Guidelines for Conventional and Thin Whitetopping (First Revision)
2015 Edition

IRC SP 76 recommends using various concrete types for Thin Whitetopping (TWT), including conventional cement concrete, fibre-reinforced variants (polypropylene, steel, nylon), and high-performance concretes incorporating silica fume (3-10%), fly ash (up to 20%), or slag (up to 70%). The water-cement ratio is generally maintained below 0.40, ideally between 0.30 and 0.38 for TWT and Ultra-Thin Whitetopping (UTWT). Strength gains are primarily improved by reducing water content rather than increasing cement content, which is typically capped at around 450 kg/m³. Workability is controlled to a slump range of 25-50 mm, often achieved with superplasticizers. Overlay thickness ranges from 100 to 200 mm for TWT, while UTWT is 100 mm or less with mandatory bonding, often requiring milling of the existing bituminous surface. Fibres are commonly used to enhance strength and durability, with joints spaced between 0.6 to 1.25 meters. Pavement placement is advised to avoid temperatures exceeding 35°C, and whitewash or curing compounds can be applied to reduce surface heat.

According to IRC SP 76, Thin Whitetopping overlays under 150 mm thick generally do not require dowel bars, whereas overlays between 150 and 200 mm thickness should incorporate dowels at joints, especially near structural elements or transverse construction joints. At asphalt transition zones, additional concrete thickness is provided adjacent to joints to compensate for diminished support. The bituminous base beneath should be at least 75 mm thick, preferably 100 mm or more. Joint spacing aims to be short, typically between 1.0 and 1.5 meters, favoring square panels (e.g., 1.0 m × 1.0 m), with the longer side not exceeding 1.2 times the shorter. Timely joint cutting—within 6 to 18 hours after paving—is critical to minimize crack development. Joints should be approximately one-third the slab thickness in depth and 3 to 5 mm wide.

IRC SP 76 details several surface preparation techniques to achieve effective bonding between the concrete overlay and the existing bituminous pavement. Direct placement involves placing the concrete over a clean, swept surface, with ruts filled by concrete to adjust thickness. Milling, typically 25-50 mm deep, is employed to remove surface defects such as cracks and ruts and to create a roughened surface that enhances bonding. When milling is difficult, light chiselling or hand grinding may be used. For uneven profiles, a levelling course made of bituminous macadam (50 mm or more) or PCC/dry lean concrete (75-125 mm) is applied, preceded by a tack coat and separated from the overlay by a membrane or nonwoven geotextile to reduce reflective cracking. Cleaning the surface post-preparation through air blasting, vacuuming, power brooming, water, or sand blasting is essential to remove debris and contaminants. Excessive roughening is discouraged to avoid high friction and premature cracking, making bonding practices especially critical for Thin Whitetopping performance.

In IRC SP 76, dowel bars are mandated when the overlay thickness ranges from 150 to 200 mm, at pavement-structure interfaces such as bridge approaches, and at transverse butt or construction joints. For overlays thinner than 150 mm, dowel bars are usually omitted. Dowel bars are specified as plain mild steel bars, 25 mm in diameter with plastic sheathing, 500 mm long, spaced at 300 mm center-to-center, facilitating load transfer across joints while allowing vertical movement. Tie bars are required at longitudinal construction joints, particularly in half-width constructions, to resist tensile forces and maintain slab alignment. These are deformed steel bars, 10 mm in diameter, 500 mm long, spaced at 750 mm centers, with a minimum concrete cover of 50 mm. Tie bars should be coated with cement slurry before placement to minimize corrosion.

IRC SP 76 addresses temperature curling stresses by recognizing the negative temperature gradient in slabs, where the concrete surface is cooler than the bottom, causing tensile stresses at slab corners. The curling tensile stress is calculated using a formula incorporating the coefficient of thermal expansion, temperature differential, slab length, and radius of relative stiffness, which itself depends on the modulus of elasticity, slab thickness, Poisson's ratio, and subgrade modulus. For example, an 18 cm thick overlay with a -2.7°C temperature differential and typical material properties results in a curling tensile stress of approximately 11.49 kg/cm². Regarding load transfer, the standard explains that whitetopping overlays act compositely with the underlying pavement due to partial bonding, shifting the neutral axis downward and placing most of the PCC slab in compression, which reduces required thickness. Reduced joint spacing further minimizes curling and warping stresses. The overlay is directly bonded to the asphalt without a debonding layer, and surface treatments like whitewash help lower temperature gradients and associated stresses.