Under long-term loading condition, concrete flexural members tend to crack over time. This can lead to corrosion, increased deflection and reinforcement debonding which can negatively affect both the structural and the architectural elements of buildings. This research investigated the effect of the addition of steel and polypropylene fibres to concrete beams as a potential solution for reducing cracking, reducing time-dependent deflection and enhancing the overall mechanical properties of Hybrid Fibre Reinforced Concrete (HFRC) beams under sustained loading.
Five concrete mixtures were strengthened internally with hybrid fibres. A sixth mixture was used as a control mixture, which did not include fibres. To monitor the time-dependent deflection, a large scale beam (4.2 m long, 300 mm wide and 150 mm in depth) for each mixture was tested under a four-point bending test for three months. The main variable of this study is the fibre volume fraction. The tested steel fibre contents are 0%, 1% and 1.5% and the tested polypropylene fibre contents are 0% and 0.2%. In addition, the concrete mixtures used in the beams were tested for flexural strength, compressive strength, splitting tensile strength, bonding strength, modulus of elasticity, compressive and tensile creep, shrinkage and tension stiffening according to building code standards.
The experimental results showed that HFRC exhibits improved mechanical properties and reduced long-term deformations compared to the control mix. Narrower crack widths and lower long-term deflections were also observed in the HFRC beams. The use of hybrid fibre significantly improved the overall structural performance compared the use of mono-fibre reinforcement.
A procedure for estimating the long-term deflection of HFRC beams is needed. An analytical investigation of the applicability of the EC2 provisions to predict the long-term deflection of HFRC beams was conducted. The EC2 equations that are used to calculate the long-term deflection of normal concrete beams were modified to incorporate the effect of the reduction in tensile creep, loss of tension stiffening and shrinkage that were observed in HFRC. Finite element simulation was performed and showed that the proposed modifications can enhance the estimation of the long-term deflection of HFRC beams.
