Lecture #10 Recycling, Noise and Dangerous Chemicals

H22 IS3 : Transport Infrastructure

Topic #10 : Recycling, Noise & Dangerous Chemicals

• Recycling

• Re-using Asphalt or Concrete as Granular Materials

• Hot Recycling of Asphalt

• The Remix Process

• Cold Recycling

• Noise

◦ Porous Asphalt

◦ Microsurfacings

◦ Whisper Concrete

• Dangerous Chemicals

Recycling, Noise & Dangerous Chemicals

Contents

• Pavement recycling is a topical subject & one which is likely to become increasingly important as pressure on the earth’s resources grows.

• Pavements have long been considered as natural locations in which to “hide” otherwise useless material.

• Indeed, bitumen itself was not considered particularly valuable before its potential as a binder for roads was realized.

• Nowadays, slags & ashes are among those materials which have found a natural use in road construction & are often found to perform as well or even better than the virgin material they replace.

• Pressure on resources is one reason why recycling is a desirable goal. However, there are others.


Recycling

• A surprisingly high percentage of the damage caused to roads is actually due to vehicles supplying material for construction, much of it road construction.

• Road recycling offers the possibility of cutting down on this destructive traffic.

• In-situ recycling also introduces the possibility of a much quicker operation than the conventional partial reconstruction.

• Unfortunately there are also negative aspects to recycling & many of these are concerned with the technical performance of recycled materials which is often inferior to conventional products.

• These problems are outlined next.


Recycling

• Both bituminous material & concrete are suitable for reuse as granular material after being crushed to appropriate sizes.

• Crushed concrete generally forms an excellent aggregate either as an unbound granular material or as aggregate to use in fresh concrete.

• The crushing action tends to release some of the cement in the original concrete which then contributes to the binding action.

• Recycled asphalt planings are certainly usable as low quality “unbound” material. Large thicknesses should be avoided because of the potential for permanent deformation as the bitumen slowly flows.

• Recycled planings are often used as surfaces for very minor pavements, for example car parks, farm access roads etc.

• The binding action of the residual bitumen leads to an acceptably stable surface.


Re-using Asphalt or Concrete as Granular Materials

• The current UK standard permits up to 30% of recycled planings to be used in a new bituminous mix.

• In principle the process is very simple. The planings are added to the new mix in the mixing drum & heated & mixed with the rest of the material.

• However, the following problems have to be overcome.

◦ The planings must not be overheated, for example by coming into direct contact with the heating flame, otherwise dangerous fumes are given off.

◦ A careful mix design has to be performed taking into account the binder content & condition of the material in the planings.

◦ In practice, a very consistent supply of planings has to be available for this sort of recycling to be 100% successful.


Hot Recycling of Asphalt

• This last difficulty illustrates one of the fundamental problems with all types of pavement recycling.

• It is always likely that the final product is more variable than the equivalent fresh material.

• However, in spite of this, a number of countries, for example the Netherlands, permit significantly higher percentages of recycled planings than the UK with no lowering of performance.


Hot Recycling of Asphalt

• Recycling material in an asphalt plant does not overcome the expenses & damage associated with delivery vehicles. This can only be achieved by an in-situ process.

• In the remix process illustrated above, the pavement surface has to be heated, to a temperature of around 80 – 90°C. At this temperature the bituminous material is workable.

• The main piece of equipment consists of a scarifier, a device which scrapes up the surface to a depth of around 20 mm, a mixer to mix the scarified material with additional fresh asphalt, & a paver.


The Remix Process

• The re-mixed material, when compacted, forms a single new 40mm wearing course.

• This treatment is suitable when the existing wearing course has started to deteriorate, either by rutting or cracking, but the underlying layers are still in a sound condition.

• Limitations are as follows:

◦ A fairly consistent existing wearing course is necessary.

◦ A careful mix design is required to establish the proportions needed for the added material.

◦ Material deeper than 20 mm below the original pavement surface is not heated sufficiently to be significantly remoulded or improved.

◦ The process can only be considered as partial recycling because of the need for new material.

◦ An increased surface level has to be accommodated.

◦ There is a danger of toxic fumes being emitted during the heating process if it is not carefully controlled.


The Remix Process

• The following 3 types of binder can be used to form a coldmix:

◦ Cement (or other hydraulic binder - slag, ash)

◦ Bitumen Emulsion

◦ Foamed Bitumen

• If cement is used than the material formed is likely to be a roadbase quality layer.

• Bitumen emulsion is a very widely used material, for example as a binder for surface dressing. It consists of droplets of bitumen suspended in water by the action of a chemical, an emulsifying agent.

• This allows the emulsion to be used as a liquid at normal air temperatures. When mixed with an aggregate the bitumen droplets begin to be attracted to the aggregate surface.


Cold Recycling

• However, this process remains a slow one until compaction of the mix greatly accelerates it. The result is a bitumen bound mix with a considerable amount of water, which has to be allowed to evaporate.

• Foamed bitumen is an alternative form of bitumen, also suitable for mixing with cold materials.

• Each of the 3 binder types can be used in a mixing plant to give a mix for use in a pavement.

• However, it would not be usual to use cement in this way except where the recycled material, for example crushed concrete, is simply being used as an alternative to conventional aggregate.


Cold Recycling

• Plant mixes using either emulsion or foamed bitumen together with recycled planings are operated commercially in the UK.

• They have the useful property that they remain workable for long periods after mixing, even up to several months, & so can be stockpiled until ready for use.

• The alternative to a plant mix is in-situ recycling. This is achieved by using a piece of equipment such as that shown below.


Cold Recycling

• The pavement is milled using a very powerful tool to a depth of up to 35 mm. This breaks the existing bound layers into quite small pieces.

• The broken pavement fragments are then mixed with whichever binder is to be used, compacted &, usually at a later date, a new surface is applied.

• The process mixes all the material together which have been milled so, in a depth of up to 350 mm, it is possible that bituminous, granular & even soil may be included.

• The quantity & type of binder added has to be suited to the materials present.


Cold Recycling

• Noise is one of the greatest environmental nuisances associated with roads & the appreciation of it has increased greatly in recent years.

• Road noise comes from 2 basic sources, namely;

◦ Engine noise

◦ Tyre/surface interaction noise.

• Engine noise is mainly a matter for the vehicle manufacturers, although the sound absorbing properties of the surface determine to some degree the amount of engine noise which escapes.

• However, tyre/surface noise is a matter for the highway engineer, & nowadays there are several noise reducing surfaces which can be used (for a price).


Noise

• This type of material has been available for many years & has frequently been used on airfield pavements as a spray reducing surface.

• It consists of a bitumen bound mixture with a fairly open grading such that, when compacted, it still contains relatively high voids.

• This structure makes the material both permeable to water & adsorbent to sound.

• The difference in noise level compared to a conventional surface is very significant indeed.

• Unfortunately, the engineering properties of the material are not nearly as good.


Porous Asphalt

• These are surfacings which are laid by a special sort of paver to a thickness of 15 – 25 mm.

• They consist of an aggregate with a grading which, like porous asphalt, is intended to ensure that the void content remains reasonable high.


Microsurfacings

• The bitumen content is also fairly high, resulting in a mix which, as well as the high void content is also durable, a very important feature if an economic solution is to be found.

• The noise absorbing properties are not as good as those of porous asphalt but are still significantly better than most conventional surfaces.

• These materials will certainly be used in large quantities in the near future.


Microsurfacings

• This term may seem like a pure contradiction.

• Concrete is perceived by most road users to be the noisiest surface of all, inducing considerable vibration to vehicles.

• However, whisper concrete (more properly exposed aggregate finished concrete) is different in that it gives to the concrete the same type of surface as an asphalt.


Whisper Concrete

• The conventional way in which the texture is given to a concrete road surface is to form grooves either by sawing the surface or by dragging a wire brush over the wet concrete.

• Either way, the result is a ridged surface with the basic skid resistance being provided by the cement mortar of each ridge. The exposed aggregate finish uses high skid resistance stones in the final few centimetres of concrete.

• The finished surface is sprayed with a retarder to stop it setting immediately &, next morning, the loss mortar is brushed off, leaving the exposed high quality aggregate.

• The effect is a dramatically reduced level of tyre noise.


Whisper Concrete

• An increasingly aware public is demanding ever increasing safety standards with regard to the use of questionable materials in road construction.

• The result is that some of the materials which in the past were regarded as indispensable are now hardly used.

• Tar

◦ Tar is the binder which gave us tarmacadam & its abbreviation “tarmac”, now both a well known firm & a word in the English language in its own right.

◦ However, recent evidence that it contains carcinogenic (cancer causing) substances has meant that it is now hardly ever used.


Dangerous Chemicals

• Cut-backs

◦ One of the commonest ways of using bitumen (or tar) in a cold form used to be as a “cutback”, that is a solution of bitumen & kerosene.

◦ Again, kerosene, as a solvent, has been demonstrated to be damaging to health, & so the alternative of a bitumen emulsion is now much more commonly used.


Dangerous Chemicals

• Sulphates

◦ Sulphates are the commonest of many chemicals in road materials which pose the possible problem of contamination of water supplies if they leach out of the road itself & find their way into drinking water.

◦ Sulphates also damage concrete & this problem has to be avoided around bridge piers for example.

◦ Unfortunately, the sulphate problem is one of those currently preventing the re-use of one of the UK’s most abundant waste materials, colliery spoil.


Dangerous Chemicals

The End of Topic #10

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Lecture #10 Recycling, Noise and Dangerous Chemicals