How much does BFRP macrofibre add to the tensile strength of concrete?

Concrete’s inherent limited capacity in tension is normally remedied by the addition of reinforcing bars, fibres, or a hybrid solution combining fibres and rebars. Fibre-reinforced concrete (FRC) is known for its improved tensile strength, fracture toughness, ductility, and enhanced post-cracking tensile residual strength compared to plain concrete [1-3]. Basalt fibre-reinforced polymer macrofibre (BFRPmf) is a novel construction material made of strands of basalt fibre woven together in a resin matrix, used as dispersed reinforcement in concrete. The enhancement of the tensile strength of BFRPmf reinforced concrete (BmfRC) is measured based on indirect testing, e.g. splitting tensile and direct tensile tests (DTT). In this study, four BmfRC mixes with different dosages of BFRPmf (0.25%, 0.5%, 0.75%, and 1%) were tested. The mixes, identified as BmfRC-0.25%, BmfRC-0.5%, BmfRC-0.75%, and BmfRC-1%, are self-compacting concrete (SCC) mixes reinforced with a macrofibre of twisted design, 43 mm length, 17 mm pitch, and a slenderness (length/diameter ratio) of about 55. The BmfRCs are referenced and compared with plain concrete with no fibre, identified as NoF, to clarify how much tensile strength BFRPmf adds to the BmfRC.

Figure 1a shows the setup of DTT with different elements of the setup noted. All DTTs were performed in a displacement-controlled manner with 0.3mm/min (5μm/s) crosshead travel (displacement) rate [4]. The tapered concrete dog bones for these tests were designed in the Civil Engineering department of Queen’s University Belfast (QUB) based on a review of the literature, basics of mechanics of materials, engineering judgement, and finite element analysis. Before the experiments, the dog bones were prepared with white and black speckle patterns required for strain measurements using the DIC technique.
Figure 1b shows the setup of the tensile splitting tests. The 200×100 mm cylinder specimens and the procedures of the experiments conform to BS EN 12350-6:2023 [5]. The tests were performed with a stress rate of 0.04 MPa/s, which is within the 0.04 to 0.06 MPa/s range prescribed by BS EN 12350-6:2023 [5] and the 0.02 to 0.04 MPa/s range prescribed by BS 1881-117:1983 [6].

The tensile splitting and direct tensile strengths of the BmfRC cylinders and dog bones were tested after 28 days of curing in a water tank. Tests on each mix were replicated three times (3 specimens), and the average and standard deviation of the measurements were calculated. The average tensile strengths are presented and compared in Table 1. According to this table, the addition of 0.25%, 0.5%, 0.75%, and 1% Vf BFRPmf leads to 17.7%, 53.5%, 53.3%, and 59.5% increases in direct tensile strength compared to the NoF mix. Moreover, the addition of 0.25%, 0.5%, 0.75%, and 1% Vf BFRPmf leads to 38.0%, 47.7%, 53.2%, and 70.1% increases in tensile splitting strength compared to the NoF mix. In general, BFRPmf significantly increases the tensile strength of the FRC mixes. This study improves the procedures for the assessment of the tensile strength of the FRCs using the direct method, which aligns with the requirements of the design codes and leads to more realistic measurements as indirect methods such as tensile splitting tests lead to higher measured values. This alleviates the design of more efficient FRC structural elements based on realistic estimates of behaviour of material in tension.