Hybrid structures using ultra high performance concrete and normal concrete for bridge applications

Abstract

The aging transportation infrastructure problem coupled with rapidly increasing traffic volumes and tightening budgets necessitates the need for cost effective and durable bridge components which can be easily implemented using current construction techniques. These solutions must also be suitable for accelerated construction in order to ensure minimum disruption to existing traffic. In this regard, Ultra High Performance Concrete (UHPC), a highly engineered cementitious material with enhanced mechanical properties lends its self as an ideal material. UHPC cost is nearly 30 times the traditional concrete, making full UHPC structures uneconomical. Through this multipart research, the emerging UHPC material and the traditional normal concrete are optimally combined to exploit both their beneficial features and yield new economical hybrid bridge components. The rebar development length in UHPC was experimentally investigated using pull out and beam specimens with lap splices. The results from these tests add significant new data on the bond stress distribution for rebar embedded in UHPC and a simplified design equation is proposed. An embedment length of 8 db (db -diameter of rebar) in UHPC with 3db clear cover was found to be sufficient to yield a Grade 60 mild steel reinforcement. A hybrid prestressed girder concept utilizing UHPC in the end zones of the girder with normal concrete in the remainder of the girder was proposed for a long-span girder with existing shapes . The endzone and shear behavior of a deep prestressed girder was investigated experimentally and analytically using four 78 in. deep, normal concrete bulb-tee (BT-78) girders. A detailed finite element (FE) model was developed in ABAQUS and calibrated using the experimental data. UHPC-NC interface behavior under direct shear and flexural loading was also experimentally investigated using direct shear testing of small-scale interface samples and flexural testing of UHPC-NC beams. A detailed finite element model for the interface was developed in ATENA and calibrated using the experimental results. Further, a detailed 3D FE model of a 205ft. long UHPC-NC hybrid girder was developed in ATENA and used to evaluate the feasibility of the hybrid girder concept. It was found that the hybrid girder concept is not only feasible but also reduces significantly the amount of end-zone reinforcement and end-zone cracking. A hybrid bridge pier system using a precast UHPC shell as permanent formwork for traditional bridge piers or as a retrofit option for existing columns was proposed. Experimental tests were conducted on 24in. long UHPC-NC columns to quantify the effectiveness of the UHPC shell in providing the confinement to normal concrete. Results obtained from the tests indicate that UHPC-shell-confined specimens exhibit a 15 to 30 % increase in peak load carrying capacity along with a 26 to 46% increase in failure strain values.