Kevin MacDonald, Cemstone Concrete Products
Photo: Tim Davis, FIGG
With only a few weeks from collapse of the old bridge to commencement of reconstruction, there was no time to develop the concrete mixes from first principles. Consequently, concrete mixtures that had been used successfully in parking structures and on other projects were utilized. In addition, the conventional Minnesota Department of Transportation (Mn/DOT) mixtures could not be used because they would not provide the required levels of long-term performance.
Although specific limits for rapid chloride permeability and shrinkage were not specified for every element, these properties were measured for some of the mixes. The four elements that required specific mixture proportioning were the drilled shafts, footings, piers, and superstructure. Each element had its own challenges and solutions.
Drilled Shafts
The concrete for the drilled shafts consisted of a ternary blend of fly ash, ground-granulated-blast-furnace slag, and portland cement. The aggregate gradation was selected to allow proper manufacture of self-consolidating concrete with a slump flow of 24 in. (610 mm). The concrete was air-entrained. This was not required for the exposure conditions, as the footing and soil would protect these elements from freezing and thawing, but was included to reduce the volume of cement required per cubic yard of concrete.
In the initial design process, the drilled shaft concrete strength was required to be 4000 psi (28 MPa). This was subsequently revised to 5000 (34 MPa). As strength was not the primary design criterion in the proportioning of this mixture, the required increase in the strength did not cause any need for reproportioning.
Cores removed from drilled shafts at an age of 21 days indicated an in-place strength of over 10,000 psi (69 MPa). The rapid chloride permeability of the concrete was approximately 750 coulombs at 28 days. This is considered to be very low permeability. Low heat-of-hydration concrete benefits significantly by being placed as self-consolidating concrete. The dispersion of the cementitious fraction results in a more efficient use of the hydratable materials; thus reducing the need for excess hydratable material. As a result, the heat-of-hydration is reduced at a fixed cement content, or, as used in this case, the observed compressive strength was increased. The use of self-consolidating concrete was the appropriate decision.
Footings
The footing concrete, requiring a compressive strength of 5000 or 5500 psi (34 or 38 MPa) at 28 days depending on location, was a similar mixture to that used in the drilled shafts, except that it was placed at a conventional slump of 8 in. (200 mm). Shrinkage of these mixes was about 0.04% after 28 days of drying per ASTM C157. This is considered to be very low shrinkage. Internal cooling was used to control the temperature rise and temperature gradient in the footings.
Piers
The more interesting concrete in the substructure was used in the elegant curved piers. This concrete had several requirements. The piers are 8-ft (2.4 m) square at the waist, expanding to more than 16 x 8 ft (4.9 x 2.4 m) at the top. The entire element was mass concrete. However unlike the footings, cooling pipes could not be placed due to the nature of the formwork. This called for a much more aggressive mixture. Concrete in the piers had approximately 15% of the cementitious material as portland cement, with the remainder being a blend of fly ash and slag. Concrete was placed at a very low water-cementitious materials ratio and a slump of approximately 8 in. (200 mm) due to the high use of a dispersant. Concrete generates approximately 24 BTU/h/ft3 (250 watts/m3) during early hydration, then heat development drops off dramatically. In contrast to many mass concrete elements, these members were required to be held at a minimum temperature of 100 °F (38 °C) for 3 days so that early age strength would be developed to facilitate construction. This resulted in the unusual situation of a mass concrete element with heat being pumped into the system in order to maintain the temperature. The concrete strengths were well in excess of 4000 psi (28 MPa) at 28 days. The concrete had a rapid chloride permeability in the range of 500 coulombs at 90 days.
Superstructure
A similar mixture was used for both precast and cast-in-place elements of the superstructure. Concretes were made at a cementitious materials content of approximately 700 lb/yd3 (415 kg/m3), with 25% fly ash and 4% silica fume. The pozzolan content was less in comparison to the other project mixtures to remove the risk of surface scaling. There were several criteria required for this mixture to gauge its performance. While a low water-cementitious materials ratio and a good air-void system were necessary, they were not sufficient for a durable structure. The concrete mix was designed to have very low rapid chloride permeability. Samples cast during production had permeability values ranging from 90 to 250 coulombs at ages ranging from 28 to 90 days. The diffusion coefficient for chlorides was approximately 4×10-8 ft2/h (1×10-12 m2/second). The average concrete compressive strength at 28 days was approximately 8000 psi (55 MPa) or 23% above the specified strength of 6500 psi (45 MPa).
One of the major design criteria for this mixture was its shrinkage. Concretes were measured in the laboratory to have shrinkage of approximately 0.04% after 56 days of drying, as determined by ASTM C157. In addition, due to the very large post-tensioning stresses, both modulus of elasticity and creep coefficients were determined—a process that the specifications gave 18 months to accomplish! The structure will have been in service nearly 8 months at that time. The whole as-built structure has a very high level of resistance to the environmental loads and the service loads it will encounter during its century or more of service, using state-of-the-art concrete technology.
Further Information
For further information about the concrete mixes, contact the author at [email protected] or (651) 686-4224.
Concrete Mixes
2. Type I low alkali
3. Meets both Class C and F
The above table lists the specified strength, water-cementitious materials ratio, total cementitious materials content, and percentages of portland cement, fly ash, slag, and silica fume used in the concrete mixes for the superstructure, piers, footings, and drilled shafts.