Creve Coeur Lake Memorial Bridge
A large double box girder cast-in-place segmental bridge constructed using a balanced cantilever method, this project provided many opportunities to explore the benefits of concrete maturity testing on a wide range of applications. The Con-Cure system was used to determine early-age in-place strength. Once sufficient strength had been reached, the contractor was allowed to begin stressing operations, moving formwork and was able to cease external heating operations. This project is being highlighted as a foremost use of concrete maturity testing in a segmental cast-in-place bridge and is featured in a paper by Bruce Kates, lead engineer on the project.
In Mr. Kates’ words: “[The Maturity] method lends itself to those conditions which require high early strength to allow the partial removal of forms for cycling of the equipment to the next placement location, or the application of post-tensioning forces into the structure. Besides providing reliable strength assessment, the procedure includes a record of the concrete temperature during the entire curing period, which helps to assess cold weather procedures when minimum temperatures are specified. The advantage is in cost savings due to reduced construction time and/or avoidance of costly repairs resulting from low-strength concrete which may be loaded too early as the result of a mishandled cylinder test.
AT&T Stadium, Dallas, TX
Precast and Prestressed concrete plant operations
Precast operations are often highly optimized already, but the use of concrete maturity reveals enormous savings in both time and costs that impress even the most advanced precaster. For example, one producer felt that they had optimized their mix design for thousands of risers for the new Dallas Cowboys stadium. They were able to turn their casting beds over with a cycle of 18-24hrs, which is impressive on such a large scale. However, concrete maturity testing showed that they could lower their cement content significantly and still achieve the strengths required in the timeframe they needed. Making this small adjustment saved the producer “one rail car tanker-load of Portland cement in the first week of operation alone.” The Con-Cure system paid for itself in the first 7 days of operation at that particular plant in cement savings alone.
St. Luke’s Hospital Parking Garage
Usually the Con-Cure system shows in-place strength to be substantially higher than field cured cylinder breaks indicate. But during a cold week in January that was not the case. Soon after the concrete was placed it was subject to a cold steady rain that lasted for three days. On the third day the testing lab called with the breaks. Given that the cylinders have been well protected from the cold, the breaks were substantially higher than the in-place strength as recorded by the Con-Cure system. With that data, the contractor postponed stressing PT for two days and avoided a dangerous situation.
Kiefer Creek Overpass, St. Louis, MO
For this project, both concrete maturity testing and cylinders were used. In one section of the bridge deck, the data from the Con-Cure system showed that the temperature of that area was virtually flat, indicating very little strength gain while other areas showed normal strength gain. While the Con-Cure system could not tell the contractor WHAT was wrong, it was obvious something was not right with the pour. Further investigation revealed that one truck driver had added 2.5 gallons of retarder instead of superplasticizer. The bridge deck remained plastic for 6 days where the retarder was placed. Since no cylinders were taken from the problem truck, reliance on cylinders alone would have been erroneous and potentially dangerous if the forms had been stripped.