CHAPTER 3: METHODOLOGY AND EXPERIMENTAL DESIGN
3.5 Testing methodology and data processing
3.5.2 Mixing and Compaction
3.5.2.1 The optimum mixing moisture content
The optimum mixing moisture content (OMMC) is different from the standard OMC for BSM- emulsion mixes. In this case, it is calculated as the sum of moisture in the aggregate, plus the bitumen emulsion water content and the residual emulsion binder content. Because water and bitumen emulsion act as lubricants in BSM-emulsion mixes, the OMMC was then determined for each mix, using the following equation:
71 | P a g e ~gg ! j Equation 3.3
Where: OMMC = optimum mixing moisture content in percentage by mass of dry aggregate Wagg = moisture content of the aggregate in percentage by mass of dry aggregate;
EWC = bitumen emulsion water content in grams in percentage by mass of dry aggregate; RBC = residual bitumen content in percentage of dry mass of dry aggregate.
In the case of BSM-foam, the total OMMC is usually taken as 60 to 85% of the optimum moisture content of the dry aggregate. More often, it also corresponds to the fluff point moisture content at which the maximum bulk volume of aggregate is obtained (TG 2, 2009). The OMMC is greatly influenced by the grading of the material, especially the amount of fine fraction (smaller than 0.075 mm).
3.5.2.2 Foamed bitumen characterisation
During foaming, the temperature of bitumen and the percentage of foaming water were maintained at values of 1550 C to 1650 and 2% respectively. The foaming properties of the bitumen were measured during each production of foamed bitumen mixes. In evaluating foam characteristics, the values of the expansion ratio and half-life were in the range of 12 to 14 and 8 to 10 seconds respectively throughout this research. This gives an indication on the quality of the foamed bitumen’s produced. The foam index parameter as proposed by Jenkins et al (2000) was not used for foam characterisation in this study.
3.5.2.3 Aggregate mixing temperature
Jenkins (2000) indicated that the temperature of the aggregates before mixing has a significant influence on the equilibrium binder-mix temperature. The aggregate should be warm enough to avoid undesirable effects such as poor collapsible rate of foam due to the transfer of heat from the foam at over 1000C to cold aggregates. In the case of this research, the aggregate samples to be treated were placed in the oven overnight at a temperature of 250C the day preceding mixing, and removed the following morning just at the time of mixing with the binder. This was done in compliance with the research project specifications.
3.5.2.4 Mixing
All BSM-foam mixes were made using the Wirtgen WLB-10 S laboratory plant shown in Figure 3.8. The calibrations of the foam plant were done for each and every mix produced. During the calibration the percentage of water and bitumen rate was checked and adjusted if necessary. A percentage of
72 | P a g e water of 2% (20 grams in 10 seconds) per mass was used at a bitumen rate of 500 gram in 5 seconds. Also the expansion ratio and half-life of each mixes was checked, measured and recorded.
Figure 3. 8: Laboratory foam plant WLB 10 S with WLM 30 mixer used for foam production
Each amount of material and binder required to produce samples were mixed at high speed in mechanical mixer (vertical shaft drum mixer for BSM-emulsion and twin shaft mixer WLM 30 for BSM-foam as shown in Figure 3.9) at the moisture content indicated earlier with the addition of active fillers. For both BSM-emulsion and BSM-foam, the procedure for mixing was as follows:
• Determine the hygroscopic moisture content of the material a day before the mixing by means of standard oven drying method (TMH 1)
h ! g# ' gh g⁄ h ∗ , Equation 3.4
Where: Whyg = hygroscopic moisture content of active filler % by mass;
Mwet = wet mass of the sample of material in grams
Mdry = dry mass of the sample of material in grams
• Determine the total amount of water to be added for optimum mixing purpose
j ! "k' h Equation 3.5 gj # ! g g ∗ j/, Equation 3.6
Where: Wadd = percentage of water to be added to the material in % by mass;
Womc = optimum moisture content in % by mass;
73 | P a g e
Mwater = total mass of water to be added in grams;
MS = mass of air dry aggregate in grams;
MAF = mass of active filler in grams.
• Introduce into the mixer the required amount of aggregate determined from Equation 3.1 and Equations 3.2
Note: An extra 1500 grams of material was added to the calculated final mass of each specimen in
order to perform moisture content check afterward.
• Add the required amount of active filler, determined as a percentage per mass of material;
g! g∗ /, Equation 3.7
Where: MAF = mass of active filler in grams;
Ms = dry mass of the sample in grams
AFC = active filler content in % of dry mass of aggregate
• Mix aggregate with active fillers;
• Add while mixing the optimum mixing moisture determined using Equation 3.3
g""k! g g ∗ \, ] Equation 3.8 ""k
• Add while mixing the required amount of binder, also determined as a percentage of dry mass of aggregate.
g ! g∗ /, Equation 3.9
Where: MBC = mass of bitumen in grams;
Ms = dry mass of the sample in grams
BC = bitumen content in % of dry mass of aggregate
Note: for BSM-emulsion, the bitumen content is the residual bitumen content including water used
for the dissolution.
• Add while mixing the compaction moisture content determined as follow:
gg! g# #' g~gg Equation 3.10
Where: MCMC = mass of the compaction moisture content;
Mweter = total mass of water to be added in grams;
74 | P a g e • Mix aggregate, cement, binder and water (see Figure 3.9)
After mixing, a minimum sample of 600 g is taken from the mixer and dried to a constant mass to determine the actual moulding moisture content using Equation 3.4
Figure 3. 9: (a) twin shaft mixer WLM 30 (BSM-foam) – (b) Vertical shaft drum mixer (BSM-emulsion)
A visual inspection was done at all stages to ensure that the mixing is done well. In the case of BSM- foam for instance, a mix of poor quality could be quickly identified by noticing the presence of bitumen concentration or clumps, exposed to elongation during mixing. In this of this research only mixes of good quality were used to compact the specimens.
3.5.2.5 Compaction
The performance of a constructed pavement layer mainly depends on the compaction quality applied during construction. As said in Section 2.3.3, laboratory compaction techniques are intended to simulate the in-place density of the mix after it has endured a couple of years of traffic. Several compaction procedures can be used on bitumen stabilized materials (i.e. Impact compactions, Kneading compaction, Gyratory compaction, Vibratory compaction). In South Africa, the most used methods for the compaction of BSM’s are gyratory and vibratory compaction. In the case of this study, the vibratory Bosch hammer was used for the compaction of all specimens because of its ability to achieve density expected on the field and to emulate particles orientation after rolling (TG2 guidelines).
In compliance with the research project specification, ITS and UCS specimens were compacted in two layers, while triaxial specimens were compacted in five layers to the required height. After
75 | P a g e compaction, the top of each layer except the last layer was scarified to an approximate depth of 10 mm before the material for the next layer is added. This was done in order to have continuity in bounding of the different layers.
After mixing the material with active filler, binder and water, the steps listed below were followed for the compaction of all specimen tested throughout this research:
• Cleaning and lubrication of the inner surfaces of the mould;
• Measurement of the amount of material required for each layer and sealing in plastic bags before compaction to avoid any moisture evaporation;
• Consecutive compaction of the different specimen layers;
• For triaxial specimens and especially specimens compacted at higher densities, the compaction of the 5th layer was done after adding an extension to the mould;
• Removal of the compacted specimen from the split mould with care to avoid any distortion; • Measurement of the exact mass of the specimen;
• Measurement and recording of the height and diameter of the specimen at 3 positions of 1200 offset around the circumference of the specimen.
The specimen dimensions were as required by the terms of reference of the project 150 mm diameter by 75 mm height for ITS specimens, 150 mm diameter by 127 mm height for UCS specimens and 150 mm diameter by 300 mm height for ITS specimens
150 mm diameter by 300 mm height for triaxial specimens.
Figure 3.2: Specimens after compaction. (a) ITS Specimens, (b) UCS Specimens, (c) Triaxial Specim
Figure 3. 10: (a) ITS specimens - (b) UCS specimens - (c) Triaxial specimens