Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 165

70

65

60

55

50

45

40

35

RAC incorporating dierent percentages of microwave-treated coarse RCA
RAC incorporating dierent percentages of untreated coarse RCA

30

0 20

Replacement Percentage of Coarse NA with RCA (%)

40 60 80 100

Figure 4.15 Variation in the compressive strength of RAC with an increase in its RCA
content. (From Akbarnezhad, A., Ong, K.C.G. et al., Construction and Building
Materials , 2011, 25 :3469-3479. With permission from Elsevier.)

a generally inverse relationship between the compressive strength of recy-
cled concrete and the replacement percentage of RCAs (both untreated and
microwave-treated RCAs) for the coarse aggregates. As can be seen, the
reduction in the mortar content of RCAs after microwave separation led to
considerable improvements in the properties of RACs. Incorporating 100%
coarse microwave-treated RCA in place of NAs led to only a 10% reduc-
tion in the compressive strength of concrete as compared to almost 30%
reduction when a similar amount of untreated RCA was used. Moreover,
the effects of incorporating up to 40% microwave-treated RCA on the com-
pressive strength of RAC seemed to be negligible.
Similarly, we observed that a reduction in mortar content of RCA
through microwave separation could result in considerable improvements
in other mechanical properties of RAC, including the modulus of elasticity
and flexural strength. The modulus of elasticity and flexural strength of
concrete were reduced by only about 10% when all NAs were replaced with
microwave-treated RCA. This is considerably better in terms of concrete
performance when compared with the up to 25% and 15% decrease in the
modulus of elasticity and flexural strength, respectively, when untreated
RCAs were incorporated. It has been debated that besides the decrease in
the mortar content, the positive effects of microwave processing of RCAs