extent. The comparison of the PC-based specimens yields the following findings: the introduction of dispersed fibre reinforcement increases the impact strength only to a limited extent (adding fibre increased the impact strength from 13.4 to 16.5 kJ/m 2 ). A similar increase was achieved by adding a polyurea layer (19.6 kJ/m 2 ). Although the PC PU specimen exhibits higher impact toughness than the PC DR specimen, it cannot be concluded that external reinforcement with a polyurea coating is generally more effective than the incorporation of fibre reinforcement. The effectiveness of the polyurea coating on increasing the impact resistance depends (in addition to the tensile strength and ductility of the polyurea itself) on the overall configuration of the element and the ratio of the thickness of the polyurea to the overall thickness of the element. The increase in impact toughness due to the polyurea coating is particularly pronounced on thin-walled elements; for thick-walled members, the benefit will be mainly in strengthening the surface and preventing fragmentation and spalling. Fig. 3 shows the example of the failure mode of PC and PC PU specimens. It is the addition of the internal reinforcement by mesh or rebar reinforcement that has a major effect on the impact resistance. The combination of internal rebar reinforcement and an external layer of polyurea appears to be an ideal combination both in terms of the load-bearing capacity of the element and in terms of