The role of hybridization on the mechanical properties of concrete
Creators: name: Nkereuwem, Samuel Etim
Publisher: name: University of Surrey
Year: 2024

description: The use of short fibre reinforcement concretes (FRC) has gained acceptance in recent years
description: with fibres such as steel, polypropylene, and glass being used to improve properties such as
description: strength, stiffness, flexural strength, etc. Initial research on FRC used a single fibre type but
description: more recently mixtures containing more than one fibre type have been investigated. Such fibre
description: combinations produce composites known as Hybrid Fibre-reinforced concretes (HyFRC). The
description: use of two, or more, fibres, with distinct stiffness and strain to failure characteristics, in a
description: cementitious composite then allows the different fibre types to provide individual responses
description: during the processes of crack initiation and growth within the concrete, thus providing
description: improved properties that are (potentially) better than the sum of the individual fibre
description: contributions.<br>
description: <br>
description: In this study, Mono-fibre and hybrid mixes were manufactured using steel (SF), polypropylene
description: (PF) and glass (GL) fibres at total volume fractions of 1% and 1.5% in combination with
description: aggregate sizes of 10mm and 20mm in the concrete matrix. The performance of these mixes
description: was evaluated in terms of compressive strength and stiffness, flexural performance, energy
description: absorption capacity and the bond between steel and concrete. In addition to this, the crack
description: propagation process and evidence of multiple cracking in concrete because of fibre
description: combinations was investigated using a non-contact imaging technique known as Digital Image
description: Correlation (DIC). Finally, the experimental work was complemented by the application of a
description: constitutive model known as the Variable Engagement Model (VEM) to single fibre and
description: extended to hybrid fibre combinations.<br>
description: <br>
description: The experimental results showed that over the range of volume fractions explored, fibre
description: additions normally produced a small increase in compressive strength when compared to plain
description: concrete without fibres, the maximum increase being recorded for hybrid concrete specimens
description: containing 1% steel fibres + 0.5% polypropylene. No significant increase in measured modulus
description: of elasticity was observed but it was found that some mixes containing polypropylene fibres
description: showed a reduction in stiffness compared to the control concrete mix.<br>
description: <br>
description: The results of flexural testing showed that for the mono-fibre mixes, the steel fibres yielded the
description: highest strength increase compared to the other mono-fibre mixes whereas a combination of
description: 1% steel with 0.5% glass yielded the highest maximum flexural strength for the hybrid mixes.
description: In the post peak region, the single steel fibre mixes outperformed its counterparts,
description: demonstrating higher residual flexural strengths at selected crack mouth openings. Hybrid
description: mixes of steel and polypropylene fibres and steel and glass fibres exhibited the highest synergy
description: as evidenced by the higher first-cracking stress and values obtained for residual flexural
description: strengths. Furthermore, both the single and hybrid fibre mixes exhibited an improved energy
description: absorption capacity as evidenced by calculated fracture energy at deflection of 3mm. For the
description: plain concrete a value of 628N/m was recorded whereas measured values 1702N/m and
description: 2271N/m respectively were recorded for the steel fibre mixes containing fibre volume fraction
description: 1% and 1.5%, indicating an increase of 170% and 260% respectively. The hybrids mixes
description: containing steel fibres in combination with polypropylene and glass fibres at total volume
description: fraction of 1.5% were found to produce the best flexural performance suggesting that the hybrid
description: effect was contributing to the overall performance of those mixes. Evidence of multiple
description: cracking in all the hybrid mixes tested was confirmed by images captured using the DIC
description: technique and supported the assumption that mixed fibre combinations can yield useful
description: improvements over mono-fibre mixes. The DIC technique was found to be a valid tool for
description: crack measurement as shown by the close agreement in measured crack opening between the
description: test data and DIC captured data.<br>
description: <br>
description: Ultimate bond stresses during tensile pull-out were improved by the addition of fibres in the
description: concrete mix. The single steel fibre mixes at 1% and 1.5% and the glass fibre mix at 1.5%
description: yielded the best improvement in maximum bond stress while the hybrid mixes containing steel
description: and polypropylene fibres and steel and glass fibres at a total volume fraction of 1.5% gave the
description: best improvements in recorded maximum bond stress suggesting that the hybrid effect was
description: leading to improved performance.<br>
description: <br>
description: Finally, a constitutive model known as the variable engagement model (VEM) was used for
description: prediction and analysis of the strength of short fibre reinforced concretes explored in this
description: research. The results obtained suggested that it could be potentially valid for hybrid
description: combinations provided all the parametric requirements are met. The VEM showed reasonable
description: agreement with the single-fibre mixes however further work in application to hybrid
description: composites is required to fully validate the model. <br>