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Cementitious artificial aggregateparticles for high-skid resistancepavementsFrançois de Larrard a , Rafael Martinez-Castillo a , Thierry Sedran a

, Philippe Hauza b & Jean-Eric Poirier ba Institut Français des Sciences et Technologies des Transports, del'Aménagement et des Réseaux (IFSTTAR, formerly LCPC) , Centrede Nantes, BP 4129, 44 341 , Bouguenais Cedex , Franceb Colas Campus Scientifique et Technique – Direction ScientifiqueCOLAS , 4 rue Jean Mermoz, 78114 , Les Hameaux , FrancePublished online: 11 Apr 2012.

To cite this article: François de Larrard , Rafael Martinez-Castillo , Thierry Sedran ,Philippe Hauza & Jean-Eric Poirier (2012) Cementitious artificial aggregate particles forhigh-skid resistance pavements, Road Materials and Pavement Design, 13:2, 376-384, DOI:10.1080/14680629.2012.666642

To link to this article: http://dx.doi.org/10.1080/14680629.2012.666642

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Road Materials and Pavement DesignVol. 13, No. 2, June 2012, 376–384

SCIENTIFIC NOTE

Cementitious artificial aggregate particles for high-skidresistance pavements

François de Larrard*a, Rafael Martinez-Castilloa, Thierry Sedrana, Philippe Hauzab andJean-Eric Poirierb

aInstitut Français des Sciences et Technologies des Transports, de l’Aménagement et des Réseaux(IFSTTAR, formerly LCPC), Centre de Nantes, BP 4129, 44 341 Bouguenais Cedex, France;bColas Campus Scientifique et Technique – Direction Scientifique COLAS, 4 rue Jean Mermoz,78114 Les Hameaux, France

For some critical road sections, a high skid resistance of wearing course is required to minimisethe risk of traffic accidents. Nowadays this skid resistance is mainly brought by the use ofspecial aggregates as calcined bauxite, a scarce and expensive material. The paper presents apatented technology, where a special high-performance mortar is produced and crushed at earlyage. These cementitious artificial aggregates (CAA) can display aggregate properties close tothose of calcined bauxite. Various models are presented for the prediction of the aggregateresistance to wear, fragmentation, and polishing, making possible the optimisation of CAA forspecific applications.

Keywords: artificial aggregate; high-performance concrete; Los Angeles; mathematicalmodels; mechanical properties; Micro-Deval; optimisation; polishing stone value

1. IntroductionSkid resistance is one of the main properties of pavement surface layers governing the safety ofroad traffic. In Europe asphalt concrete is the most common material found in wearing courses. Amixture of natural aggregate and bitumen is laid and compacted, then exposed to traffic. In the firstdays, bitumen is the dominant material involved in the contact with tyres. However, owing to theshear action of wheels, this thin layer is worn out, and the aggregate particles become apparent.Then the aggregate phase determines the macro-roughness of the layer (through its grading curve)and the bond with tyres (through its micro-roughness).

Requirements for the skid resistance of a pavement depend on the type of road section. In somespecial places, such as braking zones or dangerous bends, a superior level of skid resistance isaimed at. Artificial aggregates, most often calcined bauxite, are selected for this kind of application.These very hard particles are either used in asphalt concrete, or sprayed on as a very thin layer ofbinder, to form a surface dressing. But these materials are expensive, and the resource is limited.More sustainable solutions are required by the road industry. This paper presents an innovative,cement-based material designed to be a substitute to calcined bauxite in high-skid resistancewearing course materials.

*Corresponding author. Email: francois.de-larrard@ifsttar.fr

ISSN 1468-0629 print/ISSN 2164-7402 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/14680629.2012.666642http://www.tandfonline.com

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2. Aggregate requirements for pavement wearing coursesAccording to standards (XP P 18-545), three types of properties are critical for wearing courseaggregate particles:

(1) The material must resist shocks applied by the vehicle wheels. The resistance to frag-mentation is measured in the well-known Los Angeles test (LA) (EN 1097-2), in whicha given mass of particles is mixed with steel balls and subject to rotations in a hollowcylinder. The LA coefficient is the percentage of passing particles through a given sieve.The higher the value, the softer the aggregate. This property seems relevant for aggregatesused in surface dressing.

(2) The amount of wear caused by a shear contact in wet conditions should be limited. TheMicro-Deval (MDE) test (EN 1097-1) simulates this phenomenon. Again, the higher thevalue the weakest the aggregate. This second mechanical property, which is not directlycorrelated with the former, applies for application in surface asphalt mixtures.

(3) Finally, after a certain degree of wear, aggregate particles must keep their micro-roughness, as measured with a skid resistance test (SRT) pendulum (EN 1097-8). Thispolishing stone value (PSV) must be higher than a certain threshold, depending on thedestination of the pavement.

Natural aggregate particles come from a rock having a certain microstructure; a fine texture isoften associated with a high hardness, but with smooth aggregate surface. Meanwhile, a coarsetexture determines softer particles, with rough aggregate surface. It turns out that there is a conflictbetween strength requirements on one hand (as expressed by LA and/or MDE) and skid resistance(as expressed by the PSV coefficient) on the other hand. According to some researches (Tourenq& Fourmaintreaux, 1971) PSV of natural, two-phase aggregates is governed by the hardnessdifference between the two phases. With a low value for the matrix, a high PSV is obtained, withlimited mechanical properties.

Such a conflict can be addressed with a formulated, cementitious material. The friction betweentyres and the pavement surface can be enhanced by a suitable number and shape of fine peaks ofthe surface texture (Delanne, 1993; Do, 2005). These picks can be generated by sand grains in aspecially designed mortar. If a high level of performance is required (through a low water:cementratio) the LA and MDE values of the mature material can be low. But crushing the material at earlyage will create rough fracture aspects. Based on these ideas, cementitious artificial aggregates(CAA) from crushed high-performance mortar were proposed (de Larrard, Sedran, & Lédée,2003). In the original patent two mortar recipes were given, together with some properties ofthe obtained crushed aggregates. Table 1 compares the values obtained for these materials withconventional calcined bauxite. While the LA coefficient seems too high, results in terms of MDEand PSV were very encouraging. Figure 1 shows how CAAs act when subject to wear in theconventional PSV test.

3. Experimental programmeFrom the presented preliminary data, it was decided to further study this new area of CAA(Martinez-Castillo, 2008). The production process was first investigated. The cementitious mix-ture was produced in a laboratory concrete mixer. Then the fresh material was laid on the groundbetween steel corner-irons, forming mortar bars which were manually cut after some minutes. Apolyethylene sheet was put over the samples to provide curing. After a curing time T, the sampleswere crushed. Then artificial aggregates were sieved and kept in polyethylene bags at 20◦C. After28 days, the strength of mortar and the aggregate properties were measured.

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Table 1. Properties of artificial aggregates.

Materials Calcined bauxite CAA (preliminary recipe)

Specific gravity 3.45 2.36LA (%) 12 23MDE (%) 10 11–13PSV (%) 57 62Cost (¤/t)∗ 300–400 ≈ 200

Note: ∗According to 2008 typical cost of the French market.

Figure 1. Effect of wear on the surface of CAAs. The matrix is first worn out; then the sand particles arepolished. SRT stands for the friction measured according to EN 1097-8.

For the design of the mortar mix, five constituents were chosen:(1) The granular phase (sand grains). The sizes used were between 0.5 and 2 mm, and various

levels of hardness were tested corresponding to sand friability modulus values (FS, as describedin NFP 18-576 standard) ranging from 19 to 65. Both crushed and rounded sands were tested.

In the cementitious matrix, (2) Portland cement, (3) silica fume, (4) superplasticiser and (5)water were used. The proportions were varied so that the mortar compressive strength at 28 daysranged from 70 to 120 MPa. Also the strength at crushing time ranged from 25–35 to 45–55 MPa.

A total of 40 different types of CAA were produced. In Figure 2, the results are given in termsof mechanical properties. Areas called A, B or C correspond to the French standard classification(see Table 2). For high-skid resistance pavements, the A and Anc classes are required, with thehighest possible PSV values. In order to optimise the production of CAA, these data were used toderive a general model giving the aggregate properties from mix-design and process parameters.

4. Models for CAA properties4.1. Wear resistance (MDE)This coefficient represents the amount of worn material after a close contact with steel.Figure 3 shows the beneficial effect of mortar strength on the amount of wear. Figure 4,

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Figure 2. Mechanical properties of CAAs. For the best ones (with LA < 15 and MDE < 10), theparticles were heat cured at 90◦C during 48 hours in order to enhance the cement hydration andmicrostructure.

Table 2. Classification of aggregates according to XP P 18-545. For non“nc” classes (non-compensated), one of the two first requirements may beunsatisfied (with a maximum difference of 5 points).

Code LA MDE LA + MDE PSV

Anc ≤20 ≤15 ≤35 ≥56A ≤20 or ≤15 ≤35 ≥56Bnc ≤20 ≤15 ≤35 ≥50B ≤20 or ≤15 ≤35 ≥50Cnc ≤25 ≤20 ≤45 ≥50C ≤25 or ≤20 ≤45 ≥50

shows that above a certain strength threshold the MDE coefficient tends to stabilise, at a levelwhich depends on the quality of sand grains, as described by FS (sand friability modulus,NFP 18-576).

Finally, all data can be fitted by a semi-empirical model of the following type:

MDE = Max {a1Rc28 + a2D90 + a3(D90 + a4)FS + a5; b1 + b2FS} (1)

where MDE is the Micro-Deval coefficient; Rc28 the compressive strength of the mortar at 28 days;D90 the sieve size corresponding to 90% passing of the sand phase; FS the friability modulus of thesand; a1 to a5 and b1 to b2 are empirical parameters, with a1 = −0.199, a2 = −10.5, a3 = 0.224,a4 = −0.345, a5 = 42.2, b1 = 8.35, and b2 = 0.192. Figure 5 shows the quality of this model.

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Figure 3. Effect of mortar strength on MDE. The bottom line corresponds to pure cement paste aggregates.The top line deals with fine mortars, with a maximum size of aggregate equal to 0.5 mm.

Figure 4. Effect of mortar strength on MDE (MSA = 2 mm).

4.2. Polishing resistance (PSV)This property corresponds to the roughness of aggregate surface after a standard level of wear. Inthe case of weak sand grains the wear will quickly smooth the surface and we may expect a lowPSV, irrespective of the mortar strength.

With harder grains and low matrix strength, grains will quit the material and other ones willreappear, so that the roughness will stay at a comparable level. Then one can expect a negativeeffect of compressive strength on the PSV. As for the maximum size of aggregate (MSA), thelarger the MSA the lower the number of contacts and specific area. These trends can be verifiedon Figure 6.

Therefore we suggest a model of the following type:For FS < FS∗

PSV = (b1 + b2D90)(1 + b3Rc28) (2)

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Figure 5. Comparison between experimental and theoretical MDE values.

Figure 6. Effect of various parameters on the polishing stone value. The bottom line deals with soft sand(FS > 33) mortars. The intermediate line stands for coarse hard sand, and the top line for fine hard sand.

where PSV is the polishing stone value; FS∗ a maximum value of the sand friability; and b1, b2and b3 empirical parameters. The best found values were the following: b1 = 71.5, b2 = −1.56,b3 = −0.001 and FS∗ = 33. The mean error given by the model is about 1.4, which is lower thanthe reproducibility of the PSV test (see Figure 7).

4.3. Fragmentation resistance (Los Angeles coefficient)For this property, it is found that, for a given sand grain nature, there is a non-monotonic trendregarding the influence of matrix strength: when the compressive strength increases, the LA valueincreases up to a maximum then decreases (see Figure 8). However, in this test shocks are givenon aggregate particles by a number of steel balls. The dust produced by the shocks remains in the

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Figure 7. Comparison between theoretical and experimental values of PSV.

0

5

10

15

20

25

30

35

40

40 50 60 70 80 90 100 110 120

Rc (MPa)

LA

Andésite 0/2 mm

Alluvionnaire 0/2 C mm

Alluvionnaire 0/2 R mm

Calcaire 0/2 mm

Figure 8. Effect of mortar compressive strength on the LA coefficient of particles, for various natures ofsand grains.

mix and could act as a protection overcoming further fine production. Therefore the LA measuredin the low strength range could be misleading in terms of assessment of in-place fragmentationresistance of CAA.

If one restricts the model area to mixtures having a compressive strength greater than 85 MPa,and aggregate crushed at early age, a relationship between LA value and the tensile splittingstrength of mortar is found:

LA = c1 + c2 Rt28 (3)

where Rt28 is the tensile splitting strength at 28 days, and c1, c2 two empirical parameters. Theoptimum values found for these parameters were the following: c1 = 51.5 and c2 = −4.64.

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Table 3. Effect of mix-design parameters on CAA properties. Rcc is the compressivestrength at crushing time.

CAA property

Mortar mix-design parameters PSV LA MDE

FS −(FS < FS∗) –

Dmax –

Rc28 (for Rc28 > 85 MPa)

Rcc – –

5. Global optimisationTable 3 recalls the effect of mix-design parameters on the various properties of CAAs. The choiceof a set of parameters will be carried out on the basis of a series of specifications, correspondingto a certain destination. In most cases, the specifications shall contain maximum values for LAand MDE, and minimum one for PSV.

The FS parameter must be low enough to provide a durable microtexture to CAA. The lowerit is, the lower the value of MDE, the other parameters being insensitive. In order to maximisethe PSV value, material will be crushed at an early enough age. For the two last parameters, thereare conflicts: a large value of Dmax will produce a good attrition resistance (low value of MDE)but a limited PSV coefficient. A small value will have the contrary effects. As for compressivestrength, a high value is favourable for the mechanical properties (LA and MDE) but tends tolimit the PSV, as already pointed out. Numerical models can be used to determine a combinationproviding an acceptable compromise.

6. ConclusionBy crushing at early age a specially designed high-performance mortar, it is possible to producecementitious artificial aggregates (CAA) with superior properties that can be used in high skid-resistance wearing courses. The key parameters are the choice of sand (FS value, maximum sizeof aggregate) and the compressive strength of mortar, controlled by the water:cement ratio andthe presence of silica fume. A large experimental programme led to the construction of semi-empirical models for the prediction of critical aggregate properties (LA, MDE and PSV). Thesemodels can be used to optimise CAA for a given application.

CAAs constitute an alternative source to replace calcined bauxite, when this material becomesscarce and too expensive. Trials of asphalt mixtures including CAAs gave positive insight on thefeasibility of high-skid resistance wearing course at an industrial scale (Martinez-Castillo, 2008).Although CAAs exhibit a higher porosity, the amount of bitumen needed per unit pavementsurface is comparable to the one of conventional aggregate particles.

Referencesde Larrard, F., Sedran, T., & Lédée, V. (2005). Procédé de fabrication de granulats artificiels. Patent No.

2 858 614; 2003. Pub. No. WO/2005/016848. International application no.: PCT/FR2004/002102.Publication date: 24.02.2005.

Delanne, Y. (1993). Modélisation de la relation adhérence/texture en fonction de la vitesse. Bulletin deLiaison du Laboratoire Central des Ponts et Chaussées, No. 185, 93–98.

Do, M.T. (2005). Relation entre la microtexture et l’adhérence. Bulletin des Laboratoires des Ponts etChaussées No. 255, 117–136.

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EN 1097-1. (1996). Essais pour déterminer les caractéristiques mécaniques et physiques des granulats.Partie 1: Détermination de la résistance à l’usure (Micro-Deval). AFNOR, November.

EN 1097-2. (1998). Essais pour déterminer les caractéristiques mécaniques et physiques des granulats.Partie 2: Méthodes pour la détermination de la résistance à la fragmentation. AFNOR, October.

EN 1097-8. (2000). Essais pour déterminer les caractéristiques mécaniques et physiques des granulats.Partie 8: Détermination du coefficient de polissage accéléré. AFNOR, March.

Martinez-Castillo, R. (2008). Granulats artificiels pour couches de roulement à forte adhérence. (Doctoralthesis). Ecole Nationale des Ponts et Chaussées, February, 218 pp.

NFP 18-576. (1990). Mesure du coefficient de friabilité des sables. AFNOR, December.Tourenq, C., & Fourmaintraux, D. (1971). Propriétés des granulats et glissance routière. Bulletin de Liaison

des Laboratoires des Ponts et Chaussées, No. 51, 61–69.XP P 18-545. (2004). Granulats: éléments de définition, conformité et codification. AFNOR, 58 pp.

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