PSI - Issue 13

Martina Drdlová et al. / Procedia Structural Integrity 13 (2018) 1731–1738 Drdlová and Čechmánek/ Structural Integrity Procedia 00 ( 2018) 000 – 000

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Table 2. Description of the mixes and incorporated fibres. Designation Fibre content (Vol%) Fibre material

Length/diameter of the fibre (mm/µm)

Density of the fibre (kg.m -3 )

Tenacity (cN/tex)

Elongation (%)

1 2 3 4

1.5 1.5 1.5

Aramid

1.5/12 2.2/15 2.0/12

1,440 1,110 1,780

240 120

3.3

Polypropylene Carbon PAN

15.0

40

0.9

0

-

-

-

-

-

The mixing procedure was as follows: cement, sand and silica fume were mixed together for 60 seconds in concrete electric positive mixer for concrete (Strattentest). Then water and superplasticizer were added and stirred for another 120 seconds to achieve homogenous paste. At the end of the mixing process, the fibres were added and mixed for another 60 seconds. 500x500x10 mm slabs were cast. After being matured for 28 days, the cylindric specimens of diameter 30 mm were prepared from the slabs by water jet cutting. 2.2. Test procedures Aforementioned test specimens were used to determine the behaviour at both quasi-static and dynamic indirect tensile loading. The indirect tensile strength (Brazilian test) at quasi-static load was performed using universal strength testing machine TIRAtest 2710, R58/02, at speed of 5 mm/min. In the Brazilian test, a disc shape specimen is loaded by two opposing normal strip loads at the disc periphery. The thickness/diameter ratio should be 0.5 to 0.6 (0.5 in the presented investigation). The load is continuously increased at a constant rate until failure of the sample occurs. At the failure, the tensile strength of the specimen is calculated as follows: = 2 (1) P - applied load (N); D - diameter of the sample (mm); L - thickness of the sample (mm). Load-displacement curves were additionally captured to assess the behaviour of the specimens during static load, in particular the post peak stage. The dynamic mechanical properties were obtained using the Hopkinson Split Pressure Bar technique (HSPBT). Split Hopkinson pressure bar consists of an incident bar and a transmitter bar, with a short specimen placed between them and a striker bar that produces an impact on the incident bar to generate a longitudinal compressive pulse,  I (t) that propagates towards the specimen. The pulse is partially reflected in the border of the incident bar, stress pulse  R (t) and partially transmitted through the specimen as the stress pulse  T (t). The recorded stress pulses enable to evaluate strain-strain rate and stress in the specimen (Prachar et al. 2016).

Fig. 1. Experimental arrangement of the dynamic tensile – split test (Brazilian test).

For the evaluation of the tensile splitting strength, the position of the specimen is depicted in Fig. 1 (Brazilian test). This arrangement is a typical indirect tensile test, which was developed to determine the dynamic tensile strength of brittle and quasi-brittle materials such as rock, ceramics and concrete, as reported e.g. by Zhao (2000) or Dong (2011). Employing a proper pulse shaper in the conventional Split Hopkinson pressure bar test helps to achieve dynamic equilibrium in the test specimen (Chen (2014)). In this case the diametral loading generates tension perpendicular to the load plane, which causes the specimen to split. The strain records of the incident, reflected and transmitted pulses are used to calculate the corresponding stress pulses and the tensile stress in the loading plane according to the equation 1 for the maximum transmitted load. The strain rate can be calculated as follows: = 1 σ (2) ε – strain (-), E – modulus of elasticity of bars (MPa).