The George L. Throop Company used its fleet of volumetric concrete batch plants to place Rapid Set® concrete at the Hartsfield-Jackson Atlanta International Airport (ATL), Georgia, for its runway full panel replacement project. “The previous contractor had been kicked off the job for nonperformance, as they were trying to use normal mixers with the Rapid Set,” Jeff Throop said. “It was not a good experience. Sometimes what can happen with ready-mix trucks is that because most fast-setting concrete jobs take place at night, veteran drivers will opt for day work, leaving rookie drivers for the night shift,” he said. “They will get lost, get out of sequence, with all the issues of under-experienced drivers trying to handle an expensive product that has to be delivered quickly and placed right the first time.” That’s why Throop uses only volumetric mixers for fast-setting concrete placement.
In Atlanta, working with Kiewit, the prime contractor, Throop was able to double the production of the previous contractor. “We brought our experience to the table, and with Kiewit doing its due diligence, we doubled what anybody had been able to do,” Jeff Throop said. “The airport was happy and we got out a lot quicker, in fact, in half the time. So it was a win-win situation.”
In fact, now the Atlanta International Airport (ATL) is contemplating changing the type of mixer allowed for slab/panel replacement from volumetric as an option, to a requirement. “We like the consistency of the mix that’s made right there by the volumetric mixer, next to the panel, instead of being trucked in,” said Quintin B. Watkins, P.E. (Professional Engineer), airside area manager, Hartsfield-Jackson Atlanta International Airport, a city agency. “We had one contractor who tried to truck it in from a local plant, and it set up in his drum. It makes me sleep a little bit better knowing a volumetric mixer will mean the job will get done in the time frame.”
That’s because with ATL, much is at stake. “We are the world’s busiest airport, and can’t afford to have runways or taxiways closed very long,” Watkins said. “We need concrete that can set up in a matter of three hours. Typically for slabs we will have a 10-hour window for replacement, from 10 p.m. to 8 a.m., and we will wait until a slab is in pretty bad condition before we will undertake rapid removal of a slab.”
For these projects the Atlanta Department of Aviation will do the design, and place out for bid. Typically, for the first night, panels will be saw cut, and the second night panels removed and replaced, with the pavement reopened by 8 a.m. with a flexural strength of 500 psi. Liquidated damages are $1,000/minute for failure to open a runway on time, and $500/minute for a taxiway.
Throop stresses a team approach. “In Atlanta, when we first presented, we said we could do three panels a night,” Jeff Throop said. “They said they’d never been able to do more than one, how could we do three? We laid it out: You have the have enough equipment and people, and the right equipment and people. We ended up doing two panels per shift, but sometimes we had the time to do three.”
At ATL, after the prime contractor cut the panels, Throop replaced the panels, 25 x 25 ft. x 16 to 18 in. deep. “Nobody had been able to do more than one panel per shift,” Jeff Throop said, “but we were able to do two panels, and could have done three on some shifts, had they decided to go for it. But everyone was happy with the end result and the doubling of production over what had been done before.” Ultimately Throop placed 25 panels incorporating over 800 cu. yd. of fast-setting concrete.
“Throop was the first contractor to replace two panels in one night,” ATL’s Watkins said. “He would bring in two volumetric mixers and knock out the job right there. That was an amazing accomplishment with his machinery, and it helps out enormously because we have some panels that are 25 x 50, and others that are 25 x 75. In the past we’d cut them in pieces and deal with them in 25-ft. segments, but with Throop we can do all of it at once if we wanted to.”
The fast-setting cement is what makes it all possible. “The airport specifies Rapid Set C150 cement, but have to get approval from the city due to single-source spec requirements,” Watkins said. “And if a contractor proposes to use cement other than that specified, information relating to the proposed cement has to be submitted, indicating use of the material in five separate contracts for the previous five years. We want lots of background information on a substitute material, and no one has done that yet.
“At ATL, we were in a race to strength,” Jeff Throop said. “With this cement and our volumetric mixers, we were able to give the demolition crew plenty of time to finish slab removal. Then, with the concrete mixing for only 15 to 18 seconds, the concrete was placed within a 15-minute period. It had great workability, with a full 30 minutes for the crew to get the concrete finished, sealed, textured and cured. To see the heat rise, and know the hydration is really kicking in, is something to watch.”
Under these circumstances, the volumetric mixers give Throop tremendous control over the amount of water used for the mix. “The No. 1 benefit of the volumetric mixer is its control of water to cement,” Jeff Throop said. “The most important characteristic of good concrete is its water-to-cement ratio. It drives all qualities, from strength to shrinkage. But with Rapid Set the only change in slump that you ever see is from moisture in the aggregates. That’s why with barrel mixers we will see changes in slump.”
For any project, Throop will submit a mix design to the owning agency, which is approved when it meets agency specifications. Then each component of the mix design drives calibration of the volumetric mixer. “We calibrate the cement first, and that drives the entire calibration process,” Jeff Throop said. “We know how many ‘counts’ will be required to produce cubic yard of concrete, and then adjust stone and other ingredients by adjusting gate settings, balanced against the production rate.”
Throop also is able to heat water on its rigs. “We are able to produce concrete in colder climates,” Jeff Throop said. “We can produce in temperatures in the upper 30s or low 40s. It slows down the opening strength gains, but we have all the bells and whistles we need to produce concrete when and where we want to. On Interstate highways it’s very common to be placing concrete slabs at night, when the temperature can be in the low 40s. By adding warm water to the mix we will get curing faster than if we did not use warm water. And that can be the difference between opening on time or not.”
Keeping the quantity of cement in check is important, because a value-added concrete like that made with Rapid Set can be three to four times the price of conventional concrete.
On airport projects, despite the jawboning with airport authorities, Throop still will have to avoid liquidated damages for failure to open a pavement on time. At ATL Throop was exposed to liquidated damages of $500 a minute, Jeff Throop said.
“The damages can vary widely,” Jeff Throop said. “Taxiways have a certain value, but runways will have a much higher value. It depends on where you are. For Caltrans, it’s $1,000 a minute, but even if its $500 a minute, it’s still a lot of money. We have to have everything duplicated and backed up just for that reason. If we are late opening a freeway or taxiway or runway, it can get ugly in a hurry. But with Rapid Set we’ve never been fined.”
If the liquidated damages are high, there is a commensurate premium for the value-added service that contractors like Throop provide. “For a single slab replacement, the material cost is high, very high,” ATL’s Watkins said. “But we are replacing only one slab among many, and the premium is worth it in terms of keeping the airport open.”
Throop is responsible only for the production of rapid fast-setting concrete, but its impact goes well beyond the placement.
“The subgrade prep is the responsibility of the demolition contractor, who is doing the saw cutting, slab removal and dowel insertion,” Jeff Throop said. “We are just the ready-mix producer, but we are anxious to make sure the prep work is done correctly. So when all the existing panels have been removed, the subbase is inspected. And if there are any voids – for example, when the previous panel is picked up, it pulls up subbase with it – we sometimes are asked to pour a hot patch in the base. We will remove the retarder from our normal mix, place it and put a vapor barrier between the base and the panel to be produced. It’s just another advantage of the cement used and the equipment used to produce it.”
Dowel insertion can become a bottleneck for the whole night’s schedule. “I can’t overemphasize to airports the importance of smooth doweling in the overall plan,” Jeff Throop said. “Everything takes so-many minutes to accomplish. The saw cutting is done in shifts before our crews show up; keyways are drilled, so that when the crews arrive and lights are set up, they can go to work immediately pulling slabs that are 18 to 24 to 30 in. thick, depending on where you are, 25 ft. square. It takes a while to pull those blocks out, and it needs to be done carefully, so that no adjacent panels are damaged.”
For each 25 x 25-ft. panel, about 80 dowels have to be drilled and epoxied, Jeff Throop said. “If you are doing two panels, you may not have to do a side,” he said. “If you are up against asphalt, you may not have dowels on that side. But if you have a typical panel with dowels into the adjacent panels, there are 80.”
The dowels can be deformed, or smooth, depending on which direction they are oriented. “We don’t get involved in the dowel insertion,” Jeff Throop said, “but we like to approach work as a team effort, and if we see something that can be done quicker, we will bring it to the contractor’s attention. We all have the same goal. If we have observed something from a previous project that we can bring to the table, then we do it. That’s what it’s all about.”
The cutting of airport slabs generally takes place at night, Jeff Throop said. Impact demolition is not allowed, so giant concrete saw blades 60 in. in diameter, with 120 hp engines, are used.
“To saw up to 24 in. deep you have to have a big blade,” Jeff Throop said. “And you have to have skilled people at every place. For example, I’ve seen blades get stuck that had to be torched off because they could not get them out. You have to open the runway the next day, no matter what is going on.”
In addition to Atlanta, Throop’s done work at San Diego Lindbergh Field, LAX, Burbank, Ontario, San Francisco, and Denver International airports.
“We go to the airport authorities and make a presentation on how they can get the best value for their money, but with no sacrifice in the quality of the work or concrete,” Jeff Throop said. “We inform them on the techniques of removal, how to work with us, and schedules. For example, some airports will only allow you to work on weekends. We say if you can give us weeknights, or a combination of weeknights and weekends, we can get in and out a lot quicker, and reduce the costs of flying people in and out, and per diems.”
Throop elaborates on the essentials of speed, quality and the use of experienced crews. “We stress the need to use as many shifts per week as possible, to better utilize rented and owned equipment,” he said. “It also keeps our crews together and focused on working consistently. In addition, the project is completed in weeks, instead of months, keeping costs down.”
To further reduce costs, Throop urges the authorities to identify at least 20 to 30 panels for replacement. “When you figure in costs of mobilization and demobilization, the costs can be spread over those quantities and they are minimized,” Jeff Throop said. “We seek out the best aggregates in their area, so we can achieve our strengths in the quickest time possible, so we’re not hauling aggregates across the country, or any further than we have to. And we work with local contractors who are already familiar with the client, because we are a concrete producer who brings added value to the job, but not a local subcontractor.
Volumetric mixers are key to Throop’s successful niche market, and Throop now makes its own volumetric mixers, based on its needs. But that comes at the end of a period of experience with manufactured mixers.
“We bought our first volumetric mixer in 1987,” Jeff Throop said. “It was an all-electric unit and was a disaster. Frankly, the salesperson misrepresented what the unit could do. But we worked through that and later purchased trailer-mounted machines that have guided us to where we are today.”
Today Throop owns 15 volumetric mixers, the majority being trailer-mounted. “We are unique in that we kept making our own improvements in the machines, and sending those along to the manufacturer,” Jeff Throop said. “Because most of the manufacturers build equipment, but don’t operate their own fleet or make their own concrete, in some cases you are not getting the best components you need because of the manufacturer’s need to meet a price point. That’s why we now make our own machines, since the early 1990s.”
Throop has truck trailers and other components custom-made, but does the final assembly itself. These staff-built machines now undergird Throop’s specialty concrete business. “We felt from Day One we wanted to develop niches within the ready-mix industry, and by far the biggest niche is rapid-setting concrete,” he said. “These machines make it possible.”
Throop can monitor the volumetric mixers’ quantities and performance every five minutes, building a database that can be consulted afterward. It was the result of a demanding job the firm did in the Southeast, where it sealed nuclear waste tanks.
“About 14 years ago we were asked to do some nuclear waste tank closures in South Carolina, and they had mixes with eight ingredients,” Jeff Throop said. “It was the first time it was ever done, so there were a lot of issues with quality, consistency and documentation of what was going on. We had to develop software and install them on our machines so that a log would print every five minutes on what had been produced, and give them a live ‘snapshot’ of what was going on. That was needed because once the concrete had gone in the tank, it was gone; There was no inspecting or replacing it because of the nuclear material in tank itself.”
As the result of that job, Throop kept improving the consistency of the equipment and the components of the volumetric mixer. “By having those requirements placed on us, we had to find better ways of producing grouts and concretes,” Jeff Throop said. “And those improvements on our standard components benefit all our other projects, such as those for airports.”
By hand-selecting components for specific tasks, Throop optimizes its chances for success in the field.
“For example, air-operated vibrators on volumetric mixers are very loud, for both the operator and anybody standing near,” Jeff Throop said. “They also have a limited life. Not only would these be loud, but they would last perhaps a year with our utilization rate. We went to a continuous-duty rated 12-volt vibrator, adjustable in force. It was probably four times as expensive as the air-operated vibrator, but it it’s a great benefit to go with the improved vibrator, especially because at the time they were not offered by the manufacturer.”
But the biggest difference between Throop’s mixers and manufactured mixers is that they can be reloaded while mix is being produced. Using its own tankers, Throop can be blowing cement at the same time it’s producing concrete.
“Other volumetric mixers will have to go back to a cement silo, or aggregate staging yards to reload,” Jeff Throop said. “We’ve had projects in which we can produce hundreds of yards of concrete from one machine. We bring the materials to the job, and use a standard loader to charge the mixer while concrete comes out the back.”
Throop mixers incorporate a bin for cement, a large, partitioned aggregate bin that will hold coarse and fine aggregates separately, a surge tank for water, tanks for additives, all self-contained, perhaps sitting over a pump that will distribute the Rapid Set concrete where it needs to go.
Throop mixers must meet ASTM C685/C685M Standard Specification for Concrete Made by Volumetric Batching and Continuous Mixing. It also must meet Caltrans C109 specs for volumetric batching.
“We [volumetric mixers] are held to a higher standard in California,” Jeff Throop said. “But what’s nice is that California’s stringent standards can be sustained by us throughout the country, wherever we do work.”
Caltrans will visit a batch plant for a calibration check perhaps once a year, Jeff Throop said. “But they will be in our facility, checking calibration of our machines, once every month,” he said. “That’s understandable as volumetric mixing was new to Caltrans, and they wanted to keep it close to the vest. But we know the equipment is consistent and have our own incentives for keeping the equipment up to spec, as we are responsible if the concrete doesn’t come up to strength.”
Ultimately, Throop’s success resides in its volumetric mixers, its rapid-set cement, and its understanding of what the “race to strength” entails.
“We understand how important it is to get the concrete in the hole, quickly, and that’s why we’re at a minimum of 100 to 150 yards per hour production rate,” Jeff Throop said. “We can get that concrete where it needs to be in a very timely manner, just 20 minutes, yet the placing and finishing crew has adequate time to do everything it needs to do. Fortunately, there’s a working window with Rapid Set.”