stabilization of dune sand using foamed asphalt

168 Copyright © 2002 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.

REFERENCE: Asi, I. M., Al-Abdul Wahhab, H. I. A., Al-Amoudi, O. S. B., Khan, M. I., and Siddiqi, Z., “Stabilization ofDune Sand Using Foamed Asphalt,” Geotechnical Testing Jour-nal, GTJODJ, Vol. 25, No. 2, June 2002, pp. 168–176.

ABSTRACT: Foamed asphalt technology has increasingly gainedacceptance as an effective and economical soil improvement andstabilization technique, mainly because of its improved aggregatepenetration, coating capabilities, and handling and compactioncharacteristics. This laboratory research program was carried out toinvestigate the feasible use of foamed asphalt technology in SaudiArabia to improve the prevalent dune sands for possible use as abase or subbase material. Several variables were investigated toevaluate the relative improvement of dune sand as well as to permitthe development of design procedures for the future use of foamedasphalt technology in the harsh climatic conditions of eastern SaudiArabia. Statistical analysis of the results was employed to verify theeffects of emulsified asphalt and foamed asphalt treatment, with andwithout the addition of Portland cement, on the strength character-istics of the treated mixes. The results displayed significant im-provement in the performance of dune sand-foamed asphalt mixes,as compared to that of the emulsified asphalt mixes.

KEYWORDS: foamed asphalt, emulsified asphalt, dune sand,soaking, Portland cement

Introduction

In the 1950s, the production and use of foamed asphalt were firststudied and evaluated by Csanyi (1957, 1959, 1960, 1962), whichessentially consisted of introducing steam into hot asphalt in orderto transform it temporarily into foam. Later, Mobil (1973) modifiedthis process by replacing the steam with a controlled quantity ofcold water through a pressurized spraying nozzle, fitted inside asuitable mixing chamber. Normally, foaming asphalt is a method ofreducing the viscosity of asphalt to allow dramatic improvement inwetting the asphalt-aggregate interface. This is achieved by addinga carefully metered quantity of cold water into hot asphalt that sub-stantially increases the surface area of the asphalt and lowers the in-terfacial tension between the asphalt particles by the formation offoam (A. A. Loudon & Partners 1996a, 1996b, Abel and Hines

1979). This facilitates stiffened road-grade asphalt (now foamed)to be mixed and dispersed properly and efficiently with cold, moistaggregate, thereby creating a strong physical bond between the as-phalt and the aggregate surface without the need for evaporation(Lee 1981). The application of foamed asphalt allows the econom-ical use of sand or coarse aggregate to obtain a mix with a desirableplasticity, extended functionality, and long-term durability. Hence,the resulting foamed asphalt mix is ready to be compacted and canbe opened directly to traffic without any delay. The mix remainssoft and brown until compaction, after which it becomes harder andsomewhat blacker, ultimately reaching stiffness comparable to reg-ular asphaltic mixes (Soter International 1994).

Foamed asphalt may be used to stabilize indigenous soils and/oraggregates, such as sands, gravel, or fine crushed rock by produc-ing sufficient cohesion between the moist aggregate particles. Athigher asphalt levels, foamed asphalt may also be used to generateasphalt mixes without the need of conventional heavy and costlyhot-mix equipment. Furthermore, foamed asphalt offers an inex-pensive means of stabilization by incorporating asphalt into un-treated soils as compared to emulsified or cut-back asphalt that re-quires preprocessing, transportation, and evaporation (Boweringand Martin 1976). Advantage was taken recently of opening to traf-fic the access road for the Shaibah oil fields in Saudi Arabia di-rectly after compaction of the foamed asphalt. This 350-km roadwas built over sabkha subgrade, utilizing marl soils together withfoamed asphalt. The road is located in the extremely arid and hotclimate of Al-Rub Al-Khali desert (Al-Hilal 1997).

In eastern Saudi Arabia, there are four main types of soil: marls,sands, sabkhas, and expansive clays. The last one is not used inconstruction for obvious reasons, while marl soils are often usedwithout the need for further treatment (Aiban et al. 1997). Thesabkha soils have been chemically stabilized successfully (Al-Amoudi et al. 1995), and there is presently an investigation on theuse of foamed asphalt to improve the performance of these soils(Asi 2001). In contrast to other soils, dune and beach sands have re-ceived little attention by the construction industry, despite theirprevalence in the region. The Arabian Peninsula has three largedeserts covering about 40% of the peninsula area. Sands cannot beused directly in construction because they do not have any cohe-sion. High cement contents may be required to stabilize sands,thereby making the improvement uneconomical and impractical.Therefore, other means of sand stabilization are needed to upgradetheir performance in order to use them for the construction of baseor subbase layers in the harsh arid desert climate.

This paper presents the results of an investigation on the feasibleuse of foamed asphalt techniques to stabilize dune sands. These re-

Ibrahim M. Asi,1� Hamad I. Al-Abdul Wahhab,1 Omar S. Baghabra Al-Amoudi,1* Mohammad I. Khan,2 and Zakiuddin Siddiqi2

Stabilization of Dune Sand Using Foamed Asphalt

1 Assistant professor, professor, and associate professor, respectively, De-partment of Civil Engineering, King Fahd University of Petroleum and Miner-als, Dhahran 31261, Saudi Arabia.

2 Traffic Engineers, DOT / 405-9 Crescent Place, Toronto, Canada ON M4C5L8.

� Presently in the Hashemite University, Amman, Jordan.* Corresponding Author, e-mail: [email protected], fax: �966 (3) 860-

2879.

www.astm.org

Copyright by ASTM Int'l (all rights reserved); Tue Dec 13 09:16:02 EST 2011Downloaded/printed byDimitris Solomos (THEMELI+S.A.) pursuant to License Agreement. No further reproductions authorized.

ASI ET AL. ON FOAMED ASPHALT 169

sults were compared with those of emulsified asphalt mixes andused to determine an optimum mix design utilizing statistical anal-yses.

Materials Used

Dune sand from the Al-Jafurah area located in the ArabianGulf vicinity was used in this investigation. The gradation curveof this aggregate is shown in Fig. 1. The basic characteristics ofthe aggregate are summarized in Table 1. In addition, ordinaryPortland cement, OPC, Type I (ASTM Standard Specification forPortland Cement: C150-00), was used to enhance the bondingcharacteristics of aggregate and to reduce its sensitivity to mois-ture.

Asphalt cement from a local refinery was used in this investiga-tion. The properties of this asphalt cement are shown in Table 1.Further, a locally manufactured cationic, slow setting asphalt emul-sion with hard base asphalt (Css-1h) was used. The emulsion prop-erties conformed to the ASTM Standard Specification for CationicEmulsified Asphalt (D2397-98). Regular potable water was used inall mixes.

The percentages of foamed asphalt, emulsion, water, and OPCwere selected based on the dry weight of aggregate.

Experimental Program and Mix Design

This study consisted of two phases. In the first phase, foamed as-phalt mixes were prepared and evaluated, while the second phasewas devoted to the preparation and evaluation of emulsified asphaltmixes. In both phases, the effect of asphalt content and type as wellas the addition of 2% OPC were studied. Plain foamed asphalt

FIG. 1—Gradation curve of the untreated dune sand.

TABLE 1—Properties of the materials used in this investigation.

Physical Propertiesand Test Designation Value ASTM Limits

Dune SandSand equivalent, ASTM D 2419 79% —Bulk specific gravity, 2.593 —

ASTM C 128Apparent specific gravity, 2.661 —

ASTM C 128Water absorption, ASTM C 128 0.9% —Plasticity index, AASHTO T-88 Non-Plastic —Asphalt CementSpecific gravity at 25°C

(ASTM D 70) 1.0206 —Penetration at 25°C

(ASTM D 5), dmm 51 60–70Kinematic viscosity at 135°C

(AASHTO T-202), Cst 463 —Absolute viscosity at 60°C

(AASHTO T-202), poise 2935 —Rotational viscosity at 135°C

(SHRP), Pa.s 0.500 —Softening point in (ASTM D 36), °C 51 49–54Flash point, Cleveland Open Cup, 326 232 MinimumDuctility at 25°C (ASTM D 113) 150� —Solubility in tri-chloroethylene

(ASTM D 2040) 99.8 99.8 Minimum

—: not specified.


170 GEOTECHNICAL TESTING JOURNAL

mixes are termed “FA” and foamed asphalt mixes with 2% OPCare defined as “FAC” mixes. In the case of emulsified asphaltmixes, “EAC” corresponds to 2% OPC, whereas “EA” correspondsto plain emulsified asphalt mixes.

Phase I (Foamed Asphalt Mixes)

Specific standards to develop, test, and evaluate foamed asphalt(FA) mixes, such as ASTM and AASHTO standards, are still lack-ing. However, some private companies and laboratories, such asSoter International (1995) and A. A. Loudon & Partners (1996a,1996b), have developed their own testing procedures according totheir field experience. The most common practice in the develop-ment of FA mixes is to: (1) optimize the FA properties for a partic-ular type of asphalt, (2) prepare the aggregate samples for treatmentwith FA, (3) treat the aggregate samples with FA, (4) determine thebinder content, (5) compact and cure the Marshall-sized briquettespecimens (ASTM Test Method for Resistance of Plastic Flow ofBituminous Mixtures Using Marshall Apparatus: D1559), (6) checkthe bulk relative densities of briquettes, and (7) test the specimensfor soaked and unsoaked indirect tensile strengths, ITS (ASTMStandard Test Method for Indirect Tension Test for Resilient Mod-ulus of Bituminous Mixtures: D4123-82 (1995)). Such a methodol-ogy was adopted in this study. The objective of Phase I was to de-termine the optimum binder content of the FA-treated material, aswell as to determine the quality of the treated material based on theITS values. Other tests, such as Marshall stability (ASTM D 1559)and resilient modulus (ASTM D 4123), were also performed to pre-dict the performance of FA mixes under actual service conditions.

For the sake of this study, a laboratory-scale plant, capable of pro-ducing foamed asphalt at the rate of 120 grams per second, having

a thermostatically controlled kettle holding a mass of 10 kg of as-phalt at a maximum temperature of 250°C, and with a low-pressurecompressed air supply, was utilized. This plant was designed so asto discharge the foam directly into the mixing bowl (Fig. 2).

As a first step, FA properties were optimized. The following twoparameters were studied to assist in characterizing the FA mixes:expansion ratio, which is defined as the ratio of the maximum vol-ume of the asphalt in its foamed state to the volume of the asphaltonce the foam has completely subsided, and half-life, which is de-scribed as the time in seconds the foam takes to settle to one-half ofthe maximum volume which was attained immediately after foam-ing. Both of these factors are influenced by the type of asphalt andthe amount of water injected (A. A. Loudon & Partners 1996a,1996b). An increase in the amount of injected water normally in-creases the expansion ratio but simultaneously reduces the half-lifeof the foam. Various percentages of injected water, relative to as-phalt content, were tested in order to determine the optimum ex-pansion ratio and half-life. The aim is to select an optimum watercontent to give the maximum possible foam expansion ratio and, atthe same time, the longest possible half-life. This will maximizecoating of the sand particles and will simultaneously increase theallowable duration of mixing. The results of these qualificationtests are plotted in Fig. 3. The optimum amount of water injectionfor the used asphalt cement was found to be 2.8%.

The modified Proctor compaction test (ASTM Test Method forLaboratory Compaction Characteristics of Soil Using Modified Ef-fort: D1557-91(1998)) was carried out on sand, to determine themaximum dry density (�dmax) and optimum moisture content(OMC). The curve of the California bearing ratio (CBR) test results(ASTM Standard Test Method for CBR (California Bearing Ratio)of Laboratory-Compacted Soils: D1883-99) was superimposed on

FIG. 2—Laboratory-scale foam asphalt plant.



the compaction test results (Fig. 4), to identify the strength behav-ior of the material under the same range of moisture contents. Nor-mally, dune sand has relatively lower moisture susceptibility whencompared to silty or clayey materials. The results in Fig. 4 indicatethat the OMC value, as well as the maximum CBR value, wasfound to occur at a moisture content of about 9%.

Testing (Phase I)

First, the sand plus 2% OPC were mixed thoroughly, and the cor-responding moisture was added to bring the sand aggregate to itsOMC. The foam production unit was calibrated for the amount ofasphalt discharged per unit time before the sample preparation. The

FIG. 3—Effect of moisture content on the expansion ratio and half-life of the foamed asphalt.

FIG. 4—Effect of moisture content on the dry density and CBR of the dune sand.



specified volume of foamed asphalt was then discharged directlyon the sand mixture while being agitated in a Hobart laboratorymixer. The actual amount of asphalt cement was determined eachtime by the weight difference of the mixing bowl before and afterthe addition of FA. After proper mixing (normally for one minute)and asphalt content determination, Marshall samples were cast us-ing 75 blows of the standard Marshall hammer per face. These sam-ples were cured and soaked, after which they were subjected toMarshall stability and split tensile strength tests. The results ofsoaked ITS were considered as the primary criterion for the selec-tion of the optimum mix design.

Phase II (Emulsified Asphalt Mixes)

Unlike FA, emulsified asphalt (EA) mixes depend on the evapo-ration of water for the development of their curing and adherencecharacteristics, thereby making the prevailing environmental con-ditions considerably important. Water displacement can be fairlyrapid under favorable conditions. However, high humidity, lowtemperature, or rainfall soon after the application of the EA can de-ter proper curing (Root 1979).

In this investigation, the Illinois method for EA-aggregate coldmix design was utilized (Darter et al. 1978) to determine the opti-mum EA mixes, using class Css-1h as the emulsion type. Theamount of water present in the EA was considered in the design ofthe EA mixes.

Testing (Phase II)

To achieve the optimum EA content, the following steps werefollowed, as recommended by the Illinois method (Darter et al.1978): First, the total water content was maintained at the prede-termined OMC value determined from the compaction test results;the mixes were prepared with varying EA contents. As a result, theamount of water that was required to be added to keep the total wa-

ter quantity constant at OMC was varied. The required quantity ofwater was added to the aggregate in a thin stream. The mix was ag-itated by hand to allow initial dispersion of water, and thereafter,by the mechanical mixer for 30 s. The required amount of EA wasthen added to the wetted aggregate. Mixing was again carried out,first by hand, and then by the mechanical mixer for another 30 s.After the final mixing, the mix was compacted using the Marshallcompactor by applying 75 blows on each side of the specimen. Thecompacted specimens were then cured and soaked according to theIllinois method before testing. A soaked stability test was used todetermine the optimum residual content of the asphalt (Boweringand Martin 1976). The optimized mixes were evaluated further, us-ing indirect tensile strength, ITS, for soaked and unsoaked samples.The results were used to calculate retained ITS, which is the ratioof soaked ITS values to unsoaked ITS values.

Experimental Results and Discussion

Figure 5 shows the ITS properties of FA specimens. In FAmixes, the maximum ITS occurred at 7% and 8% asphalt contentfor soaked and unsoaked conditions, respectively. Due to the detri-mental effect of saturation on the performance of pavements,which results in stripping of asphalt, soaked conditions are nor-mally used to evaluate the optimum mixes. Hence, the 7% asphaltcontent would be selected for FA mixes. The same ITS trend wasobserved for FAC mixes (Fig. 5). However, FAC mixes had un-doubtedly produced significant improvement over the FA mixes asfar as the ITS values are concerned (18% improvement for soakedsamples and 16% improvement for unsoaked samples). Further, theretained ITS at 7% asphalt content for FAC mixes was 83% (i.e.,125 kPa out of 145 kPa), and 86% for FA mixes. Similar findingson the asphalt content and retained ITS value (i.e., 7% and 85%, re-spectively) have recently been reported when stabilizing Sabkhasoils (Asi 2001). The increase in the soaked ITS results can be re-

FIG. 5—ITS results for FA and FAC mixes.



lated to the addition of OPC, which has improved the aggregatebonding characteristics and reduced the moisture susceptibility ofFA mixes, thereby mitigating the stripping (detachment of asphaltcover) effect of water. For evaluation purposes, the Marshall sta-bility test was performed on FAC mixes, and the results indicatethat the maximum soaked and retained stability occurred at 8% as-phalt content, as shown in Fig. 6. The percent increase in the

soaked and retained stability from 7% to 8% asphalt content wasnot significant. Hence, the optimum asphalt content can be safelytaken as 7% for FAC mixes. The resilient modulus (ASTM D4123) value, using a dynamic loading of 1.0 Hz for the 7% asphalt-FAC mixes, was found to be 1745 MPa.

Similar to the FA mixes, two types of EA mixes were evaluated:with and without the addition of 2% OPC. As shown in Fig. 7, the

FIG. 6—Marshall stability test results for FAC mixes.

FIG. 7—Effect of residual asphalt content on the dry and soaked stability for EA and EAC mixes.


maximum soaked stability of EA mixes was 4.9 kN at a residual as-phalt content of 5.7%. In the case of EAC mixes, the maximumsoaked stabilities were 5.5 kN and 5.0%, respectively, indicating aslightly higher soaked stability and less residual asphalt content ascompared to the EA mixes. This indicated that the addition of OPCimproved the soaked stability. As far as optimum design is con-cerned, EAC mixes have shown improvement in both soaked andretained stability (as compared to EA mixes), leading to lowermoisture susceptibility. It is interesting to note that the dry stabilityof all EA mixes was higher than that of the FAC mixes. Neverthe-less, the soaked stability values of EA mixes were significantlylower than those of the FA mixes. This is attributed to the fact thatFA has higher viscosity than EA, leading to a thicker film of FAthan in the case of EA, thereby making FA mixes less sensitive tomoisture damage and leading to higher values of soaked stability.On the contrary, EA mixes contain lesser asphalt cement and pro-vide thinner coating than FA, but due to its lower viscosity the EAcovers all the sand particles. The result is a higher dry stability.Upon soaking, the asphalt film is stripped, making the EA mixmore sensitive to moisture damage. Considering the soaked stabil-ity as the appropriate criterion to determine the optimum mixes,EAC mix with 5% residual asphalt content was taken as the opti-mum mix. The value of the resilient modulus of the optimum EAmix was 1500 MPa, which is somewhat less than that of the opti-mum FAC mix, which was 1745 MPa.

Statistical Analysis

A statistical methodology in the form of an analysis of vari-ance, using two-factor factorial analysis, was employed to verifythe significance and reliability of the main variables in this study(FA, FAC, EA, and EAC), as well as the asphalt content on ITSand stability. The analysis of variance for each type of treatmentis summarized in Table 2. The two-factor analysis was used inthis study: the first factor includes the four treatment types, i.e.,FA, FAC, EA, and EAC, and the second factor includes the as-phalt content, depending upon the treatment type. All the testswere evaluated at the 5% level of significance. In this analysis,both factors are of equal interest. Specifically, the interest was totest the hypothesis about the equality of treatment effects (Hinesand Montgomery 1990; Montgomery 1991; Anderson and Mclean1974):

H0 : �1 � �2 � .......... � �n � 0 (1)

H1: at least one �i � 0 (2)

And the equality of column (asphalt content) effects:

H0 : �1 � �2 � .......... � �n � 0 (3)

H1: at least one �i � 0 (4)

where

H0 � null hypothesisH1 � alternate hypothesis�i � mean value of the index (ITS or stability) using treatment

type i�i � mean value of the index (ITS or stability) using asphalt

content i


The results of the analysis of variance disclosed the fact that, inthe case of FA mixes for both soaked and unsoaked ITS results, thevariation in treatment type, as well as the asphalt content, has sig-nificant impacts on ITS of FA mixes (F-value, which is used fortesting the hypothesis, is considerably greater than F-critical,which is the limit for accepting the null hypothesis). Moreover, thesignificant impacts are further evidenced by the low P-values(probability of accepting the null hypothesis), which are lower than0.8%. The effect of variation in the asphalt content is more signif-icant (F-values of effect of asphalt content are greater than those ofthe effect of treatment type, i.e., 39 and 80 versus 16 and 30, re-spectively). This means that the quantity of asphalt cement plays avery important role in the performance (both soaked and unsoakedITS) of treated mixes. On the other hand, the addition of OPC hassignificantly decreased the water sensitivity of mixes as the F-valueof soaked ITS, 80.11, was greater, nearly double that of unsoakedITS, 39.24.

In the case of EA mixes, the behavior was observed to be differ-ent. As can be seen from Table 2, the effect of asphalt content onthe soaked stability was insignificant (i.e., the F-value, 1.47, wasless than F-critical, 9.28), whereas the treatment type (addition ofOPC) showed significant effect on the soaked stability. This meansthat the addition of OPC to EA mixes has also decreased the watersensitivity. In the case of unsoaked stability, although the treatmenttype has significant effect (F-value, 10.99, is slightly lower than F-critical, 10.13), the effect of asphalt content was much higher. Ac-cording to the Illinois design method, soaked stability should beused for design considerations. Since the addition of OPC alwaysincreases the soaked stability, OPC should be considered as part ofall the optimum treatment types for both FAC and EAC mixes.When comparing the soaked stability of both FA and EA mixes, theeffect of treatment type was more significant than that of the as-phalt content. Hence, it is preferable to change the treatment typethan merely to change the asphalt content in order to improve theperformance of treated mixes. The analysis of variance indicatesthat FAC mixes have better ITS and stability characteristics thanthe other treatment types (i.e., FA, EA, and EAC mixes). There-fore, FAC mixes can reliably stabilize dune sands to be used in thelayers of the pavement structures, particularly for the low volumeroads in hot countries.

Summary and Conclusions

This research was conducted to evaluate the performance offoamed and emulsified asphalt mixes for stabilizing local dunesands with and without 2% ordinary Portland cement. Based on thefindings of this investigation, the following conclusions can bedrawn:

1. Foamed asphalt (FA) treatment significantly improved theoverall performance of treated mixes and can be effectivelyused for the stabilization of dune sands.

2. Emulsified asphalt (EA) treatment gave satisfactory results.However, according to the results of soaked stability and re-silient modulus tests, which are normally used for design pur-poses, EA mixes exhibited inferior performance as comparedto FA mixes.

3. In all the cases, the addition of ordinary Portland cementgreatly reduced the water sensitivity and enhanced thestrength characteristics of the treated materials.

4. The effect of asphalt content, regardless of the treatment type,on the performance of treated mixes was significant in most



of the cases. However, the statistical analysis revealed thatthe change of treatment type, from EAC to FAC, showedhigher effect on ITS and stability values.

5. The best performance was attained by foamed asphalt, with7% asphalt content, plus 2% Ordinary Portland cement.

Acknowledgments

The authors wish to acknowledge the support provided by KingFahd University of Petroleum and Minerals during this research.Thanks are extended to Mohammad Saud Al-Subai Est., Wirtgen

Gmbh Co., and Abdullah Abdul-Mohsen Al-Khodary Sons Co., forpresenting the laboratory-scale foam asphalt producing plant.

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stabilization of dune sand using foamed asphalt

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