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Concrete with a specified compressive strength of 9000 psi (62 MPa) was used in
the spliced girders.
HPC for the Spliced Girders of Kealakaha Stream Bridge, Hawaii
Chuck Prussack, Central Pre-Mix Prestress Co.In 2006, KSF Engineers Inc. (KSF) of Honolulu, HI, contacted Central Pre-Mix Prestress Co. (CPPC) located in Spokane, WA, to discuss spliced girder concepts. KSF had been retained by the general contractor, Hawaiian Dredging Construction Co. (HDCC), who was low bidder on the originally designed Kealakaha Stream Bridge—a cast-in-place, three span, single cell, curved segmental box girder structure. KSF’s role was to examine options that would make construction of this bridge easier. Due to site constraints, the bridge needed to be built from the top down because access into the ravine below the bridge was very difficult. The bridge is also close to an active volcano and subject to high seismic activity. KSF had seen an article in the July/August, 1997, issue of the PCI Journal about a similar project that involved CPPC. From discussions between CPPC and KSF, the concept was developed to use a longitudinally spliced, post-tensioned girder bridge that would be easier to construct given the site constraints.
The original design called for span lengths of 180, 360, and 180 ft (54.9, 109.7, and 54.9 m) using a horizontally curved, cast-in-place box girder. The value-engineered superstructure retained the same span lengths but used 150-ft (45.5-m) long straight cast-in-place box girders above each pier with 100-ft (30.5-m) long straight spliced girders between the ends of the box girders and the abutments and 205-ft (62.5-m) long straight spliced girders to complete the main span. This framing plan created five chords to provide for the curved horizontal alignment.
Due to tight road geometry leading to the site, the maximum girder segment length that could be hauled was limited to less than 50 ft (15.2 m). So, the end span spliced girders were made with two girder segments, and the main span was made with four girder segments. The girder segments were spliced together at the site, and then launched into place. The average segment length for all prefabricated segments was approximately 47 ft (14 m). Cast-in-place closures were used between the precast segments and between the spliced girders and the cast-in-place box girders.
Spliced Girder Cross Section
The spliced girder selected was the Washington State Department of Transportation (WSDOT) Standard WF95 PTG girder. This girder cross section, with a depth of 95 in. (2.41 m), had been jointly developed by industry and WSDOT to be able to span in excess of 200 ft (61 m) using splicing techniques. It is one of several depths of “supergirders” used by the WSDOT. When these “supergirders” are used in a post-tensioned scenario, the overall girder width is increased by 2 in. (51 mm) to give a 7⅞-in. (200-mm) wide stem, and 2-in. (51-mm) wider top and bottom flanges. Specified concrete compressive strengths for the precast girders were 8000 psi (55 MPa) at 28 days and 9000 psi (62 MPa) at 56 days. CPPC viewed these strengths as “business as usual” type strengths that would use a standard girder mix design.
High Strength Concrete Development
In 1996, CPPC was low bidder on the first WSDOT high performance concrete (HPC) bridge girder project called Covington Way Bridge. That project required 10,000 psi (69 MPa) for the 28-day strength and 7000 psi for strength at transfer. Until that point, a typical 28-day strength requirement for girders in Washington State had been in the 7000 psi (48 MPa) range, with release strengths in the 5000 psi (34 MPa) range. As CPPC began working on mix designs for the Covington Way Bridge, it quickly became clear that CPPC would not get 10,000 psi (69 MPa) strength, nor the required release strengths using their current mixes and approach to making concrete.
When CPPC began talking to other companies and material engineers about how to achieve these strengths, it quickly became apparent that there were many alternatives to achieving high strength concrete. Some materials engineers advocated large fly ash content, some advocated large silica fume content, some used metakaolins, some advocated “aggregate packing,” and some just used a very low water-cementitious materials ratio. It was very confusing for a while trying to reconcile all these opinions! All aspects of concrete production from optimizing mix designs, curing, moisture control, admixtures, and testing were looked at. Finally, CPPC took the approach to use a combined gradation for aggregate selections, 5% silica fume, and 20% Class C fly ash for the mix itself. A high-range water-reducing admixture was used to increase workability, the preset period was determined by the use of a hydration chamber to optimize curing, the compressive strength testing machine was upgraded to a digital readout, and cylinder capping was switched to a high strength capping compound. New moisture sensors were installed in the aggregate bins that enabled a more accurate determination of the water-cementitious materials ratio, and a match-curing system was implemented.
Since that first HPC project, CPPC has continued to improve concrete mixes such as that used for the Kealakaha Stream Bridge. By the time Kealakaha Stream Bridge girders were cast, admixtures had been switched from the original HPC mixes to the new series of high-range water-reducing admixtures, and a small amount of slag cement was used. Fly ash and silica fume were not used, and because of the newer generation of admixtures, a lower water-cementitious materials ratio was possible. CPPC also switched to the use of neoprene pads for the cylinder testing.
Concrete Mix Design
Measured concrete compressive strengths ranged from 5500 to 6000 psi (37.9 to 41.4 MPa) at prestress transfer, 10,500 to 11,300 psi (72.4 to 77.9 MPa) at 28 days, and 10,600 to 12,600 psi (73.1 to 86.9 MPa) at 56 days.
Since Kealakaha Stream Bridge, CPPC now uses the latest version of polycarboxylate high-range water-reducing admixtures, with the remainder of the mix design for HPC girders remaining the same as shown above. Strengths at transfer are well into the 7000 psi (48 MPa) range and 28-day strengths well into the 12,000 psi (83 MPa) range. The strengths that are commonplace today would have seemed impossible to CPPC in 1996, but advances in technology and a new understanding have led to significant advances.
Further information about the design and construction of Kealakaha Stream Bridge is provided in ASPIRE™ Summer 2010.
HPC Bridge Views, Issue 64, Nov/Dec 2010