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