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The photo shows a red car driving over Kualaka'i Parkway Grade Separation/>  </div>
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Fig. 1. Kualaka'i Parkway Grade Separation – No deck joints and No Shrinkage Cracks with use of SRA.
            

Shrinkage Reducing Admixture Usage in Hawaii Bridge Decks
Gerobin Carnate, Hawaii DOT

Hawaii’s climate is considered ideal for concrete construction due to its mild temperatures and moderate humidity levels year round. Rarely does the temperature rise above 90°F or drop below 65°F at lower elevations. Humidity may vary slightly on a daily basis, with an average of 63% humidity in Honolulu. Even with such a moderate climate, cracking due to creep and shrinkage is still a big concern in the design and construction of bridges. While concrete mixtures that optimize aggregate content can also minimize paste content thereby controlling shrinkage, local materials used in Hawaii have higher values of shrinkage and creep in comparison to concrete construction in other parts of the country.

The high creep and shrinkage values have resulted in cracks in concrete structures when the stresses exceed the tensile capacity of the concrete. Bridge designers address cracking by installing joints within the structure, especially within the bridge deck. The Hawaii Department of Transportation (HDOT) has continually looked for ways to reduce or eliminate the number of joints in bridge decks to cut construction and joint maintenance costs. Ultimately, the best approach in bridge deck construction is to eliminate joints.

The Kualaka’I Parkway Grade Separation structure is a newly constructed bridge on Interstate Route H-1 in Kapolei, Oahu. The structure is a prestressed concrete spliced-girder bridge that was constructed with a high performance concrete deck mixture containing Shrinkage Reducing Admixtures (SRA) and with no deck joints. No cracks have been found in the deck during recent bridge inspections.

HDOT’s usage of SRA began in early 2001 in the construction of the Keaiwa Stream Bridge on Hawaii Island. The design of the muti-span bridge included a 30-day delayed closure strip over a pier to prevent the superstructure from “lock up” due to a large skew which was analyzed to cause high stresses in the bridge deck from transverse bending. To minimize construction time the contractor requested HDOT to consider using 96 oz per cubic yard of SRA in the concrete bridge deck to reduce the shrinkage in lieu of the 30-day delayed poured closure strip. The contractor’s consultant, KSF Inc., had been consulting with Japanese engineers at Taiheiyo Cement Corporation who had used SRA quite extensively as early as 1980.

To determine its effectiveness and the effects of reinforcing, a research project was undertaken by HDOT to monitor the shrinkage strains in the Keaiwa Stream bridge deck and in eight 36 x36 x 8 inch concrete specimens. These specimens were categorized into two groups, with and without the SRA and varying amounts of steel reinforcements of 0.3 to 1.2 percent. Vibrating strain gages were used to monitor shrinkage, strain and creep for one year. Results showed a 60% reduction in shrinkage in the unreinforced test specimens with SRA. Creep was also reduced by 30 percent. The reinforced sections also showed reductions in shrinkage and creep.

In addition, the Keaiwa Stream Bridge and another single –span concrete structure in Kahuku, Oahu containing SRA in the deck mixture were instrumented with vibrating wire strain gages and data collected for a year. This data also indicated a reduction of creep and shrinkage values.

The compressive strength of concrete containing SRA was reduced as compared to the baseline concrete mixture without SRA by approximately 10 to 15 percent.

Since completion of these projects, SRA has been required in all bridge deck concrete mixtures. With the incorporation of SRA, fibers, superplastizers and synthetic air entrainment in the deck concrete, Kualaka’I Parkway was the first of several single-span bridges constructed with no expansion or contraction joints between the integral abutments. These bridges have spans ranging from 60 feet to 180 feet in length. Examinations of the decks have shown no visible cracks resulting from drying shrinkage of the concrete.

HDOT’s current concrete deck mixture has been able to achieve substantial shrinkage reduction and toughness without strength loss. Varying addition rates of SRA will result in varying costs and benefits. Local testing should be done to determine optimal addition rates for a given mix design and the desired performance of the concrete.

Figure 2 shows a graph plotting effects of SRA dosage against length change of hardened concrete/>  </div>
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Fig. 2. Kualaka'i Parkway Grade Separation – No deck joints and No Shrinkage Cracks with use of SRA .
            

Concrete mix design using SRA is only one of the elements in HDOT’s effort to reduce and/or eliminate joints. The overall design and construction of the bridge also play a major role in achieving the most crack free structure possible with the least amount of maintenance. However, SRA has proven to be a useful tool in a synergistic approach to prevent and limit cracking in bridge decks as well as eliminate or reduce joints in H bridge decks.