By Sue McCraven
Gingerbread man cookies, brown sugar, citric acid and a good rum cocktail. What do these products have in common? They all contain a sweet fluid made predominantly from processing sugar cane: molasses. Molasses is an important commodity; however, if improperly stored, it can be bad news (see sidebar “Boston Molasses Disaster & Hawaiian Spill”). Safely containing molasses is serious business, but thankfully, durable precast concrete storage tanks are up to the task.
In the Caribbean, Preconco Limited entered into an atypical public-private partnership with the government to build three new precast concrete molasses storage tanks with a capacity of more than 35,000 tons. The need for durable, strong storage for molasses had become a critical concern for the island’s controlling authorities because the three aging steel storage tanks in the port city had become structurally unsound.
“The first steel tank was in such a deplorable state that it was unserviceable based on all of the technical data,” said David Estwick, Barbados Minister of Agriculture, Food, Fisheries and Water Resource Management (1). Officials reasoned that the other two steel tanks were in similar states of deterioration.
Long-term storage solution
Construction of the precast concrete storage tanks is critical to sustain the Caribbean’s rum industry, as molasses is imported and stored in port facilities. With rum being a major Caribbean export, the Barbados Rum Committee heartedly supported the new storage tank construction.
“Without a storage facility at the Bridgetown Port to take imported molasses, we will have no rum industry,” said Chairman of the Barbados Rum Committee Frank Ward.
Preconco partnered with Britain’s Structural Systems Ltd. for design of the post-tensioned vertical panels that comprise the circular tank. Preconco designed the vertical panels and the tank foundation and is the installer of all three tanks (2). The company was also contracted to tear down the existing steel storage tanks.
Mark Maloney, CEO of Preconco, said he expects the third tank to be completed by early 2015. “Under the lease agreement, we will maintain all three tanks for their service design of 50 to 75 years,” he said.
No more flooding!
Plans called for Preconco to design and build two precast concrete molasses storage tanks with more than 13,000-ton capacity and one tank at nearly 9,000-ton capacity. The tanks range between 84 and 102 ft in diameter with a height of 23 ft. In addition to post-tensioned precast walls and post-tensioned cast-in-place (CIP) foundations, the company fabricated the roof, internal epoxy lining, piping inlets and outlets, measurement system and access ladder with roof handrails.
The client, Barbados Agricultural Management Co., selected a precast concrete tank design for long-term durability, low maintenance costs and the material’s increased ability to withstand marine environments over steel. An epoxy coating was placed on the inside surface of the tank because molasses, with a pH around five, can be corrosive to plain concrete. In fact, the acidic nature of molasses may have played a role in the deterioration of the original steel tanks.
The CIP raft slab includes a large ring beam under the walls and pads for the internal columns that support the roof. Foundation pads and buttress sections were precast and connected on site to the rest of the steel prior to casting the ground beams and floor slab.
All wall panels are produced from one semicircular steel mold, with full-height stainless steel form lining to give the tanks a smooth finish. Each wall panel is 18 in. thick and the post-tensioning anchors end in buttress sections, which are 27.5 in. thick.
Walls and buttress sections are erected on site using a vertical CIP connection that is 8 in. wide to join the pieces. The roof is comprised of inverted, prestressed T-beams supported by precast columns, and is decked with 8-in. hollowcore planks. A structural screed is then cast. To achieve the desired adhesion and remove laitance and contaminants, all surfaces are shot-blasted. Finally, an external ladder and handrails are fitted to the structure and an ultrasound measuring gauge, used to monitor the volume of molasses, is extended through the roof. Tanks are hydrostatically tested to full capacity using seawater.
A not-so-sticky situation
Molasses presents an interesting storage challenge, but precast concrete provides an effective, durable solution that is built to last. Thanks to the strength of precast concrete, molasses can follow its proper path from storage to customer, allowing us to consume the products we enjoy while keeping the sticky material out of our streets and waterways.
Sue McCraven, freelance writer and NPCA technical consultant, is a construction engineer and environmental scientist.
Sidebar – Boston Molasses Disaster & Hawaiian Spill
Molasses may be sweet, but it is not always innocuous. In 1919, residents of Boston’s harbor district heard the frightening, machine gun-like sound of popping rivets. Moments later, a 15-ft high, 2.5-million gallon wave of Puerto Rican molasses exploded out of a huge, 50-ft high steel tank that demolished buildings and killed anything alive in its sticky swath. Twenty-one people lost their lives in Boston’s North End that unseasonably warm January day.
The ensuing six-year long lawsuit against the tank owner, United States Industrial Alcohol Company, led to a noted MIT engineer determining that poor construction led to the tank’s collapse (3). As a result, state laws were enacted to require an engineer’s inspection of tanks.
In 2013, a similar disaster occurred in Hawaii when a pipeline carrying molasses from Oahu to cargo ships released 233,000 gallons into the waters of Honolulu Bay, not far from Pearl Harbor. This time, molasses created an environmental disaster that killed large numbers of marine life. Because there is no method for remediation, scientists said the only thing to do now is wait for natural forces to run their course (4).
- To view a Preconco video of molasses tank precast panel installation, see http://www.youtube.com/watch?v=Urs5T733_gg