NCMA Research Laboratory

ASTM C1782 Standard Specification for Segmental Concrete Paving Slabs provides the minimum properties for slabs in at-grade applications. With the advent of growing pedestal-set applications for roof plaza decks, a question arose on the appropriateness of this ASTM standard for slabs supported on their corners by pedestals. An ICPI Technical Committee task group addressed the question by proposing center-point load testing for slabs as this has been used by some slab manufacturers for quality assurance. Initial testing by the NCMA Research Laboratory confirmed a center-point load testing method. In addition, a tentative acceptance criterion of at least 2,000 lbs was proposed. This breaking load correlates to the 725 psi required for flexural strength in ASTM C1782.

After initial preliminary tests, the full first testing phase has been completed on 40 paving slabs donated from three manufacturers. These included 2 x 2 ft slabs, specifically 50 mm and 60 mm thick dry-cast slabs, and 50 mm thick hydraulically pressed slabs. A draft report has been submitted and accepted. Preliminary conclusions:

• Center-point load failures on three sets of full-size 2 x 2 ft slabs on (pedestal-like) corner supports were lower by an average of 7% compared to flexural testing results, i.e., roller fulcrums placed at the ends of the slabs and the load applied across the center of each unit. The coefficients of variance (COV) for center-point and flexural breaking loads were 9.6 % and 11% respectively. This indicated repeatability of the center-point load test procedure.

• The center-point loading suggests a more realistic testing approach that produces conservative values with a lower COVs compared to the current flexural strength test method in ASTM C1782/C140.

• Saw-cut, quarter-size 1 x 1 ft slabs from the 2 x 2s were tested with center-point loads resting on corner supports like that for the 2 x 2s. The 1 x 1 ft specimen size facilitates testing by a greater number of laboratories as they may not have apparatus available to test 2 x 2 ft slabs.

Further investigation indicated that travel direction through the production machine for dry cast units influenced the test results. The flexural testing indicated that slabs tested perpendicular to the travel direction through the machine had significantly lower values than those tested in the same direction as the travel path through the machine. The former slabs appear to have a weakness in the middle along a line perpendicular to the direction traveling through the machine. This could likely be due to the slabs momentarily ‘hanging up’ in the front and back during demolding thereby causing the slabs to slightly bend and weaken in the middle in the cross direction. Hairline cracks indicated this.

This data was presented to the Technical Committee and to the Foundation Program Committee and Board of Trustees at the 2021 summer meeting. The Trustees funded the second phase of testing including the additional expense of testing more dry cast slabs to identify and assess potential weaknesses, plus testing hydraulically pressed slabs. The expense covers testing to determine if this weakness can be identified by breaking quarter-size, 1 ft x 1ft specimens cut from the center of 2 x 2 ft slabs, i.e., where  hairline cracks are most likely to occur. This second phase includes testing dry cast and hydraulically pressed slabs donated by nine members and Foundation donors. Once data is analyzed from the additional testing, the task group will review the results and make recommendations on how to draft an ASTM test method and acceptance criteria which is part of NCMA’s scope of work. Results are expected in the fall of 2022.