Civil Engineering Reference
In-Depth Information
Table 4.8
Efficiency of the thermal separation method
Properties of RCA
Process
duration
(h)
24-h water
absorption
(%)
Bulk density
(OD)
(kg/ m
3
)
Mortar content
(%)
by mass
Type of RCA
Before separation
0
4.2
2370
47
After
thermal
separation
300°C
2
4.1
2380
44
500°C
2
3.8
2390
41
600°C
2
3.8
2390
38
Note: OD = oven-dried.
in the mortar as a result of rapid evaporation of the internal pore water
when RCA is exposed to temperatures above 100°C. In addition, the rapid
cooling of RCAs after heating by immersion in cold water (cold relative
to the RCA temperature) has been reported as another effective method to
improve the efficiency of thermal separation by increasing the differential
thermal stresses developed [16].
The efficiency of such thermal separation techniques has been investi-
gated and reported in a number of previous studies [30,31]. A study we
conducted (Table 4.8) showed that heating the RCA at 300°C, 500°C, and
600°C for 2 hours reduced the mortar content by 6.4% (from 47% to 44%),
12.8% (from 47% to 41%), and 19.1% (from 47% to 38%), respectively.
Thermal separation of mortar is one of oldest and most common
postrecycling separation strategies for reducing the mortar content of RCA.
However, it has significant disadvantages with regard to efficiency as well
as environmental and economic impacts. As a result of the long periods of
heating at high temperatures, thermal separation of mortar requires rela-
tively high energy consumption with the associated undesirable environ-
mental emissions. Such negative environmental impacts tend to reduce the
perceived environmental advantages of concrete recycling. In addition, as
a result of the uniform heating of RCA in the thermal separation method,
relatively high thermal stresses are expected to be developed in the original
NA present in RCA, which could damage the stone aggregate itself. For
instance, when the NA in RCA is granite, heating the RCA at 400°C and
600°C can result in about 14% and 44% reduction in the pressure resis-
tance of the granite aggregate, respectively [32]. Therefore, the removal of
mortar from RCA through thermal separation is recommended only when
NA present in RCA is substantially stronger than the adhering mortar.
4.7.1.2 Mechanical separation
Mechanical separation methods rely on mechanical forces in the form of
rubbing and impact to separate the adhering mortar. In a method known
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