Effect of self consolidating concrete (SCC) mix proportioning on transfer and development length of prestressing strands

Self-consolidating concrete (SCC), a special concrete tailored for high deformability and stability, has recently gained much interest from the precast/prestressed industry. In spite of many developments on SCC material technology, knowledge on the performance of elements built using SCC is limited. Of particular relevance in prestressed concrete is the issue of strand bond. The research objective was thus to study the effect of SCC mix proportioning on the bond of prestressing strands as it relates to transfer and development lengths in precast/prestressed girders. The study focused on 13mm (0.5 in.) diameter strands on laboratory-scale T-beams. Three SCC mix designs were strategically designed to bound the accepted proportioning methods. Comparison was made with a conventionally vibrated concrete mix (NCC). Transfer lengths were determined by strand draw-in and concrete strain measurements and development lengths were obtained through flexural beam tests. Transfer length results were within the current ACI-318/AASHTO code recommendations. Experimentally determined development lengths were longer than the ACI-318/AASHTO values. The source for this discrepancy solely on the concrete mix is questionable and the quality of the strand is being further evaluated to fully justify this findings. In spite of these doubts, the effect of SCC mix proportioning was found to distinctly affect the different mechanisms of strand bond performance influencing transfer and development length.