I J ~ ~ ~ I I J ~~ International
V A P Harris, L S Hitch, J M Jowett
Transport Research Laboratory
Berkshire RG45 6AU
United Kingdom PAI 1 3011983
TITLE: Bituminous stabilisation of
fine sands: construction of
the Baiomori-Gashua road,
by: PA1 130/83 HARRIS, V A P, L S HITCH and J M JOWETT (1983). Bituminous stabilization of
fine sands: construction of the Baiomori -Gashua road, Nigeria. Proc. Institution of Civil
Engineers Part 1, 74 (May), 2 70-300. Proc. Jnstn Civ. Engrs, Part 1, 1983, 74, May, 277-300
Bituminous stabilization of fine sands:
construction of the Baiomori-Gashua
V. A. P. HARRIS, MICE, MIHE*
L. S. HITCHt
J. M. JOWETT, BSc(Eng), FICE, FIHE*
Bitumen has been used to produce road base material in regions of cohesionless soil for
many years. The engineer, however, invariably faces the problem of reconciling the
properties of a given soil with available binders and processes to produce base material
having the required properties. The Paper describes the investigational work leading to
preproduction trials which was followed by successful road construction in an area of
aeolian sand in northern Nigeria.
Bituminous stabilization for the construction of road bases has become
increasingly used in developing countries in the last 30 years, particularly in
areas characterized by cohesionless soils and which are poor in aggregate sources.
The Chad basin in northern Nigeria is a good example and in 1960 the road
between Maiduguri and Bamna (67 kin) was constructed using the local Bamna
Ridge sand stabilized by a hot-mix process. Johnson & Gandy' of consulting
engineers Scott Wilson and Kirkpatrick and Partners have described the design
and construction of this project; Williams 2 of the Transport and Road Research
Laboratory has described the construction of experimental sections on this road
using cold-mixed cut-back stabilized material.
2. It might be reasonably argued that, notwithstanding the above experiences
and many others,particularly in developing countries, there is still a general lack
of information and understanding of bituminous stabilization: Katti, Davidson
& Sheeler' when "examining the role of water in cut-back bitumen stabilization
have stressed this general problem. Three notable areas are:
(a) characteristics of local soils
(b) selection of binder and process
(c) test criteria.
Hitch & Russell 4 have compared several mix stability procedures when applied
to the sand used for the Maiduguri Bamna experimental sections; stability criteria
Written discussion closes 15 July 1983; for further details see p. (ii).
*Ove. Arup and Partners.
tTransport and Road Research Laboratory.
277 HARRIS, HITCH AND JOWETT
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kn k1) ~ - ~ - - - -I--
0:50 r- 00 00 all (*n m# (.- ;) cl4c~l r- -. -~ -.
++ ++ tnON 9-Cl
++ ++ re~C rA 00 00 00 all (7 reCl1 -. 1~ l- V') .- : 0
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C;-) BITUMINOUS STABILIZATION OF FINE SANDS
for light traffic have been proposed. Further work by Russell 5 enabled stability
criteria to be suggested for other traffic categories. However, these suggestions
were not available at the time of the events now reported.
3. It is against this background that a road was built in an aeolian sand area
of northern Nigeria. This Paper describes the construction of a sand-bitumen
base with excellent load-bearing properties.
The Damaturu-Gashua-Geidam road, northern Nigeria
4. In 1970 Consulting Engineers Ove Arup and Partners were commissioned
by the Government of North East State, Nigeria to survey and design the
Damaturu-Gashua-Geidam (DGG) road (235 kin). Based on existing economic
haul distances the consultants proposed the following construction:
laterite base (61.5 kin)
jigilin base (50 kmn)
bitumen-stabilized sand base (123.5 kin)
Because of subsequent rapid increases in the price of the cut-back bitumen
selected, only 55 km (the Baiomori-Gashua section) was constructed using
5. At the commencement of the DGG survey and design work the Maiduguri-
Bamna road had been in service 10 years. By 1976 it was known that the Maiduguri-
Bamna road carried 250-300 vehicles per day (vpd); observers report however
that traffic had by 1980 increased to 1000 vpd or greater, since Maiduguri had
become the State capital. The road has now been in service 20 years, without
undue problems as far as is known. The performance of the experimental
sections mentioned earlier was therefore of interest when designing the DGG
road. Table 1 shows the properties of seven samples cut from the experimental
sections after 12 years in service.
6. The Maiduguri-Baina road was constructed using Bamna Ridge sands hot-
mixed with 80/100 penetration grade (pen) bitumen: the contractor was
required to achieve 95 % of the laboratory Hubbard-Field density of 1.986 Mg/in 3
124.0 lb/ft') which gave a Hubbard-Field stability of 5.34 kN (1200 lbf) at a test
temperature of 49 0C. It had, however, been estimated that the three DGG project
roads would normally carry traffic volumes of 150-200 vpd during the early
years after their construction with the exception of the crop evacuation period
of August-October when the traffic might increase by 50 %. It was also estimated
that a substantial proportion of commercial vehicles using these roads would
have axle loads in the range 15-20 t. Consequently, in the absence of any other
information, it was decided to adopt the Marshall test design criteria recom-
mended by the Asphalt Institute of America' for heavy traffic as follows:
stability at 60C 3.34 kN (750 lbf) (mmn)
flow 2.0-4.1 mm (8-16 x 0.01 in)
7. In addition, because of the anticipated heavy axle loads, the consulting
engineers selected a tyre pressure of 689.4 kN/in 2~100 lbf/in 2)for design purposes;
using the recommendations of Jackson and Brien the required stability condition
is given by:
279 HARRIS, HITCH AND JOWETT
LIJ 2 x tyre pressure (lbf/in 2
Marshall flow (0.01 in)
or = Marshall stability (kN)> 0.003 x tyre pressure (kN/M 2)
Marshall flow (MM)
8. The annual rainfall in the project area ranges from 500 to 750 mm and
occurs over a limited wet season of 3 to 4 months (June to September). It was
decided therefore that the design requirements for water absorption should be
strictly controlled in order to ensure that the stabilized base would not be
unduly water sensitive. Accordingly a design specification for water absorption
of 0-2 % maximum was adopted.
Sub grade materials
9. The Chad basin is the largest basin of inland drainage in Africa
occupying an area of approximately 1.68 million km 2in west and central Africa;
Fig. 1 shows the extent and the geology of the Chad basin in northern Nigeria.
10. The whole of the DGG road project area lies within the Chad basin
where the principal soil type consists of the Chad formation sands. These are
remarkably uniform fine sands, non-plastic and with 100 % passing the 2.36 mm
(no. 7) sieve. Typical grading curves are shown in Fig. 2.
11. Figure 3 is an electron-photomicrograph of the 600-210 mim fraction of
a sample of aeolian sand from the Damaturu-Baiomori alignment while Fig. 4
shows the 210-75 Mim fraction. Subgrade soils were found to be quite uniform over
the project area and the sample examined can thus be considered as having
properties similar to those nearer to Baiomori. The finer fraction is evidently
more angular than the coarser one and Harris 8 has drawn attention to the
BS Test sieves
QC) JQ0LO C) 00 U U) -oc'j C- C : :9otC:COaDro~C:'m
100 ii ii ii IA till 90 ~~~~~~km 16~,,
0 - 0' 50 // ~~~~~~~~~~~Trials
0 I 1 1 11 1111 illl 1111 111 0.001 0-01 0.1 1 10 100
Particle size: mm
Fig. 2. Particle size distribution; 3 sands, Damaturu-Gashua-Geidam project
4 HARRIS, HITCH AND JOWETT
theory that the finer angular particles may, in fact, not be derived from the Chad
formation but may be dust from the Sahara, transported by the Harmattan winds.
This theory is supported by a study of Harmattan dust which was done by the
US Bureau of Reclamation as part of their study of the Chad basin.' It has been
suggested that these angular particles may be responsible for the excellent load-
bearing properties of this soil: when compacted to a density of 100% BS
standard compaction 10 at the optimum moisture content, these sands resemble
soft sandstones. CBR values of 40O% are common. Harris 8 notes that on drying
out strengths improve even further and CBR values in excess of 200 % have
been recorded although the material reverts to the loose state under traffic, due
to lack of cohesive materials.
12. Bitumen grades used variously at different stages of the laboratory and
later full-scale work were:
penetration grade 80/100
cut-back grades MC 1
RC 2, 3 and 4
The cut-back grades, which may be unfamiliar, form a part of the older cut-back
classification system of the Asphalt Institute of America.' 11.2 In 1957 a system
Fig. 3. Darnaturu-Baiomori road; electron-photornicro graph of sand fraction
282 BITUMINOUS STABILIZATION OF FINE SANDS
based on kinematic viscosity (centistokes) at 60C was introduced; the older
system however is still retained in some areas.
13. Preliminary materials testing was done at the Consultants' Central
Testing Laboratory in Kano. In addition to the usual Marshall stability/flow
determinations, further Marshall specimens were examined for water absorption
by immersion in a water bath at ambient temperature for 24 h. Marshall specimens
were cured as shown in Table 2.
14. The investigation occupied a period of 2 years and involved preparation
of more than 50 000 samples of stabilized material. Harris has described the
Table 2. Curing of Marshall specimens
Binder Curing condition
80/100 pen. 24 h in laboratory
Cut-back bitumen 3, 12 and 30 days under
simulated field conditions
Fig. 4. Damaturu-Baiomori road; electron-photomicrograph of sand firaction
283 HARRIS, HITCH AND JOWETT
Table 3. Summary of laboratory research programme
Programme. Objectives Main conclusions
1 (a) To stabilize representative 1. Only a small proportion of the
samples of subgrade sand with samples achieved specified
80/100 pen. bitumen Marshall stability
(b) To study the effect of adding 2. Coarse sand improved density
coarse sand to mixtures but not stability (coarse sand
prepared in (a) above particles rounded)
2 (a) To examine the effect of jiglin 3. Fillers improved stability;
and silt fillers in the dry/mix jiglin more effective than silt
process* to standard subgrade
sandt using 80/100 pen., RC2
and MC1 binders
(b) To examine the effect of adding 4. Stabilities greatly improved by
fillers as a slurry to the sand adding fillers as a slurry
In the wet/dry mixing process++
before adding binder
3 (a) To examine the effect of clay 5. Clay filler gave higher stabilities
filler wet/dry mixed with 80/100 than jiglin and silt fillers
pen. and RC2
(b) To examine the effect of mixing 6. Mixes gave stabilities sub-
fillers and sand in the wet/mix stantially higher than the
process§ using RC2 binder specification requirements
(c) To examine the effect of sand 7. Mixes demonstrated that
stabilized with RC2 in the wet/ specified stability could be
mix process achieved and exceeded
4 (a) To examine the effect of 8. Stability of stabilized material
different compactive efforts not very dependent on
(Marshall test) compactive effort
(b) Comparison of CBR values on 9. CBR test may be of some value,
stabilized and unstabilized but more research is required
(c) Determination of optimum 10. Optimum fluids content reflects
fluids content 11 for sand- optimum moisture content of
water-bitumen mixes with a sand-water system
RC2, RC3, and RC4 binders
*In the dry/mix process both the sand and the filler were dried and, if necessary, the
filler crushed to pass a 2.36 mm (no. 7) sieve before mixing with binder
t From test programme no. 2 a subgrade sand from Damaturu was used for all tests as
typical of project
+ The wet/dry process consisted of mixing a filler slurry with sand then drying before use.
Where necessary the dried mixture was crushed to pass a 2.36 mm (no. 7) sieve before
mixing with binder
§ The wet/mix process consisted of mixing binder with a wet sand/filler mixture or wet
The optimum fluids content is the sum of the binder/water content at maximum dry
density for a stabilized mixture
284 BITUMINOUS STABILIZATION OF FINE SANDS
35 - ~~10% 30 -15% 15-
30 -25 -6.7% water
z 7% ZZ
i25 -2 j
20 -~ ~ ;,.0
20 - )U)
10 - 5 - ~~~~~~~~~0%water -
5 0 0 A
1 23 4 56 78 2 34 56 7 89 2 34 56 78 9
Bitumen: per cent Bitumen: per cent Bitumen: per cent
(a) (b) (c)
Fig. 5. Stabilization of sub grade sand w ith RC2 cutback bitumen: effect on Marshall
stability of (a) clay filler, (b) silt filler and (c) water (percentage weights as shown)
investigation in detail;' Table 3 summarizes the course of the investigation and
the main conclusions reached.
15. The most significant point in the research was undoubtedly reached in
programme 3 where it was found that the required stability could be obtained
using sand/RC2 binder in a wet mix process. Fig. 5 shows the effect on Marshall
stability produced by different bitumen contents of a subgrade sand containing
0, 5 and 6-7 % water compared with the addition of clay or silt.
16. The laboratory investigation showed that the required Marshall quotient
would be satisfied by a mix containing, for example, 5 % RC2 plus 5 % water.
Preproduction tr ials
17. Based on the foregoing study, the specification shown. in Table 4 was
adopted for preproduction trials.
18. It was considered that, of the grading characteristics, only the percentage
passing 75 pim (no. 200) sieve required monitoring; failure to meet the minimum
percentage specified would have an adverse effect on stability, while materials
exceeding the maximum value would require an increased working bitumen
content which might result in slow curing. The voids in mix limits specified may
appear to be unusually high. It was, in fact, found during the laboratory
investigation that voids could not be reduced below 10 % by volume because
of the fine nature of the sands. The Asphalt Institute has suggested criteria'`
for hot-mix sand-asphalt base which demand a voids range of 3-18 % but
concede that values higher than 18 % may be permitted provided that other
criteria are met. Other suggested values are:
285 HARRIS,.HITCH AND JOWETT
Marshall stability at 60C 0.89 kN (200 lbf) (min)
flow 5.0 mm (20 x 0.01 in) (max)
These values would give a stiffness of 10 (lbf/0.01 in) minimum. In contrast, the
consulting engineers' mix specification would give mixes with a minimum
stiffness of 66.
19. Preproduction trials were done in November 1973 and consisted of six
sections of sand-bitumen base constructed at km 7 on the Damaturu-Baiomori
road. Trials were done in order to confirm that wet/mix base materials, like
those obtained in the laboratory, could also be mixed successfully in commercial
Table 4. Specifications for pre production trials
Percentage passing 75 pm (no. 200 sieve)
CBR value at maximum dry density
(BS compaction) (soaked for 24 h)
10-40% by weight
10 %1 (min)
RC2, RC3, RC4
Stabilized material (30 days curing)
Marshall test properties (50 blows)
Stability at 60'C
Voids in mix
Water absorption after 24 h soaking
4.448 kN (1000 lbf) (mim)
2.0-3.8 mm (8-15 x 0.01 in)
10-20 % vol.
0-2 % (max)
Table 5. Sub grade sand used for trials
Grading: weight passing 3.35 mm (k in) 100
BS sieve: % 2.36 mm (no. 7) 100
1.18 mm (no. 14) 98
600pMm (no. 25) 93
425 pm (no. 36) 89
300 pm (no. 52) 85
150 pm (no. 100) 34
75 pm (no. 200) 23
Liquid limit Non-plastic
Plasticity index Nil
Linear shrinkage Nil
CBR at BS standard compaction (optimum moisture
Maximum dry density 1.906 Mg/in 3 (1 19 lb/ft 3)
286 BITUMINOUS STABILIZATION OF FINE SANDS
quantities using a conventional mixing plant and also to compare the performance
of mixes made with RC3 and RC4 cutbacks with those made with RC2.
Sub grade sand
20. The properties of subgrade sand used for the trials are shown in Table 5.
21. A Marini 1000 kg batch mixer was used; a water spray bar was mounted
over the pugmill for the purpose of adding any additional water to the local
subgrade sand which contained 2-3 % moisture when fed to the plant. During
the trials the spray-bar equipment was incapable of adding water in finely
controlled increments, but for the later full-scale production the supply system
was improved to permit increments of 0.5 % by weight.
The mixing cycle
22. The pugmill was charged with sand, any additional water was sprayed in
and then it was mixed for 15 s before the cut-back bitumen was sprayed in (10 s)
after which mixing continued for a further 35 s. The total cycle time, including
charging and discharge of the pugmill, was 70 s, but for subsequent full-scale
production the reduced water content enabled this to be shortened to 45 s.
Total fluids con tent
23. Laboratory tests had indicated that the total fluids content (water plus
bitumen) required would be 11-12 % for a mix containing 6 % by weight of
bitumen. At this level the mixture was unstable when a 35 t Albaret pneumatic-
tyred roller was used for compaction. The natural moisture content of 2-3 % was
eventually found to be sufficient for compaction purposes thus giving a total
fluids content of 8-9 % by weight under field conditions.
Spreading and compaction
24. A thickness of 215-230 mm (8.5-9 in) of loose bitumen-stabilized material
was spread between sand-cement haunches by a Blaw Knox paver in 2 x 2.8 m
(9 ft 3 in) strips and compacted to provide a base of 150 mm (6 in) thickness.
A smooth-wheeled roller was found to be quite unable to compact this material
at any stage. The lead roll displaced the material ahead of the roll in the form
of a wave which broke at intervals and formed an unbonded layer over the rest
of the base material. This roller also created tension cracks in the base at 50 mm
centres for the full width of the roll. An Albaret 35 t pneumatic-tyred roller was
successful, however, and after 4-6 passes the surface was reasonably smooth. An
attempt to finish the surface with the smooth-wheeled roller was unsuccessful
because tension cracks were formed once again. This machine was discarded in
later work. Some 10-12 passes of the Albaret roller were needed to compact the
25. The finished trial strips were primed with MC70 at approximately 0.54
kg/in' (0.10 gal/yd') and later surface-dressed as follows.
first dressing (7 days after priming): 80-100 pen at 0.88 kg/rn 2 (0.16 gal/yd')
+20 mm (Q in) basalt chippings at 17.5 k g/in 2 (70 yd 2/t)
287 HARRIS, HITCH AND JOWETT
second dressing (2 days after first dressing): 80-100 pen at 0.97 kg/rn' (0. 18 gal/yd')
± 10 mm (Q in) basalt chippings at 11 k g/rn 2 (110 yd 2/t)
The prime coat, which had been applied in order to correct an apparent drying-out
of the surface, was later considered to have been a mistake since it had softened
the top 6 mm (' in) of the base which was then marked by the wheels of the
Laboratory investigations during the trials
26. Marshall and CBR specimens were compacted during the trials using
samples taken at the Marini mixer. Harris has reported the results of this
investigation elsewhere.' Two aspects emerged.
(a) The material which were produced easily satisfied the design specifica-
(b) In general Marshall stability and compacted density increased with
increasing total fluids content. However, because of the greater efficiency
of site compaction equipment, resultant field densities and corres-
ponding stabilities would, in fact, be higher still.
27. It was also found to be possible to trace the increase in strength as curing
proceeded by the CBR test. Unsoaked CBR values were not lower than 25 after
5 days and were in the range 60-1 10 approximately after 30 days.
28. The field trials had shown that a satisfactory base material could be
obtained with a mix containing 4-5-5 00 RC4 binder with 2-3 % water. It had
been the intention to specify RC4 bitumen for the main contract, but
unfortunately the oil companies could not supply the bitumen in the required
quantities and only RC2 was available. Accordingly the production mix was
selected having a binder content of 5.5 Y% RC2 bitumen to maintain the same
residual bitumen content as the RC4 mix. Field laboratory tests on materials
from a selected borrow pit established that the required stabilities could be
achieved with 5.5 % RC2 and 5 % water and 30 days curing. In view of the results
obtained in the field trials this laboratory optimum fluids content of 10.5 % was
reduced to 8.5%/ for the site compaction equipment, i.e. total water was to be
3 % by weight.
29. Other conclusions were as follows.
(a) A 3 mm (Q in) screen would be introduced between the feed conveyor
and the mixing drum in addition to the 6 mm screens in order to
remove sand concretions.
(b) A water spray bar fitted with finer jets would be installed over the
pugmill of the Marini mixer, and the speed adjustment of the water
pump would be modified such that water would be added to the mix
in increments of 0.5 % by weight.
(c) The main drive motor to the Marini mixer would be replaced by a
heavier duty motor because of the nature of the mix (the actual weight
of each batch was 650 kg).
(d) The sand moisture content at the borrow pit would be checked frequently
and the added water adjusted so as to maintain the optimum fluid
content at the 8.5% level.
288 BITUMINOUS STABILIZATION OF FINE SANDS
(e) A 19 t Albaret smooth-tyred roller would be provided in addition to
the 35 t machine.
f)The prime coat previously specified would be omitted in full-scale
Sand-bitumen construction on Baiomori-Gashua road
30. Construction of sand-bitumen base started in October 1974 and was
completed by July 1975. Fig. 6 shows a cross-section of the road. The specification
provided for the following construction:
double surface dressing second dressing: 10 mm (3 in)
(80/100 pen. bitumen) first dressing: 20 mm (3 in)
base: RC2 stabilized sand 150 mm (6 in)
sub-base: selected local sand 100 mm (4 in)
subgrade: compacted local sand
3 1. Sand-cement was manufactured in a pan mixer to the following composi-
subgrade sand 450 kg
cement 50 kg (1 1% by weight on sand)
water 35 kg (7 % by weight on dry mix)
Because of the relatively dry nature of the mix the hydraulic kerb extruder,
designed for working with concrete, was unable to produce kerb at the required
output: the contractor therefore resorted to the use of steel forms and manual
compaction using steel punners.
Mix design in field laboratory
32. The contractors' materials engineer had previously done a laboratory
investigation using the sand intended for use, i.e. material at chainage 135 000.
Mixes were prepared at RC2 contents between 4.5-6.5 % at 0.5 % increments and
water contents between 2-7 % (1 % increments). Specimens compacted by the
Marshall apparatus were cured for 3, 12 and 30 days in 'the laboratory at
ambient temperature; they were then tested at 60C for stability and flow. The
30 days stability values showed that mixes containing 5 % water gave optimum
Surface dressing (two coats)
Sidedrain Sadcmnt haunch (250 x 250 mm)
Fig. 6. Typical cross-section Baionmori-Gashua road
289 HARRIS, HITCH AND JOWETT
stability values at 5-5.5%(, RC2 level. The effect of water, which is discussed later
in the Paper, is not entirely clear. However the field trials had demonstrated
that the apparent optimum fluids content needed to be reduced to 8.500 for the
site compaction equipment to be used. This had the effect of retaining the
bitumen content at 5.5 %/ RC2 but reducing the total water to 3 00 by weight.
Mix production and laying
33. The Marini mixing plant (Fig. 7) operated initially from a site at km 41
from Gashua (chainage 135 000) and was moved subsequently to km 16 (chainage
53 000). The borrow areas were immediately adjacent to these sites. Table 6 shows
the gradings of the subgrade sands used.
34. The critical factor affecting stability appeared to be the percentage passing
715 pim sieve. Although the material at km 16 had only 500 more passing this
sieve than that at km 41 the average Marshall stability (30 days curing) increased
from 2.20 kN (494 lb) to 5.72 kN (1286 lb).
35. The first 300 m west from Baiomnori junction was used as a second full-
scale field trial to determine the effectiveness of the plant modifications found
to be necessary during the preproduction trials. The Marini plant was operating
satisfactorily but it was found that if the sand as dug contained more than 1 00 of
moisture the screens clogged and the plant production was considerably reduced.
The stockpiles were bulldozed and graded in an attempt to dry out the sand.
36. Production was slow initially and during the first month only 2.4 km
(i.e. 100 m per wor king day) of base was laid. The contractor made an intensive
effort to complete the contract on time and by operating the plant up to 18 h
a day, 6 days a week, the production was increased to a maximum daily output
of 1350 mixes (860 t). This was equivalent to 470 m of completed road base.
The monthly production was increased to 12 750 t, or 7 km of base. A major
factor in increasing mix production was the ability to reduce the mixing cycle
from the 60 s per 650 kg batch to 45 seconds, without loss of mixing efficiency:
an additional factor was the ability to stockpile the mixed material during
Sundays (when worked) and also during the hours of darkness. Tests carried out
on the stockpiled material showed that there had been no deterioration due to
stockpiling. A thin crust which formed on the stockpile restricted the loss of mix
Table 6. Gradings of materials used in main contract
BS sieve Percentage passing sieve
Borrow pit Borrow pit
(chainage 53 000) (chainage 135000)
kml16 km 41
2.36 mm (no. 7) 100 100
1.18 mm (no. 14) 100 100
600jurm (no. 25) 98 94
425prm (no. 36) 97 91
300jpm (no. 52) 94 87
150 pm (no. 100) 56 60
75 pm (no. 200) 20 15
290 BITUMINOUS STABILIZATION OF FINE SANDS
volatiles and water prior to spreading: this crust dispersed readily in the bulk
of the mix during loading and laying. This ability to stockpile mixed materials
is a distinct advantage of the cold-mix process over the hot-mix process. A
second very obvious advantage was that of being able to roll immediately after
laying: in contrast, on contracts in that part of Nigeria which had used hot-mix
techniques it had been found possible to roll only at night due to the high
ambient day temperatures. During the construction of the work described from
Gashua to Baiomori the daily maximum ambient temperature averaged between
43 0C and 46 0C for several months. The temperature of the base at 50 mm below
the surface was recorded at 60C for much of this period and at 25 mm below
the surface temperatures of 69C were recorded.
37. The Blaw Knox paver had been used to lay the stabilized base material
during the preproduction trials. This machine, however, failed to operate
satisfactorily when construction of the main contract began: the driven wheels
of the paver dug into the sub-base, resulting in serious contamination of the base
material. Various attempts were made to solve this problem, including priming
the sub-base and laying base material in 2 x 100 mm layers but without success.
It was decided therefore to abandon the use of the paver and to spread using a
blade grader for the remainder of the contract (Fig. 8). The use of a grader
proved to have several advantages.
(a) The rate of spread of the stabilized material was much faster using the
grader and the material could be placed over the full construction
Fig. 7. Mixing plant, Baiomori-Gashua road
291 HARRIS, HITCH AND JOWETT
Fig. 8. Spreading sand-bitumen by grader
~~~~ - - - 0 -~ A
Fig. 9. 19 t pneumatic-tyred roller compacting sand-bitumnen
292 BITUMINOUS STABILIZATION OF FINE SANDS
Fig. 10. Finished surface of sand-bitumen base
wdhin one operatio coprd with the two half-widths required
by the paver.
(b) Lorries reversing up to the paver had caused damage to the sand-cement
kerbs. This damage could now be avoided using the grader since the
stabilized material would be deposited in the centre of the road thus
eliminating the problem.
(c) The grader wheels provided some initial compaction to the base material
which had the effect that the main compaction of the base could
proceed immediately after laying.
38. Use of the two pneumatic-tyred rollers proved to be amost effective method
for compacting the stabilized base. The initial compaction was carried out with
5-6 passes of the 19 t Albaret (Figs 9 and 10) and final compaction was
obtained with a further 4-5 passes of the 35 t machine. Some back-rolling with
the 35 t machine was carried out during early work to establish whether or not
the compacted density of the base was likely to increase to any great extent
under traffic. However, the results of repeated sand replacement in situ density
tests indicated that little improvement was likely to occur once the initial
compaction had been achieved.
39. While there was, generally, little difficulty in compacting the base to the
specified minimum density of 95 %, of the Marshall test density, there were some
areas where this level was barely attained. In order to avoid crazing and cracking
due to over-rolling, these sections were not overcompacted. It was considered
that the reduced road density would be preferable to the risk of extensive crazing
which had occurred on one section (km 21-22). Although it was not possible to
293 HARRIS, HITCH AND JOWETT
cut cores,. laboratory samples were made up to the densities measured in the road
and when tested these samples gave the specified minimum stability. The
minimum acceptable density was therefore reduced to 95 % BS heavy compaction.
40. There were some edge flow failures against the kerbs between km 36
and km 37. These were not continuous and were attributable to the intense
heat; the failures were cut out and replaced with fresh material. There were a
few small areas of heaving and two lengths of failure. One length was the
extensive crazed and cracked section already described and this was repaired by
scarifying, mixing in new material and recompacting. The second length was
the first 300 m from Baiomori which exhibited rutting and heaving. This was
also scarified, more material was added and then it was recompacted. All
repairs were carried out satisfactorily during the construction period. Test cross-
sections were established at various chainages and studied for any signs of rutting
or deformation in the wheel tracks; no such rutting was observed.
1 22.6 lb/ ft3(1.97 Mg/rn 3)
.132.1 ib/ ft3(2.1 2 Mg/rn 3)
1 13.0 Ib/ ft3(1.81 Mg/rn 3)
3.4 lb/ ft3(o054 mg/rn 3)
1 286 lb (583 kg)
2458 lb (1 115 kg)
607 lb (275 kg)
324 lb (1 47 kg)
In situ dry density: lb/ft 3
Mean = 2.55%
Max = 4.87%
Min = 0.69%
SD = 0.79%
-1I I 11
I f 1 0-
0) 0) 0') m'
m m- 'IT `) C' C') ~ D L o mo 0t 1
Mean -= 5.30%
Max = 6.76%
Moisture content: % Bitumen content: %
Fig. 11. Histograms of field laboratory control tests (km 0-km 27): (a) statistical
analysis of in situ density tests; (b) statistical analysis of Marshall stability (30 days
curing); (c) statistical analysis of moisture content of sand-bitumen mixes; (d)
statistical analysis of bitumen content (including cutter) of sand-bitumen mixes
---1 BITUMINOUS STABILIZATION OF FINE SANDS
41. In order to allow the curing of the base to proceed as fast as possible,
the first coat surface dressing was deferred for 2 weeks after compaction of the
base. After the first coat was applied it was trafficked for as long as possible so
that any rutting that appeared could be filled with 10 mm chipping before
application of a second coat. Very little rutting however occurred.
42. Laboratory staff made regular measurements of moisture content on sand
in the stockpile and in the borrow pit area generally, using the 'Speedy' instru-
ment. Additional water was added at the pugmill as required so as to maintain
a moisture content of 3 % by weight; the working RC2 content of 5. plus
the 3 y0water gave a total fluids content of some 8.5 0 by weight. The bitumen
and water content of the mixed material was determined at least once a day; in
addition, sufficient compacted Marshall specimens (2 x 50 blows) were prepared
twice daily so that density, stability and flow could be determined after curing
for 6, 12 and 30 days.
43. During the preliminary laboratory investigation Marshall specimens
were cured by embedding them in sand outside the laboratory in order to
simulate as far as possible curing conditions in the road after construction. This
procedure was retained therefore during construction.
44. The total numbers of tests done during construction- were:
sand grading 182
in situ density 2620
Marshall stability 5031
water absorption 320
moisture and bitumen content 733
Figure 1 1 shows histograms of some of the test data obtained.
The effect of water content on stability
45. In situ densities consistently averaged 2.0-2.02 Mg/in 3 (125 126 lb/fl 3)
(groups of 10) compared with an average value for laboratory-compacted
Marsh all specimens of 1P92 Mg/in 3 (120 lb/ft'). The moisture content used during
production was 300 instead of the laboratory moisture content of 5 % production
mixes made at this level were too wet for efficient compaction. However, since
it was consi'dered that water, other than that required to ensure good mixing
and compaction, might affect stability, samples of mixed materials were taken on
two different occasions and Marshall specimens were prepared both on material
as sampled, and on material into which additional water had been mixed in the
laboratory. Table 7 shows the results obtained on these specimens.
46. The effect of the additional water is illustrated in Fig. 12 which shows
the results obtained from mixes produced in the preproduction trials. In the case
of the RC2 mixes, the apparent benefit of increasing moisture content from 2 to
500 is particularly marked. The reasons for this improvement are not known
although Alexander & Blott' 4 observed the phenomenon during their investiga-
tions into factors affecting the structural stability of sand carpets. In an attempt
to explain the effect of water they cited the behaviour of some dispersions of
solid particles in a liquid medium which, on being allowed to rest after agitation,
295 2I 1q
0)m'~ m Cl
C) I 7
0> Cl1-4'Cl4 00
CA 0 t
04_I C1'ii9i - -~~c
_V4 i)ŽI ~ -4 l) ~
-C] !LB00 tj
-I a 4 4- --- tt-
41) o' Q-. E
Cl i -n 0
r. 0 0 r
11 BITUMINOUS STABILIZATION OF FINE SANDS
0 1 2 3
Fig. 12. Effect of moisture
(b) RC3; (c) RC4
5% RC3/2% water -.
5.8% RC3/2% water
0 1 2 L 3
4% RC4/2% water
Curing: months Curing: months
content on mix stability for 3 cutback binders: (a) RC2;
Table 8. Examination of sand-bitumen base samples after 5 years in service
Origin Binder, 0% Properties of recovered binder
Pen at 250C Softening point, '0C Penetration
(ring and ball) index
km 8 3.6 460 approx. -
kml18 4.4 60 57 + 1
km 45 5.1 72 49.5 0
km 50 3.7 27 65.5 + 1
RC3 trial, km 42 3.7 43 63 + 1
RC4 trial, km 42 3.5 28 72.5 + 2
formed gels. They stressed the differences in the relative magnitudes of viscosity
of water, cut-back bitumens and the harder penetration grades of bitumen, and
suggested that a gel structure would form more quickly when filler particles are
allowed to settle in a low viscosity medium than in one of higher viscosity. They
further suggested that 'The effect of the initial water content in the wet mixes
is maybe to bring the filler particles rapidly into a suitable state of aggregation
in which they can more readily form a structure within the oil binder.' These
suggestions were regarded by Alexander & Blott as highly speculative but
the effect of water is undoubtedly useful irrespective of the mechanism.
Hardening of binder in service
47. In November 1980 samples of sand-bitumen base were taken from the
road and sent to the Overseas Unit TRRL for testing as follows:
i HARRIS, HITCH AND JOWETT
(a) binder content (BS 598` : Part 2 : Section 4.6)
(b) recovery of soluble bitumen (BS 598` : Part 2: Section 6)
Dichloromethane was used as solvent. The results are given in Table 8.
48. Since the base construction occupied only a period of 10 months it would
seem unreasonable to try to account for the differences in the apparent
hardening of the binder recovered from materials which represent this time
separation, particularly in the context of the first 5 years of service. The
consistency of the binder recovered from km 8 must therefore remain unexplained;
otherwise the RC2 binder can be said to have hardened to a penetration range
Performance of completed road
49. The 55 km of road were inspected in June 1979, 4 years after completion.
The road base and surface dressing were in good condition and had required
very little, if any, repair or maintenance; only one pothole was seen. The amount
of rutting was negligible except for the westbound lane between km 37 and 40;
the sand-cement kerbs appeared sound although in some instances the surface
dressing had stripped from the top surface.
50. Although the testing during construction had shown that the km 0-27
base had a higher stability than the km 27-55 material there was no evidence of
this strength difference on the road. During the construction of the main contract
-two additional test sections incorporating RC3 and RC4 binders were con-
structed at km 42: both of these were also in good condition at the time of
51. At the design it was considered that during the early years after construc-
tion the DGG roads would normally:
(a) carry traffic volumes of 15-200 vpd
(b) be subjected to some axle loads in the 15-20 t range.
52. The consulting engineers made a traffic and axle-load survey near to
Gashua in April 1980 (after nearly 5 years in service). During the 7 day survey
(12 h recording daily) the average daily traffic (one direction) was 340 vpd. The
maximum multi-axle load recorded was 76 t; 7 other vehicles exceeded 50 t
laden, the maximum axle load recorded being 18.8 t.
53. Axle load data were analysed using the procedure described in Road
Note 40 16 and can be summarized as follows:
Daily traffic flow (1 direction) of 1 .2 type vehicles* (or larger) = 100
Mean equivalence factors:t to Damaturu 2.54
to Gashua 1.74
Average equivalent standard axles (esa) (daily) = 100 x 2.54 =254 esa
Annual axle loading (12 h day basis) = 254 x 365 =92715 esa
Traffic and axle-load predictions would thus appear to have been confirmed.
* 2 axles; single tyres front, twin tyres rear.
t Damnaturu figure taken.
298 BITUMINOUS STABILIZATION OF FINE SANDS
54. The Baiomori-Gashua road was one of the first roads to be designed for
heavy axle loads which incorporated a bitumen-stabilized base made with the
very fine aeolian sands of the Chad formation. The fact that it proved possible
to achieve Marshall stabilities in excess of 4.448 kN (1000 lbf) with these sands,
using RC2 binder and the wet/mix process demonstrates that it is possible to
use this form of construction for trunk roads.
55. The wet/mix method of mixing stabilized base material in commercial
quantities proved to be relatively simple provided that good quality control
procedures were maintained at the mixing plant. The stabilized base material
had very forgiving characteristics, demonstrated by the fact that it could be stock-
piled for periods up to 7 days prior to spreading; this would not have been
possible using a hot-mixed material.
56. The water-bitumen relationship in the wet/mix method is not fully
understood and requires research. Clearly, however, there is an obvious practical
advantage in establishing the optimum fluids content so as to achieve the required
stability while using the minimum bitumen content.
57. The condition of the road has remained satisfactory after the first 5 years
in service during which period it was used by vehicles in the number and axle-
loading anticipated at the design stage.
58. After 5 years in service the residual bitumen content of the stabilized
materials appears to be 4 + 0.5 % of weight; the binder has hardened to a
penetration range of 30-70.
59. The Authors wish to thank the Federal Ministry of Works, Lagos,
Nigeria and Ove Arup and Partners for their kind permission to present this
Paper. Thanks are due also to the Transport and Road Research Laboratory
for advice and assistance throughout this study; TRRLI also prepared electron
photomicrographs of different sand samples. Finally, the co-operation of the
contractors, Stirling Civil Engineering Nigeria Ltd, in making available their
construction records is gratefully acknowledged.
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Crowthorne, 1968, Ministry of Transport, RRL Report LR 239.
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299 HARRIS, HITCH AND JOWETT
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Institute, Maryland, 1963, Information Series no. 124.
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Institute, Maryland, 1980, Specification Series no. 2, 1980 edn.
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plant-mix types. The Asphalt Institute, Maryland, 1975, Specification Series no. 1,
Appendix B, 5th edn.
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carpets. J. Soc. Chem. Ind., 1945, 64, 89-101.
15. BRITISH STANDARDS INSTITUTION. Sampling and examination of bituminous mixtures for
roads and other paved areas. British Standards Institution, London, 1974, British
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loads in developing countries using a portable weighbridge. HMSO, London, 1978,
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