this information is provided by permission of Jorgensen, Hendrickson and Close Engineers, Inc. (JHCE)

The purpose of this letter is to acquaint you with tendon tank technology and show how your tank clients can benefit from this knowledge. This information is being offered to you by Jorgensen, Hendrickson and Close Engineers, Inc. (JHCE) on behalf of several tendon suppliers, precast-prestressed tank suppliers and general contractors around the U.S. that offer alternatives to Wire- and Strand-Wrapped tanks.

It is the obligation of the consulting engineer to obtain the best value for their client. One good way to do this, when the project calls for a tank of 500,000 gallons or more, is to incorporate a tendon tank alternate into the performance specifications for the project. This can be done with a simple one sentence Addendum saying essentially "Alternate bids based on AWWA Standard D115 and other applicable portions of these Plans and Specifications are encouraged and will be accepted by the owner." Alternatively, a fully developed Performance

Specification based on the sample performance specifications available free upon request from JHCE can be put out as an Addendum.

JHCE has designed over 100 post-tensioned tanks all across the country over the last 30 years, using all types of construction (wire-wrapped and internal tendon, precast and cast-in-place, circular and rectangular). We have come to the conclusion, however, that internal tendon tanks, no matter what shape or construction method, are almost always the best choice because of their superior watertightness, long-term durability and value. The purpose of this letter is to point out the differences between wire-wrapped and tendon tanks, outline the advantages of including a tendon tank alternate in the bidding and suggest ways of incorporating a tendon tank option into your project.

DIFFERENCES BETWEEN WIRE-WRAPPED AND TENDON TANKS

The fundamental difference between wire-wrapped and tendon tanks is, of course, in the way the circumferential prestressing is applied to the tank wall. In wire-wrapped tanks the 3/8 inch diameter prestressing strands or 1/4 inch prestressing wires are applied one at a time in a continuous helix. This will be referred to as "wire-wrapping" in this letter. When larger tanks or dome rings are wrapped, several layers of strands or wires must be separated from one another by layers of shotcrete. When the wire-wrapping is completed several layers of shotcrete must be applied to the entire wall surface in order to build up the shotcrete covercoat necessary for corrosion protection of the wires. It should be noted that, because of the thinness of the shotcrete layers and the susceptibility of the exposed wires to corrosion prior to covering them, this operation cannot proceed in freezing weather situations, which can delay a project for months. This process is also very cumbersome, time consuming and, therefore, expensive compared to the way tendon tanks are prestressed.

In tendon tanks several 1/2-inch or 0.6-inch diameter 7-wire strands are inserted into an empty duct and stressed simultaneously by means of hydraulic rams. After the strands are tensioned, cement grout is injected into the ducts to displace any air and help provide corrosion protection. Since the tank wall can easily be tented and heated, tendon tank construction can proceed year- round, regardless of the climactic conditions. The entire operation is very efficient, it takes only a few days, less than a week on a typical internal tendon tank. This compares to a month or more to apply the circumferential prestressing and build up the shotcrete covercoat in a typical wire-wrapped tank.

There are differences in the final corrosion protection between wire-wrapped and tendon tanks as well. In wire-wrapped tanks the corrosion protection of the circumferential prestressing wires is provided by the shotcrete covercoat. Shotcrete, like any other portland cement material, is subject to drying shrinkage. This shrinkage is restrained by its bonding to the previously placed shotcrete and wires. Therefore, shrinkage cracks develop throughout the layers of shotcrete. Since the shotcrete is applied prior to filling the tank with water, and is not circumferentially prestressed, it has to go into tension as the tank is filled. The combination of shrinkage and axial tensile stresses in the shotcrete results in numerous vertical cracks that may completely penetrate the protective layer of shotcrete. For this reason, one of the wire-wrapping contractors in the West usually calls for the circumferential wires to be galvanized, as a second layer of corrosion protection. However, this creates extra expense and is not considered to be the best solution, based on long-term durability considerations, by many. For example, this practice is not recommended by the Hot-Dip Galvanizer's Association as it may lead to concentrations of corrosion potential at cracks or holidays in the zinc coating, which would lead to stress corrosion.

Wire-wrapped tanks should have periodic field observations including "sounding" the exterior surface. The purpose of the sounding is to check for areas of potential shotcrete covercoat separation from the circumferential wire-wrapping. Shotcrete covercoat separation seems to occur without warning, leaving void spaces where water can collect. This makes the underlying circumferential prestressing wires highly susceptible to corrosion. Such periodic maintenance cannot occur, of course, if the tank is to be buried or partially buried.

Internal tendons, on the other hand, can easily be provided with "triple corrosion protection." The first layer of corrosion protection is provided by three or four inches of two-way prestressed concrete cover. The prestressing prevents tension cracking altogether in the circumferential direction, and from reaching the circumferential prestressing (the outer most layer of prestressing) in the vertical direction.

The second layer of corrosion protection is provided by corrugated plastic ducts that contain the prestressing strands. These ducts come in twenty foot lengths that are connected by waterproof methods. This way, a waterproof and, therefore, corrosion protective barrier is maintained around the prestressing strands.

Finally, the third layer of corrosion protection is provided by rich cement grout that is injected into the ducts after the post-tensioning operation is completed. The helical lay of the strands along with the corrugations have been proven to fully encapsulate all facets of the strands with the protective grout.

Contact JHCE for a reprint of the paper titled: "Corrosion of Circular Prestressed Concrete Water Tanks" for a more complete discussion of the differences in corrosion protection of wire-wrapped and tendon tanks. This paper was given at the ASCE Structures Congress XV in Portland, Oregon, in the spring of 1997 and was published in Volume I of the Proceedings of that Conference.

There are differences in the design philosophies and requirements of the various standards writing committees for wire-wrapped and tendon tanks. For example, wire-wrapped tanks are only required to have residual circumferential compression of 100 psi if above ground and only 50 psi if below ground, per AWWA D110. Tendon tanks on the other hand are required to have 200 psi residual compression, whether they are above or below grade, per AWWA D115. Also the seismic design criteria for wire-wrapped tanks is not as conservative as for tendon tanks.

Not only are the walls of wire-wrapped and tendon tanks different, so are the floors and roofs. The "membrane" (non-structural) floors of wire-wrapped tanks are only four inches thick (unless the consultant specifically requires more) and are reinforced with conventional reinforcement. The wrapped tank industry has had to write special exceptions for themselves because there is not enough room in these thin slabs to provide for the ACI 318 Code required concrete cover. Normally, reinforcing bars are required to have three inches of cover when cast against earth and one-and-one-half inches of cover when exposed to water. This would require a nearly six inch thick floor slab in order to meet these generally accepted cover requirements.

Tendon tanks, on the other hand, are normally constructed with five-inch thick two-way prestressed concrete floors. There is room in this thickness to provide two inches of cover to the bottom, cast against two layers of 6-mil plastic, and two inches to the top, exposed to water. These are the ACI 318 Code required covers for prestressed concrete. Also, the recommendations are for these slabs to maintain 200 psi residual compression, over and above any loads or slab-subgrade friction. This is obviously superior to conventionally reinforced slabs-on-grade, which contain micro-cracking and shrinkage cracks.

Similarly, the roofs of wire-wrapped tanks are conventionally reinforced two-way flat plates or domes. Tendon tanks, however, usually have post-tensioned two-way flat plate roofs. Tendon tanks can have domes, which are more expensive, when clear space is specifically needed. Post-tensioning greatly reduces cracking, both shrinkage and temperature related and structural related, thereby ensuring greater long-term durability. Post-tensioned roofs are also more economical the their thicker, heavily reinforced, conventional counterparts.

ADVANTAGES OF COMPETITIVE BIDDING

The performance specifications that most civil engineering firms use to contract for wire-wrapped tanks are provided to them, free of charge, by the wire-wrapping companies. Although slight modifications may be made by these civil engineering firms, primarily in the size parameters, the structural requirements remain virtually unchanged in almost every case. Those structural requirements are designed, by the wire-wrapped tank contractors, to exclude competitive systems, especially tendon tanks. In any given geographical region of the U.S., only one or two wire-wrapping companies offer their services. In some cases these two companies have family ties and do not appear to compete very hard with one another. In other cases, whichever one you talk to is very good at writing performance specifications that guarantee only their company will get the job. Therefore, competition is greatly reduced or eliminated when using performance specifications that limit the bidding to Wrapped Tank Contractors.

The appearance of competition is sometimes created by bidding a conventionally reinforced tank alternate against the wrapped tank. Conventionally reinforced tanks, however, are generally not cost effective over about 500,000 gallons capacity, in JHCE's experience. Some designers, competing tank builders, and suppliers of tendon systems believe that once the decision is made to use a wire-wrapped circular prestressed concrete tank as the base bid on large tanks, there may not be any real competition unless the performance specifications are "opened up."

Between 1971 and 1981, JHCE designed each of their tank projects with three types of walls, each designed to the same structural criteria. Two were wire-wrapped, based on the two competing wire-wrapped tank contractors active in the West, and one was based on internal tendons. It was possible for the tendons to be supplied by any one of two or three competing companies. In all this time, encompassing between 20 and 30 tanks, only one went to one of the wire-wrapped tank contractors, the second tank designed this way. All the others were built as tendon tanks.

One project, in the mid-1970s, involved 12 wastewater treatment tanks for the City of Spokane, Washington. This project had been bid based on performance specifications written around one of the two wire-wrapped tank contractors active in the West. After the bids were received the other wire-wrapping company complained to the EPA, who was funding the major portion of the project. They asked for the performance specifications, that effectively limited the bidding to only the first wire-wrapped tank contractor, to be opened up so they could bid on the project. The EPA agreed, the performance specifications were modified somewhat and the tanks were rebid to the overall general contractor who had already been selected. Again the excluded wire-wrapped tank contractor complained that the revised performance specifications were still not sufficiently open for him to fairly bid the project.

At this point the general contractor, who needed to get this conflict resolved, advertised for proposals from any company who could help them with a circular prestressed concrete tank. A Denver based tendon supplier read the ad, contacted JHCE and put together what was ultimately to be the winning design-build proposal for these 12 tanks. It should be noted that our proposal saved 1.8 million dollars out of an original 4 million dollar bid by the first wire-wrapped tank contractor, according to the ENR article about it.

After awhile, JHCE realized that wire-wrapped tanks simply were not as cost effective as tendon tanks designed to the same criteria. Since then, JHCE has recommended to their clients that they could save design fees with no loss in competition by concentrating only on internal tendon tanks, at least in the Colorado area. JHCE has often recommended that wire-wrapped and tendon tanks be bid "head-to-head," particularly when there was some difference of opinion on what the outcome might be.

Two separate tank projects in the Northwest were recently bid using this philosophy. In these cases the wire-wrapped tank contractor, who is generally regarded to be the most expensive in the industry (they advertise their quality not their price), bid the circumferential prestressing at less than the tendon supplier's costs. This price was less than half of the amount the same wire-wrapped tank contractor has bid on comparable jobs recently that did not contain open specifications. Some think this wire-wrapped tank contractor is simply buying these jobs in order to dispel the idea that tendon tanks are cost competitive. Nevertheless, it is certainly turning out to be a bargain for the owners.

In 1993, JHCE teamed up with a general contractor and a tendon supplier to put in a design/build bid on three tanks in Worcester, a suburb of Boston. The tanks were being bid based on performance specifications the engineer received from one or the other of the two wire-wrapped tank contractors that operate in the Northeast. It was a closed specification intended to limit the competition to these two wire-wrapped tank contractors. Our general contractor, however, knew that this was illegal under Massachusetts law, so they put in an alternate bid anyway. Our general contractor's bid was about $170,000 less than one wire-wrapped tank contractor's and about $940,000 less than the others, who appeared to be giving only a "complimentary bid."

The point of the above is to show that an owner can only be better off with the increased competition. Even if the wire-wrapped tank contractors turn out to be the low bidders, the bid will certainly be lower than if there was not increased competition. The owner will most often, however, get what some consider to be a better tank, a tendon tank, at a lower price.

PROVIDING FOR A TENDON TANK ALTERNATE

Traditionally, the structural design of wire-wrapped tanks are usually provided for by way of performance specifications. This eliminates the cost of the structural design of these specialized structures from the general civil consultant's budget. Until recently, on the other hand, tendon tanks have usually been structurally designed by consulting engineers who work for the owner, generally as a subconsultant to the owner's general civil consultant. Tendon suppliers, precast- concrete companies and general contractors do not normally have tank structural design specialists on their payroll. However, they can, and often do, contract with a consulting engineer that does have expertise in tank structural designs, such as JHCE. There are several such consultants around the country.

If there is not sufficient budget remaining in your contract as the general civil engineer to hire a subconsultant to design the tendon tank alternate, a performance specification approach could be used. This would result in the structural design fee for the tendon tank alternate being included in the contractor's bid, as it typically is for the wire-wrapped tank. This would also be the better approach when time is short, as when the wire-wrapped tank is already being bid using performance specifications. Sample performance specifications based on AWWA D115-95, in electronic format, are available for your use free from JHCE upon request.

SUMMARY AND CONCLUSIONS

Some consulting engineers, such as JHCE, and owners who have had experience with both types, think internal tendon tanks are superior to wire-wrapped tanks. This is for reasons of corrosion protection of the circumferential prestressing and other features that make all aspects of tendon tanks more durable than their counterparts in wire-wrapped tanks. Also, tendon tanks are usually significantly more economical than wire-wrapped tanks.

If the performance specifications are opened up to include a tendon tank alternate, the project is bid, and the wire-wrapped tank is low, your owner would get a wire-wrapped tank at what you have confirmed is a good price. The benefits from this increase in competition can be obtained by adding a tendon tank alternate section to the otherwise unchanged bid documents.

Thank you for the opportunity of presenting these concepts to you. If you have any questions or require additional information, please feel free to contact us. Our E-mail and snail-mail addresses are located on the [JHCE] Home Page along with all the phone numbers you may need.

Sincerely, Jorgensen, Hendrickson and Close Engineers, Inc. Steven R. Close, P.E.