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Figure 3-6 : Overview of Mix design method used for this study
Water: Cement ratio
The water cement (W/C) ratio was read off the graph (Figure 3-7) used by the University of KwaZulu-Natal for laboratory mix designs based on the Natal Portland Cement (NPC) W/C ratio curves. For this study, a W/C ratio of 0.57 was used based on the typical range stated in section 2.3.2.2. NPC Original black 42.5N cement was utilized as it was readily available at the University of KwaZulu-Natal. The corresponding NPC Plus plot was used to read off the W/C ratio for a control mix target compressive cube strength of fcu = 35 MPa at 28 days.
The study did not aim to assess the effects of using varying cement types, which can constitute a separate research topic on its own. The study instead aimed to evaluate the effect of adding various waste materials to concrete, keeping other material constituents
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(sand, cement, and stone) constant, and subsequently assessed the relative variation in concrete properties compared to a “control” mix with no waste added. The W/C ratio was not varied either because the focus was on showing the effect of waste aggregates in varying proportions on selected concrete properties and not the effect of varying W/C ratios using concrete with waste aggregates in the mix.
Figure 3-7: Graph of W/C ratio (University of KwaZulu-Natal, 2009)
152 Water content
Based on the C&CI mix design method, a water content was selected from the recommended guideline value for a concrete mix using a 13,2 mm stone size, stated in
“Fundamentals of Concrete” (Addis, 2008).
The concept of keeping water content constant and quantifying the effect on concrete properties through variations in test results compared to a control mix, was used for this study. This method has been adopted by published researchers in alternative aggregates such as Rao (2010). The effect that the waste aggregates had on the water demand was also assessed in terms of slump and compressive strength in the moisture analysis (section 4.11).
Cement content
Using the selected water content (l/m3) and the W/C ratio, the quantity of NPC original black – CEMII B-S 42.5N cement required was calculated as show below:
Water content (l/m3) ÷ W/C ratio = volume of cement (litres) per cubic metre of concrete Stone content
A 13.2 mm stone at natural moisture state, was used for this study to reduce the variance in particle size between stone and the waste aggregates to improve particle packing. The stone content (St) for a cubic metre of concrete was then found using the formula below assuming a K factor of 0.9 as per the UKZN mix design guideline (University of KwaZulu- Natal, 2009) for moderate vibration and 13.2mm stone.
St = CBDst (K-0.1FM) [kg/m3] Where;
CBDst = compacted bulk density of stone [kg/m3] K = Factor depending on workability and nominal stone size
FM = Fineness modulus of sand
The CBD was calculated as per section 3.2.2 and the FM of the sand was calculated as per section 3.2.1. The k-factor was selected from Table 2-11 on page 101 and was based on the degree of vibration and the stone size used. A vibrating table was used for the study, so moderate vibration and 13.2 mm stone size were the input variables for the table to obtain the K-factor used in the stone content formula.
153 Sand content
River sand at natural moisture state was used for this study. The sand was passed through a large 300 mm diameter, 132 mm aperture sieve to remove any large stones that may have contaminated the stockpile. Using the absolute volume method stated in “Fulton’s Concrete Technology”, the sum of the volumes had to equate to 1m3 or 1000 litres. All calculated values were converted to volume values using the respective relative densities. The volume of sand required was then calculated from the sum total of the other constituent material volumes subtracted from 1m3 or 1000 litres. All volumes were then converted to mass by multiplying by the respective relative densities.
Adjustment of trial mix
Once a trial mix was done for the control mix, the slump and cohesiveness were assessed.
If the mix was rejected, for example, if there was a lack in cohesion or it did not comply within error margins stated in Table 2-12 on page 101, then the design was modified and re-tested by reducing stone or increasing the sand quantity. If not, then the proportions were accepted as the control mix for the study.
Volumetric waste substitution
Once the control was accepted as stated in section 3.3.6, the waste mixes were calculated based on the substitution of stone and cement from the control mix proportions.
All waste materials were used at natural moisture state except for the moisture analysis, which used waste materials at oven dry state to evaluate the effect of moisture on workability and compressive strength.
The bottom ash was passed through a 210 mm diameter, 1400 m aperture sieve to remove foreign particulate matter and obtain the ash powder. The quantities for each mix were then weighed for volumetric substitutions of (2.5%, 5%, 10%, 20% & 40%).These substitutions were selected to build upon knowledge gained from past research explained in the literature review by covering both low-volume (2.5%) to higher volume (40%) volumetric substitutions.
The quantities of each mix were easily obtained by calculating the amount of concrete required to fill the desired moulds, dividing 1 m3 by this value and then dividing the proportions of each constituent by the quotient.
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Three specimens were required per a test for the compressive, flexural and splitting tests and amount of concrete volume required per mould was:
3 cubes = 0.153 x 3= 0.010 m3
3 beams = 0.1 x 0.1 x 0.3 x 3= 0.009 m3 3 cylinders = π x 0.152 x 0.3 x 3= 0.06 m3 When batching 10% was added for spillage.
The weighed materials for each mix were then added to the concrete mixer and mixed for 5 minutes.
Figure 3-8: Drum type concrete mixer
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Figure 3-9: Beam moulds on vibrating table (left) and cube mould (right)
The moulds were oiled to allow for easy removal of concrete. The concrete was then poured into beam moulds, cube moulds and cylinder moulds for each mix design and compacted with a table vibrator.
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Figure 3-10: Specimens curing in controlled curing room
The moulds were stripped after 24 hours, the date of casting and sample number were marked with chalk and the samples were placed in curing tanks. The concrete cube and beam specimens were cured for 7 and 28 days, and the cylinders cured for 28 days at a controlled room temperature of 24°C before testing