Until recently, the City of Louisville, Colo., had just one potable water storage tank. The 600,000-gallon concrete tank was built in 1950 to serve fewer than 2,000 people. Today, after consistently earning a place on “best places to live” lists, the community at the base of Colorado’s Rocky Mountains has more than 20,000 residents.
Inevitably, the tank became too small to meet demand.
Public Works wanted to increase storage capacity to 8 million gallons. In the 1980s, the department built a 3 million gallon tendon-prestressed concrete tank (American Water Works Association standard D115-06) and converted the old tank into a chlorine contact basin that feeds the new tank.
The project was successful. However, Colorado’s weather had taken its toll on the half-century-old original tank. That gave department managers two new problems to solve.
22-foot baffles serve two purposes
First, because water was able to “short circuit” through the old storage tank, the structure didn’t meet Colorado Department of Public Health & Environment contact basin efficiency requirements. Second, freeze-thaw cycles had damaged its concrete slab roof.
Public Works retained Merrick & Co., a Greenwood, Colo., firm that specializes in engineering, architecture, design-build, surveying, and geospatial solutions, to pull together and oversee a consulting team.
DRP Consulting Inc., a Boulder, Colo., firm specializing in concrete petrography (see sidebar), analyzed core samples extracted from the tank’s roof and sides. They showed that concrete below the roof’s visibly weathered surface was still adequate and the sides were in very good condition, with adequate strength and no chemical deterioration. Based on this information and the department’s criteria -- limited budget, a life-cycle of at least 50 years, space availability, and minimal down time -- the engineering team recommended modifying, rather than completely replacing, the tank.
Shoring up the structure would take a great deal of ingenuity.
To give incoming water sufficient chlorine contact time, the team recommended building a series of floor-to-ceiling concrete baffle walls within the tank. In addition to allowing water to flow from one end of the tank to the other, the 8-inch-thick walls could be used to extend the structure’s life by supporting the deteriorating roof.
If the team hadn’t decided to keep the existing roof, the baffle walls could have been placed in the traditional floor-to-ceiling manner: pumping concrete through a hose through the roof hatch to “windows” in the side of the forms, using vibrators to consolidate the mix, and placing the mix to the top of the baffle wall without pressurizing it. Placement was further complicated by the relatively thin walls and steel bracing that had been placed during an earlier repair.
Materials specialist Orville Werner II of CTL/Thompson Materials Engineers Inc. in Denver recommended using self-consolidating concrete (SCC) instead.
Concrete placement difficulties solved
Typical structural concrete requires placement in lifts and consolidation by vibrators to achieve a well-consolidated mass that’s free of surface blemishes and fills all voids near edges and around rebar.
The vibrating process can cause localized areas of high placement pressure, especially in tall walls like the 22-foot baffle walls. That ruled out using immersion vibrators and form vibrators because they would have increased the risk of form failure. And anyway, immersion vibrators would have necessitated access through the sides of the forms.
It also wouldn’t have been possible to get concrete with a typical slump to rise tightly to the bottom of the roof slab. Therefore, it would have been necessary to grout the contact area after the wall had hardened and been stripped.
SCC is a cohesive mixture that can be moved horizontally in forms to a greater distance than typical “slump” concrete without segregation of the aggregate. The mix fills voids and allows air to escape without vibration, and will, with little encouragement from a vibrator, seek a nearly flat profile similar to what water does when placed in a vessel. Consequently, the contractor was able to build forms without doors (stronger and less opportunity to leak, plus reduction of effort during placement in a confined space), and by filling the access holes in the roof slab with concrete and vibrating the concrete at the end of the placement, the concrete totally filled the void at the tops of the walls, avoiding the need for grouting after placement.
Glacier Construction Co. Inc. of Greenwood, Colo., specializes in water and wastewater infrastructure. The contractor’s crews cored holes in the roof slab over the formed baffle walls, then pumped concrete into the forms through a small, rigid tremie pipe that was long enough to reach down between the bars and one face of the forms to within 6 feet of the floor. They carefully controlled placement rate to minimize pressure on the forms.
Once the baffle wall concrete placement problems were solved, it was an easy matter to rehabilitate the worst parts of the roof’s freeze-thaw deterioration and protect it from further weathering.
Adding an insulating protective cover over the top of the roof would render the existing concrete roof viable for at least 50 more years. However, despite the strengthening effect of the new baffle walls, the team was unsure of the reinforcing bar patterns in the old roof slab. They decided to use lightweight insulation instead of the standard, but heavier, earth cover.
The tank was insulated and weatherproofed using a Carlisle SynTec Systems VapAir Seal vapor barrier, Dow Chemical Co. Styrofoam Highload extruded polystyrene foam insulation (2 inches thick), Georgia-Pacific Gypsum DensDeck Prime thermal barrier (1/4 inch thick), and Versico Roofing Systems’ VersiGard 60-mil EPDM roofing membrane. A layer of artificial grass manufactured by Denver-based PlushGrass Inc. was adhered to the roofing membrane.
The pitted main part of the roof wasn’t repaired because the damage wouldn’t affect the roof’s life span.
In addition to adding 50 years to the tank’s life, the $476,413 repair increased process efficiency from 10% to 80%. Because it used less concrete than a full replacement, the solution lessened the project’s environmental impact and reduced disposal costs. Finally, at seven months, the project took less time than a full replacement, decreasing treatment plant downtime.
