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Publication Number: FHWA-HRT-07-051
Date: August 2007
TechBrief: Current Provisions and Needed Research for Lightweight Concrete in Highway Bridges
FHWA Contact: Benjamin Graybeal,
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NTIS Accession No. of the report covered in this TechBrief: PB2007-110768.
This document is a technical summary of the unpublished Federal Highway Administration (FHWA) report, Synthesis of Research and Provisions Regarding the Use of Lightweight Concrete in Highway Bridges (FHWA Contract No. DTFH6104-C-00029), which is available only through the National Technical Information Service (www.ntis.gov).
This TechBrief summarizes a synthesis report on completed research and current American Association of State Highway and Transportation Officials (AASHTO) provisions related to lightweight concrete used in highway bridges. The synthesis report is intended to provide a reference point for an FHWA research program aimed at addressing perceived shortcomings in the AASHTO specifications pertaining to lightweight concrete. The synthesis report also makes recommendations for future research efforts, focusing on the use of lightweight concrete in bridge structures.
Significant research efforts currently are being performed both under the direction of the National Cooperative Highway Research Program and by other researchers to update and modify the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications for the use of normal weight concrete with compressive strengths up to 103 or 124 megapascals (MPa) (15 or 18 kips per square inch (ksi)). These efforts do not address many of the research needs related to lightweight concrete. Current provisions that apply to the use of lightweight concrete need to be verified and/or modified with regard to their applicability to high-performance lightweight concretes.
There is, therefore, a need to review the AASHTO specifications to identify the relevant articles that address or should address the use of lightweight concrete in highway bridges and to synthesize existing research that is relevant to those articles. This information then can be used to define further research needs.
Synthesis Report Scope
The report is a compilation of the relevant provisions of the AASHTO LRFD Bridge Design Specifications and the AASHTO LRFD Bridge Construction Specifications that address or should address the use of lightweight concrete in highway bridges. The review includes versions of both the design and the construction specifications through the 2006 Interim Revisions. For each specification, the report gives a compilation and synthesis of research relating to lightweight concrete and its use in highway bridges.
The report concludes by providing descriptions of five research needs detailing the scope and extent of work required to generate data sufficient for updating the current specifications in areas where gaps exist in the collective body of knowledge.
The synthesis indicates that the majority of provisions in the AASHTO LRFD Bridge Design Specifications are based on normal weight concrete with some form of modification factor applied for all-lightweight and sand-lightweight concretes. This modification is handled in several different ways: (i) adjusted strength reduction factors, (ii) modified factors for shear, (iii) multipliers for development length, and (iv) entirely separate provisions for lightweight concrete. In some articles, more than one modification applies. Most modifications are focused on the reduced tensile strength of lightweight concrete compared to normal strength concrete with a similar compressive strength. The modifications depend on the amount and type of fine aggregate. An assessment is needed to determine whether this approach is consistent with and best for today's materials and the ability to produce higher strength concretes.
The synthesis report recommends that future research efforts focus on the use of lightweight concrete in bridge structures. Each research effort has the objective of validating the applicability or developing proposed revisions to pertinent articles of the AASHTO LRFD Bridge Design Specifications for use with lightweight concrete having design concrete compressive strengths up to 69 MPa (10.0 ksi) or greater. In conjunction with this, an overall approach needs to be developed for concrete with densities between 1,922 and 2,162 kilograms per cubic meter (120 and 135 pounds per cubic foot). In addition, all research programs should consider that lightweight concrete can be produced with different aggregates, and this may affect their structural behavior. The research programs consequently should include several different lightweight aggregates.
Proposed future research areas include:
For additional information, contact Benjamin Graybeal at 202-493–3122 or at the Turner-Fairbank Highway Research Center, Office of Infrastructure Research and Development, 6300 Georgetown Pike, McLean, VA 22101-2296.
HRDI-06/08-07(500)E AUGUST 2007