There are more than 15 million kilometres of paved roads and highways worldwide and every year over hundred thousand kilometres of them requires major rehabilitation. The construction or improvement of the road network is rightly considered as the most effective ways of promoting economic development of a country ; infrastructure in general and transport infrastructure in particular, is undeniably one of the decisive factor in development.
A pavement serves two basic functions. First it helps guide the driver and delineate the roadway by giving a visual perspective of the horizontal and vertical alignment of the travelled path. Thus pavement gives the driver information about the driving task and the steering control of the vehicle. The second function, which is of more interest to us, is to support vehicle loads. Just as any structure, the underlying soil must ultimately carry the load placed upon it. A pavement function is to distribute the traffic load stresses to the soil (sub-grade) at a magnitude that will not shear or distort the soil.
In general, there are two types of pavement structure: flexible pavement and rigid pavement. The difference can be summarised in the fact that flexible pavement has a running pavement which is made of bituminous material while a rigid pavement has a running surface is made out of concrete.
Figure 1 - Shows the different layers in Rigid and Flexible pavement
Figure 2 - Shows the load distribution on a flexible pavement (left) and Rigid pavement (Right)
Pavement Structure
A flexible pavement is constructed with asphaltic cement and aggregates and usually consists of several layers. The lower layer is called the sub-grade (the soil itself), the upper 152-203 mm of which is usually scarified and blended to provide a uniform material before it is compacted to maximum density. The sub-base layer is just above and this normally consists of crushed aggregate (rock) since it has better engineering properties (higher modulus values) than the sub-grade in terms of its bearing capacity. The next layer is the base layer which is made of crushed aggregates but of a higher strength than those used in the sub-base. The base layer can either be unstabilised or stabilised with a cementing material which can be Portland cement, lime fly ash or asphaltic cement. The wearing surface is the top layer and this layer is usually made of asphaltic concrete, which is a mixture of asphalt cement and aggregates. The wearing course serves to protect the base layer from wheel abrasion and to waterproof the entire pavement structure. Moreover it provides a skid resistant surface that is essential for vehicle safe stop.
A rigid pavement on the other hand, is made with Portland cement concrete (PCC) and aggregate. The sub-grade (the lower layer) in this pavement type is often sacrified, blended and compacted to maximum density, as was the case for flexible pavement. The base layer however, is optional for rigid pavement since it depends on the engineering properties of the sub-grade. If the sub-grade is poor and erodible, then it is recommended to use a base layer. The top layer (wearing course) is the Portland cement slab.
General properties required for the layers
As far as base and sub-base is concerned the first property required is that they should possess sufficient strength to sustain without failure the imposed traffic stresses. The second property is that they should be of uniform and consistent quality so that they can be spread and compacted without difficulty to provide a road surface that will not be impaired by the compacting effects of traffic. An additional criterion for sub-base is that they should have a California bearing ratio (CBR) of not less than 25% when tested at the critical density and moisture content that would arise on site.
Waste in the United Kingdom
Waste can be divided into 4 main categories:
Mining and quarry waste and by-products. Common examples are:
Colliery spoil
China clay waste
Industrial waste and by-products. Common examples are:
Fly ash
Bottom ash
Metallurgical waste and by-products. Common examples are:
Blast furnace slag
Steel slag
Non ferrous slag
Municipal waste
Demolition waste
Waste rubber, plastic waste and waste glass
The total waste arising in England is around 272 million tonnes based on a survey done in 2005 by Waste and Resource Action Plan. The percentage attributed to demolition and construction is 32% while for mining and quarrying it is around 30% of total waste as shown by Fig 3.
Figure 3 - Shows the proportion of waste in England
Figure 4 - Shows the waste arising in the UK (ONS 2005)
Sustainability in construction
With the much publicised awareness of climate change, potential global warming and green building, it is difficult to imagine someone who has not experienced media exposure to the issues of sustainable development. Sustainable developments try to alter the way we design and build to limit the impacts of our built environment and throughout the entire life span of a structure. Three general categories of needs and/or impacts are generally considered; environmental needs/impacts, economic needs/impacts and social needs/impacts. Together the three categories form a 'triple bottom line' of sustainability. It seeks to find a balance between the economic, environmental and social effects of a growing population. Whenever any decision is made, the economics should not be considered solely, factors such as impact on society at large and the environments are also considered.
Figure 5 - Shows the 'triple bottom line' of sustainability
What makes a good recycled material?
It is possible to lay down some general criteria for a waste to be usable in road construction. The main factors to be considered are:
The annual quantity available at any location should at least be 50,000 tonnes (where appropriate)
Reasonable transportation distance
Materials must not be highly toxic
Materials not too soluble in water in terms of potential settlement and/or water pollution
Uniformity and consistency in their properties
Distribution
How easy it is to mix
Price of the material
Any adverse chemical reaction
Chapter 2: Recycled materials used in road construction
2.1 Mining and Quarry waste and by-products
2.1.1 Colliery spoil
Nature and Occurrence
Colliery spoil, also known as Minestone is the solid residual material resulting from the mining of coal. Colliery spoil deposits are composed of the waste products from coal mining which are either removed to gain and maintain access to the coalfaces or unavoidably brought out of the pit with the coal and have to be separated out at the cleaning plant. Waste from both sources are usually dumped on the same spoil heaps together with small quantities of coal and other washings, and the heaps are therefore very variable in composition. Combustion may also arise in the heaps resulting in a change in the chemical and physical composition. This is known as burnt spoil. Burnt spoil is only available from older tips and is also in higher demand in the construction industry than unburnt material (minestone), because it can be used in the road sub-base and base construction. The results of some physical and chemical tests made on British Colliery spoils are given below, in Table 1 and Table 2. It should be noted for this report we will focus solely on burnt colliery spoil.
Particle diameter (mm)
Burnt shales
Unburnt shales
A
B
C
D
E
F
S
T
U
V
W
>40
2
0
6
0
3
0
7
5
6
12
15
20-40
22
14
14
20
18
15
33
25
7
18
15
10-20
26
22
23
23
22
25
26
35
10
15
32
5-10
22
21
22
20
19
17
17
16
15
15
18
2-5
11
12
13
12
12
11
6
6
17
10
8
<2
17
31
22
25
32
32
11
13
45
30
12
Specific gravity
2.65
2.69
2.71
2.72
2.76
2.90
2.60
2.51
-
-
-
Table 1 - Particle size distribution and specific gravity of some British colliery shales
Burnt shales from colliery
Unburnt
G
A
B
C
D
E
F
SiOâ‚‚
57.6
56.2
60.2
55.6
56.4
45.4
51.9
Al₂O₃
31.3
31.1
21.2
26.5
23.3
21.5
19.4
Fe₂O₃
3.86
4.33
8.02
4.57
6.14
13.37
6.1
TiOâ‚‚
0.22
0.24
0.17
0.22
0.22
0.22
1.03
CaO
0.36
1.03
0.44
0.16
0.48
6.3
0.66
MgO
0.92
0.82
1.01
1.47
0.97
2.88
1.21
Naâ‚‚O
0.23
0.20
0.48
0.23
0.44
0.65
0.44
Kâ‚‚O
2.50
2.06
3.30
3.45
2.63
2.77
3.0
SO₃
0.10
1.39
0.89
1.86
2.82
4.66
0.35
S
0.02
0.01
0.10
0.02
0.10
0.05
0.02
Loss on ignition
1.9
2.25
3.80
6.3
5.5
2.6
16.13
Total
99.0
99.6
99.6
100.4
99.0
99.8
100.4
PH of shale water suspension
6.5
6.8
5.4
4.2
4.5
8.5
Not determined
Sulphate content of 1:1 shale water suspensions (% as SO₃)
0.06
0.14
0.16
0.70
0.69
0.15
Not determined
Table 2 - Composition of burnt and unburnt British colliery shales (in weight per cent)
Uses in the road making industry
Embankment fill
Burnt spoil by virtue of its granular nature and good grading can be placed and compacted up to the sub-base in almost any weather conditions. In Great Britain the only restriction on its use is that it is not allowed to be used within 450mm of the road surface if the sulphate content of a 1:1 shale water extract exceeds 2.5 g/litre due to its frost susceptibility.
In HBM for sub base and base if grading and material requirement are met
Availability and Location in the Great Britain
Most of the colliery spoil in the UK is found in Yorkshire, West Midlands, East midlands and South wale. According to the Waste and Resource Action Program the arising in England and Wales in 2001 is around 7.52 Mt with 0.81 Mt used as aggregate and an existing usable stockpile of 10-20 Mt. The arising in the England alone and the percentage used as aggregate is shown in the Table 3. It should be noted that 100% of the arising are used as aggregate as shown below.
Table 3 - Summary of arisings and use of materials - England, 2005 (in Mt)-survey2005
2.1.2: China Clay waste
Nature and Occurrence
China clay, otherwise known as kaolin owes its existence to kaolinisation, a process whereby feldspar in granite is weathered by hydrothermal activity to produce kaolinite. The other principal minerals in granite, mica and quartz remain unaltered during kaolinisation which means these form waste products during china clay extraction. The waste products consist mainly of two distinct materials; 'stent' which is basically a waste rock and 'tip sand'. China clay is traditionally extracted through hydraulic mining but currently dry mining is also employed. Overlying rock is removed to expose the underlying clay bearing rock. In hydraulic mining a high pressure jet is directed at the pit face dislodging the china clay and other minerals. This forms a slurry which is then filtered to removed the finer sand grains. The remaining clay is then refined. Conversely with dry mining clay and the associated waste materials are removed through mechanical excavation then screened, the remaining material then being disaggregated with hydraulic jets. Large quantities of waste result from the production process accounting for approximately 90% of the total quarried material. The sand and crushed rock overlying and within the clay reserves are an important supply of secondary aggregates. Numerically speaking, for each tonne of china clay produced gives rise to nearly 9 tonnes of waste consisting of:
3.7 tonnes of coarse sand
2 tonnes of waste rock
2 tonnes of overburden
0.9 tonnes of micaceous residue
All these materials above can be used in road construction except for micaceous residue with coarse sand being the most desirable.
Composition
Sand: The coarse sand waste is largest component in the extraction and it has the most desirable engineering properties. It is the quartzitic sand which is chemically inert. However even if it did contain any soluble salts, most would be removed during the extraction process and any that remained would be leached out of such a free draining material. The sand is a good quality aggregate and although the grading changes from tip to tip the specifications for aggregate can easily be met.
Stent: It can vary in size from less than 100mm to in excess of 2m in diameter, essentially consisting of massive quartz, quartz/tourmaline and partially kaolinized granite. The irregular distribution of these materials within the rock mass inevitably gives rise to variability in the waste. However they can still be used as a substitute to natural aggregate if they have been crushed and screened properly to meet the requirement of aggregate in the SHW. Typical properties reported by Hocking are:
Saturated 10% fines value 75-150 kN
Magnesium sulphate soundness value 70-90%
Soluble sulphate content 0.1%
Engineering properties
A summary of the chemical and physical properties of waste china clay sands in tabled below.
Mineralogy Analysis
Percentage by weight
Quartz
60-80
Feldspar
1-15
Tourmaline
2-10
Mica
0.5-15
Table 4 - Shows the mineral composition of waste china clay sand
Chemical Analysis
Percentage by weight
SiOâ‚‚
79-90
Al₂O₃
5-15
Fe₂O₃
0.5-1.2
TiOâ‚‚
0.05-0.15
CaO
0.05-0.5
Kâ‚‚O
1.0-7.5
Naâ‚‚O
0.02-0.75
MgO
0.05-0.5
% loss on ignition
1.2
Table 5 - Shows the chemical properties of waste china clay
Physical properties
Specific gravity
2.60 - 2.65
Water absorption (by weight)
0.5 - 1.0
Table 6 - Shows the physical properties of waste china clay
Table 7 - shows the typical grading of English china clay sand
Uses in the road making industry
The uses in road construction will be mainly for china clay sand and they are as follows:
As bitumen bound materials
In HBM for sub base and base - they can be used as part of the fine aggregate in HBM
In unbound mixture for sub base
Location and availability in Great Britain
The main location for china clay waste is South West of England around Cornwall, Devon and South West side of Dartmoor. The existing usable stockpiles in the UK, according to WRAP are around 150 Mt. The arising in the UK is as per Table 3 above.
Figure 6 - Shows the estimated total quantities of mining and quarrying by-products in UK between 1990-2004 (British geological survey)
2.2: Industrial waste and by-products
The main waste material in the industrial sector is Fly and bottom ash. We will concentrate solely on pulverised fuel ash for this section.
Nature and Occurrence
The generation of electricity from coal-fired power stations results in the production of two forms of ash, namely furnace bottom ash (FBA) and pulverised fuel ash (PFA). FBA is collected from the bottom of the furnace and accounts for 10-20 per cent of the ash produced. On the other hand PFA is a fine powder made up of individual fused ash spheres with a typical median diameter of about 10-15 micro meters. Coal burning power stations use coal which has been pulverised to a fine powder and when the pulverised coal is burned in a furnace at the power station it produces a very fine ash which is carried out of the furnace with the flue gases. This ash is PFA and it accounts for about 75-80% of the ash formed from burnt coal. Currently PFA and FBA are the United Kingdom's largest industrial and commercial waste stream. The estimated production and utilisation of fly ash is shown below. For this dissertation we will focus only on PFA.
Figure 7 - Shows the estimated production and utilisation of fly ash
Composition of PFA
PFA consists of glassy spheres together with some crystalline matter and a varying amount of carbon. There is a range of overall colour, from almost cream to dark grey and this is affected by the proportion of carbon, iron and moisture. The three predominant elements in PFA produced by burning British coals are silicon, aluminium and iron, the oxides of which together account for 75-95% of the material and the detailed composition is given in Table 8. Such ashes are known as alumina-silicate fly ash. There is another variety, known as sulpho-calcitric fly ash with a high limestone and sulphur content. Sulpho-calcitric ashes have high lime (CAO) content; they can therefore have hydraulic properties because of the pozzolanic reaction between the components of the ash. Physically, PFA is a fine powder which bears a close resemblance to Portland cement in general fineness and usually in colour. In particle-size distributions, PFA is predominantly silt-size.
Max.
Min.
Typical
Silicon as SiOâ‚‚
51
45
48
Aluminium as Al₂O₃
32
24
27
Iron as Fe₂O₃
11
7
9
Calcium as CaO
5.4
1.1
3.3
Magnesium as MgO
4.4
1.5
2.0
Potassium as Kâ‚‚O
4.5
2.8
3.8
Sodium as Naâ‚‚O
1.7
0.9
1.2
Titanium as TiOâ‚‚
1.1
0.8
0.9
Sulphur as SO₃ (soluble)
1.3
0.3
0.6
Chloride as Cl
0.15
0.05
0.08
Table 8 - Chemical analysis of British fly ash (in weight per cent)
Engineering properties
The specific gravity of PFA lies in the range of 1.9-2.3 and the dry density of material compacted at optimum moisture content varies from 1,120 kg/m3 to 1,490 kg/m3 .
Uses of PFA in road making industry
As bitumen bound materials - may be used in base course or binder course mixtures
In HBM for sub base and base - may be used as an aggregate or cementitious binder
In capping layer
Location and availability in Great Britain
The arisings in the UK is as per Table 3 above, which is around 5.0 Mt out of which 100% can be used as aggregate. The main location for finding PFA is in the Midlands, the North and Scotland where coal fired power stations are.
2.3: Metallurgical waste and by-products
The main material arising in the metallurgical sector is blast furnace slag and steel slag but for this these we will concentrate on blast furnace slag.
Nature and Occurrence
Blast furnace slag is a by-product from the production of iron, resulting from the fusion of fluxing stone (fluorspar) with coke, ash and the siliceous and aluminous residues remaining after the reduction and separation of iron from the ore. Because of its potential variability and possible inclusion of volumetrically expansive components, 'Old Bank' slag which were produced before the modern high level quality control procedures now in place, requires additional processing. There are various forms of solidified blast furnace slag produced and they depend on the rate and technique used to cool the molten material. Therefore it may be left to cool slowly in the open air, giving a crystalline slag suitable for crushing. This is known as 'air-cooled slag' and is mainly used as an aggregate in road construction. However when subjected to sudden cooling by using water or air, giving a vitrified slag. In the first case it is known as 'granulated slag' which is then processed to produce ground granulated blast furnace slag (GGBS) and this accounts to 25% of the production. Conversely when subjected to air cooling process it is known as 'pelletized slag' which is similar to granulated slag. 'Expanded slag' is the term used when it is left water cooled under certain conditions and the steam will produce the slag. The crystalline materials which produce blast furnace slag are compounds of the oxides of calcium and magnesium with silica and alumina. There is also a small amount of iron present as well as sulphur in the form of sulphides and sulphates.
Composition
The major composition of Blast furnace slag is listed in Table 9 below.
Constituent
Weight percent
Lime (CaO)
32 to 45
Magnesia (MgO)
5 to 15
Silica (SiOâ‚‚)
32 to 42
Aluminia (Al₂O₃)
7 to 16
Sulphur (S)
1 to 2
Iron oxide (Fe₂O₃)
0.1 to 1.5
Manganese oxide (MnO)
0.2 to 1.0
Table 9 - Shows the major chemical constituents in Blast furnace slag
Engineering properties
The main engineering properties are listed below in Table 10 below.
Physical state
Particulate
Mean particle size
5-30 micron
Colour
Off white
Odour
Odourless when dry but may give rise to sulphide odour when wet
PH
When wet, up to 12
Viscosity
N/A
Freezing point
N/A
Boiling point
>1700°C
Melting point
>1200°C
Flash point
N/A (not flammable)
Explosive properties
N/A
Density at 20°C
2.4 -2.8 g/cm³
Water solubility
At 20°C < 1g/l
Glass content (Vol-%)
60.4 - 100.0
True density (g/cm³)
2.796 - 3.070
Apparent density (g/cm³)
2.021 - 2.843
Bulk density (g/cm³)
0.689 - 1.427
Porosity (Vol-%)
2.5 - 31.2
Sieve size < 0.5mm (wt-%)
3.6 - 78.6
Sieve size < 3.2 (wt-%)
81.1 - 100.0
Table 10 - Shows the physical properties of Blast furnace slag
Uses in road construction
In bitumen bound materials
In concrete course aggregate
In hydraulically bound mixtures (HBM) - mainly base and sub base
In unbound mixtures for sub base
As a capping material
In embankment and fills
2.4 Municipal waste
The production of demolition and construction waste has been increasing at a gradual rate in recent years. The use of these materials as recycled base course in new roadway construction has become more common in the last 20 years. Recycled roadway materials are usually generated and reused at the construction site, thus providing increased savings both moneywise and time wise. The most widely used recycled materials are recycled asphalt pavement (RAP) and recycled concrete aggregate (RCA). RAP is produced by removing and reprocessing existing asphalt pavement while RCA is the product of the demolition of concrete structures such as building, roads and runways.
2.4.1 Recycled asphalt pavement
RAP includes three different types of recycled asphalt material. RAP could refer to the removal and reuse of the hot mix asphalt (HMA) layer of an existing roadway or it can refer to the removal and reuse of the HMA and the entire base course layer in which case it is called full depth reclamation. On the other hand recycled pavement material (RPM) refers to the removal and reuse of either the HMA and part of the base course layer or the HMA, the entire base course layer and part of the underlying sub grade implying a mixture of pavement layer materials. These three different aggregates will be collectively called RAP.
Nature and Occurrence
RAP is typically produced through milling operations, which basically involves the grinding and collection of existing HMA and FDR while RPM are typically excavated using full size reclaimers or portable asphalt recycling machines. RAP can be stockpiled but however it is usually reused immediately after processing at the site.
Engineering properties
The properties of RAP are largely dependent on the properties of the constituent materials and also the type of asphalt concrete mix (wearing surface, binder course, etc). There can be huge differences between asphalt concrete mixes in aggregate quality, size and consistency. Aggregates used in the surface course will be different to those used in binder course applications since the one used in surface course will have a higher resistance to wear/abrasion to contribute to acceptable friction resistance properties.
The particle size distribution of milled or crushed RAP will vary depending on the method of crushing or milling, the equipment used, the type of aggregate in the pavement and whether any underlying base or sub base aggregate has been mixed in with the RAP material during pavement removal. During processing, virtually all RAP produced is milled or crushed down to 38mm or less with a maximum allowable top size of 51mm or 63mm. Milled RAP is generally finer than crushed RAP. The particle size distribution can be shown in the table below.
As for the physical properties, the unit weight of milled or processed RAP is slightly lower than that of natural aggregates. Moisture content is dependent on the length of time the material has been left in the open air. The asphalt cement content is typically between 3 to 7 percent by weight which means that the asphalt cement adhering to the aggregate is harder than new asphalt cement which is due to the exposure of the pavement to atmospheric oxygen (oxidation) during use and weathering.
Physical properties
Unit weight
1.940 - 2.300 kg/m³ (120 - 140 pcf)
Moisture content
Normal: Up to 5%
Maximum: 7% - 8%
Asphalt content
Normal: 4.5% - 6%
Asphalt penetration
Normal: 10% - 80% at 25°C (77°F)
Absolute viscosity or recovered Asphalt cement
Normal: 4000 - 25000 poises at 60°C (140°F)
Table 11 - Show the physical properties of RAP
The mechanical properties of RAP depend on the original asphalt pavement type, the method used to recover the material and the degree of processing necessary to prepare the RAP. The compacted unit weight of RAP will decrease with increasing unit weight with a maximum dry density values to be around 1600 to 2000 kg/m³. The CBR will vary depending on the percentage of RAP used. This is shown in the table below.
Mechanical properties
Compacted unit weight
1600 - 2000 kg/m³ (100 - 125 pcf)
California Bearing Ratio (CBR)
100% RAP: 20% - 25%
40% RAP and 60% Natural
Aggregate: 150% or higher
Table 12 - Shows the mechanical properties of RAP
The chemical properties on the other hand are found to be similar to those of naturally occurring aggregates. This is because RAP is consisted mainly of mineral aggregate, in the range of 93-97% and only a small percentage consists of hardened asphalt cement. Asphalt cement is made up of mainly high molecular weight aliphatic hydrocarbon compounds, but also small concentrations of other materials such as sulphur, nitrogen and polycyclic hydrocarbons (aromatic and/or naphthenic) of very low chemical reactivity. Asphalt cement is a combination of asphaltenes and maltenes (resin and oil). Asphaltenes are more viscous than either resins or oils and play a major role in determining asphalt viscosity. Oxidation of aged asphalt causes the oil to convert to resins and the resins to convert to asphaltenes, resulting in age hardening and a higher viscosity binder.
Uses in road construction
The main uses will be as aggregate in asphalt.
Location and availability
The availability and location of RAP will vary since this occurs any places where there is rebuilding of roads. No information is available as to whether there are any stocks.
2.4.2 Recycled Concrete aggregate
Nature and Occurrence
RCA can be obtained from a variety of sources such as waste material from prefabricating yards, general demolition waste or demolition of individual structures. The production of RCA is done using the conventional aggregate plant but with additional features to remove some of the impurities, for example a magnetic separator to remove all steel. According to the BRE digest 433 RCA can be divided into three classes namely RCA (I), RCA (II) and RCA (III). RCA (I) defines the lowest quality material. It could have relatively low strength and high levels of impurities. It might contain up to 100% brick or block masonry or could comprise mainly of concrete but with high levels of impurities. RCA (II) defines a relatively high quality material which will comprise mainly of crushed concrete with up to 10% brick by weight but low levels of impurities, normally less than 1.5% by weight. The impurities can vary from wood, asphalt, glass, plastics or metals. In some cases it can contain an appreciable amount of natural aggregate. RCA (III) on the other hand, defines a mixed material with up to 50% brick and high levels of impurities.
Composition
The acceptable RCA quality that can be used in road construction are detailed below in Table 13 below.
Contaminant % by mass
BS 8500
BRE Digest 433 (RCA II)
Masonry
< 5% (a)
< 10%
Lightweight material < 1000kg/m³(c)
< 5% (b)
Included in other foreign material
Asphalt
< 5% (d)
Included in other foreign material
Other impurities (e.g. glass, plastic and metals)
< 1%
Included in other foreign material
Other foreign materials
Included in other foreign material
< 1%
Wood
Not quoted but should be < 0.1% as per E.N 12620
< 0.5%
Total
< 11.5%
< 11.5%
a: Limit may be increased to < 10% for exposed concrete when asphalt limit reduced to < 0.5%
b: Limit set to < 0.1% for exposed concrete
c: 'Floating stony' material only
d: Limit set to < 0.5% for exposed concrete
Table 13 - Shows the acceptable RCA quality
Engineering properties
The main engineering properties of RCA are listed in Table 13 below.
Physical properties
Specific gravity
2.2 to 2.5 (coarse particles)
2.0 to 2.3 (fine particles)
Absorption
2 to 6 (coarse particles)
4 to 8 (fine particles)
Mechanical properties
LA abrasion loss
20 - 45 (coarse particles)
Magnesium sulphate soundness loss
4 or less (coarse particles)
Less than 9 (fine particles)
California bearing ratio
94 - 148%
Table 14 - Shows the main physical and mechanical properties of RCA
Uses in road construction
The main uses for RCA are as follows:
In bitumen bound materials, either in base course or binder course mixtures
In HBM for sub base and base
In unbound mixture for sub base if suitably graded
In capping layer
Availability and Location in Great Britain
According to WRAP the arisings in the UK is around 46 Mt of which 100% are used as aggregate. The material is available nationally in Demolition sites or from suppliers of recycled aggregate.
2.4.3 Waste rubber and recycled tyres
Nature and Composition
Tyres or rubber are obtained nationally in scrap yards. The main composition of a tyre is mainly 60% rubber, 20% of steel and the rest is other material as in the figure 8.
Figure 8 - Composition of a rubber tyres
Engineering properties
The engineering properties of rubber are tabled below in Table 14.
Table 15 - Mechanical and physical properties of rubber
Uses in road construction
Using the wet process, tire crumbs are blended with hot asphalt concrete before cement is mixed with aggregate
Using the dry process tire crumbs are blended with aggregate with aggregate before they are mixed with hot asphalt cement
Availability and location
Tyres are available nationwide. It is estimated according to WRAP that around 487 000 tonnes of tyres are reused or disposed annually with around 160 000 tonnes going to the landfills. The existing stockpiles is believed to be around 300-400 kt.
Chapter 3: Uses of recycled materials in base layer
3.1 Fly ash - Fly ash bound material (FABM)
In FABM, fly ash is the main constituent of the binder with quick or hydrated lime the other constituent. FABM based on lime are generally slow setting, slow hardening and self healing mixtures. Cement can be used instead of lime but it is not as effective in mobilising the full pozzolanic and thus cementing potential of the fly ash. The use of cement instead of lime is applied when quicker hardening is required. A table representing the typical compressive strength of a treated fly ash is given below as well as a table showing the binder requirements for HBM mixtures.
Age of 1:1 sealed cylindrical specimens cured @20°C
Fly ash with 2.5% CaO
Fly ash with 5% CaO
Fly ash with 7% CEM1
Fly ash with (% CEM1
7 days
1.5
2
3
5
28 days
4
4
4
8
91 days
5
7.5
6
9
Table 16 - Shows the binder requirement for HBM mixture
Table 17 - Show minimum binder or binder constituents addition for HBM
For design purposes HBM category C is chosen from Fig. 2.1 in Appendix 2 which mean that FABM1 C12/16 is to be used. The resulting Bound base thickness which can be read from the nomograph will be 180mm.
From BS EN 14227-3 the graded mixture shall be 0/31.5mm with the grading determined in accordance with EN 933-1. The grading envelope for FABM 1 - 0/31.5 is given below and is taken from BS EN 14227-3 Fig 1.
Sieve (mm)
% passing by mass
Minimum
Maximum
40
100
31.5
85
100
25
75
100
20
66
95
10
48
82
4
34
68
2
26
58
0.5
16
38
0.25
13
30
0.063
7
18
Table 18 - Show the percentage passing
For FABM 1 the combined conditioned fly ash/lime proportion could vary between 10 to 15% by mass using, typically, a fly ash/lime ratio of 4. These figures of course depend on the grading and the cleanliness of the aggregate. However the exact amount of fly ash and lime will depends on the required mechanical performance requirements. Normally a very low percentage, 5% or less passing the 0.063 mm sieve is required for the aggregate since the binder is mainly silt sized particles and to achieve a higher strength mixture the % of silt like particle need to be minimised. A table showing the aggregate requirement for HBM is shown below as well as a table showing some typical mixes. Moreover a high percentage of fines in the mix cause dilution of the hydraulic potential of the fly ash.
Table 19 - Show the aggregate requirement for HBM
Table 20 - Show some typical mixes involving fly ash
Advantages and disadvantages of using FABM
To determine the advantages of using FABM a comparison with HRA 50 and DBM 50 will be made. Table 21 illustrates the structural thickness of each layer. The thickness is calculated using the procedures in Appendix 2 and 3.
FABM 1
HRA 50
DBM 50
Surface course
50 mm
400 mm
360 mm
Binder course
140 mm
Base
180 mm
Total
370 mm
400 mm
360 mm
Table 21 - Design comparison of FABM 1 with HRA 50 and DBM 50
As can be seen from the table FABM 1 will produce a smaller overall thickness compared to HRA 50 and almost the same thickness as DBM 50. A thickness saving of 30mm is quite consistent and it will save a lot of aggregate.
As far as recycling is concerned for FABM 1 there is the possibility of using 100% of recycled aggregate while for HRA 50 and DBM 50 only 50% recycled material can be used.
Cost wise, according to the United Kingdom Quality Fly Ash Association the cost of laying FABM1 is around £50/m3 while the cost related to DBM 50 is around £100/m3 and that of HRA 50 around £80/m3. As can be seen from above there is a big difference in the price range which consequently make FABM1 very cost effective.
However for the construction of a stretch of road, like City Road the fly ash used will have to be transported from the Midland or the North which is around 6000 km away from London. The transport cost by lorry will be very high since there is no possibility of shipping it either as there are not any river or sea nearby. Transportation by train can however be considered. Therefore overall cost will play a big part in the choosing any material for construction as well as the energy consumption for this purpose. A table showing the energy consumption by each transport mode is illustrated in Table 22 below.
As for the % of PFA can be added the maximum % is around 12% since PFA acts only as binder. However this is counteracted by the fact that with the use of PFA as a binder in FABM, recycled aggregate can be used.
As shown in Table 21 below the energy consumption for the production is the lowest among the values. Since we want to make pavement construction as green as possible, PFA will be definitely be a good choice when selecting an aggregate.
Table 21 - Typical energy consumption for material production
Table 22 - Typical energy consumption for production/transport of mixed materials and pavement construction design
3.2 Slag - Slag Bound Mixture (SBM)
SBM B, which is the one we use for this design is basically a mixture of aggregate, granulated blast furnace slag and water, where the GBFS is the activator. SBM B generally yields significant stiffness in the medium to long term. The binder constituent proportions should comply with those in the table in Fig (6.1) above.
For design purposes HBM category C is chosen from Fig. 2.1 in Appendix 2 which mean that SBM B1 C12/16 is to be used. The resulting Bound base thickness which can be read from the nomograph will be 180mm
The grading of the SBM must comply the figures listed in Table below. For the design we will select SBM B1-2 whereby the grading will be 0/31.5mm. The aggregates used in the mixture shall comply with those listed in Table (6.1) above. The grading envelope for the SBM chosen is given in Table () below and is taken from BS 14227-2.
Column
1
2
3
Line
SBM
Grading (mm)
Grading envelopes
1
B1-1
0/22,4
-
2
B1-2
0/31,5
3
B1-3
0/45
-
4
B1-4
0/31,5
-
Table 23 - Show the sub types of B1 mixture
Sieve (mm)
% passing by mass
Minimum
Maximum
31.5
90
100
16
50
85
4
25
60
2
20
50
0.063
0
6
Table 24 - Shows the % passing for SBM
As for the proportion of slag used in SBM mix, they vary between 15 to 25 % depending on the quality and cleanliness of the aggregate as well as the aggregate meeting their mechanical performance as required. In all the mixes selected the percentage of lime used as the activator is around 1%. The various possible mixes are tabled below.
Column
1
2
3
4
5
Line
SBM
Components
Aggregate
GBFS
Partially GBFS
Activator
1
B1/B2
74% to 84%
15% to 25%
-
1% lime
2
84% to 89%
10% to 15%
-
1% sulfate lime
3
87% to 91%
-
8% to 12%
1% sulfate lime
4
B3
86% to 91%
-
8% to 13%
1% sulfate lime
Table 25 - Show the composition of various mixes
Advantages and disadvantages of using SBM
When comparing the design thickness of SBM with other mixes the same example used in the case of FABM is used. The results will be the same. The total thickness for DBM 50 will be 360mm while for HRA it will be 400mm with FABM having a thickness of 370mm as shown in Table 21 above.
The production of GBS compared natural aggregate is comparatively low, around 40 as is shown in Table 21 above. However since GBS only acts as a binder in HBM the maximum addition is only between 15% -25%. A table showing how the compressive strength varies with the % of slag added is shown below.
Figure 9 -The influence of slag content on the compressive strength development of SBM
On the other hand the cost of laying SBM is around the same price as FABM, which is £50/m3 which consequently just like FABM makes it a very cost effective product to use. However the choice of SBM will largely depend on the availability of the material within the proximity of the area. Since most of the slag is found is around Yorkshire and the North East transport cost will influence the choice. Shipping of the material since there is the sea nearby or by rail will greatly reduce the cost of transportation.
3.3 Recycled concrete aggregate (RCA)
RCA forms part of unbound mixture Type 3 - Open graded according to MCHW Vol. 1 Series 800 manual. The requirements for RCA to be used in the mix is that it should not contain more than 5% asphalt (Ra) and not more than 1% of other materials (class X), including wood, plastic and metal as shown in Table 26. The material shall be tested in accordance to Clause 710 of MCHW Vol. 1 Series 700. The material requirement properties shall be in accordance to Table 29, taken from MCHW Series 800 and the grading requirements for the mixture should comply with those set out in the Table 28. The mixture, however should comply with the standards set out in Table 27.
Table 26 - Additional requirement for RCA used in Type 1, 2 and 4 unbound mixtures
Table 27 - Show mixture and grading requirement categories for unbound mixtures
Table 28 - Show a summary of the grading requirements for Type 3 (open graded) unbound mixtures
Table 29 - Show requirements for aggregates used in unbound mixtures
RCA can be used in any unbound mixes as a substitute to natural virgin aggregate as long as they comply with the requirements set out above. Conversely RCA can also be used in HBM and they need to comply with the requirements set out in the Table 26.
Advantages and disadvantages of using RCA
RCA provide engineering, economic and environmental benefits.
The main advantage of using RCA is that it can be used as a direct substitute for the aggregate used in road construction. For instance, in road construction project the use of FABM or SBM can be used along with RCA to meet the criteria of strength for the base layer. They represent a major asset if they can be screened properly to remove the steel and other unwanted material. However high class concrete will be a preferred origin as they would more easily meet the criteria set above for the requirement of aggregate but normally any concrete will provide a high strength material mostly due to the bonding of the residue cementitious material.
For the construction of a stretch of road, say City Road the material can be easily available through the various scrap yards or screening centres around London. Compared to the cost of virgin aggregate the cost related to transporting the aggregate around London will be minimal. Moreover they are helping clear the debris from scrap yards.
As for the energy consumption for the production of RCA it is relatively low, around 25 MJ/tonnes compared to other material, as illustrated in Table 21.
3.4 Recycled asphalt
Recycled asphalt is used in Type 4 Unbound mixture. According to MCHW Vol. 1 series 800 Clause 807 type 4 unbound mixture can be made from recycled aggregates containing asphalt arisings, and may contain crushed rock, crushed slag, crushed concrete or well burnt shale and up to 10% by mass of natural sand that passes the 4mm size test sieve. Asphalt arisings shall be either road planings or granulated asphalt, but excluding tar or tar-bitumen binders. Asphalt arisings are basically materials that are obtained from the asphalt layers of the pavement while granulated asphalt is asphalt bound material recycled from roads under reconstruction or surplus asphalt material which were originally meant for bound pavement layer but been granulated since it was unused.
The requirement for Type 4 unbound mixture is that the asphalt content (Ra) has to be greater than 50% while the bitumen content of the asphalt must not be greater than 10% and must comply with the requirements set out in Table 27. The aggregate properties must meet the criteria as set in Table 29. The grading requirement on the other hand must meet the values as set in Table 30.
Table 30 - Show summary grading requirements for Type 4 Unbound mixture
Recycled asphalt can therefore be substituted for natural virgin aggregate fully as long as they meet the criteria set out above.
Advantages and disadvantages of using RA
The use of RA, if it meets the requirement for aggregate will be a good substitute for virgin aggregate. The fact that RA already contains bitumen makes it better in the sense that less binder material may be needed. However there is much variability in the quality of the RA obtained whether in the gradation or asphalt content but for use in the base the quality need not be high standard. Moreover they help eliminate waste from the landfills.
In terms of the energy consumption for their production they are relatively low, around 25 MJ/tonnes which makes it a green material.
Chapter 4 - Uses of recycled materials in surfacing
4.1 Binder course
According to the Waste and Resource Action Program website the following materials can be used in the binder course layer for HRA. They are listed below:
Blast furnace slag
China clay
PFA
Reclaimed asphalt
Recycled aggregate
RCA
Recycled glass
