Individual waste materials
The fresh properties (workability) of the waste mixes were assessed using the slump test as described in section 3.4. The mixes containing individual wastes materials were first tested in varying proportions and the average workability results are shown graphically in Figure 4-5. The average workability results for the mixed waste combinations are shown in Figure 4-6 of section 4.4.2, using the critical volumes identified from the compressive strength tests in section 4.6.1.
Table 4-8 shows the average slump results (refer to Appendix D for full data) for the control (0%) and waste mixes (HDPE, SCBF, BA) in proportions of 2.5%, 5%, 10%, 20% and 40%, which are plotted in Figure 4-5.
Table 4-8: Average slump results
Average slump values for waste mixes (mm)
Control = 74 mm HDPE SCBF BA
2,50% 81 76 74
5% 91 65 73
10% 117 63 70
20% 125 57 68
40% 140 55 65
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Figure 4-5: Workability (slump) of Individual waste with varying proportions
As stated in section 2.3.3, material properties and mix proportions have a significant effect on the workability of a mix. This was evident in the workability results of the waste aggregate mixes which showed fluctuating performance with varying proportions of waste and waste material types.
The conventional mix achieved a slump of 74 mm, which was within an acceptable tolerance from the 75 mm target slump and hence the control mix proportions were accepted for the study. The conventional mix is show in Figure 4-5 as 0% substitution. The waste mixes were compared to the control slump to gauge the effects of adding the wastes to a conventional mix.
An inverse relationship between SCBF fibre content and slump was noticed as the samples containing SCBF showed a general decrease in workability with increasing proportions of fibres. These findings confirm similar findings on the effect of fibres on workability by Jorillo
& Shimizu (1992), Silva & Suely (1984) and Vandewalle (2006). The SCBF mix showed a general decline in slump when compared to the conventional mix. At 5% SCBF substitution
0 20 40 60 80 100 120 140 160
0% 5% 10% 15% 20% 25% 30% 35% 40% 45%
Slump (mm)
Percentage substitution (%)
Graph of average slump with varying waste proportions
HDPE SCBF BA
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the slump of 65mm was 12% less than the conventional mix. This further decreased to 55 mm at 40 % SCBF substitution, which was 25% less than the conventional mix. A slump of 50-100 mm is generally used for general applications as stipulated in section 2.3.3 and the slump at 40% still fell within this range (The Constructor, 2014). The general decrease in workability could have been attributed to the moisture absorption of the fibres which reduced the fluidity of the mix (literature Table 2-4 on page 57 & section 4.2.4) as well as the interlocking of long fibres within the mix, as observed by Sivarja, et al (2010). There was a slight increase of 2.7% in workability relative to the control mix, noticed at 2.5% substitution.
This could have been attributed to minor variations in the sand moisture and the lack of absorption effect from the SCBF due to the low-volume substitution. It can therefore be said that the effect of SCBF is significant at substitutions above 2.5%.
The slump test results indicated that the effect of HDPE on workability was significant at both low and high volume substitutions. An almost linear increase was noticed between 2.5% and 10%, substitutions, after which the rate of change decreased. This indicated the possible effect of particle interference at high volume substitutions. The HDPE pellets increased the workability of the mix from 81 mm at 2.5% HDPE substitution, to 140 mm at 40% HDPE substitution, which meant a maximum slump increase of 89 % relative to the conventional mix. Figure 4-5, shows a direct relationship between volumetric HDPE content and workability, as the workability increased with increasing volumes of HDPE in the mix.
This could have been attributed to the hydrophobic nature and smooth surface of the HDPE pellets having poor adhesion with the cement paste thus reducing interlocking resistance (Weymouth, 1933). This confirms findings by Ahmad, et al (2008) and Ferreira, et al (2012) on the effect of smooth surface and cylindrical shapes on workability. The poor adhesion may impact on other concrete properties such as compressive strength and elastic modulus which are dependent on the transfer of load from the cement paste to the aggregates.
The bottom ash (BA) samples did show a relatively constant decrease in workability with increasing BA substitution, however not as much as the SCBF which was a more absorbent material. The slump of BA mixes compared to the conventional mix was reduced by a maximum of 12 %, to 65 mm at 40% volumetric substitution. The relatively low deviation from the control mix indicated that BA behaved similarly to the cement it replaced in terms of its effect on workability. This was possibly because the BA was sieved before use to reduce the average particle size. The porous nature of BA compared to cement was possibly why there was a general decrease in slump.
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The workability of all waste specimens were within a reasonable range of 40 mm to 150 mm for general concrete use. However, the SCBF and BA mixes at higher volume substitutions would not be recommended for thin sections or members with closely packed reinforcing.
Mixed substitutions
Figure 4-6 shows the results of the workability tests on the mixed waste combinations and critical individual volume substitutions. The mixed waste combinations were combinations of the selected wastes using the respective critical/optimum volume substitutions from the individual waste compressive strength results, as stated in section 3.1.
Figure 4-6: Workability of mixed waste combinations
When partially substituting cement in the critical volume SCBF and HDPE waste mixes with 5% of BA by volume, the mixes remained relatively workable (between 50-150mm).
However, slight reductions of 1.6% and 2.5% from the respective SCBF and HDPE individual waste mix slumps were noticed. This indicated that HDPE was more sensitive to the inclusion of BA than SCBF. When all three wastes were added together using the respective critical volume substitutions, the resulting slump increased by 2.7% when compared to the conventional mix. This was possibly due to the poor bond characteristic of the HDPE pellets. The increase in slump was however less than the HDPE mix on its own due to the absorbency properties of BA and SCBF.
74 73
81 79
63 62
76
0 10 20 30 40 50 60 70 80 90
Critical & mixed waste mixes
Slump (mm)
Graph of slump values for critical and mixed waste specimens
Control 5% BA 2,5% HDPE HDPE + BA 10% SCBF SCBF + BA HDPE+ SCBF +BA
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To conclude on workability, it was noticed that proportions of waste, as well as the material properties (absorbency, surface texture) of the waste do have an impact on the workability of the concrete, which confirms findings by Weymouth (1933). Workability is generally task specific so this may not affect the viability of the mixes as long as the concrete’s workability is suitable for the task it is intended for. The SCBF and BA mixes were still within the S2 class (refer to Table 2-13 on page 102) generally applied to most concrete applications.
HDPE ranged from a slump class of S2 to S3. S3 is generally used for thin concrete sections or trench foundations.