Page 79 - ICJ Jan 2023

TECHNICAL PAPER COLLECTOR’S EDITION

Table 3(c) Compressive strength of concrete In general, the compressive strength of concrete
using recycled brick masonry aggregate with recycled brick masonry aggregates, Tables
with wire-cut bricks 3(c) and (d), is considerably lower than that of

Mix w/c Compacting Mean compressive the corresponding mix of concrete with recycled
2
proportions ratio factor strength, N/mm at concrete aggregate, Table 3(b). The variation in the
7-day 14-day 28-day compressive strength between the concretes with
1:1½:3 0.50 0.86 11.95 15.50 16.30 two types of recycled brick masonry aggregates is
1:2:4 0.60 0.83 11.76 14.40 16.20 rather low. The compressive strength of recycled
brick masonry aggregate concrete, even in the case
1:3:5 0.90 0.85 7.80 10.00 10.70
of a very lean mix of 1:4:7 is of the order of 9 to 10
1:4:7 1.15 0.86 5.50 6.84 10.10 2
N/mm , which may be considered equivalent to the
compressive strength of commonly available good
Table 3(d) Compressive strength of concrete quality bricks suitable for low-rise, load bearing
using recycled brick masonry aggregate masonry construction. As the mixes 1:1½:3 and
with table moulded (country) bricks
1:2:4 result in a compressive strength in the range
Mix w/c Compacting Mean compressive 2
2
proportions ratio factor strength, N/mm at of 15 N/mm (Tables 3(c) and (d)), they can be used
for RC work. The leaner mixes are suitable for PCC
7-day 14-day 28-day
works and brick manufacture.
1:1½:3 0.50 0.87 10.13 12.26 14.61
1:2:4 0.55 0.87 9.70 12.60 14.12 In the case of concrete with fresh granite and
recycled concrete aggregate, the reduction in
1:3:5 0.60 0.83 7.80 10.74 12.20
strength is gradual as the mix becomes leaner,
1:4:7 1.27 0.86 5.44 7.70 9.12
Tables 3(a) and (b), whereas this reduction is not
pronounced in concrete with both types of recycled
The strength of recycled aggregate concrete, with brick masonry aggregates, Tables 3(c) and 3(d). Such
aggregate from concrete of unknown strength, a trend may be attributed to the higher crushing
is relatively lower than that of the concrete made value of recycled brick masonry aggregates, that
with fresh granite aggregate (Tables 3(a) and (b)), is, crushing of aggregates in concrete governs the
While the strength reduction in richer mixes of failure of cubes, thereby reducing the influence of
concrete (1:1½:3 and 1:2:4)has been observed to mix proportion.
be between 15 and 20 percent, in the case of leaner A comparison of rate of strength gain with age
mixes (1:3:5 and 1:4:7) the reduction is between 30 for each of the four mixes with fresh granite and
and 42 percent. This trend of reduction in strength different types of recycled aggregates show that
may be attributed to two reasons, viz., the rate of strength gain is relatively steeper in
the cases of fresh granite and recycled concrete
(i) the relatively higher water requirement of aggregate as compared to those of recycled brick
recycled aggregate concrete (to maintain masonry aggregates.
a uniform compacting factor) than that
of corresponding mixes of concrete with The results of second stage of investigation,
fresh granite aggregate, and wherein the influence of the strength of original
concrete (from which recycled concrete aggregate
(ii) the weaker bond between fresh mortar is taken) on different mixes of recycled aggregate
and the old mortar adhering to the concrete has been studied, is presented in Table 4.
recycled concrete aggregate (this aspect Three types of interface behaviour encountered
is further elaborated in the second stage between aggregate and mortar matrix in different
of investigation). mixes of concrete are detailed through examples.

80 The Indian Concrete Journal | November 2018