RUBBER MODIFIED BITUMINOUS BINDERS

Abstract

A method of re-stiffening an over-digested rubber modified bituminous binder includes admixing a reactive, stiffness inducing organic additive with the over-digested rubber modified bituminous binder to produce a re-stiffened rubber modified bituminous binder with an increased softening point. The admixing takes place at an elevated temperature of at least 185 C. A rubber modified bituminous binder suitable for both asphalt and seal surfacing applications that includes a digested rubber bituminous admixture with a softening point of at least 55 C. and a dynamic viscosity at 190 C. of less than 2000 mPa.Math.s. A method of sealing or paving a surface includes applying a layer which includes a rubber modified bituminous binder to the surface, the rubber modified bituminous binder being in the form of a digested rubber bitumen admixture with a softening point of at least 55 C. and a dynamic viscosity at 190 C. of less than 2000 mPa.Math.s.

Claims

1. A method of re-stiffening an over-digested crumb rubber modified bituminous binder, wherein the over-digested crumb rubber modified bituminous binder is a crumb rubber modified bituminous binder with a viscosity at 190 C. of less than 2000 mPa.Math.s and/or with a softening point of less than 55 C., the method comprising: Admixing a reactive, stiffness inducing organic additive with the over-digested crumb rubber modified bituminous binder to produce a re-stiffened overdigested crumb rubber modified bituminous binder with an increased softening point, wherein the admixing takes place at an elevated temperature of at least 185 C., wherein the stiffness inducing organic additive and the over-digested crumb rubber modified binder are admixed in a mass ratio of between 0.5:99.5 and 2.0:98.0, and wherein the reactive, stiffness inducing organic additive is a terpolymer of ethylene, an alkyl acrylate and glycidyl methacrylate.

2. The method according to claim 1, wherein -the over-digested crumb rubber modified bituminous binder is completely over-digested so that the over-digested crumb rubber modified bituminous binder has a viscosity approaching that of a base bituminous binder of the crumb rubber modified bituminous binder, once the viscosity has dropped below 2000 mPa.Math.s, and/or so that the over-digested crumb rubber modified bituminous binder has a softening point approaching that of the base binder, once the softening point has dropped below 55 C.

3. The method according to claim 2, further comprising: digesting the over-digested crumb rubber modified bituminous binder for a period of time sufficient to ensure that the over-digested crumb rubber modified bituminous binder is completely over-digested prior to admixing the reactive, stiffness inducing organic additive with the over-digested crumb rubber modified bituminous binder.

4. The method according to claim 1, wherein the reactive, stiffness inducing organic additive is reactive so that reactions between one or more components of the reactive, stiffness inducing organic additive with functional groups present in the over-digested crumb rubber modified bituminous binder take place 7 to produce a re-stiffened overdigested crumb rubber modified bituminous binder with an altered composition and an increased softening point.

5. The method according to claim 1, wherein the reactive, stiffness inducing organic additive is reactive so that reactions between one or more components of the reactive, stiffness inducing organic additive with functional groups present in the over-digested crumb rubber modified bituminous binder take place to produce a re-stiffened overdigested crumb rubber modified bituminous binder with an improved J.sub.nr, and/or an improved storage stability and/or an improved stress resilience.

6. The method according to claim 1, wherein the admixing is performed at an elevated temperature of between 185 C. and 210 C., and/or wherein the admixing is for less than 160 minutes and at least for 100 minutes.

7. The method according to claim 1, wherein the stiffness inducing organic additive and the over-digested crumb rubber modified binder are admixed in a mass ratio of between 0.5:99.5 and 1.5:98.5.

8. The re-stiffened overdigested crumb rubber modified bituminous binder produced by the method according to claim 1.

9. The re-stiffened overdigested crumb rubber modified bituminous binder according to claim 8, comprising: an increased softening point to at least 56 C.

10. The re-stiffened overdigested crumb rubber modified bituminous binder according to claim 8, comprising a reduction in elasticity with an increase in a complex modulus (G*), as evidenced by reduced phase angles at in-service temperatures.

11. The re-stiffened overdigested crumb rubber modified bituminous binder according to claim 8, comprising: a compression recovery at 5 minutes, according to the MB-11 test method as set out in TG 1, 2015, of more than 70%, or a compression recovery after 1 hour, according to the MB-11 test method, of more than 70%, or a compression recovery after 24 hours, according to the MB-11 test method, of more than 40%, or a compression recovery after 4 days, according to the MB-11 test method, of more than 40%.

12. The re-stiffened overdigested crumb rubber modified bituminous binder according claim 8, comprising: a resilience at 25 C., according to the MB-10 test method as set out in TG 1, 2015, of more than 13%.

13. The re-stiffened overdigested crumb rubber modified bituminous binder according to claim 8, wherein the re-stiffened overdigested crumb rubber modified bituminous binder flows, according to the MB-12 test method as set out in TG 1, 2015, more than 10 mm.

14. The re-stiffened overdigested crumb rubber modified bituminous binder according to claim 8, comprising: a weight fraction of at least 0.13 of components with an average molar mass between 3.910.sup.6 g/mol and 4.110.sup.6 g/mol.

15. The re-stiffened overdigested crumb rubber modified bituminous binder according to claim 8, comprising: a weight fraction of at least 0.13 of components with an average molar mass between 4.110.sup.6 g/mol and 4.210.sup.6 g/mol.

16. The re-stiffened overdigested crumb rubber modified bituminous binder according to claim 8, comprising: reaction products from reactions between glycidyl methacrylate and functional groups of the over-digested bituminous binder.

17. A method of sealing or asphalt paving a surface, the method comprising: applying a layer that includes the re-stiffened overdigested crumb rubber modified bituminous binder of claim 8 to the surface.

18. The method according to claim 17, wherein the layer includes aggregate particles, and wherein the re-stiffened overdigested crumb rubber modified bituminous binder and the aggregate particles forming a seal surfacing layer or a layer of an asphalt mix.

19. The method to claim 18, wherein the asphalt mix is a medium continuously graded mix and has a maximum displacement, using a Universal Testing Machine (UTM-25) test setup for dynamic modulus testing, with gyratory compacted specimens size of 100 mm diameter and 150 mm high cored from cylindrical medium continuous specimens of 150 mm in diameter by 170 mm high, a single applied stress or deviator stress level of 276 kPa and 69 kPa confinement pressure at 40 C., with the deviator stress repeatedly pulsed in a vertical direction on the specimens using a haversine pulse load of 0.1 seconds and 0.9 seconds rest period until flow or 10,000 load cycles, equal to an identical asphalt SBS mix in terms of mix design, grading, aggregate type and binder content, based on an A-E2 conforming SBS binder, or at least 25% better than a performance of the identical asphalt SBS mix.

20. The method according to claim 18, wherein the asphalt mix is a medium continuously graded mix and has a flow number, using a UTM-25 test setup for dynamic modulus testing, with gyratory compacted specimens size of 100 mm diameter and 150 mm high cored from cylindrical medium continuous specimens of 150 mm in diameter by 170 mm high, a single applied stress or deviator stress level of 276 kPa and 69 kPa confinement pressure at 40 C., with the deviator stress repeatedly pulsed in a vertical direction on the specimens using a haversine pulse load of 0.1 seconds and 0.9 seconds rest period until flow or 10,000 load cycles, equal to an identical asphalt SBS mix in terms of mix design, grading, aggregate type and binder content based on an A-E2 conforming SBS binder, or at least 25% better, than a performance of the identical asphalt SBS mix.

21. The method according to claim 18, wherein the asphalt mix is a bitumen-rubber asphalt semi-open (BRASO) graded mix and has a maximum displacement, using a UTM-25 test setup for dynamic modulus testing, with gyratory compacted specimens size of 100 mm diameter and 150 mm high cored from cylindrical medium continuous specimens of 150 mm in diameter by 170 mm high, a single applied stress or deviator stress level of 276 kPa and 69 kPa confinement pressure at 40 C., with the deviator stress repeatedly pulsed in a vertical direction on the specimens using a haversine pulse load of 0.1 seconds and 0.9 seconds rest period until flow or 10,000 load cycles, equal to an identical asphalt mix in terms of mix design, grading, aggregate type and binder content, based on an A-R1 conforming crumb rubber modified bituminous binder, or at least 25% better than a performance of the identical asphalt mix based on an A-R1 conforming crumb rubber modified bituminous binder.

22. The method according to claim 18, wherein the asphalt mix is a bitumen-rubber asphalt semi-open (BRASO) graded mix and has a flow number, using a UTM-25 test setup for dynamic modulus testing, with gyratory compacted specimens size of 100 mm diameter and 150 mm high cored from cylindrical medium continuous specimens of 150 mm in diameter by 170 mm high, a single applied stress or deviator stress level of 276 kPa and 69 kPa confinement pressure at 40 C., with the deviator stress repeatedly pulsed in a vertical direction on the specimens using a haversine pulse load of 0.1 seconds and 0.9 seconds rest period until flow or 10,000 load cycles, equal to an identical asphalt mix in terms of mix design, grading, aggregate type and binder content, based on an A-R1 conforming crumb rubber modified bituminous binder, or at least 25% better than a performance of the identical asphalt mix based on an A-R1 conforming crumb rubber modified bituminous binder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The disclosure will now be described in more detail with reference to the accompanying drawings in which

[0063] FIG. 1-1 shows a graph of the size grading of crumb rubber particles used in studies conducted by the inventor;

[0064] FIG. 1-2 shows scanning electron microscope (SEM) photographs of the rubber crumbs, illustrating the surface texture/morphology of the rubber crumbs;

[0065] FIG. 2-1 shows a typical digestion viscosity curve of crumb rubber modified bituminous binder;

[0066] FIG. 2-2 shows digestion viscosity curves of crumb rubber modified bituminous binders or blends at different handling temperatures, as a function of time;

[0067] FIG. 2-3 shows a Black diagram of crumb rubber modified bituminous binder (CRM binder) with PAV-ageing before and after solvent-soluble recovery;

[0068] FIG. 2-4 shows digestion viscosity curves of two crumb rubber modified bituminous binders or blends at 190-200 C., as a function of time;

[0069] FIG. 2-5 shows digestion viscosity curves of two crumb rubber modified bituminous binders or blends based on base binders from different refineries at 190-200 C. as a function of time, with the solid bold horizontal lines representing the maximum and minimum TG 1 limits;

[0070] FIG. 2-6 shows a photographs of non-flushing (CRM bitumen 1) and flushing (CRM bitumen 2) areas;

[0071] FIG. 2-7 shows softening point curves with digestion time of crumb rubber modified bituminous blends or binders at 190-200 C., with the solid bold lines representing the maximum and minimum TG 1 limits;

[0072] FIG. 2-8 shows the softening point curves with digestion time of two crumb rubber modified bituminous blends or binders at 190-200 C.;

[0073] FIG. 2-9 shows a graph of J.sub.nr values at 58C for a base bituminous binder and J.sub.nr values of the corresponding crumb rubber modified bituminous binder or blend at various digestion time intervals;

[0074] FIG. 3-1 shows force ductility curves for a stiffness inducing and an elasticity inducing reactive elastomeric terpolymer;

[0075] FIG. 3-2 shows the average molar mass and molar mass distribution curves when a stiffness-inducing reactive elastomeric terpolymer additive is added to an over-digested crumb rubber modified bituminous binder;

[0076] FIG. 3-3 shows graphs of the softening point of an over-digested crumb rubber modified bituminous binder or blend before and after addition of a stiffness-inducing reactive elastomeric terpolymer (SI RET) additive in the form of an ethylene/n-butyl acrylate/glycidyl methacrylate composition;

[0077] FIG. 3-4 shows J.sub.nr results at 58C for an over-digested crumb rubber modified bituminous binder or blend before and after addition of a stiffness-inducing reactive elastomeric terpolymer (SI RET) additive in the form of an ethylene/n-butyl acrylate/glycidyl methacrylate composition;

[0078] FIG. 3-5 shows Black diagrams of over-digested crumb rubber modified bituminous binders or blends before and after addition of a stiffness-inducing reactive elastomeric terpolymer (SI RET) additive in the form of an ethylene/n-butyl acrylate/glycidyl methacrylate composition;

[0079] FIG. 4-1 shows graphs of average permanent strain accumulation vs. number of load cycles at 40 C., for an SBS mix, a repeat SBS mix, and a re-stiffened over-digested crumb rubber modified bituminous mix (MOD-CR mix); and

[0080] FIG. 4-2 shows graphs of average permanent strain accumulation vs. number of load cycles at 40C, for a crumb rubber modified bituminous mix (CRM Mix) and a re-stiffened over-digested crumb rubber modified bituminous mix (MOD-CR mix).

DETAILED DESCRIPTION OF THE DISCLOSURE

[0081] The inventor believes that although digestion viscosity curves have been used successfully as a production tool, they cannot be used as an indicator of performance of rubber modified bituminous binders because viscosity is tested at much higher handling temperatures not representative of actual field performance at lower in-service temperatures (e.g. temperatures of 20 C.-70 C.). Rheologically, disintegrated rubber polymers from the de-vulcanization of the rubber crumbs present after over-digestion are still capable of imparting elastic properties. Over-digested crumb rubber modified bituminous binder should remain useful for re-blending or continual usage in alternative products given appropriate characterisation. In addition, the potential benefits of liquid non-particulate over-digested crumb rubber modified bituminous binders include ease of handling due to their lower viscosity, ease of blending with bitumen, and energy saving during processing. The challenge is to make sure that oils and/or low molecular weight compounds present in over-digested crumb rubber modified bituminous blends do not reduce the binder stiffness and offset the enhanced elastic properties imparted by the modifier. The inventor thus reviewed earlier studies conducted to investigate the properties of crumb rubber modified bituminous binders (see Mturi GAJ, O'Connell 0.1, Marais H and Hawes N, A Brief Analysis of Over-Digested Crumb Rubber Modified Bitumen, Rubberized Asphalt: Asphalt Rubber Conference (2015) 409-419, Las Vegas, United States of America) and proceeded to propose methods by which over-digested crumb rubber modified bituminous binders can be re-stiffened.

[0082] In these studies, the typical 70/100 penetration grade bitumen conforming to SANS 4001-BT1, 2014 was used. An extender oil produced as per the requirements of COLTO, 1998 was used with the bitumen. Rubber crumbs of the grading requirements stated in TG1, 2015 were used. The rubber crumb particles used in the studies essentially pass a 1.00 mm sieve (1.18 mm was the requirement in a previous guideline document i.e. TG1, 2007) and the majority is retained on a 0.6 mm sieve. FIGS. 1-1 and 1-2 (obtained from Mturi, G., Zoorob, S. E., O'Connell, J., Anochie-Boateng, J., & Maina, J. (2011b). Rheological testing of crumb rubber modified bitumen. In Proceedings of the 7th International Committee on Road & Airfield Pavement Technology (pp. 1316-1327). Bangkok, Thailand, paper P18) depict the grading for the rubber crumbs used in this investigation.

[0083] The rubber crumb surface texture/morphology is shown in the scanning electron microscope (SEM) photographs of FIG. 1-2(a)-(c). The porous microstructures are consistent with comminution via an ambient process. The white spots seen in FIGS. 1-2(b) and 1-2(c) are most likely filler and not metal fragments since South Africa only imports non-steel reinforced tyres for rubber crumb manufacture (COLTO, 1998). The results for an elemental analysis of the rubber crumbs are shown in Table 1-1. They have a typical composition of carbon, oxygen, silicon, sulphur and traces of the metals, calcium, sodium, aluminium and zinc possibly from a vulcanization component.

TABLE-US-00001 TABLE 1-1 Elemental composition (by weight) of the rubber crumbs SPECTRUM C O Si S Ca Na Al Zn Total Sample 1 81.93 16.60 0.91 0.55 100.00 Sample 1 Repeat 1 76.54 23.46 <0.005 100.00 Sample 1 Repeat 2 86.42 11.19 0.58 1.36 0.44 <0.005 100.00 Sample 1 Repeat 3 85.91 11.52 2.08 0.50 <0.005 100.00 Sample 1 Repeat 4 88.71 8.02 0.75 2.03 0.51 <0.005 100.00 Max. Detected 88.71 23.46 0.75 2.08 0.44 0.55 0.51 <0.005 Min. Detected 76.54 8.02 0.58 0.91 0.44 <0.005 0.50 <0.005

[0084] The test results in Table 1-2 below indicate that crumb rubber modified bituminous blends used in this investigation conformed to South African Technical Guideline 1 guidelines for the A-R1 grade, which is an asphalt grade (TG1, 2015), except that the resilience of crumb rubber modified bituminous binder 1 was slightly above recommendation. The grade S-R1 is a surfacing seal grade. The test methods indicated with an MB in the name of the test method are described in TG1, 2015.

TABLE-US-00002 TABLE 1-2 Crumb rubber modified bitumen properties for asphalt (A-R1) and seals (S-R1) according to South African national guidelines. Class Results Test (TG1, 2015) PROPERTY Unit Binder 1 Binder 2 Method S-R1 A-R1 Softening Point C. 62.8 65.0 MB-17 55-65 55-65 Dynamic Viscosity at 190 C. dPa .Math. s 35 45 MB-13 20-40 20-50 Compression 5 mins % 86.6 84.6 MB-11 >70 >80 Recovery after 1 hour 88.6 88.6 >70 >70 24 hours N/A 80.7 >40 N/A Resilience at 25 C. % 42 32 MB-10 13-35 13-40 Flow mm 14 17 MB-12 15-70 10-50

[0085] The properties of rubber modified bituminous binders change with temperature, digestion time and energy consumed during the digestion process. The various stages (Stages 1 to 4) of crumb rubber modified bituminous blends or binders can be defined in terms of viscosity, as depicted in FIG. 2-1 (obtained from Mturi G. A. J., O'Connell J., Zoorob S., De Beer M., A study of crumb rubber modified bitumen used in South Africa, Road Materials and Pavement Design, Vol. 15, Issue 4, 2014, p. 774-790).

[0086] Stage 1 is characterised by an initial increase in viscosity upon blending. In this stage, the rubber particle dimension increases as the oil and/or lighter components of the bitumen diffuse into the three-dimensional rubber networks of poly-isoprene and poly-butadiene linked by sulphur-sulphur bonds. The diffusion process varies according to the amount of cross-linkages in the rubber, the molecular compatibility between the rubber and the diffusing particles as well as the molecular weight of the latter. A further incorporation of the diffusing matter into the rubber particles would possibly occur as the sulphur-sulphur bonds start to thermally dissociate and this contributes to an additional increase in viscosity.

[0087] The thermal dissociation process continues until a maximum viscosity point, referred to as Stage 2, is reached. The viscosity then decreases with digestion time in Stage 3 as the network disintegrates due to the loss of the sulphur linkages. Once the viscosity reaches the minimum recommended viscosity limit of 20 dPa.s (or 2000 mPa.S) in TG1, 2015, the bitumen rubber is labelled as over-digested, at least in South Africa. The decrease in viscosity continues until it reaches a point of relatively constant viscosity where the crumb robber modified bituminous blend is referred to as terminal. This has been depicted as Stage 4 in the digestion viscosity curve shown in FIG. 2-1.

[0088] The viscosity at Stage 4 is typically higher than the viscosity of the base bituminous binder. This viscosity increase is attributed to ageing (of the base bituminous binder) comingled with the incorporation of digested crumb rubber into the base bituminous binder.

[0089] Stage 3 typically represents an application window or period for crumb rubber modified bituminous binders or blends. Lower application temperatures can increase this application window by slowing down the digestion process, as seen in FIG. 2-2 (obtained from Mller J., Marais H., The Perceived versus actual shelf-life and performance properties of bitumen rubber, Proceedings of the Asphalt Rubber Conference (Sousa J. B., ed.), 23-26 Oct. 2012, Munich, p. 429-441) where the lower viscosities of Stage 4 are delayed. This has led to the development of warm mix additives to allow for the application of these binders at lower temperatures. However, this adds another complexity in that the effect of these extra constituents on asphalt performance needs to be investigated and controlled in guidelines and specifications.

[0090] De-vulcanised over-digested crumb rubber modified bituminous binder is potentially less sensitive to handling conditions. The over-digested binder will still be a polymer modified binder and, hence, can give superior performance compared to standard unmodified penetration grade bitumen. It has been shown that significant resilience and elastic properties remain even after 32 hours of over-digestion for South African crumb rubber modified bituminous blends. This is confirmed through the analysis of PAV-aged (pressure aging vessel aged) crumb rubber modified bituminous binders (CRM binder) in FIG. 2-3 (obtained from Mturi, G. A. J., O'Connell, J., & Anochie-Boateng, J. K. (2013). Limitations of current South African test methods for bituminous binders. Construction and Building Materials, 45, 314-323) which were shown to still exhibit a certain degree of elastic properties. FIG. 2-3 also shows the recovered PAV-aged crumb rubber modified bituminous binder displaying elastic properties without the solvent insoluble crumb rubber. This is due to the increased incorporation (as a consequence of ageing/digestion) of de-linked polymers of the crumb rubber into the solvent-soluble bitumen.

[0091] Over-digestion results in a decrease in viscosity, so previous investigators (O'Connell J., Anochie-Boateng J., Marais H., Evaluation of bitumen-rubber asphalt manufactured from modified binder at lower viscosity, Proceedings of the 29th Southern African Transport Conference, 16-19 Aug. 2010, Pretoria, p. 129-138) investigated two crumb rubber modified bituminous blends, one conforming to TG 1 guideline requirements and the other over-digested such that the viscosity is below the current range in the TG 1 guideline. The digestion viscosity curves of the two crumb rubber modified bituminous blends or binders (CRM binders) are shown in FIG. 2-4. Both blends do not show significant viscosity changes during the short-term digestion conditions. The standard crumb rubber modified bituminous binder has viscosities within the set TG 1 limits (i.e. above 2000 mPa.S) up to 20 hours of digestion. Over-digestion during asphalt manufacture is therefore unlikely, and this binder would be expected to show relatively superior and acceptable performance in an asphalt mix.

[0092] The two blends were incorporated in a standard medium continuously graded asphalt mix design and evaluated as per SABITA Manual 19, Guidelines for the design, manufacture and construction of bitumen rubber asphalt wearing courses, SABITA, Howard Place, South Africa, 2009 (hereinafter referred to as SABITA Manual 19, 2007). The mix design method was specifically developed for these crumb rubber modified bituminous binders. Interestingly, both crumb rubber modified asphalt mixes gave much poorer results from the expected performance and even inferior compared to a 50/70 penetration medium continuously graded asphalt mix (see Table 3-1). The crumb rubber modified asphalt mixes gave low Indirect Tensile Strength (ITS) and Marshall Stability test results with higher flow test values indicating increased susceptibility to deformation. The two crumb rubber modified asphalt mixes had similar gradings and the better performance of the standard crumb rubber modified bituminous binder was interestingly the higher viscosity binder.

TABLE-US-00003 TABLE 3-1 Properties of medium continuous (MC) mix at optimum binder content Standard Standard crumb Low viscosity 50/70 rubber modified crumb rubber SABITA penetration bituminous modified Manual 19, Properties bitumen binder bituminous binder 2007 Marshall 10.4 6.8 5.8 8 minimum Stability (kN) Marshall Flow 4.1 5.1 6.0 2-5 (mm) ITS (kPa) 676 317 293 600 minimum Average voids 5.7 3.3 4.2 2-6 (%)

[0093] Mturi et al. (Mturi, G., Conrad, S., & Mogonedi, K. (2011). JR 5023: Analysis of Modified Binders used in Seal Application as a Possible Cause of Observed Highway Distresses (Technical Report No: CSIR/BE/IE/MEMO/2011/0003/B). South Africa: CSIR) also prepared two crumb rubber modified bituminous binders or blends (referred to as CRM Bitumen 1 and CRM Bitumen 2 in FIG. 2-5) using the same crumb rubber and blending method as O'Connell et al. but with different base bituminous binders from two separate refineries. It was noted that the viscosity of the CRM blends conformed to the recommended viscosity range after 6-7 hours of mixing at 190-200 C. (i.e. 6-7 hours from the point referred to as 0 hour), see FIG. 2-5. The blends stayed in this viscosity range for a limited time frame before falling below the minimum required viscosity limit of 2000 mPa.Math.s.

[0094] Although the two blends displayed similar digestion curves, they still showed different field performance when used for surfacing seals. The one binder (CRM Bitumen 2 of FIG. 2-5) was a lot softer in the field resulting in flushing and atypical stone orientation (see FIG. 2-6).

[0095] FIG. 2-7 shows softening points with digestion of the same two binders (CRM Bitumen 1 and CRM Bitumen 2) shown in FIG. 2-5. Upon blending, the blend used in the non-flushing area (CRM Bitumen 1) had a much higher softening point than the required TG 1(2007) maximum limit. In contrast, the softening point of the blend from the flushing area (CRM Bitumen 2) (following blending) conformed to the set maximum criterion. After 6-7 hours of digestion, the softening point of the flushing area blend fell below the minimum specification limit. This is precisely the time the blend conformed to the required application viscosity range in FIG. 2-5. So if a contractor waited for the blend viscosity to reach the recommended range, this would explain the softer binder experienced in the field that could have contributed to the observed distress. Mturi et al. however acknowledged that other factors may have also played a role in the observed distress. It is to be noted that the softening point of the non-flushing area blend conformed to the required limits from 6-7 hours of digestion up to 30 hours of digestion.

[0096] It can be concluded that the use of digestion viscosity curves to predict performance of crumb rubber modified bituminous binders may be misleading because viscosity is tested at much higher handling temperatures (190-200 C.) which may not be simulative of field performance (e.g. in an asphalt pavement) at in-service temperatures (<70 C.).

[0097] FIG. 2-8 illustrates softening points with digestion of crumb rubber modified bituminous blends (CRM blend (CR1) and CRM blend (CR2)), re-blended with additional crumb rubber (CR) once over-digested. For this exercise, two different types of crumb rubber were used while keeping the rest of the material/process parameters constant. The figure shows softening points of the crumb rubber modified bituminous blends approaching that of the base bituminous binder with over-digestion. Once the blends were supplemented with extra crumb rubber, the over-digested binder regained the higher softening point values and retained them with prolonged digestion.

[0098] Notably, the minimum softening points of the two over-digested crumb rubber modified bituminous blends and of the two re-blends (with digestion) were relatively similar, even though the digestion curves for the two crumb rubber types appeared slightly different. It can be considered that the softening points of the base bituminous binder and the over-digested bituminous binder (considered the base binder of the re-blend) act as limits preventing any further drop in softening points with continued digestion. This proves that the properties of the base binder are just as important in the manufacture of crumb rubber modified bituminous binders and can also be specified according to climatic conditions.

[0099] Crumb rubber modified bituminous binder residues taken at various digestion time intervals were analysed for their stress sensitivity (i.e. sensitivity to traffic loading). The test method employed is referred to as the multiple stress creep and recovery (MSCR) test. The MSCR test as per ASTM D7405 or AASHTO T350 evaluates the ability of binders to maintain elastic response at the stress levels of 100 Pa and 3200 Pa using a dynamic shear rheometer (DSR). The test involves applying a 1-second creep loading followed by a 9-second recovery phase, and this constitutes a single cycle. At each stress level, 10 creep and recovery cycles are applied. The modified test procedure adopted for this investigation uses multiple stress levels of 25, 50, 100, 200, 400, 800, 1600, 3200, 6400, 12800 and 25600 Pa. The average non-recovered strain (ynr) for the 10 creep and recovery cycles is then divided by the applied stress (T) for those cycles yielding the non-recoverable creep compliance (J.sub.nr=nr/ti). The accumulation of J.sub.nr over time will eventually lead to permanent deformation.

[0100] The MSCR test was modified to adopt multiple stress loading conditions because an in situ binder in an asphalt layer experiences a wide range of stresses from the passing of light to heavy vehicles. It is the higher stresses experienced under repetitive heavy vehicle loads that would eventually be responsible for pavement deformation. Therefore, it is at higher stress loading conditions that binders are better evaluated in terms of their damage resistance properties.

[0101] FIG. 2-9 shows the MSCR test results of the crumb rubber modified bituminous binders or blends. The test results graphically illustrated in FIG. 2-9 show that the drop in crumb rubber modified bituminous binder or blend stiffness expected with digestion time is accompanied by an increase in stress sensitivity. Re-stiffening the over-digested crumb rubber modified bituminous binder with a modifier or additive should therefore be assessed as to whether it also improves stress resilience.

[0102] It is important to note that the behaviour observed in FIGS. 2-8 and 2-9 applies to the particular crumb rubber modified bituminous blends tested. This behaviour would need to be repeated and confirmed with other crumb rubber modified bituminous blends accordingly.

[0103] The foregoing investigations have shown that in order to make an over-digested crumb rubber modified bituminous binder useful again, re-stiffening the over-digested crumb rubber modified bituminous binder should specifically improve: [0104] (1) In-service properties (such as softening point instead of viscosity at 190 C.), [0105] (2) Stress resilience, and [0106] (3) Stability.

[0107] An ideal additive should recreate a network to recombine lower molecular weight oily constituents and consequently improve stiffness and storage stability properties of an over-digested crumb rubber modified bituminous binder. An investigation with a range of additives came up with the following findings: [0108] Temporary network forming non-reactive additives (e.g. fillers, styrene-butadiene-styrene, etc.) were found unsuitable because they increased the stress sensitivity of over-digested crumb rubber modified bituminous binders. [0109] Reactive additives improved the stress resilience properties of over-digested crumb rubber modified bituminous binders. [0110] Elasticity-inducing additives (e.g. terpolymers containing butyl acrylate at 28 wt % and glycidyl methacrylate at 5.3 wt %) had very little effect on stiffness properties of over-digested crumb rubber modified bituminous binder. [0111] Stiffness-inducing additives (e.g. ethylene-vinyl acetate) had the highest effect on the stiffness of over-digested crumb rubber modified bituminous binders. Ethylene-vinyl acetate is however non-reactive in bituminous blends.

[0112] Based on the investigations which the inventor was involved with, and the conclusions reached, the inventor decided that reactive elastomeric terpolymers (referred to as SI(RET) below), e.g. a terpolymer of ethylene, alkyl acrylate and glycidyl methacrylate (GMA) would be ideally suited for re-stiffening an over-digested crumb rubber modified bituminous binder. The inventor believes that the GMA group would react with available functional groups as it reacts (supposedly) with carboxylic functional groups in bituminous asphaltenes fractions:

##STR00001##

[0113] Force ductility curves of modified penetration grade bitumens however showed that only specific terpolymer formulations were highly reactive and hence sufficiently stiffness-inducing (as opposed to elasticity-inducing) for purposes of re-stiffening an over-digested crumb rubber modified bituminous binder sufficiently (see FIG. 3-1).

[0114] The addition of stiffness-inducing reactive elastomeric terpolymer to form polymer linkages with the over-digested crumb rubber modified bituminous binder (over digested CRM binder) was monitored through average molar mass and molar mass distribution determination, as shown in FIG. 3-2. In this particular case, the stiffness-inducing reactive elastomeric terpolymer additive (SI RET) used was an ethylene/n-butyl acrylate/glycidyl methacrylate terpolymer (at 9% GMA). FIG. 3-2 also shows the molar mass distribution of pure 70/100 penetration grade bitumen (Pure 10/100pen).

[0115] The crumb rubber modified bituminous binder was prepared at temperatures between 190-200 C. on a hot plate with a 4 bladed stainless-steel propeller (paddle stirrer) at a speed of 1500-2000 rpnn. The crumb rubber modified bituminous binder was maintained at this temperature until complete over digestion, notably where stable/consistent properties were achieved after 32-33 hours of stirring. 1% (nn/nn) SI RET additive was added to the over-digested blend without changing the stirring speed. The sample was stirred for a further 2 hours continuously. All blends were checked periodically and adjustments to the stirrer position done as the viscosity changed.

[0116] There was no addition of poly-phosphoric acid (PPA). When added alone or in combination with the SI RET additive, PPA was found negatively to affect the properties of the bituminous binder.

[0117] The SI RET additive increased the softening point (SP) of the over-digested binder (CRM blend (CR 1)) to a level similar to the original crumb rubber (CR) modified bituminous blend or binder. It also improved the storage stability of the re-stiffened blend as shown in FIG. 3-3. The stiffness-inducing reactive elastomeric terpolymer additive and crumb rubber modified bituminous blend (over digested binder+SI RET) also exhibited improved stress resilience as shown in FIG. 3-4 which shows graphs for the original crumb rubber modified bituminous binder (CRM binder (original)), the over-digested crumb rubber modified bituminous binder (over digested binder) and the stiffness-inducing reactive elastomeric terpolymer additive and crumb rubber modified bituminous blend (Over digested binder+SI RET), and increased stiffness and reduced elastic properties at high in-service temperatures (see FIG. 3-5), when compared with over-digested crumb rubber modified bituminous binder (over digested binder). Based on TG1 (2015) guideline recommendations, the stiffness-inducing reactive elastomeric terpolymer additive and crumb rubber modified bituminous blend was found conforming to both A-R1 (for asphalt) and S-R1 (for seals) classes for all tested properties except viscosity at 190C (as expected given it was over-digested), as shown in Table 3-1.

TABLE-US-00004 TABLE 3-1 TG1 (2015) properties of over-digested crumb rubber modified bituminous binder re-stiffened with a stiffness-inducing reactive elastomeric terpolymer (SI RET) additive. Class Test (TG1, 2015) PROPERTY Unit Results Method S-R1 A-R1 Softening Point C. 61.0-61.4 MB-17 55-65 55-65 Dynamic Viscosity at dPa .Math. s 8.0 MB-13 20-40 20-50 190 C. Compression 5 mins % 85.6 MB-11 >70 >80 Recovery After 1 hour 85.6 >70 >70 24 hours 69.9 >40 N/A 4 days 67.9 N/A N/A Resilience at 25 C. % 17.3 MB-10 13-35 13-40 Flow mm 28 MB-12 15-70 10-50

[0118] The stiffness-inducing reactive elastomeric terpolymer additive and crumb rubber modified bituminous blend or binder (re-stiffened binder) was investigated against a crumb rubber modified bituminous binder and a styrene-butadiene-styrene (SBS) modified bituminous binder in terms of permanent deformation resistance in asphalt mixes. Asphalt mixes were prepared using typical optimized mix designs used for road construction in South Africa, namely a bitumen rubber asphalt semi-open graded mix (BRASO) and a medium continuously graded mix (A-E2 SBS).

[0119] The mixing and compaction of specimens were done in accordance with CSIR test protocols. The mixes were prepared using a heated mechanical mixer into which the calculated masses of aggregate and bituminous binder were placed. Aggregates were blended in accordance with the design grading. The materials were mixed for approximately 5 minutes or until a uniform mixture was obtained. After mixing, the material was placed in an oven set at compaction temperature for four hours to induce short-term ageing, after which the mix was compacted. Gyratory specimens for performance testing were compacted to a density of between 92 and 94 percent of the Maximum Theoretical Relative Density (MTRD). The resultant cored gyratory specimens were used to perform a repeated load permanent deformation (RLPD) test.

[0120] The medium continuously graded mixes were prepared at the CSIR advanced road material testing laboratories using a standard SBS binder and an over-digested crumb rubber modified bituminous binder comprising SI RET additive (i.e. re-stiffened binder). Air voids and binder contents in the laboratory testing programme simulated the properties of the field mixes as best as possible. The original design was done using the Marshall Mix design method as per COLTO (1998) with the Interim Guidelines for the Design of Hot Mix Asphalt (2001). Table 4-1 shows the target binder and air voids contents as well as other volumetric properties of the mix.

[0121] After compaction, the densities (MTRD and BRD) were determined using the standard TMH1 C3 method. The results for the specimens tested are shown in Tables 4-2 and 4-3 for the SBS mix (specimen 14383) and the re-stiffened binder (specimen 14706). The voids content for the performance tests specimens were within 6% to 8% voids content.

TABLE-US-00005 TABLE 4-1 Summary of volumetric properties of the medium continuous mix Mix property Design value Binder content (%) 4.7% Design air voids (%), saturation surface dry (SSD) 4.6% VMA (%) 15.9% VFB (%) 70.5% Compaction temperature 145 C. Mixing temperature 160-170 C.

TABLE-US-00006 TABLE 4-2 Gyratory specimens' densities (before and after coring) for the SBS mix TMH1 C3 BRD Specimen no. MTRD BRD Voids (%) 14383-G4C 2.721 2.554 6.1 14383-G5C 2.721 2.551 6.2 14383-G6C 2.721 2.556 6.1 12651G26C* 2.669 2.490 6.6 12651G27C* 2.669 2.485 7.0 12651G28C* 2.669 2.489 6.6 12651G24C* 2.669 2.495 6.5 12651G30C* 2.669 2.484 7.1 12651G31C* 2.669 2.477 7.0 *Original mix results produced in the development of the SABITA Asphalt Mix Design Manual for South Africa (2014) (SABITA Draft Manual, Asphalt mix design manual for South Africa - provisional working document, SABITA, Howard Place, South Africa, 2014).

TABLE-US-00007 TABLE 4-3 Gyratory specimens' densities (before and after coring) for the re-stiffened binder TMH1 C3 BRD Specimen no. MTRD BRD Voids (%) 14706-G4C 2.720 2.514 7.6 14706-G5C 2.720 2.508 7.8

[0122] Based on AASHTO TP 79 (2009) and NCHRP Report 702 (2011), the rutting resistance behaviour of the mixes was investigated. The UTM-25 test setup for dynamic modulus testing was used to conduct the test. Gyratory compacted specimens size of 100 mm diameter and 150 mm high cored from cylindrical medium continuous specimens of 150 mm in diameter by 170 mm high were tested.

[0123] The permanent deformation experimental design consisted of a single applied stress (deviator stress) level of 600 kPa at 40 C. The deviator stress was repeatedly pulsed in the vertical direction on the specimens using a haversine pulse load of 0.1 seconds and 0.9 seconds rest period until flow or 10,000 load cycles.

[0124] Table 4-4 shows a summary of the test results that includes specimen air voids content, test temperatures, and the loading conditions. Specimens were tested at close to field voids. FIG. 4-1 shows the average permanent strain accumulations in the test samples against the number of load applications at the applied deviator stress level of 600 kPa and test temperature of 40 C.

[0125] The re-stiffened over-digested crumb rubber modified bituminous mix (MOD-CR Mix) showed better permanent deformation resistance than the SBS mixes (both the original SBS Mix used with this mix design and the repeated SBS Mix manufactured for this exercise) at the tested temperature.

TABLE-US-00008 TABLE 4-4 Summarized loading conditions and results of repeated load test Strain Test Cyclic Cycles Max at Permanent Sample Voids Temp. Stress to Flow Displacement flow point Deformation at ID (%) ( C.) (kPa) Termination Number (mm) strain flow point 12651G26C* 6.6 40 600 10000 945 7.332 3.720 5.455 12651G27C* 7.0 40 600 10000 982 7.441 3.080 4.583 12651G28C* 6.6 40 600 10000 697 7.428 2.872 4.266 12651G24C* 6.5 40 600 10000 1156 7.423 3.117 4.626 12651G30C* 7.1 40 600 10000 1587 7.365 3.042 4.481 12651G31C* 7.0 40 600 10000 1377 7.408 2.865 4.244 14383-G4C 6.1 40 600 10000 1569 7.575 1.880 2.848 14383-G5C 6.2 40 600 10000 1947 7.547 2.327 3.511 14383-G6C 6.1 40 600 10000 1960 7.540 2.327 3.509 14706-G4C 7.6 40 600 10000 5716 1.641 1.146 1.725 14706-G5C 7.8 40 600 10000 8655 2.574 1.651 2.484 *Original mix results produced in the development of the SABITA Asphalt Mix Design Manual for South Africa (2014).

[0126] The BRASO mixes were prepared at the CSIR advanced road material testing laboratories using a standard crumb rubber modified bituminous binder and an over-digested crumb rubber modified bituminous binder comprising SI RET additive (i.e. re-stiffened binder). Air voids and binder contents in the laboratory-testing programme simulated the properties of the field mixes as best as possible. The original design was done using the Marshall mix design method as per SABITA Manual 19, 2007 with the Interim Guidelines for the Design of Hot Mix Asphalt (2001). Table 4-5 shows the target binder and air voids contents as well as other design properties of the mix.

TABLE-US-00009 TABLE 4-5 Summary of volumetric properties of the mix Mix property Design value Binder content (%) 7.5% Design air voids (%), saturation surface dry (SSD) 5.6% VMA (%) 21.6% VFB (%) 74.3% Compaction temperature 140-145 C. Mixing temperature 170 C.

[0127] After compaction, the densities (MTRD and BRD) were determined using the standard TMH1 C3 method. The results for the specimens tested are shown in Tables 4-6 and 4-7 for the crumb rubber modified bituminous mix (specimen 14536) and the re-stiffened mix respectively (specimen 14697). The voids content for the performance tests specimens were within 6% to 8% voids content.

TABLE-US-00010 TABLE 4-6 Gyratory specimens densities (before and after coring) for the crumb rubber modified mix TMH1 C3 BRD Specimen no. MTRD BRD Voids (%) 14536 G11C* 2.455 2.289 6.8 14536 G4** 2.432 2.285 6.0 14536 G5** 2.432 2.261 7.0 *Lab sample (prepared at the CSIR) **Plant sample

TABLE-US-00011 TABLE 4-7 Gyratory specimen's densities (before and after coring) for the re-stiffened mix TMH1 C3 BRD Specimen no. MTRD BRD Voids (%) 14697 G2C 2.475 2.288 7.6 14697 G3C 2.475 2.287 7.6 14697 G4C 2.475 2.288 7.6

[0128] Based on AASHTO TP 79 (2009) and NCHRP Report 702 (2011), the rutting resistance behaviour of the mixes was investigated. The UTM-25 test setup for dynamic modulus testing was used to conduct the test. Gyratory compacted specimens size of 100 mm diameter and 150 mm high cored from cylindrical medium continuous specimens of 150 mm in diameter by 170 mm high were tested.

[0129] The permanent deformation experimental design consisted of a single applied stress (deviator stress) level of 276 kPa and 69 kPa confinement pressure at 40 C. The deviator stress was repeatedly pulsed in the vertical direction on the specimens using a haversine pulse load of 0.1 seconds and 0.9 seconds rest period until flow or 10,000 load cycles.

[0130] Table 4-8 shows a summary of the test results that include specimen air voids content, test temperatures, and the loading conditions. Specimens were tested at close to field voids. FIG. 4-2 shows the average permanent strain accumulations in the test samples against the number of load applications at the applied deviator stress level of 276 kPa with a confining pressure of 69 kPa and test temperature of 40 C.

[0131] The re-stiffened over-digested crumb rubber modified bituminous mix showed better permanent deformation resistance than the standard crumb rubber modified bituminous mix at the tested temperature.

TABLE-US-00012 TABLE 4-8 Summarized loading conditions and results of repeated load test Strain Permanent Test Cyclic Confining Cycles Max at Deformation Sample Voids Temp. Stress Pressure to Flow Displacement flow point at flow ID (%) ( C.) (kPa) (kPa) Termination Number (mm) strain point 14536 G11C 6.8 40 276 69 10000 2875 7.488 1.088 1.629 14536 G4 6.0 40 276 69 10000 4179 3.466 1.425 2.127 14536 G5 7.0 40 276 69 10000 1429 7.481 1.272 1.903 14697-G2C 7.6 40 276 69 10000 8751 0.836 0.549 0.822 14697-G3C 7.6 40 276 69 10000 7442 1.059 0.683 1.021 14697-G4C 7.6 40 276 69 10000 8237 0.656 0.424 0.633

[0132] The disclosure, as illustrated, provides a method to re-stiffen over-digested rubber modified bituminous binders or blends. Over-digested rubber modified bituminous blends re-stiffened with a stiffness-inducing reactive elastomeric terpolymer additive advantageously produced stiffer, stable and stress resilient homogeneous modified blends.

[0133] Larger volumes of re-stiffened bituminous binders were successfully manufactured and incorporated into Hot Mix Asphalt (HMA) designs. The resulting re-stiffened asphalt binder mixes, when compared with standard binders (namely crumb rubber modified bituminous binders and SBS modified bituminous binders) in identical asphalt mix designs containing the same aggregate, grading and binder content, showed superior performance against the standard mixes in terms of rutting resistance from a performance-related laboratory test.