Long-term performance of recycled steel fibre reinforced concrete for pavement applications.

Due to environmental concerns and increasing asphalt prices concrete pavements are seen as a
sustainable alternative for road construction. Steel fibres are used as reinforcement for concrete
pavements due to ease of construction, as well as improvement in the post-cracking,
tensile/flexural and fatigue behaviour of the concrete. However, cost and method of construction
are two major barriers for their use. Recycled fibres obtained from post-consumer tyres are a
new alternative due to their lower cost and potential environmental benefits. The roller
compacted concrete technique is also an alternative that enables road construction with the use
of conventional asphalt equipment. These were the two main innovations being investigated by
the FP6 EU Project Ecolanes. Understanding the durability of recycled steel fibre reinforced
concrete (SFRC) is very important before these technologies can be used in real structures.
This thesis addresses the issue of long-tenn behaviour of recycled SFRC, based on an
experimental programme divided in two main studies: I) the mechanical properties
(compressive and flexural behaviour), pore structure (porosity, density and free-shrinkage) and
transport mechanisms (penneability, sorptivity and diffusivity) and 2) the main deterioration
processes affecting the perfonnance of concrete pavements, corrosion (accelerated by means of
wet-dry cycles in chloride solution), freeze-thaw (accelerated by continuous submerged freezing
and thawing cycles) and fatigue (accelerated by flexural cyclic loads). A probabilistic analysis
in terms of service life design has also been developed.
Recycled fibres can increase the flexural strength of the concrete by up to 70% compared to
plain concrete and they can significantly enhance the post-cracking behaviour. Recycled fibres,
when added 2-6% by mass, do not affect the pore structure and the transport mechanisms of the
concrete. Exceptions apply when contents around 6% by mass lead to compaction problems or
affect the rheological properties of the concrete.
Recycled fibres improve the fatigue resistance by allowing approximately 30% higher stresses
than plain concrete for an endurance life of 2 million cycles. Fibres also contribute to slowing
down the advanced stage of freeze-thaw degradation of concrete. Both fatigue and freeze-thaw
are enhanced since these fibres control different stages of crack propagation. When subjected to
wet-dry cycles, the fibres appear to be well protected inside the concrete and the main
consequences are only in terms of superficial rust.
The coupled benefits of mechanical and long-term performance of recycled SFRC make it a
promising alternative for concrete pavements, especially in blends with industrially produced
fibres. If these advantages are taken into account in the design of concrete pavements, a 20%
reduction in the thickness of the concrete pavements should be expected, leading to less use of
natural resources and to a further 10% reduction in costs.