Natatorium naysayers argue – among other absurdities – that an ocean pool can’t survive being in, well, the ocean.
We demonstrated with a post on the ocean pools of Australia just how wrong that kind of thinking is.
Turns out, though, that we could have gone a couple of continents farther on and about 20 centuries earlier to make the point. To ancient Rome, in fact.
As the science news site Futurity reports, the Romans were whizzes at building concrete maritime structures that have – literally – stood the test of time. Wharves, breakwaters, other harbor structure – all made of Roman concrete and all still in existence two millennia later.
Here, for instance, is a photo of a sample of ancient Roman concrete. It’s drilled from a breakwater in Pozzuoli Bay, near Naples, Italy, that dates back to around 37 B.C.E.:
(Credit: D. Bartolo, courtesy of J.P. Oleson, via Futurity)
The study described in the Futurity story – conducted by engineers from the University of California, Berkeley – involved analyzing core samples from the still-standing breakwater. Modern concrete, the story says, “begins showing signs of degradation within 50 years.”
“Roman concrete has remained coherent and well-consolidated for 2,000 years in aggressive maritime environments,” Marie Jackson, a Cal-Berkeley research engineer in civil and environmental engineering, says in the Futurity story.
It is one of the most durable construction materials on the planet, and that was no accident,” Jackson adds. “Shipping was the lifeline of political, economic and military stability for the Roman Empire, so constructing harbors that would last was critical.”
The Cal researchers are working now to develop a modern version of Roman concrete that is not only durable but quick-setting. One key ingredient borrowed from the Roman recipe may be volcanic ash. Gee, d’ya think we could get any of that around here?
P.S.: “Natatorium” Etymology
Is it worth mentioning, perhaps, that “natatorium” is a Latin word? Just sayin’.
Engineer
I deal with design and implementation of repairs to deteriorating concrete in Hawaii on a daily basis. The Natatorium is definitely not unique in it’s mechanism of deterioration. This deterioration is in fact a common feature to any concrete structure in Hawaii and other places subjected to ocean exposure. Construction practices have been evolving to make coastal structured more durable for a long time, the Natatorium is an old structure and understandably (obviously) it has some issues. What needs to be said is that concrete durability in ocean environments is well established, there should be no question on this – making concrete durable in the ocean is not new technology, there are no ancient secrets that need to be rediscovered to make this a reality, although research into improvements is always ongoing. I will attempt to describe some technical aspects of the deterioration in layperson’s terms for the benefit of your readers.
The Natatorium is suffering from corrosion of embedded reinforcement, the expansive forces resulting from corrosion of the steel reinforcement that is internal to the structure is causing the encasing concrete to fail. The root cause of this is primarily the ingress of chloride ions into the concrete and probably secondarily the reaction of the concrete with carbon dioxide in the air (called Carbonation). Without ingress of corrosion promoting chemicals or carbonation, steel that is embedded in concrete will not rust, even if it is wet. the embedded metal is protected by the naturally high pH of the concrete, the presence of chlorides at the reinforcement overcomes this protection, Carbonation reduces the pH of the material, compromising it’s protective nature.
Repair of these conditions is normal maintenance for an old concrete structure, procedures are well established to create durable repairs for this type of common deterioration. To create concrete structures that are durable in coastal exposures, various construction practices have been developed, and are continually being improved upon (as noted in your nod to current research to learn lessons from ancient practices). Making concrete durable in coastal exposure is primarily a battle with salt to prevent corrosion of embedded metal, which is the bane of concrete. (ancient concrete did not use embedded metal reinforcement).
The distance between an embedded metal element and the exposed surface of concrete is called cover. Increasing this cover distance, increases the lifetime of a reinforced concrete component – there is more distance for the salt in the environment to travel before it reaches a piece of metal that can corrode, carbonation also begins from the exposed exterior, so increased cover also prevents carbonation from being problematic.
Improving the quality of the concrete decreases it’s permeability. With decreased permeability ingress of environmental salt and carbonation reactions with air are slowed. Concrete quality can be improved in various ways, including using less water in comparison to cement in the mix and including supplementary cementitious materials (an example being volcanic ash) other examples of supplementary cementitious materials currently common include waste ash from various processes (fly ash, silica fume), slag (not so much in Hawaii), and some materials of natural origin (also not too common in Hawaii). Adding certain polymers to the concrete mix can also aid in decreasing permeability.
Reducing the reactivity of the reinforcement elements themselves obviously would prolong the lifespan of a structure subjected to a corrosive environment. For this, non-metallic reinforcing (carbon fieber, fiberglass, etc) are sometimes employed. Metals with inerrant corrosion resistance are sometimes used (stainless steel of various grades are available). Finally coatings on the embedded steel (epoxy, zinc, and other metal claddings) can be used for protection.
Also, using the principles of electrical protection can prolong a structure’s life, a characteristic of corrosion is that a metallic material more reactive can be attached to the embedded reinforcing and serve as a sacrificial element to stave off corrosion, which is a principle used in passive systems. In active systems, a small electrical current to counter the corrosion potential can eliminate all corrosion as long as the system is operational, extending the structure’s lifespan indefinitely.
These considerations are so well known and predictable that there are actually a number of established simulation methods to predict the lifespan of a concrete structure. These computer algorithms can be used in a life cycle analysis to inform designers and justify (or not) the added initial expense of some of the protection options available (for example, to answer if it is really worth it to install a more expensive version of the reinforcing bars). Many important reinforced concrete structures in coastal environments are intended to have service lives of at least 100 years, a performance level that can be expected if the work is done properly. With the deplorable condition of the Waikiki Natatorium, many of the structural elements would undoubtedly need to be reconstructed, work which could be performed in a way that would last for a very, very long time.
So to conclude, your naysayer is way off on this one. Restoration of the Natatorium appears to be more a question of cultural importance and financial feasibility, NOT a question of technical feasibility. Thanks for the articles. I am hoping to swim in that pool one day, good luck!
admin
Thank you so much for this simple yet technical explanation of what is going on at the Natatorium. You’ve hit the nail on the head. Virtually no money was spent on maintenance and repair of the Memorial for the 30 years prior to its closing, which no doubt contributed to the deterioration of the deck and walls. The good news is that the pilings are still very sound and relatively speaking, the deck is cosmetic compared to those elements of the structure. Bottom line, you’re right, it is not matter of engineering (an updated pool design has already been vetted by an EIS and acquired all necessary permits). It is a matter of will. Let’s fix the pool and swim.