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Topic 5 : Surface Properties

Topic 5 : Surface Properties

Goh Boon Hoe

Senior University Teaching FellowBoon-Hoe.Goh@nottingham.edu.myTel. (office) : 6(03) – 8924 8182Room B1C20

Faculty of Engineering

Department of Civil Engineering

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Roles

• Primary

o High skid resistance - by design

o Good ride quality - requires good workmanship

• Secondary

o Low noise - by design (or good fortune)

o Little spray - by design

o Impermeable - by design

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Contents

• Roleso Ride Qualityo Material Strength, Durabilityo Skid Resistanceo Sprayo Noise

• Surfacing Materialso Hot Rolled Asphalt + Coated Chippingso Dense Bitumen Macadamo Porous Asphalto Surface Dressingo Stone Mastic Asphalto Microsurfacing (Thin Surfacings)o Conventional Concreteo Whisper Concrete

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Ride Quality

• Excellent ride quality is not always needed – but it is on high speedroads.

• Typical tolerance limits: ± 3 mm in 3 m

• So how can this be achieved?

o Pavement Quality Concrete

� No problem achieving the tolerance in a machine-laid wet-formed concrete pavement.

� Problem : concrete is hard and unable to absorb muchenergy from the tyres.

– high tyre vibration

– relatively high noise

– not so pleasant to drive on

� joints just make things worse; good texture, e.g. longitudinalgrooving or exposed aggregate finish, can help.

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Ride Quality

o Asphalt

� To get a really good finish, the surface course must berelatively thin (say ≤ 50 mm); otherwise the paver operatorwill not be able to control levels well enough.

� But then the underlying layer must also be reasonably eventoo – and this principle applies right the way through thepavement.

� The evenness of the surface of each layer can be constructedslightly better than that of the one below, but only slightly.

� UK Highways Agency tolerances (absolute maxima) at eachlevel:

– Pavement surface ± 6 mm

– Binder course ± 6 mm

– Base ± 15 mm

– Subbase + 10 mm – 30 mm

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Materials Strength, Durability etc

• A PQC surface is of exactly the same strength as the rest of theconcrete slab � not an issue.

• Asphalt Surfaces

o Asphalt surface course has to have relatively small stone size due toits low layer thickness and, consequently, a relatively high bitumencontent.

o This leads to a less stiff material but with high fatigue resistance andgood durability.

o Asphalt surface structural properties:

Mixture Type StiffnessDeformation resistance

Fatigue strength

Asphalt concrete medium high mediumHot rolled asphalt (+ chippings) medium low highStone mastic asphalt medium-low high medium-highPorous asphalt low medium-high medium-lowBASE HIGH HIGH MEDIUM

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Ride Quality

• Impact of different types of surface:

Surface TypeVibration

generationEnergy

absorptionRide quality

ranking

Asphalt concrete medium medium 3

Hot rolled asphalt + chippings

medium low 4

Stone mastic asphalt low high 2

Porous asphalt low very high 1

Surface dressing high low 5=

Concrete (PQC) fairly high very low 5=

Block paving very high low 7

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Materials Strength, Durability etc

• Block Paving

o Although blocks themselves are high-stiffness, the effectivelayer stiffness is a function of rotation and shear at joints, whichdepends on how well the joints are filled.

o It is common practice to assume a stiffness of 500MPa for acombined block-bedding sand layer.

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Skid Resistance

• Friction between tyre & road surface consists of 2components, both related to speed

o Sliding resistance between tyre & road (magnitude afunction of nature of the materials in contact)

o Loss of energy caused by deformation (hysteresis) of tyre

Surface Texture

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Skid Resistance (continued)

• Dry Conditions

o High skidding resistance

o Microtexture of surface aggregate is the main contributor

• Wet Conditions

o Skidding resistance reduced by lubricating action of waterfilm

o Drainage channels provided by macrotexture

o Drainage channels provided by pattern on tyre

o Penetration of water film (microtexture)

o Aquaplaning (safety issue)

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Skid Resistance

• Microtexture

o Microscopic properties of the surface

o Function of the aggregate (stone)

o Property which gives the basic level of friction achievablebetween the tyre and the road and thus provides skiddingresistance

• Macrotexture

o Visible roughness of the surfacing material

o Function of the surfacing (wearing course)

o This is a measure of the space available for water to use(drainage) so that direct tyre / stone contact ismaintained.

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Skid Resistance (continued)

• Measurement Methods

o Microtexture

� measured by the Polished Stone Value (PSV)

– Accelerated Polishing Machine (BS812) & Portable Skid Resistance Tester

o Macrotexture

� Measured by the texture depth (Sand Patch)

� Measured by laser-based techniques

1) Mini-Texture Meter (MTM)

»Hand propelled trolley

»Newly laid surfacing

2) High Speed Texture Meter (HSTM)

»Laser sensor in special trailer

»High speed – 100 km/h

3) High Speed Road Monitor

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Skid Resistance

• SCRIM (Sideway-force Coefficient Routine Investigation Machine)

o The principle of SCRIM is that a smooth tyre is held at 20° to the

road surface &, while the road surface is kept wet by a mobilewater tank, the loaded wheel is towed at 50 km/hr.

o The Sideways Friction Coefficient (SFC) is the force at rightangles to the plane of the inclined wheel (the sideways forces)expressed as a fraction of the vertical force acting on the wheel.

SCRIM

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Polished Stone Value (PSV)

Polishing Stone Value (PSV)

Accelerated Polishing Machine

Rubber-tyred wheel

Polishing Test Specimen

Surface

aggregate

Rubber pad

Pendulum Test

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Polished Stone Value (PSV)

• The standard method for testing is to embed theaggregates in a curved mould in cement-sand mortar &subject the sample to accelerated polishing caused by arotating pneumatic wheel.

• The specimens are mounted on a circular frame 40 cmdiameter. The size of each specimen is 45 mm wide x90.5 mm long.

• The rubber wheel is 20 cm diameter & 5 cm broad,loaded with 40 kg load at a tire pressure of 3.15 ± 0.15kg/cm2.

• Sand & water are fed to the machine when it’s rotated atan RPM of 320 – 325 for 3 hours 13 minutes.

• The specimens are thereafter tested for their polishingvalue on a British portable pendulum tester.

• This machine consists of a rubber sliding shoe which ismounted at the end of a pendulum. The slider, whenreleased, brushes past the specimen & comes to a halt.

Polishing Stone Value (PSV)

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Sand Patch

• Texture is measured by the Sand PatchMethod on small areas (i.e. areas of newbituminous surfacing surfacing prior toallowing it to be trafficked).

• The method is fully documented in BS 598: Part 105, however this method hasalready, or is soon to be superseded by avery similar method but using a standardsized glass bead in place of sand.

• The principle is fairly obvious the greaterthe texture the more the sand will be takenup by it & the smaller the circle that can beachieved from the standard quantity ofsand.

Sand Patch Method

1. Measure out

exact volume of

sand

2. Pour onto

pavement surface

3. Spread out level with

tops of aggregate

particles

4. Record diameter

of sand patch

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Spray

• Spray occurs when surface water cannot drain to the side of the road.Thus, not only is the texture depth important, but also continuity oftexture.

• Spray from surface water is a safety hazard. If water cannot easily flowacross the surface of a pavement then it will be available to form spray.The issue is not texture depth but barriers to lateral flow.

Spray on road surface

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Noise

• Surface noise is a function of the texture & of the acousticproperties of the surface material.

• High macrotexture – High noise levels

o Dense materials reflect high frequency sound

o Voided materials reflect low frequency sound

Surface noise created by road macrotexture

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Spray

• Traditional UK Hot Rolled Asphalt (HRA) with rolled-in chippings has aparticularly bad reputation for spray since each individual chipping sitsin its own small indentation (negative texture) into the asphalt surface,allowing a small ‘pond’ of water to remain around it until it eitherevaporates or is dispersed in the form of spray.

• Most other surfaces consist of protrusions (positive texture) from amore general surface level and water can flow around these protrusionsand make its way sideways. Asphalt concrete and SMA thereforegenerate much less spray than HRA. Grooved concrete is also good.

• However, porous asphalt is undoubtedly the premier material. Porousasphalt allows water to drain straight into the pavement itself and thento pass laterally through it, below the level of the tyre-surface contact.

• The result: virtually no spray at all. Of course, the pavement has to beable to cope with the presence of water within the porous asphalt.Usually the porous asphalt surface course has to overlie a dense,impermeable binder course; otherwise pavement durability problemsare likely.

Noise

• As expected, noise level generally depends on texture depth, i.e.roughness.

• Conceptually:

o Noise is caused by vibration, principally of the tyre treadelements.

o Surface type affects both the amplitude and frequency of tyretread vibration. [a rough surface will induce a high amplitude oftyre vibration and therefore high noise]

o Some of this noise will be absorbed by the surface, and this willdepend on the hardness of the surface material. [concrete haspoor ability to absorb any sort of vibration energy includingnoise and this means that it is difficult to produce a low-noiseconcrete surface]

• Porous asphalt has very low stiffness and therefore causes littleexcitation to the tyre tread elements; it also has excellent noiseabsorption properties – an ideal low-noise material.

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Surfacing Materials

• Type of surfacing materials:

1) Hot Rolled Asphalt + Coated Chippings

2) Dense Bitumen Macadam

3) Porous Asphalt

4) Surface Dressing

5) Stone Mastic Asphalt

6) Microsurfacing (Thin Surfacings)

7) Conventional Concrete

8) Whisper Concrete

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Dense Bitumen Macadam (DBM)

• Very common on minor roads

• Dense, durable but can crack

• Easy to lay

• Low skid resistance

• Lower noise than HRA

• Less spray in the wet than HRA

• Not used for high speed surfaces

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Hot Rolled Asphalt (HRA) + Coated Chippings

• Most common material used on major UK roads

• A unique British material

• Texture formed by rolling pre-coated chippings into mat

• Dense, durable but can rut

• Easy to lay

• High skid resistance

• Generates spray in the wet

HRA & Precoated Chippings

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Porous Asphalt (PA)

• Developed with the objective to reduce spray with added advantage ofreducing running noise.

• Design with drainage through the body of the material

• Single sized aggregate held together by binder with interconnecting voids

• Laid over an impermeable basecourse layer

• A relatively new concept

• Voided, therefore not so durable

• Relies heavily on binder film

• High skid resistance

• Very low noise

• Very little spray in the wet

Porous Asphalt

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Surface Dressing

• A very common maintenance treatment

• Seals surface (impermeable layer)

• High skid resistance

• High noise

• Quite a lot of spray in the wet

• Very cheap

Surface Dressing

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Microsurfacing (Thin Surfacing)

• Imported from France

• Thin layer, 15 – 25 mm thick

• Proprietary mixtures, derivatives of porous asphalt or SMA

• Relatively high voids but still durable

• Fairly high skid resistance

• Low noise

• Little spray

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Stone Mastic Asphalt (SMA)

• Imported from Germany

• Porous structure with voids filled with bitumen mastic

• Dense, durable with both good rutting and fatigue resistance

• Difficult to design

• High skid resistance

• Low noise

• Generates spray in the wet

Stone Mastic Asphalt

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Conventional Concrete

• Skid resistance depends on type & depth of surface texture

• Sawn or brushed grooves

• High skid resistance

• Often very high noise

• Generates spray in the wet

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“Whisper Concrete”

• Otherwise known as “exposed aggregate” concrete

• Surface brushed a few hours after laying

• High skid resistance

• Relatively low noise (compared to HRA & conventionalconcrete)

• Generates a little spray in the wet

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