Gary S. Greene, Jr. and Benjamin Graybeal, Federal Highway Administration
The Federal Highway Administration (FHWA) is currently conducting a study on lightweight (specified-density) high performance concrete (LWHPC) at the Turner-Fairbank Highway Research Center (TFHRC). This study includes a comprehensive experimental program to investigate the performance of lightweight concrete (LWC) with a mixture of a lightweight and normal weight coarse aggregates, commonly known as specified-density concrete. The target densities range from 125 to135 lb/ft3 (2000 to 2160 kg/m3). The research program is investigating the performance of LWHPC members in terms of their shear strength, short- and long-term prestress losses, transfer and development lengths of prestressing strands, and the bond strength of nonprestressed reinforcement. The intended goal of the study is to identify any necessary changes to the AASHTO LRFD Bridge Design Specifications pertaining to the use of specified-density LWHPC.
Under contract to FHWA, Russell summarized the articles in the AASHTO LRFD Specifications that currently address or should address LWC, synthesized the existing LWC research, and outlined the need for additional research.(1) The FHWA then developed a research program focusing on specified-density LWHPC to address many of these needs.
As part of this research program, 27 full-scale bridge girders and 40 beams with lap splices were fabricated by Standard Concrete Products, a company specializing in precast and prestressed concrete construction, in Mobile, AL. The girders and lap splice beams used three different lightweight aggregates (Haydite, Stalite, and Utelite), from sources geographically distributed across the United States. The Expanded Shale, Clay, and Slate Institute assisted FHWA in identifying existing specified-density concrete mix designs that could be used for the structural components tested in the research program. To date, all 40 splice beams and 9 of 27 bridge girders have been tested. A summary of the research program status is provided below.
Lap Splice Strength of NonPrestressed Reinforcement in LWHPC
Forty reinforced LWHPC splice beams with depths of 18 in. (460 mm) and widths of 6, 9, 12, and 18 in. (150, 230, 305, and 460 mm) were fabricated and tested to evaluate the bond strength of nonprestressed reinforcement in LWHPC. Each beam had three bottom-cast, uncoated, bars lap spliced at the same location. Eighteen of the beams also had transverse reinforcement as stirrups evenly spaced over the splice length.
These tests are significant because of the paucity of bond strength test data for this type of concrete. Key test parameters include the lightweight aggregate, bar size, splice length, and the presence of transverse reinforcement. The measured 28-day cylinder compressive strengths of the concretes in the beams ranged from 5700 to 10,600 psi (39.3 to 73.1 MPa) with densities ranging from 126 to 138 lb/ft3 (2018 to 2210 kg/m3). Applicability of the current AASHTO LRFD and ACI 318 equations for development length of deformed bars in tension to LWHPC was determined from the test results. First, the maximum reinforcement stress in the test specimens was computed from the maximum applied moment based on equilibrium and strain compatibility. This stress was then compared with the stress calculated using the AASHTO LRFD and ACI 318 equations for development lengths.
Both the AASHTO LRFD Specifications and the ACI 318 Building Code specify modification factors for use with LWC. Alternatively, the lightweight modification factor can be calculated using the splitting tensile strength when specified.
The tension development length equation of ACI 318-08 code gave conservative estimates of average bar stress at failure for all 40 splice beam tests i.e. the experimental bar stresses were greater than calculated.
For design purposes, the AASHTO LRFD Specifications multiply the basic tension development length of an individual bar by 1.7 for lap splices where 100% of the bars are spliced and the area of reinforcement provided is less than twice the area required as provided in the test specimens. Incorporating this factor resulted in conservative predictions for all 40 tests.
Development Length of Prestressing Strands in LWHPC Bridge Girders
Twelve AASHTO Type II girders were fabricated with varying aggregate types, prestressing force, strand size, and amount of shear reinforcement. The girders were made with specified-density concrete that had a blend of lightweight and normal weight coarse aggregate and normal weight sand. The average measured 28-day compressive strengths for the three girder mixes ranged between 7400 and 10,500 psi (51.0 and 72.4 MPa). Normal weight concrete decks were cast on the girders after they were delivered to TFHRC. A photograph of a strand development length test setup is shown below.
Development length tests have been completed on 9 of the 12 girders. Preliminary results indicate that the AASHTO LRFD Specifications provide a conservative estimate of the development length for 0.5- and 0.6-in. (12.7- and 15.2-mm) diameter strands in the specified-density LWHPC mixes investigated in this study.
Shear Performance of LWHPC Bridge Girders
An additional nine AASHTO Type II and six BT-54 girders were fabricated alongside the twelve girders discussed above. These girders will be tested to evaluate the shear performance of LWHPC bridge girders. Parameters including quantity of shear reinforcement, prestressing force, strand size, and type of lightweight aggregate will be evaluated. The physical testing will be completed by the end of 2010.
Prestress Losses
Long-term prestress losses are being monitored in the 15 prestressed concrete girders to be used for the shear tests. Internal concrete strains at multiple locations and long-term deflections are being measured.
References
Russell, H. G., “Synthesis of Research and Provisions Regarding the Use of Lightweight Concrete in Highway Bridges,” FHWA U.S. Department of Transportation, Report No. FHWA-HRT-07-053, 114 pp. Available as PB2007-110768 at www.ntis.gov.
Editor’s Note
The research described in this article complements the research currently being performed in NCHRP Project 18-15, High Performance/High Strength Lightweight Concrete for Bridge Girders and Decks.