Chip Seal Design by the McLeod Method
Chip seal aggregate and binder spread rates by the McLeod method.
Example
You enter
- Median particle size M (in, 50% passing) 0.375
- Flakiness index FI (%) 18
- Dry loose unit weight W (pcf) 95
- Bulk specific gravity G 2.65
- Wastage factor E (whip-off, ~1.05-1.10) 1.05
- Traffic factor T (0.85 light to 0.60 heavy) 0.75
- Surface condition factor S (gal/SY) 0.02
- Residual asphalt R (0.67 emulsion, 1.0 AC) 0.67
You get
- Average least dimension (mat thickness) 0.2785 in
- Voids in the loose aggregate 0.425496
- Aggregate (lb/SY) 30.0964
- Binder (gal/SY) 0.327541
- Aggregate ton per 1000sy 15.0482
Checked against published worked examples from: McLeod method (state DOT test procedure).
Details, formula, and sources
ScopeA chip seal is built ONE STONE THICK, so the controlling dimension is the average least dimension, not the sieve size - traffic rolls each stone onto its flattest face. 18% flat particles pull a 3/8-in aggregate from 0.329 to 0.279 in, and both rates follow. Note the direction that surprises people: heavier traffic wants LESS binder, because traffic does the embedding. All four equations quoted from a public-domain state DOT procedure.
Formula or tableALD H = M/(1.139285 + 0.011506 FI); voids V = 1 - W/(62.4 G); aggregate C = 46.8 (1 - 0.4 V) H G E lb/SY; binder B = (2.244 H T V + S + A)/R gal/SY.
Source and editionThe McLeod seal-coat design method as published in a US state DOT test procedure (public domain), by name; all four equations are quoted verbatim from that source rather than recalled.
Free accessState DOT chip-seal design procedures implementing McLeod are published free, as are the TxDOT and Caltrans maintenance manual chapters covering the same method.
What governsEstimate. AHJ and licensed professional govern.
API field namesmedian_size_in, flakiness_index_pct, loose_unit_weight_pcf, bulk_specific_gravity, wastage_factor, traffic_factor, surface_factor_gal_sy, residual_asphalt, ald_in, voids, aggregate_lb_sy, binder_gal_sy, aggregate_ton_per_1000sy
- One stone thick the average least dimension is the mat thickness; aggregate rate is geometric and independent of binderMcLeod method
- Traffic embeds higher traffic lowers the required binder via the T factor (about 0.85 to 0.60)state DOT traffic correction table
- Agency factors entered T, E, and S are table lookups that vary by agency and are user inputs, not shipped valuesuser input