Winter 2017-18

 By David Smith ENGINEER’S view InterlockDesign.org ® esign interloc 9 WINTER 2018 Strengthening Open-graded Aggregates in Permeable Pavements P ermeable pavements use open‑graded aggregates that provide two roles, supporting vehicles and storing water. Permeable interlocking concrete pavements for vehicular traf- fic typically use a 2 in. (50 mm) thick layer of ASTM No. 8 stone over a base or choker layer of 4 in. (100 mm) thick No. 57 or similar size aggregate. The latter is compacted on a subbase consisting of ASTM No. 2, 3 or 4 stone. Crushed rock is recom- mended for all three layers to help maximize interlock among the aggregate. Figure 1 shows a cross section used for vehicu- lar applications. While not shown, this section can include a perforated underdrain. Figure 1. Typical open-graded aggregate layers in permeable interlocking concrete pavement. When loaded by vehicles, open-graded aggregates behave somewhat differently than dense-graded aggregates typically used under asphalt and beneath (non-permeable) concrete pavers interlocked with jointing and bedding sand. Open- graded aggregates don’t have small particles mixed with larger particles to help confine the larger ones. Compacted, open- graded aggregates usually have 35% to 40% open space due to the absence of small particles. Compaction with a heavy plate or roller compactor helps open-graded aggregates find tighter positioning which results in better interlock among their neighbors. Engineer's View continued on page 11