Super Microsurfacings - Preprint " World Of Asphalt" AAPA, Sydney
Preprint " World Of Asphalt" AAPA, Sydney, Feb 2000
Glynn Holleran Vice President Valley Slurry Seal Company
Jeffery R. Reed President Valley Slurry Seal Sacramento Ca USA
1. INTRODUCTION
Thin lift surfacing methods such as chip seals and slurry surfacing have been used in many countries as an efficient method of preventive maintenance. In some cases they have been promoted as corrective maintenance treatments and microsurfacing has been successful as a rut filling method , where pavements are stable and rutting is largely due to chain wear or post compaction. (1,2).
Many applications however, in maintenance, involve cracked pavements. Cracking phenomena are any fracture or breaking in the pavement surface caused by a threshold stress being exceeded by either traffic loading or pavement expansion and contraction. In all cases, the effect of cracking is to allow a point of ingress for water into the pavement, thereby hastening the onset of pavement failure. (3).
Cracking can thus be defined as load associated or non load associated. The cause of the cracking may be assessed by using this criteria and examining crack patterns.
The types of cracking of interest are:
a) Fatigue ( load and pavement associated)
b) Low temperature Cracking ( non load and surface associated).
c) Longitudinal Cracking ( Load and non load surface associated)
d) Block cracking ( non load pavement associated)
e) Reflection Cracking ( load and non load and pavement associated).
f) Slippage Cracking ( load and surface associated).
Such distress mechanisms are addressed by pavement strengthening by hot mix, use of membranes for strain alleviation or by reconstruction. Slurry Surfacing has been used in association with membrane seals to address reflection cracking from any of the above sources ( 4, 5, 6). This is however, a two step process and as a result, more costly than a single application.
This project is about development of single application microsurfacing systems that could delay reflective cracking for long enough to make them economic corrective maintenance treatments.
2. DISCUSSION
2.1 Polymer Effects:
Polymer modified systems have been identified as extending the performance of bitumen binders . Fibres have also been used to improve crack resistance (7).
In microsurfacing systems polymers they have been identified as ( 8,9)
a) Improving thermal susceptibility
b) Improving stone retention and resistance to bleeding
c) Improving abrasion resistance
d) Increasing resistance to deformation
e) Increasing crack resistance
f) Higher durability due to higher binder contents
Cracking phenomena however, are not usually well treated by conventional microsurfacings. There are several apparent reasons for this.
The first is that microsurfacings generally have low polymer levels. This is characteristically 3% . It has been shown in studies with SBS for example, that a network is not formed below 4% (10). In most studies there has been shown to be a strong dependency of property modification on polymer concentration, ( 11) and that this is a function of compatibility and polymer type. The systems , being non Newtonian , especially at high polymer levels, are shear and temperature dependent, thus test conditions are important.
The second is that the systems are emulsion systems and, in the case of latex modified systems, the polymer system can act independently of the bitumen emulsion . (12).This leads to a complex interfacial system where polymer is directly adhered to the aggregate surface as well as dispersed within the bitumen phase. This could have the effect of creation of an extra phase within the total aggregate/binder matrix and effectively deplete the bitumen phase of polymer. The presence of neat polymer at the interface may be in fact a good thing but the failure point will be at the weakest part of the binder.
The answer would seem to be to improve the binder properties by increasing polymer level and compatibility and, as far as possible, making sure that there is sufficient polymer in the binder phase.
Microsurfacings are thin. They must act as a membrane to dissipate strain. To overcome reflective cracking, the material must be able to absorb strain created by movements of the cracked base. Figure 1 shows a model- for reflection cracking cause by pavement movement load and non load associated). A crack at the base of the layer moves, straining the base of the layer above. This may initiate a crack or craze in the layer above. This crack can then propagate to the surface. Clearly, in a stiff thick layer, such as hot mix overlay, this will be retarded, particularly if the binder in the system can absorb this strain by extending and then, subsequently recovering. (13).
In a thin layer the membrane must flex and absorb strain. The mechanism of strain dissipation requires the binder to be able to extend without fracture and recover after the stress is removed. Hence, criteria for such a layer, would be high elastic response, high viscosity to maintain membrane integrity, high levels of elongation before fracture, high tensile strength (especially for low temperature crack treatment) uniform isotropic morphology.
Figure 1 Mechanisms Of Reflection Cracking Source Ref 14.
Microsurfacings ,of course, are aggregate/binder mixtures. This would seem to create points of stress concentration and potential crack initiation sites , especially in higher void levels. Compacted microsurfacings tend to have lower void levels than hot mix, but in a thin membrane voids will have a significantly greater effect.
The approach then would seem to be to use as fine as possible an aggregate with as high a level of fines and as high a binder level as possible. This should allow creation of a flexible mortar with good stress absorption properties.
2.2 Fibres
It might follow ,that any mechanism of stress absorption in a membrane, would allow improvements in strain alleviation. Fibres have been used for prevention of drainage in stone mastic asphalt mixes and it might be argued that the presence of a stiffening material, such as this, in the binder and bridging the aggregate particles might assist. Fibres might well combine with the binder and the fines to increase the mortar stiffness.
2.3 Material Selection and experimental.
Elastomeric materials absorb the most energy and are able to elongate most before fracture (14). These were preferred.
Latex is by far the most popular material used in microsurfacing. Natural latex and styrene butadiene copolymers are the most used. In this study, styrene butadiene was selected because it was obtainable in cationic form and easy to use. (Source BASF)
Styrene butadiene block copolymers have long been recognized as the most effective method of modification for membrane applications. (11) A linear copolymer was selected ,as it was able to be compatibilised with bitumen more easily than radial ( source Enichem).
Asphalt Rubber was also used in the form of RG-1 (15).
Cellulose fibres were selected based on an earlier study of effective distribution of fibres in bitumen ( 16).
Microsurfacing emulsions were made using a commercial emulsifier blend . This was chosen with respect to the aggregate used. This aggregate was a basaltic commercial microsurfacing aggregate.
The binders were extracted using an evaporative technique (Caltrans Method Test 331). Studies , (17,18) have shown that the recovery method plays an important role in the morphology and hence properties of the recovered binder. The evaporative method appears to have the least effect. However, this issue is not resolved.
Binders were also characterized using the Dynamic shear rheometer and bending beam rheometer according to the new Caltrans specification for microsurfacing. ( 19)
The mixes were characterized using ISSA standard methods including crack resistance and the results expressed as a ratio. It is well understood that this gives only trends and field evaluation of the best systems. However, this test does allow a range of loading frequency,. This was roughly worked out as slow ( 20sec), fast (5 sec) and medium (10sec). This gave some indication of property improvement.
Some field testing has been carried out but this project requires more and the development of a better physical mix test for flexibility. A modification of the 4 point bending test is a possibility.
3. RESULTS
3.1 Binders
The following binder compositions were examined:
a) 3% , 5% and 10% SBR
b) 3% and 5% SBS
c) 5% SBS with 3% SBR
d) 5% SBS with 3% cellulose fibre
e) 5% SBR with 5% RG-1
3.2 Emulsion.
The emulsions made exhibited satisfactory properties but required different processing conditions and emulsifier levels. SBS based emulsions required up to 1.8% emulsifier. SBR emulsion were around 1.7% emulsifier. The cellulose modified emulsion tended to settle at a greater rate and was unstable. To overcome this, fibres were added directly into the aggregate mixture after wetting and before emulsion addition. See figure 2.
3.3 Bitumen.
A single bitumen was used but compatibility was adjusted with aromatic oil for the SBS modified binders prior to emulsification. This was optimized to a compatible system (11).
3.4 Aggregate.
A single type II aggregate was chosen. A type I might have been expected to give better membrane performance but this would be unlikely to have the surface texture to provide the required skid resistance. ( Grading shown in figure 3)
3.5 Recovered Binders.
These were measured using DSR , conventional penetration and softening point testing and some bending beam measurements.
This shows that G* was improved significantly at higher temperatures by increasing polymer content ; phase angle was reduced showing increased elasticity. All passed the Caltrans criteria for microsurfacing materials. SBS appeared to give the best results ( higher G* and best phase angle).
3.6 Mix properties.
Mixtures were required to pass all microsurfacing requirements according to ISSA. The SBS blends tended to give shorter mixing times, but in general were satisfactory.
As may be seen, the polymers gave excellent WTAT and LWT results. The flexibility was improved significantly by use of SBS at 5% and this was better with SBR. SBR based materials were not as good at 5%.
Fibres had little effect on general properties, but flexibility was improved.
3.7 Field Trials
This has been limited ,but the first trials on pavements showed that the performance was very much dependent on the surface.
In the first trial with SBR latex based, the cracks were created by active concrete slabs. The cracks reflected back within a few months.
Work with SBR combined with RG-1 (and in some cases crumb rubber dry) over aged cracked pavements, are performing well after several years.
SBS based systems have not been trialed as yet but would seem to provide the best opportunity.
4 CONCLUSIONS
Improvements in the crack resistance appear to be possible by increasing polymer content.
SBS gives the greatest apparent increase in flexibility and resistance to reflective cracking. SBR can be used and gives improvements , especially in association with asphalt rubber crumb ( RG-1).
Fibres appear to give few benefits with respect to cracking.
5 REFERENCES
1. "Microsurfacing Guidelines" (1999) California Department Of Transportation
2. "Microsurfacing : Guidelines for Use and Quality Assurance" (1996) Texas Department Of Transportation.
3. Holleran G, (1997) "Distress In Pavement Surfaces" Bitumen Asia 1997 Singapore
4. Holleran G (1996) " Cape Seals,History, Development, Design and Performance"1996 34th Annual Conference of ISSA.Phoenix
5. Holleran G ,Van Kirk J (1998) " The use of Crumb Rubber in Slurry, chip seal and hot mix part 1" REAAA Conference New Zealand Wellington
6. Holleran G, Van Kirk J (1998) "Use Of Crumb Rubber in Slurry Surfacings, Chipseals and Hotmix (part II)" Bitumen Asia Conference Singapore.
7. Technical Literature Enterprise Jean Le Febvre Neuilly France " Grip Fibre".
8. Holleran G, Van Kirk J (1997) " Use Of Crumb Rubber In Slurry, Microsurfacing and Chipseal (partIII). AAPA 10th International Flexible Pavements Conference Perth.
9. Holleran G, (1996) " Benefits and uses Of polymers and latex in slurry Seal systems" ISSA Slurry Systems Workshop.
10. Lu,X, Issaccson U (1997) " Rheological Characterization Of Styrene Butadiene Copolymer Modified Bitumens" J Const & Buildings Vol 11 No 1
11. Holleran G ( 1999) " Polymer Modified Asphalt" Proc International Materials Congress Cancun Mexico
12. Takamura,K (1999) " Spontaneous Formation of the Po;lymer Network on Curing Of The Bitumen/SBR latex Emulsion and its effects on Physical Properties" Eurobitume Workshop on Performance Related properties Of Bitumen Binders Luxemborg.
13. Holleran, G, Dullard P.P (1988) " Stress Absorption and Relaxation in Polymer Bitumen Composites. AAPA 7th International Asphalt Conference Brisbane.
14. Whiteoak D (1990) " Shell Bitumen Handbook"
15. Holleran G,(1989) " The use Of Polymer Modified emulsions in Stress Absorbing Membranes and Seals" AEMA 16th Annual Meeting Dallas
16. Holleran G, (1997)" Use Of Crumb Rubber In Slurry , Microsurfacing and Chip Seals" 10th AAPA International Flexible Pavements Conference Perth.
17. Holleran G (1988) " Fibre Effects in Microsurfacings" Internal VSS Report.
18. Takamura K (1999) Vaccuum Distillation and Superpave Analysis of Emulsion Residues- Emulsion Residues by Force Air Drying" AEMA 27th Annual Meeting New Mexico.
19. AEMA Round Robin (1998) " Emulsion Residues".
20. Caltrans Draft Specification For Microsurfacing (1999).
Glynn Holleran Vice President Valley Slurry Seal Company
Jeffery R. Reed President Valley Slurry Seal Sacramento Ca USA
1. INTRODUCTION
Thin lift surfacing methods such as chip seals and slurry surfacing have been used in many countries as an efficient method of preventive maintenance. In some cases they have been promoted as corrective maintenance treatments and microsurfacing has been successful as a rut filling method , where pavements are stable and rutting is largely due to chain wear or post compaction. (1,2).
Many applications however, in maintenance, involve cracked pavements. Cracking phenomena are any fracture or breaking in the pavement surface caused by a threshold stress being exceeded by either traffic loading or pavement expansion and contraction. In all cases, the effect of cracking is to allow a point of ingress for water into the pavement, thereby hastening the onset of pavement failure. (3).
Cracking can thus be defined as load associated or non load associated. The cause of the cracking may be assessed by using this criteria and examining crack patterns.
The types of cracking of interest are:
a) Fatigue ( load and pavement associated)
b) Low temperature Cracking ( non load and surface associated).
c) Longitudinal Cracking ( Load and non load surface associated)
d) Block cracking ( non load pavement associated)
e) Reflection Cracking ( load and non load and pavement associated).
f) Slippage Cracking ( load and surface associated).
Such distress mechanisms are addressed by pavement strengthening by hot mix, use of membranes for strain alleviation or by reconstruction. Slurry Surfacing has been used in association with membrane seals to address reflection cracking from any of the above sources ( 4, 5, 6). This is however, a two step process and as a result, more costly than a single application.
This project is about development of single application microsurfacing systems that could delay reflective cracking for long enough to make them economic corrective maintenance treatments.
2. DISCUSSION
2.1 Polymer Effects:
Polymer modified systems have been identified as extending the performance of bitumen binders . Fibres have also been used to improve crack resistance (7).
In microsurfacing systems polymers they have been identified as ( 8,9)
a) Improving thermal susceptibility
b) Improving stone retention and resistance to bleeding
c) Improving abrasion resistance
d) Increasing resistance to deformation
e) Increasing crack resistance
f) Higher durability due to higher binder contents
Cracking phenomena however, are not usually well treated by conventional microsurfacings. There are several apparent reasons for this.
The first is that microsurfacings generally have low polymer levels. This is characteristically 3% . It has been shown in studies with SBS for example, that a network is not formed below 4% (10). In most studies there has been shown to be a strong dependency of property modification on polymer concentration, ( 11) and that this is a function of compatibility and polymer type. The systems , being non Newtonian , especially at high polymer levels, are shear and temperature dependent, thus test conditions are important.
The second is that the systems are emulsion systems and, in the case of latex modified systems, the polymer system can act independently of the bitumen emulsion . (12).This leads to a complex interfacial system where polymer is directly adhered to the aggregate surface as well as dispersed within the bitumen phase. This could have the effect of creation of an extra phase within the total aggregate/binder matrix and effectively deplete the bitumen phase of polymer. The presence of neat polymer at the interface may be in fact a good thing but the failure point will be at the weakest part of the binder.
The answer would seem to be to improve the binder properties by increasing polymer level and compatibility and, as far as possible, making sure that there is sufficient polymer in the binder phase.
Microsurfacings are thin. They must act as a membrane to dissipate strain. To overcome reflective cracking, the material must be able to absorb strain created by movements of the cracked base. Figure 1 shows a model- for reflection cracking cause by pavement movement load and non load associated). A crack at the base of the layer moves, straining the base of the layer above. This may initiate a crack or craze in the layer above. This crack can then propagate to the surface. Clearly, in a stiff thick layer, such as hot mix overlay, this will be retarded, particularly if the binder in the system can absorb this strain by extending and then, subsequently recovering. (13).
In a thin layer the membrane must flex and absorb strain. The mechanism of strain dissipation requires the binder to be able to extend without fracture and recover after the stress is removed. Hence, criteria for such a layer, would be high elastic response, high viscosity to maintain membrane integrity, high levels of elongation before fracture, high tensile strength (especially for low temperature crack treatment) uniform isotropic morphology.
Figure 1 Mechanisms Of Reflection Cracking Source Ref 14.
Microsurfacings ,of course, are aggregate/binder mixtures. This would seem to create points of stress concentration and potential crack initiation sites , especially in higher void levels. Compacted microsurfacings tend to have lower void levels than hot mix, but in a thin membrane voids will have a significantly greater effect.
The approach then would seem to be to use as fine as possible an aggregate with as high a level of fines and as high a binder level as possible. This should allow creation of a flexible mortar with good stress absorption properties.
2.2 Fibres
It might follow ,that any mechanism of stress absorption in a membrane, would allow improvements in strain alleviation. Fibres have been used for prevention of drainage in stone mastic asphalt mixes and it might be argued that the presence of a stiffening material, such as this, in the binder and bridging the aggregate particles might assist. Fibres might well combine with the binder and the fines to increase the mortar stiffness.
2.3 Material Selection and experimental.
Elastomeric materials absorb the most energy and are able to elongate most before fracture (14). These were preferred.
Latex is by far the most popular material used in microsurfacing. Natural latex and styrene butadiene copolymers are the most used. In this study, styrene butadiene was selected because it was obtainable in cationic form and easy to use. (Source BASF)
Styrene butadiene block copolymers have long been recognized as the most effective method of modification for membrane applications. (11) A linear copolymer was selected ,as it was able to be compatibilised with bitumen more easily than radial ( source Enichem).
Asphalt Rubber was also used in the form of RG-1 (15).
Cellulose fibres were selected based on an earlier study of effective distribution of fibres in bitumen ( 16).
Microsurfacing emulsions were made using a commercial emulsifier blend . This was chosen with respect to the aggregate used. This aggregate was a basaltic commercial microsurfacing aggregate.
The binders were extracted using an evaporative technique (Caltrans Method Test 331). Studies , (17,18) have shown that the recovery method plays an important role in the morphology and hence properties of the recovered binder. The evaporative method appears to have the least effect. However, this issue is not resolved.
Binders were also characterized using the Dynamic shear rheometer and bending beam rheometer according to the new Caltrans specification for microsurfacing. ( 19)
The mixes were characterized using ISSA standard methods including crack resistance and the results expressed as a ratio. It is well understood that this gives only trends and field evaluation of the best systems. However, this test does allow a range of loading frequency,. This was roughly worked out as slow ( 20sec), fast (5 sec) and medium (10sec). This gave some indication of property improvement.
Some field testing has been carried out but this project requires more and the development of a better physical mix test for flexibility. A modification of the 4 point bending test is a possibility.
3. RESULTS
3.1 Binders
The following binder compositions were examined:
a) 3% , 5% and 10% SBR
b) 3% and 5% SBS
c) 5% SBS with 3% SBR
d) 5% SBS with 3% cellulose fibre
e) 5% SBR with 5% RG-1
3.2 Emulsion.
The emulsions made exhibited satisfactory properties but required different processing conditions and emulsifier levels. SBS based emulsions required up to 1.8% emulsifier. SBR emulsion were around 1.7% emulsifier. The cellulose modified emulsion tended to settle at a greater rate and was unstable. To overcome this, fibres were added directly into the aggregate mixture after wetting and before emulsion addition. See figure 2.
3.3 Bitumen.
A single bitumen was used but compatibility was adjusted with aromatic oil for the SBS modified binders prior to emulsification. This was optimized to a compatible system (11).
3.4 Aggregate.
A single type II aggregate was chosen. A type I might have been expected to give better membrane performance but this would be unlikely to have the surface texture to provide the required skid resistance. ( Grading shown in figure 3)
3.5 Recovered Binders.
These were measured using DSR , conventional penetration and softening point testing and some bending beam measurements.
This shows that G* was improved significantly at higher temperatures by increasing polymer content ; phase angle was reduced showing increased elasticity. All passed the Caltrans criteria for microsurfacing materials. SBS appeared to give the best results ( higher G* and best phase angle).
3.6 Mix properties.
Mixtures were required to pass all microsurfacing requirements according to ISSA. The SBS blends tended to give shorter mixing times, but in general were satisfactory.
As may be seen, the polymers gave excellent WTAT and LWT results. The flexibility was improved significantly by use of SBS at 5% and this was better with SBR. SBR based materials were not as good at 5%.
Fibres had little effect on general properties, but flexibility was improved.
3.7 Field Trials
This has been limited ,but the first trials on pavements showed that the performance was very much dependent on the surface.
In the first trial with SBR latex based, the cracks were created by active concrete slabs. The cracks reflected back within a few months.
Work with SBR combined with RG-1 (and in some cases crumb rubber dry) over aged cracked pavements, are performing well after several years.
SBS based systems have not been trialed as yet but would seem to provide the best opportunity.
4 CONCLUSIONS
Improvements in the crack resistance appear to be possible by increasing polymer content.
SBS gives the greatest apparent increase in flexibility and resistance to reflective cracking. SBR can be used and gives improvements , especially in association with asphalt rubber crumb ( RG-1).
Fibres appear to give few benefits with respect to cracking.
5 REFERENCES
1. "Microsurfacing Guidelines" (1999) California Department Of Transportation
2. "Microsurfacing : Guidelines for Use and Quality Assurance" (1996) Texas Department Of Transportation.
3. Holleran G, (1997) "Distress In Pavement Surfaces" Bitumen Asia 1997 Singapore
4. Holleran G (1996) " Cape Seals,History, Development, Design and Performance"1996 34th Annual Conference of ISSA.Phoenix
5. Holleran G ,Van Kirk J (1998) " The use of Crumb Rubber in Slurry, chip seal and hot mix part 1" REAAA Conference New Zealand Wellington
6. Holleran G, Van Kirk J (1998) "Use Of Crumb Rubber in Slurry Surfacings, Chipseals and Hotmix (part II)" Bitumen Asia Conference Singapore.
7. Technical Literature Enterprise Jean Le Febvre Neuilly France " Grip Fibre".
8. Holleran G, Van Kirk J (1997) " Use Of Crumb Rubber In Slurry, Microsurfacing and Chipseal (partIII). AAPA 10th International Flexible Pavements Conference Perth.
9. Holleran G, (1996) " Benefits and uses Of polymers and latex in slurry Seal systems" ISSA Slurry Systems Workshop.
10. Lu,X, Issaccson U (1997) " Rheological Characterization Of Styrene Butadiene Copolymer Modified Bitumens" J Const & Buildings Vol 11 No 1
11. Holleran G ( 1999) " Polymer Modified Asphalt" Proc International Materials Congress Cancun Mexico
12. Takamura,K (1999) " Spontaneous Formation of the Po;lymer Network on Curing Of The Bitumen/SBR latex Emulsion and its effects on Physical Properties" Eurobitume Workshop on Performance Related properties Of Bitumen Binders Luxemborg.
13. Holleran, G, Dullard P.P (1988) " Stress Absorption and Relaxation in Polymer Bitumen Composites. AAPA 7th International Asphalt Conference Brisbane.
14. Whiteoak D (1990) " Shell Bitumen Handbook"
15. Holleran G,(1989) " The use Of Polymer Modified emulsions in Stress Absorbing Membranes and Seals" AEMA 16th Annual Meeting Dallas
16. Holleran G, (1997)" Use Of Crumb Rubber In Slurry , Microsurfacing and Chip Seals" 10th AAPA International Flexible Pavements Conference Perth.
17. Holleran G (1988) " Fibre Effects in Microsurfacings" Internal VSS Report.
18. Takamura K (1999) Vaccuum Distillation and Superpave Analysis of Emulsion Residues- Emulsion Residues by Force Air Drying" AEMA 27th Annual Meeting New Mexico.
19. AEMA Round Robin (1998) " Emulsion Residues".
20. Caltrans Draft Specification For Microsurfacing (1999).
Last Updated (Monday, 20 July 2009 12:00)