ELSEVIER

J. Construct. Steel Res. Vol. 46, Nos. 1-3, pp. 69-70, paper number 56, 1998 01998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain PII: so143-974x(98)oooY3-x 0143-974xM $17.00 + 0.00

Experimental and Theoretical Study of the Behaviour of Composite Bridges During Construction

Jean-Paul Lebet and Jean-Marc Ducret

ICOM-Steel Structures, Swiss Federal Institute of Technology, CH 1015 Lausanne, Switzerland

Paper Number 56 Full paper on enclosed CD-ROM

The behaviour of steel-concrete composite bridges is influenced by time dependent effects in the concrete deck slab that can affect the durability of such structures. A detailed study of the causes of transverse cracking in con- crete slabs of composite bridges has been carried out in order to understand better the most important parameters with respect to the origin of tensile stresses in the slab. In-situ measurements and laboratory tests have enabled the behaviour of concrete slabs to be observed from the moment they were placed.

The effect of concrete hydration in a deck slab is directly connected to steel beams in a composite bridge has been studied with the aid of in-situ measurements, laboratory tests and numerical simulations. Criteria based on a restraint coefficient, p, defined as the ratio of steel beam and concrete cross- sectional areas, have been established to determine the effects of concrete hydration in relation to the type of bridge being constructed. Furthermore, a simplified method has been developed to evaluate residual tensile stresses due to concrete hydration in deck slabs.

A continuous casting sequence generates high tensile stresses in the inter- mediate support zones, in particular for large span bridges. In comparison with other actions, the most critical tensile stresses are present at the end of construction and are due to hydration effects and the casting sequence.

Three tensile stress limitation methods have been thoroughly studied: the limitation of hydration effects, a strategic casting sequence and various pre- stressing techniques. Methods for limiting the effects of hydration, such as the use of low-heat cement or cooled concrete, have been tested and shown to reduce significantly the residual tensile stresses in the deck slab. The use of the so-called piano method casting sequence significantly limits the tensile stresses in the deck slab at the intermediate supports. An analysis of prestress- ing techniques has shown that a loss of compression in the deck slab is depen-

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dent on the span length and, therefore, the restraint coefficient if the slab is connected to the steel girders before prestressing.

A qualitative relationship between tensile stresses in the deck slab after construction and the probability of cracking with time has been defined. It was shown that the use of limitation methods, as related to the probability of cracking, is dependent on the span length. The use of tensile stress limitation methods becomes more important with both longer spans and when a lower probability of cracking with time is desired. 0 1998 Elsevier Science Ltd. All rights reserved

KEYWORDS

Composite bridges, deck slab, cracking, prestressing, construction, tensile stress, hydration, durability, serviceability, in-situ measurements, laboratory tests.