W.R. Brown III, PE and Torbjorn J. Larsen, PE

Florida Department of Transportation, Gainesville, Florida 32602

Thomas A. Holm, PE

Expanded Shale, Clay and Slate Institute

Authorized reprint from the "International Symposium on Structural Lightweight Aggregate Concrete", June 1995, Sandefjord, Norway

LONG-TERM SERVICE PERFORMANCE OFLIGHTWEIGHT CONCRETE BRIDGE STRUCTURES

W.R. Brown, III and Torbjom M. LarsenFlorida Department of Transportation, United States

Thomas A. !:lQlmExpanded Shale, Clay and Slate Institute

INTRODUCTION

The long-term field performance of structural lightweight concrete bridge members constructedin Florida on U.S. Route 19 at Fanning Springs in 1964 was evaluated in an in-depth investi-gation conducted in 1992. Comprehensive field measurements of selVice load strains anddeflections were measured in 1968 and 1992 and compared to the theoretical bridge responsespredicted by a finite element model that is part of the Florida Department of Transportation

bridge mting system.

SUWANEE RIVER BRIDGE AT FANNING SPRINGS

The bridge at Fanning Springs (Figure 1), crossing the Suwanee River, incorporates a fourspan precast prestressed lightweight concrete framing system with a specified girdercompressive strength of 34.5 MPa (5000 psi) [ 1 ]. The cast-in-place concrete slabs incorporatedstructural lightweight coarse aggregate and had a specified compressive strength of 27.6 MPa(4000 psi). This two-lane bridge had, for its time, relatively long 36.9 m ( 121 ft) spans usingAASHTO Type 4 sanded structural lightweight concrete girders shown during construction inFigure 2. Continuity was developed by conventional reinforcing steel in the top of the girders,diaphragms and with continuous steel across the deck slab. Concrete specifications caned for amaximum fresh density of 1880 kg/m 3 ( 120 pcO incorporating a Florida produced rotary kilnexpanded clay in combination with a local natural siliceous sand.

-

Because there was no prior structural lightweight concrete bridge member experience in Floridabefore use on the Fanning Springs structure, an extensive research and field measurementlaboratory program was established to continuously monitor the performance of the completedstructure over its first two year history. The suspended car assemblage designed to carryresearchers for measurements under the bridge is shown in Figure 3. As a direct result of the1966-1968 testing program detailed concrete information, deflection, and strain responses ofthe bridge were available for a wide range of load conditions. The test program in 1992conscientiously attempted to reproduce the original loads and measurements to provide a directcomparison to determine any change or loss of performance over the 28 year service life of the

bridge.

LOAD TEST PLAN AND INSTRUMENTATION

The original HS20 truck configuration used in 1968 is shown in Figure 4 and the schematic ofthe Florida Department of Transportation truck used in the 1992 test is shown in Figure 5. Thetrucks are similar in overall weight, but have different axle configurations and spacings. Usingthe truck's center of gravity as the references, the load points for these tests were matched to thecorresponding original loading points. A large number of static load tests were recreated alongwith numerous combinations of lane spacings as well as positions of longitudinal spacing.

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LOAD POSITIONS

FIGURE 7. POSITION OF TRUCK (CG) DURING STATIC LOADING.

LOAD TEST RESULTS

The original loadings and measurements were duplicated with typical calculated and measureddeflection curves shown gmphically in Figure 8. Maximum deflection for one particular beamdue to a mid-point load was 7.1 mm (.28 inches), measured at 18.4 m (60.5 feet) from theunrestmined end of the span. This compares very well with the original deflection which wasrecorded to be 6.6 mm (.26 inches) measured at 15.4 m (50.5 feet). Rolling load deflectionsshown in Figure 9 were compamble, but slightly less in magnitude than the static loads.Comparison of predicted with measured deflections for the bridge profile of seveml beams areshown in Figure 10.

Stmin measurements across the bridge profile were also duplicated and are shown for oneparticular loading in Figure 11. Again, the stmin measurements compare very closely for mostlocations located in areas of significant strain. Highest stmins of 85 and 72 microstmins wererecorded for the exterior beam at 15.4 m and 18.4 m (50.6 and 60.5 feet) when loaded with atruck in the appropriate lane. Again, comparison of 1994 and the 1968 data shows bridgebehavior to be essentially similar with the profiles closely matched.

FATIGUE

It appears that the dynamic testing determination of the flexural characteristics of the 31 year oldlong-span strocturallightweight concrete bridge (Realcrete) corroborate the conclusions offatigue investigations conducted on small (Labcrete) specimens tested under controlledconditions in numerous laboratories [3], [4], [5]. In these investigations, it was generallyobserved that the structumllightweight concretes performed as well as, and in most cases,somewhat better than companion normal density control specimens. Several investigators havesuggested that improved performance was due to the elastic compatibility of the lightweightaggregate particles to that of the surrounding cementitious matrix. In structumllightweightconcrete, the elastic modulus of the constituent phases (coarse aggregate and the envelopingmortar phase) are relatively similar while with normal density concretes the elastic modulus ofmost ordinary aggregates may be as much as 3 to 5 times greater than their enveloping matrix[6]. With structumllightweight concretes, elastic similarity of the two phases ofa compositesystem results in a profound reduction of stress concentmtions and a leveling out of the averagestress over the cross section of the loaded member. Normal density concretes having a

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FIGURE 13. EXPOSED WEARING SURFACE, LIGHTWEIGHT CONCRETE BELOWTHE JOINT AND NORMAL WEIGHT CONCRETE ABOVE THE JOINT ISEBASTIAN INLET BRIDGE, FLORIDA.

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