Issue 1, 2017

InterlockDesign.org | By David R. Smith ENGINEER’S view Doing the Math 20 esign interloc ® 2017 ISSUE 1 A SNEAK PEEK AT PAVING SLAB STRUCTURAL DESIGN Thanks to a grant from the ICPI Foundation for Education and Research, design tables were developed earlier this year that provide technically conservative base solutions for paving slabs subject to vehicles. The tables were developed using finite element modeling that simulated pressures from truck tires on paving slabs of various sizes and flexural strengths. The modeling included a 1-inch (25 mm) thick sand bed under the slabs, three base materials, and three soil subgrade conditions. This article previews the design approach for pedestrian and vehicular applications derived from that modeling. For pedestrian applications, 12 x 12 in. (300 x 300 mm) units can be placed on a minimum 6 in. (150 mm) thickness of compacted aggregate base. Thicker bases (generally 8 to 12 in. or 200 to 300 mm thick) should be used in freezing climates and/or on weak clay soils (CBR < 3%). Designers should consider using a lean concrete or concrete base for larger paving units because achieving a very smooth base surface can be difficult with compacted aggregate base. Design options become a bit more complex for vehicular applications. The first step is determining the maximum number of lifetime 18,000 lb equivalent single axle load or ESAL repeti- tions. (Caltrans Traffic Indexes are provided in parentheses.) Determining ESALs or TIs can be done using Table 1. It divides them into five categories. Higher ESAL categories generally require thicker units and concrete bases. Applications exceeding 75,000 lifetime ESALs should use interlocking concrete pavers. TABLE 1. TRAFFIC CATEGORIES AND LIMITS Traffic Limits Category Symbol 20 yr ESALs (TI) Equivalent Heavy Vehicles/Day Do Not Subject to Vehicles No 0 0 Primarily Pedestrian P 1,000 (4) 0.1 Cars only (< 4,500 lbs or 2,000 kg) LT 30,000 (6) 2.0 Cars and Occasional Heavy Vehicles (>10,000 lb or 4,500 kg) OHV 75,000 (6.6) 5.0