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SELF-CONSOLIDATING CONCRETE FOR RAPID SLAB REPLACEMENTS

A refined self-consolidating concrete (SCC) mix was developed to achieve the high workability needed for faster concrete discharge and finish, and attain the Florida Department of Transportation six-hour strength requirement of 2,200 psi (15 MPa) for concrete slab replacements, report Jamshid Armaghani, Ph.D., P.E., Global Sustainable Solutions, Gainesville, Fla.; Kamal Tawfiq, Ph.D., P.E., and Steven Squillacote, FAMU-FSU College of Engineering, Tallahassee; and, Michael Bergin, P.E., State Materials Office, Florida DOT-Gainesville, in their 2015 TRB paper, Accelerating Slab Replacement Using Self-Consolidating Concrete.

 

SCC mixes exhibit very high workability and flow rate, the authors say, and are used primarily in cast-in-place or precast structural members with highly congested reinforcement. “Conventional SCC mixes are usually designed using small size aggregates (grade 89 or smaller), and incorporate high range water reducers, set retarders and workability retainers to achieve a very high workability for an extended period of time,” they write. “This will facilitate concrete flow through narrow spaces in the dense reinforcing bars to completely fill the formwork without compaction and segregation. The SCC mixes have a normal setting time and develop strength gradually.”

However, there have been no applications of SCC in highway pavements, observe Armaghani, Tawfiq, Squillacote and Bergin. Yet research has been conducted at Iowa State University’s National Concrete Pavement Technology Center to study the feasibility and application of SCC for slipform paving. In the feasibility phase, the researchers developed an SCC mix that would be adapted to slip-form paving; it possessed higher workability while maintaining good shape stability of the extruded concrete. In the application phase of a bike path and pavement, the study demonstrated the importance of using a well-proportioned SCC mix, and more attention to construction practices, to ensure durable pavements and fewer tendencies for shrinkage cracks.

Slab replacements in PCC pavements generally are done under traffic and within tough time constraints. “Precast concrete pavements (PCP) have been used in rehabilitation projects as permanent replacements or overlays for long continuous sections of concrete pavements, or in isolated individual or group slabs,” Armaghani, Tawfiq, Squillacote and Bergin write. “The PCP technology includes precast post-tensioned slabs for continuous sections, or precast reinforced panels for applications in isolated individual or consecutive slabs.”

Florida DOT-sponsored research has been under way to develop an innovative process involving the use of temporary and reusable precast slabs and SCC mix, they add. The research goal is to speed up construction of replacement slabs, increase contractor productivity during lane closure periods, and shorten the time for maintenance of traffic and overall construction. Other potential benefits from the research include mitigation of premature cracking, and cost savings to the agency and contractor.

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At FAMU-FSU Civil Engineering Concrete Lab, slump flow test is used to evaluate the consistency and workability of SCC mixes; inverted slump cone is filled with the mix, the cone is then lifted allowing the mix to flow rapidly outward.

It’s anticipated that the technology will incorporate the advantages of reusable precast panels with cast-in-place SCC mixes in replacement slab conditions. The technology would be readily available for adoption by the state highway agencies and the construction industry with minimum training.

Thus, an innovative SCC mix was developed for slab replacements in concrete pavements. The authors conclude:

  • The ability of the SCC mix for rapid discharge and finish without segregation should increase productivity of contractors during lane-closure time and may reduce maintenance of traffic time.
  • The SCC mix included grade 57 aggregate, low w/c and four admixtures including a high range water reducer, workability retainer, water reducer and set retarder, as well as an accelerator, to provide high mix workability that is sustained during the discharge and finish of slab, and early strength gain to meet the Florida DOT lane-opening requirement.
  • The SCC mix possessed a high and sustained workability without segregation, as well as high early strength. This may be considered a departure from properties of conventional SCC mixes used in structural applications.
  • The concrete workability was retained for at least 60 minutes after adding and mixing the accelerator. The workability retention will allow the truck mixers to maintain a high discharge rate during long idle times between isolated slab replacement pou
  • Vibrated and nonvibrated samples cast from the same SCC mix showed very small differences in compressive strength and weight.
  • There was no aggregate segregation in any of the multiple mixes despite high workability and the use of grade 57 coarse aggregate in the mix.
  • Adjustments to the admixture dosage rates may be necessary when the SCC mix is specified for longer concrete transportation periods, or when the admixture source changes.
  • At low temperature paving, adjustments in the cement content and accelerator dosage rate may be necessary to achieve the agency-required strength for lane opening.