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Concrete paving units are governed by ASTM C936/C936M, Standard Specification Solid Interlocking Concrete Paving Units. Regardless of size or configuration, these units are required to have a minimum average compressive strength of 55 MPa (8000 psi). This strength was originally established when ASTM C936 was first approved in 1982 based on concrete paving units having a thickness of 60 mm (2.36 in.) and a thickness/width aspect ratio of approximately 0.6. Prior to August 2012, the compressive strength testing procedures in ASTM C140, Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units, did not include consideration for the configuration of the tested specimen. This produced results that could vary solely due to specimen configuration. Because of the proliferation of shapes and sizes of manufactured concrete paving units in recent years, research was conducted to determine the effect of specimen configuration on compressive strength. The research indicates that the aspect ratio of tested specimens affects the measured strength. The results were used to support revisions to the testing procedures to standardize the compressive strength specimen configuration and institute an “aspect ratio factor” used in calculating compressive strength. A separate research project evaluated the effect of variables in capping of compression test specimens. Capping is essential for providing consistent bearing surfaces for strength testing; however, the type of material and the thickness of the cap were found to have significant effects on strength. The results were used to justify limiting the capping material for pavers to high-strength gypsum, and limiting the cap thickness to 1.5 mm (0.06 in.). This paper reviews both projects and the changes to ASTM C140 based on the results. In addition, this paper presents a useful guide for testing laboratories so that they can readily understand and comply with the new compressive strength testing requirements.
interlocking concrete paving units, compressive strength testing, capping methods for compression strength testing
Vice President, Technical Development, ACM Chemistries, Inc., Norcross, GA
Manager, Research and Development Laboratory, National Concrete Masonry Association (NCMA), Research and Development Laboratory, Herndon, VA
Smith, David R.
Technical Director, Interlocking Concrete Pavement Institute (ICPI), Herndon, VA