Oke, Oluwaseyi Olanrewaju
(2011)
A study on the development of guidelines for the production of bitumen emulsion stabilised RAPs for roads in the tropics.
PhD thesis, University of Nottingham.
Abstract
Eco-friendliness, energy efficiency and cost effectiveness are major drivers responsible for cold recycled asphalt mixtures being considered as alternatives to hot mixtures. Although such mixtures are still regarded in some quarters as second class asphalt, results from field trials on such materials under temperate climates have been reported to be highly impressive and encouraging. Some developed countries with temperate climates have since developed guidelines for the production and use of cold mixtures in road building. However, evidence from the literature shows that little or nothing has been done to ascertain the performance and suitability of such sustainable materials in developing countries located in hot tropical climates. Ascertaining the performances of such will, among other things enable the formulation of guidelines required for producing and using these alternative sustainable materials and methods in developing countries with hot tropical climates, where available funds for road building are increasingly inadequate to meet demand.
The work reported in this thesis attempts to simulate what should be expected in terms of the performance of flexible pavements containing cold mixes of bitumen emulsion stabilized RAP as road base in hot climates. Cold recycling in-plant was deemed appropriate for the obvious reason that it enables control of the quality of mixtures produced. The challenge of sourcing severely aged RAPs required for this study afforded the opportunity of developing a laboratory ageing protocol for producing RAPs with controlled properties, typical of those found in hot tropical belts (with residual binders of very low penetration). The result of the physico-chemical and rheological studies showed that ageing hot mix asphalt at 125⁰C does not degrade the binder when compared to that aged at 85⁰C, which is the conventional protocol (for temperate climates).
A target mix design based on Overseas Road Notes (ORN) 19 and 31 for 20mm DBM, which the literature suggests is suitable for road base layers of road pavements, yielded an aggregate gradation containing RAP (with residual bitumen of 20dmm penetration), 5mm granite dust and granite mineral filler in the proportion 65:30:5 respectively. Further investigations patterned after Marshall and Hveem mix design methods, indicated that a cationic bitumen emulsion content of 6.5% and pre-wetting water content of 1.5% were suitable. Unlike hot mixtures, cold mixtures due to their peculiarity i.e. intermediate nature (close to unbound granular materials in early life and close to fully bound materials when fully cured), require curing before being assessed for mechanical properties such as stiffness, strength etc.
Performances of the five cold bituminous emulsions mixtures (CBEMs), one with 100% virgin aggregate, the others including RAP with binder penetrations 5, 10, 15 and 20dmm, manufactured at 20⁰C and 32⁰C (to reflect average minimum and maximum temperatures in hot tropical climates) showed that:
• Properties of CBEM are dependent on the state of curing or maturation attained i.e. early life, intermediate life and fully cured or stable condition;
• High air void content in CBEMs appears to be inevitable;
• Mixing and compaction temperature is very important for achieving relatively low air void contents in CBEMs. For example, mixing and compacting CBEMs at 32⁰C gave better results than at 20⁰C;
• Indirect Tensile Stiffness Modulus is useful for quickly ranking the CBEMs;
• The RAP CBEMs performed better than the virgin aggregate CBEM in terms of water susceptibility;
• An increase in stiffness modulus up the range from 10dmmCBEM to 15dmmCBEM and to 20dmmCBEM, with higher values than the virgin aggregate CBEM as observed in this work gives the impression that the residual binder in the studied RAPs is active as a result of possible softening or rejuvenation. Alternatively, the stiffness enhancements could possibly have been caused by the alteration of the volumetrics of such RAP CBEMs which consequently enhanced their compactability;
• Overall, RAP CBEMs are better than virgin aggregate CBEM in mechanical performance and durability;
• Fatigue lives of the CBEMs are generally lower than those for hot mix asphalt (HMA);
• The CBEMs are stress-dependent as they all fitted the k-Θ model.
The results of the analytical pavement design showed the importance of using tools such as KENLAYER which account for the non linearity of CBEMs. Although the structural design was a hypothetical case, the results confirmed that the virgin aggregate CBEM was inferior in terms of axle loads to failure compared to the RAP CBEMs, and the RAP CBEMs were inferior to HMA.
The findings of this limited investigation suggest that the studied RAP CBEMs are suitable for low volume traffic roads, an indication of the great potential of these sustainable materials when properly harnessed. In the light of this, a short and concise set of guidelines for mix design of RAP CBEMs and structural design of pavements containing such non linear materials was proposed in the thesis.
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