Water and external systems · Decks & Paving
Bituminous Surfacing System
A reference account of the bound but unjointed hard surface - how its courses are made to act as one, why confinement replaces restraint at its edges, and why it can only ever be opened by cutting.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What bituminous surfacing is
Bituminous surfacing occupies the gap between the two other ways a hard external surface is made. A rigid paving is stiff and cannot spread, so movement is directed into a pattern of joints decided in advance. A flexible paving is a field of separate units that can be lifted by hand and confined at the perimeter. This surfacing is bound like the first and deformable like the second, and it has no joints at all: continuity comes from compacting a continuous material rather than from filling the gaps between pieces.
It is built as courses that are intended to act as one element. A lower course regulates what is beneath it and spreads load; an upper course takes traffic, weather and wear. Between them is a bond, and whether that bond exists is what decides if the construction is one thick layer or two thin ones sliding over each other.
The material arrives in a state it is not meant to stay in. It is placed loose and worked into its final condition on site by compaction, so the constructed state of every course is created rather than delivered. What lies beneath matters at that moment as much as it does later, because a course cannot be compacted properly against something that gives way.
Because nothing in it is a unit, nothing in it can be lifted out and put back. Every later access - a service trench, a repair, a new gully - is made by cutting the surface and patching it, and a patch introduces edges and joints into a construction whose whole argument was that it had none.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- asphalt surfacing
- macadam
- tarmacadam
- blacktop
- bituminous bound surfacing
- asphalt paving
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Present a continuous trafficked surface with no joints and no units to work loose.
- Spread load from the surface into what is beneath it through courses bonded into a single element.
- Deform rather than fracture when the ground beneath moves, within limits the designer sets.
- Shed all water across the top, since the surface is closed and offers no downward path.
- Be cut, removed and patched where access to what lies below is needed later.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 8. Order is meaningful.SubstrateBase or binder course
The lower bound course, spreading load and regulating the surface of what is beneath it. It is what the upper course inherits its shape from, so irregularity here reads through to the finished surface rather than being absorbed by it.
- Layer 2 of 8. Order is meaningful.Finish surfaceSurface course
The exposed layer taking traffic, braking, weather and wear. It is also the only water-shedding element the construction has, because there is no joint pattern and no downward route through the build-up.
- Layer 3 of 8. Order is meaningful.AttachmentBond between courses
The adhesion holding the courses together so they behave as one thicker element. Without it, both courses can be entirely sound in themselves while the upper one slides and shoves under turning and braking.
- Layer 4 of 8. Order is meaningful.Jointing and sealingCompaction and the few joints there are
Continuity here is achieved by compacting the material rather than by jointing it. The only planned discontinuities are where one laid run meets the next and where the surfacing abuts something else, and those are the lines a surface commonly opens along first.
- Layer 5 of 8. Order is meaningful.Edge and terminationEdge restraint and haunching
The confinement at the perimeter. The material has no tensile edge of its own, so an unsupported arris ravels inward under traffic; confinement is what an unjointed bound surface has in place of a restraint.
- Layer 6 of 8. Order is meaningful.Drainage planeThe shaped surface itself
The falls formed across the finished surface, which are the drainage. Water cannot pass through a closed bound surface, so every part of the area has to have somewhere to send water across the top, and low points are made rather than avoided.
- Layer 7 of 8. Order is meaningful.ProtectionApplied surface treatments and markings
Coatings, dressings and line markings applied over the finished surface. They change how the surface behaves at its very top - texture, colour, how it takes marking - without altering what the courses beneath are doing.
- Layer 8 of 8. Order is meaningful.Service zoneIronwork and openings within the field
Gully gratings, chamber covers and inspection frames sitting inside a continuous surface. Each one is a fixed, stiff object in a material that deforms, and each is surrounded by a line the surfacing did not want to have.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
Lose the bond between courses and the surface shoves under braking and turning while both courses remain perfectly sound in themselves. The defect is not in either material; it is in the interface between them, which is why a surface can ripple and shove without anything about the mixes being wrong.
Leave the edge unsupported and the material ravels inward from the arris. Confinement is doing here what a restraint does for unbound units and what a joint pattern does for a rigid surface, so an edge detail that looks like tidying is structural.
Under-compact and every other role is compromised at once: the bond has less to grip, the surface is more open to water, and the course will continue to compact under traffic instead of under plant. Nothing about it looks different on the day it is finished.
Because the surface is bound and continuous, what is beneath it reads through as a shape rather than as a crack at a joint. A soft area under a rigid paving announces itself as a fracture; under this construction it appears as a depression that collects water, and the water then works on the surface from within the depression.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Finish surface
Bitumen-and-aggregate materials laid hot and worked in place are the family this surfacing belongs to; the library documents that family chiefly in its jointless floor form. Which mix belongs in an external trafficked surface is a determination for the pavement designer and varies regionally in both naming and composition.
Substrate
Where the layer beneath the bound courses is itself bound, these families are commonly encountered. Whether the base is bound or unbound changes how the surfacing above behaves and is a design decision rather than a preference.
Edge and termination
Edgings, kerbs and the concrete haunching behind them are commonly met in these families. What confinement an edge has to provide depends on what crosses it and on what stands beyond it.
Attachment
Mortars and concretes of these kinds are commonly encountered where ironwork frames are bedded and haunched within a surfaced field. How a stiff frame is supported inside a deformable surface is a detail for the designer.
Jointing and sealing
Applied sealants are commonly met where the surfacing abuts a different construction or where a cut edge is closed. Whether a given sealant can sit against a bitumen-bound material without either affecting the other is a documentation question.
Protection
Applied coatings and markings of these kinds are commonly encountered over bitumen-bound surfaces, and over the ironwork within them. What any of them needs from the surface beneath, including how recently it was laid, comes from the manufacturer.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
The foundation the courses are laid on
Everything this surfacing does depends on the prepared construction beneath it, and the two are commonly designed together as one pavement. A surface course cannot be compacted against a foundation that yields, so the condition of the layer below is a surfacing question and not only a groundworks one.
Read about Pavement Foundation →Gullies, channels and low points
The surface is closed, so all water leaves across the top and arrives at whatever catches it. Where the outlets are placed and where the surface has been shaped to send water are the same drawing, and a gully in the wrong place makes a puddle rather than a nuisance.
Read about Surface Water Collection →A closed surface meeting an open one
Where a bound area drains onto or beside a surface designed to admit water, the closed area is delivering everything that lands on it to the open one. Whether the receiving construction was sized for that contribution is a drainage design question.
Read about Permeable Paving →Covers, chambers and buried runs
Access covers stand as stiff frames within a deformable field, and the pipe runs beneath were commonly laid before the surfacing existed. Both mean the surface will be opened again at some point, and neither can be reached without cutting.
Read about Buried Drainage Network →Surfacing arriving at a doorway
A bound surface meeting a building has to stop at a defined level and leave water somewhere other than against the wall. It is also the one place where a construction that is normally shaped for vehicles has to deal with people, thresholds and a damp-proof line.
Read about Door Threshold Interface →Surfacing across a rooting volume
A continuous bound surface over a rooting zone is both a lid on the ground and a material that roots can lift as a single sheet. How the surfacing is terminated around the tree, and what is beneath it, are decided together with the rooting volume.
Read about Tree Pit System →Buried services and later reinstatement
There is no unit here to lift, so reaching anything below means cutting the surface and patching it. Where services run, how often they are likely to be reached, and who is responsible for the reinstated surface are worth establishing before an unbroken surface is laid over them.
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Durability
- Wear here happens where traffic turns, brakes and stands rather than where it passes, so the parts of an area that look identical may be in quite different condition. What the surface is expected to carry, and where that action concentrates, shape the discussion.
- Movement
- A bitumen-bound material is stiffer when cold and softer when warm, so the same surface answers a load differently in different seasons. Standing loads, point loads and turning are discussed against that, and how much deformation is acceptable is a design determination.
- Moisture
- The construction offers water no downward route, which means the shaping of the surface is the whole of its drainage. Water standing in a depression, or entering at a cut edge or an unsealed abutment, works on the material from a place it cannot dry.
- Buildability
- The finished state of each course is created on site by compaction, and the window in which that can happen depends on weather, plant access and the condition of the layer beneath. It is one of the few constructions whose quality is largely settled in the hours it is being laid.
- Maintenance and access
- Because nothing can be lifted, every intervention is a cut and a patch, and every patch adds edges to a surface whose argument is that it has none. How often the area is likely to be opened is part of deciding whether this construction fits the situation.
- Interfaces
- The abutments against kerbs, buildings, ironwork and other surfaces are where this construction is most commonly seen to fail, because they are the only discontinuities it has. Each is a detail with its own drainage and confinement question.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Is the layer beneath this surfacing bound or unbound, and who is designing the pavement as a whole?
- Where does water leave this area, and has the shaping been drawn against the outlet positions?
- What confines the surfacing at every edge, including the edges nobody walks or drives across?
- How many covers, frames and fixed objects sit within the field, and how is each detailed?
- How likely is it that this surface will be opened for services, and who reinstates it when it is?
- What is the area actually used for - turning, standing, storing, playing - rather than what it is called?
- Is any applied coating or marking intended, and does it place conditions on the surface beneath it?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- It is not a rigid paving. A bound stiff surface cannot spread, so its movement is directed into a pattern of joints; this construction has no joint pattern at all and answers movement by deforming.
- It is not a flexible paving either. That is a field of separate units bedded loose and confined at the perimeter, any one of which can be lifted by hand and put back; nothing here is a unit and nothing can be taken out without cutting.
- Bound does not mean stiff. The construction is held together throughout and still changes shape under load, which is why a depression rather than a crack is the usual first sign of a problem beneath it.
- The edge is not a trim. Confinement at the perimeter is what the construction uses in place of a restraint or a joint, so an unsupported edge is a structural condition and not an unfinished one.
- A newly laid surface being smooth says nothing about compaction. A course that was not fully compacted will continue to compact under traffic, and the evidence appears long after the work looks complete.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Who has designed the pavement as a whole, and what use and traffic was it designed against?
- What is the intended bond arrangement between the courses, and how is it verified?
- How is the surface shaped to reach the outlets, and what happens at the parts furthest from them?
- What edge detail is proposed where the surfacing meets a kerb, a building and an unrestrained edge?
- How are covers and frames within the field supported, and what is expected to happen around them over time?
- What is the reinstatement standard where this surface is cut, and who is responsible for it?
- What surface treatment or marking is planned, and what does it require of the surface before it is applied?
What this page does not do
- This entry describes how the construction is organised. It is not a specification, a pavement design, or a substitute for design by a qualified professional.
- No thickness, fall, compaction requirement, mix designation or traffic capacity is stated here, and none can be inferred; all are project-specific.
- Hot-laid bituminous materials and the plant used to place them carry their own hazards, and the works are not an owner activity.
- Whether a closed surface is permitted in a given location, and what must happen to the water it sheds, varies by jurisdiction and is a drainage consent question.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- External Works · External works
- Hardstanding · External works
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Decks, Paving and Hard Landscape Systems
External surfaces people stand on, plus the substructure, edge, support and guarding that make one usable, each decided by the layer beneath and the route its water takes.
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