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The compressive strength of mastic asphalt is typically 10–25 MPa at about 20–25 °C. Mastic asphalt is viscoelastic which means strength depends strongly on temperature and loading rate. At 40–60 °C mastic asphalt often falls to ~3–10 MPa. In practice, compressive strength isn’t usually the governing criterion. Mastic asphalt specifications rely more on static indentation at 60 °C, stiffness/flow measures, and (where relevant) wheel-tracking to judge load resistance. For steps and walkways, adequacy is driven more by substrate support, thickness, and the wearing/sand-rub finish than by bulk compressive strength; correct detailing at nosings and timely renewal of the finish are key.
Compressive strength (often incorrectly called “compressible strength”) is the maximum compressive stress a material can withstand before it fails. It’s measured by pressing a standardized specimen until failure and dividing the peak load by its cross-sectional area, typically reported in MPa (N/mm²). Brittle materials fail suddenly at this limit, while ductile or viscoelastic materials (like bituminous mixes) show rate- and temperature-dependent behavior, so results vary with test conditions.
Strength is different from stiffness (modulus): strength is how much load causes failure; stiffness is how much a material deforms under load. Moisture, defects, curing/aging, and specimen geometry can shift results, which is why standardized methods are used. Engineers apply safety factors when using compressive strength for design, but real-world performance may instead be governed by indentation, creep, or buckling.
In layered floor or roof systems, substrate support and detailing can matter more than the material’s standalone compressive strength.
Compressive strength (often called “compressible strength” in error) matters for mastic asphalt because it indicates the layer’s ability to resist crushing and permanent set under concentrated loads. In roofs, steps, and walkways, sufficient strength helps the surface withstand footfall, ladders, plant supports, and trolleys without denting or wrinkling the finish. Mastic asphalt is viscoelastic, which means strength decreases at higher temperatures and under long dwell times. This means checking mastic asphalt at relevant temperatures provides a safety margin for summer conditions.
Service performance is often governed more by static indentation at 60 °C, creep resistance, and the stiffness of the supporting substrate. Adequate strength reduces local damage at nosings and thresholds and protects the sand-rub or wearing finish from scuffing. It guides thickness selection and whether to add load-spreading layers or high-density boards over softer insulation. Finally, mix design—binder grade, polymer modification, filler content, and aggregate choice—can be tuned to preserve strength while balancing workability and crack resistance.
Mastic asphalt must withstand high local pressures from ladder feet, plant supports, trolley wheels, and small base plates. Specify sufficient compressive strength and thickness to keep contact stresses below the mix’s capacity. Use load-spreading plates or high-density boards where point loads are predictable. Confirm adequacy with representative lab data and ensure continuous, well-supported substrates to avoid stress concentrations.
Dents and wrinkling arise when concentrated loads deform the surface, especially in warm weather or over soft substrates. Increase thickness and consider harder or polymer-modified binders to resist surface marking. Provide a robust wearing or sand-rub finish in turning zones, landings, and access routes. Maintain these finishes and patch damage promptly to stop minor marks from growing.
Because mastic asphalt is viscoelastic, strength drops as temperature rises and as load duration increases. Design to peak summer deck temperatures and realistic dwell times, not just room-temperature, short-duration tests. Where heat is expected, favour harder grades or polymer-modified binders and consider reflective coatings or shading. Schedule heavy static loads for cooler periods when possible.
In service, static indentation at elevated temperature and creep resistance are better predictors of durability than a single compressive strength value. Set project-specific limits (e.g., indentation at 60 °C) that reflect expected exposure and loads. Where rolling or sustained loads occur, require creep or wheel-tracking data that matches the proposed build-up. Verify performance with supplier test reports and enforce on-site QA to maintain those results.
The substrate largely governs how mastic asphalt behaves under load: softer decks allow local deflection that concentrates stress. Use stiffer layers—high-density boards, cementitious screeds, or thicker structural decks—to spread loads and reduce indentation. Ensure full, void-free support and strong adhesion; gaps or soft spots will telegraph through as damage. Verify stiffness with pull-off tests and on-site checks before laying the asphalt.
Greater thickness increases bearing area and raises resistance to point loads, but returns diminish beyond what the substrate can support. Choose thickness by use class (access routes, plant zones, steps) and expected contact pressures, not a single default value. Add load-spreading plates, pads, or high-density cover boards under known point loads to keep stresses within limits. Balance thickness against build-up height, weight, and programme.
Select binder grade and consider polymer modification to maintain strength and indentation resistance at elevated temperatures. Adjust filler content and choose harder, denser aggregates to resist marking while preserving workability. Fibre additions can curb micro-cracking and help the layer stay airtight under cyclic loading. Confirm target properties with lab tests at project-relevant temperatures and dwell times.
Edges and transitions take the brunt of impact and turning forces, so detail them for durability. Use robust nosings, reinforced corners, and tougher wearing or sand-rub finishes in landing and doorway areas. Consider local thickening or wear strips in main footfall paths, and include perimeter isolation where rigid bridges could form. Plan periodic renewal of the wearing finish and prompt patch repairs to stop minor defects from spreading.
Compressive strength is vital wherever mastic asphalt will face concentrated point loads or sustained contact pressures—HVAC legs, pedestal feet, ladder feet, and trolley wheels. It’s especially critical in warm conditions or long dwell times, when viscoelastic mixes soften and are more prone to indentation at summer deck temperatures. Edges and transitions—steps, nosings, thresholds, and landings—need higher strength (and often extra thickness) to resist dents and wrinkling. It also becomes crucial over softer substrates (insulation or lightweight screeds), where local deflection amplifies stress and calls for load-spreading boards or high-density underlays. In practice, you specify for this by meeting service criteria like static indentation at 60 °C and creep resistance wherever footfall, plant supports, or wheeled traffic are expected.
On mastic asphalt roofing, small bearing areas create very high local pressures, so adequate compressive strength is essential to prevent indentation and creep. Use load-spreading measures—plinths, base plates, or high-density protection boards—so contact stresses stay within limits. Specify mixes and thicknesses suited to summer deck temperatures and long dwell loads. Detail seals and flashings so the load path remains continuous and no point loads fall directly on the asphalt edge.
On roof access roofs and walkway waterproofing, repeated footfall and small hard wheels concentrate load, especially at turning points and thresholds. Choose a mix and thickness that limit static indentation at elevated temperature, and add a robust wearing/sand-rub finish for scuff resistance. Where trolleys are routine, include high-density boards or track plates to spread wheel loads. Keep falls, joints, and perimeter details tight to avoid soft spots that magnify local deformation.
The nosings and landings of asphalt steps and stairways experience peak contact pressures and shear from turning, so compressive strength and substrate stiffness are critical. Consider local thickening, reinforced nosings, and tougher wearing finishes in these zones. Check high-temperature indentation performance and plan periodic renewal of the wearing layer to maintain slip and wear resistance. Ensure full support, clean arrises, and sealed edges so loads don’t concentrate at weak points.
These spaces see frequent point loads and rolling loads that dwell in place during manoeuvring. Specify mixes and thicknesses that limit static indentation at elevated temperature, and verify creep/wheel-tracking data where heavy castors are expected. Use high-density protection boards or load-spreading plates under known equipment skids and along turning paths. Keep substrates stiff and fully supported, and plan prompt patching of scuffs to stop damage from propagating.
Pedestal feet focus load onto small areas, so compressive strength and creep resistance are critical. Always place high-density protection boards or load-spreading layers above the asphalt so pedestal pressures remain within limits over time. Ensure boards have adequate compressive strength/creep ratings and that falls prevent prolonged wetting at support points. Maintain continuous support at edges and corners to avoid punch-through and local cracking.
Mastic asphalt car parks and bridge decks must endure wheel loads, tight turning, and braking. This means these applications endure high local stresses and shear, especially in warm conditions. Select mixes proven for low static indentation at 60 °C and good wheel-tracking performance, and consider polymer-modified binders for stability. Increase thickness or add load-spreading layers in known hot spots such as ramps, bays near wheel stops, and entrances. Keep substrates stiff, joints tight, and detailing robust to resist wrinkling, scuffing, and long-term creep.