R. H. H . KIRKHAM
D.Sc., Ph .D., M.I.C.E., Head of Construction Methods Secti on,
England
Road
Resea rch
laboratory,
Ministry
of
Transport,
P. J . F. WRIGHT
B.Sc., A.M.I.C.E.,
formerly:
Road
Rese arch
laboratory,
now :
Civil
Engin eering
Research
Association,
Englan d
LEAN CONCRETE ROAD BASES *
(Paper No . 289)
Lean concrete which was first used as a base under a bituminous surfacing
about 30 years ago has been increasingly lIsed in the last 20 years, and
is now ail e of the more common base materials on heavily trafficked roads.
Generally the performance of these roads has been satisfactory, the most
common defect being the formation of transverse cracks through th e surfacing . The extent to which these cracks occur and th e amount of allY
deformation ha ve been. used to assess the performance of both experimental and non-experimental roads, and the requirements of the current
specification and current designs are discussed in relation .to this information.
INTRODUCTION
It was realized many years ago that
if concrete was used as a base for a bituminous surfacing it need not have the same
resistance to abrasion by traffic as would
be required for a concrete surfacing, and
a lower cement content could therefore be
used. This was considered an attractive
feature in the immediate post-war years
when th ere was a shortage of cement and
the use of lean concrete bases was therefore developed. Also, many of the problems associated with the construction of
joints and with obtaining good riding quality in a concrete surface, did not apply when
the material was used as a base and more
economical methods of construction could
th erefore be used. The type of concrete
used was mad e wit h well graded concreting
aggregates and had mix proportions in the
range 1: 12 to 1: 20 and it was laid using
plant similar to that used for unbound or
bituminous materials. This is the material
which forms the subject of this paper and
which is covered by Clause 806 of the Ministry of Transport's Specification for Road
and Bridge Works (Ref. 1).
1.
MEASURING THE PERFORMANCE OF LEAN
CONCRETE ROADS
2.
Lean concrete used as a road base
normally carries a bituminous surfacing and
itself lies on a sub-base consisting of a
granular material. The performance of such
roads, provided they have been made with
good quality materials, well compacted, and
having surfacings in the order of 4 in.
thick (of which the wearing course is asphalt), has generally been very satisfactory.
The most common defect is the formation
of transverse cracks through the surfacing
(Fig. 1) and the extent to which these
cracks occur has been used as one measure
of performance both on experimental roads
and on non-experimental roads which have
been examined.
3.
The frequency of cracking is expressed in terms of the equivalent number
of cracks extending over the full width of
the carriageway per 1,000 ft length, a crack
extending only part way across being
treated as a fraction of a full width crack.
The extent of this cracking on a number
of non-experimental sites included in a sur-
- ACKNOWLEDGEMENTS-This paper is contributed by permission of the Director of Road Research.
Crown copyright. Reproduced by permission of the Controller of H .M .S.O.
Volume 3, P art 2 (1966)
787
KIRKHAM AND WRIGHT -
Fig.
l -Typ ical transve rse crack
in
bit uminous surfacing
vey of roads with lean concrete bases is
shown in Fig. 2. Points joined by straight
lines represent sites where two or more
inspections have been made at different
times. It can be seen that appreciable cracking generally becomes apparent in three to
six years, and thereafter it increases with
increasing age. Within this general pattern
a considerable diversity exists in the frequency of cracking on different roads of
the same age and, although no complete
correlation has been found between the rate
of cracking and such variables as the traffic
788
LEAN CONCRETE ROAD BASES
over a
lean
concrete base
intensity, thickness of base, type and thickness of surfacing and mix proportions and
crushing strength of the lean concrete, some
indications have become apparent and these
are discussed later.
4.
Longitudinal or random cracking
(Fig. 3) or local crazing of the surfacing
has occurred less frequently than transverse
cracking and has generally been associated
with poor compaction of the lean concrete
or the use of materials not complying with
the specification. This type of failure is almost always accompanied by deformation
A.R.R.B .
PROCEEDINGS
KIRKHAM AND WRIGHT -
LEAN CONC RET E ROA D BASES
50
Numbers Indicate number
ot sIte s where this i s
so r -
more than one
[
.g
3
.
Co
1
I
1I
L
II
II
I
fI
LI
~
.c
;;
.~
30
~
V
/
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~
10
--'
/
JV
V
.---'
y.--
2
l---Z
~
3
5 ~
~
V
2
2
k0
10
Age at time of ins pe ction
Fig. 2-
2
2
(y ea rs J
Fre qu e nc y of transverse cracking obser ved on roads of various ages
of the pavement surface, especially in the
wheel tracks, and the extent of this deformation provides a further measure of performance. In non-experimental roads the
deformation ca n be gauged from a straight
edge laid transversely on the surface of the
carriageway, but on experimental roads it
is determined more precisely by observing
Volume 3, P a rt 2 (1966)
the changes in level of small studs embedded in the surfacing in rows across the
carriageway, the studs being placed at intervals of 1 ft. This technique provides infor mation of the type shown in Fig. 4,
which gives the average transverse profile
aEter six yea rs of two sections with 4 in.
oE rolled asphalt on 6 in . of lean concrete
789
KIRKHAM AND WRIGHT -
Fig. 3-
LEAN CONCRETE ROAD BASES
Failure of a road with a lean concrete base
Dist ance from nearside edge
10
8
4
6
'\
1\
V-V
/
~
0·8
v
14
( t~t )
16
20
18
-
i--
22
-
~v
Wheel trocks
of
neQ~ide
Fig . 4-Typical
790
r
~
I--
12
tane
transverse profile of carr ia geway surface
A.R.R.B.
PROCEEDI NGS
KIRKHAM AND WRIGHT -
that were includ ed in the Alconbury Hill
expe riment referred to later.
5.
Studies of the performance of roads
with lean concrete bases, using the frequency of cracking and the maximum defo rm ation in the average transverse profile
as measures of performance, are at present
in progress. These studies have already
yielded sufficient indications to enable some
limits to be imposed on the materials, mix
proportions and other details, but work is
continuing in order to establish such limits
with greater certainty.
MI X DESIGN
M ATER IALS
6.
The Ministry of Transport specification requires th at aggregates for lean concrete should comply with the British Standard for concrete aggregates, B.S. 882, and
that the grad in gs should fall within the
limits given in TABLE I. T hese limits are
considered satisfactory if th e gradi ng of the
sa nd fraction , i.e. the material smaller th an
3/ 16 in., falls within zone 2 or zone 3 of
B.S. 882, i.e. if the proportion passing
a No. 25 sieve lies within the range
35 to 79 per cent. If the sand is coarser
or finer than this , trial mixes are necessa ry
and a hi gher proportion of fine aggregate
where thi s has a coarse grading and a lower
proportion where it has a fine grading is
required. With coarser gradings th an those
TABLE I
A G G REGATES
S.S. Test
Sieve
AN D
LI M ITS
',4 in .
3/ 16 in.
No . 25
No . 100
GRADING S
Per Cent by Weight Passing.
l V, in .
no minal
ma x. size
3 in .
l V, in .
OF
100
95 - 100
50 - 80
30 - 40
8 - 30
0 -6
Volume 3, P art 2 (1966)
% in . nominal
ma x. size
100
80 - 100
35 - 45
10 - 35
0 -6
LEAN CO CRETE ROAD BASES
covered by the specification segregation is
liable to lead to honeycombed areas and
'hun gry' patches on the surface, whereas
fin er gradings make compaction more difficult and tend to produce a porous base unless the techniqu es are modified. With fin er
grading it is also more difficult to produce
a plane surface when the concrete is compacted with a roller as undulations occur
more readily.
7.
Provid ed th e aggregates meet the
requirements referred to above no limitation is imposed as regards the rock type.
There is some evid ence th at transverse
cracking is likely to develop more slowly
when a limestone aggregate is used than
with a siliceous aggregate because of the
smaller movements resulting fro m the lower
thermal expa nsion of concrete made with
limestone, but thi s effect is insuffici ent to
justify the exclusion of other aggregates .
PRO PO RT IONS
8.
The water content of lean co ncrete
is not normally specified but sin ce good
compaction of the lean concrete is essential , it must be such as to enable this condition to be ac hieved by means of the plant
ava ilable for the purpose. T he desirable
water content is generally the maximum
amount which can be used without free
mois ture appearing on the surface of the
lean concrete after compaction, or material
adhering to the roller where a roller is used
fo r compaction. There is a general tendency on the p art of contractors to use the
materi al too dry, since a very dry mix
gives less trouble in the fo rm of sticking
in the mixer drum and lorries and sticking
to the roller, and it is easier to obtain an
even surface free from waves. The essential
requirement, however, is to achieve compaction throughout the depth of the base
and this generally requires a water content
ranging from 5 1h per cent by wei ght of
the dry materi als in the case of a well
791
KIRKHAM AND WRIGHT -
LEAN CONCRETE ROAD BASES
graded rounded gravel and natural sand to
8 per cent with an angular material or with
a high proportion of very fine material.
9.
The cement is ordinary Portland
cement and the quantity used is normally
governed by strength requirements. The
cement! aggregate ratio should not be less
than 1 :20 or difficulty may be experienced
in achieving proper distribution of the
cement. Similarly it shou ld not normally
be higher th an 1: 15 since exoessively
strong mixes may result in severe transverse
cracking.
CRUSHING STRENGTH
10.
A wide range of strengths has been
used and there is still some uncertainty as
to the most advantageous strength to adopt.
In an experimental road at Whitchurch,
Glamorgan, laid in 1959, four mixes were
used giving average 28-day crushing
strengths ranging from 650 to 3,450 p.s.i.
with a gritstone aggregate; two of the
mixes were repeated with a limestone aggregate but this experiment has not yet yielded
More transverse
conclusive evidence.
cracking has appeared on the sections with
the stronger bases but few of the cracks
have developed to form serious defects.
There has been little deformation of the
surface so far but the deformations which
have occurred have tended to be greater in
the weaker bases. As the weaker bases may
be expected to deform more rapidly in due
course, it cannot yet be stated which will
have the longer life. Similarly an experimental road at Wheatley, near Oxford, laid
in 1963 with lean concretes having crushing strengths ranging from 550 to 2 ,200
p.s.i. , has not yet produced any useful evidence on this point. On the few sites where
serious failure of lean concrete bases has
occurred, however, the strength of the lean
concrete has been low and for this reason
mixes of low crushing strength are not recommended. At present the favoured
792
strength corresponds to an average of 2,000
p.s.i . at 28 days but for specification purposes it is more satisfactory to specify a
minimum strength th an an average strength.
If, however, a minimum strength alone is
stated difficulties are likely to arise when
an individual result falls just below this figure. Such a fai lure to meet the specification
does not usually justify the removal of hardened material , but to accept materi al of
doubtful quality brings the specification into disrepute and leads the way to accepting
further material of lower strength than is
specified. For this reason there is an increasing tendency to write specifications in
such a manner that some corrective action
may be taken before large quantities of lowstrength material are laid, and the current
Ministry of Transport specification aims to
achieve this object. With this aim in view
trial mixes are required before work on the
site is started and trial areas are required
as a preliminary to the main work. Also,
more frequent testing is required at the beginning of the work than later on. These
req uirements help to ensure that material
of the required characteristics will generally
be produced, but there may still be occasions when the strength falls below the specified figure as a result of unexpected changes
in the materials or faulty proportioning, and
provision mu st be made for these occurrences.
11 .
The present specification requires
th at three pairs of test specimens be mad e
for each 1,000 sq.yd or part thereof in
each day's work, one of each pair being
tested at 7 days and one at 28 days. Provided th e results are satisfactory after the
first thirty 7-day tests have been carried
out, the frequency of testing may be reduced to three pairs for each 2,000 sq. yd
of base. The results of crushing tests are
considered as successive groups of three
and the essential requirement is that not
more than one average of three in any five
A.R.R.B . . PROCEEDINGS
KIRKHAM AND WRIGHT -
consecu ti ve averages shall fall below 1,400
p.s.i. at 28 days. F urth er requirements on
the results of which remedial ac tion is taken
are :
(a) if more than one average in five consecutive averages falls below 1,000
p.s.i. at 7 days, the cement content is
increased and the frequency of testing
is increased to the initial rate until
such tim e as th e 28-day results are
fo und to be satisfactory, and
(b) if the average of five consecutive
groups of three test specimens gives a
strength Jess than 1,600 p.s.i. at 28
days or if the average range of five
consecutive gro ups is greater th an 50
per cent of the average, the materials,
mix proportions, plant or methods
must be changed.
The test specimens normally used for this
purpose are 6 in. cubes which are compacted to refusa l in three layers by means
of an electric or pneumatic hammer. The
LEAN CONCRETE ROAD BASES
cubes are cured in the same way as for
normal concrete.
CONSTRUCTION TECHNIQUES
12.
The mixing of lean concrete for road
bases is carried out in ordinary concrete
mixers. For machine-laid work an output
of at least 40 cu.yd/ hour is necessary and
thi s is frequently provided by one or more
batch mixers having a total mixmg capacity of at least 2 cu. yd. Sometinles continuous mixers are used with constant
weight feeders and these are capable of
Yielding much greater outputs. Most frequently the lean concrete is mixed on the
construction site at a central batching plant,
but ready-mixed concrete and truck mixers
have also been used.
SPREADING
13.
The lean concrete is normally transported in lorries, either end-tipping or sideti pping depending on the type of spreader
used. Most frequently bituminous type
fig . 5- Spreading lean concre te with a bituminous paver
Volume 3. P a rt 2 ( 1966)
793
KIRKHAM AND WRlGHT -
pavers have been used for spreading the
lean concrete (Fig. 5) and these are generally satisfactory although in some cases
slight modifications are necessary to enable
the machines to lay thicker layers than they
were designed to lay. Also, when using
these machin es, considerable care is required to achieve the recommended surface
tolerance of ± lh in. from true level. The
material may be spread between forms or
kerbs or between concrete edge beams
formed in situ. Alternatively the lean concrete may be spread without any forms , provided it is extended about 1 ft beyond each
side of the finished width of the carriageway. With this practice, however, it is more
difficult to obtain the correct levels. Sometimes hopper spreaders, (such as are used
for concrete carriageways) are used for
spreading lean concrete and these can
handle greater quantities of material than
LEAN CONCRETE ROAD BASES
the bituminous pavers (Fig. 6). These
machine s have the advantage that better
surface levels can be obtained and the full
width of a carriageway can be laid without
a longitudinal joint. Their use also avoids
damage to the sub-base or subgrade by the
lorries feeding a bituminous type paver but
requires that there is provision for feeding
from th e side of the carriageway. In some
cases thi s has been achieved by feeding
from a h ard shoulder, using side-tipping lorries, but an attractive alternative on dual
carriageway work is to feed the spreader
by means of a portable conveyor spa nning
the central reservation, using end-tipping
lorries on the opposite carriageway. Bulldozers and blade graders have also been
used for spreading lean concrete but good
levels are less easily obtained with these
and th eir use is not generally recommended.
Hand-spreading is common on small jobs.
Fig . 6-Spreadin g lean concrete with a hopper·type concrete spreader
794
A.R .R.B.
PROCEEDINGS
KIRKH AM AND W RIG HT CO M PACTION
14.
Compaction of lean concrete bases
is of the utmost importance since internal
stresses causing rupture of the m aterial may
result in a rapid disintegration and loss of
strength . In contrast, one effect of traffic
on a water-bou nd base or a base with a
tar or bituminous binder may be to improve the compaction and lead to an increase in strength , but no form of cemented
base can benefit in thi s mann er. Although
lean concrete can be properly comp acted
if the materi als and methods used are in
accord ance with the specifi cati on, there
have been occasions where departures fro m
the specification have led to poor compacti on and fa ilure of the carriageway.
15.
.Compacti on of lean concrete is carri ed out by heavy, smooth-wh eeled rolJ ers,
vibrating rollers or vibrating pl ate compactors. T he most common p ieces of equipment are 8-ton tand em smooth-wheeled
rollers and vibrating roll ers weighing about
1 ton. The present specification calls fo r a
vibrating roller having a dead load of not
less th an 10 cwt on the vibrating roll. With
th ese machin es satisfactory co mp action can
generally be obtained in bases up to 8 in.
thick but fo r thicker bases the material is
generally co mpacted in two layers. When
thi s is done th ere is a risk th at the two
layers will not bond together sati sfactorily
and often a weakness at the interface bas
been disclosed when cutting cores by th e
sep aration of the core into two parts. To
avoid thi s it is most important th at the
tim e inter val between compacting the two
layers should be kept to a minimum in
order to achieve a bond between the layers.
This is covered in the specification by requiring th at the upper layer shall be finished withi n two hours of mixing the concrete for the lower layer.
16.
After the lean concrete is compacted
the surface is sealed with a 55 per cent
Volume 3, Part 2 (1966)
LEAN CON CRETE ROAD BASES
bitumen emulsion at a rate not lighter th an
6 sq.yd/ gal. Bitumen emulsion is not recommended fo r curing concrete running
surfaces but is satisfactory for lean concrete bases th at are to receive a bituminous
surfacing. The b ase is normally left for at
least 7 days before it is used by constru cti on traffi c or before a surfacing is applied.
DENSITY REQUIREMENTS
17.
T he requirement in regard to the
dry density of the base is that, when measured by the sand replacement test given in
B.S.1377, it should be not less th an 95
per cent of the theoreti cal maximum dry
density, the criterion of acceptance being
based on averages of groups of three in the
same way as fo r cru sbing strengths. Initially, three density tests are made fo r each
1,000 sq. yd of base but after the firs t 30
tests have been carri ed out, and provided
the results a re satisfactory, the rate may be
reduced to three tests for each 2,000 sq.
yd . T he present sp ecifica tion requ ires that
th e tests be carried out not less than 4
hours nor more th an 24 hours after the
material is placed, but it h as been fo und
in practi ce th at if the tests are made on
the same day as th e concrete is laid there is
a tend ency for the concrete arou nd the hole
to slump slightly, thereby leading to erroneously high results fo r the density. It is
therefore preferable to ca rry ou t the test on
the day after placing, although within 24
hours as th e procedure becomes in creasingly more diffi cult as the concrete gains
strength . It is therefore recommend ed th at
they be done between 16 and 24 hours
after placing.
18..
T he theoretical dry density, with
which the measured density is comp ared, is
obtained from the formul a
1+a
m(l + a) + 0·32 + aid
where a is the dry aggregate/ cement rati o,
795
KIRKHAM AND WRIGHT -
m the moisture content as a proportion of
the weight of aggregate and cement, and d
the density of the aggregate. This density,
d, of the aggregate is the ratio of the weight
of a dried samp le to the absolute volume
of solid material present, i.e. C j (C-B)
where C is the weight of a dry sample and
B is the weight of the sample saturated and
weighed under water. If two aggregates are
used separately such as coarse and fin e
aggregate, the term a j d in the formula
above is replaced by
~
dl
+
a2
d2
where a 1 and a:! are the proportions of the
two aggregates in the mix in terms of parts
by weight of cement, and d 1 and d 2 are
their respective densities.
JOINTS
] 9.
Expansion joi nts are not normallv
included in lean concrete bases and at C0l1struction joints between successive days'
work an effort should be made to obtain
LEAN CONCRETE ROAD BASES
bond between old and new work and to
obtain good compaction close to the joint.
At the end of a day's work the lean concrete shou ld be compacted to a temporary
vertical stop end and the exposed face
hacked befo re laying further lean concrete.
The hacked face is then treated with a 1: 1
cement sand grout before placing further
material. This does not necessaril y produce
a permanent bond with the previous concrete but helps the new material to accommodate itself to th e irregular surface and
results in improved interlock between the
old and new work. An alternative method
in which the lean concrete is finished off
in the form of a ramp and cut b ack to a
vertical face before proceeding is less satisfactory because of the difficulty of producing a vertical face . In practice an inclined
joint is almost always obtained and this
should not be permitted either during a
day's work or at the end of a day as the
weak plane facilitates the occurrence of a
compression failure of the type shown in
Fig. 7 and 8. Where such compression
Fig. 7-A compress ion failure in a lean concrete base
796
A.R .R.B.
PROCE EDINGS
KIRKHAM AND WRIGHT -
LEAN CON CRETE ROAD BASES
Fi g. 8- Side view of a comp re ssion fail ure in a lean concrete ba se
fai lures do occur they can be repaired by
removing the damaged materia l and replacin g it with a mix of sim ilar strength but
more workable in order to faci litate compaction in a confined area .
PAVEMENT DESIGN
20.
In designing th e thickness of a lean
concrete base neither the Californi a Bearing Ratio method for flexible construction
nor the empirical meth od used for concrete
carriageways is strictly appli cable but no
separate method has been developed for
lean concrete. Most frequently , however,
lean concrete is considered as an altern ative
to other forms of gr anul ar base and in a
full-scale road experiment (R ef. 2) constru cted at Alconbury Hill in 1957, lean
concrete bases made with a gravel aggregate and a natural sand were compared
with correspondin g sections laid with
cem ent-stabilized sand , pre-mi xed waterboun d macadam (,wet-mix') made with
slag and rolled asphalt. Using each of the
Volum e 3, P art 2 ( 1966)
fo ur materials, bases of 3, 6, and 9 in.
thickness were laid with a 4 in. rolled asphalt surfacing (except that the 9 in. rolled
asph alt base was o mitted ) . The 6 in. thick
bases were repeated with 11/2, 23,4, and 4
in . roll ed asphalt surfacings (except that
the section with a 4 in. asphalt surfacing
on a 6 in. asph alt base was not repeated)
and one section of 6 in . base with each
material except rolled asphalt was laid with
a 4 in. bitumen macadam surface. The deformation of the surface of the carriageway
has been measured periodically and the relative performance may be examined in
terms of th e maximum deform ation which
has occurred in the nearside wheel track .
The deformations observed after six years
of traffic are shown di agrammatically in
Fig. 9. A number of th e secti ons , including the 6 in . thick lean concrete bases with
1 Y2 and 2 ~ in. asphalt surfacings, have
needed extensive repa ir and the deformations were greater than 11h in. and are not
shown .
797
KIRKHAM AND WRIGHT -
Surfacing
L.in
•
l.in
0
"in
Bitumen
macadam
..
Asphalt
.
9 in
6in
23A, in
lY2in
..
_ _ _ Asphalt
3 in
Base
LEAN CONCRETE ROAD BASES
.
6in
..
III
.c
o
c
,
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...."
III
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Three
sandcomont
sections
,I
I
I
t
t
Soctians
failod
h
and reconstructod
Base materi als
Asphalt
(r«~';~;l Loan concrete
I~,*:I Water bound macadam
W~:';~l Sand-coment
Fig . 9-Deformation s o b served on
21 .
In the group of sections in which
the thickness of base was varied, the 3 in.
and 6 in. b ases of sand cement and waterbound macadam have both failed, and of
798
the Alconbury Hill ex perim e nt
th ese the sand cement fai led first. In contrast the rolled asphalt bases are both still
in good condition. Between these extremes
lie th e lean concrete bases.
A.R.R.B.
PROCEEDINGS
KIRKHAM A D WRIGHT -
22.
In the group of sections in which
the surfacing was varied, the asphalt bases
again gave the best performance and the
sand-cement the poorest. Between these lie
the lean concrete and the water-bound macadam. The lean concrete bases with surfacings 11/ 2 in. and 2* in. thick have
failed and it would thus seem inadvisable
to use thin surfacings on lean concrete
bases. The 4 in. bitumen macadam surfacing, however, has performed almost as well
as the adjacent 4 in . asphalt surfacing and
this suggests that the need for a substantial
surfacing is not so much because of its load
spread ing as for some other property, probably the thermal insulation which it provides. With other base materials, which
benefit more from a surfacing with good
load-spreading properties, the bitumenmacadam surfacing has a much poorer performance than the asph alt.
It may be deduced from these re23 .
sults that lean concrete sho uld not be used
in thin layers (i.e. less than 6 in. thick) or
with thin surfacings (i.e. less than 4 in.
thick) , but that the surfacing need not be
of a ph alt throughout (though a wearingcourse of dense asphalt is recommended in
order to minimize the ingress of water).
The conclusions concerning thickness are
supported by evidence from surveys of non experimental roads which have shown that
defects other than transverse cracking occur
in a hi gher proportion of roads with 6 in.
thick lean concrete bases than in roads with
TABLE II
RECOMMENDED SURFACING AND BASE THICKNESSES
Traffic Intensity
Commercial Vehicles
per Day
Thicknesses of
Surfacing
(i n.)
Thickness
of Base
(i n.)
Up to 450
450 - 1,500
1,500 - 4,500
More tha n 4,500
2
3
6
B
8
10
Volume 3, P art 2 (1966)
-4
4
LEAN CO CRETE ROAD BASES
thicker bases and that the proportion of
roads containing such defects increases as
the combined thickness of the base and surfaci ng is reduced . These indications need
further confirmation however, and until additional information is available it seems
reasonab le to use similar designs for roads
with lean concrete bases as for water-bound
macadam , although waterbound macadam
bases require the use of a crushed rock or
crushed slag and are not permitted on
motorways.
24.
A seri es of charts has therefore been
prepared giving the recommended thicknesses of surfacing and base for different
traffic intensities, and these are published
in Road Note No. 29 (Ref. 3). The surfacing and base thicknesses are summarized
in TABLE II. The total thickness of construction and hence the thickness of subbase are determined from the CBR value of
the subgrade and the traffic intensity with
the add iti onal proviso that where the subgrade is a material likely to be susceptible
to frost-heave, the total thickness of construction should be at least 18 in.
25.
Although the thicknesses recommended in Road Note 29 for lean concrete
bases and surfacing are the same as when
water-bound macadam is used, the evid ence
above shows that the behaviour of a lean
concrete base is different. Thus the occurrence of transverse cracks in lean concrete
bases suggests that shrinkage or thermal
stresses influence the performance in a way
that does not arise with bases not containin g cement as a binder, and it has already
been suggested that thc advantage of a
thick surfacing may be due partly to the
increased thermal insulation and not only
to the increased stiffness of the pavement.
In any case, theoretical considerations indicate that because of the relatively high
elastic modulus of lean concrete compared
with other base materials, the surfacing will
contribute less to the strength of a pave799
KIRKHAM AND WRIGHT -
ment with a lean concrete base than it does
to the strength of a pavement with a weaker
base material. This difference will be more
pronounced if the base thickness is increased
or if the elastic modulus of the lean concrete is raised. Under these conditions it
seems likely that most of the strength of
the pavement would lie in the lean concrete base and, if in fact the base served
in much the same manner as a reinforced
concrete carriageway, a reduction could
probably be made in the large thicknesses
of sub-base required on poor soils. This
possibility, however, need s investigation.
26.
On roads carrying very heavy traffic, where a 10 in. thick base is used , this
has generally been compacted in two hyers
of 5 in. each, because it h as not been certain that satisfactory compaction in a 10
in . layer can be assured with the methods
used at present. A good bond has not generally been obtained between the two layers which may therefore have acted independently, and in one notable case the
failure has occurred in the upper layer of
lean concrete, which is subjected to greater
stresses th an the lower layer. In view of
this , and in order to reduce the stresses in
LEAN CONCRETE ROAD BASES
the upper Jayer of Jean concrete, It IS now
common practice to use, in place of a 10
in. thick lean concrete base, a composite
base consisting of 7 in. of lean concrete
and 3 in. of bituminous base material.
Calcu lations indicate that the change will
increase sli ghtly the tensile stress at the
bottom of the lean concrete caused by traffic but the additional thickness of overlying
bituminous material will reduce the temperature gradient in the lean concrete and
the stresses resulting from this. This change
has the added advantage that the smaller
thickness of lean concrete now required can
be compacted in one layer and may therefore be almost as strong as two 5 in. layers
acting independently.
27.
A possible alternative method of reducing the transverse cracking in roads with
lean concrete bases is the inclusion of steel
reinforcement in the concrete. This has
been done experimentally in a housing
estate road in Crawley ew Town with encouraging resu lts. On this road, which was
laid by hand , the spacing of the cracks is
influenced by the relatively close spacing of
construction joints in the base. Cracks have
occurred over all the construction joints
o
./'
./
o
No
r~intorcem e nt
(0)
./'
/'
G
./'
E xpandQd ml2ta l
./'
re inforce ment ( X I
./
o
./'
160
20
180
200
220
Dis tanc Q between construction joints (feet I
Fig.
800
IO-frequency of cracking
in
reinforced and
unreinforced sections at Crawley
A.R.R .B.
PROCEED I N GS
KlRKHAM AND WRIGHT - LEAN CONCRETE ROAD BASES
(DIS CUSSION BY SIMON)
but additional cracks have occurred between construction joints in most sections
and the equivalent numbers of full-width
cracks between successive joints are shown
in Fig. lO in relation to the distance between the joints. This diagram shows th at
fewer cracks occurred in the reinforced sections than in those without reinforcement
and also that fewer cracks occurred with
expand ed metal reinforcement than with
welded mesh. Two weights of welded mesh
reinforcement were used, 2.72 and 5.67
p.s.y. (the weight of the exp anded metal
was 5.10 p.s. y.) but this difference does
not appear to have affected the frequ ency
of cracking. Also, two thicknesses of base
were used, 10 in . and 12 in. , but this difference has not so far influ enced the rate
of cracking. The road has not, however,
been subjected to heavy traffic and the use
of reinforcement therefore requires further
investigatio n before it can be generally recommend ed.
CONCLUSIONS
28.
Bases using lean concrete made with
natural gravel aggregate are thus accepted
as giving at least similar performance to
bases of water-bou nd m acadam made with
crushed rock . To obtai n satisfactory performance the base must be thoro ughly compacted and the average strength of 6 in.
cubes of the lean concrete should be about
2,000 p.s.i. at 28 days; stronger materials
may give an unacceptable amount of cracking while weaker materi als may give an unacceptable amount of deformation under
traffic. - In order to improve the performance of this type of base, methods are bein g
sought fo r reducin g the amount of transverse cracking.
REFERENCES
1. Ministry of Transport, Specification for road and bridge works, H .M.S.O .
3rd edition (1963) .
2. Croney, D. and Loe, J. A. , Full-scale pavement design experiment on A.I
at Alconbury Hill, Huntingdonshire, Proc. I.e.E., 30, 225 (1965).
3. Ministry of Transport, A guide to the structural design of flexible and rigid
pavements for new roads, R.R.L. and Highw. E ngng Div., Rd Note No. 29,
H .M .S.O ., 2n d edition (1965) .
DISCUSSIONS
L. L. S I M 0 N , C .E., M .I.E.Aust., M .A.P .I. , Associate, Sinclair a nd Knight , Co nsultin g
Engi neers, Sydney
29.
The lean concrete bases described in thi s paper have, to the best of the
writer's knowledge, not been used extensively in Australia and it appears fro m his
investigations th at they cannot compete here economically with other pavements
of similar qu ality.
30.
The reason for this appears to be that this type of pavement, being neither
rigid nor fully flexible, cannot use the advantages inherent to these two types. The
2,000 p.s.i. cube strength in 28 days cannot be regarded to be the equivalent of
more th an approximately 300 p.s.i. in flexure. The surfacing with bituminous
concrete will contribute little to the strength of the base, as pointed out by the
authors in para. 25 of their paper. If, therefo re, the pavement is designed as a
Volume 3, Part 2 ([966)
80 1
KIRKHAM AND WRIGHT - LEAN CONCRETE ROAD BASES
(DISCUSSION BY SIMON)
fully rigid pavement in accordance with Westergaard's theory, the resultant thickness will make it uneconomical. This type of design is not recommended by the
authors.
31.
They state in their conclusions that this pavement base will give at least
similar performance to bases of waterbound macadam made with crushed rock.
It can be implied, therefore, that this type of pavement cannot be used economically where crushed rock is available at a reasonable price and should be used
where natural gravels only are available.
32.
While the writer was associated with the Commonwealth Department of
Works, he was often confronted with the problem of providing suitable materials
for high class aerodrome pavements in such areas. It was found by experimenting
and proof-rolling of experimental areas that relatively inferior natural gravels
could be brought to a satisfactory standard by th e addition of small quantities of
ce~nt. Details of this type of work base have been published elsewhere (Ref. 4).
33.
In order that such a base should behave as a flexible and not as a rigid
pavement and also to minimize the amount of reflection cracking through the
bituminous surface course, the amount of added cement is kept to the minimum
(2 to 3 per cent) and a 7-day unconfined compressive strength of 150 p.s.i. was
aimed rather as a maximum and not a minimum.
34.
It is quite possible that bases of this type, some of which have given satisfactory service under heavy loads for over ten years, may behave in a different
manner under the prevailing climatic conditions in the U.K.
35.
In order to compare relative costs of different types of pavements, the order
of cost per square yard for three alternatives has been calculated.
(a) 4 in. bituminous concrete on an 8 in. lean concrete base as recommended by
the authors for case 3 in TABLE II of the paper (1 ,500 to 4,500 commercial
vehicles per day) .
( b) 7 in. unreinforced high quality concrete pavement as recommended in
Ref. 5 (from which the prices have also been adopted).
(c) 2 in. of bituminous concrete over an 8 in. modified gravel base. This reduced
bituminous concrete thickness is based on local practice over flexible bases,
but in accordance with authors' statemen ts is not acceptable for lean concrete; the reduced total thickness is also based on th e authors' statement in
para. 25 of the paper.
It may be noted that there is also a granular sub-base required in each case but
for the purpose of comparative estimates this has been neglected. It may be noted
that in the case of weak sub-grades the thickness of this layer need be less in case
(b) than for (a) and (c).
36.
The comparative order of cost per square yard is as follows:
(a) authors' recommendation
4 in. bituminous concrete at $0.75 / in.
$3.00
8 in. lean concrete base at $11.00/ cu. yd
2.45
$5.45, say $5.50
802
A.R.R.B.
PROCEEDINGS
KIRKHAM AND WRIGHT - LEAN CONCRETE ROAD BASES
(DISCUSSION BY TA YLORj AUTHORS' CLOSURE)
(b) High class concrete pavement
7 in. concrete at $17.20/ cu. yd
Finishing and jointing per sq. yd
$3.34
0.60
$3 .94, say $4.00
(c) 2 in. bituminous concrete on 8 in. cement modified gravel
$1.50
B.C. 2 in. - $0.75 / in.
2.00
8 in. cement modified gravel at $9.00 per cu. yd
$3.50
I. C. T A Y LOR, A.M.I.E.Aust., City Engineer, South Melbourne C ity Co uncil
37.
The lean concrete bases described in this paper closely approximate a
rigid pavement with its strength of 2,000 p.s.i. at 28 days. The pavements exhibit the characteristic of shrinkage cracking that occurs in rigid pavements. The occurrence of such cracks need not necessarily be regarded as a
potential source of trouble, particularly in rigid pavements with steel reinforcement, provided to control the type, size and location of such cracks. There is,
however, a considerable degree of concern both from engineers and some members of the public when such cracks appear. They fear a failure, but with proper
design this is unlikely to occur. There is a fruitful field for research into:
(a) the incidence and control of such cracks, and
(b) the long term effect of the cracks upon the strength and riding quality of the
pavements.
38.
Have the Authors any experience of the effects of such shrinkage cracks
in the long term, and if so, what have they done about them?
AUTHORS '
CLOSURE
39.
In the di scussions on the advantages and disadvantages of lean concrete
bases, two points have been raised. The first of these is concerned with the economics of this type of pavement and particularly whether lower cement contents
might not be used to achieve economy, whiIe the second point raised concerns
transverse cracking.
40.
In connection with the economy of this type of pavement, it is important
to compare pavements which might be expected to have similar lives and similar
maintenance costs. In the conclusion to the paper, it was stated that the lean concrete base would give at least similar performance to bases of waterbound macadam made with crushed rock. It was not, however, intended to be implied that
the lean concrete base could never be used economically where crushed rock was
available at a reasonable price. Lean concrete bases have been used in such areas,
particularly since under adverse weather co nditions they may often make it easier
for work to proceed. It may also be possible in the future, as shown in Fig. 9 of
the paper, for a cheaper bituminous surface to be used with a lean concrete base
than with a crushed rock or sand / cement base. For most bases under British
Volum e 3. P a rt 2 ( 1966)
803
KIRKHAM A
D WRIGHT - LEAN CONC RETE ROAD BASES
(AUTHORS' CLOSURE)
conditions, an impervious surfacing is required , but this may be less important
with a lean concrete b ase. In this connexion, it should be remembered that in the
AAS.H.O. road test, where the bituminous surfaces would be more pervious
than those generally used in Britain, the cemented bases gave better performance
than the unbound b ases.
41.
The 8 in. modifi ed gravel base with a 2 in. bituminous surfacing, suggested
as an alternati ve to 8 in. of lean concrete with a 4 in . bituminous surfacing, wo uld
not be expected to give a comparable performance. Although the modified gravel
base has given satisfactory ser vice in an aerodrome pavement for ten years, the
traffic. conditions on such pavements are not the same as those on road s where ,
although individual loadings may be lighter, they are much more frequent. The
failure of cemented bases is probably a fatigue failure and under these circumstances the frequency of loadi ng is much more important. Fig. 9 of the paper
shows that of seven sand/ cement bases laid in the Alconbury Hill experiment,
only one survived after six years under traffic . A 9 in. sand / cement base under
4 in. of asphalt had already failed. This ev idence would not support the suggestion that a reduced total thickness would be acceptable with the modified gravel
base; in the paper the suggestion for reduced thickness was dependent on the use
of lean concrete with a higher modulus of elasticity. A pavement using an 8 in .
modified gravel, therefore, does not seem likely, under British climatic conditions
at any rate, to give equivalent performance to th e pavement with th e 8 in. lean
concrete base.
The comparison with concrete pavements is also made using a much
42.
reduced thickness than would be considered equivalent under British conditions.
An estimate of the equivalent number of 9,000 lb wheel loads to the mixed traffic
at Alconbury Hill was made to enable rough comparisons to be made between
British practice and the AA.S.H .O. test results. This gave a figure of about
450,000 such loads on the slow lane each year, and for a 20-year life thi s would
be equivalent to three times the number of repetitions for which Simon (Ref. 5)
suggested a 7 in. thick concrete pavement. For the Alconbury Hill traffic a reinforced concrete pavement on a normal subgrade would be made 10 in . thick.
Generally, British practice has required an unreinforced pavement for the same
traffic to be made two inches thicker, but this conclusion was based on early
failures of under-d esigned slabs. It may be, th at if th e frequent contraction joints
were dowelled as was done in the A.A .S.H .O. test, the sa me thickness could be
used for unreinforced as for reinforced concrete. On either basis the norm al
concrete pavement would be likely to be more expensive than the one containing
a lean concrete base. Experience in Great Britain has shown that for equivalent
performance the lean concrete base may very well be an economical form of
construction.
The cracking which occurs in the surfacings over lean concrete bases is
43.
not now generally attributed to shrinkage in the p avement. Cracking appears
more likely to be related to temperature movements and , in order to control th e
incidence of cracking, it seems desirable to consider the use of a light-coloured
curing compound in place of th e bituminous spray which has been customary up
tilt now. It also seems important to construct the bituminous surfacing on th e
804
A.R .R .B.
PROCEE DI N GS
KIRKHAM AND WRIGHT - LEAN CONCRETE ROAD BA SES
(AUTHOR S' CLOSURE)
lean concrete base at an early stage in order to give some temperature insulation
at this time. Since temperature insulation is considered to be an important factor
on the most heavily trafficked roads, a greater thickness of bituminous material
is now being used with a lesser thickness of lean concrete : 7 in . of bituminous
material on 7 in. of lean concrete is one of the current specifications for motorways in Great Britain.
44.
Generally under aU but the heaviest traffic, althougb fine cracks have
appeared through the surfacing, there has not been rapid deterioration of the
material over the cracks. However, on very heavily trafficked roads, where a
high strength gravel lean concrete (28-day compressive strength about 3,000
p.s.i.) was used, and wide cracks formed at infrequent intervals, there has been
faultin g and pumping at some of these cracks many of which occurred at construction joints. However, the frequency of cracking is related to the type of
aggregate, and lean concrete containi ng limestone aggregate of equal strength has
not shown these defects. It seems desirable therefore to relate the performance
of lean concrete bases to the tensile strength and elastic modulus rather than to
the compressive strength, and research into these properties of the material is in
hand .
REFERENCES
4. Williams and Purdam , Modification of natural gravels and soils by the addition of small amounts of cement, Construct. Rev. (January, 1963) .
5. Simon, L. L. , Reduction in the cost of concrete pavements through quality
control, Proc. 2nd Conf. , A.R.R.B., 2: 2 (1964).
Volume 3, Part 2 ( 1966)
805