Non-conventional mix for ultrathin asphalt pavements in South Africa

Abstract

Large quantities of natural materials are traditionally used in road construction, which leads to depletion of non-renewable natural resources. Concurrently, the world faces the problem of management of an increasing quantity of waste so that linking the two issues leads to a simple solution: growing and more diverse application of waste materials in road building and other areas of civil engineering alike.

Replacement of natural soils or minimisation of the use is desirable. An industrial by-product may be inferior to the traditional materials used in road pavement construction. However, the lower cost makes the by-products an attractive alternative if the required performance can be achieved. It is in the context of this study to produce results to further motivate the use of byproducts, such as Fly Ash and Chrome Slag, in road construction.

Detailed asphalt mix design and investigations were completed to evaluate the use of Fly Ash and Chrome Slag as a suitable replacement in an asphalt mixture to reduce landfill sites and, in effect, become an environmental option for road construction in South Africa. The standards in the study were combined to form a basic step by step evaluation process that can be used by designers for in-depth evaluation of the use of Chrome Slag and Fly Ash in an asphalt mixture. The design applied to ultrathin asphalt thicknesses of 20mm and is aimed at urban and rural roads. The Fly Ash in the study was used to aid with the grading elements of the Chrome Slag and for the use as filler. The gradation was developed by using a combination of standards to get an optimum grading for the designs. The Chrome Slag and Fly Ash combination showed a better density packing characteristic than the Dolerite, Natural Aggregate.

Environment is a concern when using Fly Ash and Chrome Slag in construction projects. To evaluate the potential harmful effects to the environment, the Fly Ash samples, and Chrome Slag were subjected to leaching tests. The results were compared to drinking water maximum allowable elements also found in Fly Ash and Chrome Slag with possible health effects. The Fly Ash with no treatment shows that leached elements namely: Ba, Cr, Pb are of a concern once the elements have leached into the groundwater. The Chrome Slag aggregates showed that the leached elements of concern were namely: Ba, Se, and Cu. Chemical analysis on various (FeCr) Chrome Slag samples have detected traces of heavy metals such as chromium(IV) oxide, which is very toxic and leachable. The Leaching results on the asphalt mixtures, have shown that the Fly Ash and Chrome Slag elements were “entombed” and the possibility of leachant releasing agents of a dangerous nature are to a minimal.

Volumetric design still remains the basis of this study. Marshall Stability and Flow, results was used to calculate the envisaged bearing capacity of the asphalt mixture which showed the % binders have psi that varies between 63psi and 108psi for the Chrome Slag on 50/70 binder and between 49psi and 126psi for the Natural Aggregate on the 50/70 binder. The AE-2 binder shows 87psi to 116psi for the Chrome Slag and between 104psi to 138psi for the Natural Aggregate. The VMA proved a confidence decision on the Natural Aggregate, but designers should be more aware of the Chrome Slag and Fly Ash mixtures as the VMA is on the low spectrum of the results and on the limits of the minimum required values. The Natural Aggregate conforms well above the requirements, while for the Chrome Slag and Fly Ash mixture on design voids the readings are low with 14.6 at 6% binder for AE-2 and 13.7 at 5.5% binder for the 50/70. The voids at the binder percentages for the Chrome Slag and Fly Ash mixtures are 3.4 and 3.6 respectively. The VFB has shown that the Chrome Slag and Fly Ash mixture, like the VMA, is very unpredictable. The results do conform to the minimum standard of between 70 and 80, but the voids are below 4% with 3.2% voids for the AE-2 mixture. The 50/70 binder proved more reliable at 4.3% voids. The Natural Aggregate conformed to the specifications at 5.5% AE-2 binder and at 5% 50/70 binder respectively.

Specialised testing was completed on the asphalt mixtures namely; MMLS, Gyratory, Modified Lottman, ITS and HWTT. The MMLS results have indicated ruts of between 3.26mm to 4.65mm for 50/70 binder and between 5.25mm and 5.35mm for AE-2 binder. The gyratory test has indicated voids after 300 gyratory of between 2,4% and 5.5% for the Natural Aggregate and 1.3% to 3.4% for the Chrome Slag and Fly Ash mixtures. The modified Lottman tests have shown that both Natural Aggregate and Chrome Slag have conformed to the test results with results above the minimum required in wet regions of 0.8. The results were all above 0.9. The ITS results were very favourable in showing cohesiveness. The results are indicative of toughness and durability and rutting resistance. On the 50/70 binder, the Chrome Slag shows a good ITS value at 6% binder, while the Natural Aggregate shows acceptable value at 5.5% binder. The AE-2 shows very good results with all results above 1100kPa range. The Hamburg test (HWTT) showed variable results for both Chrome Slag and Natural Aggregate mixtures. It can be that the thick compacted briquettes for the testing procedures are not suitable for the 10mm maximum size particles in the mixture. The increase in thickness for both MMLS and Hamburg have indicated that rutting is a concern with both products and binders thus the designer will have to apply further investigation research to conclude design for the required thin asphalt. The study has indicated that the Chrome Slag and Fly Ash mixture does conform to most requirements and is a viable option as a suitable replacement for the Natural Aggregate for the application in rural/urban roads as an ultrathin asphalt mixture.