Flexural behaviour of basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars

In this paper, the flexural behaviour of M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars are studied and compared with slabs made with steel rebars. The optimum percentage of basalt is 0.3% for 50mm length basalt fibres. Due to high particle packing density in concrete made with basalt fibre micro cracks are prevented due to enhanced fatigue and stress dissipation capacity. Addition of basalt fibres to enhances the energy absorbtion capacity or toughness thereby enhancing the resistance to local damage and spalling. Addition of basalt fibres controlled the crack growth and crack width. Load at first crack of M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars is more than M30 grade conventional concrete slabs made with steel rebars because the with addition of basalt and BFRP bars will make either the interfacial transition zone (ITZ) strong or due to bond strength of concrete slabs made with basalt fibre reinforced polymer rebars. The ultimate strength in M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars is more than conventional concrete slabs made with steel rebars. Deflection at the centre of M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars is almost double than the conventional concrete slabs made with steel rebars. Toughness indices evaluated for M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars indicates that basalt fibre and BFRP bars will enhance the energy absorbtion capacity of slabs.


Introduction
In the current study, M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars and with steel rebars of size 1200 mm *1200 mm *100 mm are casted and studied for flexural characteristics such as first crack, ultimate load carrying capacity and deflection at centre.

Methodology
In the present study, basalt fibres of length 50 mm and dosage of 0.3% fibre volume of the concrete are adopted based on the studies conducted by the authors. The quantities per m 3 of concrete for M30 grade concrete adopted are-Grade of the concrete -M30

Flexural behaviour concrete slabs
In the present study, flexural behaviour of M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars. Considering the limitations of the test facilities available the laboratory and handling issues of the test specimens, the size of the concrete slab specimens was fixed as 1200*1200*100mm with nominal reinforcement of 8mm diameter HYSD bars 200mm C/C both ways were casted and tested to determine the flexural characteristics of M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars. The slabs were tested in a loading frame which consists of supporting main beams and cross beams at the bottom. Load is applied through a hydraulic jack of 100T at the centre as marked. Proving ring with dial gauges was used to measure the load applied. The corresponding deflections at mid-point were measured using 50mm dial gauges with least count of 0.01mm. Crack widths were measured using crack detection microscope of least count 0.02mm. Schematic diagram of the loading system and test set up is shown in Fig.3.   The first crack load was taken as the load corresponding to the first visible crack. The load at which the deflections increased without any significant increase in load was taken as the ultimate load. The readings beyond ultimate load could not be noted since the test set up started losing stability due to excessive deflection. The test set up is so that all the four edges are simply supported.

Toughness Index
Toughness Index is measured from load deflection curves of slabs. Toughness can be quantified using the area under the curve or standardized with respect to the area under the curve up to the first crack. The Toughness (TI) is computed as ratio defined by the following equation.   The toughness indices of M30 grade basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars are presented in Table 4. It can be seen that fibre addition delays crack initiation. Load-deflection curve was linear up to first crack and non-linear in post cracking stage. On further loading this crack propagated and more number of finer cracks appeared. After subsequent load increment this propagation of crack deviated from initial path and continued to grow and fork to join the corners so as to form a failure mechanism. Meanwhile a number of smaller cracks developed and they were interconnected to each other. Failure of slabs occurred due to widening of cracks along yield lines. The rate of increase in crack width was observed higher in the case of slabs without fibre. Cracks developed in fibre reinforced slabs were closely spaced and the crack width measure-d was found to be smaller than the cracks observed in slab without fibre at all load levels. Crack pattern for slabs with different fibre content are recorded. The load was increased at regular intervals and the load at which the deflection increased tremendously without increase in load was taken as the ultimate load.

Discussions
Concrete slab specimens of M30 grades were casted to study the flexural properties of M30 grade basalt basalt fibred concrete slabs made with basalt fibre reinforced polymer rebars indicates that basalt fibre and BFRP bars will enhance the energy absorbtion capacity of slabs. So it can be concluded that the use of basalt fibres and BFRP bars in the concrete enhances its mechanical properties and their application in reinforced concrete elements improves the flexural characteristics of concrete elements significantly.