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|Federal Highway Administration > Publications > Public Roads > Vol. 60· No. 2 > The Promise of High-Performance Concrete|
The Promise of High-Performance Concrete
by David C. Smith
Much of the information in this article was drawn from papers and presentations delivered at the SHRP (Strategic Highway Research Program) High-Performance Concrete Bridge Showcase in Houston, Texas, March 25-27, 1996.
Improvement of our nation's transportation infrastructure is a top priority of the public and private sectors of the transportation community. But, it's more than that! It also ranks high among our premier national goals because our national productivity, domestic economy, and international economic competitiveness depend on fast and reliable transportation of people and goods. Among the most urgent, most costly, and most physically and technically demanding aspects of transportation infrastructure improvement is highway bridge construction.
We need stronger and more durable bridges. According to a report to Congress, 1995 Status of the Nation's Surface Transportation System: Condition & Performance, more than 12.5 percent of the bridges on our interstate, arterial, and collector road systems are "structurally deficient." That's 43,524 bridges that need significant maintenance, rehabilitation, or replacement. Although a structurally deficient bridge is not necessarily unsafe, some of these bridges are load-posted, requiring heavier trucks and buses to take an alternate, longer route.
One technological improvement that is showing increasing promise as a means to achieve long-term, cost-effective results in highway bridge construction is high-performance concrete (HPC). Materials classified as high-performance concrete construction products and components have been used for decades in the United States and in other countries for buildings, but in recent years, our pressing need for infrastructure improvement has accelerated HPC study and implementation for bridges. The enhanced strength and durability characteristics of bridges that incorporate HPC in beams, decks, and piers promise to reduce the lifetime cost and deterioration of these structures. The challenge now is to find ways to demonstrate the feasibility of widespread use of HPC and to reduce the initial cost and risk inherent in the use of any new technology.
Sue Lane, a bridge research engineer with the Federal Highway Administration (FHWA), points out, "High-performance concrete features improved impermeability, greater durability, and accelerated strength gain over normal concrete." Texas Department of Transportation (TxDOT) bridge designer Mary Lou Ralls, who engaged in several of the most advanced bridge projects using HPC, adds, "The use of high-performance concrete should greatly increase the life span of bridges. In addition, states and localities should have much lower maintenance costs for bridges that are built with high-performance concrete." Together, these statements sum up the major advantages of HPC construction materials: increased strength and durability and decreased long-term maintenance, repair, and replacement expenses.
FHWA is promoting the development, testing, and use of HPC in many ways, including funding demonstration projects by several state highway agencies. Valuable information from testing, as well as from actual bridge construction, is also coming from other countries, including Canada, France, Japan, and Norway. Disseminating this information, widely and accurately, is a primary objective of FHWA.
In March 1996, FHWA and TxDOT, in cooperation with the Center for Transportation Research at The University of Texas at Austin, sponsored a regional Strategic Highway Research Program (SHRP) HPC bridge showcase in Houston. This showcase was part of SHRP's implementation program, which includes the dissemination of HPC information and the showcasing of products developed under the SHRP Concrete and Structures Program. The goal of the showcase was to promote the use of HPC in bridges. This forum allowed the interchange of ideas and information among representatives of federal, state, and local government agencies; the construction industry; and the academic community about all aspects of this advantageous technology. A spotlight was placed on two bridge projects currently under way in Texas: the Louetta Road Overpass in Houston, which is the first highway bridge construction project in the United States to use HPC throughout the bridge (see "Texas High-Strength Concrete Bridge Project" in the Spring 1994 issue of Public Roads), and the North Concho River-U.S. Route 87-South Orient Railroad Overpass in San Angelo.
Before discussing in more detail the results of the showcase and reporting on the progress to date of the various HPC bridge construction projects currently under way, we should look back briefly at the development and diverse applications of these promising construction materials over the past several decades.
Basically, HPC is composed of the same materials as normal concrete, but it has been engineered to achieve enhanced durability or strength characteristics, or both, to meet the specific demands of a construction project. These demands, and the desired HPC characteristics, vary widely based on factors such as temperature and weather conditions and the specific member -- bridge substructure, beams, or deck -- being constructed.
As desirable as enhanced durability and strength are, engineers pursuing the benefits of HPC also must focus on the practical issues of cost and constructibility. The goal is to discover the means of constructing economical bridges that also have improved long-term performance. Balancing initial construction cost against long-term cost savings is a fundamental function in considering the use of HPC. As lessons are learned, and as standards are set, and as materials become more readily available, the economic benefits of HPC should become more readily apparent. Indeed, the use of HPC may lead to reduced construction costs through more efficient designs, faster construction, and decreased consumption of resources as a result of longer spans and fewer beams.
In response to FHWA's call for a clear definition of HPC based on long-term performance criteria, a good working definition has been developed by American Concrete Institute (ACI) committee members Charles Goodspeed, Suneel Vanikar, and Raymond Cook. This definition, published in the February 1996 issue of Concrete International, consists of four durability and four strength parameters, each one being supported by performance criteria, performance testing procedures, and recommendations to allow performance to be accurately related to specific adverse field conditions. The eight performance criteria are freeze/thaw durability, scaling resistance, abrasion, chloride penetration, strength, elasticity, shrinkage, and creep. Users of this definition can indicate the level of performance that they require for each performance characteristic, based on their own field and weather conditions, in order to determine the HPC mixture best suited to their specific need.
HPC performance characteristics consist of two categories: durability and strength. The durability and strength parameters divide field conditions into three categories: climate, exposure effects, and load. Climate varies greatly throughout the United States, with great fluctuations in temperature, freeze/thaw cycles, and humidity. No single HPC mix can be expected to perform adequately from Florida to Alaska. The FHWA definition takes into account adverse thermal expansion, moisture content, cycles of freezing and thawing, exposure to aggressive chemical agents, loading stresses, and other factors to help engineers identify an acceptable durability and strength characteristic for any specific project.
When asked if HPC is going to become the standard for concrete, Burson Patton, plant engineer at the Texas Concrete Company, which is engaged in the Louetta Road Overpass project, replied, "Oh, I think so. Especially in its resistance to the environment because in some places we use a lot of salt for deicing." Not only are the mixture and the proportions of ingredients important, the mixing sequence and curing conditions are also critical to the HPC production process. In considering the effect of climatic conditions, permeability characteristics are an additional serious concern in situations where saturated concrete will have to withstand freezing and thawing. The corrosion process is slower in concrete with low permeability characteristics because the rate at which chloride ions are diffused into the concrete is reduced.
On this subject, Lloyd Welker of the Office of Structural Engineering, Ohio DOT, says, "I think generally HPC is doing what we expected of it. We do rapid chloride testing at a certain time period after the project is poured, and those tests show extremely low permeability numbers."
In determining resistance to deterioration over the long term due to weather and climate, it is the combined effect of all characteristics that is represented in the HPC definition.
Performance against scaling due to the wide range of harsh chemicals used on highways must also be considered as a combined effect that includes many factors, such as curing history, moisture content, and salt concentration. The highway bridge designer has the responsibility of taking into consideration all the potential effects of ambient conditions. This is also true in projecting loading conditions, where structural designers may find that the strength parameters developed to meet durability requirements result in strength parameters that are greater than are needed for anticipated traffic loads.
In projecting resistance to deterioration, field conditions must be accurately understood and represented as a measure of performance for each parameter grade. With these grades of performance characteristics as a guide, engineers can determine and specify which HPC mixture will result in the concrete service life that they require for their project.
Standard performance testing methods have been determined for each of the eight HPC definition parameters, supported by specimen and curing procedures that will help ensure uniform performance evaluation. Grades have been defined for each of the eight parameters, and severity estimates have been determined to represent the complete range of field conditions throughout the nation. However, since there is insufficient data to correlate laboratory performance to actual field performance, the tables of interrelated factors presented by Goodspeed, Vanikar, and Cook as the HPC definition are currently only a guide. They will be updated and expanded over time as more is learned from research and from FHWA demonstration projects and other sources.
The Origins of HPC
As far back as 1949, beams with a concrete strength of more than 37 megapascals (MPa) (5,400 pounds per square inch) were used in the construction of the Walnut Lane Bridge in Philadelphia. This was the first prestressed, post-tensioned concrete bridge built in North America.
At that time and for the next four decades, engineers were concerned almost exclusively with strength. Specified concrete strength for buildings steadily increased from 35 MPa in the 1950s to 100 MPa by the end of the 1980s. The term "high-strength concrete" was frequently used. Today, the definition of HPC has expanded to encompass both durability and strength.
No single person invented HPC, and no single country pioneered its use. The development of the HPC materials in use today was an incremental, combined effort involving many individuals, companies, government agencies, and countries, particularly in Canada, Europe, Japan, and the United States. Since the earliest bridges using prestressed concrete beams were only constructed about 50 years ago, not enough time has passed to confidently state a durability life span for prestressed concrete bridges. FHWA has estimated the average life span of bridges of all materials to be about 42 years, but engineers are estimating that a well-constructed HPC bridge may last 75 to 100 years. Time will tell.
For decades, the construction of very tall buildings was the driving force behind the development of high-strength concrete. Economy of construction was the goal. For example, the use of 69-MPa concrete in the Interfirst Plaza building in Dallas in 1983 provided six times more stiffness per dollar than a steel-frame building. Constructors of Two Union Square in Seattle in 1988 used 130-MPa concrete to achieve a modulus of elasticity of 49,600 MPa.
Bridge designers used high-strength concrete materials on a smaller scale, including several small cast-in-place, post-tensioned bridges in Washington State built in the 1980s using 48-MPa concrete. The East Huntingdon Bridge, built over the Ohio River in 1984, is a cable-stayed bridge with a 275-meter (m) main span and 55-MPa concrete, adding tensile strength and durability to its superstructure. Concrete with similar strengths was also used in bridges constructed in Canada and France during the 1980s, including the Annacis Bridge in British Columbia with a main span of 465 m.
The largest application of HPC to date in a bridge in North America is the Northumberland Strait crossing currently under construction in Canada. The almost 13-km structure uses 55-MPa concrete. Ice shields around some of its piers will use HPC with a comprehensive strength of almost 97 MPa instead of the traditional steel shields originally specified.
Going Beyond High-Strength Concrete
The difference today is that highway designers are focusing on "high-performance" concrete, not just concrete with high strength as measured in megapascals. Durability and economical long-term maintenance have become goals along with economical construction. For example, stronger HPC materials allow for longer spans and fewer piers in initial construction, and they also enhance durability to extend the useful life of the structure and reduce the ongoing costs of maintenance.
This potential double savings is the impetus behind FHWA's encouragement of HPC research and use in demonstration projects across the nation. The advantages of HPC can easily be summarized:
The current disadvantages of HPC pointed out by some engineers include the initially higher construction bid prices to be expected with the use of any new technology. The skeptical engineers also point to quality control concerns related to various testing methods in use and the number of tests, and they are concerned about instabilities that could result from reduced stiffness. One of the advantages was increased beam spacing, but increased beam spacing may result in slightly thicker decks. Also, many designers are naturally hesitant to be the first to try something new, and some have doubts that full HPC advantages can be achieved in the field.
To encourage further research and to promote the use of HPC, FHWA is showcasing HPC in regional events and demonstration projects. Many states have expressed interest in becoming partners in this program, and, to date, eight states have become active partners with FHWA by constructing or preparing to construct bridges with HPC. The following brief progress report presents some highlights of five of the demonstration projects that are furthest along.
Many lessons will be learned from two major HPC bridge projects currently under way in Texas. Research, design, construction, and technology transfer for these projects are funded by FHWA and TxDOT. For the San Angelo project, nine other states -- California, Georgia, Iowa, Massachusetts, Minnesota, New York, Ohio, Pennsylvania, and Washington -- contributed to an HPC Pooled Fund to aid in funding the project.
The Louetta Road Overpass, two adjacent bridges on State Highway 249 in Houston, are the first bridges in the country to use HPC in all aspects of design and construction. This is also the first bridge project to use 15.24-millimeter- (0.6-inch-) diameter, prestressed strands in a pretensioned concrete application and on a 50-millimeter grid spacing. The Louetta Road structures use pretensioned concrete U-beams that are twice as strong as conventional I-beams.
The overall Highway 249 project also includes 12 normal-strength, U-beam bridges; this provides the opportunity for many informative comparisons of the conventional and HPC portions of the project. For example, these conventional bridges range in cost between $21 and $27 per square foot of deck area, while the costs per square foot for the southbound and northbound Louetta Road Overpass HPC bridges are highly competitive at $23 and $25, respectively.
While typical beam spacings for pretensioned I-girder bridges are between 1.83 and 2.74 m, the U-beams of the Louetta Road Overpass are spaced at 5.5 m. Due to the bidding procedure used, the cost differences between the HPC and normal-strength beams cannot be determined, but it is anticipated that life-cycle costs will be consistently lower when HPC becomes the standard.
Leon Wright of Williams Brothers Construction Company, the general contractor for the Louetta Road Overpass project, estimates the additional cost of using HPC as "somewhere around 50 percent to 75 percent higher, depending upon what kind of strengths you're talking about. Think about normal $50 concrete versus $80 to $90 HPC concrete." However, he adds that the higher initial cost can be justified in terms of speed and a reduction in the overall amount of concrete needed.
"We did a research project with Texas A & M on concrete paving over a weekend," Wright explains. "We closed an intersection, tore it out, poured it back, and opened up on Monday. The public hardly knew anything happened. Some of the high-strength concrete you can drive on in six hours. With other concrete, it takes seven days. "When you make longer spans and space your beams out farther, you use less concrete of a higher strength," Wright said.
According to Wright, obstacles to using HPC were minor. "We haven't had any problems," he said in May. "It's just a little different, but there were really not any major obstacles to overcome."
Bruce Williams of Texas Concrete Company, the company responsible for making the prestressed beams, agrees. "Meeting the compressive strength was a primary concern," Williams explains. "After extensive testing, it became a lot less of a problem. We also had a unit weight problem in one of the original mixes we came up with. The unit weight was too high, so we had to do some work on the batch design, redesign the mix to reduce the unit weight because of the actual load on the member itself." Williams Brothers began casting the precast pier segments and deck panels early in 1996, and the work was virtually completed by summer. As this article goes to press, the Louetta Road Overpass should be nearing completion.
The second ongoing HPC project in Texas is the overpass that spans the North Concho River, U.S. Route 87, and South Orient Railroad in San Angelo. The HPC bridge is a 290-m, eight-span eastbound structure, adjacent to a similar westbound bridge being built with normal-strength concrete. HPC was specified for the eastbound bridge largely because the 48-m span required to cross the North Concho River is beyond the typical prestressed concrete I-beam limits. Beam strengths greater than 69 MPa were required, along with concrete strengths of up to 41 MPa in the decks. Four beams with 3.35-m spacing were required for the HPC bridge, compared to seven beams with 1.74-m spacing for the normal concrete structure.
Much has been learned from extensive testing of both fresh and hardened cast-in-place concrete at the jobsite. It was also determined that special attention must be given to curing procedures, considering factors such as early age protection to prevent plastic shrinkage cracking and thermal cracking
. Texas Concrete Company developed an innovative design alternative involving two-stage stressing for the HPC beams that exceed 45 m in length. An analysis of these beams based on a commercially available program and research from The University of Texas at Austin indicated satisfactory performance for allowable stresses, design strength, and deflection history.
The San Angelo bridge is scheduled for completion in February 1997. In both Texas projects, partnering among contractors and subcontractors, university researchers, FHWA, and TxDOT personnel began as early as the pre-bid conferences and has continued throughout the projects.
Milo Cress of the FHWA Nebraska Division reports that public-private partnering has also played a central role in Nebraska's HPC demonstration project. The 120th Street and Giles Road bridge in Sarpy County was chosen because of its close proximity to a similar bridge constructed with normal concrete, which will permit useful comparison.
The HPC bridge consists of three spans of 23 m each, and it employs girder spacings of 3.81 m as compared to spacings of 2.4 m in the conventional bridge nearby. The width of both bridges is about 24.4 m. HPC products were limited to the superstructure due to county design stipulations. Nebraska University NU1100 girders are being used. In research by the University of Nebraska and also in independent studies, these new metric girders have demonstrated high simple and continuous span efficiencies, as well as excellent stability for handling and shipping.
The contractor, Ready Mixed Concrete Company, developed its own HPC mix that tested at 96.5 MPa at 56 days curing, which was more strength than was required for the project. In a trial pour, the concrete flowed well and indicated no problem for workability.
Nebraska will host the next major HPC showcase on Nov. 18, 19, and 20 in Omaha.
Before initiating the five HPC projects in Virginia, two experimental projects were conducted to support the design of high-strength and low-permeability concrete beams. American Association of State Highway and Transportation Officials (AASHTO) Type II beams were cast and tested at the FHWA's Turner-Fairbank Highway Research Center (TFHRC). In the second program, beams had composite slabs. They were steam cured, and high-release strengths between 55 and 58 MPa were obtained within 19 hours of batching. When the beams reached 69 MPa, slabs were cast and then tested at TFHRC.
"Our main interest is in the durability of structures," said Dr. Celik Ozyildirim, principal research scientist. "For over a decade, we have been testing our concretes for permeability, using both the rapid permeability test and the ponding test. [The rapid permeability test was introduced in the early 1980s.] We worked with materials like fly ash; then silica fume became available, and ground granulated slag, so we used those materials to achieve low-permeability concretes. But now, we are also concentrating on strength in our structural elements, for example, the beams."
Five HPC bridges are planned in Virginia. Two of them have already been constructed and opened to traffic. The newly proposed permeability specifications apply to all of the concrete used in three of the bridges, including one of the completed structures. The completed Walkerton bridge on state Route 629 over the Mattaponi River has 12 spans of 30.5 m each. The bridge used the smaller Type IV beams with HPC instead of Type V beams with the conventional design. The other completed bridge, on state Route 40 over the Falling River near Brookneal, which is south of Lynchburg, has four spans, each 24.4 m long. These spans required only five HPC 55-MPa beams apiece, instead of the seven beams required in a conventional design. Another bridge with 55-MPa beams, 22 spans, and a total length of 655 m is now under construction.
The HPC demonstration bridge in New Hampshire carries state Route 104 over the Newfound River in Bristol about 2.5 km from the site of a normal-performance concrete bridge that will be constructed for comparison. Both bridge sites share similar characteristics and will carry similar traffic volumes, so much should be learned from monitoring and evaluation as testing data are collected and comparison analyses are performed.
"The project should be just about completed by the fall," said Michelle L. Huppe, the New Hampshire DOT design engineer for the HPC project, in an interview in May. "The girders have been fabricated. One of the abutments has been completed, and they are building the second one. They may be able to set the girders in about a month. I would say that the composite deck should be completed by October."
The bridge is a single-span structure with a span length of 20 m and a slight horizontal curve, carrying three lanes of traffic. HPC is specified in both the substructure and superstructure. Effective partnering led to extensive use of the capabilities of the University of New Hampshire Bridge Deck Testing Facility. The contractor was allowed flexibility in designing the mix for the steam-cured girders, and three mixes were compared using testing for slump, scaling, chloride ion permeability, and freeze-thaw.
"One thing we learned," Huppe said, "is that we might be specifying too much air. New England has the freeze-thaw problem, so we typically specify 5.8 percent air. We included that in our HPC specification, and the fabricator had a difficult time meeting the specification. We think we should try to cut back on the air while still providing good durability in the concrete." As for the girders, "Everything has worked out okay. They were able to achieve the design strength for the girders, though they did have some difficulty."
James Barnhart, structure maintenance and inspection engineer with the Ohio DOT, points to the progress that Ohio has made in the use of HPC materials for bridge construction. "With the new federal initiative, we're looking for a location to try HPC. As of the spring of 1996, we have about 33 cast-in-place HPC bridge decks already in place, but decks are about the only place we've really used it. We've got about a hundred more on the books, with 50 or 60 of them to be built in 1996."
Asked about additional cost or other problems, Barnhart reveals that, "We're not finding much of a cost increase, and in some cases, it's even less expensive than our conventional concrete. The use of silica fume does create problems for some ready-mix suppliers, so we met with our producers and the Ready Mix Association in Ohio. We worked with them and explained that you need to take some special precautions to batch the concrete in the trucks in the right sequence. There was a period of time when some re-education was going on."
The Concrete Standard for the Future
Most professionals involved in highway bridge construction seem to agree with James Barnhart that some re-education has to be part of the process if the full promise of HPC is to be realized. Contractors and concrete producers have to understand the rationale behind the addition of high-tech ingredients, and designers and engineers have to feel confident in the quality control and testing procedures for new products.
This is a large part of the goal of the FHWA initiative. The long-term performance of each HPC bridge will be monitored through an instrumentation program using a variety of sophisticated gauges. Three types of measurements will be collected and evaluated: temperature, deflection, and long-term strain. FHWA's HPC definition is a working document that provides a clear guide, and the demonstration projects will be living workshops with lessons learned disseminated promptly. Throughout, public-private partnership is the key.
Will HPC be the standard in the future? Many, looking at the immediate advantages in terms of speed of construction and reduced amount of concrete needed plus the long-term savings due to enhanced durability, think it should be.
"I hope HPC becomes the standard," Huppe said. "I like using it. Design-wise you're able to use your sections better, do it more efficiently, decrease the number of girders needed, and so on. I think that, overall, using HPC is more efficient."
However, Huppe has a caveat. "I think more research needs to be done about how our design specifications should change because the old normal performance concrete design guidelines do not necessarily apply to HPC."
"I think HPC definitely is the way of the future," she concludes.
Upcoming Activities for High-Performance Concrete (HPC) for Bridges (from mid-August 1996)
David Smith is president of Amanuensis Creative Group of Vienna, Va., and a consultant with Avalon Integrated Services Corp. of Arlington, Va. He writes frequently on business and technology issues, particularly in the fields of telecommunications, transportation, and converging technologies. He received his doctorate from Cambridge University in England.
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