Ahead of the curve
- Published: Tuesday, 15 March 2011 13:17
- Written by CP Staff
Pan mixer-equipped portable plant drives high performance concrete quality for lean bridge arches above ultimate mass structure
The Arizona- and Nevada-bordering Black Canyon harbors best-in-class engineering examples of mass or high performance concrete (HPC): the 75-year old Hoover Dam and the new Mike O’Callaghan-Pat Tillman Memorial Bridge hovering high above. The bridge was built under a $114 million contract to a joint venture of Obayashi Corp. and PSM Construction USA, with lead designer T.Y. Lin International. Opened late last year with four lanes plus pedestrian walkway, the structure is part of an overall $240 million project, spanning nearly 10 years and including two- and three-mile Arizona and Nevada approaches, respectively.
Known as the Colorado River Bridge at Hoover Dam prior to the O’Callaghan-Tillman designation by Congress, the 1,900-ft. crossing rests on a structure of near-perfect shape and material composition: a cast-in-place, segmental arch with 10,000-psi design strength concrete. “It is difficult to imagine a more appropriate application of HPC than a long‐span arch such as the new Colorado River Bridge. HPC helped make this magnificent span possible and practical,” affirmed T.Y. Lin Vice President David Goodyear.
In a report for the Federal Highway Administration, lead agency on the project, he added, “Delivery of consistently high quality HPC by the contractor showed that even in the harshest of climates, HPC is an excellent choice for long‐span bridge construction.”
While certain phases of the project involved ready mixed deliveries, production of the arch concrete mixes was assigned to a portable plant joint-venture contractor Obayashi Corp. and PCM Construction USA operated on site. In lieu of volume-driven, tilt-drum or twin-shaft equipment, the contractor opted for a plant equipped with a pan mixer. In a companion report, Obayashi’s Jeff St. John noted, “Pan mixers use high-speed paddles to premix the concrete prior to discharging into the truck. They were perfect for the application since quality, not volume per hour, was the most important issue.”
In the September/October 2010 edition of FHWA’s HPC Bridge Views, dedicated to the O’Callaghan-Tillman Memorial Bridge, David Goodyear discussed “Design Aspects With HPC” and Jeff St. John “Concrete Production and Placement,” adapted versions of which follow.
Selection of the right structural system meant defining the character of a long‐span bridge that respected the pioneering work of the great dam builders and the grandeur of the Black Canyon setting. Concrete was not the only choice, but certainly the most natural in this setting.
The bridge type selection was guided through two focus groups. The technical issues were presented to a Structural Management Group (SMG), comprised of the state and federal bridge engineers and peer-review consultants. The aesthetic issues were presented to a Design Advisory Panel (DAP) comprised of state historic preservation officers, the National Park Service (NPS), Bureau of Reclamation (BOR), Native American representatives, and architectural consultants.
Both groups converged quickly on a deck arch to meet the engineering and aesthetic demands. Once the options for the arches were presented to the executives of the five leading agencies (FHWA Central Federal Lands Highway Division, Arizona and Nevada Departments of Transportation, BOR, and NPS), the unanimous selection of a concrete arch set the direction for design.
There are many characteristics of HPC that provide advantages for a long‐span arch, including superior durability, strength, and stiffness. The arch form is an ideal application for concrete owing to the primary compressive strength of a simple concrete box section typically used for the arch rib. In the case of the Colorado River Bridge, the long arch span required more than just strength. Several aspects of design were controlled by both immediate and time‐dependent arch deflections. Here, the stiffness of HPC became an important parameter, surpassing strength in its benefit to design.
As the proposal for high strength HPC was advanced, questions were raised about the ability to produce and deliver consistent quality product. Additionally, the typical questions about material properties, creep, and shrinkage were highlighted due to the arch’s 1060‐ft. span. The design team retained CTLGroup to develop a demonstration program for HPC using the local materials available to the contractor.
This allowed comprehensive testing for strength, durability, workability, creep, and shrinkage to better inform the design team, as well as give the prospective bidders a reference point for their own mix design work under the construction contract. The mix design program included a range of approaches, virtually all of which confirmed that the 56‐day strength target of 10,000 psi was achievable. Testing results showed the superior properties of HPC in terms of durability and dimensional stability.
The low permeability and low specific creep typical of high strength HPC were confirmed. The testing program also supported the project’s preference to not require job‐specific creep testing in the course of construction. Creep tests are time consuming and not well suited for the construction phase of a project that starts off with concrete production. The specific creep measured in the testing program was less than half of that typical for conventional concretes. And while the design proceeded on the basis of conventional creep factors, the dimensional stability of HPC was seen as an additional margin warranted for such a significant structure.
One of the chief topics of early discussion amongst the Obayashi and PSM team was the example of public works adjacent to the site. Discussions hit on many of the usual topics regarding Hoover Dam; how did the builders handle the intense desert heat, which can approach 130°F, and get workers, equipment, and materials to the site? The same challenges would need to be faced 70 years later.
It was readily apparent building the concrete arches dwarfed all other challenges. The main concern was the concrete itself: mix design, thermal control, delivery and placement, consolidation and, possibly above all, quality control. Work started on developing a mix design two years prior to casting the first arch segment; FHWA and T.Y. Lin had established many mix requirements.
Among parameters were concrete compressive strength of 10,000 psi at 56 days; aggregate selection to ensure long‐term durability; and, thermal control requirements to minimize cracking and ensure long‐term durability. The construction team added several others to overcome delivery and placement challenges (pumpability, flowability, and long set time) and schedule challenges (rapid strength gain to minimize the form traveler cycle time). Among additional parameters were compressive strengths of 4,000 psi for launching the form travelers and 6,000 psi for stressing the temporary stays used to support the arch during construction.
Dr. Ryuichi Chikamatsu from the Obayashi’s Technical Research Institute in Japan was brought on‐board to primarily consult on the mix design. Paul Jordan of admixture supplier Sika Corp. lent advice and helped with innumerable trial batches. Dr. Wilbert Langley from Halifax, Nova Scotia, also consulted on the mix design as well as the thermal control requirements.
A cementitious materials-rich mix (800 lbs. of Type V, 200 lbs. Class F fly ash/cubic yard) typically achieved strengths of 4,000 psi in just over a day and more than 12,000 psi in 56 days. Pumpability and flowability were addressed by the use of a high‐range, water‐reducing admixture, which resulted in slump ranges that neared those of self‐consolidating concrete. Set times in excess of 2.5 hours were achieved using a retarder.
The high cement content had a less desirable effect. The concrete in its natural curing condition would reach temperatures in excess of 190°F, far above the 155°F limit of the contract specifications. Most typical mitigation methods, such as using chilled batch water or ice chips, shading the aggregate stockpiles, and casting at night, couldn’t come close to reducing the maximum curing temperature to the target range.
The use of liquid nitrogen allowed the temperature of the concrete during the summer to be lowered from a batched temperature of 85°F to a pre-delivery 40°F. In turn, this kept the temperature at point of placement in the 60°F range, resulting in peak curing temperatures under 150°F. The high cost of liquid nitrogen during the heat of a southern Nevada summer, exceeding $100/yd., was mitigated by the minimal effort needed in other activities. No maintenance for water supply and form insulation and no mitigation efforts such as grouting of cooling tubes and leaving forms in place for an extended duration were required. The precooling resulted in a product that did not require any further thermal control measures and, with the unique bridge structure and location, offered the only viable option.
The construction team weighed two mix delivery options, pumping or—in Hoover Dam tradition—by cableway (high‐line) concrete bucket. The latter was discarded due to scheduling reasons and the size of buckets required to maintain precise control of discharge into a very small target area of traveler cover form openings.
Challenges for pumping included the harsh aggregates of the concrete mix, the long pump line, the means to place through the restricted openings, and delivery of concrete to the pump. Trailer pumps, specially modified to handle the harsh local aggregates, were selected due to their ability to fit in the tight areas available for setup. Delivery was easy on the Nevada side of the gorge, as the pump could be set up on the roadside near the arches, with concrete delivered by mixer trucks.
The Arizona side, with its tremendously steep cliffs, was another story. There, the trailer pump was set up on the base of the arch in conjunction with a 5-yd. re‐mixer. Concrete was discharged from the delivery truck into 8-yd. buckets supported by the cableway and lowered to the re‐mixer, where the buckets were discharged. Use of the re‐mixer allowed the buckets to be re‐hoisted nearly immediately to receive the next load of concrete.
Tying up the cableways for these placements was a significant issue, but no other realistic option was identified. From the trailer pump, the concrete was pumped up the arch through a 5‐in. diameter heavy wall line over a distance of 600 ft. horizontally and 250 ft. vertically to a 105‐ft.-long placing boom mounted on the arch near the form traveler. This placing boom allowed precise control of discharge. A typical arch segment placement took four to five hours.
Consolidation of the concrete was a major concern. The geometry of the arch, with many segments placed at a steep angle, required the use of top surface forms. Openings were established in the forms, not only for placement, but also to allow the use of high‐cycle concrete vibrators. In addition, external vibrators were mounted under the bottom soffit form and along the side forms to help eliminate any issues due to the lack of concrete consolidation. As a tribute to the hard‐working concrete placement crew, very little honeycombing was encountered when the forms were removed.
No delays were encountered during arch construction due to pumping or placement issues, nor were any problems encountered with the quality of the concrete. The arch construction actually went faster than anticipated and resulted in a monument that nearly rivals its neighbor.