Development of methodology to evaluate passing ability and test sample preparation for superworkable concrete

Abstract The use of superworkable concrete (SWC) and fiber-reinforced SWC (FR-SWC) necessitates some mechanical consolidation to secure proper consolidation and ensure complete filling of the formwork. This paper evaluates the extent of mechanical consolidation on the risk of segregation, surface quality, and mechanical properties of SWC and FR-SWC. In total, 10 FR-SWC mixtures were prepared using 0.5% polypropylene and steel fibers incorporated. Test parameters involved the incorporation or omission of viscosity-modifying admixture (VMA) and the addition of high-range water reducer to vary slump flow between 450 and 550 mm. The passing ability was evaluated using modified L-box and J-Ring test methods. Test cylinders were cast in one layer and underwent different degrees of mechanical consolidation using a 10-mm steel rod. A vibrating table was also used to induce high consolidation effort during 20 s and 25 s. Image analysis was carried out on longitudinally saw-cut sections along the concrete cylinders to evaluate segregation and surface defects corresponding to various consolidation modes. Sample rodding was found to enable the reduction of surface honeycombing without affecting mechanical properties or increasing the risk of segregation. The vibration consolidation enhanced mechanical properties and surface quality but led to segregation, unless the concrete was highly stable. Recommendations were made on the adequate consolidation energy on samples and test method devices using both consolidation modes. For example, test samples can be cast in one lift and consolidated with 20 strikes of rodding or vibration for 20 s. The modified L-box with internal consolidation, using a 33-mm pencil vibrator to reach a minimum L-box blocking ratio of 0.80, of 5 and 20 s is recommended for SWC and FR-SWC, respectively. Such concrete should be cast in a single lift and consolidated with 10 internal strikes using a 16-mm steel rod when performing the slump flow, modified J-Ring test, unit weight, and air volume tests.