Since its construction in the late 1960s, a less-than-one-mile section of California’s Interstate 5 along the Sacramento River has been a bit of an experiment
Steven Prokopy
Since its construction in the late 1960s, a less-than-one-mile section of California’s Interstate 5 along the Sacramento River has been a bit of an experiment. Commonly referred to as the Boat Section, the three-quarter-mile stretch earned the nickname because its bed lies below water level and had to be drained in order to complete one of the last portions of I-5. The area would flood regularly during storms without its elaborate pumping network. It took engineers in the 1960s several months to de-water the area and construct a concrete seal slab as thick as 10 feet. To hold the slab in place, pins were drilled more than 80 feet deep, supported by retaining walls, drains and pavement.
Over the decades, the constant pressure of hydrostatic forces beneath the surface has taken its toll on this section of roadway. Plugged with silt and sand, the drainage systems became ineffective, while cracks and leaks compromised the pavement. As ground water has pushed though joints in the seal slab, the water has nowhere to go but up through the road, causing surface spalls and deterioration.
When the California Department of Transportation (Caltrans) decided it was time to replace and reinforce the problem pavement, the state decided to use the project as a symbol supporting Gov. Arnold Schwartzenegger’s 2006 legislation AB 32, a comprehensive program of regulatory and market mechanisms designed to fight global climate change in a quantifiable, cost-effective manner. Although the project (commonly referred to as Fix I-5) was not subject to carbon emissions reduction standards being drafted by the California Air Resources Board (CARB), currently developing regulations to reduce the state’s greenhouse gas emissions/carbon footprint 25 percent by 2020 (mandatory caps are not set to begin until 2012), Caltrans set forth an aggressive plan to use a green mix design for much of the replacement slabs.
TIME CRUNCH
One major objective set forth by Caltrans for the $27 million project was to reduce congestion and delays as much as possible during construction. As part of its initial bid for the job, Rancho Cordova-based highway and bridge contractor C.C. Myers Inc., a company famous for rapid schedules, drew up a timeline that would have the bulk of the work accomplished between May 30 and July 24.
Having completed 17 emergency projects for the state of California, Meyers made a reputation for itself as the contractor of choice after earthquake damage and other catastrophes that required a quick rebuild. And, with most other I-5 bids coming in with year-long schedules, the Meyers’ plan would save the taxpayers money and get the job done with minimal disruption to commuters.
The plan called for significant lane and ramp restrictions on I-5 in downtown Sacramento, affecting nearly 190,000 drivers daily. The four-stage schedule allowed for closure of either the northbound or southbound lanes for 10 days each for pavement replacement (Stages 1 & 2), with an additional post-curing seven days per direction to allow for application of a polyester sealant and delineation (Stages 3 & 4).
During demolition of the old pavement, Caltrans and C.C. Meyers discovered that the original concrete was much thicker than anticipated Û in some places as deep as 48 in. This meant that the prep process for the new, fast-setting, slag-based concrete would take longer than expected. Stage 3 crews were forced to push back their start date, while still managing to wrap up the bulk of the project by the end-of-July deadline.
It is critical that this concrete be completely dry before applying the sealant and final driving surface, said District 3 Directory Jody Jones in a statement announcing the delay. To ensure a quality, long-lasting product, Caltrans and our contractor have determined the safest course of action is to move Stage 3 to July 8 and avoid impacting the public and downtown area businesses during the busy July 4 holiday.
CHEMISTRY CLASS
Two rapid-strength mixes were used as part of the Fix I-5 job, both supplied by Cemex. The primary concrete, the BASF Admixture Systems-formulated 4_4, was used for panel replacement on the I-5 approaches, which needed to achieve 1,200 psi in four hours. The design featured a straight Type III cement mix with a water-reducing admixture for a low cement-water ratio, plus a hydration control agent added just before trucks left the plant. A nonchloride accelerator was then added at the site while the mixers waited to pour. BASF supplied all admixtures and worked closely with the Cemex team and Caltrans to refine and perfect final mix designs.
Cemex enlisted two of its ready mixed facilities to supply specialty products for the I-5 work. The 4_4 mixes were produced at the company’s Joellis Way plant in Sacramento; the seal slab concrete was dispatched from the Rio Linda operation north of the city. Each of the project’s 35 mixer trucks had either R or J painted on it, so when they got to the job site, workers knew where to send the vehicles. A third Cemex Sacramento plant on Bradshaw Road was devoted to supplying regular structural concrete.
Not surprising to anyone involved in the research and development aspect of this job, the seal slab mix turned out to be the most challenging to design, since Caltrans specs called for 50 percent slag replacement and a 3,625-psi compressive strength in 24 hours. We had a month to come up with it, says Cemex’s Jim Van Nest, quality control manager for ready mixed concrete-Sacramento area. We started with the 4_4 mix and added in the slag aspect. We went through 23 trial batches before we got the right combination of admixtures to get the psi in the time required.
And every step of the way the Caltrans senior engineer was on hand to witness all the failures as well as the successes. At times, we had people sleeping in cots, and we were working 24-hour days, seven-day weeks.
The Grade 100 slag, imported from a Lafarge-operated distribution site in Seattle, met AASHTO and ASTM criteria for slag cement and required five different admixtures to meet all of Caltrans requirements. There were two different accelerating admixtures, one for shrinkage-reduction to avoid cracking and a stabilizing admixture to help keep the water-cement ratio low. At 0.28, we used only 27 gallons of water per yard, compared to 28 gal. on the regular 4_4 mix. We created the most efficient mix possible, a dense product with the minimum amount of water, explains Hernan Perez, quality control director for Cemex Northern California. We were only allowed 0.5 [percent] shrinkage and were concerned, because during the R&D we used 4-in. molds, but on the job, there were 3-in. molds. We thought that might give us problems with shrinkage, but that was not the case.
According to Perez, typical slump for slag cement mixes is 6-7 in., but with the site-dosed accelerator used for the I-5 work, slump was 9 in. We would set the slump at the batch plant knowing it would change on site, he says. We would always want to err on the side of too much water and accelerator at the plant. But, no additional water was needed beyond the plant. Temperature of the mix was controlled by the hydration control admixture.
Caltrans insisted that no load should exceed 250 revolutions in the mixer once the final admixture was introduced. They actually tossed a load that stayed too long in the drum, adds Tarek Khan from BASF’s Western Division. Because of the low water-high cementitious content, we had to refine the batching and mixing procedures to get the concrete fluid and acceptable to Caltrans and C.C. Meyers.
Curt Higgins, Cemex’s general manager of ready mix division-Sacramento District says that usually ÎgreenÌ concrete requires an extended curing age of several days, but this job did not have that luxury, since most days the trucks needed to be able to drive on slabs poured the day before. This was critical, he says. All traffic for the project had to go on one lane of concrete poured the previous day. That put extra pressure on the slag mix design, since we were creating the artery of all movement for this job. And we’re not just talking about mixer trucks. We had asphalt trucks, trucks removing torn up pavement, and admixture trailers.
A mobile lab with a curing room was always on the job for sampling and quality testing, which was carried out every 250 yards of material. With the portable curing room, samples could be processed in about four hours. In addition, truck-mounted ReadySlump meters Û which Cemex has widely adopted across its mixer fleet Û provided readings as loads left the plants and arrived on the job site.
A power screed begins the slab-finishing process, followed by manual troweling. On the seal slab surface, a broom finish is part of the process to add an anti-skid element to the structure. About one week after each direction’s pour was completed, a 3/4-in. polyester overlay and sealant were applied, prior to lane marking. One thing many people don’t realize is that the main road is considered a ÎstructureÌ and not a pavement, explains Higgins. Because it’s technically above water, the section is managed by the Structures Division of Caltrans, which also handles bridges. The approaches, however, were handled by the Paving Division.
As part of the Fix I-5 project, Caltrans also put in an entirely new drainage system to keep water from migrating to the surface and damaging the structural elements as had happened previously. The old drainage system required regular visual inspection of each pump during rainy weather. With the new system, electronic monitoring will manage the pumps, freeing up Caltrans personnel to do other work during storms.
Caltrans learned that ÎgreenÌ concrete might be tough for this type of project, but they also felt that this was the way things were going into the future, says Higgins. Caltrans is genuinely excited about the I-5 project, and they see this as a proving ground for this type of mix. For ready mixed companies with large enough technical and service departments, the prospects are great.