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UPGRADING OF AGGREGATES. by Peet Höbeda
Swedish Road and Traffic Research Institute
3-581 01 LINKÖPING
SWEDEN
This material has been used in an abbreviated
form in the General Report, themes III:4-5, at the Nice Symposium on Aggregates, May 1984.
TABLE OF CONTENTS
Page
1. INTRODUCTION l
2. MATERIAL AND CLIMATIC FACTORS l
3. DESIGN FACTORS 4
4. PROCESSING 5
5. IMPROVEMENT OF GRADING 7
6. "SALT STABILIZATION" 7 7. PRETREATMENT BY COATING AND
IMPREGNATION 8
8. TREATMENTS WITH BINDERS IN ROAD
CONSTRUCTION 9
8.1 . Lime - 10
8.2 Hydraulic and puzzolanic
stabilization , 11
8.3 Bituminous binders 12
9.
SOME EXAMPLES OF UPGRADING
'
13
9.1 Soft limestones 13 9.2 Shales 14 9.3 Laterites I 15 9.4 Other aggregates , 16 9.5 Sands , 17 REFERENCES 18
UPGRADING OF AGGREGATES
by Peet Höbeda
Swedish Road and Traffic Research Institute
8-581 01 LINKÖPING SWEDEN
l. INTRODUCTION
In several parts of the world good quality aggregates are non-existent or not freely available because of urban settlements, environmental restrictions etc. Because of this, upgrading of local low quality aggre-gates with poor mechanical properties, low durability, poor grading or particle shape, is becoming more and more important (l).
Arnould and de Mouza summarize 86 studies in France to substitute fluviatil gravels and sands with products as crushed hard and soft rocks, soils as clayey sands, moraines, superficial deposits etc. and also waste
materials. In many cases, substitution is not possible, more because of economical than technical reasons.
Low quality aggregates, treated or untreated, have been utilized especially for low volume roads (2)
and the experiences gained can not always be transfer-red to roads carrying heavy traffic. Futhermore, the utilization has been more common in countries with
warm climates; consequently the differing environmental conditions must be considered. Good knowledge of
material properties is always important so that over-specifications are avoided.
2. MATERIAL AND CLIMATIC FACTORS
diffe-rent parts of the world. Often favourable conditions exist eg. in the precambrian shield areas in the
northern hemisphere as late glaciation has eliminated the weathered mantle and deposited glaciofluvial,
clean gravels. Areas covered with young soft
sedimen-tary deposits are, on the other hand, devoid of suitable aggregate resources. In warm humid climates, chemical weathering affects bedrock to a considerable depth
resulting in a weathered mantle, sometimes with
secon-dary formations such as laterites and "duricrusts" that are widely utilized as aggregate resources.
To complicate the matter, different testing methods? are used in different countries. Jumashev and Toureng compare testing methods and specifications for weak aggregates in the Soviet Union and France. In the former country, magmatic rocks with uniaxial compres-siVe strength less than 60 MPa are considered weak, but sedimentary rocks with as low strengths as 20 MPa
are used in stabilized pavements. In France, the Los Angeles value is not allowed to exceed 40 (that is a
compreSsive strength about 60 MPa). It is also mentioned that the crushing of aggregate under load increases
significantly if weaker than stated above. Low quality aggregates should be tested for resistance to water, eg. as with the French Microdeval test. Arnould and de Mouza define as soft rocks such ones that have Los Angeles values less than 30-35 and wet Microdeval
values less than 25-27.
Aggregates perform in different ways in different
climatic conditions, this being especially true for the low-quality aggregates. Freeze-thaw action imposes the most severe environmental strain in conditions where the aggregate may reach a high degree of
satu-ration during service. In pavement and bridge appli-cations, also the use of deicing salts accelerates degradation (3).
In a cold and moist climate, the unstabilized layer in a road construction must be permeable to prevent the building-up of pore water pressures during the spring thaw (4). The most important parameter found to influence the performance of flexible roads is the fines content (<0.075 mm) in the unstabilized layers. At the base course level, it must be quite low if no cracking and rutting is tolerated (5).
In non-freezing conditions, on the other hand, the fines content can be considerably higher but instead the plasticity index of the fines form an important criterion because of the often weathered aggregates. In arid regions fines with a higher plasticity index can be tolerated than in humid regions (6). As shown by Metcalf and Wylde, even sands can perform in a
base course under bituminous seals provided the climate is favOurable and the traffic moderate.
In concreting work, the main problem in a non-freezing
climate seems to be the excessive drying shrinkage caused by some low quality aggregates (7). However,
if high-quality concrete is not required, use has
also been made of non-conventional aggregates. Laterite has been found suitable in investigations by Carvalho
(ch. 9.3).
Special problems arise in dry and hot arid conditions as the temperature variations during night and day are excessive. Also soils and aggregate deposits may be contaminated with water soluble salts which affect concrete durability (8). Roads with a thin, bituminous
surfacing, not quite impermeable, may be damaged by blistering and cracking when salts are concentrated by evaporation. Salt weathering of porous aggregates may occur because of the volume changes the salt
crystals undergo with changing temperatures and
rela-tive humidities.
3. DESIGN FACTORS
The pavement design in different countries varies a great deal depending on the traffic and climatic conditions, the materials available and the national traditions of construction. Toussaint describes dif-ferent pavement structures according to the specifi-cation in the Federal Republic of Germany which enable
the use of local resources of crushed rock, gravel and sand. The thicknesses of the stabilized and unsta-bilized materials are varied to achieve similar per-formance. If sand is utilized it is stabilized with
cement or bitumen.
When using poor quality aggregates different measures can be taken, many of which are described by Toureng. The pavement structure must be well-compacted, adequately drained and sealed with an impervious surfacing. The
necessity of good drainage of a shale pavement is also obvious from the paper of Hode-Keyser et al. The weather conditions during construction may be a limiting factor when using aggregates with plastic fines (Metcalf and Wylde, Tourenq).
A particular case is aggregates which are prone to polishing in the road surface. A solution may be to use the local aggregate in a asphalt or portland cement concrete and to apply a surface dressing with a non-polishing aggregate (Toureng).
In concreting work, grading defiencies that cause
poor workability can be counteracted by adding plasti-fying agents, fly ash etc. Shrinkage-compensating
cement can be used with aggregates that are known to cause drying-shrinkage problems (9). Unsound fine aggregate can be substituted by natural sand. Alkali-reactive aggregates, on the other hand, can often be
used in combination with low alkali-cement or puzzolanic
additives (10). In an environment with a risk ofsulphate attack sulphate-resistant cement should be
used. In a freezing environment air-entrainment is
necessary. The reduction of aggregate size can improve the durability (11).
4. PROCESSING
When working a variable, heterogenous deposit, either pit or quarry, the geological conditions and variations
in material quality must be well known as selective quarrying can be necessary (12). Grimsicar describes
a quarry where two different aggregates, a dolomite and a quartz keratophyre, are processed at the same time and Liquori et al the special processing of pumice aggregate.
The aggregate quality can be improved by several
well-known processing operations. Toussaint describes processing of a quartzite with interbedded shale
which is removed by two preliminary screenings and an intermediate crushing. The aggregates for qualified applications are processed from the plus 60 mm material. In another case, interbedded thin layers of marl and mudstone are removed from limestone through screening out material passing the 4 mm screen and replacing the fine aggregate with natural sand. Griveaux and Robert deal with the processing of a soft limestone.
PreScalping of material >20 mm is performed and a
coarser grading (0/60 mm) than specified chosen for the cement-stabilized subbase because of difficulties in processing this material. Also Carusero and Rapin describe the processing of a soft limestone by careful control of the different operations.
Washing is performed, especially when working conta-minated gravels and inhomogenous rock materials. When coatings or clay lumps are present, special scrubbing can be necessary. Arnould and de Mouza mention different replacement aggregates as moraines and superficial deposits that often must be treated
by washing (13). When treating fine aggregates,
equip-ment that works on the principle of hindered
settle-ment in water must be used. Washing may also be
necessary when the aggregate is contaminated with salts, e.g. some sea-dredged aggregates or aggregates
in arid climates (7).
Less well-known are the gravel benefication processes, employed especially for concrete aggregates when the t resistance to frost is of concern (14). In the heavy-media separation and jigging processes, porous chert,. shale etc. are removed. In the elastic fractionation, deleterious particles with a low elasticity modulus, bounce a shorter distance than the better particles and separation is possible. When treating slags,
demolition rubble, incinerated residue etc, iron
must be removed with magnets as described by Bauchard and Joubert. Light-weight contaminents can be removed by water or wind separation.
The aggregate particle shape is normally influenced more by the crushing operations-than the rock type.
shales tend to form flaky and elongated particles. äeparation of the particle shapes is possible but seldom performed (15). When processing a small-sized gravel, difficulties arise in obtaining a sufficient amount of crushed surfaces but special impact crushing may be a solution (16).
5 .
IMPROVEMENT or' GRADING
The grading envelopes given in the different national specifications warrant suitable properties of aggregates
for different applications. If binders are added,
suitable gradings are minimizing the content of expen-sive bitumen or cement. For marginal aggregates, the most suitable grading is not necessarily the specified one (17). In "mechanical stabilization" of road aggre-gate, different materials are blended to a suitable grading without the addition of binder but the water
present gives some "cohesion", Gaspar and Hegyi describe
the "mechanically stabilized" materials prescribed
in Hungary: Also quarry and pit wastes are incorporated, Hode-Keyser et al have found better performance of a
dense than an open-graded shale base. Jumasev and
Toureng demonstrate in laboratory experiments that the crushing of gravel under load is reduced when sand is added. The Characteristics of fine aggregate are also of importance as a clay content makes the material sensitive to water. ln unbound road surfaces, however, a certain plasticity is desirable.
6. "SALT STABILIZATION"
The use of hygroscopic salts such as calcium chloride and magnesium chloride for dust palliation of unbound road surfaces is well-known. Sodium and calcium chloride
road bases in the USA and it has been claimed that this salt stabilization" technique enables the uti-lization of poor quality aggregates (18). The salt
acts as a compaction aid, helps to achieve a homogenous
layer and also increased shear strength is reported. Cores carefully extracted from salt stabilized" roads have shown that sodium chloride can give some cementation. According to other fieldstudies the
salt is leached out and gives little improvement.
Aggregates in arid climates often contain water
soluble salts that cause particular problems (Ch. 2). In unsurfaced roads they give a favourable cementation. Devaux describes waste rock from potassium mines
that hardens. However, besides halite there is a high content of anhydrite that transforms to gypsum.
7. PRETREATMENT BY COATING AND IMPREGNATION
Precoating of the aggregate with asphalt or tar products has been a standard procedure for hot-rolled asphalts
and some surface-dressings (Toureng). The properties
of low quality aggregates can also be improved by a bitumen coating as shown by Gokhale et al. Lime slurry
is used for pretreatment of hydrophilic aggregates, especially those containing clay minerals (19). Pre-treatment of hot-mix-aggregate with portland cement is done in Kuwait as this promotes both the adhesion and the stability of the mix 120). Also pretreatment with e.g. silane has been studied in the laboratory
(21).
Porous aggregates have been pretreated in the labora-tory with agents such as aniline furfural, methylmeta-acrylate etc. to reduce asphalt absorption and increase the stability of the mix (22). Polymeric and other
coatings or impregnations have also been made to'
improve the wearing and polishing resistance or to counteract stripping and degradation etc (23, 24). If glossy surface coatings are formed, the aggregate polishing resistance and the stability of the mix
will decrease. '
Concrete aggregates have been treated in the same manner in the laboratory in order to improve the
durability or to counteract alkali-silica reactivity, although the compressive strength may decrease. Another approach to improve the frost durability of concrete aggregates has been impregnation with a solution of ethylene glycol (25).
Activation of manufactured fillers can be achieved by adding surface-active agents (bituminous products, cationic agents etc.) during the grinding process
for absorption on the freshly crushed surfaces (26). Better compactability, stability and moisture resistance
of the mix result according to Soviet experience.
8. TREATMENT WITH BINDERS IN ROAD CONSTRUCTION
By adding a cementing or waterproofing agent, the
mechanical properties and durability of a given aggregate can be improved. A low binder content (often <2%)
modifies the material e.g. makes its soil mortar less plastic and water-susceptible. Also segregation can be counteracted and the compactability improved. A higher amount of binder, on the other hand, stabilizes the material whereby its load bearing prOperties are fundamentally altered. The binder can be added either by a mix-in place process or, if the plasticity of
10
Hydraulic or puzzolanic stabilizations are rigid and prone to cracking and the freeze-thaw durability may be low, especially if deicing salts are used. The cracks are reflected to the bituminous surfacing and water can find an entry and cause degradation. Bitumen bound materials on the other hand, are temperature-susceptible and more or less lose stability in high temperatures. Also degradation of very weak aggregates is possible under traffic loads. It has been shown for a high porosity soft limestone treated either with portland cement and bitumen that the freeze-thaw durability and stability, respectively, will be
insufficient, although a combination of both binders
gives better properties (27).
Binders based on sulphur (28), wood industry wastes,
resins etc. are investigated but presently no alterna-tives to the lime, cement and bitumen exist.
8.l Lime
The aggregate fines must contain a certain amount of clay minerals which react with lime to react. Certain additions, e.g. of sodium chloride improves the effect
of lime. When lime is added the clay is flocculated
by a short-term reaction but cementation can be achieved by a long-term one. A puzzolan as fly ash may be
added to a poorly reacting aggregate (Ch. 8.2).
Sometimes lime pretreatment is used in combination with cement or bitumen stabilization to improve the workability of a material with plastic fines. Some lime, added as filler, improves aggregates prone to stripping in bituminous mixes (29). Also lime or
portland cement is found to minimize the degradation
ll
calcium ions (30). An unusual application is the addition of lime to salt-contaminated.aggregates (Ch. 2) as the little soluble calcium sulphate is formed (30).
Waste lime, such as kiln dust and lime from acetylene gas manufacture, can be utilized instead of commercial lime. The latter product is used in Hungary as told by Gaspar and Hegyi.
8.2 Hydraulic and puzzolanic stabilizations
Portland cement addition to an aggregate gives struc-tural strength through the hydrates formed, but also. the liberated lime can take part in a long term reaction if a puzzolanic material is present. A content of
active organic substance counteracts the binding as discussed in the papers of Devaux and Höbeda. Portland cement is by far the most common hydraulic binder
but slower reacting, blended cements can be beneficial as longer construction time is available. Also better
durability can be obtained (32). A ground slag binder'
has given good results in Swedish test roads, especially when activated with lime-gypsum (Höbeda).
In the French slag stabilization ("grave-laitier") technique, unground or slightly preground granulated blast furnace slags is used. Carusero and Rapin have found that lime activation has not given resistance against frost to a stabilized soft limestone, after 60 days of binding, contrary to alkali-sulphate acti-vation. According to the paper by Gaspar and Hegyi, slag stabilization is used in Hungary for treating both aggregates and soils. The compressive strength after 60 days should be at least 2.5 MPa, and for
12
road strengthening 5 MPa. As with portland cement, a _content of organic material is deleterious (papers
by Devaux, Höbeda) but certain additions (gypsum, calcium chloridey alkali salts) can counteract the strongly retarding effect.
Fly ash-lime is the most common puzzolanic binder.
According to Gaspar and Hegyi, in Hungary, the compres-sive strength after 60 days must be at least 5 MPa,
and for strengthening'of old roads 7 MPa. The puzzolanic reaction is slow in a cold climate, but can be accele-rated by additions in low contents. Fly ash-cement can also be used, especially with poorly graded aggre-gates (Höbeda).
8.3 Bituminous binders
By adding a bituminous binder cohesion and waterproofing of_aggregates can be achieved. In a hot laid mix,
asphalt cement is used, and these effects are obtained after cooling. Asphalt cement can also be added using a foaming process, especially when the aggregate is
rich in fines. In cold (or warm) mixes with cutback
and emulsified binders a curing period is first neces-sary. If only waterproofing (modification) is required
a low content of binder can be added (33), as shown
in experiments by Gokhale to upgrade shale.
As bituminous binders are temperature susceptible it is important that the aggregate has adequate frictional resistance. Stripping of the binder from the aggregate in the presence of water is another major problem. Lime and cement - in the form of pretreatments or *fillers - are probably the most effective remedies
with some aggregates (29, 33), but more often cationic surfactants, amines etc. are mixed in the-binder.
13
Also, a good quality limestone filler improves the
resistance to water.
The mix properties can be improved by extending bitumen with sulphur, polymers, ground rubber, adding reinforcing agents such as strips of waste plastic, "microfillers" as silica fume dust or carbon black etc.
9. .SOME EXAMPLES OF UPGRADING
9.1 Soft limestones
Under favourable climatic conditions soft limestones often perform well, especially in low-volume roads. A self-cementation effect has been found in several varieties, e.g; coral, calcrete, weathered limestone, "lime-rock" etc (35). Grid rolling or mechanical stabilization" is performed if the grading is poor.
Keyser et al. have investigated the self-binding
properties of some Florida "lime-rocks", used in
road bases. Cementation is achieved when the compacted samples are subjected to ageing and drying. The compres-sive strength of laboratory specimens is shown to
increase, as is the bearing capacity of test sections.
According to the paper by Jumasev and Toureng, a
very soft limestone (Los Angeles value 90) has degraded to fines because of frost action. Griveaux and Robert describe the treatment of a soft limestone with 4% portland cement for use in a subbase. The performance
of the road has been excellent. If the resistance to
frost is of concern, a quite high cement content-can be needed. In the UK it has been found neccessary to
add as much as 14% portland cement to make a-porous
14
bitumen binders is uncommon as the porous limestones
absorb bitumen.
Soft, non-argillaceous limestones can perform as concrete aggregates under favourable environmental
conditions. Coral detritus has been studied in Australia, limerock in Florida, kankar (calcrete) in India,
calcerous tufs in Algeria, "lime-rock" and shells mixed with sand in USA (37) etc. Arnould and de Mouza have investigated soft limestone aggregates in concrete. If the fines content is high the strength properities decrease 15% and also the volume stability to some degree.
_9.2 Shales
Shales break up in flat, elongated particles when crushed, but some vareties also swell and degrade in service. The latter ones can be treated as soils and compacted in thin lifts when building embankments,
the durable shales are handled as rockfills. Treatment'
of non-durable, swelling shales has been studied
with inorganic salts and lime and the latter procedure shows promise (38).
The use of shales in road bases was already studied in the thirties in the USA (39) and durable shales had given good service in roads with bituminous
sur-facings. The gradings are not given. Keyser et al
have, however, found in test sections in Montreal
that a shale base was not quite as good as a limestone base and the thickness must be increased. Gokhale et al report degradation of shale bases in India, however, on open grading was used contrary to the case in the Canadian tests.
15
Treatment of shale for use in pavement is possible
with lime or cement (40). Also bituminous treatments
can work, as shown in the papers by Gokhale et al
and Hode-Keyser et al. 9.3 Laterites
Laterites as described in the literature can vary from clay soils to hard, rocklike crusts (41). The clay minerals have coatings of iron and aluminium oxides and consequently the index values are very
sensitive to the degree of reworking. Hard laterite
varieties with a high sesquioxide content can even perform as surfacing aggregate (42).
Often laterites have, in spite of high plasticity indexes, performed better than "standard" aggregates
in road bases and in unbound surface courses, especially
in low volume roads and in dry climates. (41). After the removal from the borrow pit, many laterites have a self-cementation ability, probably depending on
the dehydration of seisquioxides. They also have a
certain puzzolanity and react favourably to lime and
cement treatments (43). Non-clayey laterites also
have good adhesion to bitumen.
The suitability of a Brazilian laterite as concrete
aggregate has been investigated by Carvalho. It was
found that the concrete had adequate physical charac-teristics although the compressive strength was 30% lower than with a "standard" granitic aggregate. The laterite is absorptive and the water content must be increased for adequate workability. The concrete behaved also as a lightweight aggregate concrete since failure occurred in the aggregate particles. Investigations of some African laterites have led to
16
similar conclusions (42). 9.4 Other aggregates
Camponeschi et al have investigated the geotechnical properties of puzzolanas, lithic and loose tuffs. Some of these are very fragile and degrade during construction. Liquari et al are describing pumice from Lipari that has been used in fills but alSo as light-weight aggregate. These types of aggregates are used in low-volume road construction, in the USA often after "mechanical" stabilization or grid rolling
(2). Bucchi and Righi have investigated the possibi-lities of utilizing soft sandstones. The CBR-values are high but there is a risk of degradation under traffic. Bituminous mixes with good Marshall values were obtained and also cement stabilization was
pos-sible. Road tests are, however, considered necessary to evaluate the suitability.
In some parts of the world the in-service degradation of aggregates in road bases and surfacings is a serious problem (44). The aggregates have typically been up to specifications according to standardized tests.
New test methods as wet degradation have been developed. The problems have been mainly connected with altered basic volcanics but also with other aggregates.
Treat-ment with lime.or ceTreat-ment is beneficial, but also
limestone fines is said to help (30). In New Zealand, degrading aggregates modified with 1% lime, have
sometimes performed better than high quality aggregates (45). When treating with bituminous binders, lime
pretreatment seems necessary (34).
Some of the degradable aggregates in road construction can probably function in concrete under favourable
17
environmental conditions since the degradation process probably is arrested in the alkaline mortar. The
problem may, however, be a poor volume stability
(7).
9.5 Sands
In areas lacking coarse aggregates, sands of different origins are resources widely used (46). Arnould and de Mouza describe French attempts to utilize sands as substitution for gravels. Negative factors are often a fine and single grain size, contaminations of clay, organic substances or salts. Metcalf and
wylde have investigated sands that have behaved diffe-rently during construction and service as bases in moderately trafficated Australian roads. The particle size distribution was the best criterion for suitability.
Construction problems occurred if the plasticity
index was high. The sands are said to have high iron
oxide contents and are sometimes angular, both positive
factors.
Often it is necessary to stabilize sands with a binder and at the same time add a filler or crusher fines
(47). Treatment with bituminous binders often works well. Addition of some lime or cement can improve
the resistance to moisture and the stability, especially of cold mixes. Sulphur extension of the asphalt binder
(28) also greatly improves the stability, as do certain polymeric additives.
Stabilization with hydraulic and puzzolanic binders is also common. The immediate stability (CBR-value) of the unbound material must be high in order to
avoid construction problems. In particular an active
18
a binder component (Höbeda).
Frost-susceptible silts have been stabilized in France after pretreatment with 2% quick-lime followed by 6%
portland cement (48). Silt, pretreated with 2-3%
'
cement, followed by 4% bitumen emulsion has been studied in Saudi Arabia (49);
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ARTIFICIAL AGGREGATESd by Peet Höbeda
Swedish Road and Traffic Research Institute
3-581 01 LINKÖPING
.SWEDEN
These materials have been used in an abbreviated form in the General Report, themes III:4-5, at the Nice Symposium on Aggregates, May 1984.
TABLE OF CONTENTS . Page
1.
WASTE.MATERIALS AND BY-PRODUCTS
1
1.1 Mining and quarrying wastes and by-products 2
1.1.1 Colliery spoils 2
1.1.2 Waste rock and tailings 2
1.1.3 Quarry wastes 4
1.2 Metallurgical wastes and
by-products 4
1.2.1 Blast furnace slag 4
1.2.2
Steel slag
5
1.3 ' Industrial wastes and residues 6 1.3.1 By-products from coal fired
power plants 6
1.3.2 Other products 8
1.4 Municipal wastes and by-products 8
1.4.1 Incinerated residue 8
1.4.2 Demoliation waste 9
