Mechanical Performance of Asphalt Mixtures Incorporating Slag and ...

Mechanical Performance of Asphalt Mixtures Incorporating Slag and ...

MECHANICAL PERFORMANCE OF ASPHALT MIXTURESINCORPORATING SLAG AND GLASS SECONDARYAGGREGATESG.D. Airey, A.C. Collop and N.H. ThomNottingham Centre for Pavement Engineering.University of Nottingham, Nottingham, UK.ABSTRACTThe use of glass cullet as a secondary aggregate only marginally reduces the stiffness modulusof an asphalt mixture. The increased moisture susceptibility of the material is also less thanwhat would be expected for a smooth surface-textured aggregate such as glass, with andwithout the use of an anti-stripping agent. The ageing susceptibility of the mixture is significantlyreduced, while the permanent deformation resistance, although inferior to that of a primaryaggregate mixture, is still acceptable with the fatigue performance being comparable to thecontrol mixture. The use of basic oxygen steel and blast furnace slag secondary aggregatessignificantly increases the mixture density and stiffness modulus compared to primaryaggregate mixtures. The moisture susceptibility of these secondary aggregate mixtures issimilar to that of the control mixtures, although there is an increased susceptibility to agehardening. Overall the permanent deformation resistance and fatigue performance of the slagmixtures tended to be similar to that of the control mixtures.Keywords: Steel slag, blast furnace slag, glass cullet, asphalt mixture, stiffness, permanentdeformation, fatigue, NAT1. INTRODUCTIONWith a greater understanding of the need for sustainable development, the use of fresh(primary) aggregate in the asphalt mixture layers of a road or airfield pavement is seen as awasteful use of a finite natural resource. Therefore the reuse of primary aggregates and/or theuse of waste (secondary) materials are seen as being of benefit to society. Of the various wastestreams, the by-products of the iron and steel making industries (blast furnace and steel slags)and recycled crushed glass (cullet) can be considered sensible alternative sources of aggregatefor asphalt mixture production. These secondary aggregates have similar physical properties toconventional, primary aggregate and can be processed, crushed and screened into practicalsizes for easy batching into both surfacing and base asphalt materials.This paper assesses the mechanical performance and durability of a range of both base andsurfacing bituminous materials incorporating different combinations, size fractions andpercentages of two primary aggregates (limestone and gritstone) and three secondaryaggregates in the form of basic oxygen steel slag (BOS), blast furnace slag (BFS) and recycledglass cullet. The mechanical properties of the asphalt mixtures have been measured using thesuite of tests (stiffness modulus, resistance to permanent deformation and resistance to fatiguecracking) possible with the Nottingham Asphalt Tester (NAT). The durability of the primary andsecondary mixtures has been assessed by subjecting the materials to simulative long-termlaboratory ageing and moisture susceptibility conditioning using recognised testing(conditioning) procedures and protocols.Proceedings of the 8 th Conference on Asphalt Pavements for Southern Africa (CAPSA'04) 12 – 16 September 2004ISBN Number: 1-920-01718-6Sun City, South AfricaProceedings produced by: Document Transformation Technologies cc

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICA2. MATERIAL BACKGROUND2.1 Crushed Glass CulletApproximately 2 million tonnes of container glass, such as bottles and jars, are consumedannually in the UK of which 70% is coloured glass (mostly green). Of this total only 22% (mostlyclear and brown glass) is recycled (DETR, 2001). Although crushed waste glass has thepotential to be used as a fill or drainage material, there is little or no evidence that it is beingused for this purpose (Blewitt and Woodward, 2000). The major use of crushed glass in roadconstruction would therefore seem to be as an aggregate in asphalt mixtures.The use of glass cullet as an aggregate in asphalt mixtures was developed in USA in the 1960’s(Malisch et al, 1970). However, the use of this secondary aggregate has not grown significantlydue to cost, availability and performance considerations (Collins and Ciesielski, 1994). In termsof it mechanical performance, asphalt mixtures containing glass aggregate as a replacement forprimary aggregate have tended to perform slightly worse than conventional materials,depending on the replacement ratio of glass to aggregate (West et al, 1993). As glassaggregate does not absorb bitumen, stripping of glass modified asphalt mixtures is a potentialconcern, with problems being reported by Malisch et al (1970) and West et al (1993). Inaddition, ravelling of glass particles can be a serious safety concern with Maupin (1998)recommending that glass should not be used in surfacing materials. However, asphalt mixtureswith 30% crushed glass have been successfully used in field trials in the UK as base layerswithout any detrimental effect on the properties of the mixture (Nicholls and Lay, 2002).2.2 Metallurgical SlagsMetallurgical slags consist of the non-metallic secondary products of the refining of metals frommetallic ores. Slags derived from the iron and steel industries are by far the most common typesproduced and can be used as aggregates in road construction.Blast furnace slag (BF slag) is an industrial by-product produced during the manufacture of ironby chemical reduction in a blast furnace. Approximately 4 million tonnes of BF slag is producedannually in the UK and used either as construction aggregates or as a cementitious binder inthe form of ground granulated blast furnace slag (GGBS). The BF slag is formed in a continuousprocess by the fusion of limestone (and/or dolomite) and other fluxes with the ash from thecarbon source (coke) and the non-metallic components of the iron ore. The slag floats on thesurface of the molten iron and is subsequently drawn off and allowed to cool to produce asemi-dense porous crystalline material (light weight aggregate) known as air cooled blastfurnace slag.In terms of its physical properties, BF slag is made up primarily of silicates and alumino-silicatesof calcium and magnesium together with other compounds of sulphur, iron, manganese andother trace elements (Dunster, 2002). It is as consistent in its physical properties as would beexpected for a natural aggregate and when crushed and screened produces an aggregate witha rough surface texture and relatively high porosity resulting in good adhesive characteristicswith bituminous binders. BF slag is therefore recognised as having excellent resistance tobinder stripping in asphalt mixtures as caused by the combined actions of water and traffic.However, due to its vesicular surface and high water absorption ratio, larger amounts ofbituminous binder may be required to produce an asphalt mixture containing BF slag (Emery,1982).Basic oxygen steel slag (BOS) is an industrial by-product of the steel making process withapproximately 1 million tonnes being produced annually in the UK. BOS slag is produced in abatch process when iron is converted to steel. The slag is produced by blowing oxygen intomolten iron mixed with additional fluxes and recycled steel scrap. The process refines the ironby fusion with a flux, such as limestone or dolomite, under oxidising conditions. Impurities fromthe iron, such as carbon and silicon, are either oxidised to gases or chemically combined withthe slag. Steel slag is also produced in much lower quantities as electric arc furnace (EAF) steelPaper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICAslag during the production of more specialist steels in an electric induction furnace. Both typesof slag are processed into construction aggregates in a manner similar to that used for BF slaginvolving the removal of the liquid slag that floats on the surface of the molten refined steel andits subsequent cooling.BOS slag consists primarily of calcium silicates together with oxides and compounds of iron,manganese, alumina and other trace elements. It is relatively non-porous and produces ahigh-density aggregate with high crushing strength. BOS slag is denser and stronger than BFslag and delivers a high resistance to abrasion and polishing under traffic when used in asphaltsurface layers (Lee, 1968; Dunster, 2002; Lee, 1975). However, steel slag does have thepotential to undergo volumetric expansion in the presence of water due to a reaction of theoxides of calcium and magnesium (lime and magnesia) in the steel slag with water (Emery,1982; Garvin, 1999). Expansion problems in service can be avoided by subjecting BOS slag toa long period of natural weathering in exposed stockpiles for at least a year before it is used asa secondary aggregate (Rockliff et al, 2002).The most significant difference between BOS slag and most natural aggregates is its highparticle density, which is a consequence of the presence of iron compounds in the slag. Asphaltmixtures produced using steel slag aggregates will display higher density values and generallygreater stability and stiffness values compared to conventional, primary aggregate material(Noureldin and McDaniel, 1990). Current practice recommends the use of blast furnace slag fineaggregate together with steel slag coarse aggregate to compensate for the high particle densityof steel slag.Steel slag aggregates have been used successfully in asphalt surfacing mixtures as well asbase layers in Europe, Australia, Canada and the USA (Emery, 1982; Lee, 1975; Ryell et al,1979; Kandhal and Hoffmann, 1997). Steel slag modified asphalt mixtures have tended toperform extremely well with no adverse durability problems. In addition, the surfacing materialshave shown improved skid resistance compared to conventional aggregate material. Concernsover potential expansion of steel slag aggregates have generally not materialised as thebitumen film coating the aggregate particles limits water ingress and therefore expansion.3. EXPERIMENTAL PROGRAMME3.1 MaterialsTwo asphalt mixture types (gradations) were selected to investigate the performance of glasscullet and slag (BOS and BFS) secondary aggregates in modified bituminous mixtures:1. mm size dense base asphalt mixture (DBM) as specified in BS 4987-1:2001, Table 3, usinga 50 penetration grade bitumen;2. mm stone mastic asphalt (SMA) wearing course asphalt mixture as specified in prEN131018-5:2000, using a 50 penetration grade bitumen and cellulose fibres.The two asphalt mixtures were selected to represent a typical UK base and wearing course(surfacing) material. Table 1 shows the detailed sieve size gradations of the DBM and SMAmixtures.The following primary and secondary aggregates, in various combinations, were used toproduce the above asphalt mixtures:• Gritstone aggregate (Bayston Hill) with a SG of 2.76;• Limestone aggregate (Ballidon) with a SG of 2.7;• Limestone filler (Ballidon) with a SG of 2.7;• Steel slag (Llanwern) with a SG range of 3.0 to 3.27;Paper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICA• Blast furnace slag (Port Talbot) with a SG range of 2.36 to 2.47;• Glass cullet with a SG of 2.5.As the specific gravity of the aggregates differed considerably, the DBM and SMA mixtures weredesigned volumetrically rather than gravimetrically to ensure that the mechanical properties ofthe primary and secondary aggregate asphalt mixtures were a function of aggregate type ratherthan changes in the volumetric proportions of the mixture.Table 1. Aggregate grading of 28 mm DBM and 14 mm SMA.Sieve Size (mm)37.5282014106.33.352.361. mm DBM Specification 14 mm SMA SpecificationSpecification Limits – Percentage Passing (%)10090 – 10071 – 9510058 – 8290 – 10052 – 7260 – 7544 – 6040 – 5032 – 4623 – 3328 – 4219 – 3021 – 3516 – 2614 – 2814 – 227 – 2112 – 194 – 1510 – 162 - 97 - 12The six primary and secondary aggregates and fillers, together with the two asphalt mixturegradations and one binder type, were used to produce the following six asphalt mixturecombinations (two control and four secondary aggregate mixtures) as described in Table 2.Approximately 30 cylindrical specimens (100 mm diameter by 60 mm height) were produced foreach of the six asphalt mixtures with two sets of 10 specimens being subjected to simulativelaboratory asphalt mixture ageing and moisture susceptibility conditioning after their densitieswere determined and prior to mechanical property testing. The long-term oven ageingprocedure consisted of force-draft oven ageing of compacted asphalt mixture specimens at85°C for 120 hours (AASHTO PP2). The moisture conditioning procedure consisted ofsubjecting compacted asphalt mixture specimens to saturation under a partial vacuum of510 mm Hg at 20°C for 30 minutes, followed by saturation at atmospheric pressure at 60°C for6 hours, and saturation at atmospheric pressure at 5°C for 16 hours (Scholz, 1995). Thesamples are finally conditioned under water at 20°C (atmospheric pressure) for 2 hours prior tonon-destructive stiffness testing. The moisture susceptibility of the asphalt mixture is thenexpressed as the ratio of conditioned to unconditioned stiffness modulus values.As the asphalt mixtures were designed with a constant binder content by mass, the differencesin specific gravity of the primary and secondary aggregates meant that the volumetric bindercontents differed slightly. The higher SG of the BOS slag (compared to the two primaryaggregates) resulted in higher volumetric binder contents for asphalt mixtures incorporatingsteel slag aggregate while the mixtures containing glass cullet, with its lower SG, had lowervolumetric binder contents. The differences between the slag and glass mixtures wereminimised by using low SG blast furnace slag fine aggregate with the high SG steel slag toreduce the combined bulk specific gravity of mixed aggregate. In addition, the asphalt mixtureperformance will be determined by the effective volumetric binder content and not simply by thedesign volumetric binder content. The effective binder content takes into account the amount ofbinder that is absorbed by the aggregate and therefore not available in the bulk mixture. Theeffective volumetric binder contents (V beff ) for the six asphalt mixtures are included in Table 2.Paper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICAMixtureDBMConDBMGlassTable 2. Primary and secondary aggregate DBM and SMA asphalt mixtures.CoarseAggregateFineAggregateAddedFillerM b (%) V beff (%) Target V v(%)57% limestone 41% limestone 2% limestone 4.0 9.30 4.045% limestone 50% glass 5% limestone 4.0 9.18 4.0DBM 45% limestone 50% glass 5% limestone 4.0 9.18 4.0Glass 1DBMSlag54% BOS 43% BFS 3% limestone 4.0 8.62 4.0SMA 74% gritstone 19% gritstone 7% limestone 6.0 14.44 3.0ConSMA 71% BOS 21% BFS 8% limestone 6.0 14.26 3.0Slag1 0.3% anti-stripping agentThe mechanical properties of the primary and secondary aggregate asphalt mixtures weremeasured using the NAT, developed in the mid 1980’s at the University of Nottingham (Cooperand Brown, 1989).The stiffness modulus of the primary and secondary mixtures was measured using the IndirectTensile Stiffness Modulus (ITSM) test according to British Standard DD213 (BSI, 1993):• Test temperature: 20°C;• Loading rise-time: 124 milliseconds;• Peak transient horizontal deformation: 5 µm; and• Assumed Poisson’s ratio: 0.35.The permanent deformation resistance of the different asphalt mixtures was determined bymeans of the Confined Repeated Load Axial Test (CRLAT) using a direct uniaxial compressionconfiguration according to British Standards DD185 (BSI, 1994):• Test temperature: 40°C,• Test duration: 7200 seconds (3600 cycles) with a load pattern 1 second loading on (loadapplication period) followed by one second off (rest period),• Axial stress: 100 kPa,• Confining pressure: 50 kPa, and• Conditioning stress: 10 kPa for 600 seconds.The fatigue resistance of the asphalt mixtures was determined by means of the Indirect TensileFatigue Test (ITFT) with an experimental arrangement similar to that used for the ITSM butunder repeated loading and with slight modifications to the testing modulus crosshead.Paper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICAStiffness Modulus (MPa)100008000600040002000Ratio = 1.45UnagedAged Ratio = 1.216642 6219516145740DBM Con MB = 4.0% Vv = 4.71% DBM Glass MB = 4.0% Vv = 3.03%Figure 1. Stiffness modulus results for unaged and aged DBM control and glass asphalt mixtures.Table 4. Stiffness modulus ratios after moisture conditioning for glass cullet mixtures.MixtureStiffness (MPa) – CoV / Stiffness Ratio – CoVInitial Cycle #1 Cycle #2 Cycle #3 Cycle #4DBM Con 4517 (12%) 5355 (13%)1.19 (3%)5256 (11%)1.16 (4%)5151 (10%)1.14 (6%)5057 (14%)1.12 (4%)DBM Glass 4425 (9%) 4785 (7%)1.08 (6%)4575 (9%)1.03 (6%)4377 (10%)0.99 (7%)4283 (12%)0.97 (8%)DBM Glass 1 4278 (7%) 4697 (7%)1.10 (5%)4490 (5)1.05 (6%)4534 (6%)1.06 (4%)4423 (8%)1.03 (6%)1 0.3% anti-stripping agent4.2 Permanent Deformation PerformanceThe permanent deformation results for the control and glass DBM asphalt mixtures arepresented in the form of cumulative permanent strain versus load cycles in Figure 2 and in Table5 as measures of total strain (%) after 3600 cycles and average strain rate (microstrain/cycle)between 1800 and 3600 cycles based on the average of five test results. The permanentdeformation properties have been determined on unaged as well as moisture conditioned (fourcycles) specimens.The glass cullet mixtures showed a lower resistance to permanent deformation compared to thelimestone control mixture, although their total strains were still all below 2% (a commonly usedacceptance criteria). In addition, the performance of the secondary aggregate mixtures aftermoisture conditioning was similar to that seen before conditioning with the limestone + glassmixture even showing an improvement in rutting resistance after moisture conditioning.Paper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICA2DBM Con DBM Glass DBM Glass + additiveAxial Strain (%)1.510.500 900 1800 2700 3600Number of Load CyclesFigure 2. Accumulated permanent strain results for glass cullet asphalt mixtures.Table 5. Permanent deformation parameters for glass cullet asphalt mixtures.MixtureUnconditionedMoisture ConditionedTotal strain (%) Strain rate(µε/cycle)Total strain (%) Strain rate(µε/cycle)DBM Con 0.63 0.37 0.48 0.17DBM Glass 1.51 1.01 1.00 0.53DBM Glass 1 1.42 0.78 1.38 0.871 0.3% anti-stripping agentThe greater rutting susceptibility of the limestone + glass mixtures can be attributed to thesmooth surface texture of the glass aggregate resulting in less aggregate interlock and surfacefriction. It is also worth noting that the use of an anti-stripping agent has not improved thelong-term moisture conditioned rutting resistance of the asphalt mixture.4.3 Fatigue PerformanceFatigue life versus initial tensile strain relationships have been determined for the control andglass DBM asphalt mixtures in their unaged, aged and moisture conditioned states. Althoughunique fatigue relationships have been determined for each of the mixtures, the fatiguebehaviour for the control and glass modified materials are all very similar. For this reason asingle, unique fatigue life versus strain relationship has been determined from the ITFT datagenerated for the six combinations of mixture type and conditioning state.The combined fatigue relationship is shown in Figure 3 together with the fatigue equation andR 2 value of 0.8692. The fatigue results show that based on one binder type, approximately thesame effective volumetric binder content, similar air void content and identical volumetric DBMgradation, fatigue performance is relatively independent of aggregate source and type (primaryand secondary).Paper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICA1000y = 1158.6x -0.2154R 2 = 0.8692Strain (microstrain)10010DBM Con - Uncon DBM Con - Aged DBM Con - MoistDBM Glass - Uncon DBM Glass - Aged DBM Glass - Moist100 1000 10000 100000Number of CyclesFigure 3. Fatigue functions for unconditioned, moisture conditioned and agedglass cullet asphalt mixture.5. SLAG MIXTURES5.1 Stiffness ModulusThe volumetric properties together with stiffness modulus results for the control and slag DBMand SMA mixtures are presented in Table 6. As would be expected the slag mixtures havehigher bulk densities compared to the control limestone DBM and gritstone SMA mixtures butwith the average air voids all being reasonably close to their respective targets of 4.0% and3.0%. The effect of using steel and blast furnace slag aggregate has resulted in a substantial(over 20%) increase in stiffness compared to the control mixtures. Part of this increase instiffness is inevitably associated with the lower effective volumetric binder content of the slagmixtures (8.62% compared to 9.30% for the DBM mixtures and 14.26% compared to 14.44% forthe SMA mixtures as presented in Table 2), although the chemical and physical surfaceproperties of the BOS and BF slags will also have a significant effect on mixture stiffness.Table 6. Volumetric and stiffness modulus results for slag mixtures.Mixture Density (kg/m 3 ) Air Voids (%) Stiffness (MPa)Average CoV Average CoV Average CoVDBM Con 2376 0.6% 4.32 13% 4587 10%DBM Slag 2596 0.9% 4.41 20% 5617 10%SMA Con 2420 0.4% 2.44 16% 3413 6%SMA Slag 2637 0.7% 2.84 22% 4358 7%The effect of moisture conditioning on the stiffness modulus results of the control and slagasphalt mixtures can be seen in Table 7 in terms of the actual stiffness results and as a ratio ofconditioned to unconditioned (initial) stiffness. All four asphalt mixtures show an initial increasein stiffness modulus after moisture conditioning followed by a gradual decrease in retainedstiffness with increasing conditioning cycles.Paper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICATable 7. Stiffness modulus ratios after moisture conditioning for slag mixtures.MixtureStiffness (MPa) – CoV / Stiffness Ratio – CoVInitial Cycle #1 Cycle #2 Cycle #3 Cycle #4DBM Con 4517 (12%) 5355 (13%)1.19 (3%)5256 (11%)1.16 (4%)5151 (10%)1.14 (6%)5057 (14%)1.12 (4%)DBM Slag 5581 (10%) 6156 (7%)1.10 (3%)5708 (8%)1.02 (3%)5233 (8%)0.94 (4%)4979 (8%)0.89 (4%)SMA Con 3520 (5%) 3897 (10%)1.11 (11%)3577 (12%)1.02 (11%)3397 (18%)0.96 (18%)3198 (21%)0.91 (20%)SMA Slag 4279 (9%) 4635 (12%)1.08 (5%)4592 (13%)1.07 (6%)4460 (15%)1.04 (9%)4500 (15%)1.05 (9%)In terms of the four mixtures, the best performance (highest retained stiffness after 4 cycles)belonged to the limestone control mixture, although the two slag mixtures with limestone filler(DBM Slag & SMA Slag) both performed extremely well with the retained stiffness values of theSMA slag mixture being greater than the control SMA gritstone mixture, which had a retainedstiffness of 91% after four conditioning cycles only marginally higher than the 89% of the DBMslag mixture.The laboratory aged stiffness properties of the control and slag mixtures are presented in Figure4. All four asphalt mixtures demonstrated an increase in stiffness modulus after ageing with theratio of aged to unaged stiffness being highest for the DBM slag (1.94) and SMA slag (1.68)mixtures. The increased hardening (oxidation) of the slag mixtures compared to the controlswas verified by recovering the binder from the two DBM mixtures and measuring their complexmodulus at 20°C and 1.2 Hz using a dynamic shear rheometer (DSR). The ratio of complexmodulus for the steel slag + BFS mixture’s recovered binder (3.745 MPa) versus the limestonemixture’s recovered binder (2.890 MPa) was then compared to the ratio of slag DBM hardening(1.94) versus limestone DBM hardening (1.45). The two ratios were almost identical, being 1.30for the ratio of recovered binder complex moduli and 1.34 for the ratio of relative mixturehardening. This compared favourably with the ratio of 1.68/1.29 (30%) found for the two SMAmixtures. The precise cause of the increased ageing of the slag mixtures is unclear, although itis probably linked to the chemical composition of the slag aggregate, which may act as acatalyst for excessive oxidative hardening of the bitumen.Stiffness Modulus (MPa)1400012000100008000600040002000UnagedAgedRatio = 1.4545746642Ratio = 1.94543010551Ratio = 1.2934524269Ratio = 1.68422370950DBM Con MB =4.0% Vv = 4.71%DBM Slag MB =4.0% Vv = 5.10%SMA Con MB =6.0% Vv = 2.58%SMA Slag MB =6.0% Vv = 2.59%Figure 4. Stiffness modulus results for unaged and aged control and slag asphalt mixtures.Paper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICA5.2 Permanent Deformation PerformanceThe permanent deformation results for the control and slag DBM and SMA asphalt mixtures arepresented in the form of total strain (%) after 3600 cycles and average strain rate(microstrain/cycle) between 1800 and 3600 cycles in Table 8. In terms of relative permanentdeformation performance, the slag mixture (DBM Slag) showed the lowest total strain as well asstrain rate compared to both the limestone control mixture (DBM Con) and gritstone controlmixture (SMA Con). In addition, the performance of all the secondary aggregate mixtures aftermoisture conditioning was similar to that seen before conditioningTable 8. Permanent deformation parameters for slag asphalt mixtures.MixtureUnconditionedMoisture ConditionedTotal strain (%) Strain rate(µε/cycle)Total strain (%) Strain rate(µε/cycle)DBM Con 0.63 0.37 0.48 0.17DBM Slag 0.45 0.34 0.46 0.23SMA Con 0.83 0.32 1.09 0.45The superior rutting resistance of the steel slag + BFS mixtures can be attributed to the roughand vesicular surface texture of the BOS and BF slags, resulting in greater aggregate interlockand overall permanent deformation performance.5.3 Fatigue PerformanceAs with the DBM control and glass mixtures, the SMA gritstone control and steel slag mixtureshave been subjected to fatigue testing using the ITFT in their unaged, aged and moistureconditioned states. All six asphalt mixture and conditioning combinations have produced similarfatigue functions (similar experimentally determined coefficients) and therefore a singlecombined fatigue function has been produced for both mixture types in Figure 5 with a R 2 valueof 0.8946.1000Strain (microstrain)100y = 2055.5x -0.2422R 2 = 0.894610SMA Con - Uncon SMA Con - Aged SMA Con - MoistSMA Slag - Uncon SMA Slag - Aged SMA Slag - Moist100 1000 10000 100000Number of CyclesFigure 5. Fatigue functions for unconditioned, moisture conditioned andaged slag asphalt mixture.The results indicate that the fatigue performance of both the primary aggregate as well as thesecondary aggregate asphalt mixtures are similar, although the actual fatigue data points for theslag mixture (SMA Slag) tend to lie marginally below the general fatigue function.Paper 056

8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICA6. CONCLUSIONSTwo secondary aggregate combinations have been assessed in this laboratory study. Thesecombinations have consisted of coarse primary aggregate + fine glass cullet with and without ananti-stripping agent and steel slag coarse aggregate + blast furnace slag fine aggregate withlimestone filler. The performance of these secondary aggregate mixtures has been compared tothat of conventional, primary aggregate mixtures using both a base material (28 mm DBM) anda surfacing material (14 mm SMA). All the mixtures have been designed to have similarvolumetric proportions within each mixture type and the mechanical properties of stiffnessmodulus, resistance to permanent deformation and resistance to fatigue cracking have beendetermined in the material’s unaged, aged and moisture conditioned states.The use of glass cullet fine aggregate has been limited to the base material due to concernsover road safety associated with the possible ravelling of glass particles in surfacing materials.The overall effect of replacing primary aggregate with glass cullet is a slight reduction in asphaltmixture stiffness relative to the control limestone mixture. Although the moisture susceptibility ofthe glass aggregate mixtures was greater than the control mixture, probably due to the smoothsurface texture of the glass, the retained stiffness values after long-term moisture conditioningwere still considered acceptable. In addition, the use of an anti-stripping agent did improve thematerial’s resistance to moisture damage. In terms of the materials’ ageing resistance, theaddition of glass cullet significantly reduced the ageing susceptibility of these modified mixtures.The permanent deformation performance of the glass cullet mixtures was found to be inferior tothat of the control mixture. The greater rutting susceptibility of the glass aggregate mixtures canbe attributed to the smooth surface texture of the glass aggregate resulting in less aggregateinterlock and surface friction which has not been improved by the addition of an anti-strippingagent. However, the rutting performance could still be considered to be satisfactory as the finalpermanent strains were still below an acceptable limit. Finally the fatigue performance of thelimestone + glass aggregate mixture was found to be comparable to that of the control mixture.The BOS and BF slag combination has been used to produce both the base material as well asthe surfacing material. Overall the use of BOS and BF slag has increased the density as well asthe stiffness modulus of both the base and surfacing materials relative to primary aggregatemixtures. The increase in stiffness is partly due to the slightly lower effective volumetric bindercontent of the slag mixtures and partly due to the chemical and physical surface properties ofthe slag aggregate. With regard to the moisture susceptibility of the slag mixtures, theperformance in terms of retained stiffness modulus is comparable to that of the DBM and SMAcontrol mixtures. However, the slag mixtures do show an increased susceptibility to agehardening when subjected to long-term laboratory ageing with a 30% larger increase in stiffnessmodulus after ageing compared to the control mixtures.The permanent deformation performance of the DBM slag showed a high degree of resistanceto permanent deformation compared to the control. As with the glass cullet mixtures, the fatigueperformance of the slag mixtures was found to be comparable to that of the control mixtures.7. REFERENCESBlewitt, J. and Woodward, P.K., 2000. Some Geotechnical Properties of Waste Glass.Ground Engineering.British Standards Institution, 1993. Method for Determination of the Indirect TensileStiffness Modulus of Bituminous Mixtures. DD 213, BSI, London.British Standards Institution, 1994. Method for Assessment of Resistance to PermanentDeformation of Bitumen Aggregate Mixtures Subject to Unconfined Uniaxial Loading. DD185, BSI, London.Paper 056

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