TRANSPORT RESEARCH LABORATORY ICjTHR TITLE by Prevention of salt damage to thin bituminous surfacings: design guidelines B Obika, R J Freer-Newish. M Woodbridge and D Newill .. n- -- t-.1 - ~ . Overseas Centre Transport Research Laboratory Crowthorne Berkshire United Kingdom IA OBIKA,13, R J FREER-HEWISH, M WOODBRIDGE AND D NEWILL. (1 995). Prevention of salt damage to thin bituminous surfacings: design guidelines. In: Sixth International Conference on Low Volume Roads, Minneapolis, Minnesota 25-29 June, 1995. Transportation Research Record, Volume 2. Washington DC: Transportation Research Board, National Research Council, 3-1 2. Prevention of Salt Damage to Thin Bituminous Surfacings: Design Guidelines B. Obika, Botswana Roads Department R. J. Freer-Hewish, University of Birmingham, United Kingdom M. Woodbridge and D. Newill, Transport Research Laboratory, United Kingdom A design method to prevent soluble salt damage to thin bituminous road and runway surfacings based on labora- :ory tests and field trials is proposed. Remedial treatments for damaged surfacings are also suggested. The damage, :haracteristic of warm islands and arid and semiarid inland regions of the world, occurs when salts crystallize in the pavement, physically disrupting the bituminous surfacing and causing premature deterioration of the road. Bitumi- nous primes have been found to be more susceptible to damage than final surface dressings. The design method has been developed from laboratory studies in the United Kingdom and field trials in the West Indies and Botswana. rhey apply wherever a waterbound or a chemically sta- bilized pavement layer is covered with a thin bituminous ;urfacing. Soluble salts in pavement layers, groundwater , or both can migrate upward to the surface and dam- Sage the thin bituminous surfacing. This migration through capillary action is mainly caused by evapora- tion at the surface. At or near the surface, the salts in solution become supersaturated and crystallize. This creates pressures with associated volume changes that can lift and physically degrade the bituminous surfacing and break the adhesion with the underlying pavement layer. The damage may appear in the form of "blister- ing," "doming," "heaving," "fluffing," and "powder- ing" of the bituminous layer. Soluble salt is defined (1) as "basically those minerals that are most soluble, notably salts of magnesium and sodium." Gypsum (calcium sulfate), an abundant salt, is only slightly soluble and does not cause damage. This type of damage has been reported in southern and west- ern Australia, southern and northern Africa, West Indies, India, Chile, United States, and the Mivddle East. By su- perimposing the location of recorded cases of surfacing damage from soluble salts onto Meig's (2) classification map of world climates, it is apparent that salt damage of bituminous surfacings is restricted main--ly to warm islands and arid and semniarid inland regions where evaporation exceeds precipitation. Moisture is drawvn to the surface where any soluble salts present can be precipitated. The published work on soluble salt damage to bitu- minous pavements was reviewed by Obika et al. (3). The published papers deal with local climates and ma- terials, and the recommendations cannot necessarily be universally applied. Whereas very small amounts of sol- uble salt (0.2 percent) can cause damage, higher per- centages have been used without subsequent damage. 3 1 4SIX T~SIXH INTERNATIONAL CONFERENCE ON LOW-VOLUME ROADS There appears to be a requirement to standardize methods of salt content analysis so that maximum salt limits can be compared. The sulfate values of Weinert and Clauss (4) are based on the analysis of water ex- tracts, whereas Fookes and French (5) refer to acid- soluble sulfates. Netterberg et al. (6) quote sulfates as S0, and others [e.g., Weinert and Clauss (4) and Net- terberg (7)] as S0,. To relate S0, to S0,, multiply the SO, value by 1.2. Cole and Lewis (8) used NaCI, and Weinert and Clauss (4) measured chloride. The recommended salt limits refer to the initial salt content of the materials used in the pavement structure. Howevei, tests have shown that, depending on temper- ature and relative humidity, the final salt content at the pavement surface before surfacing can be significantly higher than the initial amount. Ideally, it is desirable to specify the maximum salt content a t the surface before bituminous surfacings. Practically, this value may have to be estimated as discussed later. The design criteria developed in this paper are based on a review of previous work, laboratory simulations in environmental cabinets at the University of Birming- ham in the United Kingdom, salt damage investigation and pilot trials in the West Indies associated with air- field pavement damage, and full-scale trials for a new road in Botswana constructed through known saline natural ground. The laboratory work initiated to examine problems with bituminous surfacing in the West Indies (9) indi- cated that bituminous prime coats were particularly sensitive to damage from small amounts of salts con- tained in the pavement materials or groundwater, whereas bituminous surface dressings were more resis- tant to salt damage. The lack of traffic was considered to be a problem that is particularly important for air- field runways and sealed road shoulders. Field trials in the West Indies and Botswana were con- structed to confirm these findings and also to investigate the benefit of several preventive treatments. The details and results of these trials have been reported by Obika and Freer-Hewish (1 0) and Woodbridge et al. (1 1). OBJECTWvES A design method is proposed to prevent soluble salt damage to thin bituminous road and runway surfacings in warm islands and arid and semiarid inland regions. It is based on laboratory simulation testing of field con- ditions and field trials. FACTORS INFLUENCING SALT DAMAGE The factors influencing salt damage are climate, geology and hydrogeology, materials characteristics, pavement surfacing design, and construction practice. Climate Temperature, relative humidity, wind speed, and rainfall all influence salt damage. They affect evaporation sig- nificantly and hence the potential for upward salt mi- gration. Temperature and relative humidity also deter- mine whether salt crystallization thresholds are crossed. These aspects are discussed by Obika et al. (12). Pre- cipitation influences the net water balance at a given location and also whether there is a seasonal or peren- nial moisture deficiency that could provide the condi- tions for a net upward saline moisture migration. Where rainfall is insufficient to leach out minerals from weathering rocks, in situ. accumulation of mineral salts generally occurs. Geology and Hydrogeology The depth and quality of groundwater contribute sig- nificantly toward creating bituminous surfacing damage from salts. Saline groundwater may result from the so- lution of minerals present in sediments or from the in- gress of seawater to the host material. The predominant type of salt depends on a variety of geochemical pro- cesses, the source of the salt, and the local climate. The most commonly encountered salt in many arid and semiarid zones is sodium chloride, known as halite. In arid and semiarid zones, the capillary moisture rise can be more than 10 m (13). The height of the rise depends on a variety of factors, including porosity and temperature gradients. Materials Characteristics The various salt types that can contribute to the damage of pavements in dry lands include but are not limited to sodium chloride (NaCI), sodium sulfate (NaS0 4), sodium carbonate (Na 2.CO,), and magnesium sulfate (MgS 04). Fine-grained porous materials can encourage delete- rious filamentous crystal growth, and the pore charac- teristics of the individual particles can influence the movement of saline moisture in the pavement layers. Obika et al. (12) discussed the nature and magnitude of salt crystal pressures. For materials of equal mechani- cal strength, those that contain large pores separated from each other by micropores are the most liable to salt weathering. Pavement Surfacing Design The type of bituminous surfacing and its applicationl rate influence the rate of evaporation from the pave- 4 OBIKA ET AL. menit surface and therefore the rate of upward salt mi- gration. Pavement damage from soluble salts appears to be confined to thin bituminous surfaces, generally less than 50 mm thick. However, a few exceptions have been recorded in Algeria (14) and Australia (15). In southern Africa, Netterberg et al. (6) discovered that damage from sulfates in mine waste pavement ma- terial could be prevented by applying a bituminous sur- face seal that had a ratio of permeability to thickness not exceeding 30 (permeability in millimeters per sec- ond, sufacing thickness in millimeters). Thick surfacings minimize evaporation and hence reduce migration and crystallization of salts at the surface. Obika and Freer-Hewish (9) and Woodbridge et al. (1 1) showed that bitumnen emulsion primes perform slightly better than bitumren cutback primes in reducing salt damage. The emulsion "sits` on the surface rather than penetrating into the base, thereby forming a less permeable surface than cutback primes. However, emul- sion generally gives a poorer bond to the underlying pavement layer. Construction Practice The intervals between the construction of the pavement layers-a waterbound or cemented material, a bitumi- nious prime coat, and a final surfacing, such as a surface dressing-can be critical if evaporation is high and if salts are present in the pavement material, the shallow ground- water, or both. Substantial salt accumulation may occur it the exposed surface in periods longer than 24 hr. Brackish water is often used for compaction or cur- nig of pavement layers. This method can lead to a sig- nificant precipitation of salt on the surface of the comn- pacted layer. Also, there is evidence from laboratory studies and field observations to suggest a high risk of surfacing damage when salts in a pavement layer are subjected to repeated wetting and drying (solution and recrystallization). RISK EVALUATION FOR BITUMINOUS SURFACINGS IN SALINE ENVIRONMENTS Salt damage risk evaluation is recommended whenever a thin bituminous surfacing is proposed for a pavement in a warm island or arid or semniarid inland region. Clearly, the damage process is dependent on a complex interaction of many variables, but the proposed design method is based on two significant parameters, salt con- tent and climate, which can be measured relatively easily. Further design parameters can be added as other varia- bles can be linked to the damage process in qualitative terms. The procedure (Figure 1) first allocates values to salt levels in the pavement and subgrade materials, comn- paction, and, in some situations, groundwater and then allocates values to climatic conditions. The values are combined to provide an overall rating that indicates the damage risk for bituminous surfacings. Salinity Levels of Materials and Water Salinity values are required for pavement and subgrade materials, imported and in situ, and compaction and groundwater. Methods for determining salt content are given by Obika and Freer-Hewish (10), and it is im- FIGURE 1 Risk analysis for salt damage to bituminous surfacings. 5 6 ~~~SIXTH INTERNATIONAL CONFERENCE ON LOW-VOLUME ROADS portant to adhere to these methods for consistency and comparability of results. Salt Measurement In the first instance, total soluble salts (TSS) will nor- mally be measured; however, it Is important to ascertain the dominant salt type for more detailed design and construction control, particularly if salt levels are significant. Most of the analytical techniques available to deter- mine salt content are time-consuming, and some require a considerable degree of skill. For this reason, methods have been developed whereby the electrical conductivity (EC) of an aqueous solution of the material is measured using a standardized procedure and the values are re- lated to the percentage of TSS of the sample. The ab- solute relation of EC to salt content is complex, and although it is not possible to obtain a correlation on a global basis, it has been proposed on a regional basis, for example, by Doornkamp et al. (16) in the Middle East. The EC is measured in millisiemens per centimeter (a siemen is the reciprocal of the electrical resistance in ohms) at 25'C, and the TSS content is measured in mass percentage. The correlation between the rapid EC and the TSS measurements for the Botswana field trials, where the TSS was measured according to the method given in BS 1377: Part 3 (17), was TSS = 0.04 + 0.16EC. A corre- lation coefficient of 0.9 was obtained for this relation- ship. All determinations were carried out on the minus 20-mm fraction of the samples, corresponding to about 75 percent by mass of the borrow pit material; it is generally recognized that the major proportion of the salt is contained in the fines. Materials and Water Rating Using the appropriate salt levels for materials and water, the weighting value M is determined from Figure 2. An M value of 10 should be adopted if the pavement or subgrade salinity exceeds 0.8 percent TSS regardless of compaction water salinity. The salt content value used should be the maximum value obtained from the pavement or subgrade materi- als and may be measured in terms of either TSS or EC if calibrated locally for the materials used. Ideally, the salt content at the surface before bituminous surfacing should be used. Climatic Conditions Characteristics of the regional climare, seasonal precipi- tation pattern, and seasonal temperature pattern are required. Climate Rating The project site can be classified regionally as extremely arid to others in Table 1. A value (C1) appropriate to the regional climate is assigned. Similarly, Tables lb and c are used to assign values (C2and C3) appropriate to the seasonal precipitation and temperature variations. The overall rating for the climate (C) is obtained by multiplying the sum of C2and C, by C1. Combined Risk Evaluation (M X C) The combined risk value is obtained by multiplying M by C. For MC values greater than 20, special design and construction measures may be required in order to pre- vent damage to bituminous surfacings. High MC values relate directly to high risk of salt damage. If the MC value is marginal, it is recommended that a detailed in- quiry of the history and performance of existing thin surfacings be implemented. A visual inspection of ex- isting surfacings, particularly for lifting and loss of ad- hesion between the bituminous surfacing and the un- derlying layer, is also desirable. DESIGN PROCEDURES FOR MC > 20 Types of Bituminous Surfacing For reasons discussed earlier, only surfacings less than 50 mim thick appear to have been damaged by salts, and the degree and rate of damage vary according to the type of bituminous surfacing. Selection of Bituminous Primes and Priner Seals The prime surfacings are the most susceptible to dam- age from salt crystallization, primarily because they are the thinnest and the least effective in reducing evapo- ration from the underlying pavement. Damage can oc- cur within 2 days of application. Type and Application Rate Primes made from penetration-grade bitumen cutback with a highly volatile fluid such as kerosene are more susceptible to salt damage than primes made from an emulsion. Although emulsion is useful to alleviate the onset of salt damage, it can create a tacky surface that cannot he trafficked before final surfacing unless dusted with fine aggregate. Increasing the prime application] rate and providing a thicker barrier to reduce evapo-,. 6 1 OBIKA ET AL. 7 Compaction water salinity (see key) / / / / 2 4 6 8 10 M value Key: - -- -- Fresh 0 -0.5% - - - - Brackish 0.5 -1.0% Saline >1.0% For pavement/ subgrade material, with TSS levels in excess of 0.81% use an M value of 10, irrespective of compaction water salinity. Salt values are bulk and not surface values -see Fig. 4. FIGURE 2 Materials risk rating: salt damage to bituminous surfacings. ration from the pavement may also create a tacky surface. The salt thresholds shown in Figure 3 provide guide- lines for the use of either cutback or emulsion primes. The salt values in Figure 3 refer to the surface (0 to 50 mm) of the pavement just before sealing, and Figure 4 provi'des a relationship, obtained from the Botswana field trials, between initial salt content within the pave- menit material at construction and salt content at certain time intervals after construction. Ideally, trials on site to check this relationship are recommended. The values given in Figure 2 for initial risk assessment are initial bulk salt values. Intervals Between Bituminous Surfacing Pavement Layers From the foregoing discussion, it Is clear that the rime intervals between compaction of the road base and the prime and the surface dressing are important. Ideally, primes should be covered immediately if salts are pres- ent in the pavement. 1.0 0 EQ)V0 .02 I-( 0 E0 a. 0.8 -0 U) U) 0.6 (- U) .0 0 I- 0.2 / / / / / / / 5.0~ CD(005 ,0 3.0 0a 2.00 0 2.0 (0CL 0 0 C. 0) 0 0 / E U) E- w 4- 0 4-W / /// / / 0 0 0 1 OBIKA ET AL. 7 1 8 ~~~SIXTH INTERNATIONAL CONFERENCE ON LOW-VOLUME ROADS TABLE 1 Salt Damage: Risk Evaluation for Climate a) Regional Climate: Cl Value Extremely a-dd AridSemid-aridIsland coastal Others b) Precipitation 44650 C2Value No marked season of precipitation 2 Summer precipitation 3 Winter precipitation c) Temperature Range C3Value Zone Ti >300C 3 Zone T2 20- 300C 2 Zone T3 10-200C When the pavement materials have a negligible salt content and there is the possibility of ingress of salt from the water table or subgrade, or both, through cap- illary action, vulnerable primes and primer seals should be covered within a week by a more substantial surfac- ing. Actual rates of moisture rise from the Botswana field trials appeared to be of the order of 5 mm/day (1 0). Figure 3 incorporates the time constraints for vari- ous conditions. Control of salt movement is another op- tion and is considered below. Selection of Permanent Surfacings There is no evidence to indicate salt damage to thick surfacings (> 50 mm), and it is reasonable to assume that no precautions are required for these surfaces. Damage to thin permanent bituminous surfacings takes considerably longer to develop than damage to primes. This period can vary from one week to several years and may depend on the condition of the prime when covered by the permanent surfacing, the type and po- sition of harmful salts in or below the pavement, and trafficking of the surface. The importance of the impermeability of the bitu- minous surfacing as a means of retarding the upward rise of salt was mentioned earlier. Surface dressings ap- pear to be impermeable; however, upward movement of moisture has occurred on some of the Botswana field trial sections. Moisture rise beneath impervious pave- menits has been noted before by Tomlinson (18) and Horta (14). Cracking of a surface caused by shrinkage, oxidation, or traffic would encourage localized evapo- ration and salt crystallization. The salt content thresholds and time intervals be- tween surfacing operations are shown in Figure 3. These values have been designed for protection of primes and are too conservative for the performance of single and double surface dressing seals, but, at present, precise salt-level thresholds for long-term performance of final surfacings have not been substantiated. The maximum salt content thresholds recommended for Botswana are shown in Table 2; the trials are still being monitored for longer-term performance. The Botswana trials and other damrage sites, how- ever, highlight the importance of identifying whether the surfacing will be trafficked or untrafficked. Sealed shoulders and large portions of runway pavement areas are examples of the latter. These trials indicate that stricter limits are required for untrafficked surfaces. The kneading action of traffic on surfacings appears to be very important in prevent- ing damage and increases the resistance of surface dress- ings to salt damage. The road trials in Botswana showed detachment of single sealed shoulders alongside the intact double-seal trafficked carriageway. Control of Salt Movement When salts are inherent in the subgrade or groundwater, an impermeable plastic fabric can be introduced at the subgrade-pavement interface. This method was found to be effective in preventing the rise of saline water to the pavement surface and thus preventing surface dam- age, even though adjacent sections were damaged. A thick bitumen layer placed in the same position was not successful in preventing damage. In Botswana thin bi- tuminous surfacings constructed on saline subgrade! without an impermeable plastic fabric were damaged b) salts. REPArR OF DAMAGED SURFACING Prime Surfaces Damage detected in its early stages can be arrested b, rolling, which may control further blistering until mor layers can be added and adhesion with the underlyin base can be regained. For severe damage, rolling will not be successful an the surface must be broomed to remove the primft When the underlying base is still sound, a new spri technique following the guidelines above can be use( but if the base consists of soft aggregates, broomning cl 8 BITUMINOUS PRIME TYPECUBK(C30 SUBGRADE SALINITY SALT CONTENT AT PAVEMENT SURFACE BEFORE PRIMING. mSlcm at 251C (% TSS) PERMISSIBLEDURATIONTO SEAL <2.0 (0.36) 30 days 2.0 -5.0 >5.0 .d.0 (0.36-0.84) (0.84) (0.36) I I I 2 days seal imm ediately no limit 2.0 -5.0 (0.36-0.84) 2 ays >5.0 (0.84) sel mediately see, not* 2 BITUMINOUS PRIME TYPE SUBGRADE SAUINITY SALT CONTENT AT PAVEMENT SURFACE BEFORE PRIMING. MS/cm, at 250C EMU1 3.4 3.4 -8.0 >8.0 <3.4 3.4 -8.0 (1.32) se li m diately (0.60) no limit (0.60-1.32) (1.32) I I 10 d ys ea l im mebdiately Total of time delays between base compaction and seaing should not exceed 30 days. This is the estimated time taken for selts from the saline eubgrad. to migrate through a 150mm baa.. Thickter pavements may permit longer delays to sea. * 2 Longer duration to seal may be possible if the base surface remains dry with moisture contents typically below 6% FIGURE 3 Permissible intervals between prime and final surfacing in relation to subgrade salinity and pavement surface salinity. (% TSS) PERMISSIBLEDURATIONTO SEAL (0.60) 30dys (0.60-1.32) 5 days NOTES: 1 >0.0 3A factor of safety of 2 should be applied to salt contents. CUTBACK(MC30) 10 ~~~SIXTH INTERNATIONAL CONFERENCE ON LOW-VOLUME ROADS 20 1 9 18 In E U')E E0 0 Cl) c E 0. CL 0Cm c. C.a) 17 16 15 14 13 12 1 1 1 0 9 8 7 6 5 4 3 2 0 0 2 4 6 8 10 1 2 14 Time after compaction (days) FIGURE 4 Electrical conductivity readings at surface of a layer with time for initial conductivity levels of bulk material for Botswana trial site. damage the base surface. It may then be difficult to re- gain the same surface level and quality without scari- fying.to at least 100 mm. Final Bituminous Surfacings Small failures should be treated locally by removing the surface and hand spraying a new surfacing, possibly re- Z placing cutback bitumens with emulsions and increas- ing the application rate without risking severe bleeding. CONCLUSIONS In warm arid and semiarid climates, where eaoaif exceeds precipitation, soluble salts accumulate inthj 10 OBiKA ET AL. 11 TABLE 2 Suggested Minimum Salt Limits for Botswana Project Surface Traffic Type Status SubgradeCondition Maximum jowal soluble salt conical at surface prior 10 priming % TSS 1)0-50 mm sample depth) Emulsion Primc Cutbackr Prime Prime 0.30' 0.20. SinglcSurface Unirafficked Saline 0. 1( 0.1 5 Non Saihne Trafficke~d Saline 0 9)) 0.70 Non Siline > 1,20 >0.80 DoubleSurfaceTieatment Trafficked >2.10 >1.0 Notes: I . A factor o/safely of 2 has been applied. 2. For the Boilsano calcrete TSS % = 0,04 0,16 x Elewtical Condunctivity (E.Cl) 3. Salts ca. be inherent in pase~nie nuoeiaily or iaroduiced .nitil brackishlsalimi coni~a~ctwn ~aer. 4 . if iniiial sali contents only are knou'n. iibtain an estimeile of sue/oce salt conieiitfor ihe appropriate time delay using Fig. 4. Increased if coasiructioii process speeded up to .iaiisfy, Fig. 3.. upper layers of the road pavement and can damage thin bituminous surfacings such as prime coats and surface dressings. The results of laboratory and field studies have identified the importance of climatic factors such as humidity and temperature, intervals between the con- struction of each pavement layer, surfacing types, and trafficking. The design procedure shows that single values of salt limits, as suggested in other reports, are niot appropriate for all surfacing types and construction procedures. A procedure for risk evaluation of potential salt dam- age has been developed based on the laboratory and field trials. Risk ratings are assessed for materials, comn- paction water and groundwater, and climatic conditions for different surfacing types. Bituminous prime coats are very sensitive to salt damage and can be damaged if the soluble salt content exceeds 0.3 percent TSS in the roadbase material as a whole. Cutback prime is more sensitive to damage than emulsion prime. Surface dressings are more resistant than bitumen primes to salt damage because of their increased bitu- men thickness. The Botswana trial indicated that traf- ficked single and double surface dressings would not be damaged by roadbase TSS contents up to 0.5 and 1.0 percent, respectively. Trafficking appears to increase the resistance of surface dressing to salt damage. Surface- dressed road shoulders and large areas of runway are especially vulnerable to salt attack at lower salt content levels. For salt levels at the upper acceptable limits, a prime coat should preferably be excluded, and trials would be advisable to determine the effectiveness of bonding the surface dressing directly onto the roadbase. Alternately, the prime coat could be surface dressed within 2 days of application, but this may be impractical in contract situations. When traffic is withdrawn from hitherto sound sec- tions, the surfacing becomes damaged. Methods of preventing the upward rise of salts in solution were incorporated into the trials in Botswana, and an impermeable fabric (plastic) placed at the bot- tom of the roadbase prevented the upward rise of salt and protected the bituminous surfacing from salt dam- age without compromising road performance. A thick bitumen layer placed in the same position was not successful in this respect. The technique of a cutoff membrane has been applied to the new road in Botswana. Remedial treatments for salt damage consist of roll- ing at early signs of damage followed by further sur- facing immediately or removal of damaged material and replacement to the requirements of the design method described in this paper. ACKNOWLEDGMENTS The research was carried out by the School of Civil En- gineering, University of Birmingham, in collaboration with the Overseas Centre of the U.K. Transport Re- search Laboratory for the Overseas Development Ad- ministration and the Ministry of Works, Transportation and Communications, Botswana. The latter two organi- zations funded the work. Ove Arup and Partners, Bot- swana, and Wallace Evans and Partners, U.K., also sup- ported the work described in the paper. REFERENCES 1. Nerterberg, F., and L. J. Maton. Soluble Salt and pH De- terminations on Highway Materials. Presented at 6th Re- gional Conference for Africa on Soil Mechanics and Foundation Engineering, Durban, 1975. 2. Meigs, P. World Distribution of Arid and Semi-Arid Homoclimates. Arid Zone Hydrology. UNESCO, 1953, pp. 203-209. 3. Obika, B., R. J. Freer-Hewish, and P. J. Fookes. Soluble Salt Damage to Thin Bituminous Road and Runway Sur- faces. Quarterly Journal of Engineering Geology, Vol. 22, 1989, pp. 59-73. 4. Weinert, H. H., and M. A. Clauss. Soluble Salts in Road Foundation. In Proc., 4th Regional Conference for Africa on Soil Mechanics and Foundation Engineering, Cape Town, 1967, pp. 213-218. OBIKA ET AL. 1 1 12 ~~~SIXTH INTERNATIONAL CONFERENCE ON LOW-VOLUME ROADS S. Fookes, P. G., and W. J. French. Soluble Salt Damage to Surfaced Roads in the Middle East. journal of the Institution of Highway Engineers, Vol. 24, No. 12, 1977. 6. Netterberg, F., G. E. Blight, P. F. Theron, and G. P. Mar- ais. Salt Damage to Roads with Bases of Crusher-Run Witwatersrand Quartzite. In Proc., 2nd Conference on Asphalt Pavements for Southern Africa, Durban, 1974, pp. 34-53. 7. Nctterberg, F. Salt Damage to Roads: An Interim Guide to its Diagnosis, Prevention and Repair. Institution of Municipal Engineers of South Africa, NITRR, CSIR, 1979, p. 4. 8. Cole, D. C. H., and J. G. Lewis. Progress Report on the Effect of Soluble Salts on Stability of Compacted Soils. In Proc., 3rd Australia-New Zealand Conference on Soil Mechanics and Foundation Engineering, Sidney, 1960, pp. 29-31. 9. Obika, B., and R. J. Freer-Hewish. Salt Damage to Bitu- minous Surfaces for Highways and Airfield Pavements. Final report. Overseas Development Administration, Uni- versity of Birmingham, 1988. 10. Obika, Bl., and R. J. Freer-Hewish. The Control of Solu- ble Salt Damage to Bituminous Road Surfaces in Tropical Environments. Final report. Overseas Unit, Transport and Road Research Laboratory, University of Birming- ham, 1991. ii. Woodbridge, M., B. Obika, D. Newill, and R. J. Freer- Hewish. Proc., 6th Conference on Asphalt Pavements for Southern Africa, Cape Town, 1994. 12. Obika, B., R. J. Freer-Hewish, and D. Newill. Physico- Chemical Aspects of Soluble Salt Damage to Thin Bitu- minous Road Surfacing. Presented at International Con- ference on the Influence of Ground Chemistry in Construction, IGCC '92, Bristol University, 1992. 13. Cooke, R. U., D. Brunsden, J. C. Doornkamp, and D. K. C. Jones. Urban Geomorphology in Dry Lands. Oxford University Press, 1982. 14. Horta, J. C. de 0. 5. Salt Heaving in the Sahara. Geo- technique, Vol. 35, No. 3, 1985, pp. 329-337. 15. Januszke, R. M., and E. H. S. Booth. Soluble Salt Dam- age to Sprayed Seals on the Stuart Highway. In Proc., 12th Australian Road Research Board Conference (ARRB), Part 3, 1984, pp. 18-30. 16. Doornkamp, J. C., and H. A. M. Ibrahim. Electrical Con- ductivity and Saline Concentrations in Arid Land Groundwaters. Quarterly Journal of Engineering Geol- ogy, Vol. 19, 1986, pp. 249-250. 17. British Standard Methods of Test for Soils for Civil En- gineering Purposes, Part 4: Compaction-Related Tests. British Standards Institution, 1990. 18. Tomlinson, M. J. Engineering Problems Associated with Ground Conditions in the Middle East. Quarterly Journal of Engineering Geology, Vol. 2, 1978. 12