Why would a general contractor want to buy and operate his own concrete batch plant?

The answer, according to Don Clark, partner in Denver-based Lillard & Clark Construction Co., is simple: convenience.

Clark explains:

• With its own batch plant on the job, the project superintendent can order concrete and have it delivered within a few minutes, saving downtime and loss of production in transitioning his crews from one job to another.
• Further, there is no wait time by a concrete pump or crane and placing crew as a result of concrete trucks arriving late for a pour that is scheduled for a specific starting time.
• Spacing of truck deliveries is controlled to match the crew's optimum rate of placement. There is no waiting for trucks and no trucks are being rejected for going past the time limit.

Currently, Lillard & Clark is some seven months into a major expansion of the Big Dry Creek wastewater treatment facility for the northern Denver suburb of Westminster. The plant, at 13150 Huron Street, treats about half of the city's wastewater and has a daily treatment capacity of approximately 10 million gallons. The expansion, designed by the Denver office of CDM, will ultimately raise that capacity to approximately 27 mgd.

Concrete placement makes up a substantial portion of the expansion work, including construction of new aeration basins, a new secondary clarifier, new circular anaerobic digester, new headworks building and influent pump station, numerous other new features and modifications to nearly all existing facilities. Caissons support several of the facilities, with 20 beneath the new headworks alone. Slabs 2-1/2-feet thick for the new aeration basins each require 250 cubic yards of concrete. In all, the expansion will utilize some 12,000 cubic yards of concrete, and Lillard & Clark expects to be on the job site for two to two-and-a-half years.

Combine the large amount of concrete needed over an extended period with the current cement and concrete shortage facing the Denver metro area (as well as many other parts of the country), and this seemed to Don Clark an ideal project on which to employ his own batch plant. The caissons alone made the idea of a captive batch plant attractive. Clark notes that when drilling and placing caissons it is essential to have the concrete available to fill the hole immediately after drilling is finished, for only bad things happen as long as the hole is left empty. With the batch plant on the job, the superintendent can order the concrete batched when the hole is completed and have it setting at the pump by the time the reinforcing steel is installed. Conversely, if the concrete must come from an off-site plant, it must be ordered early for it to be there when needed, but then, if some kind of difficulty is encountered in completing the hole, it could sit there until it is past the time limit and be rejected. If the concrete is late in arriving, then there is lost time for the drilling rig, the concrete pump and the placing crew, plus the risks of losing the drill hole while it sits without being filled. All of this can get very expensive.

Lillard & Clark went looking for a batch plant that would suit its needs and learned of Divide Construction and Port-a-Pour of Berthoud, Colo., through the companies' website. They contacted Jerry Doherty, the owner, who said he had a new portable plant just being completed. He suggested a turnkey agreement where he would put the plant on the job, set it all up and operate it with his people like a joint venture. In a matter of two or three weeks, Divide had the Port-a-Pour plant set up with aggregate bins built; four secondhand mixer trucks bought and on the job; a 120-kilowatt, 3-cycle, 480-volt generator hooked up; and materials on site, ready to run test batches. The mix designs were approved by the engineer, and test batches were run that were good from day one. Most of the concrete used on this project is 4,000-psi strength at 28 days, and fly ash is being substituted for a portion of the cement, thereby lessening the impact of the cement shortage.

Yet another advantage of the on-site batch plant, according to Clark: Every concrete placement brings on a dilemma at the end of the pour. It is not difficult to calculate the volume of most pours, but with most mix designs and batch plants the volume produced is not precisely the amount the batch plant reports, so it is likely that if the exact calculated amount of concrete is ordered, it will come up long and concrete will be wasted, or it will come up short. When it is short, a reorder is necessary for the amount it takes to finish the pour. When the batch plant is not on the job, it may take an hour to get the final quantity (depending on availability of trucks at the plant and distance from the plant to the job). Sometimes the last truck on the job must return to the plant and come back with the final quantity because no other truck is available. All that time, the expense of the placing crew and the concrete pump or crane (whichever is being used) continues. With the plant on-site, the contractor can purposely stay short on the quantity and order back what is necessary to finish, as it only takes a few minutes to produce and deliver.

The low profile of the Port-a-Pour plant, with its pressurized horizontal storage bins, advantageous in areas with limited clearance, has proven quite popular with nearby homeowners at this wide-open site, and most are completely unaware that such a "NIMBY" (not in my back yard) installation is even operating on the project.

Lillard & Clark has employed two of its own Terex American HC80 cranes on the project, one to handle forms and rebar, the other to place rebar and handle the concrete placement bucket. Six months into the project, after placing concrete all through a particularly mild winter, 7,500 cubic yards of the eventual 12,000 have been placed. And project superintendent Greg Bedin reports that the work has gone even more smoothly than expected. Originally, it was planned to pour one of the aeration basin walls per week, but under the direction of assistant superintendent Randy Falconer, two or more were done routinely.

It's not for every project or every contractor, but the on-site batch plant can be ideal for projects requiring between 7,000 to 20,000 cubic yards of concrete, and where they are pouring almost every day. And the Port-a-Pour plant is probably not the right choice for main line paving, where a thousand yards a day is called for, but it works well where the contractor must meet high specifications and regularly needs between 150 and 500 yards per day. Although the Port-a-Pour plant Lillard & Clark is using is rated at 150 yards per hour, it can actually produce much more in a day if needed.

The time factor is also important. On the Little Dry Creek WWTP expansion, specs call for concrete to be placed within 60 minutes from the time it is batched. With over a 30-minute delivery time from most fixed plants in the area, it doesn't leave the contractor much time to unload the truck if there is even the slightest delay.

The low profile of the Port-a-Pour plant and all the additional cement and fly ash storage on this site without the use of a vertical silo is a major advantage when it comes to permitting issues. The use of cement bins with their own dust collection systems keeps everything below 15 feet.

Port-a-Pour's Jerry Doherty sums it up: "Seeing the contractor's realization of how efficient it is having dedicated service to their project and what that means to increasing their production schedule is really what it's all about. It's always rewarding to hear a contractor say, 'I don't know why everybody doesn't use this type of set-up.'"