Hurricane Katrina struck the Mississippi and Louisiana coastline on Aug. 29. Over 100 days later, the Mississippi Department of Transportation (MDOT) had completed two major emergency bridge repair projects.
Popps Ferry Road Bridge is the main north-south traffic artery for west Biloxi over the back bay to the mainland. It is a drawbridge that was constructed from 1976 to 1979 and on the average day carries 20,000 vehicles and is raised 10 times primarily to allow coal barges access to Mississippi Power’s coal-fired electric generating plant 2 miles up the river.
Hurricane Katrina damaged the Popps Ferry Road Bridge and I-10 at Pascagoula and destroyed two major MDOT bridges. The latter two are the Oceans Springs to Biloxi and Bay St. Louis to Pass Christian (U.S. 90) bridges.
MDOT determined that I-10 and Popps were capable of emergency repairs. The I-10 project was a 30-day, $5.4 million contract and was completed in 20 days with traffic flowing Sept. 30. WL Wallace, Columbia, Miss., was the prime contractor. For the Popps Ferry project, the city of Biloxi hired Neel-Schaffer Inc., a Laurel, Miss., bridge design and engineering firm, as their consultant. This was the second emergency repair as a result of Hurricane Katrina. On Oct. 20 Key Constructors, a Madison, Miss.-based company, was awarded a 100-day contract for $7.65 million. The contract contained a penalty/bonus clause of $50,000 per day with a maximum bonus of $500,000. Key is a regional bridge and commercial concrete contractor established in 1975 and is licensed in the states of Mississippi, Arkansas, Alabama, Louisiana and Tennessee with about 100 employees.
MDOT had allowed the use of IntelliRock concrete maturity, an in-situ method of estimating the early strength in concrete on the I-10 bridge project as well as some previous paving projects. In the interim the materials division had instituted a process to publish a concrete maturity specification.
Neither Neel-Schaffer nor Key Constructors had previously utilized the maturity method as a way to estimate the early strength in concrete placements and thus were unaware of the benefits. Richard Sallee, vice president of sales and market development for Engius LLC, approached MDOT, Neel-Schaffer and Key about the possibility of utilizing the concrete maturity method on this project in the same manner it had been successfully used on the I-10 project. He explained what had been accomplished on I-10 and the benefits of an enhanced critical path, improved quality control and improved quality assurance. After a discussion with John Sones, P.E., Neel-Schaffer’s structural engineer, and Jason Henry, Key Constructor’s project engineer, it was decided that Key also would implement the use of the maturity method on this project. Given the penalty and bonus clause the main driver was the critical path, but not at the expense of concrete durability and good construction practices.
The forces of Katrina had offset 21 consecutive spans of the Popps Ferry Road bridge. Rick Webster, the CEO for Key Constructors, moved his crew on site on Saturday, Oct. 22, two days after the MDOT/city of Biloxi awarded the contract. This job had numerous challenges to overcome. A primary challenge was that initially it was thought the concrete for the deck placements would have to be barged out to the site and pumped, a time-consuming process.
The job consisted of repairing and replacing the 21 spans. Nine had to be repaired and 12 had to be replaced. Subsequent to demolition, the first phase consisted of resetting the nine spans. The next phase consisted of replacing 12 spans. As part of this, eight pier caps would have to be cut off and replaced with deeper caps. This is the first procedure where the maturity system demonstrated its worth. The specification allowed for the removal of the forms on the pier caps when the concrete strength reached 2,000 psi, and it allowed for setting of the beams when the concrete had reached 3,000 psi.
“[The] maturity sensors allowed us to estimate when the cap’s concrete placement attained the necessary strength, and thus when we could proceed with the placing of the beams. Since this was our first experience with maturity, we were simultaneously breaking traditional companion cylinders as a comparison. Having no previous experience with the use of concrete maturity we were somewhat skeptical,” said Rick Webster, Key’s CEO. “It was during this pier cap placement process that we became convinced that the maturity process worked. Based on information from the calibration curve [ASTM C 1074], as compared to the in-situ information, we were able to begin stripping forms and placing beams 24 hours after the pouring of the caps.”
The next phase where the maturity system benefited the project was in the placement of the deck sections. Originally, Key had planned to barge the concrete out to the bridge sections and pump it. It was during the cap and beam operation that the contractor saw the opportunity to possibly eliminate the planned barging operation.
The deck specification called for 100% strength, or 4,000 psi, before construction traffic was allowed on the surface.
The approved mix design specified by the contractor called for a minimum strength of 2,500 psi in 12 hours. This is a similar mix design that was used on the I-10 project and developed by Gulf Concrete. The mix design included Type III cement and mid- and high-range water reducers, No. 67 limestone and a 0.37 water/cement ratio.
The Popps Ferry maturity calibration curve developed by Bobby Dowdy, regional Q/A manager for Gulf Concrete, showed that pier caps forms could be removed when sensors indicated a maturity of 331?C-H [2,155 psi], and beams could be placed when the 24-hour maturity of 363?C-H [2,507 psi] was reached. The 4,000 psi, or 100% strength, correlated to a maturity reading of 536? C-H, and this was the point when construction traffic could be allowed on the deck.
“Utilizing the Type III deck mix, combined with the use of the maturity sensors, we were able to run the concrete operation from land by trucking the concrete from the south shore. Having this real-time strength information allowed us to place a deck every other day, alleviating the need to work from a barging operation. This was very beneficial to our operation,” said Webster.
Key constructors started the project on Oct. 24 and finished it on Dec. 23, completing the job in 39 construction days and collecting the maximum bonus of $500,000.
Busy at the bay
MDOT still has the two large bay bridges to completely rebuild as a result of Katrina. MDOT Materials Engineer Mike O’Brien, P.E., working in conjunction with the bridge design section, took the initiative based on their I-10 and Popps Ferry experiences and under AASHTO guidelines drafted and instituted a concrete-maturity specification as well as a mass-concrete-materials specification that was included in the a final addendum to these upcoming bay bridge’s construction specifications.
These two are to be completed by December 2007. Not only is the critical path important to all involved, but so is the bridge durability.
“With the additional benefits of concrete maturity in the areas of quality control and quality assurance, this will support the concrete operations and the residents of Mississippi will benefit,” said O’Brien.
On Dec. 14, MDOT’s materials division held its first mandatory two-hour continuing education course for all MDOT, contractor and engineering testing lab personnel who will be involved in projects where concrete maturity will be used.