Slurry Seal and Micro-Surfacing: The Difference and the Design

Nothing in the process is heated. There is no drying drum, no hot storage, no paver, and in normal road work no roller. The mixture does not exist before the machine makes it and it cannot be transported — it is produced continuously, a few seconds ahead of being laid, and the material in the box is minutes old. That is the defining constraint of the whole technology, and almost every practical difficulty in it follows from the fact that mixing, placing and setting are happening within a few metres of each other on a moving machine.

The four streams, and the fifth

A slurry surfacing machine carries the ingredients separately and meters each one into a twin-shafted pugmill:

• Bituminous emulsion from a heated or ambient tank, metered by a positive displacement pump linked to the aggregate feed so that the ratio holds when the machine speeds up or slows down.
• Graded aggregate from the hopper, drawn off by a belt or drag conveyor over a calibrated gate.
• Water, metered separately, which controls the consistency of the mixture and therefore how it flows in the box and how it strikes off.
• Mineral filler — Portland cement, hydrated lime, limestone dust or fly ash — fed dry from its own small hopper into the aggregate stream.

The fifth stream is the one that is easiest to overlook and the one that most often decides whether the job works: a liquid set-control additive, dosed into the mix water at a fraction of a percent, which retards or accelerates the break. On a well-run job the additive rate is the only thing the operator is permitted to vary within a range fixed by the approved mix design, and it is varied because the pavement gets hotter through the day, not because the mixture looks wrong.

What comes out of the machine

The pugmill discharges into a spreader box towed immediately behind, sitting on the pavement on flexible rubber side seals and spanning the full width being treated. Inside the box, augers or a spreader screw move the mixture across the width and hold a rolling head of material against the front seal. At the rear, a rubber strike-off blade meters the layer and sets its thickness. Better boxes carry a secondary strike-off, a second blade behind the first, which does not add material but drags a fine texture over the surface and removes most of the ridging that a single blade leaves.

The layer that results is thin. In a single-course surfacing it is struck off at approximately the thickness of the largest aggregate particle in the grading — the box is not building a slab, it is laying one stone deep and filling the voids between the stones with the mastic of binder, fines and filler. Neither treatment is rolled in normal highway work; the mat is densified by traffic once it will carry it. Some airfield and specialist work does use a pneumatic-tyred roller on slurry seal, and where that is required it is in the specification.

What both treatments do, and what neither does

• They seal. A continuous bituminous mastic over the surface closes the fine voids and the surface porosity, which keeps water out of the layers below and slows oxidation of the binder in the surface course.
• They restore texture and friction. A polished or bleeding surface gets a new layer of angular, crushed, hard-wearing fine aggregate at the tyre.
• They correct minor surface irregularity. Ravelled surfaces, minor deformation and open texture are filled and levelled. Micro-surfacing extends this into genuine rut filling; slurry seal does not.
• They extend the life of a sound pavement. Both are preventive maintenance treatments, applied to a pavement that is still structurally adequate, to postpone the point at which it stops being adequate.

What neither of them does is add structural capacity. A layer typically six to twelve millimetres thick, made of fine aggregate and emulsion residue, contributes nothing measurable to the load-spreading capacity of a pavement, and no pavement design method credits it with any. This matters commercially because both treatments are regularly proposed as an answer to a road that is failing structurally, and they are not one. Placed over fatigue cracking, pumping, or deformation coming from a weak base, the treatment will hide the defect for a season and then reproduce it, because the movement that caused the crack is still there and the new layer is far too thin to bridge it. The cracks come back through, usually within the first winter, and the money is gone.

The documents that describe them, and what kind of documents they are

Four families of document govern this work, and it is worth being precise about the status of each because the field is unusually loose about it.

• ASTM D3910, the practice for design, testing and construction of slurry surfacing, and ASTM D6372, the practice for design, testing and construction of micro-surfacing. These are ASTM practices — they describe how the work is designed and carried out and they reference the test methods, but they are not product specifications with a pass line for every property.
• ISSA A105, the recommended performance guideline for emulsified asphalt slurry seal, and ISSA A143, the recommended performance guideline for micro surfacing, published by the International Slurry Surfacing Association. These carry the gradation tables, the residual binder ranges and most of the numerical limits that specifications actually quote.
• ISSA Technical Bulletins, numbered TB-100 upward, which are the test methods themselves. Almost every acceptance figure in this field is produced by a TB method rather than by an ASTM or AASHTO one, and this is the single most important thing to know when you are asked to procure testing for a slurry surfacing job.
• EN 12273, slurry surfacing requirements, with the test methods in the EN 12274 series, parts 1 to 8. This is the European route and it is organised differently: performance classes declared by the producer rather than a single set of numerical limits.

Note carefully what the ISSA documents call themselves. They are recommended performance guidelines. They are not national standards, they were not written by a standards body with a formal balloting process of the ASTM or CEN kind, and they carry no force of their own. What they are is the industry’s consensus description of a well-made system, and highway agencies overwhelmingly adopt them by reference and then amend them — tightening a gradation band, changing a wet track abrasion limit, adding a local durability test. Where this page quotes an ISSA figure it names the guideline that carries it. The document that binds your job is the project specification, and if that specification says "per ISSA A143" without further qualification, you should read A143 itself before you price the work.

What an emulsion is holding together

A bituminous emulsion is bitumen dispersed in water as droplets of the order of one to ten micrometres across, kept apart by an emulsifier that adsorbs at the droplet surface. In a cationic emulsion — which is what essentially all slurry surfacing uses — the emulsifier gives every droplet a positive surface charge, and the droplets repel one another electrostatically. That repulsion is the only thing preventing a tank of emulsion from becoming a tank of bitumen. Remove it and the droplets touch, coalesce and form a continuous film, and the process is irreversible.

Two separate things then have to happen for a surfacing to become a road. The break is the coalescence itself: the droplets merge and a continuous binder film forms around the aggregate. The cure is the subsequent loss of the remaining water and the development of full strength. The two are often confused, and the confusion is exactly where slurry seal and micro-surfacing part company. Further background on emulsion structure, grades and their designations is on the bitumen emulsion and emulsion grades pages.

The slurry seal route: concentration by evaporation

A conventional slurry seal emulsion is formulated for mixing stability. It has to survive being violently agitated with a large surface area of fine, charged aggregate for long enough to get through the pugmill and out along the box without breaking on the way. The emulsifier package chosen to do that is, by construction, one that resists breaking. So what breaks it?

Principally, the departure of water. Once the mat is on the road, water leaves it in three directions: upward by evaporation into the air, downward by absorption into a porous substrate, and to a much smaller extent by chemical interaction with the aggregate and filler surfaces. As water leaves, the droplets are forced closer together. Eventually the concentration is high enough that the electrostatic repulsion cannot keep them apart, they coalesce, and the mat turns from brown to black. Adsorption of emulsifier onto the aggregate surface helps, and mineral filler helps, but the rate-controlling step in normal conditions is the loss of water.

That is a physical process governed entirely by ambient conditions, and it is why a slurry seal is genuinely weather-dependent in a way that is not a matter of contractor diligence:

• Air and surface temperature set the vapour pressure driving evaporation. A pavement at 40 °C in the sun dries a mat in a fraction of the time that a pavement at 12 °C in shade will.
• Relative humidity sets how much water the air will accept. At high humidity the driving force is small regardless of temperature, which is why a warm, humid coastal evening is worse for a slurry seal than a cool, dry, breezy morning.
• Wind removes the saturated boundary layer sitting on the mat. Still air is a much bigger problem than most people expect.
• Substrate porosity gives the water somewhere to go downward. An open, oxidised, porous surface pulls water out of the mat from below; a tight, flushed or concrete surface gives it nowhere to go.
• Layer thickness, which is the one the contractor controls, and the one that produces the hard limit discussed below.

The consequence is a treatment that has to be programmed around weather forecasts, that stops for the day earlier than anyone would like, and that is normally kept closed to traffic for hours. Traffic on an unset slurry does not merely mark it. The mat is still a wet mixture; tyres displace it, pick it up, and carry it away, and what remains has no chance of achieving its designed binder distribution.

The micro-surfacing route: break on a designed timetable

Micro-surfacing was developed to remove the weather from the critical path, and it does so by making the break a chemical event that is designed and tested during mix design rather than an incidental result of drying.

The mechanism is ionic. Cationic emulsifier molecules carry a positive charge; the surfaces of most mineral aggregates carry a net negative charge in water. When the emulsion meets the aggregate, emulsifier is adsorbed onto the mineral surface and stripped from the droplet, which destabilises the droplet. The mineral filler contributes directly: Portland cement and hydrated lime release polyvalent calcium ions into the mix water, and polyvalent cations are far more effective at collapsing the electrical double layer around a droplet than the monovalent ions already present. The very fine fraction of the aggregate contributes surface area for the same reaction. And the liquid additive shifts the balance in the other direction, giving the operator a controlled retardation.

The whole system is designed so that these reactions proceed slowly enough for the mixture to be mixed and spread, and then quickly enough that cohesion develops on a known timetable — measured, not guessed, by the ISSA TB-139 wet cohesion test during mix design. Water still has to leave for the mat to reach full strength, so cure is still partly evaporative. But the initial cohesion, the part that decides whether a lane can be opened, comes from the chemical break. A micro-surfacing mat becomes traffic-bearing while it is still visibly damp, and that single fact is what the product is for.

What the polymer contributes

ISSA A143 requires a minimum of 3 percent polymer solids based on the mass of the bitumen, and the polymer may be milled into the base binder before emulsification or added to the emulsifier soap solution. Styrene-butadiene rubber latex is the most common system in this application; styrene-butadiene-styrene block copolymer milled into the base binder is also used. The general behaviour of modified binders is covered on the polymer modified bitumen page; what the polymer does specifically here is four things.

• It raises the softening point of the residue. ISSA A143 requires minimum 57 °C by ASTM D36, which is well above what an unmodified residue of 40–90 dmm penetration would give. A thin, dark, high-friction layer on a road surface reaches temperatures that would flush an unmodified binder of the same penetration.
• It gives the residue elastic recovery. The mat has to accept traffic within the hour and then survive tyre scuffing at junctions and roundabouts without shoving.
• It builds early cohesion. The polymer network forms as the droplets coalesce, so the mat gains strength faster than the binder alone would give it, which is what makes the short traffic interval achievable.
• It holds the aggregate. A tougher, more cohesive mastic resists the plucking of individual stones out of the surface, which is the mechanism of early ravelling.

Why thickness follows the set mechanism

This is the point at which the mechanism turns into an operational rule, and it is worth stating plainly because it is the answer to the most common question about these two treatments.

A slurry seal laid thick behaves badly for a specific physical reason. The mat loses water fastest at its top surface. If the layer is thick, the top coalesces into a continuous binder skin while the material underneath is still an emulsion in water. That skin is close to impermeable. The water beneath it now cannot evaporate upward, and unless the substrate is porous enough to take it downward, it stays there. What you have built is a cured crust over an uncured, water-bearing layer. Under traffic the crust breaks and the soft material below shoves, tears or is pulled out in sheets. This is why slurry seal is struck off at roughly one stone thickness and why the correct response to a rut is never to increase the slurry seal thickness over it.

Micro-surfacing does not have that constraint in the same form, because the break that gives it its initial strength is not a drying process and proceeds through the depth of the layer rather than from the top down. A thicker layer still has to lose its water eventually, and it takes longer to reach full strength, but it develops cohesion through its thickness rather than forming a skin over a wet core. That is the mechanical licence to place a scratch course, a full stone-and-a-half surface course, and a rut fill of substantial depth in a wheel path.

The two failures at the ends of the same lever

Because the break is a controlled reaction, it can be controlled in the wrong direction. Too much retarder, too little filler, cold reactive aggregate, and the mat behaves like a bad slurry seal: it sits there, does not gain cohesion, and is destroyed by the first vehicle or the first rain. Too much cement or lime, a hot reactive aggregate, a pavement at 55 °C, and the reaction outruns the machine: the mixture breaks in the pugmill or in the box, and instead of a homogeneous mat you get lumps of coated aggregate dragging across the surface. Both of those are mix design failures expressed as site problems, and both are addressed in the failure section below.

Choosing between the gradings

The selection is made on what the surface needs, not on what is available:

• Type I is a sealing grading. It is fine enough to penetrate and fill surface voids and hairline cracking, and it leaves an almost smooth finish. Use it where sealing is the whole objective and traffic is light: car parks, service areas, footways, airport shoulders, and as an underseal beneath a chip seal where the objective is to close the surface before the chips go on. It carries the highest residual binder of the three because it has the greatest surface area per unit mass to cover.
• Type II is the general-purpose grading and the one most slurry seal work uses. It seals, it restores a moderate texture, it fills minor ravelling and open texture, and it holds up on residential and collector roads. In micro-surfacing it is the usual choice for a scratch or levelling course placed ahead of a surface course.
• Type III is the coarsest and most durable. It gives real surface texture and therefore real wet friction, it takes traffic better than the finer gradings, and it is the standard micro-surfacing surface course on arterials and highways. It is also the grading used in a rut box, because the coarse skeleton is what gives a thick placement its stability.

A great many jobs are specified as a Type II when the pavement needed texture, or as a Type III when the pavement needed sealing. Neither error shows on the day. The first shows up as a polished, low-friction surface within a couple of years; the second as a surface that is still open and still admitting water.

Thickness, and why it is expressed in stones

A single-course surfacing is struck off at approximately the thickness of the largest aggregate particle. For a Type III with a 9.5 mm top size that is a layer of the order of eight to ten millimetres; for a Type I it is a few millimetres. This is not a convention, it is the physical consequence of the set mechanism discussed above, and it is why the quantity that gets specified and measured is a mass per unit area rather than a thickness. Mass per square metre is measurable behind a machine — it is the delivered tonnage over the treated area, checked against the calibration — whereas the thickness of a thin cold-laid mat over a surface with its own texture is not a number anyone can measure honestly.

Micro-surfacing is normally placed in more than one pass where the surface needs it. A typical sequence on a rutted arterial is a rut fill in the wheel paths, then a full-width scratch or levelling course with a Type II, then a full-width Type III surface course, each pass allowed to break and gain cohesion before the next. That is three separate placements and three separate mix designs’ worth of thinking, and a bill of quantities that carries only a single line for "micro-surfacing" has not described the work.

Rut filling: the thing slurry seal cannot do

Rut filling is done with a rut box: a narrower spreader box, typically sized to the wheel path, with an adjustable strike-off that can be crowned. The machine makes a pass along each wheel path, filling the rut, and the strike-off is set to leave the filled rut slightly proud of the surrounding surface so that traffic compaction brings it down to level rather than leaving a depression. Common practice leaves a crown of a few millimetres, and common practice also limits a single pass to somewhere around 40 mm of fill, with deeper ruts filled in more than one pass. Both figures are practice rather than requirements, and the governing figures are in the specification and the manufacturer’s guidance for the machine.

Two things about rut filling are consistently misunderstood, and both are commercially significant.

First, filling a rut does not deal with the cause of the rut. If the rut is surface deformation in an asphalt layer that has densified or shoved under traffic, and that layer has now stabilised, filling it is a reasonable and durable treatment. If the rut is coming from consolidation or shear in the base or subgrade, the deformation is still active, and the fill will simply be pushed down again while the pavement continues to move. Establishing which of those you have is a coring and investigation exercise, and it belongs before the treatment is specified, not after it has failed.

Second, a rut fill is placed thick and it is placed in the wheel path, which is the most heavily loaded strip of the pavement. That is precisely why the ISSA TB-147 loaded wheel displacement requirement exists in the micro-surfacing guideline: a mixture that is stable at one stone thickness may flow laterally when it is 30 mm deep under a truck tyre. A mix design that has not been tested for lateral displacement has not been tested for the thing a rut fill actually asks of it.

The machine, and the calibration nobody checks

A continuous-run machine meters five streams and is only as good as the calibration of each. Each feed — aggregate belt, emulsion pump, water, filler, additive — has to be calibrated individually against mass or volume, and the machine has to hold the proportions across its working speed range, because a machine that is correct at one belt speed and wrong at another produces a mat whose binder content changes with the operator’s foot. Calibration records for the specific machine on the specific job, dated, are a reasonable thing to require before work starts, and they are the first document to ask for when a mat ravels.

The spreader box is the other half of the outcome. Rubber side seals must be in contact with the pavement along their whole length or material escapes and the mat runs thin at the edge. The strike-off must be straight, of even hardness, and not worn into steps. The augers must actually deliver material to both ends of the box against the crossfall. A box that has done a season’s work without the rubber being changed will produce streaks on any mix design in the world.

Surface preparation, and the tack coat question

Neither treatment is placed on a dirty surface and neither of them bonds to dust. The preparation sequence is unglamorous and it is where most delamination is prevented: sweep the surface mechanically, remove vegetation from cracks and edges, repair structural failures and potholes and let the repairs cure, treat working cracks with a proper crack sealing operation beforehand rather than expecting a 9 mm layer to bridge them, and mask or protect ironwork and drainage.

A tack coat is not routinely used under slurry surfacing, because the mixture is wet and the emulsion in it wets the surface directly. It is specified in defined situations: on a very dry, porous, badly oxidised surface that would otherwise pull water out of the mat and shock the break; on Portland cement concrete; on a milled surface; and where the guidelines or the agency call for one. Where it is used it is normally a slow-setting emulsion diluted heavily with water — a common dilution quoted in this application is one part emulsion to three parts water — applied at a light rate and allowed to break before the surfacing goes on. Dilution arithmetic, residue content and rate conversion are covered on the prime coat versus tack coat page, and the grade normally named is on the CSS-1 and CSS-1h page. Those dilutions and rates are practice and guideline material, not standard requirements.

Weather windows and traffic

Common practice, and the requirement in many agency specifications, is a minimum air and surface temperature of about 10 °C and rising, no rain forecast within the cure period, and no expectation of freezing within 24 hours of placement. Those figures vary between agencies and the binding ones are in the specification. What does not vary is the underlying logic: a slurry seal placed late in the day in autumn may not set before the dew falls, and a mat that has taken dew before it broke is unlikely to recover.

On traffic, the two products differ exactly as the mechanism predicts. Micro-surfacing is commonly opened within about an hour in favourable conditions and slurry seal takes several hours or an overnight closure. In both cases the first hours of traffic should be slow and straight. Turning and braking traffic on a mat that has broken but not fully cured is the classic source of scuffing and early ravelling at junctions, and a well-run job manages that with signing and speed control rather than by hoping.

Joints

Longitudinal joints between adjacent passes are the weakest line on the finished surface. An overlap that is too wide leaves a raised ridge that takes the traffic wear, wears through first and then ravels from the exposed edge; a gap leaves an unsealed strip. Transverse joints at the end of a run, where the box is lifted, leave a heavy line unless the mat is cut back and the next run started against a clean edge. Look at the joints first on any completed job; they show what the crew’s standard actually is.

What is absent, and why saying so matters

Slurry seal and micro-surfacing are cold processes throughout. There is no hot binder, no bitumen kettle, no torch, no hot storage tank, no drum mixer and no solvent-cutback fraction anywhere in the operation. The hazards that dominate hot bitumen work and roofing work — severe thermal burns from binder at working temperature, bitumen fume, flammable solvent vapour, torch ignition, hot-tank confined space entry — are simply not present on a slurry surfacing job. Naming that explicitly is not padding. A crew that has been briefed on hot bitumen burns and then handed a slurry job frequently relaxes, because the material is cold and looks like wet mortar, and the specific hazards below are exactly the ones that get missed as a result.

The emulsion is acidic

This is the hazard most often underestimated. A cationic emulsion is manufactured by protonating the emulsifier with an acid, which is what puts the positive charge on the droplets, and the product therefore carries a distinctly acidic pH. The figure for a given product is on that product’s safety data sheet and it differs between producers and grades, so read the sheet rather than assuming a value.

The practical consequences are that contact irritates skin, that splashes into the eye can cause serious and lasting damage, and that neither effect is intuitive to somebody who associates bitumen with heat rather than with chemistry. Fresh emulsion also stains and adheres to skin and clothing, and the urge to remove it with a solvent introduces a hazard that was not otherwise on the job. Chemical splash goggles rather than safety glasses, a face shield when connecting or disconnecting emulsion hoses or when working over an open tank hatch, chemical-resistant gloves, and covered arms and legs. For eye contact the response is immediate and prolonged irrigation with clean water followed by medical attention, and eyewash has to be at the machine, not in a cab a hundred metres away. Emulsion hose couplings under pump pressure are the classic splash point, and they should be broken only with the pump stopped and the line depressurised.

Mineral filler is caustic, and the injury is delayed

Portland cement and hydrated lime are not inert powders. In contact with moisture on skin they are strongly alkaline, and the resulting chemical burn characteristically develops over hours with little or no pain at the time of contact, which is precisely why it is allowed to continue. Hydrated lime is the more aggressive of the two. Dry dust causes serious eye damage and irritates the airways.

The exposure point is loading dry filler into the machine hopper, which is a dusty operation performed at head height into a moving airstream. It needs goggles rather than glasses, gloves with the cuffs worn outside the sleeves so that powder cannot funnel in at the wrist, respiratory protection appropriate to the material, and a wash facility on site. Filler that has got inside a boot or a glove is removed and the skin washed straightaway rather than at the end of the shift.

Dust from aggregate, filler and sweeping

Dry fine aggregate, some fillers and the sweeping operation that precedes placement all generate airborne dust, and where the aggregate or filler is siliceous that dust can contain respirable crystalline silica. Occupational exposure limits for respirable dust and for respirable crystalline silica are set nationally and differ between jurisdictions, so work to the limit and the exposure assessment that apply where the job is, and do not carry a number across a border. The controls that actually reduce exposure are ordinary: sweep with a suction or wet sweeper rather than dry brushing, damp down stockpiles, avoid free-fall transfer of dry filler in wind, and keep people upwind of the loading point.

The set-control additive and the washout water

The set-control additive is the most concentrated chemical on the job and it arrives in the smallest container, which is a combination that invites careless handling. It is a formulated surfactant product, handled neat, and it is dosed by someone leaning over a tank. Read its own safety data sheet — not the emulsion’s — and handle it with the eye and skin protection that sheet specifies.

Machine washout is an environmental control as much as a safety one. The water that comes off a washed-out pugmill and spreader box carries emulsion, the acid from the emulsion, alkaline filler and additive residue together. It must not go to a watercourse, a gully or a surface water drain. It is contained and disposed of as the site’s environmental controls require, and the same applies to unused emulsion, which is never tipped on the verge.

The machine, and the reaching-in accident

The twin-shafted pugmill, the aggregate conveyor and the augers in the spreader box are the mechanical hazards, and they share one accident. When a mix flash-sets in hot weather and material begins to lump and cling, the instinct is to clear it by hand while the machine is running, because stopping means a washout and lost production. That is the injury that recurs in this trade. Nobody puts a hand, a bar or a shovel into a pugmill, a conveyor or a spreader box until the machine is stopped, isolated and locked off, and the job programme has to be built so that a washout is an accepted cost rather than something a crew is under pressure to avoid.

Working in live traffic

On a whole-job view this is the largest risk on most slurry surfacing contracts and it has nothing to do with the material. The work is done on the carriageway, often under traffic management rather than full closure, with a slow-moving machine train and operatives walking alongside and behind it in the live lane. It needs a traffic management plan appropriate to the road and the speed limit, high-visibility clothing to the applicable standard, a briefed and enforced separation between the crew and running traffic, and control of the treated lane until the mat will carry a vehicle. The short traffic interval that makes micro-surfacing attractive commercially is also a pressure to reopen a lane while people are still working next to it, and that tension should be resolved in the method statement before it is resolved on the road.

The documents to have on site

A generic reference to a safety data sheet is not enough on this work, because the emulsifier system, the acid used and the additive chemistry differ between producers, and a sheet for one producer’s CQS-1hP does not describe another’s. Before work starts, hold the current safety data sheet for the specific emulsion as supplied, for the set-control additive, and for the filler, and brief from them. Where this page and a safety data sheet disagree, the safety data sheet governs, and where national regulation imposes more than either, the regulation governs.

1. Ravelling from an under-asphalted mix

Ravelling is the loss of aggregate from the surface of the mat, starting as a fine dusting of loose stone that sweeps into the gutter, then as a visibly open, scoured texture, and finally as bare patches where the treatment has gone entirely. It usually appears first in the wheel paths, at longitudinal joints, and wherever turning traffic scuffs.

The root cause is almost always that the residual binder is below what the aggregate’s surface area needs. That has several routes:

• The design binder content was set too low. This is exactly what wet track abrasion by ISSA TB-100 exists to prevent, and a mix designed close to the abrasion limit rather than comfortably inside it will ravel as soon as production varies.
• The aggregate is more absorptive than the design assumed. A porous aggregate takes binder into itself, so the effective binder available to coat the surface is less than the mix proportions suggest. A change of stockpile face can do this without any change in gradation.
• The delivered gradation drifted fine. Finer material has more surface area. The same binder content that worked at the coarse end of the band starves the mix at the fine end.
• Excess mix water. A wet mixture segregates in the box; binder-rich mastic drains toward the bottom of the layer and the surface is left binder-poor. The mat can be correctly proportioned overall and still ravel from the top.
• Traffic before cohesion. A mat opened too early loses stone mechanically, and no amount of binder prevents it.
• A break that outran the coating. If the emulsion breaks before it has properly wetted the aggregate — too much cement or lime, reactive fines, a hot dry stockpile — the binder is present in the mix but not where it needs to be, on the stone.

The diagnosis is documentary before it is forensic. Get the mix design report and check where the design binder content sits between the TB-100 floor and the TB-109 ceiling. Get the delivered gradations against the job mix formula. Get the machine calibration record. Get the emulsion certificates and check the residue figure against the one the design assumed. In a majority of ravelling cases one of those four documents contains the answer, and the material in the ground is confirming rather than revealing it.

2. Streaking from a worn spreader box

Streaking is longitudinal: continuous ridges and grooves running in the direction of travel, sometimes as fine corduroy over the whole width, sometimes as one or two heavy lines. It is a placement defect, not a material defect, and it is worth saying so directly because the argument on site almost always starts by blaming the mix.

The causes, in rough order of frequency:

• A worn, hardened or stepped strike-off. The rubber blade is a consumable. Once it has hardened with age or worn into steps it cannot meter an even layer, and it will streak on any mixture.
• Damaged or lifted side seals. Material escapes at the edge, the head of material in the box is uneven, and the mat runs thin or ridged along the edge.
• Oversize material lodged under the strike-off. A single oversize stone or a lump of broken emulsion trapped behind the blade drags a continuous groove for as long as it stays there. This is where the ASTM D6933 sieve requirement on the emulsion and the top-size control on the aggregate earn their place.
• An uneven head of material in the box. Augers that are not delivering to both ends, particularly against a crossfall, leave the box starved at one end and overfull at the other.
• The box not sitting correctly. A box riding on a crown, hung at the wrong angle, or dragged too fast for the mix to fill behind the blade.
• Consistency out of range. A mixture that is too stiff tears behind the blade; a mixture that is too wet flows past it and then segregates. Consistency by ISSA TB-106 is the design control, and mix water is the site control.

Streaking is not only cosmetic. Ridges take the traffic wear first, wear through, and then ravel from the exposed edge, so a streaked mat has a shorter life as well as a poorer appearance. The fix is mechanical and it belongs at the start of the job: change the rubber, check the seals, check the box geometry, and run a trial section before the first production run of the day rather than after the first complaint.

3. Delamination from a dirty or unprimed surface

Delamination is the loss of the whole treatment as a sheet, rather than stone by stone. It shows up as patches lifting under a turning tyre, as a plough blade taking a strip off, or as areas where the treatment can be peeled by hand at the edge of a crack. The diagnostic is unambiguous and takes ten seconds: turn the lifted piece over. If the underside is dusty and the pavement beneath it is clean, the treatment never bonded to the road at all — it bonded to a layer of dust that was sitting on the road.

The causes are almost all preparation failures:

• Inadequate sweeping. A mechanical sweep, not a nominal one. Milled surfaces are the worst case because the grooves hold fines that a light sweep will not lift.
• Clay, soil or agricultural deposits carried onto rural roads, which are frequently worse than construction dust and are not always visible on a dark surface.
• Curing compound or laitance on concrete, which is a release agent in all but name.
• A flushed or bleeding surface. A fat, binder-rich surface gives the mat nothing mechanical to key into, and in hot weather the underlying binder softens and becomes a slip plane in its own right.
• Omission of a specified tack coat on a dry, porous, oxidised surface or on concrete, or applying one and covering it before it broke, which puts water at the interface instead of binder.
• Water at the interface from any source: dew, a damp surface after rain, a surface fogged too heavily ahead of the box.

Note how little of that has to do with the materials in the machine. The remedy for delamination is a broom and a programme that allows time for preparation, and the specification clause worth enforcing is the one requiring the engineer to accept the swept surface before placing starts.

4. Setting too fast in heat

This is the micro-surfacing failure, and it is the direct consequence of the chemistry that makes micro-surfacing worth using. The break is a designed reaction, and reaction rates rise with temperature. On a pavement at 50 °C or more, with a hot dry stockpile and reactive filler, the reaction can outrun the machine.

What it looks like, in escalating order: the mixture stiffens visibly in the pugmill; balls and lumps of already-coated aggregate appear in the box; those lumps drag under the strike-off and leave grooves and scars; the mat tears; and in the worst case the machine has to be stopped and washed out, which on a continuous-run unit is a substantial loss of production. The mat that has been placed in the meantime is not homogeneous, its binder is not evenly distributed, and it will ravel and streak.

The contributing factors:

• High pavement surface temperature, which is far higher than air temperature on a dark surface in the afternoon.
• Too much reactive filler. Portland cement and hydrated lime are set accelerators, and an operator who has increased the filler to stiffen a wet-looking mix has also accelerated the break.
• Reactive fines in the aggregate, which is what the methylene blue test by ISSA TB-145 and the sand equivalent test are looking for.
• A hot, dry stockpile. Dry aggregate at pavement temperature absorbs mix water immediately and concentrates the emulsion at the moment of contact.
• Insufficient set-control additive, or an additive dose that was set at nine in the morning and never adjusted as the pavement heated.
• Low mix water, which reduces the dilution the emulsion is working in.

The legitimate responses are all inside the approved mix design or inside the programme: increase the retarder within the permitted range, reduce the filler within the permitted range, pre-wet the stockpile, fog the pavement surface lightly ahead of the box where the specification allows it, and move the work to night or early morning. The illegitimate response, which is common, is to add water beyond the design range to keep the mixture moving. That buys workability now and pays for it with segregation, a delayed set and a ravelled surface later. Any real change to filler or additive outside the approved range is a mix design change and needs the designer’s approval, not the operator’s judgement.

The related failure at the other end: flushing

Over-asphalted mixtures produce the opposite defect: a fat, dark, slick surface that loses texture and wet friction, tracks onto tyres in hot weather and can bleed into wheel path stripes. This is the failure the loaded wheel sand adhesion test by ISSA TB-109 exists to prevent, and it is the ceiling of the bracket described earlier. It is more common than it should be because ravelling is the failure everyone fears, and the instinctive defence against ravelling is more binder.

How to write the specification so these are controllable

• Name the guideline and the amendments. "Micro surfacing to ISSA A143" with a schedule of the specific limits you are changing is enforceable. "Micro surfacing" alone is not.
• Require the full mix design report before mobilisation, with every ISSA Technical Bulletin result and the aggregate source, emulsion producer and additive named. Require the TB-100 and TB-109 curves, not just the selected binder content.
• Tie the design to the stockpile. State that a change of aggregate source or stockpile, emulsion producer, filler or additive requires a new mix design, and mean it.
• Require machine calibration records for the specific machine, dated, for every feed, and a trial section at the start of the work.
• Specify the acceptance of the prepared surface as a hold point. This is the single clause that prevents delamination.
• Specify the traffic criterion, not just a time. Reference the cohesion values from the mix design rather than writing a bare number of hours into the contract.

What we supply, and what we do not

This page describes a treatment we do not sell, and it is more useful to say that plainly than to imply otherwise.

We are a bitumen exporter. We do not supply micro-surfacing or slurry seal systems, we do not supply the machines, the spreader boxes, the aggregates, the mineral fillers or the set-control additives, and we do not carry out slurry surfacing work, issue mix designs, or provide laboratory testing, machine calibration or site inspection services. The polymer modified quick-setting emulsion at the centre of a micro-surfacing job is a formulated product matched by mix design to one aggregate stockpile and one filler, and emulsion in general has a limited storage life, is damaged by freezing and by prolonged storage, and separates on standing. For those reasons micro-surfacing emulsion is normally manufactured close to the work by a producer who can support the mix design, and shipping it across the world in drums is not a sensible way to buy it.

What is exportable, and what we do supply, is the material further up the chain: penetration grade bitumen as base binder to an emulsion plant’s specification, and polymer modified bitumen where the plant mills polymer into the base binder rather than adding latex to the soap solution. If you are running an emulsion plant that serves slurry surfacing contractors and you need base binder of a defined penetration and source consistency from the Middle East, that is a conversation we can have usefully. If you need a micro-surfacing system for a highway maintenance contract, your supplier is an emulsion producer with a slurry surfacing laboratory, and you should be asking them for the ISSA Technical Bulletin results listed above.