Didier Lesueur

The mechanisms of hydrated lime modification of asphalt mixtures: a state-of-the-art review

Although already known for a long time, hydrated lime (HL) attracted a strong interest as an asphalt additive during the 1970s in the USA, when moisture damage and frost became some of the most pressing pavement failure modes of the time. Given its extensive use in the past 40 years, HL is known to be more than a moisture damage additive: it is an “active filler” that also reduces the chemical ageing of the bitumen and stiffens the mastic more than a normal mineral filler above room temperature. These properties impact durability, and HL is now seen as an additive that increases asphalt mixture durability. This article is a literature review on the fundamentals of the effect of HL on asphalt mixtures. The reasons for it being so effective lie in the strong interactions between both the aggregate and the bitumen and a combination of four mechanisms, two on the aggregate and two on the bitumen. HL modifies the surface properties of the aggregate, allowing for the development of surface composition and roughness more favourable to bitumen adhesion. Then, HL can treat the existing clayey particles adhering to the aggregate surface, inhibiting their detrimental effect on the mixture. Also, HL reacts chemically with the acids of the bitumen, which in turn slows down the age hardening kinetics and neutralises the effect of the “bad” adhesion promoters originally present inside the bitumen, enhancing the moisture resistance of the mixture. Finally, the high porosity of HL explains its stiffening effect above room temperature.

Cold mix design: A rational approach based on the current understanding of the breaking of bituminous emulsions

ABSTRACT The general environmental concern all over Europe constitutes a strong incentive towards cold technologies and therefore designing cold mixes with bituminous emulsion has become a key subject in the last few years. However, cold mix design is not an easy task and necessitates first a good knowledge of the physicochemical aspects involved in cold mix properties, from the characterization of the emulsions to the final application with a given aggregate. Therefore, this paper presents an up-to-date description of the chemistry and physics of bituminous emulsions and their interactions with mineral matter, based on recent published results. In particular, the key parameters involved in the breaking process are highlighted. From this overall picture, a general method to design cold mixes is proposed. This highlights the work needed in terms of testing and standardization in order to develop a rational mix design method, without which further development of cold technologies will be difficult.

Foamability and Foam Stability

ABSTRACT Foamed bitumens were obtained by injecting water into hot bitumen using the Foamlab apparatus from SAE (Fayat Group). Three bitumens were studied and the volume of foam obtained under fixed conditions of bitumen temperature and water content, was quantified through the maximum expansion ratio ERm. This parameter reached a maximum value ERmax(T) versus water content, that increased with temperature and varied to a lesser extent depending on bitumen origin. The optimal water content to obtain ERmax(T) increased linearly with temperature. In all cases, the ratio of ERmax(T) to the optimal water content, called “coefficient of foamability” M, was independent of temperature for a given bitumen, and ranged between 2 and 3. ERm below the optimum water content were directly proportional to the coefficient of foamability and the water content. This is in agreement with simple thermal arguments. Thus, foam formation is controlled by the quantity of water vaporized in contact with the hot bitumen, and the yield and kinetics of this heat transfer, which is a function of the nozzle. The foam half-lives first decreased with increasing water content and were then almost constant at a given temperature once the water content exceeded half of the optimal water content. Moreover, half-life was found to be proportional to bitumen viscosity, in agreement with current theories of foam stability. Lastly, a slight ageing was observed on some of the bitumens after foaming, but it remained much less severe compared to current specifications on RTFOT ageing.

Skin Formation During the Drying of a Bitumen Emulsion

ABSTRACT This article presents a study of skin formation at the free surface of a drying bitumen emulsion. The presence of a skin slows down the evaporation process and may even stop it. High initial bitumen content in the emulsion and fast drying rates are shown to favor skin formation. The effect of mineral fillers is also studied and it is shown that depending on the amount of cations released, the process of skin formation may be greatly accelerated. This may explain former results of very slow drying of slurry seals made with limestone aggregates. A simple theoretical argument is presented to explain skin formation as a consequence of the competition between an advancing drying front that accumulates particles at the surface, and Brownian diffusion that tends to homogenize particle concentration throughout the drying emulsion. Therefore, particle size and drying rates are the governing parameters. Finally, practical consequences are discussed, and it is concluded that too fast drying rates can be detrimental in applications such as surface dressings.

THE RHEOLOGICAL PROPERTIES OF BITUMEN EMULSIONS. PART I - THEORETICAL RELATIONSHIPS BETWEEN EFFLUX TIME AND RHEOLOGICAL BEHAVIOUR

ABSTRACT This article is the first of a series of two that presents a comparison between the rheological properties of bitumen emulsions as measured either by means of efflux time viscometers (NF T66-020 and prEN 12846) or by more fundamental methods using a constant-rate rheometer. In this first part, the basic theory behind efflux time measurements is derived as a function of the rheological properties of the emulsions, assuming first Newtonian behaviour and then a more realistic Casson-type behaviour. Kinetic energy corrections in both cases are included. Thus, it is shown how the yield stress behaviour of bitumen emulsions, to be described more thoroughly in Part II of this series of articles using a Casson law, need to be taken into account in order to accurately describe efflux times measured with conventional devices such as the Engler or STV viscometers (NF T66-020—prEN 12846).

On the impact of the filler on the complex modulus of asphalt mixtures

Mineral fillers have been used in asphalt mixtures from their very beginning. Rapidly, formulators observed that the filler could have a strong influence on the final mixture. Nowadays, mineral fillers are essentially characterised by their ability to stiffen the bitumen, leading to the development of the celebrated “Rigden air voids” RV (EN 1097-4), now the corner stone of the European specifications for fillers in asphalt mixtures. If the way the filler modifies the rheological properties of mastics is now well described, the relationship between mastic and mixture properties remains poorly understood. Therefore, this paper describes a systematic study on the impact of increasing filler contents on asphalt mixtures. Experimental results are presented for two very different asphalt mixture formulas, a regular semi-dense Asphalt Concrete (AC) and an open-graded AC for very thin layers (BBTM). The complex modulus of each formula was measured as a function of filler content with two materials having very different stiffening behaviour, that is, limestone filler (LF) and hydrated lime (HL). For both formulas and regardless of the type of filler, the modulus versus filler content curves exhibited a well-defined maximum. The filler content at which the maximum was found was strongly filler dependent, with HL formulas peaking at 4u2009wt.% and LF formulas peaking at 10u2009wt.%. An original interpretation based on reduced variables was proposed, showing that the maximum in modulus occurred for reduced volume fraction of filler close to 66%. This suggests that, before the peak, modulus increased with filler content as a consequence of a stiffer mastic, and after the peak, modulus started to decrease due to poor aggregate coating as a consequence of too-viscous a mastic. As a practical consequence, the studied mix formulas could accept higher filler loadings than currently specified, which would potentially stiffen the mixtures by ∼20%. Therefore, one way to benefit from these findings without changing the specifications would be to use blends of HL and LF. Of course, these findings must be validated with other mixture formulas and raw materials, and similar systematic study on the filler impact on the resistance to fatigue and moisture damage would be needed before implementing the conclusions based on modulus only.

THE RHEOLOGICAL PROPERTIES OF BITUMEN EMULSIONS. PART II - EXPERIMENTAL CHARACTERIZATION

ABSTRACT This article is the second of a series of two that presents a comparison between the rheological properties of bitumen emulsions as measured either by means of efflux time viscometers (NF T66-020 and prEN 12846) or by more fundamental methods using a constant-rate rheometer. In this part, it is shown that bitumen emulsions behave as Casson fluids, described by only two parameters: a (very low) yield stress and a high-shear limiting viscosity. Thus, it was possible to superpose all rheograms for all emulsions on a single master curve of reduced stress (stress divided by yield stress) versus reduced strain rate (strain rate multiplied by high-shear viscosity and divided by yield stress). Comparison between efflux times calculated using the theoretical results derived in part one of the paper, and measured one, showed that reasonable estimates of the efflux times of bitumen emulsions are obtained neglecting their yielding behaviour. Finally, efflux times of bitumen emulsions in standard geometries typically measure their high shear limiting viscosity which is representative of industrial phenomena such as pumping, but do not give information about the behaviour at low strain-rates under static conditions like those found in the coating or drainage processes. Therefore, they remain good quality control tools, but it must be kept in mind that they only characterize part of the rheological behaviour of a bitumen emulsion.