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Feasibility of Utilizing High Performance Lightweight Concrete in Pretensioned Bridge Girders and Panels

Sylva III, Gilbert S.

2001

The use of high performance lightweight concrete in Texas prestressed concrete bridges has potential advantages and disadvantages. Advantages include reduced dead load, crane capacity, and shipping costs. Disadvantages include higher prestress losses, deflections, camber, and material costs. Prestressed concrete bridge girders can be designed with lightweight concrete that has compressive strengths of 6000 psi and 7500 psi and unit weights of 118 pcf to 122 pcf, respectively. Comparisons of AASHTO Type IV girders made from normal weight concrete and girders made from lightweight concrete, both with various composite concrete deck combinations, reveal that higher prestress losses and lower allowable stresses reduce the possibility of having fewer prestressing strands in the lightweight girder. The design of the lightweight concrete girder was controlled by the allowable stresses and not by strength requirements. The lower modulus of elasticity of lightweight concrete results in higher camber and deflections.

Testing of 3/8-inch prestressing strands in precast concrete panels to determine the transfer length showed that the AASHTO provision of 50 times the strand diameter is conservative for these panels. The transfer length in the lightweight concrete panel was slightly higher than the transfer length in the normal weight concrete panels, but both were below the AASHTO criteria.

Lightweight concrete material costs are higher than normal weight concrete. However, the higher costs are somewhat offset by reduced shipping costs. Larger shipping savings for girders can be realized by shipping two girders at the same time, but this is only practical for the smaller Type A girders. The precast concrete panels made from lightweight concrete also provide opportunity for reducing the shipping and handling costs.

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