Top cast effect: Influence of bond length on splitting mode failure

John Cairns*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)


It is widely considered that bond strength of bars cast near the top of a pour is weaker than that of bars cast near the bottom. The lesser strength of top cast bars is attributed to: (a)The increase in binder/cement ratio toward the top of a pour due to consolidation of fluid concrete prior to setting. (b)The formation of voids beneath bars as solids in fluid and setting concrete consolidate when the bar is restrained from vertical movement. The difference in bond strength bars cast near the top of a pour and those cast near the bottom varies widely between individual studies, with little difference reported in some studies and reductions in excess of 50% reported in others. A markedly greater top bar effect observed in pullout type specimens with short bond lengths compared to that in studies based on conditions more representative of practical construction. Through a simple analytical model of behavior of a lapped joint based on the local bond slip model in fib Model Code 2010, it is shown that the casting position effect diminishes as bond length increases. This influence is supported by analysis of trends in the top cast section of the ACI408 bond test database. Conclusions demonstrate the casting position factor in current design Codes significantly overestimates the top cast effect and justifies a casting position factor of 1.0 for laps designed to develop the full design strength of Grade 500 bars. Increased bond lengths would still be required for top cast laps designed for lower bar stresses.

Original languageEnglish
JournalStructural Concrete
Early online date17 Dec 2021
Publication statusE-pub ahead of print - 17 Dec 2021


  • anchorage
  • bond
  • casting position
  • lapped joints

ASJC Scopus subject areas

  • Civil and Structural Engineering
  • Building and Construction
  • Materials Science(all)
  • Mechanics of Materials


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