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The photo shows the erection of the girders at night.

The use of high strength lightweight concrete reduced shipping weights.                     

High Strength Lightweight Concrete for Use in Precast, Prestressed Concrete Bridge Girders in Georgia
Paul Liles, Georgia Department of Transportation and R. Brett Holland, Georgia Institute of Technology
The development of high strength concretes has allowed for the use of longer precast, prestressed concrete bridge girders throughout the United States. In Georgia, the increased lengths result in girders that often are too heavy to transport across some existing bridges and require a super-load permit if they are to be transported at all. The use of high strength, high performance lightweight concrete (HSLWC) can result in longer span lengths and lighter weight girders.(1) Previous research at the Georgia Institute of Technology (Georgia Tech) showed that HSLWC bridge girders can be constructed with 10,000 psi (69 MPa) compressive strength concrete with a very low permeability, while achieving up to a 20% decrease in shipping weight.(1)

To determine the practicality and in-place performance of HSLWC bridge girders, the Georgia Department of Transportation (GDOT) designed and constructed a bridge with two spans having HSLWC in the girders. The center two spans of the four-span I-85 Ramp crossing State Route 34 in Newnan each consist of AASHTO BT-54 girders made with HSLWC using expanded slate coarse aggregate and manufactured granite sand and a composite deck with normal weight concrete (NWC). The girders have a span length of 110 ft (33.5 m) and a concrete design strength of 10,000 psi (69 MPa). The girders were the first use of HSLWC by the GDOT and were part of a research project to monitor the performance and material properties of girders constructed with HSLWC, as discussed in the next article.

The results of the previous research were compared to the results from the field production of the girders. The 56-day compressive strengths exceeded the required 10,000 psi (69 MPa) design strength and the chloride ion permeability tests(2) showed very low values (284 to 360 coulombs at 56 days).(3) Therefore, the research knowledge was successfully transferred to field production. The girder construction emphasized the importance of adequately soaking the lightweight aggregate prior to batching, otherwise early and later-age strengths were reduced.

Design and Construction Considerations with HSLWC
The GDOT was concerned about the camber of the girders and the deflection due to the dead load of the deck slab so that the roadway profile was accurately constructed with a minimum of grinding. An accurate estimate of the modulus of elasticity and girder stiffness was essential in order to predict deflections as well as prestress losses. For HSLWC with expanded slate aggregate, the modulus of elasticity is less sensitive to the unit weight than the AASHTO LRFD(4) equation predicts. The modulus of elasticity of the HSLWC with expanded slate aggregate was estimated by Meyer’s equation,(1) Ec = 44,000 [f 'c (wc /145)]0.5 where f 'c is compressive strength in psi and wc is unit weight in lb/ft3. Georgia Tech load tested five of the lightweight concrete bridge girders to verify their stiffness; results of these tests verified the Ec value.

Prestress losses affect the camber in the girder, as well as the service load stresses. The figure below shows a comparison of the calculated total prestress losses of a 10,000 psi (69 MPa) HSLWC girder versus a 10,000 psi (69 MPa) NWC girder using the Tadros method.(5) The Tadros method is the basis for the current AASHTO LRFD method for refined estimates of time-dependent losses. HSLWC undergoes increased elastic shortening losses compared to NWC due to the lower modulus of elasticity. In addition, GDOT research at Georgia Tech has indicated that HSLWC has similar long-term losses due to creep and shrinkage as a high performance NWC.(6) The long-term evaluation of the I-85 Ramp Bridge is designed to determine if this research prediction is correct. To date, results have shown that the current AASHTO LRFD bridge design specifications may be safely used for HSLWC girders as long as the appropriate value for the modulus of elasticity is used and appropriate care is taken in girder construction.

The graph shows calculated prestress losses versus concrete age for lightweight and normal weight concrete.
Comparison of total calculated losses for high strength lightweight and normal weight concrete.

Summary
The construction and monitoring of Georgia’s I-85 Ramp crossing State Route 34 using HSLWC girders demonstrated that the HSLWC research can be applied to construction practice and that lightweight concrete provides an effective material for reducing the weight of a bridge; thus permitting longer spans to be efficiently constructed. The construction has verified that special attention needs to be given to soaking the aggregate prior to batching and that attention needs to be paid in evaluating the modulus of elasticity so that deflections can be accurately predicted. Otherwise, it appears that HSLWC girders can be used in routine design applications for highway bridges.

Further Information
For further information on HSLWC use for precast, prestressed concrete bridge girders in Georgia, please contact Paul Liles at pliles@dot.ga.gov or Brett Holland at rbholland@gatech.edu.

References
1. Meyer, K. F., “Transfer Length and Development of 0.6-inch Diameter Prestressing Strand in High Strength Lightweight Concrete,” Doctoral Thesis, Georgia Institute of Technology, 2002, 616 pp.

2. Standard Method of Test for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration, AASHTO T 277, American Association of State Highway and Transportation Officials, Washington, DC.

3. Dunbeck, J., “Evaluation of High Strength Lightweight Concrete Precast, Prestressed Bridge Girders,” Master’s Thesis, Georgia Institute of Technology, 2009, 186 pp.

4. AASHTO LRFD Bridge Design Specifications, 4th Edition, American Association of State Highway and Transportation Officials, Washington, DC, 2007.

5. Tadros, M. K., Al-Omishi, N., Seguirant, S. J., and Gallt, J. G., “Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders,” NCHRP Report 496, Transportation Research Board, Washington DC, 2003, 73 pp.

6. Lopez, M., “Creep and Shrinkage of High Performance Lightweight Concrete: A Multi-scale Investigation,” Doctoral Thesis, Georgia Institute of Technology, 2005, 530 pp.

HPC Bridge Views, Issue 61, May/June 2010