Question:
With a cast-in-place concrete deck placed on precast, prestressed concrete deck panels that are supported on recast, prestressed concrete beams, can I assume full composite action between the deck, panels, and beams in calculations of service load stresses and strengths?
Answer:
The answer to this question can be illustrated by the results of recent tests on four specimens, conducted at the Ferguson Structural Engineering Laboratory at The University of Texas at Austin. Each test specimen consisted of a Type I AASHTO girder with a depth of 28 in. (711 mm) and a composite deck with a thickness of 8 in. (203 mm) and a width of 78 in. (1.98 m). Two specimens used a full 8-in. (203-mm) thick cast-in-place, normal weight concrete deck. The other two specimens used 4-in. (101-mm) thick precast, lightweight concrete panels with a 4-in (101-mm) thick cast-in-place, normal weight concrete deck. The panels were placed immediately adjoining each other with no special treatment of the joint. The specimens were tested in flexure with a span of 24 ft (7.3 m).
The load-deflection curves for two pairs of test specimens are shown in the figure. Specimens 1 and 3 had a full-thickness, cast-in-place deck. Specimens 2 and 4 had precast panels with a half-thickness, cast-in-place deck. Specimens 1 and 2 were tested with a shorter shear span than Specimens 3 and 4. Consequently, Specimens 1 and 2 had less deflection for the same load and a higher load capacity.
The load-deflection curves and strengths for each pair of specimens are almost identical. Strain gages placed across the width of the slab showed that the full width of the slab was effective for both the full-thickness, cast-in-place deck and the composite, precast panel deck. Failure was identical for both cross sections with the strain at the top of the deck exceeding 0.003 before surface spalling and concrete crushing occurred at the maximum load. The measured flexural strengths exceeded values calculated either using the provisions of the AASHTO specifications or by strain compatibility analysis.
In terms of analysis at the section where the precast panels abut each other with no special treatment of the joint, the test results show that no special analysis is needed. Based on the test results, full composite action, with or without the use of precast, prestressed concrete panels, can be assumed for both service load and strength calculations.
Answer contributed by Ned H. Burns of the University of Texas at Austin. He may be contacted at [email protected] or 512-471-1619 for further information.