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Renewing A Beachfront Barricade

Located less than an hour west of Honolulu, Pililaau Army Recreation Center serves as a quiet beachfront getaway for military personnel and their dependents based at nearby Schofield Barracks and other installations. Considered one of the best beach facilities on the Hawaiian island of Oahu, this popular vacation spot on Pokai Bay boasts white sandy beaches, gentle waves, and spectacular sunsets.

The Pililaau center features guest cabins, a dining facility, a police headquarters, an administrative office, a recreation center, and other structures. To protect the complex from the threat of beach erosion, an approximately 2,000-foot-long stone sea wall serves as a barrier between the beach and the structures. The wall measures nearly 11 feet from top to bottom, and the natural lava stones are held together with a concrete sand mortar mix. Also, a concrete and sand mix cap measuring 1.5 inches thick by 12 inches wide covers the top of the seawall.

The Army will use a novel concrete additive to repair crumbling sections of the sea wall at Pililaau Army Recreation Center on Pokai Bay, on Oahu. Photos courtesy of Army Corps of Engineers ERDC-CERL.

Unfortunately, the barricade itself has fallen prey to the effects of pounding surf. During the winter of 2007-2008, storms washed out a portion of the seawall. In addition, much of the wall's concrete cap and other areas have fallen victim to cracking and other types of deterioration. Furthermore, a section of sidewalk straddling the top of the seawall and steps leading to the beach have deteriorated.

Although temporary stabilization measures have been applied to the damaged sea wall, the Army is seeking a more permanent solution to preserve structural integrity and safeguard the recreation centerís visitors and employees. Otherwise, the seawall will continue to crumble, and foundations supporting various structures could be compromised and even collapse. The Construction Engineering Research Laboratory (CERL) of the Army's Engineering Research & Development Center (ERDC) has proposed a solution that will retain the sea wall's natural lava stone appearance while making it more resistant to degradation.

Engineers at CERL have launched a demonstration project to repair the damaged retaining wall with a concrete and mortar that resists saltwater penetration. Although concrete is a very versatile building material, its porous structure allows it to absorb water and dissolved salts like a sponge through a process called capillary absorption. The water and dissolved salts within the concrete will corrode the sea wall's critical steel reinforcement. The laboratory is using a commercially available hydrophobic concrete additive called Hycrete DSS in the concrete and a mortar mix that will be applied to repaired sections of the sea wall. The admixture will make the concrete waterproof, blocking the absorption of water and dissolved salts. As a result, the structure's steel reinforcement will be protected from corrosion.

According to CERL, the admixture reacts with calcium in the concrete and prevents capillary absorption. In addition, the Hycrete molecule is designed to produce a layer of protection at the molecular level by bonding with polar particles such as iron in the steel reinforcement. Meeting strict environmental criteria and human health standards, the corrosion-inhibiting admixture technology has proven itself in projects such as basement walls, sewer tanks, swimming pools, and bridge overpasses.

In addition to repairing and reconstructing washed-out areas of sea wall, CERL will use the hydrophobic concrete to replace unserviceable sections of stairwells, handrails, sidewalk, and seawall cap. The repair and replacement project is expected to begin in April of 2010.

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