Roads Department - Government of Botswana

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Roads Department 5. PREVENTATIVE DESIGN PROCEDURES FOR MC > 20 5.1 Types of Bituminous Surfacing For reasons discussed earlier, only surfacings less than 50 mm thick are normally damaged by salts, and the degree and rate of damage varies according to the type of bituminous surfacing. 5.2 Selection of Bituminous Primes The prime surfacings are the most susceptible to damage from salt crystallisation, primarily because they are the thinnest surfacings and the least effective in reducing evaporation from the underlying pavement. Damage can occur within two days of application. Prime 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. Whilst the use of emulsion is useful to alleviate the onset of salt damage, it can create a ‘tacky’ surface, which cannot be trafficked prior to final surfacing, unless dusted with fine aggregate. Increasing the prime application rate and hence providing a thicker barrier to reduce evaporation from the pavement may also create a “tacky” surface. The salt threshold values showen in Figure 5.1 provide guidelines for the use of either cutback or emulsion primes. For long term performance of final surfacings, the maximum salt content thresholds recommended for Botswana are shown in Table 5.1. which refer to the surface (0-50 mm) of the pavement just before sealing. Figure 5.1. Provides a relationship, obtained from Botswana field trials, between initial salt content within the pavement material at the time of construction and salt content at certain time intervals after construction. Ideally, trials on site to check this relationship are recommended. The salt content thresholds and time intervals between surfacing operations, as given in figure 5.1, have been designed for protection of primes. Note that the values presented in Figure 5.2. (used for initial risk assessment) are initial bulk salt values. Cattle will congregate and lick salt from the road shoulders if salts are present in materials. This causes a traffic hazard as well as erosion of the road shoulder. Caution Salt values at Surface (0-50mm) are required for ascertaining appropriate preventative measures whereas bulk salt content (i.e from sample of whole material) is used for the initial risk assessment. Construction - Time Intervals between bituminous surfacing and construction of pavement layers From the above, it is clear that the time 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 present in the pavement. In the situation when the pavement materials have a negligible salt content and there is the possibility of ingress of salt from the water table and/or subgrade through capillary action, then vulnerable primes and primer seals should be covered within a week by a more substantial surfacing. Figure 5.1 incorporates the time constraints for various conditions. Control of salt movement is another option and is considered below. Actual rates of saline moisture rise in pavement from the Botswana field trials appeared to be of the order of 5-mm/day. Guide to the Prevention and Repair of Salt Damage to Roads and Runways Chapter 45 Preventative Risk Evaluation Design in Procdures Saline Environments for MC > 20 33
Roads Department Double Otta Seals (between 30 - 40 mm in thickness) are relatively impermeable. In areas where there is a potential for salt damage, Otta seals should be considered as an alternative to other seals 5.3 Selection of Permanent Surfacings 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 position of harmful salts in or below the pavement and trafficking of the surface. The importance of the impermeability of the bituminous surfacing as a means of retarding the upward rise of salt was mentioned earlier. Surface dressings appear to be impermeable, however, upward movement of moisture has occurred on roads in Botswana field with double surface dressing. Cracking of a surface caused by shrinkage, oxidation and/or traffic encourages localised evaporation and salt crystallisation. Stricter limits are required for untrafficked surfaces. The kneading action of traffic on surfacings is very important in preventing damage, and increases the resistance of surface dressings to salt damage. Cases of salt damage in Botswana show detachment of single sealed shoulders alongside intact double seal trafficked carriageway. 5.4 Control of Salt Movement When the salts are inherent in the subgrade and/or groundwater an impermeable plastic fabric can be introduced at the subgrade/pavement interface. This has been found to be effective in preventing saline water rising to the pavement surface, thus preventing surface damage. A thick bitumen layer placed in the same position is usually not successful in preventing damage. BITUMINOUS PRIME TYPE CUTBACK (MC 30) Shoulder deterioration can also be caused by other factors such as insufficient binder spray rate. SUBGRADE SALINITY SALT CONTENT AT PAVEMENT SURFACE BEFORE PRIMING. m S/cm at 25 o C (% TSS) PERMISSIBLE DURATION TO SEAL SALINE SUBGRADE NON SALINE SUBGRADE 5.0 5.0 (0.36) (0.36 - 0.84) (0.84) (0.36) (0.36 - 0.84) (0.84) 30 days 2 days seal immediately no limit 2 days seal immediately See note 2 Bituminous surfacings constructed on saline subgrades without an impermeable plastic fabric have a high probability that salt damage will occur to the surfacing. BITUMINOUS PRIME TYPE SUBGRADE SALINITY SALT CONTENT AT PAVEMENT SURFACE BEFORE PRIMING. m S/cm at 25 o C (% TSS) PERMISSIBLE DURATION TO SEAL SALINE SUBGRADE EMULSION (Cationic) NON SALINE SUBGRADE 8.0 8.0 (0.60) (0.60 - 1.32) (1.32) (0.60) (0.60 - 1.32) (1.32) 30 days 5 days seal immediately no limit 10 days seal immediately Figure 5.1 Permissible intervals between prime and surfacing in relation to subgrade salinity and pavement surface salinity. 34 Guide to the Prevention and Repair of Salt Damage to Roads and Runways 34 Chapter 45 Risk Preventative Evaluation Design in Saline Procdures Environments for MC > 20
Page 2 and 3: Roads Department The Prevention and
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Page 6 and 7: Roads Department 3.3.3 Field Determ
Page 8 and 9: FOREWORD Roads Department The preve
Page 10 and 11: Roads Department 1. INTRODUCTION 1.
Page 12 and 13: Roads Department 2. OCCURRENCE AND
Page 14 and 15: Roads Department Preliminary field
Page 16 and 17: Roads Department tained up to 0.5%
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Page 26 and 27: Roads Department 3. LABORATORY AND
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Page 44 and 45: Roads Department APPENDIX B Appendi
Page 46 and 47: Roads Department Appendices Stage 3
Page 48 and 49: Roads Department APPENDIX E - ABBRE

Roads Department - Government of Botswana

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