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Penobscot Narrows Bridge.
Introduction of HPC by the Maine DOT
Michael Redmond, Maine Department of TransportationWhile the Maine Department of Transportation (DOT) may not use all the traditional definitions of a high performance concrete (HPC), Maine does incorporate and benefit from many of the improved performance characteristics realized from the use of HPC. This article contains a brief history of how Maine DOT came to incorporate HPC into its everyday concrete requirements.
In the early 1990s, Maine DOT was invited to participate in an alkali-silica reactivity (ASR) workshop sponsored by the Federal Highway Administration and hosted by the New Jersey DOT. At this conference, Maine DOT took the first step towards understanding the value of pozzolans and their beneficial effects when added to a standard concrete mix. While a good portion of the workshop focused on identification of ASR in concrete pavements and bridge structures, preventative measures were also presented. These included the use of ground granulated blast-furnace slag and pozzolans such as silica fume and coal fly ash. Following the workshop, Maine applied for and was granted research money from the Federal Highway Administration's Office of Priority Technologies and purchased the necessary equipment to screen the concrete aggregate sources. The following year (1993) Maine DOT implemented an ASR specification that required the use of pozzolans in all bridge concrete mixes containing potentially reactive aggregates.
During the same time frame, Maine also began to implement its first Quality Control-Quality Assurance (QCQA) specification. Initially the intent was to introduce this specification to the bridge construction general contractors and give them a greater role in the design and control of concrete mixes. However, it also enabled Maine DOT to reduce inspector staffing at concrete batch plants and inspector testing at jobsites. Under QCQA, these duties became the responsibility of the contractor’s quality control personnel and enabled department engineers and inspectors to focus more on material and construction quality issues rather than the day-to-day testing of concrete.
Perhaps the most important aspect was the specification that was developed with the advent of QCQA. The DOT initially focused on test properties such as slump, water-cement ratio, entrained air content, compressive strength, and chloride permeability. Once QCQA was fully implemented in 1996, the test properties evaluated were reduced to entrained air content, compressive strength, and chloride permeability. Maine continued to specify these test properties until 2004 when a new version of QCQA was adopted known as Quality Level Analysis (QLA). Under QLA, the test properties remained the same but were all evaluated under the Standard Deviation Percent Within Limits Method, which generated a Composite Pay Factor (CPF). The current CPF consists of 20% compressive strength, 40% entrained air content, and 40% rapid chloride permeability. The resulting pay factor is then multiplied by the cubic yard quantity of concrete in the lot and the dollar value assigned per cubic yard for the concrete in question. This generates a penalty or bonus payment for the general contractor.
While there have been instances of contractors being penalized on QLA projects, most have been due to variations in control of the entrained air contents. Maine specifies an air content higher than most states due to its proximity to the Atlantic Ocean. Because of this proximity, Maine also experiences more freeze-thaw cycles than most states. This requires a higher quality and more durable concrete for use in bridge structures. Over the life of our QCQA program, the concrete industry in Maine has consistently provided better concrete year after year. The driving force behind this has been the use of pozzolans and blends of pozzolans in our everyday bridge concrete and also in concrete used at the many precast concrete plants providing products.
Recent blends of ground granulated blast-furnace slag and portland cement have resulted in cast-in-place compressive strengths approaching 10,000 psi (69 MPa), while limiting the total cementitious materials content to 660 lb/yd3 (392 kg/m3), and precast concrete girders with compressive strengths in excess of 13,000 psi (90 MPa). While these test results would suggest the ability to lengthen spans, design thinner sections, or use fewer beams, Maine is more focused on durability of structures, again due to the severe environmental conditions.
Perhaps the greatest benefit from the mixes utilizing pozzolans is the resistance to chloride penetration, which is a critical tool in the current QLA specifications. Because corrosion of reinforcing steel continues to be the number one cause for structures not achieving their design life expectations, the resistance to chloride penetration is very important. Maine DOT will remain committed to using HPC and any other technologies that extend the service life of concrete bridges.
For more information about HPC in Maine, please contact the author at firstname.lastname@example.org.
HPC Bridge Views, Issue 56, July/Aug 2009