aggregate breakdown as a cause of chip seal flushing

25th ARRB Conference – Shaping the future: Linking policy, research and outcomes, Perth, Australia 2012

© ARRB Group Ltd and Authors 2012 1

AGGREGATE BREAKDOWN AS A CAUSE OF CHIP SEAL FLUSHING

P.R. Herrington, Opus International Consultants Ltd, New Zealand

G.F.A. Ball, Retired, New Zealand

J.E. Patrick, Opus International Consultants Ltd, New Zealand

J.I. Towler, New Zealand Transport Agency, New Zealand

ABSTRACT

Flushing in chip seals is one of the main factors affecting seal lifetimes in New Zealand. This paper explores the hypothesis that a major cause of flushing in chip seals is the build- up of fine aggregate material in the seal layer, largely produced by surfacing aggregate breakdown and abrasion under traffic. The volume of fine aggregate particles and bitumen present, together eventually exceed the void volume available resulting in a flushed surface.

Results of calculations based on measured void volumes in very dense aggregate particle packing configurations and ‘textbook’ bitumen application rates, showed that multiple seal layers seals should not in theory flush. Dense packing of various aggregate grades and combinations showed that void volume of approximately 40% would be expected in heavily trafficked (compacted) seals- more than sufficient to accommodate the bitumen used in sealing.

Data from volumetric measurements made on a large number of cores taken from flushed multiple layer seals showed the presence of significant quantities of aggregate material passing a 4.75 mm sieve (material which in theory should not be present). The volume of bitumen and fine material present was close to the 40% value predicted at which flushing of well compacted seal layers should occur. Measurements made of two cores showed that the contribution of tyre rubber and other non-aggregate particulates to the fine material was negligible.

Data from cores taken both in the wheel tracks and on the shoulder, at the same site, indicated that breakdown of aggregate due to over chipping during construction may be a significant contributor to fines generation. This contention is supported by preliminary laboratory experiments to measure breakdown under a loaded tyre.

INTRODUCTION

Chip seal flushing is a longstanding and widespread problem and is the single greatest cause of seal failure on New Zealand roads (Transit New Zealand 2005). Flushing is not only an expensive problem but results in slippery, unsafe surfaces and binder pickup on vehicles from bleeding bitumen.

Flushing is defined as low texture (as distinct from bitumen tracking or bleeding-although flushed seals often give rise to both). Flushing thus occurs when the available void volume in a seal is insufficient to accommodate the volume of bitumen, aggregate fines and other detritus present. It is shown below that the void volume of a fully compacted multi-seal layer should in theory be sufficient to accommodate the volume of bitumen used to construct the seal (even allowing for moderate levels of over application). In this paper the hypothesis is put forward that loss of void volume leading to flushing is primarily a result of the build-up of fine (<4.75 mm) aggregate particles produced by wear and breakdown of sealing aggregate during construction and under traffic. Aggregate embedment and possibly even migration of fines from the base course may also exacerbate the process. Aggregate breakdown has been identified as a possible cause of flushing in Australian seals (Alderson 2008). It is also conceivable that fine

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Table 2: Summary of seal void data

Chip grades in seal: 2 2/4 2/4/3 2/4/3/5 2/2 3/2/3/3

Surface thicknessa mm

13.0 22.0 33.0 33.9 26.0 42.0

Total voids Lm-2 6.5 10.9 16.0 13.6 12.3 16.7

Percent voids Lm-2 46.0 47.8 46.9 39.5 48.0 39.5

Available voidsb Lm-2

4.5 8.9 14.0 11.6 10.3 15.0

Percent voids allowing for basecourse embedment Lm-2

31.8 39.0 41.0 33.7 40.2 35.5

Calculated total spray rate Lm-2

2.13 3.60 5.40 6.34 4.26 7.53

Percent voids allowing for basecourse embedment and applied bitumen Lm-2

16.8 23.2 25.2 15.3 23.5 17.7

Available voids/calculated total spray rate

2.1 2.5 2.6 1.8 2.4 2.0

Notes: a) Surface thickness is taken as the depth for which all the chips are just covered. b) Available voids are the total voids minus voids in the bottom layer that would be lost due to embedment into the

basecourse. A figure of 30 percent of the voids in the bottom layer has been taken, i.e. 0.3 � 6.5 2.0 L/m2 for surfaces with a Grade 2 seal at the bottom; for a Grade 3 seal, a value of 2.0 (10.5/12.5) 1.7 L/m2 has been assumed. Potter and Church (1976), examining eight single-coat seals, found that between 20 and 40 percent of the potential voids were filled with basecourse material.

Available voids are typically about twice the volume of bitumen expected from normal spray rates. On this basis, there should never be a problem of flushing. In order to fill available void volume with bitumen twice the normal application rates would be required. If the hypothesis that flushing is caused by the build-up of fine aggregate is correct then it can be inferred from the calculations in Table 2 that in a flushed seal such fine material should exceed, assuming correct spray rates, about 15-25% of the seal volume. Also the total volume of bitumen plus fines plus residual air voids should be at least about 30-40%. These results prompted further research to study the volumetric composition of flushed, multiple layer seals, as discussed below.

VOLUMETRIC ANALYSES OF SEAL CORES

Sixty four seal cores from eight different flushed State highway sites around the country were analysed to determine the volume of aggregate and bitumen present. Only seals with grade 3 or larger chip were examined to avoid possible confusion between generated fine particles and material that may have been added as part of the sealing aggregate itself

The base of the (150-200 mm diameter) cores was cut with a concrete saw to remove any basecourse aggregate. The resulting cores were typically 30 to 110 mm thick and usually comprising 3-4 seal layers (Figure 2). Care was taken to exclude any cores that appeared to have asphalt mix present. Asphalt (probably used as a smoothing layer) was obviously present in several cases despite there being no record of such treatment in the RAMM database. The bulk volume of the core was measured using ASTM D 2726-09, Bulk Specific Gravity and Density of Non-absorptive Compacted Bituminous Mixtures. Surface air voids were calculated by measuring the weight of fine sand, of known bulk density, needed to fill the texture.

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Figure 3: Volume of aggregate passing the 4.75 mm sieve

The volume of fine aggregate plus bitumen and air voids, ranged from 33 to 53%, with a mean value of 41.3% (Figure 4). These values are in good agreement with the void calculations in Table 2 which predict that a flushed seal must have at least 30-40% volume of fine aggregate plus bitumen and air voids. Notably none of the flushed seal specimens were found to have low volumes of bitumen-fines.

Figure 4: Volume of aggregate fines (passing the 4.75 mm sieve) +bitumen +air voids

Non-aggregate fines

The aggregate fraction from two multi-layer seal cores taken from flushed sites on State highways 3 and 4 were examined to determine if non-aggregate material (e.g. tyre rubber, brake linings, organic matter) were a significant contributor to the fine material present. Petrographic

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analyses showed that both samples consisted of a mixture of basaltic igneous rock and quartzose arenitic sandstone (greywacke) as shown in Table 3.

Table 3: Summary of petrographic results

Type (% particles)

Aggregate (> 4.75 mm) Fines (<4.75 mm)

SH3a SH4b SH3 SH4

Balsaltic 30 55 85.9 68.9

Greywacke 70 45 8.3 22.8

Uncertain 0 0 3.4 7.8

Non-aggregate 0 0 2.4 0.5 Notes: a) 6/08/64 SH3 Core 7 b) 6/08/64 SH4 Core 17.

Very little non-aggregate material was identified. Interestingly the proportion of the two mineral types in the two fractions was not the same for a given site. The SH3 site had for example 30% balsaltic material in the aggregate fraction but 85.9% in the fines. This may just be due to differences in the traffic level experienced by different seal layers in the cores, or may reflect real differences in wear behaviour of the aggregate types.

It was also considered highly likely (by the angular shape of the particles) that the fine material had originated from the seal aggregate rather than windblown from the adjacent landscape. The possible contribution of windblown material and aggregate from activities such as gritting in winter however needs further investigation.

Comparison of wheel track and shoulder specimens

The results given in Table 4 were taken from cores from the same seal but at two locations; in the (flushed) wheel tracks and from the sealed road shoulder.

There was no significant difference between the overall volume of bitumen+fines+air voids, found in the wheel tracks and the shoulder. Similarly the aggregate gradings were very similar except for the slightly higher proportion of grade 3 size chips in the shoulder specimens. The mean percentage of aggregate retained on the 9.5 mm sieve being 46% and 58% for the shoulder and wheel tracks respectively. This suggests that the wheel tracks had had at some time an extra application of chip compared to the shoulders. The mean core depth also supports this contention as that of the shoulder is about 20 mm lower than that of the wheel tracks.

In the first instance it would be expected that there should be a higher level of <4.75 mm fines in the wheel paths compared to the shoulder due to tyre wear. One explanation for this apparent inconsistency is that fines generated in the wheel path may be redistributed over the seal by the movement of traffic, rain and wind. This is feasible once the surface of the bitumen is coated with a fine layer of fines and has lost its ‘tack’. Alternatively the findings could imply that most fines generation occurs during construction (where the shoulder and wheel paths receive approximately equal treatment) and that subsequent traffic action adds comparatively little material. Differences may become more apparent for more highly trafficked seals. The common practice of over- chipping during construction provides an opportunity for stone to stone abrasion and fracturing. This seems the most likely route for production of the 2-5 mm size chip fragments found in all the cores studied. Once chip re-orientation is largely complete traffic action is more likely to have a ‘polishing’ effect producing sub-millimetre size particles.

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the mean bitumen/stone ratio was 9.8 % (std dev 1.3) (Jones 2012). The latter figures agree well with the results in Figure 7.

Figure 7: Bitumen/stone mass ratios

Table 7 gives the results of two simulations in which calculations of bitumen/stone mass ratios are made for a hypothetical sequence of seals. An estimation of the seal texture depth after a variable period of time was made using the basic equations from NZTA P17 Performance Based Specification for Reseals (Transit New Zealand 2005, NZTA 2012).

The chip is a grade 2 (12 mm ALD) applied to a surface with a sand circle texture depth of 0.64 mm (NZTA 1981). This gives a bitumen application rate for 1000 v/d of traffic of 1.97 Lm-2. The volume of aggregate in each seal was calculated using a ratio of 800/ALD and converted to mass by assuming bulk density of 1300 kgm-3. The total binder and aggregate masses were then used to calculate the bitumen/stone ratio.

The simulation assumed that the grade 2 seal was resealed after one year. The texture depth from the P17 equations was estimated and converted to a sand circle texture depth of 3.05 mm (second row) and the calculations repeated. It can be seen in this simulation that the bitumen/stone ratio for this extreme case is approximately 11%. When the calculation is repeated for a grade 2/4/3/5 sequence, the binder/stone ratio is approximately 11.5%. These values are consistent with the data in Figure 7 (note that the calculation of aggregate mass is conservative (lower than may be occurring in the field) which results in higher calculated binder/stone ratios). If the binder application rates assumed in this study were increased by 25% then the ratios increase to around 13-14% (Table 8). Clearly to obtain the very high ratios seen in a few cases in Figure 7 then application rates well in excess of the design values would be required.

Except in a few extreme cases the quantity of bitumen present in the cores analysed is consistent with expected (standard) application rates and the conclusion drawn from the analysis in Table 1 (that the void volume available in the seal layer should easily accommodate the bitumen likely to be present) appears valid.

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Binder /stone ratio (%m/m)

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Table 7: Binder/stone mass ratio calculations (traffic = 1000 v/l/d, elv = 2000 v/l/d)

ALD (mm)

Sand circle

texture depth (mm)

Bitumen application

rate (Lm-2)

Age (year)

Volume of aggregate

(m3)

Weight of aggregate

(kg)

Total bitumen

(kg)

Total mass of

aggregate (kg)

Total bitumen/stone mass ratio

12 0.64 1.97 1 0.015 2.0 1.972318 19.5 10.1

12 3.05 2.24 1 0.015 4.2 4.213737 39.0 10.8

12 2.80 2.21 1 0.015 6.4 6.425304 58.5 11.0

12 0.64 1.97 10 0.015 2.0 1.972318 19.5 10.1

7 2.21 1.36 5 0.007 3.3 3.328183 28.6 11.6

10 1.24 1.72 10 0.013 5.1 5.050327 44.9 11.3

5 1.76 0.99 1 0.006 6.0 6.039439 53.0 11.4

12 1.11 2.02 1 0.015 8.1 8.064123 72.5 11.1

12 3.01 2.24 1 0.015 10.3 10.29973 92.0 11.2

Table 8: Binder/stone mass ratio calculations using bitumen application rates 25% above design (Traffic = 1000 v/l/d, elv = 2000 v/l/d)

ALD (mm)

Sand circle

texture depth (mm)

Bitumen application rate (Lm-2)

Age (year)

Volume of aggregate

(m3)

Weight of aggregate

(kg)

Total bitumen

(kg)

Total mass of

aggregate

(kg)

Total bitumen /stone mass ratio

12 00.64 2.47 1 0.015 19.5 2.47 19.5 12.6

12 2.58 2.73 1 0.015 19.5 5.20 39 13.3

12 2.30 2.70 1 0.015 19.5 7.90 58.5 13.5

12 0.64 2.47 10 0.015 19.5 2.47 19.5 12.6

7 1.73 1.63 5 0.007 9.1 4.09 28.6 14.3

10 0.97 2.12 10 0.0125 16.25 6.21 44.85 13.8

5 1.38 1.18 1 0.005 6.5 7.39 51.35 14.4

Materials requirements for sealing chips

The rate of breakdown and wear for a specific aggregate must be related to the physical and mineralogical properties of the material. In the present study specific aggregate properties (such as strength or polished stone value) were not determined, however all the aggregates involved would have met the minimum material requirements for sealing aggregates used in New Zealand that are given in Table 6. The effect of specific aggregate source (i.e. mineralology) on the rate of aggregate breakdown is the subject of on-going research; that work is also considering the usefulness of properties such as Polished Stone Value and aggregate abrasion tests as predictors of the rate of wear in the field.



Table 6: Minimum Requirements for sealing chip in New Zealand, from NZTA M/6, Specification for Sealing Chip (NZTA 2011)

Property Method Requirement

Crushing resistance NZS 4407 Test 3.10 <10%

Weathering resistance NZS 4407 Test 3.11 AA or BA

Weak particles test AS 1141.32-1995 <1%

Polished stone value NZTA T10 53-57* *Typical values.

CONCLUSIONS

Generation of fines from abrasion and breakdown of sealing aggregate during construction and under traffic appears to a major cause of seal flushing. Over application of bitumen (except in extreme cases) or build- up of detritus (e.g. tyre rubber, brake linings) are likely to be secondary factors in most cases.

The role of water vapour as a contributor to flushing through the formation of blisters has not been considered in this paper and needs to be investigated to determine whether the effect is significant compared to fines build up. In particular the mode of water ingress into the seal needs to be established. Fine aggregate material may play a part in the process as the very high surface area presented may allow significant quantities of water to be absorbed into the bitumen mastic.

Further research is needed to identify the role of construction practices (e.g. over-chipping), aggregate mineralogy (source) and seal design (e.g. two-coat versus single-coat seals) on the rate of aggregate breakdown. However even at this stage it would be beneficial to consider practical steps that reduce the likelihood of chip breakdown during construction.

REFERENCES

Alderson, A. J. (2001), Aggregate packing and its effect on sprayed seal design, Austroads Project RC2009-D. 24 pp

Alderson, A. J. (2008), Flushing distress mechanism in seals, Presented at the 1st International Sprayed Sealing Conference, 21–29 July 2008, Adelaide

Ball, G.F.A. and Patrick J.E. (2005), Resealing strategies to increase seal life and prevent seal layer instability, NZ Transport Agency Research Report, 372. 39 pp

Dickinson, E. J. (1990), Sprayed seal design using the voids, and the void distribution with depth in layers of cover aggregate, ARRB 20, no. 2: 38–53

Houghton, L. D., and Hallett, J. E. (1983), An analysis of single-coat seal design, In New Zealand Roading Symposium 1983 2: 249–263. Wellington

Jones, A. (2012), Opus International Consultants Napier, Personal communication

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AUTHOR BIOGRAPHIES

Philip Herrington has 20 years’ experience in bitumen and roading materials -related research. This work has involved fundamental studies into bitumen, its chemistry (in particular oxidation) and rheology, in addition to applied bitumen production asphalt and chip seal-surfacing research. Mr Herrington currently leads a large, multi-year research programme investigating new road construction technologies.

George Ball received his PhD in physics from Canterbury University. He has thirty years of experience in research on road materials and road surface performance. His research interests include the effects of additives on road bitumens, sealing and paving binder quality assurance, factors determining the lifetimes of asphaltic concretes and chip seals, and the environmental impact of road surfacing processes.

John Patrick is the Pavements Research Manager at Opus International Consultants Central Laboratories Lower Hutt New Zealand. John has over 40 years' experience in roading investigations and research. He has been associated with a wide range of research into pavement materials including hot mix asphalt, granular basecourse, aggregates, and chip sealing and bitumen properties. He has also performed research into pavement performance and methods of measurement including roughness and skid resistance. John has been responsible for technical input into revisions to NZTA specifications and developing performance- based specifications for chip seals and hot mix asphalt. Practical experience has been gained during three years' employment with a roading contractor.

Joanna Towler holds the position of Operations Engineer at the New Zealand Transport Agency’s National Office in Wellington. Joanna is responsible for specifications relating to road surfacings and delineation for New Zealand state highways and has been involved in a wide variety of projects, ranging from performance based chipseal specifications, skid resistance, to road markings. Joanna has also had local authority experience, working at the Wellington City council prior to joining Transit New Zealand (now the NZ Transport Agency). Joanna holds a degree of Bachelor of Environmental Engineering and has a Masters of Pavement Technology.

Copyright Licence Agreement

The Author allows ARRB Group Ltd to publish the work/s submitted for the 25th ARRB Conference, granting ARRB the non-exclusive right to:

• publish the work in printed format • publish the work in electronic format • publish the work online. The Author retains the right to use their work, illustrations (line art, photographs, figures, plates) and research data in their own future works The Author warrants that they are entitled to deal with the Intellectual Property Rights in the works submitted, including clearing all third party intellectual property rights and obtaining formal permission from their respective institutions or employers before submission, where necessary.

aggregate breakdown as a cause of chip seal flushing

Documents