[american society of civil engineers geoshanghai international conference 2010 - shanghai, china...

Effect of Aggregate and Asphalt on Pavement Skid Resistance Evolution

Dan ZHAO1, Malal KANE, Minh Tan DO

Laboratoire Central des Ponts et Chaussées, LCPC, Route de Bouaye, 44341 Bouguenais Cedex, France 1PhD, LCPC, Tel.: +33 2 40 84 57 17; E-mail address: [email protected] (D. ZHAO) ABSTRACT: When designing pavement, engineers must optimize some requirements such as user safety (skid resistance), environmental impact (noise, rolling resistance…)... However, this skid resistance evolves during the entire pavement life. So it is a common practice to perform laboratory tests to forecast the evolution of skid resistance. Previous works done in the French Laboratory of Bridges and Roads (Laboratoire Central des Ponts et Chaussées, LCPC) have identified phenomena such as binder removal, aggregate polishing and seasonnal variations to be responsible of these variations. This paper focuses on the polished stone values of aggregates and the aging of asphalt on the evolution of pavement skid resistance. Skid resistance of different specimens of nude aggregates and asphalt mixes that are submitted to polishing and aging was studied. On skid resistance point of view, aging of aggregates can be neglected in comparison to those of asphalt. Rocks with high polishing resistance offer less variation of skid resistance. Aging of asphalt tends to increase skid resistance until 12 month and remains this latter constant after. INTRODUCTION Skid resistance is one of the fundamental requirements that provide a safe road (Diringer and Barros 1990; Roe and Hartshorne, 1998). But, unfortunately pavement skid resistance evolutes during the whole pavement life due to change on pavement surface characteristics. In the case of asphalt pavements, skid resistance is governed by, among other factors, asphalt types and aggregate properties (Michelin Company, 2000].

Research has been launched at LCPC since 2004 to investigate the polishing phenomenon of asphalt pavement. As results of this investigation, two parts was clearly observed from the evolution tendencies (see FIG.1): the friction coefficient increases firstly until reaching a maximum then decreases (Minh-Tan Do et al, 2007).

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Copyright ASCE 2010 GeoShanghai 2010 International Conference Paving Materials and Pavement Analysis

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FIG. 1. Friction coefficient versus number of passes with WS machine

Comparing skid resistance evolution of asphalt mix specimen and specimen of nude aggregate (see FIG.2), the most important point is that the aggregate and asphalt curves coincide after the asphalt has reached the maximum friction. This result was explained by the fact that once the binder layer of asphalt pavement is removed, the aggregates are exposed little by little, and the surface of asphalt pavement behaves as the aggregate in this moment. For summarizing, it can be said that the skid resistance evolution is controlled by the aggregates after the binder removal [Tang, 2007; Minh-Tan Do et. al. 2008; EN 1097-8. 2000; Y. Brosseaud and V. Le Turdu., 2005].

0,2

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0 50000 100000 150000 200000

Number of polishing

Fric

tion

coef

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nt

Asphalt core Aggregate disc

FIG. 2. Friction coefficient evolutions simulated by WS machine of asphalt and

aggregate

In this paper, the effects of asphalt and aggregates characteristics on the evolution of skid resistance are analyzed. The experimental program is based on two parts. The first one focuses on the aggregate effects whereas the second consists on studying the effect of the asphalt.

GEOTECHNICAL SPECIAL PUBLICATION NO. 203 9


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EXPERIMENTAL PROGRAM Wehner-Schulze machine The experimental program is based on a set of tests with the Wehner-Schulze machine. This machine contains two stations for respectively performing polishing and measuring friction (see FIG.3).

FIG. 3. Wehner-Schulze machine The polishing station contains three rubber cones mounted on a rotary disc and rolling on the specimen surface. Test parameters of this station are: • load on all polish cone rolls is 40kg; • revolution of polishing head is 500 r.p.m; • flow of abrasive water mixture is 5 l/min; • surface is polished on a ring of roughly 16 cm diameter and 6 cm width; • temperature of abrasive water mixture is 20°C; The friction measuring head composes of three small rubber pads (4 cm²area for each pad) disposed at 120° on a rotary disc. Test parameters at measurement station of friction coefficient with friction-measuring head: • load on all measurement rubber is 26kg; • start velocity of measurement rubbers is 100 km/h; • flow of water is 20 l/min; • contact surface is 82 cm²; • temperature of water is 12°C; Specimens are cores of 22.5cm diameter. Aggregate specimens Six types of aggregates characterized by their Polished Stone Value (PSV - see Table 1) were used in this study. The used procedure for performing the polishing process can be found in the following reference (Tang, 2007). Circular specimens are prepared in laboratory with 7.2/10 aggregate size (see FIG.7 (b)). They are fabricated by placing manually the aggregates in a single layer as closely as possible, with their flattest faces lying on the bottom of a mould, then filling the

GEOTECHNICAL SPECIAL PUBLICATION NO. 20310


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mould with resin.

Table 1. Aggregate characteristics

Aggregate A B C D E F Type Rhyolite Spilite Gabbro Diorite Leptynite Limestone PSV 56 53 51 50 49 41

To study the effects of types and aging of aggregates, the following test program is elaborated (see FIG.4).

FIG. 4. Aggregate test program Asphalt specimens This part is planned to study the effect of asphalt on skid resistance. The studied specimen is a very thin asphalt concrete (VTAC) with STYRELF 11-40 bitumen (see Table 2) [AFNOR, NF EN 12697-33, 2004] taken on an experimental road section.

Table 2. Description of Asphalt Mixes and Aggregates

Site Traffic(105 trucks/year) Mix Binder Aggregates PSV

Experimental section 1.14 VTAC STYRELF

11-40 Gabbro 51

The program is shown by the flow charts in FIG.5.

Aggregate

Specimens exposed outside of a LCPC building

Specimens with 6 different PSV

Polishing simulation

Ageing effect



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FIG. 5. Asphalt mix test program

In this road section, ten cores of 22.5cm diameter are taken from the road side just after the road construction (see FIG.6(a)) in September 2004: • one specimen is stored in the laboratory as a reference; • the other specimens are used for polishing tests. Other cores are taken every 6 months of traffic in site (see FIG.6 (b)): • three from the right wheel path; • four from the road side (see FIG.7(c,d)).

(a)just after construction (b)every 6 months

FIG. 6. Planning of in situ samples extracted Part Ⅱ

Friction measurements were made using the Wehner-Schulze machine as described in previous paragraph.

VTAC

Every 6 months (4 cores) Every 6 months (3 cores)

Road side Right wheel path

After road construction (10 cores)

Traffic effect

Ageing effect

One for reference

One exposed outside of a

LCPC building

8 for polishing



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(a) (b)

(c) (d)

FIG. 7. Examples of Specimens

TEST RESULTS Influence of aggregates Polishing of aggregates The evolutions of skid resistance of different aggregates are shown in the FIG.8. It can be seen that the initial values are almost the same, however, after some polishing passes; each of them shows a different evolution tendency. Three groups of evolution curves can be distinguished from the PSV value. The first group consists specimen F with PSV very low (41); the second group is composed of four aggregates (specimen B to E), with an average PSV between 49 and 53; and the third group is a single specimen A with the highest PSV (56).

0,1

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0 50 000 100 000 150 000 200 000

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Fric

tion

coef

ficie

n

A B C D E F

FIG. 8. Evolution of friction coefficients of different aggregates

(a) specimen cored just after the road construction; (b) specimen of nude aggregates fabricated in laboratory; (c) specimen cored after twelve months of road side; (d) specimen cored after forty eight months of road side.



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FIG.9. shows the correlation of friction coefficient versus the PSV of aggregate after 180000 polishing. There is reasonable correlation between the aggregate PSV and their final friction coefficients. A mathematical relation can be proposed as follow:

29.0100

18.1 −= PSVWSμ ( 66.02 =R ) (1)

y = 1,18x - 0,29R2 = 0,66

0

0,1

0,2

0,3

0,4

0,5

0,6

0,4 0,5 0,6

PSV/100

Fric

tion

coef

ficie

nt (1

8000

0)

FIG. 9. Relation between friction coefficient and PSV after 180000 polishing

passes

This indicates that aggregates of higher PSV may offer less decrease of skid resistance, especially at the start of the polishing process. This study confirms perfectly a linear relation of PSV versus friction coefficient of previous studies where other kinds of devices were used. For example, with the British Pendulum Number measured, Diringer proposes:

PVC)PVe1(C 2

C0min

1 +−=μ (2)

with, PV: minimum polish value; and 210 ,, CCC : nonlinear regression coefficients (Diringer K.T. and Barros R.T., 1990). And Roe proposes a relation representing the influence of traffic by SCRIM:

C)CVDln(BPSVAmin +⋅−⋅=μ (3)

with, PSV: Polished stone value of aggregate; CVD: Number of commercial vehicles per

day and per lane; A, B, C: coefficients determined by fitting (P.G.Roe and S.A.Hartshorne, 1998).



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Aging of aggregates For studying the aging effect, the friction coefficient is measured at every 6 months on specimen made with aggregate C (Table 1). This latter is exposed outside, then submitted to all kinds of attacks of the nature (rain, sun, temperature…). The aggregate friction curve presents a slight decrease of only 0.05 from 0 to 24 months (see FIG.10). This low variation of friction due to the nature attacks indicates that, for this aggregate, we can ignore the aggregate aging effect.

0,2

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0 6 12 18 24 30Time (months)

Fric

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coef

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FIG. 10. Friction coefficient evolutions of aggregate

Influence of asphalt Aging effect without traffic FIG.11 shows the friction measured on specimen cored at each six month on the road side (untrafficked part of the road). A significant increase of the friction (+ 0.2) is observed from zero to 12 months following by stabilization.

0,2

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0 6 12 18 24 30 36 42 48Time (months)

Fric

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coef

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FIG. 11. Evolution of the friction coefficient of asphalt pavement due to aging

alone

Pink curves with full squares of FIG.12 and FIG.13 show the friction measured on specimens extracted at 48 months after road construction. The friction coefficient starts from high value (higher than those of specimens extracted at 0 months) then decreases without ever reaching the curves of the new asphalt and the specimen of nude aggregate. These results suggest that the mechanism of hardening of asphalt increases the friction



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that tends to remain even with increasing the polishing passes.

0,2

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0 50000 100000 150000 200000Number of passes

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48 months 0 month

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Number of passes

Fric

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nt48 months aggregate

(a) (b)

FIG. 12. Friction coefficient evolution with polishing mumber of specimens

without traffic Aging effect with traffic We have observed that friction evolution stabilizes approximately from 12 months after road construction in site (see FIG.11). Just after road construction, the binder of bitumen is soft and easy to be removed by polishing or traffic, so we can observe, in laboratory simulation with Wehner-Schulze machine, that the evolution curves of nude aggregate specimen (orange dotted curve) and of new asphalt specimen (green curve) coincide after a given number of polishing cycles (see FIG.12) (X Lu and Ulf Isacsson, 2002; Y.Brosseaud and J. Bellanger. 1997). And this phenomenon confirms the previous conclusion (see FIG.2). However, after some years, the chemical changes of asphalt harden the asphalt binder, and make it difficult to be removed (Fabienne Farcas, 1996).

From the part after 50 000 number of polishing passes in curves of FIG.13, the different gaps can be seen. The gap in FIG.13, between the asphalt pavement specimen with traffic (pink curve with hollow squares) and without traffic (pink curve with full squares), occurs by asphalt aging. After 48 months road traffic service, asphalt binder is removed and this makes the aggregates expose. The texture of pavement can be changed by exposed aggregates, residual asphalt, dust, etc… So the gap between the new specimen (nude aggregate and new asphalt mix) and the specimen with traffic of 48 months is less obvious but can’t be neglected.



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48 months with traffic 48 months no traffic0 month aggregate

FIG. 13. Friction coefficient evolution with traffic and without traffic Summary of findings The aging effects of aggregates may be neglected comparing with those of asphalt on skid resistance. Traffic tends to polish the aggregates and makes them slipper and that rocks with high polishing resistance may offer less variation of skid resistance. Without traffic, the aging of asphalt tends to increase skid resistance until 12 month and remains this latter constant after. On pavements submitted to traffic, the removal of asphalt exposes the aggregates on the pavement surfaces step by step to polishing. Conclusions In this paper, several studies have been applied to investigate the effect of aggregate and asphalt on the evolution of skid resistance. The experimental campaign was undertaken with specimens fabricated in laboratory and extracted from experimental road. All the polishing tests and skid resistance measurement are carried out with the Wehner-Schulze machine. These results obtained allow showing the importance of aggregate nature, especially the PSV (greater PSV aggregate may offer a low great variation of skid resistance). The test results also show that asphalt had a significant effect on the measured skid resistance. We have observed the importance of asphalt aging, and this effect have a tendency to increase the skid resistance. This study allows exploiting the influence of some factors of aggregates and asphalt on the evolution of skid resistance, and indicates us the importance of considering these influences in analysis.



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REFERENCES Diringer K.T. and Barros R.T. (1990). Predicting the skid resistance of bituminous

pavements through accelerated laboratory testing of aggregates, Surface characteristics of roadways: International Research and Technologies, ASTM STP 1031, American society for testing and materials, Philadelphia, pp. 61-67.

P.G.Roe and S.A.Hartshorne (1998). "The polished stone value of aggregates and in-service skidding resistance." TRL Report 332, 28.

Michelin Company. (2000). " Le pneu/l’adhérence." Clermont-Ferrand, Michlin Technology.

Minh-Tan Do and Zhenzhong Tang and Malal Kane and François de Larrard. (2007). " Pavement polishing: Development of a delicated laboratory test and its correlation with road results." Wear, 263(2007), 263:p36-42.

Tang. Z (2007). "Polishing and skid-resistance of road pavements." PhD thesis (in French) of ENPC (Ecole Nationale des Ponts et Chaussées), LCPC.

Minh-Tan Do and Z. Tang and M. Kane and F. de Larrard. (2008). " Evolution of road-surface skid-resistance and texture due to polishing." Wearing Press , Corrected Proof.

EN 1097-8. (2000). "Tests for mechanical and physical properties of aggregates Part 8: Determination of the polished stone value."

Y. Brosseaud and V. Le Turdu. (2005). "Adhérence des revêtements de chausses routières" Bulletin des laboratories des Ponts et Chaussées, 225: 71.

AFNOR, NF EN 12697-33. (2004). "Test methods for hot mix asphalt Part 33: Specimen prepared by roller compactor."

Xiaohu Lu, Ulf Isacsson. (2002). "Effect of aging on bitumen chemistry and rheology." Division of Highway Engineering Royal Institue of Technology, Sweden.

Y.Brosseaud and J. Bellanger. (1997). "Les nouvelles formulations d’enrobés pour couches de roulement evaluation de leurs caractéristiques de surface" Revue générale des routes et autoroutes, 752 : 51-56.

Fabienne Farcas. (1996). "Etude d’une méthode de simulation du vieillissement sur route des bitumes." Thèse de doctorat de l’Université Paris VI.



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