�XATRANSPORT RESEARCH LABORATORY

"TIThE, The use of caicrete in paved roads inBotswana

by A V, Lionja fnga, T Toole and P A K Greening

Overseas CentrTransport' Res'e'arc'h" Laboratory.-Crowthorne Berkshire- United Kingdom

. - - 1

LIONJANGA,A V, T TOOLE and P A K GREENING (1 987) The use of caicrete in pavedroads in Botswana. In: AKINMUSURU, J 0 et al (Eds). Soil Mechanics and FoundationEngineering. Ninth Regional Conference for Africa, Lagos, September 1987, Volume 1.Rotterdam: A A Balkema, 489-502.

9th Regional Conference forAfrica on Soil Mechanics and Foundation E'igineernng/Lagos,' Seprei;ber 1957

The use of caicrete in paved roads in Botswana

ANV.LionjangaMinistry of Works and Communications, Gaborone, Botswana

T.Toole & P.A.K.GreeningTransport andRoadResearch Laboratory, Crowthorne, Berkshire, UK

ABSTRACT: This paper describes the studies undertaken in Botswana which have led tothe development of local specifications for the use of calcretes in paved roadconstruction. The studies have examined their use as road bases, untreated shouldermaterials and as surface dressing aggregates. A new specification for lightly-trafficked roads based on the findings of the studies is included.

The results of recent research concerning the use of cement and lime stabilisationis presented and recommendations made.

Finally, the results of the research will be applied to the construction of over1200 km of road to be constructed in the Kalahari region over the next decade. Thiswill enable massive cost savings to be made..

1 INTRODUCTION

Calcrete deposits exist as virtually theonly sources of hard or gravellymaterials available for use in pavementlayer construction throughout most of thecentral and western areas of Botswana.Their occurrence is generally confinedto those areas overlain by Kalahari sandsalthough they are also found further eastoverlying older rocks and along drainagelines, (;4allick, Habgood and Skinner1981).

The engineering properties of calcreteare extremely varied. They range fromslightly calcified uniform sands andsoft powdery deposits to hard gravel androck layers. For use in pavement layerconstruction only the best depositsnormally satisfy traditional specifica-tions although the use of the softertypes has been successfully demonstratedin low-volume roads (Overby 1982,Lawrence and Toole 1984).

A number of major paved roads areplanned to be built in the next decade inthe Kalahari region in which calcreteswill be used. These projects areidentified in Figure 1.

The completion of these routes willlead to an expansion of the paved roadnetwork from its current length of 2000km to approximately 3200 km.

It is likely that calcrete will beused as the main pavement material in

the construction of all routes. It mayalso provide a source of surfacingaggregate.

In the earlier development of the pavedroad network, calcrete was seldom used.The reason for this was the abundance ofnatural gravels and weathered rockdeposits exposed in the east of thecountry where most of the road andeconomic development has taken place.Whenever calcrete has been used as a

base material on the main road networkstabilisation by cement and/or lime hasbeen undertaken. The main purpose of thestabilisation has been to reduce theplasticity of the calcrete. In each casethe calcrete material has been relativelycoarsely-graded prior to treatment.

The occurrence of the harder varietiesof calcrete however is not widespread andhas led to the need to develop experiencein using marginal and low grade types.In order to obtain the necessary exper-

ience and evidence to justify new designstandards, the Botswana Ministry of Worksand Communications CMOWC) has conductedresearch into the use of calcretes with anumber of research organisations. Thishas led to the construction of four majorfull-scale road experiments in whichcalcretes ranging in classification fromcalcifi,-ed sand to hardpan and boulderdeposits are being examined as sub-bases,bases and surface dressing aggregates.

The construction of roads under Rural

489

Major study and constructi~ot

yrojnict, 11986 Onwards)

Sites ot tl scale road

Q- ','®5 i~15rr~fr and construction

- Main bitumen surlaced roadS

- -- Main earth/gravel roads

A Caianoula

Fig. 1 Map of Botswana showing the location of the experiment

Roads Project (Overby 1982), in whichuntreated powder calcretes and calcifiedsands were used as roadbases, has alsoassisted in the development of localexpertise.

The objective of this paper is to bringtogether past and recent experience inthe use of calcretes in paved roads forthe benefit of highway engineers involvedin the design and construction of roadsin calcrete areas.

2 THE OCCURRENCE AND DISTRIBUTION OFCALCRETES IN BOTSWANA

Calcrete deposits occur throughout theKalahari region of Botswana, within theKalahari sand. They are associated withtopographic features of the sand surfacesuch as pans, inter-dune hollows, topo-graphical depressions and former drainagelines, but also occur in association withgrey-coloured patches of Kalahari sand'and with rocks near the surface. In thelast case they occur as cappings tocalcareous or basic igneous rocks, whichform a local source of calcium carbonate.

Calcretes and the geological formationswith which many are associated, have been

mapped in Botswanta by the use of remotesensinG techniques. Black and whiteaerial photographs have been used to maplandforms associated with calcrete, andLandsat imagery has been used to mapfeatures showing colour differences. The`Photogeological Map of Botswana`(Mallick, Habgood and Skinner 1979) wasproduced in this way, and illustrates themain surface materials and exposedgeology of Botswana. Figure 2 which isbased on this map, shows the presence ofcalcretes and other materials that arepotentially suitable for road pavementconstruction. The remarks in Fig 2 arebased on field investigations conductedin the TRRL-MOWC study of the occurrenceof calcretes (Lawrence and Toole 1984).As part of this latter study detailedmaps of calcrete occurrence covering anarea in the Central Kalahari, and whichare suitable for estimating materialresouces, were produced. Use was alsomade of natural colour aerial photographyin mapping grey sand areas.

3 PREVIOUS USE OF CALCRETES IN PAVEDROADS

Calcrete has been used in the sub-baselayers of sections of the main paved roadnetwork, in the Rural Roads Project roadsbuilt in the Kalahari region and invarious experimental pavement sections.The road sections which incorporatecalcrete in the road base are listed inTable 1.

Roads built as part of the main roadnetwork have been constructed withcrushed or mechanically stabilisedcalcrete bases, lime modified caicretesor cement and lime stabilised calcretes.These roads have given good performanceover periods up to ten years.

In the construction of these roads thepavement thickness designs and materialstandards have been based on either TRRL,guidelines for tropical countries (TRRL1977) or South African guidelines(National Institute for Transport andRoad Research, 1980a, 1980b).

Roads built as part of the Rural RoadsProject have used various thicknessdesign guides. In the selection of pave-ment materials, trial specificationsbased on research conducted by NITRR(Netterberg 1971) and those developedfollowing the 'forced-trafficking' oflengths of- project roads have been used.These are described later in this paper.

490

Fig. 2 Distribution of the main material types in Botswana(After Mallick, Habgood and Skinner 1979)

Table 1. Use of calcretes as road bases in Botswana

Project/Route Length Type of construction(kin)

1. Main road network

(a) Francistown - Nata

(b) Palapye - Serowe

(c) Nata - Kazangula

2. Rural road projects

(a) Sehitwa - Tsau

(b) Tsabong - Makopong

(c) Molepolole - Letlhakeng

(d) Mopipi - Rakops

28 Lime stabilised calcrete base with a doublesurface dressing

45 Two stage lime and cement stabilisation of acalcrete with a double surface dressing

180 Lime stabilised calcrete with a double surfacedressing

50 Untreated calcified sand/powder calcrete witha double surface treatment of a graded sealand a sand seal

95 Untreated and mechanically stabilised calcretebase with a double surface treatment comprisinga single graded aggregate or single sizedaggregate followed by a sand seal

50 Untreated calcrete base with a double surfacedressing

50 Untreated calcrete base with a double surfacedressing

491

Ky Geoogica unit Calcrete occurrence

PeKahri rocks and soils Few calcrete deposists occurNatural gravels and weatheredrocks abundant

Kaaaisand areas Powder calcretes occur ininterdlune hollow% and pansand beneath patches of greysand

Alluviu ~~~~Poorly developed calcretesoccur on fringe areas

J Lacustrine deposits Calcretes may occur betweenfowlying areas and on former

Major areas with calcrete at Hard, pavement qualityoutcrop or wi-th a very thin calcrete occurs. Calcretes also

M E sand cover found along river courses

/55 XE X+~ Oistribation limit of most of Hard calCretes occur oin pan5 the major calcrete rimmed rims. Poorly developed

4x5N pans calcretes occur on pan floors

and in sand areas

4 THE CLASSIFICATION AND ENGINEERINGPROPERTIES OF CALCRETES IN BOTSWANA

4.1 Description and classification

The calcretes which exist in Botswana arethe result of a pedogenic soil formingprocess, in which carbonates of calcium(and often magnesium) have accumulated inthe Kalahari sand. Solution andredeposition over time have given rise toa variety of calcareous deposits, allcalled calcrete, varying from loose,calcified sand to massive, hard "rock".

in Botswana a classification of thecommon calcrete types based on the SouthAfrican groupings CNetterberg 1971) hasbeen adopted and these are distinct bothphysically and in their engineeringproperties. The groups are; calcified(or calcareous) sand, powder calcrete,hardpan calcrete (including "boulder"calcrete) and nodular calcrete (Figs 3and 4). They differ by the amount andtype of carbonate present and its degreeof crystallinity, the proportion and hard-ness of any gravel sized particles, andthe degree of induration. They may alsocontain varying amounts of mineral finematerial, eg water soluble salts and clayminerals.

4.2 Profile characteristics

Any one or a number of types of calcretemay be expected to occur in a calcreteprofile. The thickness, position andhardness of the layers will also vary.Thicknesses from a few centimetres to overtwo metres have been xeecorded and theseoften determine the cost of production,the mechanical plant necessary forexcavation and construction and whether adeposit is workable.

Because of the possibility of variablelayer quality and thickness particularcare needs to be exercised during excava-tion to prevent contamination and mixingof materials. The availability of anaccurate description of in situ conditionsand test data representative of thecalcrete profile can assist in theidentification of the most suitabledeposits.

4.3 Properties of calcretes

4.3.1 Engineering tests

The properties of some Botswana calcretesrelated to their use in paved roads areshown in Table 2. Typical gradings forthe four common types of calcrete

Fig. 3 Sampling calcified sand. Note the extreme loosenessof the material. The upper part of the profile is morecoherent, approaching soft powder calcrete

*¼�

Fig. 4 Hardpan calcrete. The horizontal laminar structure istypical. The material grades downwards into toughpowder calcrete at the bottom of the photograph

492

.TabL£ 2. Laoaoyteat ProP.rtie. l -cone ..t.an cacrt related to ace in roa d cons truction

Soil .. cnt.nts S'"oed clas..ifct ionSample Psig p..alng APV CM Calr... according to lio in full-Scal

no Pt L. ~ L l 475 jun 7pi (2) cod at typo B-t..ana Road ta eietPI LL (% (%)~ AA SRTo Oig. Miac.l (3) iflbtan

883 (4j SP - 2.6 210 - 70 172 Rardpa~N/Sd. Roud cone (C4l Road bae ... : os-aaepr~e

FNI II 5.3 2.5 21 - 70 67 Nodalar Subun.. (05) or lin- Roa sae: Prnistm-aa eprs.oddi~ed road none. (untreated and ie-oile

B01 8 28 3.7 2.2 3d 6 23 110 Hardpon Satbuan (05' or- lin- Road b..e : K.nYe-Jco.... ea persetcodi'fed road bas..

504 20 ad 8.5 2.1 35 II 53 105 Nodular Lne e.dified ta... Road base Knn.snngepron

JSSB 24 50 8.0 2.3 22 a '0 - SotlrLi- :udirfied sane or Roa base: Jaoneng-S5lonk epeientu-bae

Q81 12 49 8.3 2.3 23 2 43 103 Nodla Sub-bue (GS) or Ine- -codified bs

806 7 36 2.9 1.9 ad Is 8 90 Powder S.nl-ted .ungrade (C6) Road base : K..Ye-J.-anon .. ....ri.n.

807 17 41 3.6 1.1 50 20 28 so P10tc Fi ISC1)Rod 65:- Kasys-Jan1engeonisn

calcifie.sd (utretd and Inn1 cn .n.u.bi..e.

805 19 38 8.0 1.5 70 18 - 42 Plautic Powder FiLl 100151

87 16 38 6.3 1.2 79 8 5 66 Powder Fill (SCISI

ibd 26 49 10.6 1.1 73 28 - 56 Cal~ifitid ...od Fill (SCIS) Roa b...n: J ...annc-5elo ... romn

Psr dofimitiss of Godi- Modulus (ON) GM0 30-8. K.tr. p..,. 1200-.045-.005- i..

APO - aggrealte Pliers Valu. (Natt.rber& 19711

Ease d Os .at.rlml stand-ard ecd p-s...ent cstegsries in Cbspter.. 5-107 and 5-307 )9ROWC 18821

Tn. 119018 lImit c s f s..mpi. -0 F83 m-d FRI -ers deeri uing SI£TeS test methods and need.cerrectieg cc

acco rdance s ithb-P Sapsn s Setterbeg 11984).

.B........... G 1 - Hardpan calcrete…- 8G 4 -Nodular calcrete

. . .-. . O. G 6 - Powder calcrete- - - 8G 7 - Plastic calcified sand

_____________Recommended limits for minus37.5mm material (TRRL AN 31)

800

0

0L

63 150 425 1.18 2.36 5 10 20 37.5 75

(on5B sieve sizes (mm)

* Hardpon calcrete * Nodular calcrete0 Powder caicrete 0 Calcifieoa-and

0Low Traffic Roads (MOWC 1982)

0NITRR )Netterberg 1971)

01 0

I TRAL RN 31 Botswanaso MOWCI ID *m[~~~~~~~1982).

0 50 O00 150Aggregate pliers value x grading modulus

Fig. 5 Mean particle size distributions of samples of four of the Fig. 6 Relationship between laboratory test properties of somecommon types of calcrete Botswana calcretes and type (After Lawrance end Tool.

1 984)

493

Wtlotn: I .

2 .

3.

4 .

m

0.

Table 3. Results of testing calcrete aggregates used in bituminous surface dressings

Aggregate Modified Aggregate 10% Fines Aggregate Watersource Impact Value Crushing Value absorption

(%) (kN) (%)Dry Soaked Dry Soaked

Nata (used in Nata-Kazangula experiment) 22 23 270 210

Sekoma (used inJwaneng-Sekoma 24-28 -210-175 110-100 2.6-3.1experiment)

are shown in Fig 5. In comparison withconventional specifications for pavementmaterials the usefulness of the materialsranges from subgrade and embankment soilto road base quality (Ministry of Worksand Communications 1982). As a result ofresearch in Botswana however, their use-fulness can be extended, particularly inlow volume road construction.

Because of the variability of calcreteseven within the recognised types it issuggested that the full range of lab-oratory tests be performed prior toselection. In common with traditionalguidelines the properties which definethe usefulness in road construction of acalcrete are its grading, the strengthand proportion of the hard particles andthe proportion and plasticity of the soilfines (material passing the 0.425 mmsieve). These four properties have beencombined in Fig 6 in two compositeexpressions comprising the product of thelinear shrinkage and the amount ofmaterial passing the 0.425 mm test sieveand the product of the grading modulus(Table 2) and the aggregate pliers value(Netterberg 1971) to illustrate thespread in properties obtained from test-ing the various types of calcrete.Superimposed on the figure are variousacceptance limits for road bases(Netterberg 1971, Lawrence and Toole 1984,TRRL 1977).Calcretes suitable for use as surfacing

aggregates also exist in some parts ofBotswana although their full potentialis still being investigated. The testvalues of two such deposits are given inTable 3. These particular materials arecurrently being assessed as surfacedressing aggregates in on-going roadexperiments.

4.3.2 Cement and lime stabilisation

In cases where untreated materials suit-able for pavement layer construction are

unavailable, or their use is not costeffective, local soils are oftenstabilised by the addition of low percent-ages of cement or lime to effect animprovement.The purpose of the stabilisation needs

to be clearly defined since the reactionof cement or lime is quite different.Lime (hydrated lime) is usually added toreduce the plasticity of a material byinitiating a flocculation reaction withany clay minerals present. In some casesif additional lime remains available apozzolanic reaction may follow. Cementis usually added to effect a direct gainin strength by creating cementitiousbonds between soil particles. Whenmeasured in the laboratory after spe-cified curing periods high strengths havebeen obtained with calcretes using eithercement or lime. These can usually bemaintained on soaking. In the field how-ever, the success of stabilisationdepends on a number of factors. Theseinclude the time between mixing and com-paction, the efficiency of curing andeffective sealing over long periods andthe initial consumption of lime (ICL) ofthe unstabilised material.

The difficulties of compacting cementstabilised materials after long delays(usually greater than 3 hours) are welldocumented. It has been suggested thatsimilar reaction times may occur withcertain lime-stabilised calcretes due tothe presence of non-plastic pozzolans inthe form of reactive silica (Netterberg1971). The only current means of determ-ining the response of a material in thisway is to perform a test programme where-by the density-strength-delay timerelationship is determined. No specificexamples are available from Botswanawhich demonstrate a rapid lime reactionalthough the data contained in Fig 7illustrates the change in density achievedat constant compactive effort at differenttime delays. Corresponding cured

494

____________Powder calcrete (samnpcent hydrated lime

- - ~~Powder caicrete (sampcent cement

95 1

901

10min 1 hourrime delay

Fig. 7 Per cent relative compaction obtaineotime del-ays following the addition ofagent

Strength loss due to loss of density and eldelay on cementitious products

- - - 7 day cure

28 day cure

10min 1 hourTime delay

Fig. 8 Effect of time delay on the strength ostabilised calcrete: Powder calcrete (Illper cent cement

le 087) +3 per strengths at the likely compaction levelare shown in Fig 8.

le 087) +3 per More recently problems related to thepossible carbonation of stabilised layershave been brought to the attention of high-way engineers (Paige- Green 1984). The

~~~ ~ carbonation reaction is most likely tooccur beneath permeable surfacings in thetropics particularly in areas of low tomedium humidity, and can take place in bothcement and lime stabilised materials.Recent, as yet unpublished, research hasshown that with calcretes carbonation canretard, and in extreme'cases, reversestrength development in materialsstabilised with cement and lime. It isnot known however whether the initialflocculation reaction which is responsiblefor reducing the plasticity of soil-limemixtures can be reversed in a similar

2 4 6 a manner. Evidence does exist however toshow that poorly-graded calcretes whichhave flocculated to different degrees and

l after different failed to show a permanent strength gainthe stabilising are in fact less stable than untreated

materials (Lion~janga, Toole and Newill(1987).Good efficient curing techniques and

early sealing are probably the best meansto combating the problem and research isin progress to determine the most efficienttechniques. The choice between cement andlime stabilisation may therefore depend onthe initial design objectives ie modifica-

Ifect of time tion of plastic components or cementing thematerial. In either case the same curingmay be required.

A further consideration in stabilisationdesign is the initial lime consumptionrequired to produce and maintain suffi-ciently high alkaline conditions for theformation of cementitious products. Thisis the minimum content required for longterm durability (Klaus and Loudon, 3.971).Often it is more efficient to introduce atwo-part stabilisation process, first add-ing lime (at least up to the determinedICL value) and then adding cement. Thishowever depends on costs particularly ofdouble handling etc, weighed againstmaterial costs. With calcretes it islikely that the ICL would remain constantwith time.

4.3.3 The role of calcium carbonate

Calcium carbonate is the main mineralLJL.J ~component introduced into calcrete during

2 4 6 8 its formation. The presence of the carbon-ate is thqught to be one of the mairnmineralogical influences on the often

f cement stabilised satisfactory behaviour of some otherwiseimpleQ087) +3 poorly-graded and plastic materials. It

495

100

~ -1%E

0

0

CL

E.

7

C7

z

2

E

CL

0cU

6

5

4

3

2

I I

N

1

is believed that a minimum amount ofcarbonate is required to achieve thesatisfactory response of a material.The calcium carbonate content of Botswanacalcretes ranges from less than 10 percent in calcified sands to over 50 percent in some hardpan and powdercalcretes.Simple equipment can be used for the

measurement of the carbonate content egthe 1Collins` calcimeter (Road ResearchLaboratory 1952).

4.3.4 Soluible salts

Highly soluble salts such as sodiumchloride may be present in some calcretes.The salt can migrate to the surface of abase layer and disrupt the bond with theprime and bituminous surfacing (Netterberg1979). The various construction tech-niques available to deal with salt-relatedproblems are discussed, later in thispaper.

The determination of the quantity ofsoluble salts present in a material isregularly undertaken when testingcalcretes. A simple soil paste conductiv-ity test is commonly used (NationalInstitute for Transport and Road Research1979). In Botswana provided that ground-water does not gain access to the roadpavement and provide a source of saltsafter construction then testing of theintended base material with the requiredamount of the compaction water to be addedis normally done.

5 EXISTING GUIDELINES FOR THE USE OFCALCRETES IN PAVED ROADS

Until recently the specifications used inthe selection of calcretds for use inpaved roads all derived from the adaptionof European and North American specifica-tions (eg TRRL 1977) or South Africanexperience (NITRR 1980a, 1980b). Theywere neither calcrete-based nor were theyderived exclusively for use in the semi-arid areas which extend across almost thewhole of Botswana. The specificationsgenerally require strict control of soilplasticity, grading, CBR and aggregatehardness (see Table 4).

As a result of studies of calcreteperformance in South Africa and Namibiaguidelines specific to their use in pavedroads have been published by NITRR(Netterberg 1971). These are also con-tained in Table 4 and allow for the accept-ance of materials with higher plasticitycharacteristics and poorer gradings thanwould normally be considered mechanically

stable.%The latter specifications have been

incorporated into the Botswana Road Design'Manual (MOWC 1982) and have been appliedin the construction of secondary and ruralfeeder roads receiving less than 200 000equivalent standard axles (ESA) in a designperiod.

The lower calcrete specification howeverexcludes the majority of calcrete depositsand may still be too stringent in manyareas of Botswana where traffic levels areless than 100 vehicles per day (VPD) anddesign traffic levels can be as low as100 000 ESA.

The development of locally-based specifi-cations which take greater account of thetraffic, climatic and subgrade conditionswhich exist in Botswana was thereforerequired. This need was met by under-taking studies to examine the use oflocally available calcretes in all pave-ment layers and by developing appropriateconstruction techniques.

6 THE DEVELOPMENT OF APPROPRIATE SPECIFI-CATIONS AND CONSTRUCTION TECHNIQUES

Local expertise in the use of calcretes inpaved roads has been obtained as a resultof the construction of parts of the ruralroad network and through the constructionand monitoring of four major full-scaleroad experiments constructed over the lastten years on the main road network'. Theprojects are identified in Fig 1.

6.1 Full-scale road experiments

A summary of the design and performance ofthe four full-scale road experiments ispresented below. Monitoring is carriedout at regular intervals by means ofmeasurements of surface deformation, crack-ing, longitudinal roughness and surfacedeflections. Panel inspections have alsobeen conducted. In addition traffic andaxle load surveys have been performed,climatic data recorded, and in someinstances the density, moisture contentand strength of the pavement layers andsubgrade have been measured.

(1) Francistown-Nata. (Project 1 inFigure 1). Eight test sections were builtwhich incorporate both lime stabilisedand untreated calcretes as base materials.Calcretes were also used in the otherpavement layers. Surfacing comprised adouble surface dressing with a fog spray.

The base materials consist of coarselygraded nodular and hardpan calcretes (seeTable 2). The worst material has a 4-daysoaked CBR of 60-70 per cent and a PI

496

TABLE 4 Existing guideline specifications for the selection of light-trafficked calcrete bases

Notes. 1. Details of gradings and appropriate maximum sizes given in TRRI. (1977)

2. Can be raised to 12 in arid and semi-arid areas (TRRL 1977)

3. At 100% BS, 2.5 kg rammer method. (In practise a higher field compactionof 97% BS 4.5 kg rammer method is often specified and CBR is based on this).

4. A minimum 10% Fines Aggregate Crushing Test value has been suggested by TRRL (Hoskingand Tubey 1969), or alternatively a maximum Modified Aggregate Impact Value of 40% issuggested.

5. Test methods described in NITRR (1979).

6. An Aggregate Pliers Value of 50 per cent

between 9 and 16 determined usingCasagrande apparatus (NITRR 1979).

The experiment has remained in a satis-factory structural condition since it wasopened to traffic in 1977. The surfacinghas however required maintenance followingloss of stone, which led to the basebecoming exposed. The cause of this is-thought to be the progressive absorptionof binder by the caicrete base. Remedialtreatment has included the application ofdilute emulsion in 1983, as a fog spray,with subsequent resealing in 1985.

Traffic volumes on this road are of the

is usually specified.

order of 100-200 VPD. The route is themain road link to north-west Botswana andZambia. The estimated axle loading overthe life of the road is in excess of500 000 ESA.

(2) Kanye-Jwarneng. (Project 2 in Figure1). Eleven experimental calcrete basesections and four control sections inwhich quartzite and granites are used inthe base are incorporated into thisexperiment.

The surfacings include conventionaldouble and single surface dressings andbituminous sand seals.

497

NITRR Bulletin 10 (Netterberg 1971)

TRRL

Test Desig Ntraf3i Expected Trsffic Category (VPD)<~ 20%> 3 tonnes

2.5 x 106ESA

< 500 500-1000 1000-2000 2000-4000

Maximum size of msterial 37.5 (1) 19-37.5 37.5-53 37.5-53 37.5-53(mm) or size range

Minimum grading modulus Not specified 1. 5 1.5 1.5 1.5(ns)

Percentage passing the 12-25 15-55 15-55 15-55 15-55425..ajm sieve

Percentage passing the 5-15 ns ns ns ns63Aim sieve

Maximum liquid limit (%) 25 40 35 30 25

Maximum plasticity index 6 (2) 15 12 10 B

Maximum linear shrinkage 4 6 4 3 3(LS) (%)

Maximum LS x % passing Not specified 320 170 170 170425,umMinimum CBR after 4 days 80 60 at 2.5 mm 80 at 2.5 mm 80 at 2.5 mm 80 at 2.5 mmsoaking (3) 80 at 5.0 mm 80 at 5.0 mm 100 at 5.0 mm 100 at 5.0 mm

Minimum dry 10% FACT value Not specified ns (6) 80 110 110(KN)

Minimum eoaked 10% FACT 50 (4) na s 50 50 svalue (Kq4)

Minimum relative field 100% 85 (3) 98% mod AASHTO (5)compaction _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Total soluble salts (%) (5) Not specified 0.2 0.2 0.2 0.2

The four main types of calcrete wereincorporated as base and unsurfacedshoulder materials. Their particle-sizedistributions are contained in Fig 3 and asummary of the results of laboratory testsin TFable 2.

Two of the experimental base sectionscomprised a plastic calcified sandstabilised with Portland cement andhydrated lime. Another was constructed inwhich a nodular calcrete was mixed with anequal proportion of the local Kalaharisand to determine whether savings could bemade in the haulage of materials. Inaddition a powder calcrete which possessedan optimum compaction moisture content ofaround 30 per cent was compacted at amoisture content considerably below thenormally required level as a means tosaving water in dry areas. A furthersection was constructed in which a nodularcalcrete was left unsurfaced for a numberof months to determine whether any gain instrength on drying would be maintained insubsequent years.From the results of the experiments so

far, seven years after construction, theuntreated road bases have performed satis-factorily when contained in the pavement.The poorest calcrete (the plastic calcifiedsand) however has not proved suitable asan unsurfaced shoulder material, havingdeteriorated badly within two years afterconstruction.

Neither the mechanically-stabilisedcalcrete nor the cement or lime-stabilisedcalcrete have proved to be a success.Deep rutting has occurred in parts of eachof these sections. The cement and lime-stabilised bases have also carbonated overtheir full depth. The failures have beenattributed to the lack of stability in thebase layer which id the case of the sand/calcrete mix is a result of an excessivegap in its grading and in the stabilisedbases is a result of the stabilisationreaction producing a material whichresembles a single-sized uniformly gradedsand with~a low amount of silt and clayparticles. Since opening to traffic in1979 all remaining road-bases have per-formed satisfactorily and have so farcarried up to 160 000 ESA in the moreheavily loaded traffic direction(Lionjanga et al 1987). Daily trafficflows have varied between 150 and 250 VPD.

The performance of different bituminousseals applied to two different calcretebases was also examined. Sand seals usingthe local Kalahari sand have not performedwholly satisfactorily, being best on thebetter quality calcrete, which containedan appreciable quantity of coarse hard

particles. These provide a surface finishthat permits sufficient penetration of thebinder film. Extensive potholing occurredon a powder calcrete with a sand seal.The single surface dressings performedreasonably well on both although theyallow little margin for error in theirdesign and their performance can bevariable.

(.3) Nata-Kazangula (Project 3 in Figure1). This experiment comprises a total offour test sections which incorporate acrushed calcrete aggregate in the bit-uminous surfacing. The calcrete aggregateswere applied in the following surfacetreatments to a primed crushed stone base:(a) Double seal 13.2 mm first layer

6.7 mm second layer.(b) Double seal 13.2 mm first layer,

graded crusher waste second layer.(c) Single seal 13.2 mm with a fog spray.(d) Single seal 13.2 MM.The mechanical properties of the aggre-

gates are contained in Table 3.The performance of the sections with the

double seals and the single seal with afog spray has been satisfactory. Thesection with a single seal only has suff-ered a substantial loss of stone as aresult of the lack of a second binder sprayto make up for the deficiencies in thecondition of the primed base.

The experiment was opened in June 1983and carries the heavily loaded trafficmoving between Botswana, Zambia and Malawi.Vehicle flows are of the order of 40-70VPD.

(4) Twarieng~-Sekoma. (Project 4 inFigure 1). This experiment comprises atotal of eighteen test sections andcalcretes are represented in the bit-uminous surfacing, base and sub-base.

In the experiment various surfacedressings (both single and double) andgraded aggregate seals (using the 'Otta'concept (Thurmann-Moe and Ruisten 1983))were applied to both a plastic calcifiedsand base and a nodular calcrete base.The experiment has also examined the useof an armour course of large chippings(>20 mm) applied to the calcified sandbase prior to priming and the applicationof a bituminous seal to the shoulders ofthe calcified sand sections.

The properties of the calcrete surfacingaggregate which was obtained from aKalahari pan deposit,are contained inTable 3.

Of the 18 original sections, the 7 whichreceived double surface treatments haveperformed reasonably well on both types ofbase although these sections required afog spray after 15 months to enrich a

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binder-hungry surface. These consist ofdouble surface dressings, using alterna-tively a single sized chipping or acrushed waste in the second seal, anddouble. 'graded aggregate seals'. Theremaining 11 test sections have beenextensively patched and a slurry sealapplied.

The experiment has identified a numberof factors not adequately covered inexisting guides. These include the needto take account of absorptive aggregatesand the absorption of surfacing binder bya primed base. An analytical basis forthe design of graded aggregate surfacirngsneeds also to be pursued.The experiment has shown that the con-

struction of an armoured base is partic-ularly difficult and costly and a similardesign effect can be achieved successfullyat a lower cost using a double surfacetreatment.

The experiment has also highlighted theneed to take into account the slow curingproperties of diesel cutbacks when used assurfacing binders.

Since completion of the construction in1984 the shoulders of the plastic calci-fied sand secticns have performed wellhaving received a bituminous seal incorp-or i--ing a graded crusher waste.

The traffic on the experiment has variedbetween 40-80 VPD.

6.2 Construction of rural roads

During the construction of roads builtunder the Rural Roads Project (Overby1982) trials using different materialspecifications and construction techniqueshave been undertaken. These were necessaryto enable economic solutions to be devel-oped in areas where conventional materialswere absent. Four project roads in theKalahari region were constructed usingcalcretes (see Table 1). Of these, twoprojects have been more closely examinedand reported.

(1) Sehitwa-Tsau. (Project 5 in Figure1). The construction of this road waspreceded by the construction and "forced-trafficking" of trial sections. In thearea only poorly-developed, slightlyplastic, calcified sands existed andexperience of their use was previouslyconfined to unpaved roads. The purpose ofthe trials was therefore to determinewhether they could be used successfully asa base material, both untreated and follow-ing stabilisation with lime. Sectionswere also constructed where an "armour-course` was applied to the untreated baseprior to priming and sealing. The surfac-

ing of the experiment comprised a doubleseal using a graded aggregate in the firstlayer (an 'Ottal-type seal), followed by abituminous sand seal as the second seal.

Following the construction of the trialsin December 1980 35 000 ESA were appliedby "forced trafficking". This was consid-ered to be equivalent to ten years traffic.No differences in performance wereobserved during this period. As a result,the local calcified sands were used asroad-base material for the construction ofthe road. The surfacing adopted comprisedthe graded aggregate seal followed by alight sand seal.

The road has been monitored at regularintervals since construction and the per-formance of the test sections and the mainroad project has remained satisfactoryover a period of six years although acrack pattern, possibly associated withseasonal and long term moisture changes,of a rectangular form has developed on theunstabilised sections. No surface deforma-tion has been observed.

(2) Tsabong_-Makopong. (Project 6 inFigure 1). This road was constructedcommencing in 1980 and the road basematerial comprised a mechanically-stab-ilised hardpan/nodular calcrete mixed withKalahari sand. CBR values were in excessof 80 per cent after four days soaking andthe material was of low plasticity.Problems encountered on the project were

.associated with high amounts of solublesalt in the calcretes bases. The solublesalt contents ranged between 0.1% and 0.5%,and various techniques were used to remedythe problems of surface disintegrationwhich occurred. The most successful tech-nique was to use an MC70 grade primesprayed at 1.3 litres/in' immediately afterbase construction and to complete the sur-facing as soon as possible thereafter. ifdelays occurred to the priming operationthe alternative method used was to skimthe top of the base by a grading operationonce the salts had migrated to the surface.This latter method resulted in a roughsurface and required extensive patching.

7 RECOMMENDATIONS FOR THE USE OF CALCRETEIN PAVED ROADS

(1) Untreated or lime-modified road bases.The results of the, studies have shown thata wider range of calcretes can berecommended for use in lightly traffickedroads than those specified previously (seeTable 1). A revised specification whichapplies, to a cumulative traffic loading of160 000 ESA is shown in Table 5 and isbased on a more critical examination of

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the performance of calcified sands, powder.calcretes and gap-graded materials exam-ined in Botswana (see Lionjanga et al1987). In particular the importance ofthe distribution of sizes in the soil finesi~s highlighted. A minimum calcium carbon-ate content is also recommended which mayhelp to classify caicretes furtheralthough the role of calcium carbonate inthe performance of calcrete as a roadbuilding material is not fully understood.There has been no evidence to suggest thatself-stabilisation occurs as has beensuggested elsewhere (Netterberg 1971).

For higher traffic levels it wouldappear that the maximum plasticity in thebase material could be increased up to aplasticity index of 15. It is not knownhowever whether grading requirements andCBR criteria could also be relaxed. Un-treated and lime modified calcretes whichsatisfy the specifications of TRRL RoadNote 31, provided that the criteria foraggregate hardness and soluble salts arealso met, should be suitable for trafficlevels up to 2.5 x 106 ESA.

(2) Cement and lime-.stabilised road bases.Specifications exist in Botswana forcement and lime-stabilised bases. Theyare largely based on South African designguides (NITRR 1980a, 1980b). They havehowever been applied in circumstanceswhere the untreated materials were rel-atively well-graded to begin with and inthese cases they have generally met withsuccess. In an efficient design processmore poorly-graded materials would normallybe used, particularly at current designtraffic levels in Botswana. A number offactors can however affect the success ofthe stabilisation and these include:inefficient curing, which can lead to poorhydration of the cemented material, car-bonation and delays between initial mixingand compaction. To ensure that some amountof mechanical stability is maintained aminimum grading modulus of 1.5 and a co-efficient of uniformity greater than 5, andpreferably greater than 10, is recommended.A minimum unconfined crushing strength of1.7 MPa atfil -dnity is also recommend-

ed. The curing period should be 7 days forcement-stabilised materials and 28 dayswhen lime is used.

(3) Shoulder materials. As a result of theunsatisfactory performance of some calci-fied sands as unsurfaced shoulder materialsseparate guidelines for their use arerequired. For low volume roads these arecontained in Table 6 and include restric-tions on the maximum allowable amount ofmaterial passing the 425 pim sieve and a

minimum calcium carbonate content.Where shoulders receive a protective

bituminous seal the appropriate basematerials for the design traffic levelgiven in Table 5 should be used. At highertraffic levels (as covered in Table 1) thebase should be extended through the shouldereither unsealed or sealed. This latterconsideration will depend on local policy.

(4) Surfacing materials. Hard calcreteswhich satisfy norm:Rl aggregate requirementscan be used successfully in bituminoussurfacings up to high traffic levels. Theyshould possess a minimum Ten Per Cent FinesAggregate Crushing Value of 210 kN in thedry test and 160 kN after a period of soak-ing (NITRR 1j80b). For low-volume roadsthose calcretes with lower particleStrengths (eg a wet 10% FACT of 100 kN) andwhich often possess a high water absorptionvalue may still be used although they maygive a more variable performance as a resultof the variation in the strength of indi-vidual particles.

Calcrete bases often require more bitum-inous prime to adequately seal the surfacethan other road base materials. Studieshave shown that trials may be necessary atthe time of construction to choose thecorrect type and application rate. Coarsely-graded calcretes are often the most `hungry.'materials.

A conventional double surface treatmentprovides an adequate surfacing for allcalcrete bases and from evidence in Botswanathis can last for between five and sevenyears. Following this the application of afog spray, to rejuvenate the surfacing,may add a few years to the life of a seal.

Single surface dressings and sand sealsare not suitable for surfacing all typesof baseand require high maintenance andresealing within 2-3 years.

The use of graded aggregate seals hasbeen successfully demonstrated on ruralroad projects using conventional aggre-gates although a quantitative designmethod does not exist and site trials arenecessary to establish the technique tobe used.

8 ACKNOWLEDGEMENTS

The work described in this paper formspart of the joint research programme ofthe Ministry of Works and Communications,Botswana and the Overseas Unit of theTransport and Road Research Laboratory,and is contributed by permission of theDirector. The assistance given by theNational Institute of Transport and RoadResearch and the Norwegian Road ResearchLaboratory is also gratefully acknowledged.

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Table 5. Revised guideline specifications for lightly trafficked calcretebases (Note 1)

Percentage passing the 425 pmsieve (2)

Test10-50 50-65 65-85

Maximum particle size range (mm) 75-10 75-10 75-10% passing 63 pm sieve 5-25 15-35 20-35Maximum ratio of % passing 425 pm and 63 pm Not specified 3.5 3.5Linear shrinkage C)<10 <6 <6Plasticity index <25 <15 <15Maximum LS x % passing 425 pm 600 500 500LS x % passing 63 pm Not specified 30-150 120-210Minimum 4-day soaked CBR at field density() 40 40 40Minimum calcium carbonate content (%) of thematerial passing 425 pm 12. 12 12

Notes. (1) Up to a design traffic level of 160 000 ESA(2) After compaction using wet sieve analysis methods

Table 6. Guideline specifications for unsurfacedlightly trafficked roads

calcrete shoulder materials in

Percentage passing the425 pm sieve (l)

Test10-50 50-65

Maximum particle size range (mm) 75-10 75-10% passing 63 pm sieve 5-25 15-35Maximum ratio of % passing 425 pm and 63 pm Not specified 3.5Linear shrinkage ()<10 <6Plasticity index <25 <15Maximum LS x % passing 425 pm 600 500LS x % passing 63 pm Not specified 30-150Minimum 4-day soaked CBR at field density C)50 50Minimum calcium carbonate content (%) of thematerial passing 425 pm 25 25

Notes. (1) After compaction using wet sieve analysis methods

Any views expressed are not necessarilythose of the British Government's Depart-ment of Transport, nor the OverseasDevelopment Administration.

9 REFERENCES

BSI 1975. Methods of tests for soils forcivil engineering purposes. BS 1377.London: British Standards Institution.

Hosking, .J.R. & L.W.Tubey 1969. Researchon low-grade and unsound aggregates.TRRL Laboratory Report 293. Crowthorne:Transport and Road Research Laboratory.

Klaus, K.A. & P.A.Loudon 1971. The in-fluence of the initial consumption oflime on the stabilisation of SouthAfrican road materials. Proc. 5th Reg.Conf. for Africa on Soil Mech. and Fndn.Engn: Launda.

Lawrence, C.J. & T.Toole 1984. The loca-tion, selection and use of calcrete forbituminous road construction in Botswana.TRRL Laboratory Report 1122. Crowthorne:Transport and Road Research Laboratory.

Lionjanga, A.V., Toole, T. & D.Newill 1987.The development of specifications forthe use of calcretes in lightly-

501

trafficked roads in Botswana. 4th. Intl.Conf. on Low Volume Roads, New York:Transportation Research Board.

Mallick, D.I.J., Habgood, F. & A.C.Skinner1979. A geological interpretation ofLansat imagery and air photography ofand air photography of Botswana. Over-seas Geology and Mineral Resources No.56. London: Institute of GeologicalSciences.

Ministry of Works and Communications 1982.Road design manual. Roads Department,MOWC, Gaborone, Botswana.

NIT.RR 1979. Standard methods of testing.road construction materials. NITRRTMH1. Pretoria: National Institutefor Transport and Road Research.

NITRR 1980a. Structural design of inter-urban and rural road pavements. NITRRDraft TRH4. Pretoria: NationalInstitute for Transport and RoadResearch.

NITRR 1980b. Standards for road construc-tion materials. NITRR Draft TRH 14.Pretoria: National Institute forTransport and Road Research.

Netterberg, F. 1971. Calcrete in roadconstruction. NITRR Bulletin 10.Pretoria: National Institute forTransport and Road Research.

Netterberg, F. 1979. Salt damage toroads - an interim guide to its diagnos-is, prevention and repair. Pretoria:National Institute for Transport andRoad Research.

Overby, C. 1982. Material and pavementdesign for sealed low traffic roads inBotswana. NRRL Internal Report No.1042. Oslo: Public Roads Administra-tion.

Paige-Green, P. 1984. A laboratory in-vestigation into the influence of car-bonation on the strength of lime-stabil-ised materials. Proc. 8th Reg. Conf.for Africa on Soil Mech. & Fndn. Engn.Harare, 1, 403-406.

Road Research Laboratory 1952. SoilMechanics for road engineers. London:HM Stationery Office.

Sampsoni, L.R. & F.Netterberg 1984. Acone penetration hiethod for measuringthe liquid limits of South African soilsand its implications. Proc. 8th Reg.Conf. for Africa on Soil Mech. & Fndn.Engn. Harare, 1, 105-114.

Thurmann-Moe, T. & H.Ruistuen 1983.Graded gravel seal (Otta surfacing).Proc. 3rd Intl. Conf. on Low VolumeRoads. Tempe: Transportation ResearchBoard.

TRRL 1977. A guide to the structuraldesign of bitumen-surfaced roads intropical and sub-tropical countries.

TRRL Road Note 31. Crowthorne: Trans-port and Road Research Laboratory.

(C) CROWN COPYRIGHT. Published by per-mission of the Controller of Her BritannicMajesty's Stationery Office.

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