One night a 2-hour traffic delay built up as Redmond Construction's volumetric concrete trucks poured a median barrier just south of San Francisco. The project, scheduled to last 4 hours, took only about 3 hours to complete. However, traffic lines grew so fast that the California Highway Patrol police commander required updates from the project engineer.
The engineer decided to specify a change in the mix design for the next night's pour, using a proprietary fast-setting hydraulic cement that would reduce the concrete's set time and cut the onsite time down to less than an hour.
While highway users still want great roads, they also want them repaired without any delay or inconvenience. In high traffic areas, project engineers are now telling contractors and producers that one key performance requirement is to keep traffic moving.
Using data from a late-1990s University of California research study, Dave Holman from the California Cement Promotion Council (CCPC) estimates that 75% of the California highway system, including 90% of the rigid pavement, is 24 to 39 years old. Unfortunately, most of these roads were designed for a 20-year project life using load factors based upon substantially lower traffic counts than they actually carry today. So it's not surprising that some studies estimate that 80% of the rigid pavement in urban Southern California will require some form of rehabilitation in the immediate future.
Anticipating the challenges that such massive rehabilitation presents from both a materials and public inconvenience standpoint, state engineers began to investigate economical options for rebuilding or repairing the roads with concrete in the late 1990s.
In 1998, Caltrans engineers and concrete producers held a series of seminars to try to answer how state transportation could write a specification for the repair of outside traffic lanes of the deteriorating pavements.
It was obvious that transportation engineers would need a different approach to their typical repair procedures and have to tailor their materials requirements.
To determine which mix would best meet the rehabilitation requirements, Caltrans worked with Dr. Jeff Roesler, at the time a concrete researcher at the U.C. Berkeley Field Station in Richmond, Calif. In addition to strength and workability tests, U.C. Berkeley researchers also tested the durability of in-place concrete pavement.
The producers advised state officials to distinguish between high-early-strength concrete (HESC) and high-strength concrete. This distinction enabled specifiers to allow water reducers and nonchloride accelerators. It also opened the door to considerations of different cements.
What would seem to be a concrete promoter's dream situation--a client wanting to use concrete and willing to specify a performance mix that demonstrates concrete versatility--has become an unfortunate battleground for our industry. Many of the participants are debating which cement to use for the project. When members of the CCPC and the Western States Chapter of the American Concrete Paving Association saw that that proprietary cements were becoming the preferred choice, many contractor members saw only potential problems.
In late 1999 Caltrans constructed a test project on a section of Route 71 in Pomona. Caltrans engineers were so satisfied with project results that they later allowed the tear-out and replacement of 3500 cubic yards on a 2-mile stretch of I-10.
The decision to allow fast-setting hydraulic cement in the specifications is not without its critics. Hydraulic cements are about four times as expensive as standard Type I or Type II portland cements, and the different chemistry necessitates different mix designs, which include higher water content. Finally, the chemistry also causes contractors to revise their standard placement methods and finishing procedures.
Despite these differences over which cement is best for fast-setting construction techniques, the fact remains that our industry has a product and mix design for this type of job.