Impact of an Inclined Work Surface on the Measured Slump of Concrete

Abstract ASTM C143/C143M-15a, Standard Test Method for Slump of Hydraulic-Cement Concrete, states that the slump test is to be conducted on a level work surface. However, the standard does not provide a method for determining if a work surface is level, a tolerance for levelness, or guidance on what to do in the event that a work surface is not level. More importantly, field technicians need to know at what angle(s) and slump(s), if any, an inclined work surface will significantly influence the measured slump. A study at Valparaiso University included 114 simultaneous measures of slump on a horizontal and separate inclined work surface. This study indicates that inclination angles greater than 6° will influence the outcome of a slump test for slumps of 5 to 7 in. (127 to 179 mm). There is no evidence of concrete “leaning over” for lower inclinations or slumps. At high angles and high slumps, the fluidity of the concrete negates any influence of the inclination. Consequently, it is recommended that ASTM Committee C09.60 on Fresh Concrete Testing add language to C143 requiring the slump test to be conducted on a work surface with an inclination less than 6°, which equates to a 1:9.5 gradient and 10.5 % slope, regardless of the slump of the concrete.