1. BACKGROUND

During the late 1960’s and early 1970’s German scientists began experimenting with emulsion/aggregate mixes to find a way to use stable, thicker applications which could be applied in narrow courses to fill wheel ruts, without destroying expensive road marking lines on the autobahns.  They found that if carefully selected aggregate was combined with an emulsion containing special polymers, the resulting mix remained stable and resisted deformation, even when applied in multi-stone thickness.  The result was the development of Microsurfacing.

Microsurfacing was introduced in South Africa in 1982 when Petrocol obtained the licence from Raschig in Germany to produce and lay the Ralumac Microsurfacing system in Southern Africa.  Although originally formulated as a rut filling system, it is now commonly used as a surfacing system to solve a variety of road surface problems throughout the world. When Colas South Africa was taken over by Colas France during early 2000, Colmat and Colpave replaced the existing brand names Ralumac and Ralupave respectively as the new Microsurfacing products with an improved performance. 

 The latest overseas developments in Microsurfacing to date have been:

• The use of reinforcing synthetic fibres giving the end product improved fatigue properties and surface texture.

• Continuously variable spreader boxes allowing adjustment of the spreader box whilst the Microsurfacing unit is in motion. The latter thus ensures that longitudinal joints are kept to minimum and handwork is reduced.

• Fully automated mix and lay machines

• The use of pigmentable binders for demarcating cycle lanes and pedestrian walkways   

This paper reviews some of the above developments in context of the local usage of Microsurfacing in Southern Africa.

2. MATERIAL COMPONENTS

A Microsurfacing system consists of a quickset cationic emulsion mixed with selected continuously graded crushed mineral aggregate, ordinary Portland cement and water. In exceptional cases additional mineral filler may be required to adapt the grading or improve the reactivity of the aggregate towards the emulsion.

It is essential to select a bitumen emulsion and aggregate system which will perform well together given the local prevailing climatic conditions. The economics favour a variation of the emulsifier used rather than changing the bitumen or aggregate source. The preference is to stick to well known sources of bitumen and aggregates and try avoiding constant formulation changes. 

2.1 Bitumen emulsion

The type of bitumen used has an influence on the mixing and setting properties of the emulsion produced from it. The key specifying parameters being the acid value and penetration of the bitumen. In South Africa use is made of 80/100 bitumen for overlays and 60/70 bitumen for rut filling purposes, derived from Middle East crude which have low acid values. Similarly the type and concentration of emulsifier, acid used and pH, and particle size distribution of the emulsion will affect the performance of the system. The incorporation of an elastomeric polymer like synthetic rubber latex will improve the consistency and cohesion of the Microsurfacing and ultimately the durability of the surfacing. 

2.2 Aggregate

The first requirement is to ensure that the candidate aggregate can be reliably supplied to the specified grading. The ‘reactivity’ of the aggregate influences the emulsifier type and concentration required to give an adequate mixing time. Various tests can be used to investigate aggregate ‘activity’. Below are the two most commonly used gradings by Colas in South Africa.

2.3 Filler

Cement is used to minimise the risk of segregation, adjust the grading curve and as a setting agent. The type and amount needed must be determined by a laboratory mix design. This can vary between nil and 3 % by mass of dry aggregate. 

2.4 Water

Water is added as a wetting agent and to improve workability of the mix. Potable water, free of harmful salts must be used.  

2.5 Fibre

The purpose of the fibre addition is to improve the structure of the wet Microsurfacing mix during the spreading process so that a good macro texture is obtained. This is normally achieved by adding a cellulose fibre around 0.3% m/m of dry aggregate. If the intention is to also improve the tensile strength of the final product then glass, polypropylene or other fibre can be used.

2.6 Pigment

The incorporation of pigments with light coloured synthetic binders can be used to produce coloured slurries. The recommended practice is to disperse the pigment in a liquid phase to avoid on-site handling of dry powder pigment.

3. TYPES OF MICROSURFACINGS

Colmat is generally used as a remedial treatment for existing surfacings to improve skid resistance and decrease pavement permeability, whilst ensuring low noise levels. In South Africa, Colas offers 4 types of Microsurfacing products, which have the following uses, namely: 

Table 2

Grade	Description	Use
COLMAT ‘N’	Quickset Microsurfacing without elastomeric polymer	Overlays of 5 – 20 mm of existing surfaces carrying medium level of traffic and where traffic accommodation can be a problem for conventional slurry
COLMAT ‘L’	Quickset Microsurfacing modified with elastomeric polymer	Overlays of 5 – 20 mm of existing surfaces carrying a wide range of traffic
COLRUT	Quickset coarse graded Microsurfacing modified with elastomeric polymer	Filling of ruts > 20 < 50mm to restore road profile
COLPAVE	Medium set Microsurfacing modified with elastomeric polymer	Resealing small areas by handStockpile for coldmix

4. PROPERTIES

4.1 MATERIAL CHARACTERICS

A well formulated Microsurfacing system will, after placing, rapidly set up, allowing pedestrian traffic within 10 – 15 minutes and vehicular traffic within one-hour of placement during normal weather conditions.  Whereas conventional slurry relies on the evaporation of the water phase for the mixture to cure, Microsurfacings derive their superior performance properties mainly due to the chemical reaction which takes place between the positive charged bitumen droplets in the emulsion (and latex if used) and the free negatively charged ions of the aggregate. The product sets and develops cohesion through a complex electrostatic/chemical reaction between the aggregate, cement and polymer modified emulsion.  The bond between the aggregate and the binder is irreversible and once early setting has occurred, the product is unaffected by unexpected rain. Its performance is further enhanced when the emulsion is further modified with SBR latex. The latter results in an improvement of the temperature susceptibility of the binder, improved adhesion of the aggregate which reduces the loss of aggregate especially in the early life of the seal. Colmat will however not prevent cracks from reappearing and neither does it add structural strength to the pavement. Localised areas of fatigue cracking must be repaired prior to placing of Colmat.

Figure 1

4.2 FUNCTION PROPERTIES 

The aggregate gradings used in Colmat result in enhanced surface roughness and micro skid resistance characteristics.  Below is a comparison between the skid resistances measured on Colmat and continuously graded asphalt surfacing on the M3 highway in Cape Town.

Figure 2

The overlaying of an existing bituminous surfacing with Colmat will also limit further deterioration of the pavement by preventing:

• The ingress of water into the underlying layers by filling the voids and cracks in the existing surfacing,
• Further oxidation of the aged binder in the existing surfacing.

5. MIX DESIGN

To ensure optimum performance of the Microsurfacing system, it is important that all the components be evaluated in the laboratory before full-scale production can commence.  The following tests are performed on the components:

• Aggregate  – grading, sand equivalent and methylene blue value
• Emulsion  – binder content and residue on sieving

As the material is mixed and applied at ambient temperature, the Marshall Hot Mix design method is not suitable for determining the optimum binder content of the mix.  The binder content range within which the mix will be functional and is determined by using the following methods:

1. The minimum binder content is determined by using the Wet Track Abrasion method.  Samples of the mix are prepared at various binder contents on circular discs of roofing felt.

After a curing and soaking period, the samples are abraded with a rubber hose for five minutes.  The mass loss of the samples is determined after drying the sample to constant mass.  The binder content level at which the abrasion loss stabilises is recorded as the minimum binder content.

2. The maximum binder content is determined by placing Microsurfacing mixes, prepared at various binder contents in a Marshall compaction mould.  After a curing period the mixes are compacted at a temperature of 60ºC at 150 blows per side.  The compacted samples are then inspected for signs of richness or bleeding.   A clear indication is normally obtained at the binder content where the material has a tendency to bleed.  Very good results have been obtained in the field using this simple approach.

The diagram below shows the effects of the variation in binder content on the above laboratory tests.

Figure 3 

Alternatively the Loaded Wheel Test can be used to determine the maximum binder content.  A strip of Microsurfacing mix prepared on a steel plate is trafficked by a loaded wheel for a certain period.  Sand is then spread over the trafficked area and the quantity of sand adhering to the surface, gives an indication of the maximum binder content that can be tolerated by the mix.    

For very heavy traffic conditions, the minimum binder content determined within the range can normally be safely used, whilst for lighter traffic conditions the higher binder content level can be used without any possibility of bleeding. 

6. APPLICATION METHODS

 Microsurfacing is produced and placed by a purpose-designed machine which carries all the components.  The machine accurately proportions these components on a continuous basis and applies it onto the road surface through an agitated spreader box.  The setting rate of the mix can be controlled by the addition of a special chemical additive.

6.1 MACHINE APPLICATION

Due to its rapid setting characteristics, the Colmat system has a limited mixing time of approximately 120 seconds, before the mix starts setting up.   The Colmat application machines are equipped with special mixers and spreader boxes that are continuously agitated to prevent material from prematurely setting up before application on the road.  The setting rate can be controlled by the addition of a chemical additive to facilitate handwork application.  The spreader boxes are mounted on long skids that will “ride” over imperfections on the road surfaces, resulting in improved rideability of surfaced layers.  The width of the spreader boxes can be hydraulically varied to accommodate widths ranging from 2,5 to 3,8 metres for normal overlay applications. 

For rut filling applications a special narrow, agitated box is used that concentrates the coarser parts of the mix in the centre of the rut, whilst it moves the finer parts of the mix towards the edges of the box to ensure a thin feathered edge.  The box is also capable of placing the mix with a slight camber to compensate for return compaction under traffic.  The rut filling box width can be varied from 1.5 to 1.8 metres.     

6.2 HAND APPLICATION

For hand application, Colpave emulsion can be used to prepare mixes in a builder’s concrete mixer.  These mixes are suitable for the patching of potholes, surfacing of parking areas, driveways, and intersections and difficult to reach areas.  The Colpave emulsion has an extended mixing time of approximately four minutes, allowing the preparation of small sized mixes that can be placed before the mix sets up.  The mix is easily applied with rubber squeegees and textured with a damp hessian drag before it sets up. Setting time of a Colpave mix is usually approximately 60 to 90 minutes.   Fresh material that has set prematurely before placing can be loosened and used as a patching material for potholes and trenches.

7. COST EFFECTIVENESS  

Microsurfacings are a cost effective remedial solution for improving the profile and skid resistance of an existing road. . Conventional slurries have limitations in that they cannot be placed in a single layer with a thickness > 1.5 its maximum aggregate size. With the developments in microsurfacings it has become possible to apply thicker layers to accommodate higher traffic densities and due to its fluid nature to fill surface irregularities such as wheel ruts. Microsurfacings are designed to be opened to traffic in one hour, whereas slurries however are weather dependant making them slow curing. 

Below is a table comparing the key parameters (typical values) of the various types of Microsurfacing products with conventional slurry.

Table 3

	Slurry	Colmat ‘N’	Colmat ‘L’	Colrut	Colpave
Emulsion binder content	60 – 63	62 – 65	62 – 65	62 – 65	62 – 65
Softening point, °C	43	43	53	55	53
Emulsion in mix, l/m3	230 – 260	200	180	170	200
Curing time, hours	5	< 1	< 1	< 1	1 – 2
Cost index per cubic metre*	100	100	130	140	100

  *Cost index expressed as a percentage against the cost of mixing and laying conventional slurry under similar conditions.

In summary the main benefits of Microsurfacings versus other surfacings types are:

Environmentally friendly

• Low surface noise
• No harmful emissions
• Cold application

Minimal traffic disruption

• No lifting of kerbs and manholes required
• No tack coat required
• Minimal construction plant required on-site
• Traffic within one hour

By Lindsey Roussouw (Colas SA Marketing Manager) and Kobus Louw (Colas SA Research and Development Manager)

January 2008