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TECHNICAL PAPER

table 5: part three: Mix proportion of recycled aggregate concretes containing nano silica and ultra fine
fly ash

MIx DEsIgnAtIon MIx ProPortIons In kg/m 3 slUMP
(mm)
oPc UltrA fInE fly nAno ncA rcA nfA wAtEr sUPErPlAs-
Ash (UffA) sIlIcA (ns) tIcIzEr
Control 430 - - 1200 - 650 172 - 100

50 RCA 430 - - 600 600 650 172 - 90
50 RCA+2 NS 421 - 8.6 600 600 650 172 4.3 95
50 RCA+ 10 UFFA 387 43 - 600 600 650 172 3.7 110

4. CaSting, Curing anD tESting the fly ash due to its spherical shape. Also, since a by weight
replacement of slag and fly ash was used and slag is less
All concretes were mixed in a pan mixer using a constant dense than OPC there is more paste volume in the mixes,
water-to-binder ratio of 0.40. The mixing sequence of the which improves the workability. It can also be seen the effect
concretes was as follows: both fine and coarse aggregates of SF on the workability of recycled aggregates concretes
were mixed dry for 2–3 minutes, OPC and other binders and containing high volume fly ash and slag. In order to maintain
one third of the water required were then added to the dry mix the slump value between 80 and 120 mm, superplasticizer
and mixed for another 3 minutes followed by the addition of dosage is increased with increase in SF contents, because
the remaining two-thirds of the water and mixing of the whole due to very high surface area and smaller sizes of SF particles
mix for a further 3 minutes. The reason for adding one third of the workability of the mixes is reduced. It can also be seen in
the water at the second stage was to minimize the dust during Table 5 that superplasticizer is used in the recycled aggregate
dry mixing. Slump tests were conducted immediately after concrete containing NS and UFFA to improve the workability
mixing the concrete to measure the workability of each mix. At in order to achieve proper compaction during casting of the
least three specimens were cast and tested in each series. All specimens.
specimens were water-cured until the day before the test dates.

6. CoMprESSiVE StrEngtH
The compressive and indirect tensile strengths were measured
at 7, 28, 56 and 91 days in each mix. The compressive strength Figure 4 shows the compressive strength results of all 14 types
test was carried out on ø100 × 200 mm cylinders and the of concretes measured at 7, 28, 56 and 91 days. It can be seen
indirect tensile strength on ø150 × 300 mm cylinders. The in Figure 4a that the partial replacement of NCA by 50% RCA
compression and tensile tests were conducted in accordance significantly reduced the compressive strength of control
with the relevant Australian standards (AS1012.3.1 and concrete at all ages, which is also reported in other studies
AS1012.10). and is due to the presence of additional pores in old mortars
in the RCA and fissures formed during crushing of the RCA.
The effects of 50% fly ash and 50% slag on the compressive
5. rESultS anD DiSCuSSion strength of concrete containing 50% RCA are also shown in the
same figure and it can be seen that the compressive strength
workability
at all ages is decreased by 22-44% due to addition of 50% fly
The measured workability in terms of slump values of all mixes ash and by 6-35% due to addition of 50% slag. It can be seen
are shown in Tables 3-5. As expected, the workability of the that the 7 days compressive strength is significantly decreased
recycled aggregate concrete containing 50% RCA is reduced. by 44% and 35% due to addition of 50% fly ash and 50% slag,
The reduction in workability is caused by the high angularity respectively compared to 37% and 11% reduction at 28 days. At
of RCA and high content of fines in the RCA. The possible later ages, e.g. at 56 and 91 days, the gap is reduced gradually.
formation of smaller particles due to breaking of adhered old However, recycled aggregate concrete containing 50% fly ash
mortars during mixing of concrete in the mixer might have showed higher strength loss than that containing 50% slag and
also increased the smaller size aggregates and as a result total is due to slower pozzolanic reaction of fly ash than slag and the
surface area of aggregates in increased and the workability presence of lower amount of Calcium Oxide (CaO) in fly ash
is decreased. As can be seen, the introduction of fly ash and than in slag. Significant reduction in compressive strength of
slag improves the workability of recycled aggregate concrete concrete containing high volume fly ash compared to control
mix, and this can be attributed to the lubricating effect of concrete containing 100% OPC is reported in many studies.

The IndIan ConCreTe Journal | SepTember 2019 59