Don’t Be Fooled

Spring 2017

The misapplication of H-20 loads

by David R. Smith


Don’t Be Fooled

Having worked with segmental and monolithic pavements for a few decades, one growing notion popularized in marketing/technical information for competing pavement systems is H-20 loading. Product literature for plastic pipes, chambers, stormwater storage crates, grids, etc., state confidently that their products can receive an H-20 load.

While that may be true, here’s a more complete picture. The H-20 load notion comes from bridge design and not from pavement design. The concept is found in the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges. Most bridges that receive trucks label such loads as H-20 or HS-20 in the bridge design process. The ‘20’ stands for a 20-ton vehicle, i.e., 4 tons on the steering axle and 16 tons on the drive (rear) axle. Adding an ‘S’ means the truck is a tractor-trailer combination. This adds 16 tons to a third (rear) axle, making a 36-ton vehicle.

The H-20 load is used in bridge structure design, a process that examines how the structure deflects under the weight of the bridge itself (dead load) and the applied truckloads (Iive). Computerized structural design models find the right size beams that limit their movement (deflection) under truckloads. The deflection in the structure is analyzed under H-20 and HS-20 loads, and likely other loading options depending on the anticipated traffic.

H-20 or HS-20 is a single truckload used in the analysis of bridge designs. These designations don’t apply to pavement design. Pavements are not designed to receive a single truck on them. In fact, my grassed front yard could easily receive an H-20 load. Obviously, that lawn is not a suitable pavement structure for repetitive loads from trucks.

Whether grass or something else, pavements do not typically fail from one H-20 or HS-20 load. Their limitations are defined by their ability to receive thousands or even millions of axle loads from trucks. Since axle loads vary with every vehicle, the AASHTO 1993 Guide for Design of Pavement Structures provides a process to equalize them to 18,000 lbs using a pavement concept developed in the 1950s called equivalent single axle loads or ESALs. Applying the ESAL concept, one H-20 load equals about 10 ESALs and one HS-20 load equals 26 ESALs.

Pavement failure (an unserviceable pavement) is typically seen as rutting in asphalt and cracking in rigid concrete pavements. In either case, the pavement surface slowly fatigues from repetitive loads over time due to tension and resulting horizontal strain at the bottom of the pavement surface. This eventually causes the pavement surface to bend or break.

Interlocking concrete pavements have a much higher compressive strength than conventional concrete (8,000 psi versus 4,000 psi). This makes the paving units especially resistant to fatigue from repeated loads compared to conventional asphalt or concrete surfaces. Most interlocking concrete pavements wear out deeper in the pavement, i.e., from repeated compressive strain within the bedding sand, base or at the top of the soil subgrade layer. This suggests that the base thickness needs to be sized and then constructed correctly, as well as testing conducted on soil subgrade compaction.

ICPI provides guidance documents to help with design on ICPI Tech Spec 4 Structural Design of Interlocking Concrete Pavements is one such document. Another is ASCE 58-16 Structural Design of Interlocking Concrete Pavement for Municipal Streets and Roadways. Both documents provide structural designs up to 10 million ESALs, a very busy major urban thoroughfare.

In the meantime, don’t be fooled. When the term H-20 appears in product literature, ask what happens when that load is repetitively applied? How long does the pavement last? When does it become unserviceable and fail from rutting or cracking? These are the core pavement design questions that require an answer for designers to create reliable pavements.

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