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The Solar Reflectance Index (SRI) of mastic asphalt varies ~0 to 110. The SRI of mastic asphalt is dependant on its finish: ~0–5 for bare black asphalt, ~30–60 with light-coloured chippings or grey coatings, and ~80–110 with white solar-reflective coatings. SRI is calculated (e.g., to ASTM E1980) from solar reflectance and thermal emittance; bituminous surfaces usually have high emittance (~0.90–0.95) but need light colours or reflective coatings to raise reflectance. Texture, pigmentation, and weathering can shift SRI by ±10–20 points over time, so cleaning and periodic recoating help retain performance. In practice, mastic asphalt provides the waterproofing; the chosen surface finish controls SRI.
Solar Reflectance Index (SRI) is a single number that indicates how hot a surface will get in the sun compared with standard “black” and “white” reference surfaces. It combines two measured properties—solar reflectance (how much sunlight is reflected) and thermal emittance (how effectively heat is re-radiated)—into a heat-balance calculation.
By convention, the standard black surface has SRI = 0 and the standard white surface SRI = 100, though real materials can score below 0 or above 100. Higher SRI means a cooler surface temperature under the same sunlight. SRI is calculated per ASTM E1980, using reflectance measured by tests such as ASTM C1549/E903 and emittance by ASTM C1371/E408. Results depend on the climate parameters used in the model and on the material’s colour, texture, and surface roughness. SRI usually drops over time due to soiling and weathering, so many programs ask for both initial and aged values. Designers use SRI to compare roof and paving finishes, meet cool-roof codes, reduce heat-island effects, and improve summertime comfort.
Solar Reflectance Index (SRI) tells you how hot a mastic asphalt surface will run in sunlight, which directly affects risks of flow, ridging, indentation, and accelerated ageing. Specifying higher SRI finishes (e.g., light chippings or reflective coatings) lowers peak deck temperatures, improving warm-weather stability and worker comfort. In the case of roofs, high SRI finishes potentially reduce cooling loads. SRI also supports compliance with cool-roof and heat-island targets and guides finish choices for exposed roofs, balconies, ramps, and car parks. Because SRI drops with soiling and weathering, it informs maintenance plans—cleaning and periodic recoating—to keep temperatures and performance in check.
SRI is a direct proxy for equilibrium surface temperature: a finish with SRI ~80 can run 15–25 °C cooler than a dark SRI ~5 surface under strong sun. Because bituminous layers already have high thermal emittance (~0.90–0.95), the reflectance/colour of the finish is the main lever—light chips or white coatings lift SRI dramatically. Orientation, shading, texture, and soiling all shift real temperatures, so use both initial and aged SRI values when selecting finishes. Choose SRI early, alongside softening-point targets and detailing, so you aren’t forced into an over-hard mix just to cope with heat later.
Lowering peak deck temperature reduces flow, indentation, and creep, keeping edges stable at upstands, thresholds, and ramps. Cooler running can let you avoid excessively hard binders, preserving workability while still meeting warm-weather stability. It also trims thermal expansion/contraction cycles, cutting the risk of ridging and crack initiation at stress points. For chip finishes, ensure proper embedment and cleaning to maintain reflectance; for coatings, plan periodic recoating and confirm skid resistance where foot traffic or vehicles are expected.
Many planning policies and rating systems (e.g., heat-island credits, local authority guidance, BREEAM/LEED pathways) set minimum SRI targets for exposed roofs, terraces and hardscapes. Specifying higher-SRI finishes lets you meet these thresholds without resorting to overly hard binders that hurt workability. In specifications, state target SRI (initial and aged) alongside thickness, softening-point, slip resistance, and detailing; note the test basis (e.g., ASTM E1980). Keep documentation ready—manufacturer data sheets, test reports, and sample submittals—to satisfy compliance checks. Align the chosen SRI with acoustic, thermal, and durability goals so the finish supports performance on multiple fronts.
SRI declines with soiling, chalking, and chip embedment, so plan a maintenance cycle from day one. Schedule periodic cleaning (low-pressure water and neutral detergents) and, where coatings are used, recoating intervals based on exposure and traffic—retest or reference aged SRI after works. Keep drainage clear and detail edges to avoid dirt traps that accelerate reflectance loss; protect high-traffic paths where tyre marks or footfall concentrate. Balance cleaning/recoating with slip resistance requirements and confirm skid values after treatment. Use simple IR spot checks during hot spells to identify “hotter-than-expected” zones and target maintenance where it delivers the biggest temperature drop.
Applications with the highest need for SRI are exposed, sun-facing roofs, balconies, and terraces—especially dark or south/west-facing surfaces—where cooler running cuts flow, ridging, and ageing. Ramps, steps, and busy walkways also benefit from high SRI to limit warm-weather indentation and surface marking on slopes and turning zones. In open car parks and bridge decks, high SRI helps manage peak temperatures and can support heat-island compliance while improving durability. By contrast, below-grade tanking and podium decks under pavers see little benefit from high SRI because the asphalt isn’t directly sun-exposed.
Exposed asphalt roofs, balconies and terraces required the highest solar reflectance indexes. These areas receive full solar exposure and can run well above air temperature, so higher-SRI finishes are a primary control on peak deck heat. Cooler running keeps the binder stiffer, reducing flow, ridging, and indentation—particularly on dark colours, south/west aspects, and sloped surfaces where heat gain and gravity combine. Light-coloured chippings or reflective coatings typically cut peak temperatures by ~10–20 °C (finish, colour, and soiling dependent); use compatible primers and ensure good chip embedment so reflectance is durable. Specify both initial and aged SRI (reflectance falls with soiling/weathering), and include a cleaning/recoating plan in the O&M manual. Pair the finish with an appropriate softening-point target so thermal relief from SRI is complemented by intrinsic hot-weather stability.
Ramps, asphalt steps and busy walkways also need high solar reflectance indexes. These applications have, slopes and tight turning concentrate shear and point loads, so keeping surfaces cooler with higher SRI meaningfully reduces warm-weather indentation, scuffing, and surface marking. Combine reflective finishes with a robust wearing/sand-rub layer and consider local thickening at nosings, thresholds, and main turning zones to spread stress. When using coatings or light chips, confirm slip resistance (pendulum/skid values) and specify a texture that balances reflectance with safe micro-texture. Detail perimeter isolation and avoid hard bridges at edges that can promote ridging, and use high-density boards over softer substrates where sack-truck routes are expected. Maintain performance with periodic cleaning and selective refresh of the finish, noting that soiling diminishes SRI and raises surface temperatures if left unmanaged.
Open car parks and bridge decks are other mastic asphalt applications which benefit from high solar reflectance indexes. Large, unshaded expanses such as exposed car park decks heat rapidly, so higher SRI finishes help moderate peak temperatures and reduce heat-related deformation under wheel loads, braking, and turning. Bridge decks and asphalt car parks should make use of reflective chippings/finishes which also deliver reliable skid resistance. Before using chippings/finishes you should confirm performance alongside softening point, static indentation (~60 °C), and wheel-tracking/creep tests. You may still need polymer-modified binders and local thickening at ramps, tight turns, and wheel-stop zones, but high SRI reduces how often the deck reaches critical temperatures. Detail drainage, joints, and edge restraints to manage thermal cycling and de-icing salts, which accelerate wear. Plan maintenance for soiling, salts, and tyre marking, and specify initial and aged SRI so the performance target is clear over time.
Mastic asphalt finishes which are dark of faceing in a South or West direction need high solar reflectance indexes. Orientation and colour drive solar gain; dark finishes on south/west aspects benefit most from higher SRI to curb temperature spikes. If a dark aesthetic is required, use tinted reflective coatings or higher-reflectance chips which preserve the look while lifting SRI. Applications such as asphalt flooring or walkways where foot or vehicle traffic is expected should always verify slip resistance. Combine surface reflectance with higher softening-point binders, modest filler adjustments, and, where feasible, local shading/ventilation to keep deck temperatures within safe limits. Specify initial and aged SRI and include a simple cleaning/recoating schedule in the O&M manual to maintain reflectance. Occasional IR spot-temperature checks during hot spells help target upkeep where it delivers the biggest cooling benefit.
The best way to increase the solar reflectance index of mastic asphalt is to apply a high-albedo finish. This means to apply a finish with a surface which has high solar reflectance to bounce a large share of sunlight back. This results in the mastic asphalt staying cooler. High albedo finshes include; light-coloured stone chippings with good embedment and solar-reflective coatings which are compatible with bitumen. SRI for both states; initial (new) and aged (after weathering) should be specified. Additionally, you should plan routine upkeep: regular cleaning and, for coated finishes, occasional recoating. The reason for this is dirt and weathering reduce reflectance, this will lower SRI over time. If a darker appearance is desired, use tinted “cool” coatings or higher-reflectance chips to raise SRI without changing the look, and always verify slip/skid resistance. The chosen high-albedo finish should be combined with an appropriate softening point and detailing so the cooler surface translates into real gains in stability and durability.