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Research Results Digest 360 August 2011 INTRODUCTION The flow table is described in AASHTO Standard Specification M 152, âFlow Table for Use in Tests of Hydraulic Cement,â as an apparatus â. . . used in making flow tests for consistency of mortars in tests of hy- draulic cement.â In brief, a mortarâs flow value, which is an indication of its work- ability, is measured by unmolding a sample of the mortar on the flow table, dropping the table a specified number of times, and mea- suring the increase in diameter of the sam- ple, expressed as a percent of its original diameter. Use of the flow table is referenced in AASHTO T 71, âEffect of Organic Im- purities in Fine Aggregate on Strength of Mortarâ; T 106, âCompressive Strength of Hydraulic Cement Mortar Using 50-mm or 2-in. Cube Specimensâ; and T 137, âAir Content of Hydraulic Cement Mortar.â Footnote 3 of the 2003 revision of AASHTO M 152 states that a reference material for calibration of a flow table is available from the Cement and Concrete Reference Laboratory (CCRL) at the Na- tional Institute of Standards and Technol- ogy, Washington, DC 20234. This refer- ence material is compounded in batches by CCRL from a white mineral oil and a finely ground silica powder. The oil viscosity is not measured, but the value reported by its manufacturer is recorded. The powder fine- ness is obtained by a permeability method similar to the Blaine method (e.g., ASTM C204, âFineness of Hydraulic Cement by Air-Permeability Apparatusâ), although in recent years the particle size distribution of the powder has also been measured using a laser diffraction particle size analyzer. Each batch of silica powder is ground at CCRL from an Ottawa sand meeting the require- ments of ASTM C778, âStandard Sand,â until its measured fineness matches that of the previous batch. The powder is then mixed with the oil and the flow value of the batch is determined using the flow table at CCRL. This flow table is identical to com- mercially available units. Thus, flow val- ues obtained with the CCRL flow table are considered reference values for use in the calibration of all other flow tables in the United States. Although the CCRL flow table is well maintained and has been successfully used for preparation of all flow table reference RHEOLOGICAL CHARACTERIZATION OF FLOW TABLE REFERENCE MATERIAL This digest summarizes key findings of NCHRP Project 20-07/Task 272, âRheological Characterization of Flow Table Reference Material,â conducted by the AASHTO Materials Reference Laboratory under the direction of the principal investigator, Haleh Azari. The digest is based on the project final report authored by Haleh Azari and Chiara Ferraris of the National Insti- tute of Standards and Technology. The full text of the project final report is available for download at http://apps.trb.org/cmsfeed/TRBNetProject Display.asp?ProjectID=2600. Responsible Senior Program Officer: E.T. Harrigan NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM