Resin Bound Over Tarmac or Concrete: Judging the Base First

Cracks you fill, cracks that end the conversation

Crack triage is where most of these jobs are won or lost, and the distinction is not width, it is pattern. A single transverse crack in a mat that is otherwise tight is thermal movement in a surface that still works. Interconnected polygons in a wheel path are fatigue: the base is flexing, the surface is a passenger, and every one of those polygons will reappear in the resin. The distress vocabulary in ASTM D6433 Standard Practice for Roads and Parking Lots Pavement Condition Index Surveys is worth using on a domestic drive even though nobody will ever ask for a PCI score, because it forces you to name what you are looking at before deciding what to do about it.

Cracks that survive triage get routed out to a clean, dry, uniform section and sealed — not smeared over. Sealants for this are specified by ASTM D6690 Standard Specification for Joint and Crack Sealants, Hot Applied, for Concrete and Asphalt Pavements, and anything wide enough to swallow material needs a backer rod so the sealant bonds to two faces and stretches, rather than three faces and tears. On a drive that is about to be overlaid, sealant has to be flush or slightly below the surface. A proud bead telegraphs through 18 mm of resin as a visible ridge, in a straight line, forever.

Say the honest thing about reflective cracking in writing before anyone signs. A repaired crack is still a joint in the base with movement across it, and a bonded topping with no reinforcement and no bridging capacity will eventually trace it. Clients accept that when it is a sentence in a quotation and do not accept it when it is a phone call in March.

Getting an area off a drive nobody set out

Almost no domestic drive is a rectangle. There is a splay at the footway, a return past the bins, a hardstanding that grew when the extension went in, and a border that has been shaved twice. Break it into rectangles and triangles on the survey sheet, measure each one twice, and keep the arithmetic visible — because this single figure multiplies through the aggregate, the resin, the primer and the labour, and a five per cent error at the top of that chain is a whole kit at the bottom of it.

Waste behaves differently from unit paving. There are no cuts, so nothing is lost at the perimeter, but there is spillage at the mixer, there is material left in the barrow, and there is the depth you actually achieve rather than the depth on the drawing. Screeding to a nominal 18 mm and averaging 19 mm across 74 m² is not a rounding error; it is a fraction over five per cent more material, which is another four square metres' worth nobody ordered.

Depth follows the stone, and the percentage follows neither

The working rule is that laid depth is at least three times the largest particle. A 1-3 mm dried aggregate makes a footpath finish; a 2-5 mm makes a drive; going deeper than the stone needs wastes material and going shallower produces a surface that ravels in the first frost because there is no interlock to hold a stone that has lost one contact point. This is why the 6 mm sample board proves nothing about how the drive will behave — it is a colour swatch that happens to be made of the same ingredients.

The aggregate must be kiln-dried and must stay dry. Two-part polyurethane reacts with water: free moisture consumes isocyanate, produces gas, and the visible result is a foamed, pale, weak matrix that never develops strength. That is the mechanism behind most catastrophic first-week failures, and it comes from damp aggregate, a damp substrate or rain inside the cure window far more often than from anything the crew did with a trowel.

Quantities come out of density, not out of a coverage chart copied off a forum. Aggregate per square metre is laid depth multiplied by the loose bulk density of that specific stone, and loose bulk density is a measured property — BS EN 1097-3 Tests for mechanical and physical properties of aggregates: determination of loose bulk density and voids, or ASTM C29/C29M Standard Test Method for Bulk Density and Voids in Aggregate. Different stones at the same nominal size do not weigh the same, so a system priced on one supplier's granite does not transfer cleanly to another's marble.

Resin content is a percentage of aggregate weight, and it is the one number on this page that should never be estimated. Too little and the surface ravels; too much and it closes the voids, drowns the porosity, and puddles resin at the bottom of the layer where it goes shiny and dark. The figure belongs to the specific system, in its own technical data sheet, at the ambient temperature you are working in — and there is no British Standard for resin bound surfacing to fall back on if the data sheet is missing, which is a good reason not to buy from anyone who cannot produce one.

Mix in whole kits. Two-part systems are supplied as matched Part A and Part B units precisely so that nobody has to measure a ratio on site, and splitting a kit by eye to stretch it across an extra barrow is how a batch cures soft, on a drive where every batch is visible next to the one beside it. Set the day up so the last mix of each run is a full one.

A porous surface on a sealed box

Resin bound is porous. A resin bound drive is only porous if everything beneath it is. Those are two different statements and the gap between them is where this product gets mis-sold. Water passes through the voids in the matrix in seconds, reaches the old tarmac or the concrete slab, and then does exactly what water does on an impermeable plane: it runs sideways along the interface, following the fall of the base rather than the fall of the surface, and emerges wherever the section runs out. Usually that is the lowest free edge, which is usually the footway, which is usually the thing the client wanted to avoid.

There is a second cost to that trapped layer. Water sitting at the bond line in winter freezes, and the expansion works directly against the adhesion the whole system depends on. Delamination that appears as a lifted plate in February is very often not a priming failure at all — it is a drainage failure that spent a year loading the primer.

So the decision is binary and it is made before anything is ordered. Either the base is impermeable and the design has to collect what comes through the resin — a channel at the low edge, a positive fall built into the retained surface, and a legitimate outfall for it — or the base is rebuilt permeable and the water goes down instead of sideways. Where the drive is a front garden in England, the planning position turns on which of the two you chose, under the Town and Country Planning (General Permitted Development) (England) Order 2015 and the Communities and Local Government guidance on the permeable surfacing of front gardens. Read both before telling a client no application is needed, because the exemption is about where the water ends up, not about what the surface is made of.

If you have chosen the permeable route, size the reservoir on measured properties. Void ratio is a property of the gradation you actually ordered, storage is depth multiplied by area multiplied by that void ratio, and the design storm has to leave the box within the drawdown period the reviewing authority sets. Note also that no test method has been written for infiltration through resin bound: the nearest published procedures, ASTM C1701/C1701M for pervious concrete and ASTM C1781/C1781M for permeable unit pavement systems, are not written for this material, but running one of them on the same ring, on the same spot, each year is a repeatable way to prove the surface is still passing water — which is what the client is actually paying for.

When the answer is new black

A base that is structurally sound but cosmetically finished — oxidised, ravelling, patched into a quilt — does not need excavating. It needs a new binder course over it, and then the resin over that. This is the middle outcome of the three, and it is frequently the right one, because it buys a single continuous surface at a known thickness with a known bond, laid and compacted to a standard, instead of a repaired mosaic that the resin has to be honest about.

Two things then govern. The mixture is specified under BS EN 13108-1 Bituminous mixtures: material specifications — Asphalt Concrete, and the laying and compaction under BS 594987 Asphalt for roads and other paved areas: specification for transport, laying, compaction and product type testing protocols. Neither of those documents knows anything about resin, so the third governing document is the resin system's data sheet, which will state a minimum age for new bituminous work before overlaying — new asphalt carries a rich surface film and continues to lose light oils for a period, and both interfere with the bond. That waiting period is measured in weeks, not days, and it belongs in the programme and in the client's expectations from the first conversation.

Then check levels before, not after. A binder course plus a resin course is a real rise in finished level, and everything fixed in elevation gets a vote: the garage threshold, the door sill, the gully gratings, the kerb upstand at the crossover, the step at the front door, the clearance under the gate. Where the rise cannot be absorbed, the answer is planing out first, which is a different price and a different day. The asphalt side of this operation — tack coats, joints, temperature, compaction — is covered in the resurfacing guide rather than repeated here.

Digging out to build the permeable version properly

The full rebuild exists for two reasons: the base has failed, or the drive has to infiltrate. The construction is not the same as the overlay with more stone under it. Resin bound cannot be trowelled directly onto open-graded stone — the mix has nothing to bear against and the resin drains into the voids — so a permeable build-up carries an intermediate bound layer, usually a porous asphalt, between the reservoir and the resin. That layer is what the trowel works against, and leaving it out is the most expensive misunderstanding available on this job.

Below it, the sub-base is open-graded and specified as an unbound mixture to BS EN 13242 Aggregates for unbound and hydraulically bound materials for use in civil engineering work and road construction; the open-graded Type 3 material used for this in the UK is defined in the Specification for Highway Works within the Manual of Contract Documents for Highway Works. It is ordered by weight at a compacted thickness, and the subgrade under it is not proof-rolled to a dense finish, because compacting the formation is how a designed infiltration surface is quietly converted into a tank with no outlet. Whether infiltration is viable at all comes from a field test — BRE Digest 365 Soakaway Design sets out the pit method used for this in the UK, and CIRIA C753 The SuDS Manual covers the wider design case.

Excavation depth is set by the reservoir the design storm needs, not by the pavement. That is the moment the job stops being a resurfacing and becomes a drainage scheme with a surface on top: skips, muck-away, a formation that must stay clean, and a programme that no longer fits inside a dry weekend. Price it as such, or do not offer it.