Asphalt
A drawing program in hot mix. Shovel lumpy, sticky asphalt onto a gravel base or lay an even mat with the paver, rake it level, and roll it flat before it cools. Every layer is kept with its thickness, air voids and temperature, and the strip under the picture cuts through them. Rolling squeezes the air out only while the mix is hot; once it drops below about 80 °C the roller just cracks it. Add the flaws real crews fight: cold lumps, stray stones, puddles, boot prints and stony patches.
Things to try
- Beat the clock. Shovel a heap, rake it level, and roll it while the thermal view is still yellow. Wait until it turns purple and roll again: it does not get denser, it cracks.
- Thin lifts cool faster. Lay a 25 mm mat next to a 70 mm one and watch which goes dark in the thermal view first. In cold wind both shrink the time you have.
- Count the passes. The pass map shows what an intelligent-compaction roller's screen shows. Most mixes need five to eight passes; the density map turns green at about 93%.
- Spoil it. A cold lump will not flatten. A big stone rides proud and the drum bridges over it. Water on hot mix boils off and leaves a cold, loose spot. A stony, segregated patch stays open however much you roll it.
- Cold joints. Lay a second strip beside the first once the first has cooled: they do not merge, and the cross-section shows two separate layers meeting at a seam.
How it works
The patch is 2.4 by 1.6 metres in 5 mm cells. Each cell keeps a stack of layers, and each layer its solids (stone and bitumen), its air voids and its temperature; its thickness is the solids divided by one minus the voids. Hot mix is laid at about 150 °C and cools to the air and to the ground below it at a rate inversely proportional to its thickness, so a 50 mm lift takes about twenty minutes to reach 80 °C and a 25 mm lift half that. Water cools it far faster.
Each pass of the roller removes a share of the air voids that are left, scaled by temperature: all of it above about 130 °C, none below about 80 °C, which is the window paving crews work in. The drum rests on the high points, so they take more of its weight and flatten first; hot mix that still stands proud is shoved ahead of the drum. Rolling cold, loose mix cracks it. Material is never created or lost by raking or rolling, only moved and squeezed.
The picture is drawn per pixel from that state: stones of the mix's size stand out of the binder with dark gaps while it is loose, and are pressed flat and level with the binder once rolled; hot binder is glossy; puddles mirror the sky and steam on hot mix. The numbers are simplified but chosen to stay in realistic ranges; they are not a design tool.