Roads Department - Government of Botswana

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Roads Department to temperature. The solubility of sodium chloride, the most ‘common’ salt, is only slightly temperature-dependant, whereas other salts show rapid changes in solubility with change in temperature. In practice this means that these salts can re-crystallise several times a day with disruptive crystal growth pressures at the road surface. 60 50 Na 2 SO 4 Solubility of salt per 100g. H 2 O. 40 30 20 10 Na 2 SO 4 7H 2 O Mg SO 4 6H 2 O Mg SO 4 7H 2 O Na 2 CO 3 10H 2 O Na 2 SO 4 10H 2 O TRANSITION POINTS Mg SO 4 Na 2 CO 3 10 H 2 O + Na 2 CO 3 H 2 O Na 2 CO 3 10 H 2 O + Na 2 CO 3 7H 2 O Mg SO 4 6H 2 O Na 2 CO 3 H 2 O Na C1 Na 2 SO 4 Mg 2 CO 3 Mg SO 4 H 2 O Na 2 CO 3 0 10 20 30 40 50 60 70 80 90 10 TEMPERATURE ( O C ) Figure 2.4 Solubility of some natural salts in relation to temperature Sodium chloride (NaCl) crystals are stable at relative humidity below 76%. Above this humidity (typically at night) the crystals will attract moisture from the air (hygroscopic) and go into solution. As the humidity drops during midday they re-crystallise creating disruptive pressures sufficient to disintegrate road surfacings. The magnitude of salt crystal pressures generated by crystal growth is sufficient to heave over one metre thick concrete slabs and buildings. Prevention of salt damage to roads must, therefore, rely solely on methods of stopping migration and crystallisation of salts. See Pincher and Hawkins, 1986. for more detail on the magnitude of salt crystal pressures. An alternative is to ensure that supersaturation does not occur. High supersaturation results in the formation of the most deleterious type of salt crystals with high growth pressures. These crystals are known as filamentous crystals or salt ‘whiskers’. Obika (1989, 1992) provides a detailed description of the physico-chemical mechanisms of salt damage including the magnitude of crystal pressures which are outside the scope of this guideline. Guide to the Prevention and Repair of Salt Damage to Roads and Runways Chapter 2 Occurrence and Characteristics 23
Roads Department Cubic NaCl crystals cause little damage. Whisker NaCl crystals cause maximum damage. 24 Chapter 2 Guide to the Prevention and Repair of Salt Damage to Roads and Runways Occurrence and Characteristics
Page 2 and 3: Roads Department The Prevention and
Page 4 and 5: Roads Department ROADS DEPARTMENT U
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%
Page 18 and 19: Roads Department In addition to the
Page 20 and 21: Roads Department The various salt t
Page 22 and 23: Roads Department 2.5.4 Pavement Sur
Page 26 and 27: Roads Department 3. LABORATORY AND
Page 28 and 29: Roads Department E.C can also be co
Page 30 and 31: Roads Department 4. RISK EVALUATION
Page 32 and 33: Roads Department The salt content v
Page 34 and 35: Roads Department 5. PREVENTATIVE DE
Page 36 and 37: Roads Department 20 19 12 11 10 9 8
Page 38 and 39: Roads Department 6. REPAIR OF DAMAG
Page 40 and 41: Roads Department 8. REFERENCES Refe
Page 42 and 43: 41 Guide to the Prevention and Repa
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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