Summary of Tests and Results

Lehigh University

Lehigh University

At Lehigh University in 1984, Professors Daniels and Slutter tested two Exodermic® panels, one with 2" of concrete and one with 3" of concrete. Substantial static testing was done before and during extended fatigue tests. The 3" panel was subjected to 3.3 million cycles of simulated HS 20 loading. Although a small amount of surface cracking eventually developed in the slab, this was considered typical of concrete decks in general. Daniels and Slutter concluded that "...an infinite in-service fatigue life can be expected..."

West Virginia University

West Virginia University

Professor Hota V.S. GangaRao, with Penmatsu Raju and K. Ramesh, conducted extensive tests of various grid decks at West Virginia University beginning in the late 1980s at West Virginia University's Constructed Facilities Center. Four Exodermic® panels were tested: two newly constructed panels of fairly light design, and the two original panels from the Lehigh tests.

An attempt to test the 'ultimate strength' of the older panels was inconclusive; the strength of the Exodermic® panels exceeded the capacity of the university's testing equipment -- 90,000 lbs loading. The test successfully demonstrated the strength and safety factors inherent in Exodermic® design.

Static and fatigue tests were conducted on the new panels with excellent results. Once again, Exodermic® design was shown to offer excellent internal composite behavior. In the final report for the Federal Highway Administration, the researchers concluded that:

  1. "...full composite action exists between the reinforced concrete slab and bare steel grid."

  2. "The strength and stiffness .. are nearly identical for .. traffic flow parallel or perpendicular to the main bars."

  3. Internal welds are fatigue category C in "a redundant load path structure. "Predicted and measured stresses within the steel grid are low enough such that the category C designation indicates that they will have an 'infinite' fatigue life.

Clarkson University

The "revised" design of Exodermic® decks is simpler and less expensive to fabricate, and with the elimination of the tertiary bars, contractors have more working room for installation of shear studs.

Historically, the Exodermic® deck evolved from traditional concrete-filled grids. The idea was to move the concrete from within the grid to the top of the grid in order to make more efficient use of the two components. Putting the concrete on top also allowed the use of reinforcing steel in the slab to significantly increase the negative moment capacity of the design, and to move the fabrication welds of the grid closer to the neutral axis of the section. A shear connecting mechanism was required between the grid and the slab, and this was provided by the addition of "tertiary bars" to which were welded short, 1⁄2" diameter studs. The tertiary bars are welded to the grid during fabrication of the Exodermic® grid panels, and extend up 1" into the structural slab.

In the "revised" design, the standard for all Exodermic® decks since 1997, the tertiary bars are eliminated, and their function is taken over by the extension of the main bars of the grid 1" into the slab. 3/4" diameter holes are punched in the top 1" of the main bars, to aid in the engagement of the bars with the concrete.

Testing of the revised design was conducted by Dr. Christopher Higgins, then an Assistant Professor at Clarkson University (now at Oregon State University). Professor Higgins and graduate student Heath Mitchell completed both static and fatigue tests of the revised design, and went on to do push out and pull out tests, looking specifically at the shear connecting mechanism between the concrete slab and the structural grid.

Results of the static and fatigue tests were published in the January/February 2001 issue of the Journal of Bridge Engineering. Results of additional testing have been submitted for publication.

The first test, in the autumn of 1997, involved testing a panel spanning 8' between supports to failure. Load was delivered through a load patch sized to simulate an HS-25 (20,000 pounds plus 30% impact factor) double truck tire footprint at 100 psi. The deflection and main bar strain were linear to 80,000 pounds loading, indicating full composite behavior to at least that point. The load/deflection curve gradually "softened" to 123.5 kips, when there was a punching shear failure of the concrete. Far from dramatic, a rectangular area around the load patch dropped approximately 1⁄2", and the panel was still carrying 66,000 pounds of load. Concrete used was a standard 3500 psi mix, using 3/8" maximum coarse aggregate.

The second test was a fatigue test, consisting of two million load cycles delivered to a two span continuous panel through two 9.1" x 22.8" steel loading shoes simulating a full simulated HS-20 truck axle. The tested spans were 7'2". Static tests were conducted at intervals of 250,000 cycles. No significant difference in behavior of the panel was observed from start to finish of the test. For example, strains measured at the bottom of the grid main bars at maximum load did not change meaningfully over the course of the test.

After completion of the fatigue test, the panel was cut in half, and a static test conducted in which it was sought to fail one half of the deck, with the test setup being as close as possible to the first such test.

Despite the two million fatigue cycles, the panel performed well, demonstrating a linear load-deflection response similar to that of the earlier panel. Due, in part, to the high performance concrete (NYSDOT Class DP, a 5000 psi mix) used in this test, the limits of the test setup were reached before the panel failed.

The applied load was increased gradually to 143,000 pounds, the limit of the load cylinder, and the panel did not reach punching shear as did the first one at 123,500 pounds. Surface cracking did not appear until approximately 118,000 pounds, versus 80,000 pounds in the first test. The crack pattern, when it did appear, was similar.

The testing was in accordance with the ASTM specification D6275-98, Standard Practice for Laboratory Testing of Bridge Decks. In addition to the report published in the Journal of Bridge Engineering, a comprehensive test report was prepared by Professor Higgins. For further information, please contact D.S. Brown.

Russell Road Bridge Field Test

One of the earliest Exodermic® decks, the Russell Road Bridge over the New York State Thruway in Albany, NY, was field tested by Professor Darlow and other researchers from Rennselaer Polytechnic Institute (RPI) with assistance from Neal Bettigole, the inventor of the Exodermic® design. Static live load testing was conducted before and after redecking. "Good agreement" between AASHTO design and measured strains was found. A major goal of the testing was to evaluate the properties of the composite girders. The researchers concluded that "Composite behavior was verified for an effective deck width..." equal to center-to-center stringer spacing, and "...with 't' equal to the full depth of the deck." This result makes intuitive sense in that the steel grid, which is transverse to the girders, does an excellent job of mobilizing a large portion of the concrete slab, allowing it to work compositely with the girders. The results of this research were published in the American Society of Civil Engineering's Journal of Structural Engineering in October of 1989.

High Street Bridge Field Test

In 1992, an Exodermic® deck on the High Street Bridge over the Metro North railroad in Dobbs Ferry, NY underwent field testing. On this structure, the Exodermic® deck spans 11' between floorbeams without stringers or other supports. The tests were conducted by M.G. McLaren, P.C., the consultants to Westchester County on the project, with assistance from EBDI. Static live load testing involved placing strain gauges on one of the trusses, a floor beam, and on the bottom of several deck main bars. According to McLaren's detailed report, "...testing and analyses show that the steel floor beams act compositely with the Exodermic® system. "Testing confirmed that the width of deck acting compositely with the floorbeams was at least 12t (with t = the full thickness of the Exodermic® deck). The consultant's report concluded that "this structure does not need to be posted." NYSDOT agreed, and the 10 ton limit sign, posted when the bridge carried a timber deck, was removed.

Galvanized Coating Testing

Hot dip galvanizing has been specified for Exodermic® decks for many years, and provides excellent protection from corrosion. Due to the 5.5 to 6.0 mils of zinc typically deposited on steel grids during hot dip galvanizing, coating life is expected to be at least 55 to 60 years.

In 1994, Chuck Wofton of Young Galvanizing conducted tests of coating thickness on the Exodermic® deck on the New York State Thruway's Russell Road Bridge. The grid portion of the Exodermic® deck was fabricated and galvanized in 1986, and the deck was erected in 1987.

Despite seven years of exposure to salt spray from the Thruway below it, the galvanized steel was visually in excellent condition, exhibiting the mottled appearance of newly galvanized steel. A few quotes from Mr. Wofton’s report to the New York State Thruway Authority follow:

  1. "After scraping the glue [from some haunch forms] off a bearing beam [of the grid] to expose galvanized surface, measurements showed a minimum zinc loss in the adjoining areas. The area protected by the glue was bright and lustrous in appearance, without any zinc oxides forming on the galvanized surface. This is indicative of recently galvanized steel."

  2. "Measurements taken on adjoining unprotected areas were, on the average, .55 mils less than the protected galvanized area. This calculates to only 5% loss of the galvanizing over approximately eight years."

  3. "With knowledge of the existing exposure condition and the amount of zinc remaining on the structure, one can make an accurate prediction of the service life of any hot dip galvanized item."

  4. "A conservative estimate of the Russell Road Bridge would be an additional 60 years to 5% rust."

Future Testing

Testing of the Exodermic® design will continue as the design is further refined to minimize total cost to the owner by reducing grid fabrication cost and simplifying field installation.