Research for Revisions to AC276

Grantee: NCMA Research and Development Laboratory
Principle Investigators: Tim Jones
Year: 2021
Project Number: 2021.012

This research will support updating a key industry specification for segmental retaining wall systems.


ICC Evaluation Services has developed AC276, Acceptance Criteria For Segmental Retaining Walls, originally approved in October 2004 (editorially revised January 2018). This criteria establishes procedures for qualifying segmental retaining walls (SRW) constructed as dry-stacked gravity structures consisting of SRW concrete units or blocks, with or without geosynthetic reinforcement materials. This criteria includes test and performance requirements for the SRW units and the interaction of geosynthetic reinforcement materials with the SRW units.

For interaction between SRW units, AC276 requires testing in accordance with ASTM D6916-03, Standard Test Method for Determining the Shear Strength between Segmental Concrete Units (Modular Concrete Blocks). This version is historical, and the current version of this test method was published in 2018. The main difference between versions of this standard is the loading rate. A constant rate of displacement of 1.0 ± 1 mm/min. and 5.0 ± 1 mm/min. is specified by the D6916-03 and D6916-18 respectively.

ASTM D6916 testing consists of a total of seven individual tests, at five unique normal loads, with one of those loads repeated twice to verify the repeatability of the testing performed for each set. Moreover, with a slow loading rate as required by AC276 this testing method becomes very time-consuming. In addition, it can be confusing for testing labs and their clients to have to perform the test using different versions of the standard for different purposes.

Work Plan:

In order to effect changes to AC276, it must be shown that the differences in test method versions do not significantly affect tested results. This project aims to perform research to justify updating AC276 to reference the most recent version of ASTM D6916.

This research will investigate difference between the shear strength results tested in accordance with ASTM D6916-03 and ASTM D6916-18. A set of flat SRW units without geosynthetic reinforcement will be tested at a constant normal load (approximately for 10-15 ft. height), with two different loading rates as specified by the ASTM D6916-03 and D6916-18 to see the difference. For each loading rate, the SRW units shear strength will be tested fifteen times. This data will be provided to the ICC evaluation services to support the proposed changes.

In addition, ASTM D6916 requires evaluating the service-state shear strength only when required by the client but the displacement that defines the serviceability strength is not specified. However, AC276 outlines the deformation criterion to either be 0.75 inch or a value equal to 2 percent of the block height, whichever is less. This study will also investigate the deformation criteria for serviceability shear strength.