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Publication Number: FHWA-HRT-09-068
Date: Octorber 2009

 

Structural Behavior of A Prototype UHPC Pi-Girder

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FHWA Contact: Ben Graybeal, HRDI-06,
202-493-3122,benjamin.graybeal@dot.gov

This document is a technical summary of the unpublished Federal Highway Administration report, Structural Behavior of a Prototype Ultra-High Performance Concrete Pi-Girder, available only through the National Technical Information Service (NTIS).

NTIS Accession No. of the report covered in this TechBrief: PB2009-115495

Objective

This TechBrief highlights the results of a research program that evaluated an ultra-high performance concrete (UHPC) pi-girder cross section prototype developed for use in short- and medium-span highway bridge applications.

Introduction

UHPC is an advanced cementitious composite material which has been developed in recent decades. When compared to more conventional concrete materials, UHPC tends to exhibit superior properties such as exceptional durability, high compressive strength, usable tensile strength, and long-term stability.(1,2)

This experimental investigation focused on the structural behavior of a newly developed highway bridge girder cross section, the pi-girder. This girder was developed and optimized to exploit the advanced mechanical and durability properties of UHPC. Structural testing was completed on girders to investigate their flexural response, shear response, transverse flexural response, and lateral load distribution capabilities. Through this work, topics related to the design and fabrication of the pi-girder were also addressed.

UHPC

Advances in the science of concrete materials led to the development of the next generation of cementitious materials, namely UHPC. As a class, these concretes tend to contain high cementitious material contents, low water-to-cementitious material ratios, compressive strengths above 21.7 ksi (150 MPa), and sustained tensile strength resulting from internal fiber reinforcement. Table 1 presents a select set of material properties for the type of UHPC investigated in this study.(1)

Table 1. UHPC material properties.

Property
Value

Unit weight

156 lb/ft3
(2,500 kg/m3)

Modulus of elasticity

7,600 ksi
(52,400 MPa)

Compressive strength

28.0 ksi
(193 MPa)

Post-cracking
tensile strength

1.0 to 1.5 ksi
(6.9 to 10.3 MPa)

Chloride ion penetrability
(ASTM C1202)(3)

Negligible

UHPC PROTOTYPE PI-GIRDER

The advanced properties of UHPC provide opportunities for the development of new structural forms focused on addressing any number of important transportation infrastructure issues. Topics such as creating longer lasting bridges through enhanced durability or allowing for the spanning of longer distances with shallower superstructures can be addressed through the use of UHPC. In a systematic sense, UHPC also presents the opportunity to create new structural forms which facilitate accelerated construction and rapid renewal of the highway infrastructure.

The development of a decked girder prototype member which utilized UHPC as the primary structural component was initiated during the early stages of the Federal Highway Administration's (FHWA) UHPC research program. It is evident that UHPC's mechanical and durability properties allow for beneficial modifications to conventional concrete bridge component solutions. It also allows for the development of previously impractical components. Given the prior applications of conventional concrete decked girder members in infrastructure applications and the construction of the Peace Footbridge in Seoul, South Korea, a decked pretensioned girder with slender cross sectional dimensions was envisioned as a prototype solution.

The cross section dimensions of the UHPC pi-girder prototype were established through an analytical study completed at the Massachusetts Institute of Technology.(4,5) The models implemented in the design included one-, two-, and three-dimensional analyses of the predicted response of the girder to the loadings prescribed in the AASHTO LRFD Bridge Design Specifications.(6)

The girder prototype is a modular component designed to span 70 and 100 ft (21 and 30 m). Figure 1 shows a UHPC pi-girder cross section prototype, which is 96 inches (2.4 m) transversely and 33 inches (0.84 m) deep. It can also be prestressed by up to 15 strands in each bulb.

This figure shows a transverse slice of the prestressed ultra-high performance concrete (UHPC) girder investigated

Figure 1. Illustration. UHPC pi-girder cross section prototype.

TEST PROGRAM

This research program included two phases. The first phase focused on the fabrication of four UHPC pi-girders. These 70-ft (21.3-m)-long prestressed girders were fabricated at a conventional precast bridge girder production plant and then transported to FHWA's Turner-Fairbank Highway Research Center (TFHRC). The second phase of the research focused on the physical testing of these full-scale girders through the application of structural loads. Three tests including primary flexure and shear, transverse flexure, and load distribution were conducted. Figure 2 shows a full-scale shear test of the pi-girder prototype underway in the laboratory. Observations from the girder fabrication and results of the structural tests are discussed in detail within the corresponding main report.

This photo illustrates the setup used in the shear testing of the ultra-high performance pi-girder. Two hydraulic jacks are applying a load to the top surface of the girder, and each end of the girder is supported by rollers. One end of the girder is seen in the foreground, and the bottom of the bulbs is supported by rollers sitting on fixed abutments. The angle of the photo shows both the end of the pi-girder as well as part of the side and top of the girder.
Figure 2. Photo. Full-scale shear test.

 

CONCLUSIONS

The design, fabrication, and testing of a UHPC pi-girder prototype demonstrated that the concept of decked UHPC modular girders for bridge construction is viable. Girders of this type can be fabricated in existing prestressed girder production facilities. The structural response of the girder prototype exceeds design requirements in terms of ultimate shear capacity and meets flexural design requirements with the inclusion of appropriate prestress force. The load distribution capability of the girder is limited, so distribution of live loads beyond adjacent girder legs is not recommended. The transverse flexural capacity of the girder is insufficient to elastically resist service-level wheel loads.

FURTHER PI-GIRDER DEVELOPMENT

The research results presented in the corresponding report have led to the further development of the pi-girder concept. A 2nd generation pi-girder has been designed, full-scale test girders have been fabricated, and testing has begun. The 2nd generation pi-girder retains the initial concept, but it has been modified to provide significantly increased transverse flexural capacity while also allowing for simplified fabrication. Results of this follow-up program will be published in the near future.

References

  1. Graybeal, B.A. (2006). Material Property Characterization of Ultra-High Performance Concrete, FHWA-HRT-06-103, Federal Highway Administration, McLean, VA.
  2. Graybeal, B.A. (2006). Structural Behavior of Ultra-High Performance Concrete Prestressed I-Girders, FHWA-HRT-06-115, Federal Highway Administration, McLean, VA.
  3. ASTM. (1997). Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration, American Society for Testing and Materials Standard Practice C1202, Philadelphia, PA.
  4. Park, H. (2003). Model-Based Optimization of Ultra-High Performance Concrete Highway Bridge Girders, 139, Massachusetts Institute of Technology, Cambridge, MA.
  5. Soh, M. (2003). Model-Based Design of a Ultra-High Performance Concrete Prototype Highway Bridge Girder, 64, Massachusetts Institute of Technology, Cambridge, MA.
  6. AASHTO. (2007). AASHTO LRFD Bridge Design Specifications, 4th ed., American Association of State Highway and Transportation Officials, Washington, DC.

Researchers—This study was completed by Ben Graybeal of the Federal Highway Administration's (FHWA) Turner-Fairbank Highway Research Center. Additional information can be gained by contacting him at 202-493-3122 or in the FHWA Office of Infrastructure Research and Development located at 6300 Georgetown Pike, McLean, VA, 22101.

Distribution—The unpublished report (PB2009-115496) covered in this TechBrief is being distributed through the National Technical Information Service, www.ntis.gov.

Availability—The report will be available in November 2009, and it can be obtained from the National Technical Information Service, www.ntis.gov.

Key Words—Ultra-high performance concrete, UHPC, Fiber-reinforced concrete, Bridges, Precast concrete, Prestressed concrete, Bridge design, Accelerated construction, and Durable infrastructure systems.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' names appear in this TechBrief only because they are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration provides highquality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

 

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