Page 355 - New Trends in Eco efficient and Recycled Concrete

312 New Trends in Eco-efficient and Recycled Concrete

The tensile strength can be computed using Eurocode 2 simplified approach
(11.19).

2=3
f ctm 5 0:30 f ck (11.19)
Silva et al. (2015a) showed that the same equation can be used for RAC mixes,
regardless of the replacement ration of RA type. Therefore, it is possible to express
α 5 in function of α 1 (11.20).

2=3
α 5 5 α 1 (11.20)

The relationship between creep and compressive strength in concrete is a com-
plex phenomenon and the prediction models are usually difficult to establish (Silva
et al., 2015a e). Of the existing models, the one proposed by ACI is one of the
most recognised, but it requires other properties not considered in this work. The
used relationship between α 66 and α 1 can be seen in Eq. (11.21) and was obtained
from the average trend lines of each of this fundamental parameter. Isolating the
replacement ratio (x) of the properties and matching both formulae leads to the
given equation.

α 6 5 2:55 2 1:55α 1 (11.21)

The validity of this equation is in the range of α 1 from 0.0 to 1.0. For higher α 1
values, α 6 goes below 1.0 which is physically inaccurate.

11.7.2 Case studies

The results conducted in 32 slabs cases (Dobbelaere et al., 2016) have proven that
all comply with the tested limit states, using the available fundamental parameters.
The slabs were tested with three geometry and boundary conditions and each of
them lead to reasonable results. All examples in RAC have a smaller computed
rebar cross-section (A s ) than the corresponding NAC cases.
Fig. 11.8 presents the obtained K-values for each tested slab case. The K-values
range from 1.0 to 1.37 and:
13 examples have a K-value below 1.1;
9 examples range from 1.1 to 1.2;
6 examples range from 1.2 to 1.3;
4 examples have a K-value above 1.3.
The compliance checks for slabs made with several RAC gathered from the liter-
ature review with or without all fundamental parameters available can be seen in
Tables 11.3 and 11.4, respectively. The results prove that some of the RAC case
studies show better compliance checks than the corresponding NAC, which may
indicate over conservatism of the method, meaning that a lower cross-section in