Ceco’s Justin Bowen, a project manager based in Tampa, Fla., recently co-authored a technical paper published in the American Concrete Institute’s Structural Journal. “Reinforcing Bar Pullout Bond in Tremie-Placed Concrete Cast in Drilling Slurry Environments” appears in the July 2019 issue, pages 183-192.
The technical paper discusses the effects of submerged shaft construction on reinforcing steel bond where groundwater or drilling slurry is displaced during concreting. According to the paper, drilling slurry made from a mixture of water and mineral or polymer powder is often used to stabilize deep excavations, in which cast-in-place reinforced concrete foundation elements, such as drilled shafts, are constructed. At the time of concreting, drilling slurry is displaced by heavier, highly fluid concrete tremie placed at the bottom of the excavation from within the reinforcement cage. The concrete must build up within the reinforcing cage to a sufficient height before it can radially press into the annular cover region. This flow pattern can trap slurry near the steel reinforcement, affecting rebar bond.
From 2012 to 2013, Bowen worked with a research team from University of South Florida’s Department of Civil and Environmental Engineering to perform rebar pullout tests on 227 specimens tremie-placed in commonly used mineral, polymer or natural (water) drilling slurry. Test results showed how each slurry type influences rebar bond strength. Based on their analysis, the team recommends that current development length values be increased by 1.8 for bentonite slurry environments and 1.9 for polymer slurry environments for rebar to achieve the same level of reliability as in dry conditions.
The technical paper was co-authored by Bowen and University of South Florida’s Gray Mullins, PhD (professor), Kelly Costello, PhD (postdoctoral researcher), and Sarah Mobley (doctoral candidate).