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Equivalent functional unit in recycled aggregate concrete 303

ω

ξ 5 1 2 0:5 ω 2 (11.5)
χ

A s f yd
ω 5 (11.6)
bdf cd
In Eq. (11.5), it is possible to confirm that the height of the cross-section increases
as the compressive strength factor (χ) decreases. As an example, for ω 5 0.1 (usual
in slabs), the increase in height for a RAC concrete with a compressive strength of
50% of NAC, would correspond to an increase in the effective height of 5%. This
conclusion is in accordance with the consensual idea that for pure bending under nor-
mal moderate load levels, the class and area of the steel reinforcement is the govern-
ing factor for the performance of reinforced concrete cross-sections.

11.4.3 Discussion of a multi-criteria approach (based on
structural functionality)

Even though a simple approach to determine the EFU based on the compressive
strength alone is attractive in practical terms, it lacks the depth required to fully
address the different performances of RAC and NAC. As an example, RAC concrete
structures will not only require a larger section because compressive strength is
lower, but also it will require a larger section, because the stiffness of RAC is also
lower than that of NAC. Therefore, it is imperative that a multi-criteria approach is
conducted, considering all the conditioning factors affecting SLS, ULS and durability
of RAC structures. To consider this, Dobbelaere et al. (2016) developed a flowchart
that provided the required steps to define the EFU based on structural functionality of
RAC (Fig. 11.2). The first step corresponds to establishing the new required cover of
the reinforcement, as a function of durability parameters (carbonation and chloride
penetration). Based on this new cover and using simplified criteria, a new cross-
section height is defined (h RAC ). Using this new cross-section, the structural element
is sequentially checked for deformation SLS, bending ULS and finally for cracking
SLS. At each intermediate step, h RAC can be adjusted if it does not comply with the
limit state in question. At the end of this sequential computations, a design friendly
parameter (K) is determined, as a function of the most conditioning verifications and
the EFU can be calculated depending on the fundamental parameters.

11.5 Proposal of an equivalent functional unit for
recycled aggregate concrete

11.5.1 Practical rules to compare different properties of RAC and
NAC
The determination of an EFU based on a multi-criteria approach requires the defini-
tion of several ratios (α), which will address each of the performance properties