Performance of Bridge Decks and Girders with Lightweight Aggregate Concrete

Structural lightweight concrete is a very versatile material and Haydite and Minergy lightweight aggregates can be utilized in the mixture development of concrete for use in girders and decks in bridges. More widespread use of lightweight aggregates (LWA) would result in savings in construction cost due to considerable dead load reduction. The materials phase of this research study evaluated the fresh concrete properties (slump, unit weight, and air content), the mechanical properties of hardened concrete (compressive strength, tensile strength, flexural strength, static and dynamic modulus of elasticity, Poisson’s Ratio, and temperature development), and durability related parameters (air void distribution, freeze-thaw resistance, chloride permeability, resistance to scaling, and drying shrinkage). In addition, selected properties (24-hour absorption, bulk specific gravity, size distribution, and internal pore structure) of the lightweight aggregates were also determined. Lightweight aggregate mixes used in this study were proportioned using water-cement ratio/strength method in order to better address the issue of producing mixes that would satisfy both durability and strength requirements. Target compressive strengths for bridge girder concrete were 42 and 60 MPa (6000 and 10,000 psi), and 31 MPa (4500 psi) in the case of bridge deck concrete. In the structural phase of the study the shear strength of reinforced and prestressed concrete lightweight concrete (LWC) beams was evaluated. In the Kettelhut Structural Engineering Laboratory twelve reinforced and four prestressed concrete beams were tested to failure. The experimental shear capacities were compared to calculated shear strength values from the 1995 American Concrete Institute Building Code and the 1994 AASHTO LRFD Standard Specifications. Based on the test results from this study it is recommended that water-cementitious ratio be used for proportioning of lightweight concrete mixes and that trial batches be prepared and tested before actual field application. To ensure adequate durability, lightweight aggregate concrete should be allowed to dry before exposing it to cycles of freezing and thawing. In the structural side, both the ACI and the AASHTO LRFD Specifications were found to yield conservative estimates of the shear strength of LWC reinforced and prestressed beams provided adequate detailing of the reinforcement is provided. More work is needed regarding the minimum amount of shear reinforcement for high strength lightweight concrete beams