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• Optimized Sections for High-Strength Concrete Bridge Girders—Effect of Deck Concrete Strength

Optimized Sections for High-Strength Concrete Bridge Girders—Effect of Deck Concrete Strength

CHAPTER 1. INTRODUCTION

BACKGROUND

For more than 25 years, concretes with compressive strengths in excess of 41 megapascals (MPa) (6,000 pounds per square inch (psi)) have been used in the construction of columns of highrise buildings. ⁽¹⁾ While the availability of high-strength concretes was limited initially to a few geographic locations, opportunities to use these concretes at more locations across the United States have arisen. With the increase of such opportunities, material producers have accepted the challenge to manufacture concretes with higher compressive strengths. Although the technology to produce higher-strength concretes has developed primarily within the ready-mix concrete industry for use in buildings, the same technology can be applied in the use of concretes for bridge girders and bridge decks.

For precast, prestressed concrete bridge girders, compressive strengths in excess of 41 MPa (6,000 psi) have rarely been specified. However, strengths at release have frequently controlled the concrete mix design so that actual strengths at 28 days are often in excess of 41 MPa (6,000 psi). In recent years, a strong interest in using concrete with higher compressive strengths for bridge applications has emerged at a few geographic locations in a manner similar to the developments in the building industry. Several research studies have addressed the application of high-strength concrete in bridge girders and have identified the potential benefits of this approach (e.g., the use of fewer girders per cross section, longer span lengths, and more economical structures). (See references 2–6.)

The durability of concrete bridge decks has been a concern for many years, and numerous strategies to improve the performance of bridge decks have been undertaken. These include the use of greater cover to the reinforcing steel, the use of epoxy-coated reinforcing steel, the use of special admixtures in concrete to reduce permeability, and the use of sealers to reduce the penetration of chlorides into the concrete. Most codes and specifications now recognize that a more durable concrete can be achieved through the use of a low water-to-cementitious-material ratio, appropriate air entrainment, and appropriate cementitious materials to produce a low permeability concrete. These concretes are now becoming known as high-performance concretes where high-performance includes durability and ease of placement as well as strength. Many of the factors that enable a durable concrete to be produced also result in a high-strength concrete. Consequently, if a concrete for a bridge deck is designed to be durable, it will probably also have a high compressive strength. This research program was initiated, therefore, to investigate the cost and structural advantages of using high-performance concretes in bridge decks.

OPTIMIZED CROSS SECTIONS FOR BRIDGE GIRDERS

In the early applications of prestressed concrete, designers developed their own ideas of the "best" girder cross section to use. As a result, each bridge used a different girder shape, making it impossible to reuse girder formwork on subsequent contracts. Girder shapes were subsequently standardized in the interest of improving economy of construction which, in turn, led to the development of the standard American Association of State Highway and Transportation Officials—Prestressed Concrete Institute (AASHTO–PCI) sections for bridge girders. Girder types I through IV were developed in the late 1950s, and types V and VI in the 1960s.

Adoption of the AASHTO standard bridge girders simplified design practice and led to the wider use of prestressed concrete for bridges. Standardization resulted in considerable cost savings in the construction of bridges. However, following the original adoption of the standard AASHTO–PCI shapes, individual States again developed their own standard sections for improved efficiency and economy. In l980, the Federal Highway Administration (FHWA) initiated an investigation to identify new optimized sections for major prestressed concrete girders.

In an FHWA study published in 1982, Construction Technology Laboratories, Inc. identified the Bulb-Tee, Washington, and Colorado girders as the most structurally efficient sections.^{(7, 8)} A cost effectiveness analysis recommended the use of the Bulb-Tee girder (with a 152-millimeter (mm) (6‑inch) web) as a national standard for precast, prestressed concrete bridge girders in the United States for span lengths ranging from 24 to 43 meters (m) (80 to 140 feet (ft)).

Subsequently, the PCI Committee on Concrete Bridges developed a modified section for use as a national standard. ⁽⁹⁾ The modifications resulted in a slightly heavier section that was easier to produce and handle. This cross section was later adopted by several States and is identified as the Bulb-Tee (BT-72) in this report. Several other versions of the Bulb-Tee have also been developed in different geographic locations.^{(10, 11)}

A recently completed report for the FHWA entitled Optimized Sections for High-Strength Concrete Bridge Girders concluded that the use of existing girder cross sections with concrete compressive strengths up to 69 MPa (10,000 psi) will allow longer span lengths and more economical structures. ⁽⁶⁾ In order for concrete with compressive strengths in excess of 69 MPa (10,000 psi) to be used effectively, additional prestressing forces must be applied to the cross section. Report conclusions were based on analyses performed using the computer program BRIDGE which determines relative unit costs and maximum span lengths for different prestressed concrete bridge designs. All analyses were based on the assumption that concrete in the deck had a compressive strength of 28 MPa (4,000 psi) and that the prestress losses have a constant value of 310 MPa (45,000 psi). However, these assumptions may not reflect true behavior and current trends in the usage of high-performance concretes.

HIGH-PERFORMANCE CONCRETE IN BRIDGE DECKS

The American Concrete Institute (ACI) has defined high-performance concrete as concrete meeting special performance and uniformity requirements that cannot always be achieved routinely using only conventional constituents and normal mixing, placing, and curing practices. ⁽¹²⁾ These requirements may involve enhancements of the following:

• Ease of placement and compaction without segregation.
• Long-term mechanical properties.
• Early age strength.
• Toughness.
• Volume stability.
• Long life in severe environments.

For bridge decks, high-performance concrete needs to have enhanced long-term mechanical properties, enhanced toughness, and long life in severe environments. Volume stability is also desirable. From a construction standpoint, ease of placement and compaction without segregation is also essential. Therefore, a concrete to be used for a durable and long-lasting bridge deck needs to meet all the requirements of a high-performance concrete as defined by ACI.

In the Strategic Highway Research Program (SHRP) C-205, high-performance concrete for pavements and bridges was defined as concrete with the following characteristics:

• A maximum water–cementitious material ratio of 0.35.
• A minimum durability factor of 80 percent as determined by ASTM C666 Method A.
• A minimum strength criteria of:
  1. 21 MPa (3,000 psi) within 4 hours of placement.
  2. 34 MPa (5,000 psi) within 24 hours.
  3. 69 MPa (10,000 psi) within 28 days. ⁽¹³⁾

For bridge decks, items 1 and 2, as defined by SHRP, are clearly essential and desirable for long-term durability performance. However, the use of a water-to-cementitious-material ratio of 0.35 will result in a concrete compressive strength at 28 days well in excess of the 28 MPa (4,000 psi) that is often specified for today's bridge decks and could well be in the range of 41 to 55 MPa (6,000 to 8,000 psi).

The use of high-performance concretes in bridge decks is receiving an increased amount of attention as a means of improving durability. High-performance concretes provide higher resistance to chloride penetration, higher resistance to deicer scaling, less damage from freezing and thawing, higher wear resistance, and less cracking. Many of the methods used to increase the durability of concrete result in a concrete that has a higher compressive strength. However, the higher concrete strength is rarely considered because the design of long-span prestressed girders is controlled by service load stresses caused by dead load, live load, and impact.

In a recent project for the Louisiana Transportation Research Center, four full-size, prestressed concrete girders were tested to destruction in flexure. ⁽¹⁴⁾ While the specified strength of concrete in the girders was 69 MPa (10,000 psi), the bridge design for these girders required concrete with a compressive strength of only 29 MPa (4,200 psi) in the deck. However, analyses of the cross section indicated that failure in flexure would occur by crushing of the deck concrete. Consequently, a concrete compressive strength of 41 MPa (6,000 psi) was specified for the deck to ensure flexural failure by fracture of the strands. Although the flexural strength of the section was reached when the strands fractured, evaluation of the test results indicated that the section was close to failure by crushing the concrete deck even when 41 MPa (6,000 psi) concrete was used in the deck.

It was noted in a previous report that as girder concrete strength increased, a point of diminishing benefits was reached. ⁽⁶⁾ The primary cause of these diminishing returns is decreasing strand eccentricity. Once strands have to be placed within the web, the efficiency of additional strands decreases rapidly. Finally, a point is reached where no more space is available for additional strands. The only benefit, therefore, is an increase in the concrete tensile strength. Another factor contributing to the reduced benefits was the deck concrete strength. In calculating the composite section properties, transformed girder–deck section was used. As girder strength increased and deck strength remained constant, the composite section properties decreased with a corresponding increase in service load stresses in the girder for the same span length and girder spacing. The use of a high-performance concrete in the deck with a higher modulus of elasticity will result in an increase in the composite section properties.

EFFECT OF HIGH-STRENGTH CONCRETE ON PRESTRESS LOSSES

When compared with the properties of conventional strength concretes, high-strength concrete has a higher modulus of elasticity, a higher tensile strength, and reduced creep. The higher modulus of elasticity results in less elastic shortening in prestressed concrete girders at time of release for the same stress level. This reduction in shortening may be offset by the use of higher prestress levels with high-strength concrete. The higher tensile strength does not have a direct effect on prestress losses but allows high-strength girders to be designed for a higher permissible tensile stress. This, in turn, results in higher service load design moments.

Creep per unit stress of high-strength concrete is lower than the creep for conventional strength concretes. ⁽¹³⁾ Thus, the direct substitution of a high-strength concrete in place of a lower strength concrete will result in less prestress losses. However, the utilization of a higher level of prestress will offset the reduction in creep per unit stress. The magnitude of the net result will depend on the reduction in creep per unit stress and the increase in stress level.

Prior to casting the deck, prestress losses depend on the properties of the girder concrete alone. After the deck is cast, prestress losses depend on the properties of the deck concrete as well as the girder concrete. When the deck concrete has a strength significantly lower than the strength of the girder concrete, the deck concrete may have a major influence on the magnitude of prestress losses in the bridge. Consequently, the utilization of a higher-strength concrete in the bridge deck can be beneficial in reducing prestress losses. A reduction in prestress losses means that for the same amount of initial prestress, a greater force is available for design at service load. Since the amount of force available at service load controls the design of long‑span girders, reduced prestress losses will be beneficial in the more effective utilization of high‑strength concrete.

Another factor related to elastic shortening, creep, and prestress losses is the change in camber. Girders produced with high-strength concrete are likely to have less initial camber at release and less change in camber. Bridges produced with high-performance concretes in the decks and girders may undergo less deflection changes after the deck is cast.

OBJECTIVES AND SCOPE

Based on the above background, the objectives of the research were to evaluate the following:

• Effect of using high-performance concretes in the deck on the cost per unit area.
• Effect of using high-performance concretes in the deck and girders on flexural strength and ductility.
• Effect of high-performance concretes in the deck and girders on prestress losses and long-term deflections.

The objectives were accomplished in three separate tasks. Each task used a different research approach as described in the following chapters.

For each task, the analyses were based on a PCI Bulb-Tee (BT-72) cross section with a depth of 1.83 m (72 inches). In the previous investigation, the PCI Bulb-Tee was identified as the most cost-effective cross section for span lengths up to 45.7 m (150 ft) at all concrete compressive strength levels. ⁽⁶⁾ For span lengths greater than 45.7 m (150 ft) and for all concrete compressive strength levels, the Florida BT-72 and Nebraska NU-1800 were the most cost effective. In the present investigation, some analyses were also performed using the FL BT‑72. The cross sectional dimensions of the BT-72 and FL BT-72 are shown in figures 1 and 2, respectively.

Figure 1. Cross section of girder analyzed—PCI Bulb-Tee (BT-72).
All dimensions are in millimeters (inches).

Figure 2. Cross section of girder analyzed—Florida Bulb-Tee (FL BT-72).
All dimensions are in millimeters (inches).