Disclaimer

The contents of this report reflect the views of the authors, who are responsible for the facts and the accuracy of the information presented.  This document is disseminated under the sponsorship of the Department of Transportation, University Transportation Centers Program, in the interest of information exchange. The U.S. Government assumes no liability for the contents or us thereof.

Abstract

The concept of combining wood and concrete in layered composite bridge deck was investigated.  A shear key/anchor detail recently used in Europe to construct floors in office buildings was adapted for this study.  This connection detail provides the interlayer shear transfer between the layers. Laboratory testing included anchor pull-out tests, interlayer slip tests on various key/anchor details, preliminary load tests of full-scale rectangular layered beam specimens and pilot tests of two full-size layered deck specimens. The deck specimens were realistic for short span right and skewed longitudinal deck bridges, respectively.  A rigorous analytical model successfully predicted the beam behavior.  Analytical work is in progress to rigorously model the composite behavior of the decks.  Results show that under static loading, a high degree of composite action was achieved in the beam specimens, as compared to use of ordinary mechanical connectors.  An initial analysis shows an extremely high efficiency for the deck specimens, but is overestimated in the model.

Executive Summary

The objective of this project was to configure a notched shear key/anchor detail for a composite wood-concrete bridge deck. The goal was to achieve a high degree of systems behavior to result in a viable short span bridge concept and possible strengthening method.

Commonly, deteriorating wood bridge decks are completely replaced without serious consideration of a possible retrofit to strengthen them.  This likely is due to lack of potential approaches to strengthen such decks.  Those tried have been limited in number and have proven structurally ineffective.  One approach to strengthening a wood bridge deck is to add a concrete deck layer and interconnect it to the wood deck.  While some time ago, other researchers attempted to achieve such composite decks, the structural effectiveness was inadequate.  Principally, shrinkage of the concrete and wood during hydration (curing) of the concrete resulted in a loss of bond with the mechanical connectors used to join the layers. This rendered the system essentially non-composite in a structural behavior sense.  Hence, the intended strengthening was not achieved.

Recently, in Europe, new technology has been used successfully in overlaying wood deck floors in office buildings with a concrete layer, thus creating a concrete-wood composite floor. An innovative notched shear key with a steel dowel tension anchor is used to join the layers.  The technique involves a unique, but readily done, interlayer connection method.   While a mechanical connector is involved, it is not relied upon for interlayer shear transfer needed to affect the desired composite behavior. Instead, a notched shear key is used to rely on wood to concrete shear and bearing to achieve the interlayer force transfer.  The mechanical connector tightens the concrete to wood-bearing surfaces after hydration drying of the concrete has taken place.  It is not affected itself by curing of the concrete, as it is anchored into the wood by gluing.  Thus, deficiencies of the past approaches apparently are overcome.

It was reported that this method produces a high degree of composite action, exceeding 80 percent. Consequently, it was a promising idea for application to short span bridges.  In this MPC project, extensive laboratory load testing was conducted to study this concept.

First, geometry of the shear key connection was examined in load tests of full-scale specimens.   A preferred geometry and adhesive type were established from these tests.

Second, a series of full-scale beam specimens were tested in different basic cross-sectional geometries.   They were representative of a portion of the width of full size prototype deck systems.  These enabled successful determination of its effectiveness before constructing full-scale deck specimens.  It also was a basis for developing a computer-based analytical model of the system behavior.

Third, two layered bridge deck specimens incorporating the connection detail were load tested.   They were loaded by point loads in various locations to simulate concentrated wheel loads.  One was for a right bridge geometry and the other a skewed bridge geometry.  Assessment of the degree of composite behavior was made based on test results for the bare wood deck and the retrofitted specimen resulting from the addition of a concrete layer.

Compared to past tests of layered wood-concrete T-beams interconnected by mechanical connectors, the beams with notched shear key anchor detail performance was dramatically superior.  The former produced efficiencies of about 10-20%, whereas the latter exhibited a range between 54.9 percent and 77 percent.  The two deck specimens exhibited efficiencies of 81.1 percent and 92.2 percent, but these may be overestimated due to limitations of the computer model used in calculating them.

Nationwide, about 20 percent of highway bridges nationwide involve the use of timber decking.  In Region 8, state and local bridges predominately are on secondary roads and are critical to the movement of the vast agricultural and mineral production. The dispersed rural area and low tax base makes saving every possible bridge repair and replacement dollar a critical need.  The technology examined in this project has the potential to save existing deteriorated wood decks and increase the overall bridge load capacity.  The outcome is a feasible concept based on laboratory studies.  Further work is recommended on durability aspects, including humidity (moisture) and temperature changes, repeated loading and creep effects.   Ideally, an experimental bridge on an actual roadway site is desirable as a proof of concept.