Recommendations About Overlays on Cement Concrete Pavements
1977 Edition

IRC SP 17 recommends specific overlay types based on traffic intensity: for light traffic (under 150 commercial vehicles/day), options include 7.5 cm bituminous macadam topped with 4 cm asphalt concrete or a 15 cm granular layer with 4 cm asphalt concrete. For medium to heavy traffic (151-1500 CV/day), combinations such as 7.5 cm bituminous macadam with 2 cm premix and seal coat, built-up spray grout with premix, or granular layers with asphalt concrete are advised. Very heavy traffic zones (above 1500 CV/day) require thicker granular layers, typically 7.5 cm water bound macadam plus 7.5 cm built-up spray grout, or alternatively coated macadam with bitumen content around 2.5-3%. Prior to overlay application, it is essential to treat rocking slabs, mud-pumping, and seal existing cracks with bituminous materials to ensure durability.

Prior to overlay application, existing concrete slabs should be thoroughly assessed and treated as per IRC SP 17. Defective slabs exhibiting rocking or mud-pumping must be repaired or removed. Broken slab material may be reused as base aggregate with bituminous grouting (approximately 30 kg per 10 cubic meters) or as granular sub-base. The overlay type should be selected based on crack severity: fairly to moderately cracked slabs are suitable for partially bonded overlays, while moderately to badly cracked slabs require unbonded overlays with at least a 5 cm bituminous separation layer. Overlay thickness design factors in pavement condition through a coefficient C, with thickness formulas ensuring adequate structural support and bonding.

Rigid overlays typically have a thickness of approximately 8.75 cm, constructed from cement concrete with matched joints. Surface preparation involves removing bond-preventing substances, acid etching using dilute hydrochloric acid, and applying a cement-sand slurry or epoxy resin for bonding. Reinforcement with mild steel mesh at 3 kg/m² is recommended under heavy traffic or high rainfall conditions. Flexible overlays vary between 5 cm and 22.5 cm thickness, depending on traffic and soil conditions, often including a minimum 15 cm granular base beneath bituminous layers such as bituminous macadam and asphalt concrete. Tack coats are applied to enhance bonding, particularly when water bound macadam is placed directly on concrete slabs.

Drainage quality and subgrade characteristics significantly affect overlay durability. Poor drainage causes water accumulation, weakening the subgrade and base layers, which leads to distress manifestations such as cracking, rutting, and surface degradation. Weak subgrades, including black cotton soil or marine clays with high plasticity indices, reduce load-bearing capacity and necessitate thicker overlays and reinforcement to maintain pavement integrity. To mitigate these challenges, IRC SP 17 prescribes reinforcing overlays with a mild steel mesh at 3 kg/m² in high rainfall or weak subgrade areas and recommends a minimum 15 cm granular base beneath bituminous overlays to enhance support and drainage.

Bonding procedures vary with overlay type. Unbonded overlays require a bituminous separation layer after cracks are cleaned and sealed. Partially bonded overlays involve removing loose debris, detergent washing (using 1 kg powdered soap per 10 m²), wire brushing, and thorough water flushing, followed by crack sealing with bituminous materials. Fully bonded overlays demand removal of all bond-inhibiting materials like joint sealants and grease, light chiseling to roughen the surface, acid etching with a 1:1 diluted hydrochloric acid solution applied at 4 kg per 10 m² twice, and thorough flushing. A bonding medium of approximately 1 mm thick cement-sand slurry (1:1 mix) is then applied before placing the overlay on saturated surface dry slabs. Although precise tack coat rates are not explicitly specified, thin bituminous layers or a 1 cm non-plastic soil layer are recommended beneath overlays, especially between water bound macadam and concrete slabs to ensure proper compaction.