THE DEVELOPMENT OF INITIAL STRENGTH IN SLURRY SURFACINGS BY THE FORCED COALESCENCE OF ASPHALT FILMS

Glynn Holleran1, Irina Motina 2

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1. INTRODUCTION

 The formation of films during the process of coalescence build up in cold mixes is a key factor in mix success in the field. The effect of compaction and use of coalescing agents was reported at the International Slurry Surfacing Association meeting in 2003 (1). This is part of ongoing research to understand the mechanisms of coalescence and curing in aggregate emulsion mixtures and how they may be controlled.

This work was designed to examine the design and field considerations related to humidity, night work and temperature variation during curing. The aim was to be able to predict mixture performance under a range of conditions with a simple test and to use this test to enable design improvements.

The effect of emulsion formulation and use of co-emulsifier systems was examined and ISSA test 139 was modified to determine curing effects over a wide range of conditions. The results have been applied to field jobs in high humidity areas and wet and dry freeze areas. It has also been used to facilitate filling ruts, especially in damp cool conditions.

The work demonstrates that mixes may be modified by use of coalescing agents and compaction to product faster curing materials under a range of conditions while retaining good mix workability and not resorting to more expensive microsurfacing emulsifiers.

This paper also examines the affect of compaction, as a means of forcing film formation and cohesion build up in systems and the effects of wet aggregate on curing.

The basic theory of film formation, and the variables which affect it, is discussed.
2. BREAKING AND FILM FORMATION

 A slurry surfacing requires two mechanisms to operate for it to become a useful road surfacing. The first is adhesion development and the second is cohesion build up. Adhesion development is a chemical function of the interaction between the asphalt and the aggregate (and pavement surface) ,and the cohesion build up is the creation of internal strength of the binder and is a function of the emulsion properties and the application conditions. Quick set and microsurfacing systems are designed to allow a chemical interaction between the emulsifiers used in the emulsion, and the aggregates. Evaporation of water is usually a confounding factor for most applications.

The adhesion development and cohesion build up would appear to operate as follows (1, 2,3):
  1. The free emulsifier in the emulsion is attracted to the electronegative sites of the aggregate forming an amine silicate salt, or to carbonaceous aggregates, forming an insoluble amine carbonate.
  2. The loss of free emulsifier changes the equilibrium of the double layer, causing some emulsifier to migrate into the aqueous phase. This ultimately destabilizes the emulsion causing flocculation and coalescence.
  3. The asphalt particles near the surface of the aggregate pack in an ordered structure at the interface. The particles then flocculate and coalesce.
  4. The process continues by water expulsion and evaporation until the asphalt is a continuous phase.
  5. In the case of latex modification, the latex also undergoes this mechanism and is distributed through the asphalt phase.
The detail of film formation mechanisms in emulsion systems seems to have several steps. (4)
  1. The double layers undergo significant interaction after being destabilized, due to aggregate interaction and evaporation.
  2. The particles come into irreversible contact. If the emulsion flocculates too quickly at this stage, it can occur before the particles are ordered into a compact form. This process is mostly evaporation controlled. Adhesive bonds are formed at this point.
  3. A continuous film forms as the remaining water leaves the film. This occurs by diffusion via inter particulate channels. Evaporation at this stage has slowed to the rate of diffusion. It is at this point where the mechanical properties of the film (cohesion) are developed.
  4. In the developing film three regions may be observed. The first, is a dry region where the emulsion has coalesced and coated the aggregate; the second is an intermediate region of partly flocculated and coalesced emulsion. The third is a wet (emulsion) region where the emulsion is largely unchanged. The second region has some strength, but the wet region has none. Drying occurs from the inside outwards. This is shown in figure 1.

Figure 1 Three Regions of Film Formation (after reference 3)

The rate at which this process occurs determines the traffic time and initial strength of the slurry surfacing. This process seems likely to depend on several variables.
  1. Binder type: Different binders have different minimum film forming temperatures (MFFT). Harder binders or binders below their Tg- glass transition temperature- may not form continuous films.
  2. Particle size: The film formation process may be slower in coarser emulsions, which may trap water as they come together (4). Also the particles pack better and form a more ordered structure prior to flocculation and coalescence. It has been shown (4) that coarser particles produced films in latex systems that were more permeable.
  3. Reaction rates: It may be slowed and therefore is emulsifier and aggregate dependant.
  4. Ambient Temperatures: Lower temperatures and the coalescence rates of the binder at the aggregate surface slow the rate. Therefore, the rate is dependant on asphalt grade and additives. For example, diluents such as kerosene will increase coalescence rate; impermeable or hydrophilic additives will slow the rate of film formation. Higher temperatures can lead to flocculation before ordering has occurred and result in poor adhesion and a non-continuous film.
  5. Particle Size Distribution and non-homogeneity of aggregate: In reality, slurry surfacings are complex interactive mixtures. The range of particle sizes in the aggregate means that reaction rates are different for different size fractions.
  6. At higher temperatures, i.e. above the glass transition temperature (Tg) the mechanical aspects of film formation may be interrupted by excess water preventing floccs from coalescing.
It is not unreasonable to expect that squeezing the binder film will increase water loss at the diffusion stage and thereby accelerate film formation, leading to and increase in the rate of cohesion build up.

3. EXPERIMENTAL

To effectively design mixtures, the above mechanisms may be used as a guide to improving performance. The determining factors of performance are often not related to the standard laboratory or ideal conditions. These factors include humidity, night work, and temperature during curing. Variables such as traffic (compaction) and emulsion formulation are important contributors to the overall results. Aggregate is an important factor and wet aggregate is a perennial problem in some locations. This work too included examination of wet aggregate and its effect on curing. It was found that water that had soaked the aggregate in the stockpile was not equivalent to the same water concentration added ion the mixing phase to dry aggregate. Wet aggregate had much poorer cohesion build up.

3.1 Test Methods

Two tests were employed; firstly wet cohesion testing TB 139. Different curing conditions were used and drying times from 15 minutes to 90 minutes. Also samples were cured 10C/High Humidity and Lab Humidity, 25C/High Humidity and Lab Humidity, and 40C/High Humidity and Lab Humidity. Some samples were compacted, where possible, using a hand roller.

It is anticipated that an automated version of this test will provide more reproducible results. One operator was used in all this work and the results replicated 3 times. Thus, the results are an internally comparable set only and allow the observation of trends only.

The second test was TB 100. The standard TB 100 was carried out and compared to results obtained after compacting the samples using a hand roller. Compaction was carried out on the samples when they were cured to a firm consistency. Unfortunately, this is variable, depending on the mixture, and required a judgment assessment as to when the sample could take the roller without excessive deformation. This was assumed to be where the binder had at least all reached stage 2 in the film formation process. It is anticipated that some repeat work will be done using the Screg wheeled wet track test. (5) For non-standard curing conditions as this is expected to produce more reproducible results for cohesion development measurements.

Samples were cured at 25C (laboratory) and 60C(oven) overnight (16 hrs). Samples were also cured at 10C and high humidity and 40C at high humidity for 16 hours to give an idea of cohesion development. This was done with and without coalescing agents for different emulsion types, with and without compaction. The effect of wet aggregate was also examined for standard conditions and high humidity cure for 25C and 10C.

Some samples were subjected to 1 hour, and also 6 days soaking in water at 25C. Most non-oven cured samples fell apart in 6 day soaking.

3.2 Effect Of Emulsion Formulation

Asphalt (bitumen) emulsions are usually formulated from a combination of performance and convenience. In mixtures such as slurry surfacings, performance characteristics are important. In many countries restrictions on weather, bitumen sources aggregates and transportation make the convenience aspect often as critical as performance. For example, in one market the bitumen is less dense than the latex modifier in the system and the latex separates to the bottom of the tank and the bitumen to the top! Compromises must be made, and so fine-tuning ,and the ability to do it, is important.

G&J have developed a system referred to as Tri-emulsions to address these issues. This approach used a combination of co-emulsifiers to address different property requirements such as aggregate compatibility, transport stability, control of separation and in field performance. Cost is also an issue, so such optimization is useful if it can lead to property improvement without using very expensive emulsifiers (such as specialty microsurfacing emulsifiers). Such emulsifiers may not even give the best property balance given all circumstances.

The emulsions are all optimized for particle size and particle size distribution (6). This is done by equipment selection and set up, manufacturing conditions selection, optimization of exit temperature and optimization of formulation.

In this study, a single aggregate is reported (although many were tested). This was a volcanic source granite material sourced in northern Europe. The main problems associated with this aggregate for field purposes were consistency and maintaining a good level of fines. Figure 2 shows the grading and table 1 shows the aggregate properties. This is a fine standard ISSA type III and the mixes were intended for a wide range of traffic conditions and climates ranging from winter freeze / hot summer to humid hot summer with winter freeze thaw.

Test Result Specification
SE6565 min
Soundness8% NaSO415% max
LA Abrasion25‹30

Table 1: Aggregate Properties


Figure 2 Aggregate Grading (best available source)

The emulsions were:

X: tri-emulsion quick set slurry system latex modified
Y: tri-emulsion modified microsurfacing system
Z: tri-emulsion full microsurfacing system

The emulsions were modified with coalescing agent at 1-3%.

Test X X Ca Y Y Ca Z Z Ca
Binder 62.161.9 62.562.364.263.0
Sieve 0.0 0.0 0.1 0.050.2 0.2
pH2.22.3 2.12.42.32.5
Settlement
1 day		0.2 0.1 0.4 0.3 0.6 0.5

Table 2 Emulsion properties.

The standard mix optimization was carried out with each emulsion i.e. a full mix design to the A 143 guidelines (7). Slurry surfacings are aggregate binder mixtures. As they are spread and leveled, only by moderate pressure from squeegees, they have high voids levels in their pre trafficked form. If un-compacted, they will retain this high void structure. Usually this is closed up in traffic lanes by vehicles. Compaction was carried out using a press in the case of cohesion samples and a weight of 100kg for 5 seconds. In the case of wet cohesion, the samples were rolled using a hand roller (Vialit test roller) when the samples could just take the weight of the roller without lateral flow. This was done on samples taken from the oven usually after 2-3 hours.

These mixtures were designed designated as Ca- Coalescence agent added and C, compacted e.g. Xca- Coalescence agent added to emulsion X formulation; XC- compacted sample, and; XcaC- Coalescence agent added to emulsion X formulation, with sample compacted.

3.2.1 Cohesion Results

Figure 3 shows the TB 139 results for all the emulsions and it is indicated that this test showed the differences between the emulsion types.

The cohesion rate build rate up was clearly better for the microsurfacing type emulsions. Coalescence agents seemed to improve the results, but at these standard cure conditions, it was not pronounced. When compaction was added to the system, the cohesions were higher.


Figure 3 Cohesion Results Different Emulsions

3.2.2 Wet Track Results

Figure 4 shows the wet track results for the three emulsions under standard curing conditions and 1 hour and 6-day soak. It is clear that as all were optimized and had added latex in the formulation, that wet track results were similar. Addition of coalescing agents improved the retention of stone in this test. When compaction was added ,significant improvements in stone retention were noted.


Figure 4 Wet Track Results effect Of emulsion and Compaction

3.3 Effect of Curing Conditions

The curing conditions were varied to simulate different field conditions. This was done by curing at different humidity using a simple apparatus employing hot water, ambient water or ice water contained in a lower chamber with the samples cured in an upper air gap as per figure 5. Temperatures could be kept at 10C, 25C or 40C.


Figure 5: Simple apparatus for Determining humidity and temperature effects.

3.3.1 Cohesion Testing

The samples were subjected to the following curing conditions:

10C/High Humidity and Lab Humidity
25C/High Humidity and Lab Humidity
40C/High Humidity
This was done with and without compaction. The results for emulsion Y with coalescing agent and compaction are shown in figure 6. All the mixes gave the same trends.


Figure 6: Effect Of Curing Conditions on Cohesion

They show that humidity slows build up of cohesion, and that this may be improved by use of coalescing agents. The microsurfacing type emulsions worked best in this regard. Compaction appeared to have a greater effect than coalescing agent, especially for high temperature and humidity.

3.3.2 Wet Track Abrasion Testing

The standard TB 100 was carried out and compared to results obtained after compacting the samples using a hand roller. Samples were cured at 25C (laboratory) and 60C(oven) for 16 hours. Samples were also cured at 10C and high humidity and 40C at high humidity for 16 hours to give an idea of cohesion development. This was done with and without coalescing agents for different emulsion types, with and without compaction. The effect of wet aggregate was also examined for standard conditions and high humidity cure for 25C and 10C.

Some samples were subjected to 1 hour, and also 6 days soaking in water at 25C. Most non-oven cured samples fell apart in 6 day soaking.

Some of the results are shown in figure 7 for emulsion Y with coalescing agent and mixtures compacted (similar trends were observed for all systems). The conditions were:

A = Curing at 25C for 16 hours and 1 hour soak.
B = Curing at 25C for 16 hours and 6 day soak.
C = Curing at 60C for 16 hours and 1 hour soak.
D = Curing at 60C for 16 hours and 6 day soak.
E= Curing at 40C 16 hours High Humidity 1 hour soak
F= Curing at 10C High Humidity 16 hours 1 hour soak
G= Curing at 10C Lab Humidity 16 Hours 1 hour soak

Designations on emulsions coalescence agents and compaction are as before.


Figure 7 Wet Track Abrasion Results


The coalescing agent improved abrasion loss in all cases. This was less pronounced for compacted samples. The compaction improved loss under all situations. The test appears to show the same general trends as the wet cohesion test indicating that this cohesion build up is the primary issue in fresh mixes.

3.4 Effect Of water in aggregate

The effect of water in aggregate is a practical one and an engineering one. Bulking effects and segregation effects in some aggregates are well known and the practice of adjusting water addition is also well known. However, field indications seemed to be that the effect of water was not the same for wet aggregate, as for water addition to dry aggregate in the field. The wet aggregate mixes seemed to take longer to cure despite water levels being similar to the mix design. The difference was that the aggregate was wetted in the stockpile.

Work was carried out to examine this effect. The focus was on cohesion, so the TB139 was used and samples of aggregate made up with various levels of moisture from 1 to 5%. This was carried out with emulsions X and Y, and results for standard cohesion are shown ,and some representative results for high humidity and low temperature curing (10C). These were kept in sealed containers with no air-gap and allowed to marinate. The mixes were made, and cohesion measured for a range of curing conditions. Some of the results are shown in figure 8,these are for 10C cure temperature at laboratory humidity and the same total moisture in all samples.


Figure 8 Effect of Moisture in Aggregate On Cohesion Build up


It is evident that the water that was allowed to soak into the aggregate slowed down cohesion build up, and affected even overnight strength. This is an unexpected result but may be explained by the potential for the aggregate to be porous and lock moisture in. The moisture cannot then assist in mixing, and during cure, may exit the aggregate pores causing binder to coalesce more slowly. This is conjecture and will need to be checked. Re-dried aggregate it was found returned to its original cohesion build up characteristics in the mix (see Figure 8).

4. FIELD WORK

 The tri emulsion system now has been used successfully in several countries and application types. This has included areas that freeze in winter (down to -200C) and are hot in summer (up to +350C), areas that have high humidity during application (95%), areas that have heavy traffic (40,000 VPD), and areas where the original pavement was rutted (50mm).

The use of coalescence agents has allowed successful application, even with damp aggregates (1-2% moisture) down to pavement temperatures of 80C with acceptable traffic times (less than 2 hours).

Compaction has been shown to protect surfaces against a combination of heavy traffic (30% tucks) and rainfall, in instances where non-compacted surfaces suffered high stone loss.

Using a combination of emulsion formulation techniques and compaction has allowed applications in high rainfall areas under cool conditions and the capability to place thick layers in ruts with acceptable curing rates.

5. CONCLUSIONS

 The main conclusions from this work are as follows:
  • Film formation appears to be key in emulsion adhesion and cohesion.
  • Film formation is controlled by temperature, humidity, aggregate water content and the emulsion formulation and aggregate type.
  • Cohesion build up rate and, stone retention may be improved by use of coalescing agents. Compaction appears to be better, maybe as it also closes voids and reduces water ingress.
  • Fieldwork is so far consistent with this approach, and has allowed quality surfacing to be carried out in adverse weather and traffic conditions.
6.0 REFERENCES
  1. Holleran, G, Petrov , Pozdnyakov ,V International Slurry Seal Association Conference 2003 Phoenix Arizona
  2. French Society Of Bitumen Emulsions " Bitumen Emulsion" Paris 2000
  3. Holleran G " Modified Emulsions- Benefits and Uses of Polymers In Slurry Seal and Microsurfacing" International Slurry Surfacing Association Workshop Las Vegas 2003.
  4. Steward, PA " Literature Review Of Polymer Latex Film Formation and Particle Coalescence" www.intium.demon.co.uk/ff_nf.htm 1995
  5. Deneuvillers,C, Samanos J (2000) " A methodology For Studying and Designing Microsurfacing- Applications" International Slurry Surfacing Association Proceedings Amelia Island 2000
  6. Holleran, G, Hicks, R.G, Reed. J.R, " Effect Of Particle Size and Distribution on Slurry Surfacing Performance" International Slurry Surfacing Association International Conference Berlin 2002.
  7. International Slurry Surfacing Association Bulletin A143 2000

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